NCLT Dichotomy

[gyroplanes]

(I meant to put a question mark after "Dichotomy")
I copied this here from another thread. Please help me with this.
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Quote:
Originally Posted by Screw
Screw-In

1. The Bensen, as designed, was a very close to CLT machine with a Mac or VW direct drive engine.
Screw-Out


This and other statements cause me great concern.

Chuck Beaty has recently been touting the "safety" of the old Bensens and our old method of learning to fly one.

I have to agree with Chuck about "Self training" I haven't seen a decline in fatalities since dual came along. I don't condone self-training any more. That's a subject for another thread.

Our chapter did the old double hang test vertical C of G determination on a member's VW Bensen. This old Bensen (with a very short mast by today's standards) came in real close to CLT. I expected it would.

What causes my heartburn???

I remember the old days. Gyro carnage didn't start with Dennis Fetters Air Commands.

Don't get me wrong. I believe in the benefits of CLT. I just can't understand how these NCLT early Bensens still managed to PIO / PPO as often as they did..

Our beloved gyroplanes didn't earn their horrid reputation with the advent of the Air Commands, it was very well established before then.

When I was a student at the FAA academy in Oklahoma City, OK. thirty years ago, I spent my spare time in their library sifting through all of the gyro accident records. There was a staggering number of accidents, virtually all of the fatal accidents displayed the tell tale signs of PIO / PPO.

The FAA library did not have a copy machine, so I hand copied each and every accident for my records.

When I was the PRA Safety Editor we published these reports in the PRA magazine. I still have them and maybe after the Chistmas confusion I could dig them out.

[scottessex]

See what you did Screw! If you would have come out to the house today, you would not have had time to stir the pot here on the forum!!!!:eek:
I think everyone has thier undies in a twist (see more Screw posts) because summer is over and they can't fly as much!:mad: :cool:

[GyroRon]

Tom, my guess is that the old Bensens were close but not quite CLT. They were still a few inches off of being CLT, and with no horizontal stabs they were easy to over control in pitch resulting in PIO and possibly PPO.

The double hangtests and other ways of testing for VCG on a old type Bensen these days would more than likely show better CG alignment than the real early machines due to the early machines using the boat type fuel tanks mounted down low and most machines now having switched out to seattanks which places the fuel up higher.

I think alot of the earlier accidents could be explained by the lack of proper training, the key word being PROPER. Properly self training may not have been bad, but I think alot of people rushed it. If you rushed the self training you probably didn't learn all you needed to.

[C. Beaty]

Bensens did tumble out of the sky in the old days, Tom. In every single case where I have personal knowledge, it was someone that had skipped the Bensen training program. As president of the Sunstate Rotor Club for 10 or 15 years during the late 60s and through the 70s and early 80s, I tracked all accidents in Florida closely. I have yet to find a fatality that I’m aware of that is not listed on the NTSB site.

A stock Bensen, because it did not have a horizontal stabilizer, was poorly damped in pitch. The lag and overshoot inherent in such a configuration inevitably leads to PIO for most individuals without training.

The time spent on a towline was a start toward developing the reflexes that enabled the pilot to stay ahead of the machine. The remainder was acquired during the early hours spent balancing on the main wheels and then wallowing down the runway at a few feet of altitude.

The people who killed themselves in a Bensen were generally loners and as often not fixed wing pilots who thought the procedures outlined in the Bensen training manual silly. Any club member was dissuaded from simply strapping in and flying.

Even so, most of the fixed wing pilots who killed themselves more than likely would have survived had their Bensen been equipped with a Ron Herron “T” tail or similar.

I have seen both Bensens and AirComands tumble out of the sky. The Bensens would go through 3 or 4 cycles of porpoising before tumbling. AirComands would simply bunt with no obvious porpoising beforehand, reflecting the difference between CLT and a large offset of the propeller thrust line.

Individuals who have never flown a CLT machine with proper tail surfaces simply can’t comprehend how crisp the response and how easily such a machine flies.

Furthermore, most gyros being sold today are Bensens in everything but name; seesaw rotor, pusher engine, same gimbal rotorhead with the same bearing Bensen used, same 2 x 2 aluminum tubes rearranged slightly and generally, even the same mast angle. Some got it right, some didn’t.

[Screw]

Screw-In

As it's my understanding, the old Bensens and Ken Brock KB-2s when equipted with a Mac or Vw engine was very close to CLT. I think the thrustline was only 2 inches above the CG.

If I'm wrong, please tell me.

Of course, the use of a Horizontal is a must!

Sorry Scott. I don't mean no harm.

Screw-Out

[Timchick]

John,
How many hours did you fly your KB-2? Did you ever feel like you were starting into porpoising?

[Hognose]

The original Bensen B-8M plans (if you haven't got a set, you can see a set in the museum in Mentone) show the CG located precisely on the thrust line.

I am told by guys from the Bensen Era that that depicted more of a wish than a reality.

By the way, a friend of mine decided to teach himself to fly a Bensen he bought at a yard sale. He rolled it up in a tight little ball (it sounds from the description like he tried to take off with too little RRPM and flapped the blades badly). He survived the accident, and now thinks he was not only lucky to survive, but lucky to have crashed.

"Or I'd have kept trying to fly it..."

He is a highly experienced FW pilot and was a PPC pioneer as well.

cheers

-=K=-

[Screw]

Screw-In

I flew it over 40 hours and no. I had a "T" Tail. She flew very stable.

Tim, I think there is a misunderstanding between what is HTL. I think the misunderstanding is that alot of folks think that all Bensens or KB-2s are HLT machines when in fact they are not.

Over the years of course people wanted bigger props and 2 stroke engines with more power. As a result, Air Command, Ken Brock Mfg (KB-3) and others truly made HTL Machines by simply raising the engine and mast to swing a larger prop.

I don't know who first came up with the idea of dropping the keel and CLT, but it's been a winner so far. If you look at what the most popular gyros out there, most incoorperate some type of drop keel, powerful 2 stroke engines, large props, and Horizontals.

To further answer your question Tim, I think it would have porpoised had I not had the horizontal, and I also think any gyro would without a horizontal.

Screw-Out

Screw-Out

[mceagle]

The early Air Commands had the ability to reach higher speeds, where they became nose down to the extent of running out of back stick. A low center of pressure combined with a high thrust line and no back stick meant that a bunt over was mandatory, limited only by airspeed and pilot experience. New chums often had the lethal combination of too much of one and not enough of the other.
I dare the same could be said of the standard RAF, although I do not know whether they got close to full back stick or not. They were certainly well nose down.

[Hognose]

Furthermore, most gyros being sold today are Bensens in everything but name; seesaw rotor, pusher engine, same gimbal rotorhead with the same bearing Bensen used, same 2 x 2 aluminum tubes rearranged slightly and generally, even the same mast angle. Some got it right, some didn’t.

You know, not having built a gyroplane, it was only when I went to Mentone in 2003 and saw my first RAF 1000 did the Bensen heritage of the lead sleds strike me. OK, so I'm dumb as a keg of nails.

I just reread Jean Fourcade's paper and the abstracts of Stewart Houston's studies. I notice that the physics of many of the problems we have been discussing so heatedly are explained by theory in Fourcade's work, and the theory is validated by experimentation in Houston's.

Much of the argument concerns whether the mishaps we are seeing are instantiations of the class of mishaps predicted by this research, or something else.

Science is a wonderful thing. I wish more of my countrymen had more education in it.

Here is what I take away from all that stuff -- no, never mind. Here are things for people to read them themselves.

