Knife edge pitch trim ?

[Dean Pappas]

Hi Paul,
The short answer is no. That wasn't very helpful, now was it!
Your conjecture about the positive incidence requiring some down trim is good. The trim change is probably not the result of rudder displacement making the elevatore more or less effective.

The difference between level flight and a vertical downline is that when the wing is "loaded" the downwash that leaves the wing and strikes the stab, in effect, produces a small bit of up trim. When the wing is unloaded, that up trim effect goes away. In the upline, the effect of the thrustline (usually above both the center of gravity and the center of drag) comes into play. In the knife edge, the yaw of the plane produces small pitching effects from different sources ranging from the size of the plane's chin, how big and how far back the canopy is, to the distribution of the fin and rudder area and to the effect of wing dihedral. That's why, in my opinion, the usefulness of a trimming chart is sometimes limited. I have seen a dihedral change fix a knife pitching problem, and a canopy placement change create one. Stab versus elevator height has a very big effect on how much the downwash induced trim adds into the mix, and a simple CG change has dramatic effects, especially if the plane is borderline tail-heavy.

The result is that you start with the trim chart (it usually works) and then experiment. The state of "proper Pattern trim" is actually a fairly tricky thing.

Sorry but there is no "one size fits all" answer!
Regards,
	Dean P.

 

-----Original Message-----
From: discussion-request at nsrca.org
[mailto:discussion-request at nsrca.org]On Behalf Of Paul Horan
Sent: Saturday, June 26, 2004 7:51 PM
To: discussion at nsrca.org
Subject: Knife edge pitch trim ?


    The NSRCA site's trim states that during knife edge if the model pitches to the canopy wing then increase wing incidence.
    I assume what they are trying to accomplish is by increasing wing incidence the elevator will carry more down.
    OK, how does elevator position tie in with rudder ?  Does rudder deflection cause the elevator to become more effective - thus
the pitch coupling ?

Thanks,
Paul

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