Bunts, PPOs, neg G

[Doug Riley]

Joe/Spiff comments in the current accident thread about Greg Gremminger's discussion of "Buntover." Like much other gyro lingo, the terminology here is not perfectly consistent.

"Bunt" was until recently a mostly British expression, when used in the aeronautical sense (rather than in reference to baked goods). It referred to an intentional nose-over maneuver commanded by the pilot, ordinarily of a FW machine. At the conclusion of the investigation of Pee Wee Judge's crash (which happened in England around 1970), the report stated that it appeared that a "bunt" maneuver was hazardous in a gyroplane.

Bensen always claimed that the many accidents in which a gyro pitched upside down, ate its rotorblades and fell out of the sky resulted from "negative G" induced by a pilot throwing the stick forward -- bunting in the proper Brit sense. Never fully explained was why things went so very badly when these foolish pilots supposedly did this thing. Was the rotor really tipped forward and lifting downward? If so, why didn't it push the top of the mast back and rotate the gyro nose-UP? Why did the machines go all the way inverted and tumble every time?

When the role of a prop thrustline offset from the CG was finally widely recognized around 1990 (as a result of thinking and investigation in several countries, including especially Finland, England, France and the U.S.), Bensen's self-serving theory of the ham-fisted pilot was revealed to be a half-truth. The term "power pushover" (PPO) was a good choice for a replacement. Confusingly, "bunt" crept in as an alternate term for the same thing. That is, THIS version of "bunt" is not a pilot's attempt to start an outside loop, but an UNcommanded nose-over induced by engine thrust when the counterbalancing force of the rotor takes a temporary vacation.

Greg Gremminger has adopted "bunt" in yet a third sense, to mean something approximating "precession stall," as nearly as I can tell. That is, I think he means any sudden pitching rotation of the airframe that is too fast for the rotor to keep up with. This use makes PPO a large subset of the set of "Greg Bunts." Other possible G.B.'s could, we believe, be caused by draggy landing gear or a big body pod with a negative aerodynamic moment. A third, theoretical but unproved possibility is a G.B. caused by a very large LOW thrustline offset.

Such an extreme LTL offset would cause large pitching rotations in what we think of as a "stable" direction -- power down, nose down; power up, nose up. There may be such a thing as too much of this. Large pitching excursions induce large cyclic inputs into the rotor, not necessarily limited by the flap stops (as our manual stick inputs pretty much are). The sector of the rotor that gets a cyclic-pitch increase in such a frame-driven input could experience cyclic blade stall and teeter down into some part of the frame.

None of this means that HTL is as good as any other thrustline (it sure isn't) or that LTL is always bad. A modest LTL simply imitates the effect of a down-loaded HS that happens to sense throttle setting. Large LTL offsets probably should be avoided; HTL ones are a very well-established killer. Cierva advocated and patented CLT, as we now have re-descovered.

Maybe we should consign "bunts'' to the church bake sale and get some clearer nomenclature.

[C. Beaty]

English isn’t precisely the same everywhere, Doug. There are many regional colloquialisms that are gradually disappearing in this age of mass communication. Until you enlightened me, I had no idea a bunt was an item of pastry.

Until I had actually lived on Cape Cod for a while, I had no idea a milkshake was a frappe or that a black clam was a quahog (pronounced ko-hog).

In the Deep South, a bunt was either something one did to a baseball or a forward flip off a diving board; sometimes what a goat would do to you if he caught you with your back turned. The posture assumed preparatory to a forward flip from a diving board was reminiscent of a charging goat.

I do agree that the terminology ought to be standardized, however.

[Doug Riley]

Never heard of a diving bunt. "Frappe" is Mass. talk, but when I lived in Providence, R.I. the same drink was called a "cabinet." Order a "milkshake" there and you'd get a glass of milk with chocolate syrup in it.

The church-supper cake is actually spelled "bundt," I gather from the 'Net.

The point of my initial drivel was that Greg is probably right about multiple causes of bunts, IF "bunt" is more broadly defined than usual. A CLT or LTL gyro is physically incapable of bunting itself (in the PPO-only sense of the word), since the engine continually applies a neutral or nose-up torque.

[spaceman spiff]

All this talk about cakes and shakes is making me hungry!

Thanks for taking the time Doug.

Lots to talk about in your post. I accept your opinion that there are multiple modes of "bunting"; one where there is a flip, but seems those are extreemy rare. i remember having a legthy discusion with Ron about it, there were links to various crashes on the thread, and i couldn't find one that had the PPO tumble everyone talks so much about, which makes me wonder if the PPO isn't a self serving label aplied to every crash the same as the old Benson one. Anyway there were a few incidents listed where there is something less than a flip, reports where the gyro noses down and just keep going. The nose down and keeps going one is the one i am most interested in.

I have an idea that what locks you into one of those dives has more to do with negative loading on the disc, than anything else. Normally you have a rotor that has formed a coning angle facing upward. The disc is of course balanced by a trim spring, a center of drag that is higher than the hub bar because of the coning angle, and presumably the back of the disc is being supported by some prop blast. The thing keeping the cone from just flopping all the way back like a paper funnel being held by the small end, is that the teeter bolt, which is pulling the whole thing thru the air, is raised above the hub bar a few inches so the disk is being pulled near its center of drag.

What happens in negative G is the coning angle turns inside out so the teeter bolt now is creating a lever instead of removing one, so any drag on the disk is now trying to pull the stick forward. If the negative coning angle is great enough, the back of the disk is still in prop wash which also is forcing the stick forward and you are locked into a big outside loop going to the ground.

Recall a week or so ago, we had a long talk about velocity related forces and acceleration related forces and how the two do not necissarly exist to gether at all times (the moving sidewalk analogy) that same distinction comes into play here.

So basically if you are flying 60 MPH and there is no acceleration, there is no TL moment, if something happens, pilot caused or otherwise that gets you into strongly negative loading, the disk cones negative, you dive, there will be some acceleration at this point, mostly due to gravity (600 pounds) but there is no moment associated with acceleration of gravity. For a very breif period before it reaches terminal velicy, you may get some acceleration moment fom your prop, might be enough if you were going slow initially, have lots of room, lots of LTL and jam the throtle as you dive, if not your probably screwed.


Theres my theory. Feel free to scoff, belittle, harass all you want. My fire suit is on!

[Udi]

Joe - I don't want to scoff belittle or harass you but there are so many conclusions in your post that are based on misconceptions, that I don't even know where to begin. Please give me and others here the benefit of the doubt - we know what we are talking about.

Take a piece of paper and draw all the forces that are acting upon a gyro in flight (yes – steady state, un-accelerated flight). You may choose the gyro CG or any other point as your datum. To make it easier, ignore airframe drag, and ignore the stabilizer. Draw vectors for rotor thrust, engine thrust, and weight.

Now - figure out what determines the pitch angle (atitude) of the gyro in flight and (this is a harder one) WHY is it that the gyro is not just spinning in space? What is keeping the gyro right side up? Keep in mind that the rotor is NOT hanging from the sky, like you would hang it from the ceiling in your garage. The gyro is not a pendulum.

You have to figure this out before you can really understand why a HTL gyro will pitch nose down - sometimes violently so - upon loss of rotor thrust. By the way - you are making the rotor dynamics way too complicated. The teeter bolt is like a universal joint, which means that there is only one rotor thrust vector, it is passing right thru the center of the disc, and it is perpendicular to a plane formed by the tips.

Udi

[GyroRon]

Spaceman is using eye witness reports in the accident reports to base his assumptions on. I know if has been discussed before, but in most cases the eye witness accounts are not much to go on.

In a Buntover or power pushover, not all gyros will be flying straight and level forward and just tumble straight over. Some are going to roll over due to engine torque, some are going to go completely out of control in any direction. But if they didn't buntover, then what did they do?

I am about done with these discussions with Spaceman. How can a person sit here and argue with someone like Chuck Beaty, Doug, Udi and everyone else over a subject that is so simple to understand? That is like a 5th grader going to Albert Einstien and telling him he is wrong about some simple math formula..... It is none of my business what he does with his gyro, and frankly at this point I don't care. I began by seeing someone new to this sport with a gyro just like the one that crashed this past weekend, and tried to offer simple free advise to help make sure that doesn't happen to him. But instead of going with the flow, and putting on the CLT kit, he has gone out and laid his gyro on it's side on bathroom scales and tells us that the CLT kit from aircommand puts the thrustline extremely below the CG and that he is alot closer to CLT than he would be if he put on the CLT kit. I suppose Aircommand and Dominator, Mayfield and the boys over at Sparrowhawk, Larry Neal and his Butterfly and Monarch line of gyros, and many more, Are ALL Wrong and we are all still flying machines that can nose over or roll over and have the rotors eat the tail cause thrustline to CG placement is not important. Also if we just had a stab these accidents would not happen, and even though some of the gyros that we believe did a power push over had stabs, it was a UFO's tractor beam or someother mysterious force that flipped these gyros and caused them to tumble to the ground, not the thrustline!

[Doug Riley]

Now, now, Ron -- it's good to be skeptical. This is not a religion, and for a mechanical engineer to question the "received doctrine" is fair game.

If Joe wants to flip the engine and raise the seat to get the Air Comm. CG where it's supposed to be, I say have at it. He's a mechanical engineer and knows how how to check the CG when he's done. (I may be biased, as a somewhat similar project is sitting in my garage.) I think such a mod could be enough on an A.C. with no pod or extra tanks. The pod would make the low CG more severe, and I imagine Air Comm's more extreme kit takes that worst-case scenario into account.

