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Bob, There are a couple of things going on at high AOA when the tail enters the wake. Unfortunately some of the component contributions are more easily isolated in the wind tunnel. The low energy air reduces the natural restoring force or the force to push the nose down. The elevator also sees this lower energy flow. Given a fixed elevator position this would result in less downforce as you describe. The low energy flow also reduces the hinge moment. If the pilot is holding a constant pressure on the stick, the elevator will actually deflect more. You can see an increase in elevator deflection per unit force in Figure 10. The curve steepens quite a bit. The springs add another interesting element. I would expect them to add some degree of stability.
I am not familiar with all the geometric differences between the 235 and 320 airframes, but it certainly sounds like the published aft CG limit has no margin on the 235. Thanks for sharing your observations. Chris Sent from my spiffy iPad
Chris:
First of all, thanks for an excellent writeup and a fine demonstration of scientific measurement-based engineering. You've done a great service to the LML community by your work and by reporting it so well.
I fly a small-tail 235/O-320. At the aft CG limit published by Lancair, static stability is just about gone. I flew there one time and choose not to raise the landing gear for fear of the CG moving a tiny bit further aft. Maintaining level flight was difficult. Dropping the flaps about 1/2 inch restored some pitch stability. Fortunately my passenger was able to move some gear from behind his head to the footwell. Without that, landing would have been challenging. As soon as I landed, I revised the CG limits in the operator's manual 0.5 inch forward of Lancair's suggested value.
This airframe is also known to oscillate when slipping with full rudder and full aileron. In a left slip, the nose describes clockwise circles with about 1-2 second period. Relaxing the slip just a bit stops the oscillation. That is probably a rudder blanking effect.
I cannot say that I have observed stick *force* reversal at high AoA, but rather stick position reversal. The trim springs may be masking a true force reversal at the elevator hinge. What I do know is that as nose comes up in a flare, the stick must move forward to stabilize at the higher AoA. Holding a fixed stick position in the flare would have the "nose pitching up on its own" as you say. I do not think that is a masking effect. If the stabilizer were moving down into the wing's low-energy wake, I would expect to have to pull more to continue raising the nose. The opposite is true, so either the stabilizer is normally in the wake and drops into higher energy air at high AoA, or some other effect is responsible. I have assumed that it is an increase in the wing's pitching moment at high AoA, but haven't measured it.
-bob mackey
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Scott, Our little Lancairs with a wing aspect ratio around 6.6 are definitely in the Hawk category - so you do have an appropriate call-sign.
Horizontal stabilizers have a few other issues. They live in a terrible place aerodynamically speaking. First there is the intersection and proximity to the fuselage, then the downwash off the wing that diminishes their effectiveness. Then, at high angles of attack, they can enter the wake of the wing. This low energy air further diminishes their effectiveness and hinge moments start to fall off. It is this region that is of most interest. This is where all the anecdotal reports of stick force reversals in the flare get started. The stories about the nose pitching up on its own, told at the Lancair fly-in after folks arrive with the plane packed full of people and baggage .
I think the most important chart in all of the reports is Figure 12 in the stability and control write-up. It shows a well behaved increase in AoA with stick force throughout, even with an aft CG. Chris
Chris Zavatson N91CZ 360std
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