Jean Fourcade, engineer, theorist:
http://www.asra.org.au/L_Stability.htm

Stewart Houston & Douglas G. Thomson, professors of engineering, experimenters:
http://www.aero.gla.ac.uk/Research/Fd/Project5.htm

[I think the actual papers are these:
Houston, S.S., "Longitudinal Stability of Gyroplanes". The Aeronautical Journal Vol. 100 No. 991, pp. 1-6 (1996)

Houston, S.S., "Identification of Autogyro Longitudinal Stability and Control Characteristics". AIAA Journal of Guidance, Control and Dynamics Vol. 21 No. 3, pp. 391-399 (1998)

Houston, S.S., "Identification of Gyroplane Lateral/Directional Stability and Control Characteristics from Flight Test". Proc. Inst. Mech. Engrs, Part G, Vol. 212 No. G4, pp. 271-285 (1998)

Houston, S.S, Thomson, D.G., Spathopoulos, V.M., "Experiments in Autogyro Airworthiness for Improved Handling Qualities", Presented at the American Helicopter Society 57th Annual Forum, May 2001.

Houston, S.S., Thomson, D.G., "Identification of Gyroplane Stability and Control Characteristics", NATO RTO, RTO Meeting Proceedings 11: RTO-MP-11, AC/323(SCI)TP/7 ëSystem Identification for Integrated Aircraft Development and Flight Testingí, March 1999, ISBN 92-837-0006-6

Spathopoulis, V.M., Thomson, D.G., Houston, S.S., "Flight Dynamics Issues Relating to Autogyro Airworthiness and Flight Safety", Paper No., Proceedings of the 54th American Helicopter Society Annual Forum, Washington, May 1998

Houston, S.S., Thomson, D.G., "Flight Investigation of Gyroplane Longitudinal Flight Dynamics", Paper 108, Proceedings of the 23rd European Rotorcraft Forum, Dreseden, Germany, September 1997.

Houston, S.S., Thomson, D.G., "A Study of Gyroplane Flight Dynamics", Paper No. VII-6, 21st European Rotorcraft Forum, St Petersburg, Russia, August 1995.]

I note that although Houston reaches a very firm conclusion w/r/t thrustline, the CAA (which funded him to the tune of half a million quid) has not actually acted on his research, and instead (thru PFA) not only permits, but requires, high thrustline gyros to be flown without even a stabilizer.

Sometimes it's a relief to see that bureaucracy isn't just local to the country I happen to be in and battling with at any given moment.

cheers

-=K=-

[GyroRon]

Screws KB-2 would fly nose down at high speeds and not sure if the 2 inch or so HTL accounted for that or possibly the large horizontal stab being at the wrong angle and causing the nose lowering at high speeds.

[Aussie_Paul]

Screws KB-2 would fly nose down at high speeds and not sure if the 2 inch or so HTL accounted for that or possibly the large horizontal stab being at the wrong angle and causing the nose lowering at high speeds.
..... I believe that it would be the parasitic, or as it is sometimes called these days, form drag, would be responsible for that increasing nosedown attitude as you got faster.

Aussie Paul. :)

[scandtours]

Just from my experience.
My first gyro was a Bensen with 70 Hp McCulloch and 50 in prop. Stick
rudder and stab were replaced later from Ken Brock. With some modifications I moved KB stone guard stab which is semisymmetric to the aft and with the flat surface on top ( upside down). The gyro behaved completely different
and become much more stable. I could not believe it myself.
George K

[C. Beaty]

A fixed wing aircraft flies more and more nose down with increasing airspeed. As forward speed increases, more forward stick is required to reduce the wing’s angle of attack and to keep it from climbing.

A CLT gyro controlled with an elevator would behave in exactly the same way as a FW.

A CLT gyro controlled by cyclic pitch is prevented from assuming as great a nosedown fuselage attitude by the fixed horizontal stabilizer.

With increasing airspeed, the rotor disc flies flatter which moves its thrust line rearward with respect to the CG since the fuselage attitude tends to be held level by the stab. This increases static stability* about the pitch axis and gives an increasingly solid feel with increasing speed. *Static stability depends upon the strength of the restoring force following a disturbance.

[Hognose]

Interesting. It appears that the UK CAA issued a directive -- a Mandatory Permit Directive, which is like an AD for non-TC'd aircraft that in Britain get a "Permit to Fly," rather than a special C of A as in the USA -- in August requiring all UK single-seat gyros to be limited, those with pods more so than those now, in:
- Never Exceed airspeed (Vne 61kt)
- Permissible Weather Conditions (max surf wind 15; VNe 55kt in turb)
- Pilot Experience (PPL-G and 50 post PPL solo hours to fly pod)

And the limitations can be lifted if you can demonstrate to the PFA that you are within 2 inches of CLT.

In addition, all single-seat gyros must add an "acceptable horizon reference" and open-frame gyros are restricted to a minimum speed of no less than 26kt, except in the landing flare. These requirements in the new directive are not waiverable.

Have we had a discussion here of CAA MPD 2005/008 here? If so, can someone point me to the discussion?

I don't remember talking about this. Some parts of it are clear to me
- separating podded from open machines
- pilot experience requirements
- airspeed limitations on non-CLT gyros
- max wind limits on non-CLT gyros (there are gust delta limits too)
- horizon reference

I'm not sure I "get" why the minimum speed limits on the open-frame gyros.

cheers

-=K=-

[Aussie_Paul]

Interesting. It appears that the UK CAA issued a directive -- a Mandatory Permit Directive, which is like an AD for non-TC'd aircraft that in Britain get a "Permit to Fly," rather than a special C of A as in the USA -- in August requiring all UK single-seat gyros to be limited, those with pods more so than those now, in:
- Never Exceed airspeed (Vne 61kt)
- Permissible Weather Conditions (max surf wind 15; VNe 55kt in turb)
- Pilot Experience (PPL-G and 50 post PPL solo hours to fly pod)

And the limitations can be lifted if you can demonstrate to the PFA that you are within 2 inches of CLT.

In addition, all single-seat gyros must add an "acceptable horizon reference" and open-frame gyros are restricted to a minimum speed of no less than 26kt, except in the landing flare. These requirements in the new directive are not waiverable.

Have we had a discussion here of CAA MPD 2005/008 here? If so, can someone point me to the discussion?

I don't remember talking about this. Some parts of it are clear to me
- separating podded from open machines
- pilot experience requirements
- airspeed limitations on non-CLT gyros
- max wind limits on non-CLT gyros (there are gust delta limits too)
- horizon reference

I'm not sure I "get" why the minimum speed limits on the open-frame gyros.

cheers

-=K=-

Hi Kevin, boy the UK officiates conduct all sorts of knee jerk reactions over there!!!. If they had said that, "And the limitations can be lifted if you can demonstrate to the PFA (PopulayFlying Assn which is all aircarft types) that the craft meets the US ATSM pitch stabnility standards". That would have been a sensible way to handle the situation.

Scenario. A person could have the CoM to thrust line offset at 2" and have a very poorly designed inefficient stab with no moment arm, or even no stab at all, and they will lift they limits!!!

Aussie Paul. :)

[Harry_S.]

It is my opinion...there is no other method, nor less expensive method, nor safer method, nor idiot proof method, to learn all there is to know about basic rotor control, than with dual instruction in a two seater towed Bensen Glider.

I wonder how many of the young gyronaut aces, have even been in a towed glider. Land or water??


Cheers :)

[gyroplanes]

I have to play the Devil's Advocate now and again. I grew up in a time when we were taught to question everything. I glad no one misconstrued my post as questioning the need for NCLT or H Stabs.

Chuck Beaty and I have very similar recollections regarding the past.