That's not to say that it won't be more work than Air Comm's bolt-on kit. You have to lengthen the tail boom, raise the seat at least 8 inches, move the pedals and control stick up the same amount, and be sure you have brakes because your feet can't be used, Flintsone-style, any more. Flipping the engine involves also moving it back, which requires alteration of the engine mount and may screw up rotor-prop clearance. Raise the rotor and you'll need new pushrods. The prerotator shaft and lower end may or may not fit anymore. One change begets ten.

I don't understand Spiff to be saying that CG location is irrelevant even when one of the forces keeping the gyro in static equilibrium disappears. That's basic stuff that you get in Mechanics of Solid Bodies, a first-year Engin. course.

[Udi]

...So basically if you are flying 60 MPH and there is no acceleration, there is no TL moment, if something happens, pilot caused or otherwise that gets you into strongly negative loading, the disk cones negative, you dive, there will be some acceleration at this point, mostly due to gravity (600 pounds) but there is no moment associated with acceleration of gravity. For a very breif period before it reaches terminal velicy, you may get some acceleration moment fom your prop, might be enough if you were going slow initially, have lots of room, lots of LTL and jam the throtle as you dive, if not your probably screwed...
Have you read this, Doug...

[C. Beaty]

Engineer, Udi, is a loosely defined term. A guy that drives a locomotive is an engineer. A person who tends a boiler is a stationary engineer. In the UK, a guy who mends broken automobiles is an engineer. Draftsmen/designers call themselves engineers. In English, anyone who messes with engines or machinery can call himself an engineer.

The Germans have the right idea in attaching DiplIng. after their names if they're university graduates. Avoids much confusion.

[spaceman spiff]

Spaceman Spiff AKA Joe Jensen (dpling) mechanical engineer, class of 88 BSME UofA, Principal engineer, inventor of the durtadur tube, amature dogma slayer, at your service!

[C. Beaty]

Ok Spiffy, perhaps you can follow this:

The propeller thrust line of an RAF-2000 passes 10”-12” above its CG (measurements performed at the University of Glasgow in a study of gyroplane stability funded by the UK CAA, contract #7D/S/1125), dependant upon loading.

The propeller thrust required for steady flight at 60 mph is ~400lb.

The nose down moment about the CG is then ~400 ft-lb.

If the all up weight is 1200 lb., the rotor thrust line must pass 1/3 ft. (4 inches) in front of the CG to maintain equilibrium. This is a simple tail heavy situation that causes response to disturbances to be in an unstable direction.

The pitch axis moment of inertia of an RAF-2000 is ~75 slug-ft².

Suppose rotor thrust is suddenly removed.

The angular acceleration of a body about its CG is equal to: torque/moment of inertia. Result is radians/sec².

An RAF-2000 with rotor thrust suddenly removed accelerates in a nosedown direction at a rate equal to:

400/75 = 5.33 radians/sec².

The angle traveled is 1/2at²

The time to invert (rotate pi radians) is:

(2pi/angular acceleration)^0.5 = (6.28/5.33)^0.5 = 1.085 seconds.

A neck snapping flip.

The University of Glasgow subcontracted wind tunnel testing to the University of Prague (Czech Republic).

They built a ¼ scale model of the Magni with the propeller driven by an electric motor. The finding of the University of Prague was that the horizontal stabilizer wasn’t very effective except at severe nosedown yaw angles where the horizontal stabilizer was immersed in the prop wash. The lift slope of a horizontal stabilizer in the shadow of the fuselage structure is much shallower than with the same airfoil exposed to the free stream.

But lets discount the wind tunnel testing and say the stab acts just like it’s exposed to the free stream. Discount also the negative pitching moment of a sloping windshield and landing gear drag well below the CG.

A low aspect ratio stab will develop a maximum lift coefficient of perhaps 1.0 to 1.2 at stall.

If a stab had an area of 10 ft², it could develop a maximum lift of 89 to 107 lb. at 60 mph. (lift = 1/2pV²ACl)

To balance a 400 ft-lb moment, the stab would have to be on a moment arm of: 400/100 = 4 ft.

I don’t think any RAF has a 10 ft² stab on a 4’ moment arm. So even discounting wind tunnel testing and negative moments from cabin and landing gear, it’s nearly impossible to get enough stab power to balance the propeller pitching moment. And even if possible, the machine is needlessly burdened with an extra 100 lb of load.

The University of Glasgow’s Dr. Stewart Houston created a computer simulation of a gyroplane that had more degrees of freedom, such as rotor rpm coupling, than can be addressed in a simple presentation like this. The computer simulation indicated that best stability was obtained with propeller thrust line passing several inches below the CG. Such a configuration ensures that the CG is always ahead of the rotor thrust line.

The computer simulation was verified by testing conducted on an instrumented Magni. The measured and predicted behaviors were essentially identical.

The process of a gyro catastrophically tumbling generally involves more than a simple forward flip. As the rotor becomes unloaded, the machine will roll in a direction opposite to propeller rotation. This rotation could be eliminated with contrarotating props but a tall tail works nearly as well.

The roll-over may, in some cases, be a greater culprit than the pitch over. The torque that must be applied to a propeller receiving 160 hp at 2400 rpm is 350 ft-lb. And the roll axis moment of inertia will be considerably less than the pitch axis moment of inertia. In either case, the angular rate is greater than the rotor can follow (known as precession stall) so the rotor stalls and chops up the airframe; the airframe, in effect, tumbles into its own rotor.

[Doug Riley]

Udi, to give Joe his due, I think I understand what he means. In steady-state flight, the aircraft is in static equilibrium. There are, IOW, no unopposed forces or couples in or about any of the three axes. The engine thrust is necessarily being opposed by SOMETHING, or we wouldn't have straight unaccelerated flight. As long as the engine thrust is fully opposed, there is no "moment" in the strict definitional sense.

This boils down to saying that a HTL, no-HS craft CAN fly in equilibrium. We know that. It is the "argument" that RAF advocates use, though they may use different words.

The problem is that this equilibrium is too fragile in the real world. Remove one of the forces that create the equilibrium, and obviously you're then OUT of equilibrium. At that point of DISequilibrium, the thrust force then produces an acceleration. A force whose line does not pass through an object's center of mass is the equivalent in effect of a force that DOES pass through the C.M., plus a couple. The moment of the couple is the PPO moment. The moment equals the force multiplied by the distance of the line of the force from the C.M., measured long a line perpendicular to the line of the force. This couple causes an angular acceleration -- a pitchover.

The lesson is that we must not try to maintain our equilibrium using forces that aren't reliable in real life. It turns out that the thrust of the lifting surface in an aircraft comes and goes, even in normal flight. We simply can't rely on this force always to equal X, and therefore to oppose an engine thrust of Y. We need to arrange the line of the engine's thrust so that, when rotor thrust shrinks for some reason, the moment of that couple that pops up equals zero. That's what CLT is.

LTL means that there IS a non-zero couple when you hit disequilibrium, but the angular acceleration it produces is in a statically stable direction. Even so, this angular acceleration should not be too violent. A little LTL is plenty.

Sorry for the jargon.

[spaceman spiff]

Thanks,

I think the problem is in the statement :

"The propeller thrust required for steady flight at 60 mph is ~400lb.

The nose down moment about the CG is then ~400 ft-lb."


That is all based on the assumption that the thrust is always acting relative to the CG of mass. The moment you speak of requires acceleration to exist. Simple F=MA if there is no acceleration guess what? no force.

@ 60 MPH steady state, the thrust is being gobbled up by a different set of forces completely independent of mass center, put simply, the drag and various momets above the line of thrust have to equal the the drag and associated moments below the thrustline, suddenly take away the drag of the rotor and you are going down in a head snapping pitch, no matter where your Mass CG is.

There is some benefit in raising CG in that instance, it will slow the pitch down so you might get to say "oh shi" instead of just "oh s"

[Udi]

...@ 60 MPH steady state, the thrust is being gobbled up by a different set of forces completely independent of mass center, put simply, the drag and various momets above the line of thrust have to equal the the drag and associated moments below the thrustline, suddenly take away the drag of the rotor and you are going down in a head snapping pitch, no matter where your Mass CG is.
You're still not getting it, Joe. Have you drawn the balance of moment in flight?
This is not so hard - only three vectors about a mass, in balance. Where these vectors pass in relation to the mass CG will determine whether and how the mass will spin when one of the vectors is taken away. I don't know why I am bothering, but the attached diagram may get you started.

Udi

[Doug Riley]

Spiff, you may be getting drawn into the dreaded "Drag Fallacy." Thankfully, this has nothing to do with cross-dressing. It's common among advocates of HTL gyros, though.

When the rotor enters a low-G state, both lift AND drag are reduced. (Lift and drag are vector components of a unitary force, rotor thrust). The diagonal line in Udi's picture is this unitary force. If the horizontal force (engine thrust) is ABOVE the CG, angular acceleration upon loss of rotor thrust will be nose-DOWN around the CG. If the engine thrust line is BELOW the CG, the angular acceleration will be nose-up.

This checks out quite well in practice. Fly a LTL gyro in turbulence and it will nose down into updrafts like (dare I mention?) a dowser's rod. It will nose UP into downdrafts. If reduction of rotor thrust always caused nose-down pitching, then the opposite would occur in a downdraft. A downdraft is a low-G event.

[spaceman spiff]

I called a guy in phoenix area to get a flight on a Sparowhawk, get to try one in a couple weeks.

[spaceman spiff]

" When the rotor enters a low-G state, both lift AND drag are reduced. (Lift and drag are vector components of a unitary force, rotor thrust). The diagonal line in Udi's picture is this unitary force. If the horizontal force (engine thrust) is ABOVE the CG, angular acceleration upon loss of rotor thrust will be nose-DOWN around the CG. If the engine thrust line is BELOW the CG, the angular acceleration will be nose-up."


