Jim Wilson wrote:
Excellent, thanks for the clarification. Just looking at the cub you can see down-wash is a major design feature. The DC-3 has a high tail, but I can see the incidence in the main wing is pretty high. I wonder what happens when you increase the wing incidence and set the horizontal stabilizer to 0 or whatever it is supposed to be.
I'm getting seriously out of my depth here, since I didn't even take high school physics, but as far as I understand the most important part of lift is the suction created by the partial vacuum *above* the wings -- that means that wings are pulling air down more than pushing it down, effectively, and the hstab will be in downwash even if it is level with or slightly above the wings. Only a very high hstab, like the one on a t-tail, will be clear of it.
Now Jon, Tony, or Andy can step in and explain how I've totally misunderstood the aerodynamics.
For some reason the designers seem to be going the opposite direction (positive tail incidence). I'd like to understand the reason in order to make a decision on the p51d fdm config. It did seem to handle better with the negative incidence number. But down-wash certainly isn't present on the hstab and the diagram appears to show positive incidence.
It's very hard to see this with the naked eye because you cannot tell the angle of the downwash. I imagine that downwash would be more dramatic in a plane with higher wing loading, so the hstab could have a less negative (or more positive) incidence angle and still have effectively a lower angle of attack.
The other related problem I'm not sure of is that with or without reducing the
incidence value, I can't seem to take off in a moderate cross-wind (> 12kts). The tail always blows around and the rudder/brakes can't stop it. Does
anyone know if this is normal behavior?
The vstab is probably too effective at low speeds, and I have no real-life taildragger experience, but a 12 knot crosswind component is not small -- the Cessna 172, for example, has a maximum demonstrated crosswind component for landing of only about 15 kt, if I recall correctly.
Can you lock the tailwheel so that it doesn't castor? If so, try to keep backpressure on it to hold the plane straight. You might also want to shift the CG back a bit so that the tailwheel doesn't go too light.
All the best,
David
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