Model Forum / General / Models / August 2005

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.

http://www.f15sim.com/operation/f15_hydro_mech.html
http://home.att.net/~jbaugher1/f15_2.html

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.

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
http://www.davidbevan.co.uk