Yup, I agree that there could be a momentary nose up or down moment associated with acceleration on loss of rotor drag. (while the craft is accelerating to a new equilibrium speed), but that moment will NOT be total thrust times the CG - thrustline distance, it will ONLY be that portion of newly available thrust that is causing that acceleration moment. The remainder is still getting eaten up in any drag that still remains. It is also important to note that the center of drag will drop. The million dollar question is which moment wins? If the temporarily missing rotor drag is at a lever of 4 feet relative to the new center of drag and the thrustline is at a lever of 6 inches relative to the center of mass, I believe the temporaily missing rotor drag has controling advantage.

[scottessex]

Why do people that don't fly, know so much more than those of us who do fly?

It amazes me at the amount of people that think they can "re-invent the wheel" with out any experience.

[spaceman spiff]

Not inventing anything. I beleive in stabs i believe in thrustline.

I spent 8 to 10 hours a day for 5 and a half years studying. Thats more hours devoted to the topic of engineering, that you will ever have flying.

I also dont have to blip the throttle or go thru a thermal a milion times over a decade to notice a nose up or nose down and observe corectly. If i am paying attention, once is enough.

[GyroRon]

Yeap, all the times gyros have Power Pushed over it was cause they were trying to go faster when the rotorthrust has disappeared for a momment, Poor guys and gals would still be here if they had just installed cruise control.....

If you stop to look at the bigger picture Scott, you will see that none of this matters. Why would you have to have any of this make sence in your own head? If You fly a Aircommand and the factory claims that you need to upgrade the gyro with a kit that basically flips the engine and raises the seat, why question it? Especially when all the other NEW successful designs to hit the market in recent years all more or less have the same shape and layout as a Aircommand with this CLT kit installed? Greg G who Spacey apparently has more faith in than any of the regulars here did his own CLT conversion for the aircommand years before Aircommand came out with their own. Why would greg re-build his aircommand if he felt it wasn't needed.

[Brent_Brown]

3 to the power of 5 and e= MC(pie) carry the one and Mu 2 you.

[spaceman spiff]

The standard CLT kit is not what i want.

It comes with a prerotator cable, trouble is my bendix is rusted, the lower end has a cracke, and i just got a motor with electric start, so i will have a battery anyway and i have become accustomed to the electric prerotator. i am going with electric, so i don't need the cable.

It comes with a keel extender, the thing fitts poorly, whole tail flopps around more tha half a degree. no thanks, i'l make my own.

The standard tail is a flat plate, i want it to be bigger and have winglets to improve its ability to stabalize and help it track around corners. Placed an order for the Sport copter HS a few days ago.

I measured my CG, at present i only need a few inches of seat rise, once i extend the mast it will need even less. I will have a true CLT and a sturdy front wheel, and you will not.

got a problem with any of that?

[C. Beaty]

People confuse themselves by thinking of rotor drag as a force existing in isolation.

A gyroplane rotor is no different from a helicopter rotor or a propeller. It produces a LINE of thrust along its tip plane axis. With freely hinged blades, a rotor can apply no torque (moment) to the rotorhead.

The thrust of a rotor can be resolved into rectangular coordinates; lift and drag, or in the case of a helicopter, lift and propulsive force. These are virtual forces that don’t have to be resolved into vertical/horizontal components, we could resolve them into lines of force running in any direction of our choosing if we wanted to seriously confuse ourselves.

If the wind was blowing 14 mph out of the northeast, the weather bureau could correctly state the wind out of the north was 10 mph while at the same time, the wind out of the east was also 10 mph but thankfully, they don’t.

The only real force is rotor thrust. A vector coaxial with the tip plane axis. It’s as though a gyro is hanging from a rope running in the direction of the tip plane axis.

Cut the rope and what happens?

If there is a residual torque (moment) about the CG, the machine begins a pitch axis acceleration; for good or ill. Moment is short for moment of force (force + moment arm) and has precisely the same meaning as torque.

But broken ropes aren’t the problem.

A reduction of rotor thrust caused by a gust can initiate a sequence of events that mimics a broken rope.

When the propeller thrust line passes above the CG, the rotor thrust line must pass forward of the CG to maintain equilibrium. A reduction of rotor thrust upsets equilibrium and initiates a nose down pitch acceleration, tilting the rotor disc forward, farther reducing rotor thrust, etc. It can rapidly become self sustaining and irreversible as can be seen from all the smoking holes in the ground.

Tail heavy airplanes, those with CG behind the aerodynamic center, suffer similar problems. They are unstable in pitch and can enter an unrecoverable flat spin.

A horizontal stabilizer can serve as a bandaid for offset of the propeller thrust line but is an imperfect solution.

The primary purpose of a horizontal stabilizer, other than providing damping, is to balance aerodynamic offsets such as landing gear drag and sloping windshields and to have enough force left over to provide weathercock stability. The only correct way of balancing aerodynamic offsets is with a horizontal stabilizer, ensuring that balance is not upset by changes of airspeed.

A large offset of propeller thrust line balanced by a stabilizer of sufficient power, introduces undesirable side effects. Engine power changes cause pitch motion and airspeed change.

With stick locked, changes of engine power should cause the machine to climb or descend at constant airspeed.

According to Peter W. Brooks in his book; “Cierva Autogiros,” the first fatality in an Autogiro occurred after the accumulation of 35,000 hours in 1932. The fatality rate of general aviation in the US in 1939 was one per 5,000 hours.

Some might say this outstanding safety record was because Cierva’s machines were tractors but that is only indirectly so. Tractors, by nature, provide nearly correct propeller thrust line placement and also
enable the installation of satisfactory stabilizer surfaces on an adequate moment arm. Cierva found it necessary to provide a slight nosedown tilt of the engine to assure passage of propeller thrust line through the CG and received a patent for this arrangement.

Tractor Vs. pusher of its own accord is irrelevant. The engine/propeller assembly is bolted to the airframe and applies propulsive force along a fixed line. It is totally unrelated to the wheeled “Quacky Duck” toys we pulled by a string as children. With the toys, propelling force of variable direction is applied to a point ahead of the center of resistance and the toy follows the string.

[scottessex]

Look Spiff, I am not trying to get in a pissing contest, I mean no disrespect.
But changing one thing leads to other changes bla bla bla,
It's good to question and understand how things work etc. But after building and flying a couple of gyro's and helping other people on thiers, I feel you would be happier getting the machine flying, and then make small changes a little at a time, where you can tell if the change is for better or worse, a "work in progress" It is much more enjoyable to be able to fly, then tweak the machine, then test it out, than staring at the sky everytime you want to fly while your machine is down for numerous modifications at the same time,

Been there done that.

Theory and book work is fine, knowledge is good. practical application is good also.
example:
You can look at a power curve chart on paper and it seems to all make sense,
BUT when you actually experience behind the power curve, it really makes sense!

Keep us posted on your project.

[Heron]

Thank you all for bringing this important subject back and taking time to explain one more time. There are new people here and some of the older ones still need this refreshing dialogue.
(place for a sentence directed to somebody that ignores all my posts) :D
Thank you all!
Heron

[spaceman spiff]

At the very least it is stimulating isn't it.

C Beaty, well said. Stumbling on this statment though.

"With stick locked, changes of engine power should cause the machine to climb or descend at constant airspeed. "

Even if everything else is perfect, Seems like the rotor itself would have to do somthing to acheive that. Do you adjust Jesus bolt offset? (the pivot point that sit above the hub bar)

[Student]

I have to agree that although I'm not touching this thread with a 28' rotorblade, I'm learning a lot. If it can "remain" a discussion and not a mud slinging fight, I'm sure I'll continue to learn from this thread.

[dapartlow]

Spiff, The CLT kit DOES NOT come with a pre rotor cable and the keel extender does fit tight ( but if you have not bought one how would you know?) and the tail or HS is a whole different kit I think you sould really know you facts before you DISS some ones products.

[C. Beaty]

The mechanism of “flapping” produces speed stability.

An increase of power tends to increase airspeed and an increase of airspeed would increase cyclic flapping, the net effect being the rotor cone appears to “blow” back. This tilts the rotor thrust vector rearward, producing a nose up torque about the CG.

The machine noses up and climbs at the trimmed airspeed with flapping angle back at its original amount.

This sequence of events does not occur in discrete steps although there can be some bobble until speed and attitude stabilize.

Flapping is another term that leads to confusion. Viewed along the rotorhead axis, the rotor certainly appears to flap. Viewed along the tip plane axis, the rotor does not flap but instead undergoes a cyclic variation of pitch, the amount of which is equal to the flapping angle. The teeter bolt in conjunction with a seesaw rotor is a form of universal joint that allows rotation of the rotor about its own axis.

[spaceman spiff]

A buddy here has one, its loose.

Either way the round keel isn't going to work for my HS.

Anyway, i will be more carefull. Forgot that others feel the same way about thier toys as i do about mine, Sorry to offend.

[PW_Plack]

Ron said,

"If You fly a Aircommand and the factory claims that you need to upgrade the gyro with a kit that basically flips the engine and raises the seat, why question it?"

My short answer would be, because at least one Canadian marketing firm recommends flying with a 10-inch high-thrustline offset and no horizontal stab.

The best thing about Light Sport Aircraft is that objective stability standards will soon emerge. When there's a consensus on how a gyroplane should behave, and how stability is measured, every newbie won't need to reinvent this wheel. Even if some kitmakers don't play, the tests and results themselves will become apples-to-apples.

