Ailerons

[CTho9305]

In a design with swept wings, would ailerons normally rotate about an axis parallel to the trailing edge of the wing, or perpendicular to the direction of flight?

[stevem1928]

I tried to look this up in Andy Lennon's book, Basics of Model Aricraft Design. He does not seem to address it. However, from what I have observed. It appear that they rotate along the axis parallel to the TE.

Hopefully someone can give you a definate answer.

[davidfee]

For a "toy airplane" it probably doesn't matter all that much. almost every balsa kit ever made uses constant cord ailerons, with a hingeline parallel to the trailing edge. This is simple to do with manufactured trailing edge or aileron stock.

Molded planes and other "high performance" models more often use an aileron which is a certain percentage of the wing cord. So, for a tapered wing the aileron gets narrower as you move out towards the tip.

I do not believe I have read anything regarding the hingeline being (or not being) perpendicular to the airflow/direction of flight... but there is often spanwise airflow anyway, so this may be really hard to quantify.

have fun,
David

[Sparky Paul]

There's two reasons the aileron hinge line follows (more or less) the trailing edge.
Structurally, it's easier/cheaper/lighter to do it that way.
A hinge line perpendicular to the airflow on a swept wing would result in an aileron shape that gets larger further out the span. This adds structure where you don't want it.

[CTho9305]

So, do ailerons create any significant sideways forces when raised/lowered since they do not just push air straight up and down?

[Sparky Paul]

There is a lateral effect from deflected ailerons.
Generally the downgoing surface creates a certain amount of drag, which acts at the center of the surface's area, which of course has a moment arm relative to the plane's centerline.
The upgoing side OTOH tends to reduce drag (relatively), which adds to the unbalance from the other side.
This summed unbalance is termed adverse yaw, and is countered with differential aileron travel to try to balance the drag effect of the surfaces.
A scale Piper Cub for instance will exhibit a severe amount of adverse yaw, requiring the use of the rudder to help point the nose in the direction of the turn. Just like the real one does.
Another extreme example is the use of flaperons. When these are fully deflected as flaps, when a turn is attempted, the aileron which should be going down is already at its maximum deflection, and doesn't move, while the aileron on the upgoing side can move up. This reduces the drag on that side, and the airplane turns/yaws to the down side, which is opposite to the input command. A very disconcerting feeling when you first experience it, let me tell you!

[BMatthews]

Sparky, I think he was asking more about any sideways reaction due to the surfaces being at the sweep angle as opposed to adverse yawing.

In this case no there is no sideways effect. If you think about it both surfaces being deflected are pushing the air outwards slightly in addition to the normal control surface flow. Since they both are pushing outwards the net result is nothing. But if the aircraft was using differential where (usually) the upward moving aileron moves more than the downward moving one to help counter the adverse yaw then I suppose there would be a slight sideways force away from the upward moving side. But it would be pretty small.