Why seperate elevons from ailerons?

[FishInferno]

Title says it all. Why do most planes have separate elevons from ailerons? Why cant the ailerons control both pitch and roll? Probably a stupid question but I just wondered:P

[Dodgey]

I assume you mean elevators. If you look at an aircraft you will notice that the ailerons are typically at the outer edge of the wing. Same with the elevators they aer as far back as they can be. This is so they can achieve the greatest leverage around the COM/COL. The Concord is an exception to this.

[Starman4308]

Control surfaces have to be far from the CoM in the appropriate axis to be effective. Many ailerons, for example, are not very far from center-of-mass for pitch: trying to use them for pitch will mostly just cause greater drag, while providing very little pitch authority. Elevators aren't used for roll authority for the same reason: they're too close to centerline.

Now, if you had a long delta design, you could have elevons (elevator-ailerons), but with traditional wing and tail layouts, there's a good reason that ailerons and elevators are separate.

[softweir]

What they said, but in more detail: Delta-wing planes such as some fighters, Concord and the space shuttles HAVE had control surfaces with combined functions, known as elevons, so it can be done if the wing-plan is appropriate for it.

The reason they are usually separate is because their jobs are very different and so they are best placed at different points: ailerons need to be spaced fairly far apart and out from the CoM of the craft, while the elevators are better placed as far back as possible. In most conventional subsonic craft, the best place for the ailerons is on the main wing where they can be placed far apart, while the elevators are best placed on the tail where they are as far back as possible.

On delta-wing craft the shape of the wings means there is only one place to put control surfaces, and that is at the back of the wing - which is fine because they can be placed far apart enough to act as ailerons, and far back enough to act as elevators.

(Modern fighters are rarely these days built as simple delta-wing craft. This is because the simple delta-wing shape is too stable, and prefers to fly straight because the wings are so far back; what is wanted is a craft which is very nearly unstable, usually to the extent that the fly-by-wire avionics are fighting to keep it stable! This makes for a much more maneuverable and responsive fighter. As a result, the elevon model is becoming rarer.)

EDIT: I type so slow I got ninja-ed twice.

[Bill Phil]

Aren't elevons elevators on the wings rather than the tail plane? Yeah. Ailerons are placed for roll, elevons are placed for elevating.

[Leszek]

Elevators are pitch controls in the tail. Rudder is Yaw control in the tail. Ailerons are roll controls at the tips of the wings. Flaps are trailing edge wing adjustments for lift/drag. Slats are leading edge wing adjustments for lift/drag. Spoilers are top of wing controls for spoiling lift and adding drag. Airbrakes are for adding drag without spoiling lift.

Elevons are dual purpose elevators and ailerons. Sometimes there are also ailerons and elevons with the elevons being the whole horizontal tail plain like in some fighters.

Other possibilities for mixing control surfaces exist such as flaperons. (Flap/Ailerons) Some planes use spoilers for roll control instead or in concert with ailerons.

It all depends on what you need the craft to do and with what restraints.

Edited December 1, 2014 by Leszek
Language edit

[King Arthur]

Aren't elevons elevators on the wings rather than the tail plane? Yeah. Ailerons are placed for roll, elevons are placed for elevating.

Elevator + Aileron == Elevon. A elevon is a flight surface that functions as both an elevator and an aileron.

[Cdub-178]

The elevators push the tail down, forcing the nose up. Using the ailerons as elevators would essentially do what flaps do. Increase drag and increase lift. If you do that high up you'd probably stall

[Snark]

Hello, and welcome to the forums! 

5 hours ago, Cdub-178 said:

The elevators push the tail down, forcing the nose up. Using the ailerons as elevators would essentially do what flaps do. Increase drag and increase lift. If you do that high up you'd probably stall

That's as may be... but the person you're answering wrote that question nearly five years ago, and presumably has long since found an answer and moved on.  Any further discussion at this point would be moot.

Accordingly, locking the thread to prevent further confusion.  If anyone has further questions on this topic, feel free to spin up a new thread.  Thanks!