Until then, you take the word of one kitmaker, you get a safer machine. You take the word of another kitmaker, you get a smoking crater. "Question Authority" is, at this point, a kit gyroplane survival skill.

[scottessex]

Either way the round keel isn't going to work for my HS.

Greg Spicola used a 2x2 sq tube for the aft section of keel on his Air Command, and it does seem to be beefier than the round tube. Either way there is still going to be a fair amount of twisting that goes on with a long rear keel section. Most gyro's without a tall tail have a bit of flex/wobble at the rear fin/rudder. doesn't seem to be a problem in flight.

[GyroRon]

Yeap, I have seen many new and old gyros with aircommand type tails - a all flying rudder that is held onto the gyro with one big bolts going through the keel - and several of these had alot of play in the tail if you grabbed it and pushed or pulled on it. Chris Wilsons brand new Monarch had alot of play in it when I saw it at Bensen Days. Apparently there is bushing in this assembly that wearout and need adjustment and replacement.

Space you do what you think is best for you. I don't think anyone here can change your mind about any of this. Raising the seat flipping the engine and using a taller mast sounds fine to me. Also sounds just like what the kit from Aircommand does too.... And as for money, there is nothing stopping a any aircommand owner from copying the parts from someone elses CLt kit and making these parts yourself with raw materials.

Scott is right, it may be better to just have a machine that flys and experiment later....

[Dale Young]

Joe, I don't exactly know how to take your response to Scott's statement. Every day at work, I find myself having to explain simple solutions to "highly educated" over-engineering, better-than-you types. Sometimes the book stuff cannot replace life experience. Don't get me wrong, I'm not knocking an education. But don't belittle someone else because they did'nt study engineering in college.Maybe...just Maybe...Scott understood it all by just using common sense. I know of one particular outstanding neuro-surgeon that I would'nt trust with a screwdriver in fear that he'd probably hurt himself with it! I have also known at least a dosen music majors that could tell ya ANYTHING technical about the arrangement of music,but could'nt play an instrument...Go figure...

[gyrogreg]

Great thread guys. My ears were burning and I found my name referenced in this thread - so, I'll offer a point or two:

I did the "High Command" modification with the intent to do the "CLT" thing on my stock AC. I basically moved the landing gear 8 inches lower (with a drop keel for the tail wheel), increased the prop diameter to 68", and lowered the upright engine 4 inches. This had the effect of lowering the Prop thrustline by 4 inches - about what I thought the AC needed! I lengthened the tail boom 8 inches by splicing the old, round tail keel tube to a drop keel joint under the prop. I used the standard "flat" AC HS.

When Steph transitioned to gyros, with the high CG and no larger wheelbase, her instructor and I were worried about tipping the gyro over on landing with the amount of torquing it did with power changes on landing. Instead of increasing the wheelbase, I opted for the Dominator Tall-Tail to reduce the torquing effect of power changes. (I also changed the caster arm and spring rates of the nosewheel to avoid the old "Air Command DART" when touching the nose with large rudder input!) Steph transitioned to this configuration with no problems - nice flying.

Now, let me point out a bit of a negative with this thrustline, T-Tail configuration. I now understand that the "Static Power stability" of this configuration indicated that the Prop thrustline was actually a bit lower than the CG - LTL. This gave it the nose-up, CG forward super stable effect - WHEN POWER WAS APPLIED! When power was not applied, at higher airspeeds without the nose "pushed" higher, and with the low LG dragging the nose lower, the CG was positioned more aft relative to the RTV, to very noticably reduce it's pitch stability - loss of the "power augmented" pitch stability! In otherwords, it was very uncomfortable to fly at higher speed and LOW power such as a 80 mph low power descent - at the end of a long and very comfortable cross-country at high speed. At cruise power and above, the "High Command" was very comfortable at higher airspeeds in even rough windy turbulence at mid-day over the rough Missouri hills - lots of turbulence. However, upon reduction of power to descend at even 60 mph, the gyro "felt" very noticeably affected by the turbulence - scary to me! As a result, I found I often continued at high power (and high airspeed) directly over my destination, then reduced power AND airspeed for a vertical descent - which "felt" much more comfortable in the turbulence than a low power, high speed descent!

I also built and flew 60 hours in (a very popular LTL gyro). This gyro had the same high airspeed, low power "perceived" reduced stability. (I quoted the word "felt" and "perceived" above because I now know you can't judge an aircraft's stability by "feel". But, the "feel" of these characteristics influenced me to research all this more.)

What I now feel was going on with this LTL configuration is that stability is "augmented" by power in the LTL machine - because higher power in flight pushes the nose up and the CG forward relative to the RTV - much improved stability. Also, with higher power applied, the immersed T-Tail HS was much more effective in the accelerated air of the propwash - AT HIGHER POWER SETTINGS. However, at lower power settings, the "sum of the static moments" caused the nose to lower - higher "trimmed" airspeed and the CG to move aft relative to the RTV. I'm not saying it moved all the way aft of the RTV (I don't know) to become airspeed and G-Load unstable, but it definitely "felt" to be much reduced stability as "perceived" in rough air! Also, with the lower propwash accelerated air over the totally immersed HS, both the dynamic and static stabilizing capacity of the HS is significantly reduced. To me, this indicates a large change in the overall stability - especially at high airspeeds - with large changes in power. I maintain this is not a good thing - to require less experience pilots to have to contend with. The stability of any aircraft ought to remain essentially the same for all airspeed, power and loading combinations - otherwise why would FW pilots get additional training and flight time when they move from a utility/normal category airplane to an aerobatic Pitts or such? FW pilots don't expect to be able to safely fly a radically differing stability airplane. Why would we want less experienced gyro pilots to "switch" from a very stable gyro to a much less stable gyro with simply a change in throttle position - or uncommanded power loss!?

I'm not saying this changing stability affect is a major cause of accidents. Obviously, the LTL "power augmented" stability reduces the PIO/"buntover" exposure of the older, non-stabilized HTL gyros - accident records verify that! I'm simply pointing out that with too much LTL "power augmented stability" it will be difficult to design out stability margin changes with power changes - especially if the very low LTL configurations is overly utilized to achieve high stability at high airspeeds. This may be better, much better than a totally unstable gyro at high airspeeds. But, we need to recognize that things might change drastically if power is reduced or lost or even changed!

Also, if the gyro pitches severely upon any severe change in power, enough so to pitch the gyro suddenly as much as 10-15 degrees either way - as upon a sudden loss of power - that severe cyclic input to the rotor disc can readily initiate "precession stall" - instantaneous rotor blade AOA exceeds blade stall. Even if the pilot responds with available aft cyclic input, say on a nose-down sudden pitch, that aft cyclic input may immediately move the RTV forward of the RTV, immediately making the gyro negative G-Load stable - initiate a "buntover" or PIO? This all says to me, that the "sum of static pitching moments" ought to be balanced for both changes in power and changes in rotor thrust. A way to do this ussually requires a minimal prop/CG offset and a large volume HS.

One more point: The prop offset is not always the only way to "skin the cat". Magni maintains a relatively steady CG forward of the RTV simply by using a very effective, large volume HS that essentially maintains airframe flight attitude (AOA) through all variations of power and rotor thrust. Flight airframe attitude, edicted by a powerful HS, essentially positions the CG relative to the RTV - irregardless of prop offset, power or rotor thrust changes. It so happens that this "sum of static moments" over-powering HS approach essentially "balances" the nose-down moment of the slightly high prop thrustline with a slightly down-lifting HS. The tail-down moment of the HS provides the "airspeed stability" as in a FW with the CG forward of the lift of the wing. The CG forward of the RTV avoids G-Load instability that is root of a self-supporting buntover.

(There is a very instructive article (very basic concept) in the latest EAA Sport Pilot magazine on how the HS works - and the erroneous presumption by many that an up-lifting HS is OK. Airspeed stability is achieved with the CG forward of the RTV - achieved aerodynamically in gyroplanes through the "sum of static pitch moments" - a nose up aerodynamic moment from something - prop thrust, HS down-load airframe aerodynamic moments, or some combination of all these! For gyroplanes, this is even more important because the CG forward of the RTV also imparts the very important G-Load static stability - the major contributor to buntover avoidance.)

Thanks, Greg Gremminger

[spaceman spiff]

Many appologies for my grumpy behaviour guys.

[gyrogreg]

Correct me if I'm wrong. The terms "bunt" and "buntover" are also applied to helicopters - for essentially the same thing as for a gyro buntover. I hope I did not invent the use of these terms for gyros!

I had not intended that the term buntover be limited to a "precession stall" event. Maybe I'm wrong, but I use the term "buntover" to include essentially any destructive forward pitching result from pitch instability issues. This would include a "precession stall" or a PPO. But, a PPO is just a form of a "buntover" - commonly envisioned when the prop thrustline is higher than the CG where it may easily be described as the prop "pushing" the nose over when the balancing RTV decreases. Maybe I've applied the term "buntover" too widely, but we should understand that PPO is not the only variation of a "buntover" and that a gyro can "buntover" even if it is not "pushed" over by the prop!

I think this concept and delineation is important so that people don't have undue total confidence in any gyro simply because it follows all the popular stability prescriptions! All gyro configurations should be respected and their characteristics understood. Some configurations may be better or better in some flight regimes, but maybe any gyro configuration can be "bunted".