I too have witnessed several fatal gyro flights. The Old Bensen style gyros did mimic a divergent, roller coaster path before the fatal outside loop. Air Command and the Marchetti fatals I saw bobbled very little before the fatal plunge.

A loveable fellow from Wisconsin, vice president of our chapter and good friend, learned to fly an enclosed Air Command low rider 532 with factory stock H Stabs. He flew it home from Oshkosh and flew it at several air shows around the midwest. Dave was in love with our sport and converted a step van to haul his ship around the country in.

Dave wanted to get another Air Command for his son. He bought another 532 without enclosure and stabilizers. I watched him test fly it. It was a gusty, windy day and he bobbled around the sky quite a bit.
After he landed, he commented that this machine didn't fly anything like his. He said he was going to go through the engine and convert it to CLT over the winter. By spring, all he had accomplished was the engine work.

He and a fellow club member went to the airport in the spring to install the engine. They decide to hear it run, then he decided to taxi it. The next thing the other guy saw was him taking off (hey, it was a warm spring day) he flew it like a man possessed according to witnesses. He made a fast pass, waived to some people in an airplane taxiing out and plunged to the ground.

I would have done the conversion for him had I known.

[Doug Riley]

I'm afraid that some of what has been said here (and elsewhere by fairly respectable gyro people) will lead to needless confusion. Here are two things that are FALSE:

1. PIO causes PPO. If a gyro PIO's, it's always in danger of PPO.

2. There is something inherent in all gyroplanes that makes them subject to a mysterious malady called "buntover." IOW, gyros would rather be upside down, if we would only let them.

Pilot-induced oscillation -- PIO -- happens when pilot and machine, taken together, lack damping. They bob up and down at their natural frequency. All types of aircraft (and, in fact all kinds of machinery operated by humans) can and do PIO. PIO does not necessarily lead to a PPO or any other kind of crash; the system may settle down after awhile, especially if it has at least a LITTLE damping and the operator has the sense to hold the controls still. PIO occurs when the pilot-machine system lacks dynamic stability and has a lag in its response.

(Even a non-laggy machine like car steering can PIO if the driver is drunk. It's the familiar "weave," caused by lag in the driver, not in the car. You have to look at the total system, machine plus operator.)

OTOH, PPO is an example of STATIC instability. A force on the airframe pushes or pulls the whole craft over. No oscillation is involved in PPO, and no oscillation necessarily precedes it.

NO tendency to push over is present in any properly designed gyro rotor. There is NO force generated by the rotor itself that is continually trying to flip the rotor over. Even if the rotor is dipped low enough in the front that the air hits the "disk" on its upper side, the rotor will not "dig in" and continue flipping like a coin. Instead, the rotor will simply generate lift in a downward direction. This may actually push the gyro's tail down and restore positive angle of attack.

To borrow an old campaign phrase, "it's the airframe, stupid." PPO happens if, and ONLY if, the (1) prop thrust line is above the CG AND (2) there is no other force that counters the nose-down torque created by the high thrust line.*

Pilot skill can often completely compensate for poor dynamic stability -- remember, PIO is a lack of damping in the man-machine SYSTEM. If the pilot has plenty of "training, training, training," than the pilot can sometimes do the damping job with no help from the machine. It's a lot of work, but you can learn to do it automatically with enough practice. All pilots of tailless gyros -- old Bensens included -- develop this skill, if they survive the learning process.

The seeming link between PIO and PPO is this:

Teeter-rotor, cyclic-controlled gyros, PPC's, trikes and weight-shift hang gliders have as their primary control a device that points the lifting surface in various directions to carry out the pilot's wishes. This setup makes the aircraft simpler. It does, however, deprive the pilot of control during moments of zero "G."

HTL machines would all be un-flyable unless something countered the nose-down effect of the prop thrust. Since HTL machines do fly, something does counter the prop thrust. If the "something" is rotor thrust, however, watch out. Rotor thrust is interrupted by zero G events. The "PIO connection" is simply that PIO will produce a zero G event if it persists through a few cycles. The PIO lets the PPO genie out of the bottle. Without a built-in tendency to PPO, PIO will not lead to PPO.

OTOH, if a gyro experiences a PPO after PIO-ing, then that gyro was susceptible to PPO all along. This means the gyro became statically unstable at low G -- a frame-layout defect that could have been corrected in the design.

*(There is one other kind of pitchover that I know of besides PPO -- the drag-over. If the frame layout is such that the pod, wheels, radiator or what-not tend to pull the nose down, then they can act in a way that mimics HTL, even if there is no HTL. This is most likely to happen at high airspeeds, especially if the nose tends to get lower at such speeds. Note, however, that the airframe, not the rotor, is again the culprit. The vague term "bunt" suggests that there are any number of other varieties of pitchover. I know of no others besides these two.)

[gyroplanes]

Nicely put Doug.
The thrust line and Center of gravity mis-match is often represented as a torque arm. Your explanation above also describes a torque arm below the gyro acting in the SAME direction as a high thrust line would. Drag below the center of gravity compounds the high thrustline problem.

Now we have both nozzles of Chuck Beaty's lawn sprinkler.


I saw, what I believe was a very graphic example of something similar to this, an "anti-horizontal stabilizer".

I knew a guy who flew Bensens for years. He built a Marchetti Avenger (H stabless) he added an Air Command two place SxS pod enclosure.
Flying it solo, he said it "acted strange in gusts" and that "it was the closest to PIO as he ever came".
He took Duane Hunn flying with him one day and said that with the extra weight "it flew great".

He added some lead shot to the nose and went out and enjoyed himself. He flew it around and had a great time.

The next day a far more experienced pilot went with him for a "ride" to check it out. Several club member ta twere present aske if they were going to remove the shot ballast. The experienced guy said it wasn't necessary.

The take-off looked normal. Things soon became "hairy" as I observed the gyro bobbling around a bit during climbout. It just didn't look right. They levelled off on downwind, gained speed and there was a slight pitch up followed by a nose over "bunt". Both were killed.

[Hognose]

This is the order that says what they are requiring, but not why. It will fit in the file limits. The explanatory Q&A I'm not sure of.

cheers

-=K=-

[Hognose]

This one is actually smaller, it turns out but it contains the CAA's responses to community questions including many of the questions I asked myself after reading the first document.

Most annoying that one is locked out from cut and paste on it.

cheers

-=K=-

[PW_Plack]

So, the UK's CAA will limit a single-seat Magni to 61 kt, but still won't let you put a horizontal stabilizer on an RAF. Incredible.

[C. Beaty]

As a general rule, all unstable vehicles exhibit similar behavior, whether land, sea or air. If a vehicle responds is a way that magnifies a disturbance, it is unstable. It is stable if its response following a disturbance tends to minimize the disturbance.

Tail heavy road vehicles behave in a similar way to tail heavy airplanes and gyros.

A Chevrolet Corvair, with 70% of its weight on the rear wheels, was tail heavy and unstable. Negotiating a curve, the rear end tended to swing out, increasing the turning rate beyond that commanded by the driver. That’s called oversteer.

GM’s bandaid for the Corvair was to specify inflation pressure for the front tires at 16 psi and 32 psi for the rear tires. That masked the instability up to a point.

Stable aircraft always head into the relative wind in both vertical and horizontal planes.

An upward gust, say a rising air column from a thermal, shifts the angle of the relative wind from head on to one that’s rising at an angle. The wind can’t blow in two different directions at the same instant and same place.

The correct response is for the aircraft to tilt nosedown and head into the relative wind. If it had a horizontal axis weathervane, there would be no indication of a wind shift. Eventually, the aircraft will acquire the upward velocity of the thermal and return to level flight. The time required depends upon the inertia of the machine and the loading of its lifting surface.

