Hello all
A group of us were wondering about why aircraft pull to canopy or wheels while in knife edge flight. Can't quite figure out what the dynamic forces are that cause the long axis pitch changes when the rudder is deflected. I'm sure it's not too complicated, but would appreciate insight from anyone who knows.
Thanks
RJM

Actually...........it's pretty complicated.

1) When the rudder deflects, the air tries to flow spanwise from the center, just like a very low aspect wing. The elevator acts as a fence, and recieves a bit of lift off the spanwise flow. In lay terms, a low placement of the elevator in relation to the vertical tail will cause a pitch to the canopy....A high elevator will cause a pitch to the gear, and there is a neutral location if you have the patience to find it.

2) Empenage shape affects the flow of air and can produce lift that causes a pitching moment. A curved upper turtledeck will create lift in a slip, pitching the airplane to the gear. If the top of the empenage is flat with sharp edges, it will tend to spoil lift in a slip, causing a pitch to the canopy (maybe). The roundness of the lower empenage will have the same effects, with opposite pitching tendencies. Cowl shape can have similar effects, but to a slightly degree, since the moment arm is shorter to the aircraft CG.

3) CG makes a difference. A forward CG will cause the plane to pitch to the canopy in knife edge. Think of it this way; when the plane is nose heavy, you have to trim up elevator to fly level in upright flight. Then when you roll into knife edge, the up trim pulls the nose to the canopy. A tail heavy plane works the opposite, and will tend to pitch to the gear in knife edge. There is a neutral point, but it may be outside the usable CG envelope for stable flight.

4) Wash from the wing. Depending on the placement of the tail and wing, the airflow from the wing may wash over the horizontal stabilizer, changing it's angle of attack. In knife edge, the wing is not producing lift, so the airflow over the tail will no longer be affected by wing wash.........that is if it was to begin with, and with some planes it is not.

5) Prop wash. Here is a topic for the aero guys. You have up or down thrust that can influence the horizontal stab. You also have the spiral airflow induced by the rotating prop that can influence the horizontal and vertical stab. Non of this is predictable...........by us anyway.

6) Airfoil shape. An asymetric wing airfoil will always tend to pitch to the canopy in knife edge. Hopefully your aerobatic plane has a symetric airfoil........so you can at least disregard this factor.

Totally confused?? There are more factors, but I think this is enough to throw out.

Luckily we don't need a working knowledge of all this to trim our planes. Try to adjust the pitching tendency by changing the CG. When you can't move the CG any more..........or don't want to for other reasons, then mix the pitch out. If it takes more than about 10% mix to fix it, then you can relocate the horizontal tail up or down to fix it (hard way) or slightly alter your wing incidence (easy way).

regards.........CJ

Oops..........I'm lost in the forum..........again

While we are talking about "knife edge" mixing, my question is should you mix the rudder so that you have no coupling in knife edge or should you mix the rudder so there is no coupling in straight flight?

Thanks

Rob

Hey Rob,
Trim it for knife edge. That will be the most useful for pattern flying.........four points rolls, eight points rolls, rolling circles, etc. The only use you would have for trimming in level flight would be crosswind landings.......if you use the wing-low technique (dip the wings toward the wind with aileron, and counter the aileron with opposite rudder). Personally I land in a crab. Not pretty, but functional.

CJ

CJ,

That was the best explanation of pitch coupling issues that I've ever read in one place.
You mentioned:


If it takes more than about 10% mix to fix it, then you can relocate the horizontal tail up or down to fix it (hard way) or slightly alter your wing incidence (easy way).


Could you explain how wing incidence affects pitch coupling, and in what directions (increasing wing incidence causes pitch in what direction)?

Thanks
--Derek

If you change the angle of the wing, it will affect the downwash over the horizontal tail. This, in turn, will require you to adjust the trim for the plane to hold level flight. Now, when you roll into knife edge, the wing is no longer producing lift, so the downwash is reduced.........and the trim you added will adjust the pitch tendency.

The amount of effect the wing incidence change will have depends on the airplane design. On a high T-tail, it will likely have no effect at all. On a design with the tail lower than the wing (like a bipe), it will probably have a large effect. Most other designs will be somewhere in the middle. It is NOT the most efficient way to adjust pitch coupling, but it is relatively easy (compared to moving the tail!) and usually works.

If a 1 or 2 degree change in wing incidence doesn't help, it would be best to use another method to fix it. This is because - as with all things - any change you make effects other flying characteristics.

Have fun, CJ

Thanks for comments. Very informative. I thought of some of the points raised, but not the span wise flow, and not the really neat thoughts about the shape of the top and bottom of the fuse and cowl. I am still wondering about CG and incidence issues and for that matter - prop effects because I have noticed that pure rudder inputs cause the same (more or less) pitching moments even when the plane is straight and level and even when the power is at idle (level or knife edge).
Thanks for the info!
RJM

Derek, not sure if this thread is active still, but I have just completed a series of experiments where I changed rudder shapes dramatically, wing incidence about 2 degrees from neg to positive, and cg by about 1", aft to fore. Also played around with dorsal fin shapes, added to the top of the fuse, from the base of the fin forward. I was able to significantly change the pitch mix to belly on my pattern design from around 16% down to 4%.

One main thing is do not change the location of the horizontal stab, since it will result in virtually no change to pitch mix, and its alot of work for zero gain. If you want details contact me at [email protected].

MattK

I agree with all but the last sentence. I have always been able to obtain huge gains in correcting a models pitch in knife edge by raising or lowering the stab. On the jekyll by moving the stab down an inch we were able to go from 30% up elev with rudd to about 8%. What I have never gotten to work was to change incidense to trim for knife edge, without really messing up everything else.

With the plane in a stand with the fuse at the angle you want it to fly at your wing should never be much more then 1 degree positive with the stab 0 to that. Then after a little trimming, (Level Flight, and Correct CG) you should be able to check knofe edge accuartely. Make sure to adjust the stab to get the elevators level after checking your CG. Most airplanes will go slightly to the belly at this time. I have adjusted this out with stab position, but have never like the feel of the elevator thru square corners. Maybe because the wing wash is doing something to the stab when its that low. Regardless I leave the planes requiring around 5% up with rudd. and I never know its there.

Thats just been my experience I am sure there are other ways but I know this works.

Chip

One main thing is do not change the location of the horizontal stab, since it will result in virtually no change to pitch mix, and its alot of work for zero gain. If you want details contact me at [email protected].

MattK
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Maybe ANHEDRAL in the stab can help??? (Who remembers Prettner!?!? LOL)

Principal behind Anhedral on Mr. Prettner Curare was that at least some part of elevator was in clear, non turbulent air behind the wing in differing Angle of Attacks. Might have something to do with other flying gharacters too on recent manouvers. Mr. Akiba flew one in WC.

Rudder hinge line tilt can be used to reduce the elevator mixing. Difficult with ready built model but if you know that certain design needs up trim, you should build rudder hinge tilted back on top and forward in case of down trim need.