Power Push Over (PPO)

Udi, I’ve never so much as had a ride in a SH so couldn’t confirm whether it has runaway airspeed properties or not following a sudden propeller thrust reduction. I can confirm that the rotor blades have a sizable negative pitching moment coefficient if the sample section I have is representative.

Both Greg and Stan appear to have stated that it has runaway speed characteristics.

I’m only trying to explore reasons why that might be so. Whether true or not, I don’t believe LTL is a factor.
 
Tell me, Greg, how does propeller thrust line location affect steady state glide angle or the propensity for airspeed runaway?

The effect of a sudden loss of propeller thrust is entirely transient and has no effect on airspeed after things have stabilized.

Only two things could cause runaway airspeed; an excessively draggy cabin and a rotor with negative pitching moment. Some gyros might have both.

The location of airframe drag center vs. CG plays a secondary roll.

Chuck, what Udi said. Thaks Udi! I do believe it is the excessively nose-down windscreen aerodynanic moment - along with the fact that the HS looses a lot of it's airflow at lower power. The HS also appears to be highly shaded by the cabin - so as airspeed is increeasing, the HS may not be statically balancing the increasing windscreen and cabin destabilizing static moments. But, the initial increase in airspeed from power reduction is what Udi described. (And in this case, if Terry had indeed become incapacitated, and Bill wanted to try to slow the gyro down, he would likely have reduced power - he knew enough to figure to slow down reduce power. IMHO, this would have just started higher and higher airspeed dive - and reducing power just makes it worse.)

FYI: Not only Stan tested and reported this effect - over about 80 mph at idle power the airspeed just keeps increasing. But, a mutual friend of Bill Finnegan's and myself had a SH and flew a lot with with Bill. Bill's friend was at wits end when he found out that day and went out in his SH to see if he could find a way to duplicate a gradually steepening dive to the ground. He did this way above the groud. He found that when he reduced power to idle from a S&L normal cruise, and without compensating cyclic movement - aft movement - the aircraft picked up speed and when it got to 80 mph, it just kept getting faster and faster. This friend reported that the stick forces to pull the nose up even made him warry of pulling so hard. He pulled out of it by ADDING power to help push the nose up - LTL and maybe better lift on the hS from the increased airflow? He said it took about 500 ft, but adding power raised the nose and returned it to below 80 mph flight with aft cyclic force he felt comfortable to apply..

I believe this is also what Stan reported.

- Thanks, Greg
 
Greg, right after Terry’s crash, I checked with a couple of people that surely knew whether a SH had runaway speed characteristics or not and was assured that it didn’t.

I suspect now that it may. Whatever the case, it is a critical problem that needs to be addressed.

I would like to know whether an RAF-2000 with SH mods and RAF rotor has the same problem. I expect not but it would be worthwhile to see a comparison between RAF and Sportcopter rotorblades on an RAF with SH mods.

I have a Sportcopter 9” blade section from which I very carefully extracted the ordinates on my Bridgeport milling machine table using an electrical probe with light bulb to indicate contact. The paint is pretty tough and required a lengthy soak in chemical paint stripper. X-Y ordinates were read out using a Mitutoyo optical readout system. I could repeat any measurement within 0.001”.

The sample section I have has a substantial negative pitching moment coefficient. Admittedly, my blade section may not be representative.

A rotorblade with negative pitching moment suppresses cyclic flapping; the advancing blade tends to twist nosedown more than the retreating blade as a result of the airspeed differential. The reduction of flapping leaves the rotor thrust line more nearly concentric with the rotorhead axis instead of moving it forward with increasing airspeed and requires a significant increase of trim spring tension. Some people misinterpret this as an indication of low drag. It is not.

The plots are attached. The airfoil program I use may not equal NASA’s supercomputers in predicting lift and drag polars but pitching moment coefficient is quite accurate.

One of the views in the attachment shows a Sportcopter airfoil superimposed over a NACA 8H12 section. They are identical from the leading edge to the backside of the spar. After that, the Sportcopter blade flattens out and neglects the reflex.
 

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With stable rotor, speed runaway is probably impossible because of the rotor’s inherent velocity stability, AKA rotor blowback.
 
Chuck,

I think Ron Menzie has flown both RAF and SC blades on his modified RAF; you might contact him and see if he can offer any info.
 
You guys rock!!! It's slowly starting to sink in?
My plate is full until Jully but I'd like to start learning the best simulator program to test these theories out as you all pound more into my head.

