Power Push Over (PPO)

Udi, perhaps the SH designer is too smart to chime in and start wrestling alligators.

Stan, I’ve always appreciated your straightforwardness. Most likely if you had a secret, you’d simply keep your mouth shut.
 
Stan is one of the most tell it like it is guys, I've ever met in my life!
There are no secrets regarding rotorcraft or even any of his mistakes in life, he tells the good, bad, and the ugly 100% of the time.

I'd bet all I've got; on Stan's words!!!!
 
Thanks, Chris. It’s great to have at least one person that doesn’t disagree with me.

No, Chuck, I would never dare to disagree with you on rotorcraft subjects. :D

Just trying to understand.
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.

Now I got it, thanks.

Kai.
 
Udi- No offense taken! I said myself that I was inexperienced, but I was just doing the stability tests that Greg Gremminger had outlined. The SparrowHawk did excellent on the other tests...except for this speed test. No way would it stay within 10% of the initial speed.

I am simply saying that if you cut power after flying straight and level....it will just keep speeding up if you leave the stick alone....until I cry uncle. Maybe a test pilot that knows what he is doing can ride this machine faster than I can , but I know my limits.

I dont say everything I know at times. Rob Dubin called me back when I had my SparrowHawk, and wanting me to do this test and give him the numbers. Well, I already had done the test and it was the report I have typed here last few posts. But....I refused to give it the Rob at the time simply because he was a Xenon dealer and I was leary of giving out any negative information on the SparrowHawk, especially since I loved the way it flew for the most part.

Stan
 
Stan - don't feel bad for the SH. I believe the SH is not alone; also the Xenon does not meet a 10% or even 20% airspeed stability test upon power reduction. It also accelerates. And the Xenon is "supposed" to be CLT, not LTL. Is there something we don't understand here?

Udi
 
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 .....
I too am very wary about disagreeing with Chuck because when I do, if I go back and read his post more thoroughly I realise that I was wrong.
I do however have a problem with the above - perhaps I have taken it out of context??
 
Chuck, If a portion of the problem of AS instability in the SH is 2* to "drag over" as the long sloppy wind screen suggests it could be, then yawing as Steve M. suggests could lessen this IF FLOWN WITH BOTH DOORS OFF????
 
Tim, I'm with you on this one and I'm sure Chuck will clear it up. It seems to me that the initial response to a power chop on a HTL machine would be a nose up pitching moment, not nose down????
 
Udi- I really enjoyed my SparrowHawk, and it still was my favorite of all the gyros I had flown or ridden in.

Stan
 
I too am very wary about disagreeing with Chuck because when I do, if I go back and read his post more thoroughly I realise that I was wrong.
I do however have a problem with the above - perhaps I have taken it out of context??
Tim, Walt and all; I was bass akwards on that one; should have been LTL. My poor frazzled brain couldn’t keep up with my fingers which aren’t all that fast either.
 
Chuck, If a portion of the problem of AS instability in the SH is 2* to "drag over" as the long sloppy wind screen suggests it could be, then yawing as Steve M. suggests could lessen this IF FLOWN WITH BOTH DOORS OFF????
Walt, the bottom line is that all gyros in a power off glide are merely a glob of stuff hanging from a rotor.

Sloping windshields, tilted horizontal stabilizers and whatever may affect the angle by which it dangles but as long as the cyclic controls aren’t up against a stop, there will never be a runaway situation with a stable rotor. I suspect “dragover” is a hypothetical that exists only in our minds. It’s a new ball game if drag or whatever forces cyclic controls against a stop.

A flapping rotor is automatically stable vs. airspeed. The faster you go, the more it flaps back and resists a speed change.

The angle of descent at any given airspeed is controlled by the overall lift/drag ratio. The draggier a machine is, the steeper the glide angle.
 
Regardless Chuck, I would still swap you "frazzled brains" though.
In my case, my RAM has limited short term capacity, my hard drive is full and I have to delete some past information just to have the capacity to soak up anything new.
 
------Sloping windshields, tilted horizontal stabilizers and whatever may affect the angle by which it dangles but as long as the cyclic controls aren’t up against a stop, there will never be a runaway situation with a stable rotor. I suspect “dragover” is a hypothetical that exists only in our minds. It’s a new ball game if drag or whatever forces cyclic controls against a stop.

Chuck, "Fixed Stick", or even pilot restricted stick movements are the worst case for all stability issues. If the stick is allowed to float completely free, the gyro is truly a "glob of stuff hanging from a rotor" - the "glob" is no longer "dangling". But, a "fixed" or restricted stick is essentially the situation where the cyclic controls are "against a stop" and any airframe pitch change is coupled to the rotor through cyclic action of the spindle - no matter how airspeed stable the rotor is. Also, the trim spring does not fully allow the rotor to float relative to airframe pitch actions anyway - not truly dangling.

To account for both the trim spring coupling from airframe to rotor, and a novice restricting the stick, aircraft stability criteria generally require stability - including static Airspeed - with fixed stick as well as free stick. The worst condition where the sloping windscreen, shaded HS, draggy LG, etc. can be a source of airspeed runaway is with a fixed stick - not with a stick that can freely float, if any could really float (without a trim spring?)

Just saying the worst case is with a fixed stick - I'm not sure Stan was even describing a "fixed" stick. The pilot experienced in flying a gyro that does not have a stabilized airframe (HS), will allow the stick to float - to minimize coupling unstable airframe pitch action to the rotor and forcing the rotor to also pitch in that divergent direction. (This may be one reason why pilots experienced in their unstable gyro are probably safely flying that gyro - they have learned to allow the stick to float and apply only pressures to the moving stick.) But, a novice pilot will likely restrict or freeze or over-react on the cyclic to a sudden nose-down pitch. And the trim spring coupling to the rotor may essentially "fix" the stick anyway.

