Power "chop" test - A
Power "chop" test - A
Hi Chuck and all. I’m afraid I must comment on this, and I really appreciate the opportunity to discuss these issues in depth – I think they are important to understand. And, perhaps my concepts are questionable. Pardon me for detailed explanation below – I’m not trying to patronize anyone who does not need the basic explanations, but I am trying to help everyone understand these basics and to further a constructive discussion.
---- there is a simpler way of evaluating bunt resistance.
From trimmed cruise flight, snap the throttle shut while holding the stick as stationary as possible. If the machine pitches noseup, you’ve got a potential bunter. If the machine stands on its tail and does a mid-air flare, you’ve got a ticking timebomb.
I do not recommend an average pilot “chopping” the. I believe this can be dangerous. First a bit of a definition:
“Unbalanced Propeller Thrustline”: This is when the HS, reacting to propwash, is not able to compensate for an offset propeller thrustline (from CG). On a HTL or a LTL (deviation from the actual CG), the static prop thrust pitch moment will change upon change of engine power, requiring the airframe attitude to adjust accordingly – until the RTV offset again adjusts to balance all the remaining moments on the airframe. It is possible to embed the HS in the propwash enough to compensate for some or all of HTL or LTL. However, perfection of this “balance” is also affected by the other (aerodynamic) pitching moments on the airframe, so there may only be one airspeed where the HS in the propwash can be arranged to perfectly “balance” a prop thrustline. The airframe reaction to a power change is therefore the result of the “effective prop thrustline” when the propwash on the HS is included in the “balance” of the prop thrustline.
Changing the power quickly, for an unbalanced prop thrustline, causes the airframe to pitch suddenly to the new balance point of the “sum of moments” on the airframe. For a badly unbalanced HTL, this could cause the airframe to pitch suddenly and severely nose-up – the reason for the old instructor adage to reduce power if you think you are getting into trouble. This would also indicate as you suggest, Chuck, the possibility that the gyro might be prone to PPO (buntover). But, for a badly unbalanced LTL, the pitch reaction to a “chop” in power will be suddenly nose down. A sudden nose down pitch reaction may not be so good:
Especially with the stick held “fixed” during such a radical airframe pitch reaction, the spindle will input a sudden cyclic action to the rotor. In either the nose-up reaction, or the nose-down reaction, the sudden and severe cyclic input could “precess stall” the rotor – suddenly stall more of one or both rotors enough that the teeter range is exceeded. In the nose up direction of an unbalanced HTL, a precession stall might only cause severe mast bumping. In the nose-down direction of an unbalanced LTL, a precession stall might not only cause a severe mast bumping, but the sudden reduced rotor loading could also suddenly slow the rotor at the same time. This is especially severe at higher airspeeds where the aerodynamic airframe moments can cause more severe nose-down pitch reaction. This is all what could happen if the stick is fixed or the pilot does not react quickly or adequately enough with opposite cyclic reaction to avoid severe cyclic input to the rotor.
But, also in the nose down reaction of a LTL, if the pilot does allow or commands a quick compensating cyclic input – not “fixed” stick - the nose-down airframe reaction cab cause the airframe CG to suddenly move to a new position aftward – statically AOA less stable or even unstable condition. (With the power at idle or off, there is no longer a LTL artificially holding the RTV aft of the CG, so the new steady state trimmed condition will likely be with the RTV forward of the CG because of the other nose-down aerodynamic moments on the airframe – especially at higher airspeeds! If at any time in this transient, the RTV becomes forward of the CG – AOA unstable - and if the cyclic input has not fully corrected the reducing G-Load on the rotor, that gyro is in danger of a buntover! It would not be a good situation to have the nose still dropping, the rotor load still reducing, at the same time the RTV is forward of the CG.
In a severe power “chop” on this LTL condition, without immediate and adequate cyclic input, the pilot could end up in an AOA unstable condition with the nose still dropping – a possible initiator of a buntover – rapid AOA static divergence in the nose-down direction! Especially with the gyro suddenly in a statically unstable situation, rapid pitching could easily excite the pilot into over-reaction – possibly inducing a PIO pilot reaction.
There are issues with the power “chop” on a severely unbalanced HTL also – but at least it is moving the CG in the more stable direction and gyros do not really have a “bunt-up” reaction – but the precession stall and inducement of a pilot PIO reaction is still possible.
I do not recommend a power “chop” as a test for other than a professional test pilot. I did this once in a standard 618 Dom ONCE! I was familiar with the power “chop” reaction of my original Air Command and with my High Command modification. During the 40 hour Phase I flight testing on the Dominator I had built, and before I understood much about gyro aerodynamics, I tried a power “chop” at 70 mph and 20 ft over the runway. Upon the “chop” in power, the airframe suddenly pitched down about 15 degrees and I was looking at eating the runway! My reaction was, of course, an immediate aft cyclic and I avoided contacting the runway! I over reacted somewhat with the stick and ended up PIOing a bit as I added power back in – I was totally unfamiliar with, and unexpecting of those reactions in that gyro! Later, after I though this over a lot, I realized that I could have precess stalled the rotor, that I had probably experienced a point of AOA instability accounting for the sudden handling sensitivity – realizing that perhaps my commanded cyclic input could have been less than ideal to save that situation!
Another indication that this normally docile-in-turbulence Dom could have conditions where it was not so AOA stable is when I would reduce power at high airspeed and attempt to continue that high speed in an idle power gliding descent. In turbulence, this was a very uncomfortable condition. I believe this is because, without the LTL artificially holding the CG well forward of the RTV, the nose-down aerodynamic moments actually presented a statically AOA unstable condition – the RTV was forward of the CG – at least it did not have the large margin of AOA stability one gets used to in that LTL at normal high speed cruise! Without the LTL to augment the static AOA stability, that gyro flew rather uncomfortably in turbulence! I had learned by then to not “chop” the power at higher airspeeds, but I also avoided high speed glides with reduced power simply because it felt very uncomfortable! I think I realize now that was because I was flying a marginally AOA stable gyro when LTL power was not augmenting AOA stability by holding the CG well forward of the RTV.
The ASTM standard has a section to check for this “unbalanced prop thrustline” – the “Static Power Stability” test. This “Power Stability” test was originally suggested by the FAA Rotorcraft Directorate for the standard – probably because they recognized the issue’s severe airframe pitching might cause pilot over reaction. This test does not suggest “chopping” the power for the above reasons. However, a slow power change will slowly adjust the pitch attitude and trimmed airspeed to indicate an unbalanced prop thrustline condition just as well as a “power chop”, but without the dangerous implications above.
IMHO, even if this test was safe to perform, it does not allow for a conclusive determination of static AOA stability or susceptibility to buntover – the judgment of the severity of the pitch reaction is subjective – we are looking for an objective determination that is not dependant on the pilot subjective evaluation or stick reaction. At a minimum, for an objective evaluation, the stick would have to be mechanically fixed, a situation in which I would certainly not suggest a sudden power change – for a non-professional test pilot!
Thanks, Greg