Tail heavy aircraft pitch noseup upon entering a thermal, magnifying the disturbance.

As a general rule, gyros with the propeller thrust line well above the CG are tail heavy. The rotor thrust vector must lead the CG in order to generate a noseup torque that balances the nosedown torque of the offset propeller thrust line.

The dangle angle of a gyro has no primary effect on the relationship of rotor thrust line to CG.

The only real purpose for setting dangle angle is to center the cyclic stick. Some RAF pilots dupe themselves into believing their machine climbs better or goes faster in one mast adjustment hole or another but that’s not so. Changing the dangle angle of these machines alters the pressure in the poorly placed static pressure ports and produces fake readings. Using a standard NACA style static/pitot probe would have shown no change.

If a gyro is equipped with a horizontal stab, the dangle angle affects the angle of attack of the stab and exerts a secondary influence on the relationship of the rotor thrust vector to the CG.

The preceding is a thumbnail sketch of static stability. A machine can’t possess dynamic stability unless it first has static stability.

If a machine possesses strong static stability, it can be dynamically unstable.

The restoring force of dynamic stability tends to return a machine to its undisturbed state but since it has mass and has acquired momentum as a result of returning to its undisturbed state, overshoots unless heavily damped, the same as a weight on a spring. If the overshoot exceeds the amount of the original disturbance, the oscillations will grow and the machine is dynamically unstable.

The lack of damping of stabless gyros has led many people to believe that gyros fly entirely differently from fixed wing aircraft, however, that’s a false conclusion.

Move the stick of a stabless gyro and nothing seems to happen at first but that’s an illusion. The rotor follows the stick position within 2 or 3 revolutions (½ second or less) and the machine begins to climb if the stick was moved in a noseup direction. The fuselage attitude lags behind but the forward shift of the rotor thrust vector relative to the CG starts a noseup acceleration of the airframe. The airframe eventually catches up with the rotor but in the process, has acquired momentum and overshoots.

A horizontal stabilizer of sufficient power eliminates the lag of fuselage alignment with a change of rotor flight path and snubs the overshoot. Such a machine handles about like an ordinary airplane except the response is faster.

“Fly the rotor and not the airframe” was a bit of folklore that I heard from day 1 of my gyro exposure. The rotor can’t distinguish whether the rotorhead was tilted by the pilot or by the airframe. The Bell gyroscopic stabilizer bar isolates the rotor from the airframe but I don’t know of a single gyro using one.

[Ga6riel]

I read some good things here, and some great advice. But I believe what is lacking is any form of analytical design parameters.

As many of you would know, there is a wealth of knowledge available for anyone attempting to design or better understand a fixed wing aircraft. For instance, the criteria for static and dynamic stability are relatively easy to get your head around, and to work out for yourself.

It may be my blundering guess but, it seems crazy to me to desire to fly a machine without a stabilizer, I even shudder when I see the flat plate stabs that seem common around gyro stables. It seems underdeveloped and unprofessionally guided, and has the air of….’it may be this, it may be that’.

With this in mind, has anyone seen anything like the lift and drag charts common to fixed wing profiles, but for a fairly average rotor? Ive been around a little bit, I already have many of the pdf files seen mentioned in this thread for instance, but I have never seen any data on a 2 blade teeter rotor.

Id really like to see some hard data to make some adequate conclusions about what is going on here, though the ‘general’ drift of the thread seems right. Someone somewhere needs to be sure.

[gyroplanes]

. But I believe what is lacking is any form of analytical design parameters.

. I even shudder when I see the flat plate stabs that seem common around gyro stables. It seems underdeveloped and unprofessionally guided, and has the air of….’it may be this, it may be that’.



Aerodynamically shaped stabilizers may be more esthetically pleasing than the "flat plate" slab design, but don't sell the slab short.
Quite a few factory airplanes have flat stabs.

I really doubt that replacing the flat plate stab with an aerodynamically contoured one would make any difference at all in the performance department. The frontal area isnt that significant and I doubt the airflow is anything other than chaotic a mere few inches from the fan.

Personally, I build a shape that is convenient to the material I'm working with. If my Barnett has a 1/2" round tube frame stabilizer. It will have a 1/4" radius leading and trailing edge.
The sheet metal tall tail I'm about to build will have a pointy leading and trailing edge.

I made my first Bensen wooden tail flat plate.
I made my second Bensen metal tail flat plate
I made my third Bensen tube and fabric tail with a radiused leading and trailing edge
I made my fourth and fifth Bensen composite tails with an elongated tear drop shape

I never felt any difference between them.

It would be nice if there was a gyroplane design manual, I agree.

[Ga6riel]

I just want to take issue with the flat plate or board stab, and im not giving you a bashing here, its just one area of gyrocopter performance that seems to me to be deficient. I really dont care how it looks, although maybe some people see it that way. This ties up a bundle of ideas and I will attempt to be short but comprehensive.

A form such as NACA 0009 or 0012 will enable a degree of beam strength latterly across the stab. You wont get this out of a flat ally plate unless you care to add weight and thickness; same deal for plywood. The foil already has 10% or so on you, thats going to be hard to beat with any ply I can think of.

Also the lift performance, and the lift/drag performance is far better with an airfoil shape. The form carries a greater lift coefficient and less drag coefficient. Read, less stab required for the same lift, and less drag EVEN where the areas are the same. That means less stab is less weight, and where you probably have less weight for the same strength anyway. Also it will face the airflow at higher angles of attack before stalling, although this is a function of aspect ratio as well.

But primarily what would interest me is the difference in the stall performance of either the flat plate or the airfoil form. For the airfoil form will provide more power when airspeeds are low, and just when you need that performance most, more especially on NCLT and HTL machines. Consider the case where your at an airspeed of say 15mph, and open the throttle. Thats when you need all the lift from the stab that you can get, and even that may not be enough.

Stabs need to be effective, they need to be well placed, they need to be tuned to counter adverse forces on your machine. And its a fact that it needs to be as good as we can make it.

[Al_Hammer]

I'm aware that Doug Riley has tested some airfoils and the flat plate isn't the very best, however the performance of a flat plate is not too shabby at low aspect ratios, as you alluded to.
The attached plot of test data from NACA is for aspect ratio =1. The red line is the flat plate, and as you can see, it stalls at whopping 37 degrees and in fact the lift doesn't fall off to nothing after that. Beyond 30 degrees more than 1/2 the drag is actually acting in the desired direction to stabilize the gyro.
The lower coefficient of lift can be made up for by sizing the stab about 20% larger in span and chord.

The stab's effectiveness in damping goes up as the square of the distance form the CG, so placement is probably as important as airfoil shape. Most stabs are necessarily on a short moment arm in pushers. Proper incidence, placement in propwash, whether its full span, etc , also needs to be considered. A flat plate is primitive looking, but a nice looking airfoil can have problems, too.
If fabricated with a sharp leading edge, much of the advantage of an airfoil shape is lost, and I 've seen a few built that way.
BTW, some helicopter stabs use a gurney flap to increase effectiveness when size is limited. I wonder if it might be useful on a gyro?

[gyroplanes]

Al, Ga6riel,

I do not question the effectiveness of aerodynamic shapes. I just doubt their ability to make a difference on low speed pusher gyroplanes.

I think we may be confusing aerodynamic shapes and airfoil shapes.

The "Stabilizer" on a helicopter isn't really a stabilizer, it is a "Lifting" surface designed to impart a downward force on the tailboom as forward airspeed increases and as such isn't usually symmetrical. The asymmetrical airfoil shape of a helicopter trim stabilizer has it's "lifting" surface on the bottom.