The internal mechanisms of what happens in a "buntover" event may vary widely - but all resulting in the same type damage and fatality! I personally believe that very few "buntovers" actually pitch over onto their backs. I personally believe that the damage mechanism in most "buntovers" is a "precession stall". Once the spindle rapidly changes angle more than 10-15 degrees, the severe blade precessionstall causes the rotor teeter stops and pitch/roll pivots to hit and the rotor bend or hit things on the gyro! This sudden spindle angle change can be in pitch or in a strong roll induced by loss of rotor thrust to counteract engine torque - as mentioned in a previous post. Or, the rotor teeter and spindle hitting its stops in one direction will rapidly further change the spindle angle for even more "precession stall" spindle cyclic input. Either way, or even if the gyro makes it all the way onto it's back, the results are the same!

It's been said before, but we should realize that we don't really need to get "air on top of the rotor" or "negative Gs" to initiate a buntover. On a G-Load unstable gyro - CG aft of the RTV - the suddenly reducing G-Load of even a slight forward stick or down-gust transient can self sustain into further rapid G reduction and a self-sustaining and accelerating forward pitching. So, for an unstable gyro, it really only takes a sudden G-Load repoint of no return depends on the degree of G-load instability, airframe/rotor inertias, and probably the skill and reactions of the pilot. But, I doubt the rotor load really needs to get to zero before the "precession stall" occurs!

It's also interesting to note that Chuck describes the self-sustaining pitching rate to be a "neck breaking" rate. I contend that on a true "buntover", the sudden pitch or roll rates are so sever that the pilot loses consciousness immediately and can have no hope to pull a parachute launch handle! The best way to save lives from "buntovers" is to design gyroplanes that are highly resistant to self-sustaining and divergently accelerating "buntovers" - keep the CG forward of the RTV!

For a static G-Load stable gyroplane - the CG forward of the RTV - any G-load disturbance will cause the gyroplane airframe to pitch in the direction to restore G-load toward the original 1G airframe load. This means that any G-Load disturbance will not be self-sustaining or diverging. (It may be oscillatory - but this is "dynamic stability, another story!) When the CG is forward of the RTV, any G-Load transient, pilot induced or wind induced, will not self-sustain and statically diverge, but will tend to restore or converge toward the normal 1G load. This does this by forcing the airframe to pitch in the direction to impart a spindle cyclic input to change rotor AOA/load toward the original 1G load. This G-Load stable gyro will be very difficult to "buntover"! In fact, if the airframe pitching rates - HS stabilizing power and inertia - are matched to the rotor pitching rates properly, it may not be even possible to actually buntover! Such a gyroplane will "fight back" when the pilot tries to force a "buntover" - this is through strong stick feedback forces as the airframe pitches against the pilot pitch input.

A real life example of how difficult it may be to actually force a "buntover" on a well stabilized gyroplane: Our long-time Missouri gyro flier Jim Smith once suddenly pulsed the cyclic forward to dive quickly under sudden wires. Jim "knew" that to punch the stick forward, to get "air on top of the rotor", would buntover - but in the circumstance that was his only option other than catching the wires around his body! As he went under the wires he heard a "twang - twang". He flew out the other side wondering why he was still alive. He also heard the rotor hit the wires and figured he must have broken the wires. After landing the wire strike marks on his polished Dragon Wing blades were found ON THE TOP of both blades. They went back to see the wires and they were not broken! Jim had dived so severely under the wires that his rotor disk had enough negative AOA to strike the wires ON THE TOP of the blades! (Don't believe this, Jim will gladly show you the wire marks on his blades! Jim's stock 90 HP Mac powered Bensen gyro had a Dominator T-Tail - a powerful HS when his engine was running hard! The lesson we learn from this is that with enough HS stabilization, even "air on top" or "negative G-load" will not necessarily cause a gyro to "buntover"! If the CG had been aft of the RTV - negative G-Load stability - the strong negative Gs from the negative disk AOA would likely have initiated a self-sustaining diverging nose-down pitch reaction "buntover". The point is, it's not the negative Gs that get you - it's the self-sustaining pitching reaction of even a partial loss of positive Gs in a G-Load unstable gyro.

Thanks, Greg Gremminger

[C. Beaty]

I gather from reading posts in a couple of places that the UK’s CAA has sent gyro pilots a notice of a proposed rule change (I don’t know the correct nomenclature) that would require gyros with cabins to be modified for CLT. Evidently, existing gyros would be grandfathered in and would still be permitted to be flown by the original owner in calm weather

Does anyone know anything about this?

[GyroRon]

So Greg to sum it up, your saying a gyro like a Dominator or CLT aircommand is unstable at lower power settings and maybe able to buntover if the airspeed is high and you chop power? What would I or another CLT pilot have to demo to you to prove to you to show that this is not true?

[gyrogreg]

So Greg to sum it up, your saying a gyro like a Dominator or CLT aircommand is unstable at lower power settings and maybe able to buntover if the airspeed is high and you chop power? What would I or another CLT pilot have to demo to you to prove to you to show that this is not true?

Hi Ron, Not saying it is unstable. Just saying it could be significantly less stable at lower power settings. This is analogeous to some HTL gyros becoming less stable at higher power settings! And, any time the nose reacts to any disturbance with a rapid pitching motion, there is concern for "precession stall" if that sudden pitching extreme could cause extreme flapping where the rotor stops are not adequate!

How to show me!?? Don't show me, just evaluate your aircraft so you know - I've already seen the results of similar configurations. You can't easily tell if the "sum of static pitch moments" results in an effectively CLT, HTL or LTL gyro - it must be flight tested.

- Conduct the Static Power Stability flight test to see if and how much the trimmed airspeed changes with power changes. The full test starts at level flight at MPRS and tests the resulting trimmed airspeed at full power, idle power (and no power, but I don't suggest turning the engine off).


"Trimmed airspeed": This is the airspeed the gyro tries to return to with no pilot input. If your gyro is not statically airspeed stable (another stability criterion), the gyro might have very light stick forces and you can't establish "trimmed airspeed" by hand. If your gyro won't trim hands-off to a "trimmed airspeed", you will need to fix the stick to establish the "trimmed airspeed". You can't just hold the stick steady - have to prevent pilot inputs completely - especially for a very light stick! You can hold a short stick between the cyclic stick and the instrument panel (or some other fixed hard point). This is to "fix the stick" so it won't be changed when you test full power or idle power. Choose the length of the "fixing stick" so the gyro flies at MPRS (minimum power required speed). A gyro that is relatively Power Stable should not fly at an airspeed more or less than 10% different from the original "trimmed " MPRS. (at no power the criterion in the gyroplane standard is 20%)

Change power slowly from MPRS power and allow the "trimmed" or "fixed stick" airspeed to settle out to the new "trimmed" airspeed. Don't just "chop power" like the first time I tried it in a LTL gyro - scared myself to death when the nose dropped badly!

What this tells you is whether the "sum of static pitch moments" changes much with changes in power. It can tell you whether high power or low power affects stability significantly. Under the power condition where the aircraft "trimmed airspeed" is slower than MPRS, that aircraft is more stable - nose-higher, CG further forward relative to RTV. Under power conditions where the "trimmed airspeed" is higher than MPRS, that aircraft is less stable - nose lower, CG further aft relative to RTV.

A true CLT gyro, one with no drag line offset and no aerodynamic pitching moments from LG, windscreens, etc. - will have perfect POWER Stability. A HTL or LTL without other perfectly balancing aerodynamic moments (HS, LG, windscreen, etc.) will show degrees of Power Stability deviations depending on the amount of LTL or HTL and other pitching moments. Generally, a HTL without properly balancing HS will fly at higher "trimmed airspeeds" with higher power and lower "trimmed airspeeds" at lower power. Generally also, an unbalanced LTL will fly at lower "trimmed airspeeds" at higher power settings and higher "trimmed airspeeds" at lower power settings. These changes in "trimmed airspeeds" indicate a corresponding change in the relative positions of the CG and RTV, and the corresponding changes in stability margins.

Do this and report the results if you want to show what is true. But mostly you should do this so that you would better understand any flight regimes in your gyro at which you ought to be aware of different risks such as what might happen if power suddenly stops. For the traditional unbalanced HTL, a reduction in power is the traditional prescription to restore stability - this is because on a power reduction that HTL gyro CG repositions to a more stable position relative to the RTV. The opposite CAN happen for an unbalanced LTL gyro where a sudden power reduction can actually worsen the stability or even initiate a "buntover" or PIO - such gyros can get more stable with increased "power augmented" stability - but get less stable when there is less "power augmentation"! I maintain you should determine, understand and respect these effects. Almost all "buntover" accidents are from the lack of this understanding and respect! - What you don't know you don't know can hurt you!

Also please note: You may have trouble actually getting the gyro to settle out at a "trimmed airspeed" - and return to that airspeed after a disturbance - with the stick in a "fixed" position. If this is the case, the aircraft is not statically Airspeed Stable - does not seek to hold or return to a "trimmed airspeed". This is another issue that can be the result of either rotor characteristics or a HS that is actually lifting upwards. Please read the latest HS article in EAA Sport Pilot if you don't understand this. Or, read the stability section of aerodynamics for airplanes in any Private Pilot training course. Many people fly gyros that are Airspeed unstable, but aerodynamic engineers and FAA certifications all put strong value on static airspeed stability. Many helicopters, especially those without a HS are not airspeed stable - attesting to a higher level of skill to fly them also!

Please report the results so we all know. - Greg Gremminger

[Dean_Dolph]

Hey, Greg, you mentioned in an earlier post about your ears burning but this isn't the only thread where your name was mentioned!

Please go to the thread about Ralph's Digipod and see if you can contribute what inputs would be required to data log stability testing. If you believe that the static/dynamic tests as you have described them without data logging is sufficient then that is okay too. But, the Digipod is being given CPR and now would be the time to address this issue.

