How do model flying wings w/o vertical fins turn?

I know real flying wings like the B2 turn by using split flaps on one side to make more drag there, but do models use that same configuration? If not, how do they do it?

From : Don Stackhouse

Mike, what you're referring to is a Northrop-style drag rudder. The Northrop flying wings (and some others) use a split drag brake on the trailing edge on each wingtip to control yaw by creating extra drag on one wingtip or the other. Other flying wings have used other schemes, but usually it is some form of differential drag producing device. One exception is on the B2. The control surfaces in the "W" tail in the center can be deflected in opposite directions so that they cancel each other in the pitch sense, but because of their sweep they collectively create a sideways force that acts like a rudder. It's similar to the way a V-tail works, except in this case it's because of sweep of the hinge lines, not tail dihedral.

That said, this is NOT how a flying wing turns, drag rudders or not. In fact, with very few exceptions, aircraft do not turn as a direct result of rudder, even on 2-channel rudder/elevator models. Rudder controls yaw, but an airplane almost always turns as a result of BANK ANGLE, otherwise known as "roll". If the aircraft has ailerons, they are deflected down on one side and up on the other, causing the aircraft to roll towards the up aileron. This tilts the lift force of the wing (which is normally perpendicular to the wing) to one side, so that some of the wing's lift is pulling the aircraft around in a turn. The turning causes centrifugal force on the aircraft away from the direction of the turn, which exactly balances the sideways portion of the wing lift. Meanwhile, the upward portion of the wing's lift balances the downward force of the plane's weight. The ailerons are returned to approximately neutral once the plane has rolled to the desired bank angle, and because the various forces are all in perfect balance, the aircraft "thinks" it's in level flight. It continues turning until you give it an aileron input the other way to roll the wings level again. It is NOT at all like steering a car!

Flying wings usually have "elevons", which are a combination of elevator and ailerons. They can move up or down together to act as elevators, and they can move in opposite directions to act as ailerons. Control mixers, either mechanical or electronic, combine the elevator and aileron commands from the control stick to get just the right movement on each elevon for the combined control functions, just like the way rudder and elevator are mixed for a V-tail's ruddervators.

On aircraft that do not have ailerons, the rudder is normally used to indirectly control roll. These aircraft normally have lots of dihedral. When the rudder is deflected, it causes the plane to yaw to one side. Because of dihedral, the angle of attack increases on one wingtip and decreases on the other, causing a difference in lift between the two just like what ailerons would create. The model begins to roll, and continues to roll in that direction as long as the rudder keeps it yawed, just as if it had ailerons.

There are a few aircraft with either side-lifting airfoils, or else a fuselage with gobs of lateral area, that can generate enough sideways lift from these to do a turn without banking first. Generally though, a wing is a much more efficient lift producer than a fuselage, so it's much more efficient to generate the sideways lift required to turn the aircraft by banking the wing. If fuselages were a better way to make lift, all aircraft would look and fly like a flounder. I guess that's also why most of the times I've seen somebody try to turn without banking first, the aircraft just seems to flounder!

Don Stackhouse
DJ Aerotech