F-15 rudder movement question

[David Hingtgen]

While playing Ace Combat 6, I noticed that they modeled the rudders moving asymmetrically. If you yaw left, the left rudder deflects more than the right. The "outside of the turn" rudder always moves less than the "inside" one. Is this correct? (can't imagine they'd do it for no reason). If so, is this an F-15 thing, or twin-tailed jet thing? As the airspeed increases, the difference in the rudder movement decreases (in addition to the overall decrease in movement).

As the developers seem to have almost unlimited access to the JASDF's planes, the F-15 is always among the best-modeled planes in the series. (Though they don't have the rudder auto-deflect when you fire the gun)

Of course, there's lots of errors with other planes, but based on how subtle a thing this is, plus it being a plane they usually have lots of access to, I wondered.

Edited November 20, 2007 by David Hingtgen

[Ol Crew Dog]

With CAS the rudders as with all flight controls on the F-15 deflect to augment the others, so in roll they might deflect diferentially. As the airspeed increases rudder deflection decreases. With CAS on, flight controls are controled by a computer with inputs from AOA, Airspeed and stick deflection, its allot more complicated but you get the jist.

Cheers

Dave

[MattC]

My assumption would be something to do with adverse yaw. If you are trying to create a yaw movement, you want to keep the most drag on the "inside" of the turn. By letting the rudders move differentially, it is possible to minimise that effect and thereby increase the resultant yaw effect. Otherwise there would be a more significant "braking" effect.

It would also make sense from a flow turbulence perspective. If you look at the rear view of an F-15, efficiency dictates that the air flow between the vertical stabilisers should be as clean as possible, so any intrusion into that "box" between the fins is pretty undesirable.

In fact, I can't really see why the rudders cannot be set with 100% differential so that only the "inside" rudder moves at all

[Ol Crew Dog]

Not making any assumptions, the rudders are controled through CAS (Control Augmented System) when it is turned on. All flight controls are put in motion in flight to control all axis of the aircraft with CAS on through the PRCA and ARI. The ARI (Aileron Rudder Interconnect) uses the rudders to control the aircraft to augment the Stabs and Ailerons. The Rudders will decrease deflection as the airspeed increases in level flight but in turning the rudders will augment other flight controls to make the turn crisper without the yaw effect of normal aircraft.

Any more questions feel free to ask....

Dave

[MattC]

The Rudders will decrease deflection as the airspeed increases in level flight but in turning the rudders will augment other flight controls to make the turn crisper without the yaw effect of normal aircraft

I presume that system is switched off during combat manoevers? I can imagine that it would be more useful to be able to operate flight control surfaces independantly in that scenario? Example, it wouldn't be possible to hold a knife edge and maintain altitude because of the need for "top" rudder to be held in, despite varying amounts of aileron and elevator to maintain attitude and heading?

I also assume that in the rudder is always a slave function, with aileron and elevator as the primaries?

Couple of questions

Is aileron differential variable or is at a preset value?

Does the flight control system have the sophistication to "dial out" yaw/roll couple, throttle/pitch couple and the like so that the controls are pure? (ie, a bootful of rudder only gives a yaw response, not an associated roll or pitch, increasing throttle only increases speed, not pitch)

I use a system like that on RC aircraft where each channel is programmed in with each other channel, so that, for example, if I drop the flaps, I get a preset amount of elevator compensation so there is no pitch change and so that if I stick in some rudder, I get a slight amount of elevator and aileron compensation from the programme to keep the wings level (for aerobatics)

[Ol Crew Dog]

CAS is on during any manuevers, from start up to land unless there is a malfunction, all inputs go through an air data computer, which inturn sends a signal to the PRCA, which controls all the manual inputs to the flight control hydraulic actuator.

Depending on the aircraft attitude is what determines deflection of any flight control to augment the others that will loose the authourity in that certain flight attitude or wash out.

As the airspeed increases any slight deflection of the flight control will be magnified, thus as the airspeed increases, deflection decreases.

All flight controls can have a pure response from the stick or the pedals thus the ARI.

There is also on the stick what is called the stick force sensor, so instead of actually moving the stick you are just moving the grip, a minor input will produce a greater reaction at higher speeds with CAS on.

cheers

Dave

Retired F-15 Crew Chief/Engines/Hydraulics/ Crash recovery (flight controls)