[gyrogreg]

Hey, Greg, you mentioned in an earlier post about your ears burning but this isn't the only thread where your name was mentioned!

Please go to the thread about Ralph's Digipod and see if you can contribute what inputs would be required to data log stability testing. If you believe that the static/dynamic tests as you have described them without data logging is sufficient then that is okay too. But, the Digipod is being given CPR and now would be the time to address this issue.

Dean, I looked over that thread and I don't see a good place to jump in with a new subject on stability testing.

Some thoughts on instrumented stability testing:

First, we intended in the ASTM gyroplane standard to find ways to test gyroplanes WITHOUT expensive instrumentation. With the help of some good test pilot types, I think we mostly mangaed to do so.

Static stability testing: We use airspeed and stick poszition and pilot foces to identify positive Airspeed and G-Load stability. This is on the premis that negative stability has been the root of the problems and ensuring positive static stability will eliminate the vast number of instability induced accidents. For static airspeed and G-Load stability, we make no effort or criteria for margin of positive stability - if you can verify it in airspeed, stick position and forces, it is better than not!

Power static stability: Even though the FAA expresses desires for minimum airspeed/pitch reaction to power changes, this static test is most helpful for gyros in establishing the power/airspeed envelope corners that a lesser degree of airspeed and G-Load stability might be encountered. The Power stability test measures a percentage of "trimmed airspeed" deviation with power changes.

Dynamic pitch stability: Although we specifiy a criteria in the aSTM standard for this - can be measured with airspeed, G-Load, deck AOA or RRPM - we do not recommend amateurs trying to do this test. The problem is to test it you may excite a PIO or worse in a dynamically unstable gyroplane. Test Pilots have careful methods to walk up on any potential resonant frequency point that might induce PIO. The digipod might be useful in doc8umenting professional testing on dynamic pitch stability. I prefer to use RRPM. You can record the pulses from a digital rotor tach onto a tape recorder. The resulting hum pitch changes can readily be heard and the period and damping time measured just by listening to the tape. The RRPM deviations are the result of step inputs to excite pitch oscillations as the aircraft seeks to re-establish equilibrium. The ASTM standard simply states there should be no oscillations at cycle periods faster than a certain amount. I don't beleive we want non-professionals to try to excite such pitch oscilations in a frequency range that might induce PIO.

The good story about dynamic stability. Even though we don't recommend actually flight testing dynamic stability, we can get a very good confidence factor that it will be OK IF we meet the static stability criteria with the use of a good size HS. A HS is the key to longer period natural pitch oscillations. To achieve good Dynamic stability, it is probably more than adequate to use a large HS that is effective for static purposes. We are hoping gyroplane manufacturers will, in time, be able to concur with that idea.

In the meantime, the ASTM static stability criteria really need no instrumentation. To document the static stability testing, I recommend using a tape recorder to record each test run set of parameters and results.

Thanks, Greg

[Douglas Riley]

Greg, what force do you believe causes a LTL gyroplane to bunt over? I gather (but correct me if I gather wrong) that you believe this possibility is not limited to the "drag-over" scenario (draggy gear, windshield) or true negative G.

I agree heartily that an arrangement of forces about the CG that allows large uncompensated moments to pop up under common flight conditions is undesirable. It's undesirable because, as you say, any very rapid rotation of the fuselage may force more cyclic pitch change per rev into the rotor than it can tolerate without stalling a blade.

I just don't picture such a large imbalance occurring in the typical LTL gyro, unless it's EXTREMELY LTL or has an unstable fuselage.

BTW, could a big, rapid stick movement by itself directly cause a precession (or cyclic) stall? I think it's unlikely. When you shove the stick about in large/faster movements than the rotor can follow, the rotor hammers the flap stops. Unless you're a maniac, this tells you to stop and physically pushes your hand back. A cyclic input from frame rotation, OTOH, is not within the immediate control of the pilot. What's more, the frame has enough leverage to overcome the stops and thus over-pitch the blades -- as was obvious in the crash I just looked into yesterday.

[Udi]

Any gyro - LTL, CLT, or HTL can be set up to fly stable (i.e. CG in front of the RTV) without power. All that needed is a correct steup of hang angle in flight and stab AOA. The most efficient and easiest to design configuration is CLT with an airspeed-proportional downloading stab.

People confuse gyroplane aerodynamics with FW aerodynamics. Gyros have other factors that work for static stability - in addition to CG/CLT. The proof is in the pudding - even the most stable gyroplanes have some regiments within their airspeed envelope of positive stability, in which the HS is clearly up-lifting.

We should not make any generalizations - CLT, LTL, whatever. There is more than one way to skin this cat and any of them can be safe and achieve the mission it was designed for (well, maybe not for the cat).

The Dominator, the CLT Air Command, the Sparrow Hawk and the Magni are all good, safe, and viable designs. You know what - I can improve on each and every one of them.

Udi

[gyrogreg]

Greg, what force do you believe causes a LTL gyroplane to bunt over? I gather (but correct me if I gather wrong) that you believe this possibility is not limited to the "drag-over" scenario (draggy gear, windshield) or true negative G.


Doug, I believe any gyroplane can buntover. The basic scenario for any gyro is that if/when it happens to have the CG aft of the RTV with a decreasiing G-Load. At this point, the sum of moments may sustain or accelerate a forward pitching rotation - further reducing the G-Load and accelerating the nose-down pitching and aft movement of the CG. As you have said, this airframe induced spindle cyclic input is not readily overcome with pilot input. Actually, pilot input would probably be aft stick - moving the RTV further forward - of the CG!

A gyro with an unbalanced HTL, at high power and especially at high airspeed, probably has the CG continuously aft of the RTV - setting it up for a forward bunt with just some initiating forward pitching disturbance.

A gyro with a LTL though is not so continuously in the precarious state of CG aft of the RTV. Normally, under power, the LTL gyro CG is well forward of the RTV - super stable and highly impervious to a forward bunt from pilot input or wind disturbance! In otherwords, for most of the time, some other initiating radical initiating disturbance must occur. That disturbance can be sudden power loss or reduction. For most LTL gyros, especially if they have a rather smallish and propwash augmented HS effectiveness, under power and at higher airspeeds, the HS is lifting UP (nose-down) to balance the LTL nose-up moment. (The nose-up moment forces the HS to a positive AOA). At higher power and airspeed, not only is the HS providing a strong nose-down moment, but probably draggy LG is also providing a strong nose-down static moment.

In this scenario, a sudden loss or reduction of engine power initiates a rapid nose-down moment from the now unbalanced aerodyanmic moments of the HS, LG, windscreen, etc. The nose rapidly drops moving the CG aft (less forward) relative to the RTV. Depending on airframe inertia, the nose could drop suddenly enough to actually result in the CG moving aft of the RTV. Also, the pilot reaction to the nose drop would likely be a sudden aft stick - moving the RTV forward while the airframe CG is moving aft! In the extreme, the CG could move aft of the RTV with the nose still dropping. Once the CG is aft of the RTV and the nose is dropping, the forward tumbling becomes self sustaining - especially if the stick range has hit the aft stop where the pilot cannot stop the decreasing G-Load on the rotor. One more phenomena that doesn't help stop the forward pitching is that an embedded HS, once the propwash has quit, has much less effective moment to stop the forward pitching once the HS actually does re-establish a negative AOA.

This may just be speculation, and the scenario may only occur on an LTL gyro if there is a sudden loss of power at high power and high airspeed. This is not presenting a continuous hazard of CG aft of the RTV as in the unbalanced HTL gyro. For the LTL gyro, another thing must happen - sudden or rapid loss of power. This may not happen often, but may have happened at least once. It might be a dangerous proposition to believe a LTL gyro cannot buntover "by definition". Any gyro can sustain a diverging forward pitching buntover if its CG somehow gets aft of the RTV.

Another scenario, one that I often experienced in my High Command (and in the other LTL gyro I built and flew). At high airspeed and low power, I believe the lack of power augmented nose-up stability coupled with the reduced effectivenss of now not immersed HS, and with the nose-dowon drag of the LG and windscreen, these gyros flew more like my old tailless HTL Air Command than the super stable gyro I was flying before I reduced power! With reduced power at high airspeed, I felt very susceptable to PIO or buntover. Flight in turbulent air in this corner of the power/airspeed envelope was something I instinctively avoided.

I don't believe you can objectively assess stability by "feel" - it must be tested. I did not test these gyros under this condition for even static stability - I did not know what I did not know at the time! But, experienced pilots have a certain "seat of the pants" developed feel. "Seat of the pants" flying is when the pilot has developed his/her own G-meter in the seat of their pants. Once developed, this is a pilot's built-in G-load feedback sensor, much like eyes sensing pitch attitude. When this sensor is developed and assimilated into pilot control reactions, the pilot is much more proficient at flying that machine. This "seat of the pants" G-meter heightens the pilots awareness of what his/her stick movements are doing - immediate G load feedback. A pilot feels comfortable when the G-meter is properly in-phase and reacting to stick inputs. When the phase of the G-loads and stick inputs stop correlating, as in a G-Load unstable gyro, the pilot recognizes this, perhaps subconsciously as an uncomfortableness. Long story short, experience pilots can "feel" when their aircraft is not responding as ingrained in the brain. I beleive this "seat of the pants" feel is a big reason pilots, well experienced in their unstable gyro, don't have PIO or buntovers as often as less experienced pilots do - the experienced pilots have the "feel" that says don't venture into this area! That's what I felt when I flew these particular LTL gyros at high airspeed and low power - a high speed glide.