What is the name of the best programs (most accurate) is it still the one that NASA created? (Can't remember it name either)
Is there a program that you pros would trust?
 
In 2003/4, I received some flight training in Buckeye on a AAI modified RAF with sport copter blades. I remember clearly one time I was coming back from some practice, descending from altitude with little power (maybe 3000 rpm), holding 100 mph airspeed. The gyro was solid. The stick was light and responsive in pitch. No control reversal.

While practicing emergency landing, we would chop power downwind abeam the numbers and enter a steep 180 for landing - all without power. After chopping the power the gyro would require some back stick pressure to maintain airspeed (my single place AC needed the same). I never felt the amount of force or back stick displacement needed was excessive. That was done at ~60-65 mph. That is all of my personal experience with the SH design.

I am curious. With all of the SH flying out there - if the problem was really as bad as Greg is reporting (second hand) - how come we don't hear any complaints?

Udi
 
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Udi- I can only speak from my limited experience in my SparrowHawk and flying Terry Eilands as well.

I was wanting to buy a SparrowHawk from Terry...so at Bensen Days 2005, I received my first ride in one. I distinctly remember the considerable back force needed on the cyclic when doing moderate turns....a tight turn...really had to have much more force than I was used to flying my other gyros.



Now fast forward to Bensen Days 2006. I was getting my flight review with Terry in his SparrowHawk. He had me fly him to the airport on the north side of Lake Okeechobee. While enroute... I was commenting on Terrys electric trim that he had installed. Terry was all enthused after reading how I had installed electric trim on my SparrowHawk,,,so much so that he now had it. Anyway...Terry commented that he liked to not trim out all the back pressure as he liked to feel it in the cyclic more. I being a greenhorn with the SparrowHawk was reluctant to respond back to someone I admired both as a person and as a skilled flyer...but I had to. I said..." I like to have all that backstick trimmed out just in case the motor goes silent...cause that nose wants to head to the earth abruptly the way it is! I took his advice and flew the rest of my flight review holding what I felt unneccessary backstick when the trim could have handled it.
Anyway, the next passenger was Bill Finnegan....and we all know about their tragic accident. We unfortunatel may never really know the cause of their crash.

I was so obsessed with the extras forces my SparrowHawk needed during tight turns...that I kept hounding Dan Banks at GBA to come out with a heavier upgrade on those 1/4 rodends that were on my cyclic. After Terry and Bills accident....I just couldnt relax until that upgrade was installed. I finally after almost a year of whining received my upgrade tp 5/16 rodends. Maybe I didnt need them,,,but it sure looked better. I was just so aware of the higher stick forces needed...and this couldnt hurt.

Steve McGowan had given me some advice over the phone about a scenario of loosing that back cyclic...and the best possible way to get out of it. If say that rodend failed...or that teeter stop bolt jammed the rotorheads back travel....the SparrowHawk enters a swan dive if you cut power....and Steve suggested practicing giving a hard rudder input...turning the SparrowHawk akmost sideways...and then using sidestick to stop the dive. Steve....step in here and correct me if I am misquoting your advice. Steves advice by the way is as good as gold to me and I doubt he would ever tell anyone wrong on anything pertaining to gyro flying.

Anyway....I would go up to straight and level.....60 mph....and then ease off the power. My SparrowHawk would start accelerating...and I would then pretend I could not use any backstick...simulating a control failure or jam. I would very shyly apply rudder at first...but Steve said I had to be aggressive. That was hard for my engrained reflexes to make a sudden and hard rudder input turning sideways. I never got proficient at this recovery method...but if I checked the airspeed early...before it ran away....I could turn the SparrowHawk sideways.....left rudder by the way.....and then use left cyclic to check my airspeed. It wasnt pretty...but at least it wouldnt be a runaway S-W-A-N---------D-I-V-E....... and I could keep wollering it around half way.


I loved the way the SparrowHawk was so easy to fly...but this speed runaway always kept me a little edgy.....as it just loved to head nosedown with no power. Thats why I liked my electric trim there to catch most of it when I cut the power. Towards the end of my SparrowHawk career....I actually liked trimming it out so that I had to hold forward stick while cruising so that when the power was reduced....the trim was holding most of the backstick for me.

Hey....take the above information from my limited experience and point of view...but thats how I perceived the situation.

