WhackityWhack wrote: ↑
Sun May 13, 2018 7:02 am
Ok, 1st, a disclaimer: I grew up in the Show-me-state so I don't automatically believe everything I'm told (or shown). On several occasions, I've heard several very experienced pilots refer to a glider's "pitch curve". They are usually talking about the characteristics of a certain glider's design. I readily admit to them that I have no experience or knowledge of this aspect of HG aerodynamics so I have no idea what they are talking about. In other words, "save your breath until I figure out what you're talking about".
Now, I am familiar with Polar Curves and L/D curves and so forth. So, is a "Pitch Curve" a real thing or is it just pilot-speak
Every airfoil has a pitch-curve. You can look them up on various aeronautical websites, but maybe you should use a wind tunnel to conduct your own evaluations, with pitch curves from your own tests. In general, the pitch curve of any airfoil would give an aircraft designer some idea of how large the tailplanes should be, or how far aft, on the proposed aircraft.
If you put a hang glider on a pitch-testing truck (or a wind tunnel, ideally), you can measure the force it takes to hold the glider at any certain Angle of Attack (with airspeed held constant). Plot the various AoAs on a graph, and the force needed at the control bar to hold it there, and you will have a pitch curve for that glider. It may be somewhat different, at various airspeeds (especially if the airframe deforms under load). The glider should pull up (recover from a dive) at any normal AoA. That would show as a positive pitch curve value. As the AoA goes negative, the glider should continue to pull up (still a positive value on the pitch curve). At some point, though, when the pitch angle (AoA) goes too far negative, the pitch force values will go negative, and the glider would dive itself, or even "tuck" violently in that case, if flying. The longer the pitch curve has positive values, as the AoA decreases, the better the glider will resist "tucking" in flight, and the more effort (from the pilot) will be needed to fly fast.
With some early HGs, the pitch curve was neither positive or negative (e.g. Cirrus-III, and the Standard Rogallo). Their pitch curves were just zero, at any normal AoA. The glider did nothing to pull out of a dive, from about 30 degrees nose up to 30 degrees nose down, on the test truck. Some early HGs on the test truck would rip the tail out of the tester's hands and tuck, even when at a positive AoA. These dangerous early hang gliders put the words "tuck" and "divergent" into our HG vocabulary.
Feel free to doubt anything said here, and verify those things for yourself.