Most modern airplanes utilize an asymmetrical airfoil with the "lifting" surface on the bottom. Flight instruction books generally refer to horizontal stabilizers as "providing a down force" instead of "lift" as that might be confusing to a ab initio pilot.

Our gyros need a stabilizer that reacts to the inflow of a changing angle of attack like the feathers of an arrow. IIRC the word Empennage comes from the word Empenner, the person that feathers the arrow.

[Ga6riel]

Tom do you mean like a servo operated elevator ?

[Al_Hammer]

Tom, I think its fair to say that a horizontal stab on a helicopter not only functions to keep the nose level (static trim). It also provides dynamic stability, even if it's an inverted airfoil.

"In a maneuver, the lift on the tail switches from download to
upload. What we hope is that the tail always has enough "lift" to counter the
rotor's wish to just keep digging in and flipping the helicopter. Usually aft
cg is the worst case, and
we test for positive maneuvering stability, looking
for the tail to keep the nose in place in a high G pullout. The way we can
tell is to note if we need to push forward on the stick as we hold the G in a
turn. If we have to use cyclic to keep the nose from accelerating its pitch
rate, then the tail is not big enough yet. We then make it bigger, or reduce
the aft cg, or add a Gurney flap to make it more effective." -N. Lappos

[Doug Riley]

Al and all: A friend and I did test a sheet-metal airfoil-section HS about 6 years ago. We intended it to be NACA 0012, but the dimensions of the available tube spars made it sort of a 0011. When I get back to my office computer, I'll try to remember to post some pictures and lift curve results.

We got a maximum C.L. at about 14 deg. AOA, but, aside from a couple of wild data points that I just threw out, that CL. was only about 0.8. This is a good 20% lower than the lift curves given by NACA for the 0012 (which are based on an infinite aspect ratio).

I attribute this low outcome mostly to low aspect ratio. Our span-chord ratio was 2:1. At those low A.R.'s, the difference between a flat plate and an airfoil are much less than, say, in a 40:1 sailplane wing. So much lift is lost around the tips that the HS's performance is low, no matter what sectional shape it is. Unfortunately, we didn't have time to test a piece of flat plywood on our rig to put some numbers on this general idea. We also didn't test tip plates. Some have reported C.L.'s around 1.4 (?) on this type of HS when plates are added.

One advantage of a low A.R., however, is that it has a soft stall. There's so much air already leaking around the tips that stall isn't a sudden cessation of lift. In our case, with our 2:1 airfoil, lift at 16 deg. AOA (past stall) simply retreated to the same number as 12 deg.! There was no stall "break" at all.

It's possible that an airfoil shape with a suitably rounded leading edge will not stall as soon as a flat plate. I recall seeing a lift curve in an old EAA magazine that showed the maximum pre-stall AOA of a flat plate to be only 0.4 -- half of what we got for our max pre-stall AOA.

Structurally, there are tremendous advantages to an airfoil shape when you use a skin that has some strength. These skin materials include thin plywood, sheet aluminum and fiberglass. With fabric, it doesn't matter structurally, except insofar as you can use a thicker spar. But classic "Cub" construction for fabric tails, possibly with wire bracing, works beautifully. When in doubt, add some area if using this type of construction, given our short tail lever arms.

[Ga6riel]

indeed the design Cl Max for FW stab is usually around .8
this is so the mainplane is guaranteed to stall before the tail
how that works with a gyro I havnt the first idea !

endplates are more than just usefull for recovering Cl here
endplates fashioned like those used on helicopter elevators provide 2 things
they assist in maintaining direction particulary where the rudder is obscured and where it is suspected that a twin tail is NOT the answer

and they offer additional aerodynamic bite for a decending machine, especially where a vertical surface is more or less obscured by an elevator/stabiliser
and ditto for twin tails as before.

[Doug Riley]

I've posted these pics before. I described our test HS in an earlier post. The boom on the truck is pivoted in its center. The HS is mounted on a pivot in front, and the back end presses down on a bathroom scale. The person riding in the back can alter the HS's incidence with a "stick." The "stick" is calibrated in 2-degree increments.

To start a test run, we would select a speed and move the "stick" until the scale read zero. That was our assumed 0 degrees AOA. We then would increase AOA 2 degrees at a time and read the lift off the scale. We did the test on a calm day and tested this by driving in both directions along the same road. We increased speed by 5 mph at a time.

Our numbers are quite crude, but the maximum CL at 14 degrees of around 0.8 is fairly solid. The lift curve for a wing of such low aspect ratio isn't a straight line (as lift curves are in theory) -- it "sags" lower as AOA increases. We found the slope of the curve at low AOA (dC.L./dAOA) was around .06 and at high AOA only 0.02 -0.03. Of course it's zero at the stall point, just above 14 degrees AOA. The slope for an ideal 2-dimensional 0012 is 0.1, and is pretty constant all through the range of AOA.

Just for fun, I pretended that the lift curve WAS a straight line and did a spreadsheet showing pounds of lift/sq. ft. of HS at various speeds and AOA's. This chart UNDERstates lift at low AOA, so it's safe and conservative, but very rough indeed. Airspeeds are on the vertical scale and angles of attack are horizontal. (Oops, the system won't take a spreadsheet. I'll convert to .pdf and re-post.)

[Doug Riley]

OK, here's that lift/AOA table. Again, AOA in degrees is read across the top row. Airspeed in MPH is read down the left column. The data are HS lift in pounds per square foot of HS area.

These are calculated numbers, based on a straight-line lift curve that passes through the data point C.L.= 0.8 at 14 degrees. It fits our data at that AOA, but is very pessimistic for low AOA. Our actual test results showed lift 20-30% higher than this chart at 4 deg. AOA, for example.

Despite all this crudity, the numbers at high AOA are especially interesting. They help give a sense of whether a HS alone can possibly compensate for the 12-inch HTL of a RAF-2000, for example. It's very, very hard to accomplish that at 3-4 or 5 lb./sq. ft.... or even 10-12 lb./sq.ft.

[Ga6riel]

nice work Doug
just to be clear .... so you figure that at low speed stability in the instance you stated would be marginal or below required?
or put another way, at the low speed end of the spectrum, there becomes a zone that cannot offer enough stabilising force?

and that therefore, a heavy power application would create a nose down moment.

[Al_Hammer]

Doug, that's interesting stuff. To nitpick, which is my job, you said that the lift goes to zero at the stall. Actually, as far as I understand it , the stall is the point where drag starts to go way up, but it doesn't mean that lift goes to zero. A wing that stalls will slow down and lose lift, so its generally assumed that stall=0 lift and that's not really the case.

At higher angles of attack, (corresponding to higher pitch angles of the gyro), a significant component of drag is acting as "lift" if we define lift as the force normal to the stab, acting to raise/lower the nose.
If the stab were pitched up to 90 degrees, all the drag would be considered lift, since it is acting to push the stab back down. A flat plate is just as good as an airfoil at these high angles in terms of static moment and probably in terms of damping.
This is not to deny the advantages of the airfoil in terms of L/D, strength, etc.

[Doug Riley]

Gabriel: Yes. If the tail arm (distance from aircraft CG to HS 1/4-chord) is, say 5 feet, the prop thrust line is 12" above the CG and the engine produces 500 lb. of thrust, then the HS must produce at least 100 lb. of (down-) lift. At 50 mph and 14 degrees (right at the stall point) , we need a HS area of 10 sq. ft.! actaully, the HS area should be much larger, since our HS really should be run at an AOA of perhaps 3-4 degrees, so it doesn't stall the first time it hits a downdraft.

