Ken,
Nice post. Good questions and observations. I will try to answer them below.
John
You've described the character of the vertical axis (yaw) but can you tell us more about the longitudinal axis (roll)? As a previous post points out, this is most likely a dutch-roll mode and I suspect it is as not many configurations produce pure yaw moments. Would you say the yaw divergence grows faster than the roll divergence?
The yaw divergence is definitely more pronounced. There is a roll component but it is not dominant.
Also, you need to test the spiral stability - tendency for the airplane to recover, maintain, or increase AOB during a stick free turn.
Have not done this. I guess we'll have to go out and fly. Darn. Will report back.
As stated Dutch Roll is a complex combination of many factors - from a pure configuration point of view I cannot believe the RV8 exhibits negative dynamic yaw stability - maybe weak but not negative. Its it actually hard to even design for that mode in conventionally configured fixed wing airplanes...function of the area behind the CG vs forward of the CG provided you remain in the CG envelope.
This is what has me puzzled. There is nothing unique about this 8. Stock wheel pants, cowl, etc. The only thing unique is it is sporting a tail wheel fairing. Hmmm, extra mass swinging around? The tail wheel is not directly linked to the rudder. It has chains. The fairing is actually fairly long.
Now however, if your spiral mode is divergent (roll tends to increase from a fixed AOB when stick free) and is combined with weak dynamic yaw stability then the dutch roll mode can become divergent. There is a term called the spiral determinant which is the difference between the product of the yaw control derivative and Dihedral derivative and product of the Roll Control derivative with the yaw stability derivative. If this term is positive = damped dutch roll, negative = divergent dutch roll. Zero = dutch roll continues (or something like that...its been awhile)
Like I said. Standard Van's. I have not noticed any divergence in AOB but if you're not looking for it you tend to compensate without thinking. We'll test next time out and report.
Also, it would be important to characterize the frequency of your rudder doublets as every mode also as a natural frequency...if the Fn is high enough to not be in the range of normal pilot inputs (which are of low frequency) then its generally of no consequence. 1-2 Hz rudder inputs are a good guess as to the Fn of the airplane.
Your guess is about right. I'd say my inputs we're in the 1-2 Hz range with pedal deflections of about 2". This produced results exactly like perturbations caused by turbulence.
Also, another way to help determine the more offending mode in the dutch roll is to input aileron doublets of slightly higher freq (but keep the amplitude low) with fixed rudder at neutral. See if the dutch roll is excited by this input.
This is another good test! Will do and report back.
A minor point: adding friction intentionally to the system is generally not going to be a good thing. While it may help with apparent stability by simulating a fixed pedal condition - you will have altered the mechanical characteristics of the flight control system. Specifically adding friction increases the breakout force (which is just that force required to start the pedal moving from a static position). Why is this important? Because higher break-out forces lead to over controlling in high gain tasks such as yaw control during gusty crosswind landings. The pilot will want to make many small inputs but the friction forces the pilot to push harder just to get the pedal to move, this increased force causes a slight overshoot (since static friction will be higher than dynamic friction), requiring a compensatory input in the opposite direction..and so on - this results in PIO (pilot induced oscillations) which is essentially a rudder doublet and therefore excites the dutch-roll mode...and we are back to the reason we added friction in the first place..make sense?
No, adding friction was never a consideration. Friction in a control system is not a good thing. It was brought up by another poster as a possible cause of the yaw instability.
So to summarize your inputs and those of another A.E. who posted: What we are experiencing is probably a divergent "dutch roll" mode with the dominant divergence in the vertical axis. However, further testing needs to be done to see if the roll mode is also divergent. In addition some testing on AOB stability is in order including inducing some aileron doublets to evaluate roll stability.
whew that hurt my head
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