For instance, if the controls in Terry's SH broke or locked, the rotor may not be free to float, or may respond stangely to airframe motions - essentially the fixed stick scenario - and a continued "drag over" of the airframe due to a HS that does not compensate for a windscreen or enclosure (for instance), would force the rotor to follow the airframe's continuing nose-down attitude.

If Terry had become incapacitated, as some scenarios of the accident suggest, the very novice Bill Finnegan, upon increasing airspeed and nosing down, would likely have had the intuitive presence of mind to reduce the throttle - unexpected by him, that would have made any "drag over" effect worse. As Stan and Bill's other SH friend have reported, if you don't do anything upon reducing power, the gyro tended to nose over and pick up speed until, when the airspeed exceeded around 80 mph, the stick forces to raise the nose become formidable - especially for a novic pilot who would be afraid to pull that hard on the stick.

Another point. If the loss of LTL thrust is not the cause of this reported characteristic, but ROTOR instability is, why would the gyro not tend to also diverge to increasing airspeed with power applied. I have heard no reports that the SH had this diving tendency at normal power levels. I suggest this is because both the LTL is helping to hold the airframe nose up, and the extra prop blast on the HS is enhancing the HS effectiveness to balance the enclosure and windscreen and LG.

It is a simple matter to test this - if someone flying a SH would do - just repeat Stan's description of cruising S&L at 60 mph, and then reduce power without moving the stick - or with letting the stick float free - to see if the gyro remains at the trimmed airspeed or starts to accelerate in an increasing dive. That's Static Airspeed stability or instability. If the speed and slope of dive increase beyond where you feel comfortable pulling on the stick, add power to force the LTL nose up and recover the airspeed. Start at high enough altitude, because recovery with power from over 80 mph is reported to take as much as 500 ft.! It would be helpful if someone with RAF blades would also do this test - the reports of this phenomena, I believe were with Sport Copter blades. As Chuck explains, there are other rotor characteristics that could also present airspeed instability or divergence from trimmed airspeed - such as "blade runaway" due to torsional flexibility of the blades.

- Thanks, Greg
 
Chuck
Did you mean to say that Sportscopter rotors have no neg or positive pitching moments? I ask because I believe you've said Sportscopter blades cause a nose down attitude on Dominators.

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
 
Greg, if airframe drag exceeds rotor drag as inevitably it must with large cabins at cruise speed and above, sudden propeller thrust reduction causes pitchover in excess of that required to maintain trimmed airspeed. The excess pitchover amount depends nearly entirely upon the ratio of airframe drag to rotor drag. Exact CLT will have excess pitchover. Moderate HTL will have excess pitchover.

I have don’t know whether a SH is LTL or not. I think it might be nearer to CLT than is credited. Do you know where it is?

If the rotor is stable, the airspeed will not run away fixed stick but will stabilize at some airspeed in excess of trim speed. Simply the nature of the beast. Fixed wing criteria isn’t blindly applicable to gyros.

I’m out of here for this day.
 
Walt, the bottom line is that all gyros in a power off glide are merely a glob of stuff hanging from a rotor...

Chuck, do you implicitly assume the pilot is floating the stick when you say this? The reason I'm asking is because some unusual attitudes can result when the wind hits the canopy in a strange and unintended way, which would probably lead the pilot to apply some sort of "correction" using the stick and thereby feeding back into the cyclic.

-- Chris.
 
I read in this forum many assumptions about the involvement drag fairing and windscreen in instability. To remove doubts it is very easy to carve out a model in polystyrene, with the SH in cardboard. The side crossed by a knitting needle horizontally assumed in G. After that, put your weathervane in the wind . If the tail does not stay behind: Not good
 
Chris, the blob theory assumes the pilot is allowed to make inputs.

Viewed as a flying machine with sufficient tail surface to provide weathervane stability about all axes with and without propeller thrust, sloping windshields make no difference. Windshield and tail move through the same air and are subject to the same laws of aerodynamics although flow separation can produce kinks.

But what’s needed is more fact and less speculation. I don’t know whether a SH is CLT, LTL or somewhere in between. All I am certain of is that the rotor blade sections I have show the RAF blade to have a zero pitching moment coefficient and the Sportcopter blade to have a negative pitching moment coefficient.
 
Chuck
Did you mean to say that Sportscopter rotors have no neg or positive pitching moments? I ask because I believe you've said Sportscopter blades cause a nose down attitude on Dominators.

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
Russ, I don’t recall having said anything about Sportcopter rotors on a Dominator.

For rotorcraft applications, a rotorblade airfoil should have no tendency to twist either nose up or nose down.

The attachment sort of illustrates what I mean.

The NACA 0012 section was used on many early helicopters because it has no propensity for twisting one way or the other and was simple to fabricate. Being symmetrical, it is as capable of producing up lift as well as down lift, a valuable thing to have on Pitts specials where the ability to fly up side down is important.

The Clark Y is essentially the same as an 0012 if the curvature is removed. A favorite for many years for aircraft having no need to fly inverted. Notice the curvature of the mean line, the line midway between upper and lower surfaces.

It would be worthless for rotorcraft use. If it would fly at all, it would immediately dive into the ground.

It could be used for a rotor if the trailing edge was reflexed as shown in the sketch. With 5% of the trailing edge bent upward at an 11-degree angle, the nose down pitching tendency is eliminated.

But there are modern airfoils that offer superior performance for rotors.
 

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