If we put the HS entirely inside a 120 mph propwash, we can get our 100 lb. at 4 degrees AOA with a 12 sq. ft. HS. That's still pretty extreme.

This tells us that the current HS installations on stock RAF-2000's are marginally adequate at best.

Al, to nitpick your nitpick (nitpick squared), what I said was that the SLOPE of the lift curve (dC.L./dAOA) goes to zero. In the case of these very low aspect ratio wings, all that happens after this point is that lift retreats to the same number as at a lower pre-stall AOA. For example, lift at 16 deg is the same as at 12, etc. We couldn't feel a stall break at all... and that's a good thing in a tail! The only way we could tell it had "stalled" was that the lift started going down instead of up with increased "back stick."

The Mighty Mazda pickup couldn't feel the increased drag, of course.

[Al_Hammer]

Oh, right, slope = 0 means no change in C.L with angle. Boink! :D

[Rehan K.Janjua]

Hello Doug.

Thank you for the cal/info.
I'm looking for info on HS for a A/C 582 single 68" Warp, L. Pod, ex range tanks, tall mast, elevated seat.
I like the one from the factory which is for square tube keel, but mine has the round extended keel.
Would greatly appreciate if I could get some surface area, AOA calculations required for this particular machine.

Best wishes.

Rehan Janjua

[Doug Riley]

Rehan, you'll need much more information about the gyro before you can use numbers to select a HS.

First, where is the CG in relation to the prop thrustline? Above, below or perfect CLT?

Second, what is the aircraft's static thrust?

Third, how long will the lever arm be from the CG to the 1/4-chord point on the HS?

Fifth, what do you know about the aerodynamic characteristics of the front pod? Does it produce a nose-down moment, and, if so, how much is the moment at various airspeeds?

Sixth, what are the chord and diameter of the rotor blades?

Seventh, what is the gross weight of the gyro?

Eigth, will the HS be located entirely in the propwash, entirely outside it, or partially in and partially out?

If the gyro has perfect CLT and there is no aerodynamic moment created by the pod, you can apply the "Cierva formula" to arrive at a rough size for the HS.

If there is some amount of HTL and/or moment from the pod, you should use lift curve data to arrive at a HS size and incidence that will compensate for these items. Then compare the HS size needed for this compensation to the size dictated by the Cierva formula, and use the larger of the two.

[Udi]

Rehan,

Nobody can give you a theoretical "required" square footage for your stab. The only way to know for sure if your stab is adequate or not is to perform the stability tests that Greg G. has published. If you have the original Air Command stab I can say with good confidence that this stab is marginal for your machine. I had a very similar machine, with a large instrument pod but without an enclosure, and I know the stab was marginal on my machine. Having an enclosure requires a larger stab.

If you are going to install a larger stab, I recommend that you change the keel tube from the 2" round tube to a 2" square. The 2" round tube is not stiff enough for larger, and heavier, tail feathers.

Udi

[Vance]

A local fellow who designs and builds airplains always starts with the tail too big because he feels that the penality for too big is very small compared to the penality for too small. During his test program he cuts both the rudder and horizantal stabilizer down till he gets the characteristics he is looking for.

A large tail on a long arm can be made very light and it is fairly simple to fabricate.

I was wondering why there seems to be such an aversion to a large tail on a long arm given it's aparent value. What is the down side?

Thank you, Vance

[Doug Riley]

Vance, getting the HS onto a really long arm on a pusher requires a strange, curving tail boom. The boom has to get around the prop, while still allowing the aircraft to rock back for takeoff and landing. The tail tube ends up looking like a scorpion's tail. It's just kinda ungainly and difficult to build.

I don't disgaree with Greg Gemminger or Udi that a tail configuration must be tested. I do disagree with any implication that there is nothing more precise to be done than to stick SOMETHING back there and go fly it to see what happens.

I think that all the talk of flight tests has misled many people into believing that the numbers can't predict anything at all. This notion is an unintended consequence of the publication of test methods. It feeds right into the agenda of the "them engineers don't know nuthin'" crowd. These ignoramuses have already butchered too many people with their eyeball-designed killer machines.

Organized flight tests are the LAST step in a design process that starts with quantifying your goals and conditions. Numerical design will get you close in most cases; the more data you have, the better you can make the design BEFORE you fly. I bet that even Vance's airplane-designer friend has a good feel for the tail-volume ratios that are common on airplanes similar to the ones he's designing. The Cierva formula prescribes analogous ratios for rotorcraft, based on rotor dimensions.

It's especially true that a HS that is not big enough "by the numbers" will not be big enough in practice.

[Rehan K.Janjua]

Hello All.

Thank you for all the input, it feels great to read and experience the valuable info.
There are lots of A/C 582 like mine flying in the US.
The same machine built by Larry Neil (Aug 1999) with 25 ft Skywheels and wonder if anyone installed a larger H/S on the extended round tritube heavy keel (for SxS)

Regards.
Rehan

[Vance]

Thank you Doug. I am still wondering what the down side of too much rudder/horizantal stabilizer/moment arm is? Ugly is in the eye of the beholder! Is there some part of the performance envelop that is degraded by too much tail?

I have only talked with this local fellow a few times and yes, he does have a good sense of how much tail is required. I don't know him well enough to call him my friend, but I truly admire him. He uses calculations more and better than anyone I know. He can tell within a few pounds how much a new design is going to weigh.

He told me he does it because, for a fixed wing aircraft, too much tail is much better than too little and making it smaller and a shorter moment arm is easier than making it bigger and longer.

He also spent a lot of time explaining why he couldn't help me because of liability. Very nice man even if he does build plastic, funny looking aircraft.

Thank you, Vance

[Hognose]

Vance asks about the downside of a large tail on a long arm.

On gyros, a principal problem is risk of tail strike, particularly when the rotor decelerates. Now, if it decelerates that much inflight, the pilot is beyond caring withing seconds, tail strike or no; but the rotor must decelerate after every flight and we can't have it striking the tail willy-nilly.

The rigidity of the rotor in flight is provided by centrifugal force. The rotor gets flexible at low RPM.

That apart, there are stability and control limits on VS/Rudder/HS size. The important thing is for the forces to be in balance in all forseeable flight conditions. For example, Huey helicopters are highly susceptible to mast bumping when flown sideways at speed because of the side load on the airframe -- especially with one or both doors closed. There is enough tail rotor authority to fly it that way, but the airframe wants to do something entirely different. This is a consequence of airframe drag (which some of the fly-the-rotor guys say to disregard).

Oversized tail surfaces can make in aircraft so gust-stable that it always weathercocks, making xwind landing problematical. They can do ugly things to control forces also, and size adds weight (the least of our concerns).

A well designed aircraft would not have radically different flying characteristics with doors off and doors on, assuming that it is approved for flight like that. Some gyroplanes have lateral stability problems with doors on. That means that they lack v-stab and/or rudder authority.

I don't know that I like all-flying tail surfaces in an aircraft flying in three dimensions, or should I say, directions. It's one thing on a Cherokee which is always going within a few degrees of straight forward, but another on rotorcraft which seem to invite brisk handling. I wonder if you could get it far enough out that the stab reverses on you after a stall -- very bad.

If elevator is oversize and/or arm too long, the machine might be prone to pilot overcontrol in pitch.

There are established sizing equations that are used to do preliminary/conceptual design for fixed wing aircraft. I think they might work with gyros as well. One source for these that is in print and reasonably priced is Dan Raymer's conceptual design textbook.