What I beleive was happening in this high speed, low power glide condition was that the CG was no longer held by prop thrust so well forward of the RTV At best, with low power it is a true CLT gyro - CG exactly on the RTV! However, the other aerodynamic nose-down moments at high airspeeds may even position the CG aft of the RTV with no prop thrust to balance them. In this condition, the CG may well be aft of the RTV - explaining the "seat of the pants" uncomfortableness with the unstable G-Load condition. When the CG is aft of the RTV, the nose responds to G-Loads in the wrong direction! When the CG is aft of the RTV, the pilot must excercise extreme "stabilizing" proficiency - similar to what the old Air Command pilots (and others) have to do all the time. For the pilot of this normally super-stable LTL gyro, the challenge of this sudden proficiency reaquirement may result in over control (PIO) or even a buntover in reaction to a strong wind disturbance - not unlike the less experienced pilots in some unbalanced HTL machines we read about far to frequently!

Now, put both scenarios together - high speed and a sudden loss of power. The nose is dropping, AND the pilot suddenly needs flying skills he/she might not have developed. Couple this with the popular guidance to reduce power to stop PIO or a buntover! We should realize that power reduction is not the way to re-establish stability (CG forward of the RTV) in some gyro configurations.

Another concept to consider. Normally the cyclic stick pitch (and roll) range is limited to 9-10 degrees from centered stick. This normally does physically limit the pilot input to extremes that are not likely to induce a precession stall simply from pilot input. But, in a LTL gyro at high power the stick must often be held further forward than center to keep the airspeed high - nose-up moment of LTL slows the gyro requiring forward cyclic to maintain airspeed. This does mean that the pilot now has aft stick range of more than 9-10 degrees - possibly entering the precession stall range if fully used. Gyros do have this sort-of nice "stick shake" warning when the pilot is getting overly agressive with the stick - flap hitting the teeter stops! This would normally discourage such severe cyclic inputs. But, upon sudden nose-down pitching, the pilot reaction is unpredictable - does he/she reactively pull the stick the full range aft? - if they react quickly enough and the right amount they might actually stop an airframe induced precession stall. But, they have to react with some precision - not likely if not practiced. (When I "chopped power" at 70 mph on the un-named LTL gyro, my reaction was certainly not something I had practiced!). But, even if the sudden aft cyclic reaction is perfect, that aft stick movement is still radically moving the RTV - perhaps forward of the rear moving CG?

This is all along your proposed concept - you can't allow sudden changes to result in sudden large pitching moments. This is the main reason I see for the criterion of Static Power Stability - changes of power should not radically alter the flying conditions or stability margins. We can't completely depend that the pilot will not make sudden control inputs. We can't control the disturbances the wind can present. And we can't say for sure the engine won't quit (or the pilot won't "chop power"!) - always provide "augmented stability". Sounds to me that we need to provide stability that won't won't change or quit suddenly - that sounds like a large and effective HS that isn't unduly affected by a sudden change in power. You gussed it - a HS that is not totally immersed in the propstream - immersed only enough to compensate any offset prop thrustline.

I have to say, I'm not trying to "prescribe" solutions to PIO, buntover, etc. I do believe there may be good enginering ways to attack all of these stability issues - not just the Magni way! What I am proposing is that all gyros should be evaluated for these issues - flight testing that either allows us to improve the situation, or flight testing that makes us aware of conditions where we could get into trouble. Knowledge is what will prevent fatalities. - Flight testing can provide that knowledge. HTL, CLT or LTL - its the results that count!

Thanks, Greg

[gyrogreg]

Any gyro - LTL, CLT, or HTL can be set up to fly stable (i.e. CG in front of the RTV) without power. All that needed is a correct steup of hang angle in flight and stab AOA. The most efficient and easiest to design configuration is CLT with an airspeed-proportional downloading stab.

I agree with Udi - to consider only CLT, LTL or HTL is only part of the issue. there are certainly other factors in gyroplane aerodynamics - pendulum effect, "blowback?', rotor coning, etc. With other, much more difficult to factor in effects of the rotor, it may be possilbe to achieve all three static stability criteria with CLT and even with LTL. Considering only the static moment effects of RTV, HS, prop thrustline and airframe aerodynamic moments - difficult enough as that is - does not take into acount the other more difficult effects. Considering only the moments I have been pointing out, I find it difficult to come up with a CLT or LTL configuration that meets all three static criteria.

BUT, I beleive also that these criteria can be met with at least a CLT design. I bemieve this because it looks like that has been successfully done - at least with the Sparrow Hawk. Jim Mayfield has reported for some thime that the SH meets all the static criteria - and the dynamic criteria! Jim is very professional, knows the theory and knows what he is doing. I would think the SH is a pretty good CLT gyroplane. With the simple analysis I have offered above, apparently that is not the whole story. The SH has very strong airspeed and G-Load static stability! It also has VERY good Power pitch static stability.

I am hoping that other people actually do these static tests on their gyro configurations. It would be very encouraging to find and figure out these other factors that go into achieving the static stability criteria. I hope people will do the tests and report the results. We are making great headway in gyroplane stability safety. Since the total answer is probably not just LTL or CLT or HTL or HS, let's get a better handle on it. When someone with a true LTL gyro can show it meets the static stability criteria, we'll have a lot more we can start to analyze as to how it does that.

As a start, my simple explanation, considering only the basic static airframe moments, cannot explain how an up-loaded HS achieves static airspeed stability. I know there are other effects - I've seen rotor effects that completely destabilized (static airspeed) an otherwise statically stable gyro. If a rotor can make it much worse, I would guess a rotor can somehow make it better. Udi, could you explain your rotor "blowback" stability theories?

Thanks, Greg

[StanFoster]

Greg: It is so nice to have your inputs along with Doug Rileys...Al Hammers...Chuck Beatys....Udi......you guys are a think tank.

The order of those names I posted are at random. I would lump all equally.

Good reading Greg.....see you at Mentone.


Stan

[rehler]

Dr. Greg,

My Symptom is: when I decrease my throttle, while lowing my altitude, my air speed increases. I have to pull back on the stick to keep the speed constant.

What is the cure? Should I adjust my HS with more leading edge down (more down force)?

Thanks. Please send your bill to my insurance company. :)

[Udi]

Greg - I am away from home, using a Mac with a browser that shows all the frames scambled-up and no spell checker - so please fogive any errors due to my dislectic brain... Although I am flattered, the rotor blowback stability theories are not mine - I am just picking them up as I go. The credit should go to people smarter than me. I know you know all this, but I am going to re-cap anyway.

As you mentioned in your post, depending on the type of rotor, most gyroplane rotors flap, or blow-back, with airspeed. This is simply the result of the advancing blade teetering up, and the retreating blade teetering down, due to lift dissimetry. As the gyro speeds up, the advancing blade "sees" a higher airspeed and the retreating blade sees a lower airspeed. So the advancing blade teeters up to, in effect, reduce it's AOA, and the retreating blade teeters down to increase it's AOA. A new equilibrium is achieved when the lift of the advancing blade is equal the lift of the retreating blade.

The result is that the rotor disc is tilted back (blown back) - RELATIVE to the rotor head - at higher airspeeds. When the rotor is tilting backwards, the RTV is moving forward relative to the CG (because the RTV is always parallel to blade tip axis). When the RTV is moving forward, the nose is pitching up, and the gyro slows down. This is the most basic mechanism for airspeed stability in a stabless Bensen gyro.

Other factors involved in pitch stability are the offset gimbal head, which adds a G-load (or AOA) stability to the stabless gyro, and, of course, the horizontal stabilizer, which adds another measure of G-load (and airspeed) stability due to it's steeper lift curve relative to the rotor. When properly set up, the stab may also add an additional measre of airspeed stability, as you are describing very well in many of your papers. The stab is also adding a significant measure of dynamic stability - which is very lacking in any stabless gyro. Another very impotrant purpose of the stab - which is not mentioned very often - is rotor/airframe coupling - in essense making the airframe weathercock, giving the pilot immidiate feedback on directional changes.

All these factors are adding up to the CG/RTV relationship, which you are rightly preaching for, as an important measure for G-load stability.

I hope to see you in Mentone, Greg, and we can talk more about all this stuff. I hope you can find some ground time for chatting (let Steph be the driver for a while). I will also love going flying with you again, if I can find an open slot in your busy schedule.

Regards,

Udi

[Udi]

Ken - Although I am not a Dr., I think the symptom you are discribing is typical of a low thrust line. Reducing power is resulting in a nose-down pitching moment. I would pitch your immersed stab 0.5-1.0 nose up to counter the engine nose-up moment. I would also re-balance your hang angle to be more nose down by the same amount, to prevent the stab from lifting due to airspeed.

By the way - the AAI modified RAF that I have flown had the same tendency you are describing.

Udi

[gyrogreg]

Dr. Greg,

My Symptom is: when I decrease my throttle, while lowing my altitude, my air speed increases. I have to pull back on the stick to keep the speed constant.

What is the cure? Should I adjust my HS with more leading edge down (more down force)?


Ken, More down force on the HS might make this effect worse. Sounds like the sum of static moments is essentially a LTL. This means the effect of increased power is nose higher - good for stability at high power. It also means the effect of reduced power is nose lower (higher trimmed airspeed) - less stability at lower power setting. If the HS is reacting significantly to propwash, more HS down angle would mean even more nose-up with power and more nose down reaction with loss of power.

This isn't the exact way to test static Power Stability. You should fix the stick (or hands-off trim) when power is changed for the test, and then see how much the airspeed changes with the power change. Start at about MPRS power in level flight. With stick fixed or trimmed hands off at MPRS, increase power to full power (slowly) and see what the resulting (steady state) airspeed settles out to. For yours, this probably will be a lower airspeed. Do the same for decreased power to idle - for yours this sound like it will be a higher airspeed. The change in airspeed from MPRS level should be no more than 10% at either full power or idle power. If it is within this 10%, the nose-drop on a sudden power loss should not be excessive enough to cause a real problem.