By the way...my weight and balance were in the envelope nicely...as I gave Groen Bros...the weight of my three wheels with just myself...and with my 270 pound son in it. My experiences above are my solo flying...and would be even more pronounced if I had done these tests with my son in the passenger seat.


Stan
 
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What you’ve just described, Stan, is the effect of rotorblades having a negative pitching moment coefficient.

As for pitching moment coefficient, a rotorblade behaves as though it was a thin plate running midway between top and bottom surfaces of an airfoil. If that midway line is curved in the shape of a venetian blind slat, it tends to twist nose down when being dragged through the air at several hundred miles per hour.

That’s fine for propeller blades that are so short and stiff that elastic twist can be ignored but not for long and skinny rotors. A cambered rotor airfoil requires a reflex that serves as an up elevator to balance the nose down pitching tendency.
reflex.jpg
On a rotor, the advancing blade, having greater airspeed than the retreating blade, twists more nose down than the retreating blade. This equalizes lift between advancing and retreating blades and suppresses cyclic “flapping”.rotorhead.jpg

It also increases necessary trim spring tension and can cause runaway airspeed.
 
A stable rotor, one having zero pitching moment coefficient; meaning the blades have no tendency to twist either nose up or nose down, possesses velocity stability. It resists a change of airspeed as a result of cyclic “flapping”.

As forward speed increases, so does cyclic “flapping”, presenting the appearance of the rotor disc having been blown back in proportion to forward speed.

It’s hard to imagine a situation where a stable rotor could be subject to airspeed runaway. Blanking of the horizontal stabilizer by the cabin seems unlikely to be a cause since a stock RAF-2000 has no horizontal stabilizer and as far as I know, no tendency for airspeed runaway.

I fail to see the relevance of propeller thrust line location in any of this except during the second or two following a sudden thrust reduction before things have stabilized. To be sure, a HTL machine will pitch nose down in excess of the amount necessary to maintain constant airspeed following abrupt propeller thrust reduction but with a stable rotor, will stabilize at perhaps 10-15 mph faster than trim speed. Not the end of the World.

Static “dangle angle” has no effect so long as there’s sufficient control travel to keep the stick off a stop.
 
While John is saying, he understands, I'm more and more loosing it:

Chuck, you say
Tail heavy rotorblades, those with CG aft of the aerodynamic center, cause angle of attack instability that is often mistakenly blamed on other factors.

Nose heavy rotorblades can produce angle of attack stability that is often mistakenly credited to other factors.

Rotorblade airfoils that produce a nose down twisting force as a function of airspeed over the blade can cause non-recoverable dives, often mistakenly blamed on other factors.
All of this makes sense, so Magni puts lead into the blade-tip to pull the CG forward towards the center of lift.
I furthermore understand that Magni may even be over-doing it, putting the CG even more forward, which attribute to the resistance against change you can feel on the stick.

However then you say:
A cambered rotor airfoil requires a reflex that serves as an up elevator to balance the nose down pitching tendency.

Now we mostly use the same 8H12 airfoil which clearly has this reflex, but why is it necessary? I thought nose-down is good?

Kai.
 
Nose down in response to an upward gust is good; nose down in response to airspeed is bad.

But we are dealing with two different and unrelated phenomena.

An airfoil has an aerodynamic center where all aerodynamic forces can be considered to act. For normal airfoils, this is at ~25% of chord behind the leading edge. Nearly all rotorcraft utilize airfoils mass balanced about the aerodynamic center. A gust produces no pitching reaction.

A tail heavy airfoil, one having mass balance at a point aft of the aerodynamic center, produces a nose up twisting torque when subjected to an upward gust. The gust acts on the aerodynamic center and inertia does the rest. Such an airfoil is unstable vs. angle of attack. Mildly underbalanced, such a rotor produces the feel of a balloon as described previously. Strongly underbalanced, flutter is likely.

A nose heavy airfoil, one having the CG forward of the aerodynamic center responds in the opposite manner. An upward gust tends to twist the airfoil nose down, alleviating the effect of the gust. A cyclic pitch input is no different than a gust. An overbalanced airfoil responds at a lower rate than an exactly balanced airfoil, producing a heavy stick.

Bensen in one of his experiments, over balanced a wooden rotor to the extent that it would not respond at all to a cyclic input.