Raymer's own recommendations are here: http://aircraftdesign.com/books.html I would warn you that unless you are a working engineer or otherwise meet your "maths recency and currency" requirement you will find them heavy sledding, and a community college review of algebra->trig->calc may be required. Where is book is written for "average joes" rather than for the trade, Raymer indicates.

Some of these theories are incorporated in Raymer's software and in the visual Airplane-PDQ by Da Vinci Softworks. I have not used either program.

I am told that gyrocraft modeled in Plane-Maker (a component of X-Plane flight simulator) are modeled fairly accurately. Carter Aviation Technologies has used this, although I think more for pilot training than for design.

cheers

-=K=-

[Udi]

Doug,

I don't consider the Cierva rule of thumb for stab volume an engineering calculation. Rules of thumb are just that - a general guideline.

As far as I know, there is no good way to scientifically size a stab for a CLT gyro. What assumptions would one make? Rehan's question is a good case in point. He is flying a common gyroplane with, what I believe an undersized stab. Yet, his gyro is flying just fine! How would you calculate a stab that will make the gyro statically and dynamically stable enough to meet the LSA guidelines?

The question is more complex than just sizing a stab. A gyro that is to meet the LSA guidelines shall have the correct combination of stab size, stab location, stab AOA vs flight path, stab AOA vs. prop wash, not to mention the dynamics between the rotor system and the stab.

As an engineer I, for sure, don't want to short change the value of careful design but I just think we currently don't have the tools to properly size a stab. Unfortunately, Vance is correct. One would have to experiment with a few stab sizes and configurations before they can choose the "perfect" stab for their gyro.

Rotorcraft scientists and large heli corporations probably have simulation programs that can help in the design of a gyro but I doubt any of them will help us with our home brewed toys.

Udi

[Doug Riley]

Udi, use the best numerical information you have and it'll still beat the obscurantist/nihilist approach every time.

Sure, the Cierva formula is empirical, but experience shows that it's actually pretty conservative for small gyros. It's a couple orders of magnitude better than telling people to take down a bedroom door and strap IT on to see what happens... or, even worse, having people test the RAF-2000 with some irrelevant gizmo like a magic mast, thinking THAT might do the trick.

Here's how the Cierva formula would apply to an old short-tail Air Command.

Rotor Diameter = 23 feet
Blade Chord = 7.5" = 0.625 ft.
Tail moment arm (to 1/4 chord of the factory HS, with short tail tube) = 4.5 ft.

Rotor volume = 23 x 23 x .625 = 331 cu. ft.

10% of rotor volume = 33.1 cu. ft. For 4.5 ft. arm, HS area = 7.35 sq. ft.
12% of rotor volume = 39.7 cu. ft. For 4.5 ft. arm, HS area = 8.8 sq. ft.

These are very conservative numbers, but they are in the upper end of the range that works to provide good stability in practice. The Gyrobee, which is not very different from a small Air Command, shows very solid AOA, airspeed, and power stability with a tail volume of 30 cu. ft., for example. It obtains this volume with a 6 sq. ft. HS on a 5 ft. lever arm. I think we agree that the original Air Command HS -- the one that bolts to the rudder -- make handling nicer but is too small. Its area is about 4.25 sq. ft., or 20 cu. ft. volume. Aren't we bracketing the acceptable values fairly well here already?

The Dominator has, I think, a tail volume of only about 5% of rotor volume. It has its stability enhanced to (mostly) acceptable levels by means of two "tricks:" First, the prop thrustline is a few inches below the CG, adding power stability and keeping the rotor thrust line ahead of the CG while power is up. Second, the HS is deeply immersed in the propwash, giving it an airspeed of over 100 mph at WOT. Still, the Dom. has a couple of flight characteristics that could be improved with more HS area -- especially power stability and power-off pitch behavior.

We could (and probably should) come up with more elaborate formulas based on a target position for the rotor thrustline relative to the CG, and taking into account airframe mass, moment of inertia, height of rotor above CG, HS immersion and maybe other factors. Also, tail volume of X obtained by using a short lever arm and large HS is not really the same as the same tail volume obtained with a long lever arm and smaller HS (the latter provides better damping). I'm impressed with how well the Cierva formula works on small, direct-cyclic pusher gyros, however, even though it originally applied to large tractor gyros, some of which used elevators.

The one thing the Cierva formula definitely WON'T do is tell you what to do about pitching moments caused by body pods or high thrustlines. HS power to deal with these items has to be figured out separately. The pitching characteristics of a body pod really ought to be tested in isolation, on a truck boom rig, for example. In the case of HTL, the moment is easily quantified.

Testing vs. a priori numerical design is not an either-or proposition. Asking someone to choose only one is like asking him to choose between the left wheels and the right wheels on his car. My point is that your first cut for testing purposes can be sized numerically, and you'll have an excellent chance of getting it in the ballpark. You can, and may have to, tweak the design based on test results. By the same token, you can be quite sure that some home-brewed design you see at a flyin with a HS volume of 3% of rotor volume doesn't have enough HS. This may lead to other relevant questions about the designer's knowhow.

[Ga6riel]

What Doug says is absolutely true, that will get you in the ball park, and I have to say, it is apparent that, many machines are somewhere outside the ballpark. Relying on pilot skill to make the combination safe. Some pilots have more skill than others, and by the numbers, its not really working out well is it?

You can create the 'right' solution via analytical design, this has been happeneing since before WW2, and on planned safe testing, determine the level of success.

It can work out in field testing that more area needs to be added, that has happened many times on FW, where spin resistance has meant additional area needed to be placed on the underside, a very common fix. Wherever unusual configurations are encountered, extra grey matter is required. Step outside the envelope, and more skill is required.

You dont need 'rules' for this, there is only 1 rule, dont die. What you do need is a method, a complete and verifiable method. What scares me a bit is, I dont see any evidence of that. The days of suck it and see should have ended in WW1.

[Rehan K.Janjua]

Hello Doug and all.

You guys are wonderful. Now I have some where to start from.
Feel real good to know that I can get help about something thats just impossible out here.
Thank you very much and keep posting, love the response.

Regards.
Rehan Janjua

[Vance]

Thank you Kevin, That helps.

I am still confused about too large a horizantal stabilizer. It doesn't apear to me that rotor clearance is the limiting factor on most designs. I feel that there must be something that I am not understanding or people sould have larger horizontal stabilizers.

O well, I am confused on a higher level now, Thank you, Vance

[Vance]

Thank you Ehud, I always try to understand the benifits and costs of any compromise.

I have found that even in something as simple as motorcycle design it comes down to compromise. How does it feel? I have built motorcycles for road racing that change direction very fast, but would be much too squirly on the street. Lengthing the wheelbase would make it more stable on the street, but slow it's ability to rapidly change direction on the race track. Other people that would try one of my race bikes would feel that it handled badly because it never felt stable. For me, and the purpose of the bike, I felt that it handled great. It worked for my riding style.

I would asume it would work the same way with a gyroplane, that for it to feel good to a paticular pilot it would need to have the compromises in a particular direction. The part that puzzles me is the tendency to have too little horizantal stabilizer. Operating well below the predictated size doesn't seem to have a benifit. It is natural for me to wonder what the down side of being on the large side of horizantal stabilizer volume.

A gyroplane operates over such a wide speed range that it is dificult for me to understand how aerodynamic controls can always be sized corectly. Fixed wing aircraft use a lot of tricks to widen the speed range that they can safley operate. Gyroplanes have a lifting devise that takes care of the problem of stall, but control is still largly velocity dependent.

I am less confused if I have a way to find a ball park to play in, but I would never dismiss the value of a good test pilot.