You've been flying this gyro for a long time - you have probably developed the stick reactions to keep disturbances minimum with power changes. The only question might be how radically will the nose drop if the engine suddenly "chops power". You can work up to seeing how bad this might be, by reducing the power from MPRS power to idle in quicker and quicker steps. If it starts getting a severe nose drop on quick power reductions, maybe don't go any further.

How to correct this?: Don't know a good way - more HS down AOA will likely worsen this effect. It might not be so bad if the HS is not heavily immersed in the propwash. Adding more HS up AOA may not be a good fix either - but should help the nose-drop issue. With less HS AOA, if the AOA becomes positive, you may notice the gyro does not want to self-return to trimmed airspeed when disturbed (Airspeed static stability). I do suggest you try a little more up AOA on the HS - see how this works. If the nose doesn't drop severely with sudden power "chop", I would opt to keep the slightly negative HS AOA to maintain good Airspeed static stability.

You should also check G-Load stability at all these HS AOAs. At any speed, but especially at higher airspeeds, you don't want the "trimmed" airspeed to drop when in a banking turn. The test for this is simply to stabilize level airspeed and then enter a banking turn at the same power setting (about 30 degrees is enough), and note that it does require aft stick pressure to maintain the initial level airspeed in the slightly descending spiral. G-Load stability is your assurance against buntover from a disturbance - wind gust or pilot cyclic input. Do this at higher and higher trimmed airspeeds - buntover issues often get worse at higher airspeeds. G-Load stability assures the CG is effectively forward of the RTV, thereby pulling the nose down to higher airspeeds when G-Load is increased in a banking turn. This doesn't necessarily protect from a sudden loss of power that could swing the CG aft of the RTV, but it does statically assure the gyro will try to return to 1G loading upon a wind or pilot disturbance - it will not diverge into a buntover from a disturbance.

I believe with an effectively LTL, it may be difficult to achieve both Airspeed Stability and good power stability at the same time. This is because, for a LTL, it may require an up-lift on the HS to "balance" the nose-up moment of the LTL. Compromise may be necessary.

Please report your results of all testing.

thanks, Greg

[gyrogreg]

Hi Udi, Very nice summary of stability factors other than just the simple CG/RTV concepts - Offset gimbal, HS vs rotor lift curves, and the weather vaning effect of the airframe so the pilot is presented with a pitch attitude reference that is not mis-representing the actual transients. It is easy to see how difficult it can be to do a total paper design to achieve a stable gyro - a lot of very difficult variables to contend with. Flight testing is the only way to verify the overall results!

Another issue I have experience, and I have not exactly figured out why this is. but it had a big effect on static airspeed stability. A very early prototype aluminum rotor (shall remain un-named here) apparently had inadequate trailing edge geometry. That rotor on my stock 582 Air command (HTL, no stab) would try to fly at higher and higher airspeeds if airspeed increased above "trimmed" airspeed. At "trimmed" airspeed, the stick forces were very light. But, if you flew faster or slower than "trimmed" airspeed, stick forces increased rapidly to cause the gyro to fly at faster and faster, or slower and slower airspeeds. This was an example of extreme negative static airspeed stability! - it would not hold an airspeed, but had to be constantly balanced by the pilot at the "trimmed" airspeed so as to not start deviating badly from "trimmed" airspeed - you could not let go of the stick! My point is, even the rotor blade pitching moments can affect airspeed stability - another difficult parameter to assess on paper!

I think I now do see how the "blowback" - more forward RTV at higher airspeeds - can be forcing the nose higher, causing more stick forces to fly at higher than trimmed airspeed, and inducing a form of airspeed stability that tries to restore the airspeed back to original trimmed airspeed condition. I think this is why you are maintaining that static airspeed stability might be achieved even with a slight up-load on the HS. It will be real interesting to see if flight test results verify this effect. I think that would mean that even an effectively LTL gyro (at least a small amount of LTL) as verified by the Power Stability test, might still test to have a degree of positive airspeed stability - this "blowback" airspeed stability effect offsets the effect of a somewhat up-lifting HS. I hope we see some test results on this - maybe Ken's Rehler's testing can show us this effect - positive static airspeed stability even after he corrects the Power stability with a bit of up-lift on his HS.

Ken, I do suggest you try a bit less (more positive) AOI on the HS. Then test for both Power stability and airspeed stability. Always need to be sure G-Load stability is positive at all conditions also. Please keep us informed of results - we need more flight testing on a lot of different gyros to verify these concepts.

(Airspeed stabilty is verified simply by the requirement of forward stick pressure and displacement to go faster, and aft stick pressure and displacement to go slower - than "trimmed" airspeed. Another way is simply that a specific stick fixed position results in a specific and constant airspeed.
Or another way is that, hands-off, the airspeed returns to trimmed condition from both a higher and lower airspeed.)

Knowledge is safety! Thanks, Greg

[rehler]

Dr. Greg,

Thank you very much. I really appreciate your help.

I changed my HS angle (more leading edge down) not too long ago - to make sure my CG was in front of the RTV. It is 4.25 degrees down - I probably over did it. I can reduce the angle and still maitain a negative pitch.

Thanks again.

[Dean_Dolph]

Ken, I think that you can provide some help to everyone by doing the tests before you change it back and then test it again. Greg seems prepared to compile this type of info into something useful.

[rehler]

Dean,

Good idea - to test before and after.

[Doug Riley]

Negative HS incidence and LTL really do act somewhat the same. Both my Dominator tandem (LTL, HS parallel to keel) and Gyrobee (slightly HTL, 3 deg. negative on a large HS) behave the way Ken reports. That is, they both somewhat overcompensate for power changes. Increasing power brings the nose up enough that you'll slow down if you don't add forward stick pressure or forward trim. When power is cut, both of them nose down enough to pick up speed unless you add back trim or back pressure. Both could stand a touch LESS of the thing that's providing these reactions. That means LESS LTL in the Dom. and LESS neg. HS incidence in the 'Bee.

The wild card in here is the amount of HS area that's energized by propwash. The 'Bee's is partly immersed, while the Dominator's is centered on the prop hub. To avoid any possible tendency to "tuck under" at high speed-low power (the effect that I believe Greg is concerned about), I think the partly-outside approach is better. And bigger is better, too -- which argues for a HS that's NOT mounted on the rudder, since that setup limits the practical size and span of the HS. Mounting on a stationary vertical fin would be OK. Some tests indicate a significant gain in HS efficiency when tip plates are added to the HS. Ken's U-tail may well benefit from added HS power thanks to this effect.

If a rotor blade is underreflexed and 1/4-chord balanced or over-balanced, it will have a net nose-down pitching moment. This moment, like lift and drag, is a function of the SQUARE of the blade's airspeed. IOW, it gets big fast as the blade speeds up ... and the blade speeds up as it (the blade, not the gyro) noses down. Speeding RRPM up reduces blowback. Thus the airspeed stability effect that Udi mentions is apt to be muted in a rotor that has this characteristic.

I found that my old no-HS Air Command with McC blades tended to run away at airspeed either below or above trimmed speed, too. It was most airspeed-stable if you set the spring for about 35 and set power a little above cruise. This was the setup I used for hands-off aerial photo runs.

On the Dominator with current, reflexed Dragon Wings blades, I can fly around with my arms folded indefinitely at a wide range of airspeeds, by adjusting the in-flight trim. Unlike the Air Comm., it will not go out of trim and "run away" when disturbed by a puff of turbulence.

[spaceman spiff]

think i understand 1/4 cord ballanced, but lost me on overballanced. How does cord ballance cause rotor pitching?

[Doug Riley]

A gyro rotor blade is a little tailless glider. Much the same design issues come up in a rotor blade as in something like a Mitchell Wing ultralight, but with the extra "twist" that the wing's/blade's flexibility is important.

If you OVER-balance, you add weights so as to bring the blade's CG forward of the 1/4 chord point. If that same blade is a cambered foil, it will have a nose-down pitching moment about the 1/4 chord point (and also obviously even more of one about any point forward of the 1/4 chord).

With BOTH of those moments trying to pitch the blade down, the blade's own torsional rigidity is what's keeping it from twisting and depitching itself. As the de-pitching moment grows with airspeed (as in a high-G maneuver or just the gyro going faster and thus adding airspeed to the advancing blade), there will come a point where the moments tending to de-pitch the blade overcome the blade's own rigidity. At that point, the blade twists leading-edge down, picks up RPM and hence airspeed, twists some more and so on.

An over-balanced blade with sufficient reflex to just offset the nose-heaviness at normal RPM can work OK. In fact, it'll be very stable, like an airplane with a forward CG. Take away the reflex, though, and it'll become unstable with increasing blade airspeed.

A blade that's got the right amount of reflex but a CG way aft of the 1/4 chord point will be unstable with angle of attack, like an airplane with aft CG. McCutchen Skywheels seem to have this condition, though it crops up mostly when they are used on heavy gyros. I guess they are tosionally stiff enough to behave themselves fairly well when used on light gyros, as my A.C. was. Even on mine, they seemed to twist in a bit of pitch and have a "ballooning" tendency when you hit an updraft -- you could hear a distinct whoosh and get a delayed "second lift" when that happened.

Note: I'm talking about BLADE CG location here, not the CG of the whole gyro. Same for "nose-up, nose-down" etc. -- this refers to the nose, or leading edge, of the individual blade, not the gyro.