Flaperons on an F15?

Hi, David.
The F-15 uses a tailplane. When using a tailplane (empennage), the balance point of the wing is much further back from the leading edge than on a full delta airplane (no empennage). This causes the control surfaces at the trailing edge of the wing to provide a different kind of function (fulcrum) than one would imagine.
Even though they look similar, they behave differently.
Ed Cregger

The F-15 control surfaces are ailerons, rudders and stabilators. Stick and pedal inputs are mixed in the hydromechanical flight control system so that all surfaces, ailerons, stabilators and rudders, deflect appropriately when roll is commanded. Pitch control is by stabilators and yaw by rudders.

Sir, Length of the wing's chord and the position of the wing are not relevant. The wing of statically stable aircraft produces in addition to lift and drag aerodynamic torque tendig to decrease pitch, that torque is countered by force generated on the horizontal tail. When both flaperons as deflected down they will act as plain flaps and will cause a nose down pitch that will have to be countered by changing the incidence of the horizontal tail. If I understood correctly the comands on Your model have been coupled i.e. aileron and tailplanes both deflect differentially for roll control and when tail is used for pitch control flaps are lowered or deflected upward. Coupling of ailerons and tailplane should improve rate of roll and differential tailplane is used on actual F-15 to improve roll control at high AoA. However coupling of flaperons and tail (tail down and flaperons up for pitch up) will decrease wing's lift and increase drag significantly and could cause a stall. In the other case (tail up and flaperons down for pitch down) high negative loads are possible because the wing torque may not be enough to counter the pitch down that will keep increasing as long as the comand is held. I think that such an arrangement will decrease controllability of the model. The other possibility (flaperons down and tail down for pitch up) will have an adverse effect on the rate of pitch, the torque produced by the tail will be reduced by that from the flaperons in addition flaperons will increase the drag significantly, reduce speed and thus impare the rate of pitch even further. This will cause the model to act more like an F-5 (sluggish in pitch, excellent roll). Flaps are used only when extra lift is needed at a cost of increased thrust needed e.g. reducing the landing speed and boosting maneuverability at lower speeds. If possible flight test and please post results.
Nemanja Vukicevic aeronautical engineering student

The term elevon is only really appropriate to tailless delta's, where the trailing edge controls must combine both elevator and aileron functions. On an aircraft with all-moving dual-purpose tailplanes, they are usually called stabilators although tailerons has also been used.
Flaperons are ailerons that can also droop together as part of the flap function, often used in conjunction with leading-edge droops to provide the effect of variable camber. Flaperons have nothing to do with pitch control. Deflecting both ailerons in the same direction on an F-15 wing, being close to the center of pressure, will probably cause an increase or decrease in lift and a direct translational force up or down, rather than a change in pitch.
The F-15 is a tailed delta, with a large wing and low wing loading leading to high maneuverability. The wing employs simple flaps inboard of plain ailerons. There are not even any leading-edge droops or slats, the wing design avoiding these complications.

Definitions vary. The roll control spoilers on the A-6 Intruder and EA-6B Prowler are also called Flaperons; no flap function, no droop or downward deflection...

Which is why we only did flaperon rolls, were the jet rolls around a wing tip instead of aileron rolls vice the centerline for those that care.
Pugs

Is a flaperon roll the same as a barrel roll?

As I understand it the axis of a barrel roll lies parallel and "above" the centreline of the aircraft. The plane of the wings is tangential to the "circle" of the roll.
I've never seen one, but it seems to me that a flaperon roll would have the plane of the wings rotating radially around the roll axis - which is located on a wingtip.
Corrections and clarifications welcome
Roger

OK, thanks everyone for your answers and opinions, I test flew the model the other day and as I expected the additional lift generated by the flaperons going down was outweighed by the nose down pitch that having the flaperons so far from the CofG caused and full up elevator (down flaperon) resulted in a nose down pitch.
Luckly I was able to switch the flaperon mixing on/off in flight and so land the model safely.
Thanks
David Bevan