This is easily explored with an electric drill and a strip of thin sheet metal to mimic a rotor. Aluminum flashing material, ~0.010” thick works well. Run the drill backwards to avoid unscrewing the spindle bolt.drill.JPG

Cut a strip of thin sheet metal about 1” wide x 12” long and run in your electric drill. Without nose weights, it will flutter violently. Single nose weights will probably prevent flutter. As the amount of nose weight is increased by adding more nose weight screws, the “rotor” will respond more and more slowly to the tilting of the drill motor.

A rotor with a nose down aerodynamic pitching moment is bad in that it tends to run away with airspeed. The faster it goes the greater the tendency to twist nose down.

Bensen wooden rotorblades had excess reflex and a nose up tendency which increased cyclic flapping and added to velocity stability. Not necessarily a bad thing because it limited top speed; a Bensen with wooden blades would begin to run out of forward stick travel at 60-70 mph. Additional power would cause it to climb at the same airspeed. My first Bensen with wooden blades didn’t require a trim spring for hands off flight at 45 mph.
 
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Kai, let me jump in here and try to explain. I'm sure Chuck will fill in the holes and correct any slip ups of mine.

First, "nose down" (i.e. a negative pitching moment of the airfoil) isn't necessarily good if you've got too much of it. Generally it will tend to keep the AoA constant. If you hit an updraft (increasing the AoA), the aerodynamic force will tend to twist the blade nose down, thereby lowering its AoA. On the other hand, this overbalanced blade make for a more sluggish rotor response (c.f. the "Magni" effect, or at least some part of it). Too much overbalancing leads to a very dangerous situation: when you want to pull out of a dive, you don't have enough control authority because the blades resist your attempt to increase their AoA. It's a non-recoverable situation.

The other thing to keep in mind is that even if an airfoil has zero pitching coefficient (as the 8H12 with reflex does), it will still exhibit a negative pitching moment if it is overbalanced. So, to have a truly neutral pitching behavior, you need an airfoil section with zero pitching coefficient and a CG which coincides with the CoL.

-- Chris.

P.S.: Oops, just realized that Chuck beat me to it :)
 
No disagreement with the last 2 posts (Chuck and Chris). And I also agree that a balanced rotor that is airspeed stable (i.e. has a flapping angle that is proportional to airspeed) is a very good thing for a gyro. What I am not convinced of is that 1. SH has a real airspeed runaway problem and 2. that whatever problem is has is mostly due to a specific brand rotor.

To me, airspeed runaway means that, with the stick locked, and idle power, if left alone the gyro would continue accelerating indefinitely until it exceeds a critical speed limit and breaks apart. It is possible the SH will reach a new steady state - maybe not very comfortable for the pilot, but I don't think it will run away.

Has anyone tried swapping the Sport Copter blades with RAF blades to see if it makes any difference?

Chuck - static hang angle makes little difference other than stick position in a stab-less gyro. It can make a difference in a stabbed gyro since the stab is affecting the CG/RTV relationship and is contributing to aircraft AOA stability - which is a prerequisite for airspeed stability.

Udi
 
Udi, I have no first hand knowledge whether a SH is airspeed stable or not. Some say yes; others say no.

My only factual knowledge is that the sample RAF blade section I have is very nearly ¼ chord balanced and has an essentially zero pitching moment coefficient. The Sportcopter section I have is neither.

I’m sure you know I’m not an RAF apologist.

Another thing that leaves me puzzled is why would Steve McGowan advise Stan to yaw the machine sideways if unable to otherwise arrest a runaway speed encounter. If aerodynamic “dragover” was a factor, turning the machine sideways would exacerbate the problem.

The only reason I could see for turning it sideways would be to act as a drag brake. A rotor after all is round and doesn’t care which way it goes. Perhaps there’s more cyclic travel about the roll pivot.
 
Chuck- Thanks once again for your excellent posts that are so detailed. The situation where i turn sideways was if the pitch cyclic broke or became jammed, one of many possible scenarios for Terry and Bills crash. The idea is to yaw sideways and use the side stick as your new pitch cyclic. Steve could better explain it as I never accomplished much in my training. As far as the SH chopping power, it most definately will keep speeding up past my danger line of 95 mph. Maybe it will level stabilize speed eventually, but I am too inexperienced and too scared just to ride it out to find out the numbers. Stan
 
I would have liked to see the SH designer and test pilot chime in here. I would value his opinion on this more than the opinion of amateur pilots, or even CFIs (No offense intended Stan and others). In my experience he does not tolerate unresolved safety issues. Which is another reason I don't think there is a big safety issue here. Hope I am correct.

Udi
 
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