Road racing motorcycles are still refined with a test rider even though they are a lot simpler than an aircraft. When I did design work we always worked at getting divergent opinions so we could find the best compromise.

Thank you, Vance

[Vance]

Thank you Doug, that is my point exactly. The local fellow starts with a tail that he knows is too big and tests it and cuts it till it feels just right. Kevin described well the down side of too much rudder, but I am still wondering about too much horizantal stabilizer.

I love the way you share information and I am always gratefull for your posts.

I apoligize for being a little slow, I have a very wobbly foundation when it comes to rotorcraft, so things that may be clear to you are a mystery to me.

Thank you, Vance

[Udi]

Hello Vance,

The benefits of a larger stab are known. The penalties for having a stab that is too large are higher weight, higher cost, and higher drag. A larger stab requires a beefier keel, which further adds weight. I agree with you that one should err on the large side. I also agree with Doug that the Cierva rule of thumb may be our best first shot right now, although I think we can do better. Unfortunately (or fortunately, depends on your side), gyros fly reasonably well with a tiny stab so people save money and weight by slapping a ping pong racket on the back keel.

The best stab, in my opinion, is one that is installed as far as practical behind the CG. The benefits of having the stab mounted further back from the CG outweigh, IMHO, the benefits of a tall tail. To help counter engine torque I would use a T-tail with differential incidence or opposite airfoils to achieve the required counter-toque (and I have not yet given up on using counter-rotating props).

Udi

[Vance]

Thank you Ehud,

It is interesting to me that it is based on rotor volume rather than weight, length or surface area.

In your opinion, how far into the prop wash does the horizantal stabilizer need to be?

I love counter-rotating props. There have been many attempts to realize the benifits. As far as I know, non of them have been paticularly sucsesful.

Thank you, Vance

[Doug Riley]

Udi, I imagine that a horizontal "tall tail" would do the same thing as a vertical one. In theory, it doesn't matter whether your flow-straightening vane is horizontal, vertical or diagonal, as long as it spans the whole slipstream symmetrically.

I'm guessing that Cierva (and Ron Herron) needed to use differential incidence on their HS's because of the long tail cones on their tractor gyros, which result in the slipstream's being partially diluted or dispersed by the time it reaches the HS.

I also think that, when trying to design a HS, it's helpful to run the machine tethered on the ground and use a portable airspeed indicator to "map" the slipstream. Put the A.S.'s pitot tube on a broomstick and move it about inside the slipstream while someone else tends the throttle (and the kill switch!). Only then do you have numbers for slipstream speeds at various locations and throttle settings for your particular prop and engine.

If you're a real white-coat type, you'll also do test flights with this special airspeed indicator. Attach the pitot to a different spot on the tail with each flight. (The slipstream isn't quite the same in flight as in a static run.)

None of this substitutes for pure flight testing per the ASTM standards, but it does provide a guidance in the design stage, and confirmation of flight-test results. If I got flight-test results that complied with the standards, but I couldn't support these results based on design numbers, I'd worry until/unless I could figure out why.

[Udi]

Well, the number of vanes, the type of vanes, and their distance from the prop affect their ability to straighten the slip stream and recover some of the rotational energy. A Dominator-type tall tail is very close to the prop and has 2 almost full-span vanes. I would think that a T-tail, having only 1 full-span vane, located twice or three times the distance from the prop will be less than half as effective in recovering torque as a tall tail. But using differential incidence, opposite airfoil (ala Herron), or gurney flaps can make it just as effective.

By placing the gyro on weigh scales - one under each main wheel - you can measure the net rolling torque at different RPMs. This will allow you to tweak your stab until there is no net torque.

Vance - the location of the stab vs. prop wash depends on what you are trying to achieve. Placing the stab inside the prop wash can help recover some of the prop rolling torque, as discussed above. The more traditional reason for placing the stab in the prop wash it to give it a boost, with regard to control power. This, in turn, allows a smaller stab, which saves money and weight. Which approach is best is in the eyes of the designer - and I just described my preference above.

The people at Air Command believe the stab should be located outside the prop wash, so it gets "clean air". Ernie and many on this forum believe in the tall tail concept.

Have you formed an opinion yet, Vance? I've heard you are working on your own design.

Udi

[Ga6riel]

The CL 20 Westland, a Cierva license built machine. had a tailplane in 2 halves. With one side with negative incidence (airfoil inverted) and the opposed with positive incidence, this compensated for the propeller torque.

[Doug Riley]

The downside of a smallish HS deeply buried in the prop wash is that it isn't very effective at idle power or engine-off. This is true because (1) it's small (2) it has a short lever arm and (3) its location right in the center of the wake of the fuselage puts it in turbulent or stagnant air. The Dominator has a distinct loss of control crispness when you power down, even if you hold the same airspeed. Some HS area outside the prop circle (with negative incidence to keep the CG ahead of the rotor thrust line) would probably lessen this effect.

Bottom line: partial immersion may be best of all. This is one of those fine points that should be worked out for each new design, however. The FW designers test and adjust HS incidence, size and location, too; it's not just us.

[Ga6riel]

I posted other views of CL 20 in my Pic File
It seems that the stb stab is the inverted side
being British, perhaps the prop rotates the other way

[Vance]

Thank you Ehud,

No, I have not formed an opinion. I am always trying to achieve confusion on a higher level. I have found it self defeating to believe that I know something.

Yes, I am working on a design with the help of a number of people much more learned then I. I am on major redesign number three, caused by a severe funding interuption. Redesign number two was caused from learning that compromises on design one might not be the best choice.

My funding interuption is nearly resolved so I hope to start making parts soon. I sold my shop building, but the people defaulted on the second, so my wife is working at getting it back. I sold most of my large tools, lathe, mill, tig, plasma cutter, ect, but I have a lot of good friends with tools so I don't expect that to stand in the way of progress. I hope to have the details of the interuption cleaned up by the end of February.

I love to learn and I love to transform ideas into something that can test my understanding of the theory. A gyroplane is a wonderfuly complex test of interacting theory. Yes, I have a test pilot who has agreed to test it when it is real, asuming he is not to old by then.

Part of the fun is forming the relationships that the desire to raise my level of confusion causes. Thank you for our relationship.

Thank you, Vance

[Vance]

Thank you Doug,

I feel like we have come full circle.

My initial question was realy about what the other side (bigger than calculations) looked like.

I believe that test and adjustment is a critical part of any experment, and perhaps the most fun. It is also the most dangerous, so a lot of thinking and calculating on the front end lowers the risk to the tester.

Thank you, Vance

[Aussie_Paul]

The downside of a smallish HS deeply buried in the prop wash is that it isn't very effective at idle power or engine-off. This is true because (1) it's small (2) it has a short lever arm and (3) its location right in the center of the wake of the fuselage puts it in turbulent or stagnant air. The Dominator has a distinct loss of control crispness when you power down, even if you hold the same airspeed. Some HS area outside the prop circle (with negative incidence to keep the CG ahead of the rotor thrust line) would probably lessen this effect.

Bottom line: partial immersion may be best of all. This is one of those fine points that should be worked out for each new design, however. The FW designers test and adjust HS incidence, size and location, too; it's not just us.

Doug, with Firebird I am trying 1/2 in and 1/2 out of the prop wash. The stabs have independent incidence choice.

Just thought I would try this as I have seen a couple of articles that convinced me it would be worth trying.

On day, when I catch up!!!! I will try 4 stabs. two in the center of the prop wash and two down under the prop wash. That way I will be able to compare apples with apples. Try the 4 and try 2 at either level. Time, oh for another one of me without the bad bits!!!:eek:

Aussie Paul.:)