Manipulating aileron linkages for perfect rolls?
 

Hi everybody! I was just reading about bipes, because I ordered a small 25" FF bipe kit to convert to r/c.

Now, let's say the engine crankshaft is right on the centerline between the wings, when looking from the side. If all the controls are set with pushrods and surfaces at right angles, then technically, perfect rolls should be acheived. Well, what if the engine is off center, like it is many times - could a linkage be changed up to accurately return the rolling to the centerline?

Bipe Flyer posted these drawings in another thread. Somebody noted the throw differences in the first drawing. This reminds me of how I use different linkage positions to get accurate Ackerman angles in r/c cars (I even set it up so that when the offroad truck banks to the side, the outer wheel turns into the turn more, and pushes harder- maybe it's not needed).

In the first drawing, when the bottom aileron goes down, the throw for the pushrod is increasing until it is 90 degrees with line created by the connection point and hinge point of the surface. Then it is decreasing, but by that time your surface is already at its max. While this throw is increasing, the throw of the top aileron is decreasing, and it's running out of room to pull with. This means that the bottom wing will push harder than the top wing. On the other side of the plane, it will be the same, except reversed- the top wing will push harder than the bottom wing. Could this linkage be modified to reflect an off-center engine?

In the second pic, the control surface/linkage connection point is in the same relative place on the surface, so throws will always be the same, and the plane might start doing a barrel roll... well, barrel roll when judging by the engine centerline.

I would like to create a system where this connection between engine crank centerline and control surface deflections is accurate, and can be accurately adjusted. It probably isn't worth it, but I want to make it perfect. Anybody done this before, or thought about it?

RE: Manipulating aileron linkages for perfect rolls?
 

I would say the relationship to the aircraft's longitudinal center of gravity is more of a concern than the relationship to the crankshaft. An object in free space will rotate around it's center of gravity (center of mass) in whatever axis you choose. To not do so requires an additional force. So for the aircraft to happily roll about the engine crankshaft line the crankshaft should be on the longitudinal CG line.

Trying to compensate with fancy aileron throw manipulation may result in rolls at one airspeed that are fairly axial, but to the best of my mental figgerin' this situation will deviate at lower or higher airspeeds. I think it is better to think CG line location.

Note to aero engineers - I am using the term longitudinal CG because that's all I can think of, it might have a different term associated with it, but hopefully what I mean is evident. The longitudinal line that passes through the center of mass.

MJD

RE: Manipulating aileron linkages for perfect rolls?
 

You might wish to also post this intriguing question in the Aerodynamics forum.

jess

RE: Manipulating aileron linkages for perfect rolls?
 

Rather than trying to create differential throws mechanically, it is easier to use 2 servos and adjust it with the radio. The other way is to offset the linkages on the servo(s). Typically you need more up than down to eliminate adverse yaw. A large factor affecting the throw of the upper aileron in the first picture is the distance between the wings themselves, which is something you wont be able to adjust. On the one plane I have with the setup in the first picture, a DP Ultimate 40, I cannot detect what effect the linkage has, if any. The wings seem to by far enough apart that the amount that the linkages is offset makes very little difference. All my other biplanes are setup like the second picture.

RE: Manipulating aileron linkages for perfect rolls?
 

BTW, here is a picture of the linkage on the DP Ultimate review here on RCU.

http://www.rcuniverse.com/magazine/a...?article_id=49

http://www.rcuniverse.com/magazine/r...e/IMG_1463.jpg

RE: Manipulating aileron linkages for perfect rolls?
 

A large part of achieving truly axial rolls is to ensure that the CG is located back very close to or right on the neutral point of the model and that the side area of the fuselage and fin is distributed with a 25-75% split on either side of the CG as well.

Much of what you are concerned with is related to the normal setting of the CG ahead of the NP in order to have some pitch stability. However that same stability results in a strong pitching towards the upper side of the model when it's not actually flying upright.

A secondary but still important point is to ensure that the major drag producing elements all have their centers of drag located so that the overall drag center of the model is as close to the true 3 dimensional CG (as opposed to the balance point) as possible.

Do all this and you end up with something that looks remarkably like a modern pattern model. Or in the case of a biplane much like Chip Hydes show bipe. From there it's just a matter of playing with the control surface throws to ensure you get even and linear throws just like you're doing.

RE: Manipulating aileron linkages for perfect rolls?
 

Dihedral makes rolls barrelly too.

RE: Manipulating aileron linkages for perfect rolls?
 

Too bad the micro servos that I play with don't come with circular output wheels to allow custom linkage work. I guess a hunk of 1/16" plywood can be screwed to the stock servo arm. More up than down has cured funky rolling on some of my planes.