What do ailerons do and is it normal for them to go up and down if controls are moved when the aircraft is stationary? I use FS2002 and moving the joystick left or right causes ailerons to go up or down on the wings....

From : Don Stackhouse

Yes, they should move whenever the stick is moved. There is a direct mechanical linkage from an R/c model's aileron servo to the ailerons, or from the control stick to the ailerons in the vast majority of full-scale aircraft, just as there is between the elevator servo (models) or the control stick (full scale) and the elevator, and the rudder servo or rudder pedals and the rudder. If they don't move when you move the stick, it means something is broken. In fact checking for that movement is usually the very first item in the pre-flight inspection and pre-takeoff check.

I recommend going to your local library and checking out a book on how to fly full-scale aircraft. A particularly good one is "The Student Pilot's Flight Manual" by William K. Kershner, pub. by the Iowa State University Press, ISBN no. 0-8138-1610-6. This book explains how the controls on a full-scale aircraft operate and how the pilot uses them. The theory and techniques involved also apply to model aircraft, with few exceptions.

To answer your question, first we have to back up a little and discuss the relationship between the wing's lift and the rest of the airplane, and the airplane's behavior before, during and after a turn.

The wing makes lift. This lift is a force perpendicular to the direction of flight when you look at the plane from the side, and perpendicular to the wing when viewing the plane from behind (or from in front). If the wing has dihedral (i.e.: it's bent up in the middle) or anhedral (i.e.: bent down in the middle), draw an imaginary straight line from one wingtip to the other. The average of the lift will be perpendicular to that line.

If the wings are level, parallel to the horizon, and the airplane is in upright level flight, the lift will be pointed straight up.

Now let's say we want to turn around. If we were on the ground, we could push on a rudder pedal and steer the airplane that way, using the steering effects of the rudder and/or nosewheel or tailwheel plus the traction of the wheels against the ground to turn the plane. If you try that in flight, you will probably notice that it doesn't work, or at least not very well. Notice when you try to turn in a car, especially a tight turn at a fairly high speed, there's a force trying to pull you sideways across the seat, away from the direction of turn? That force is commonly (although somewhat incorrectly) called "centrifugal force", and anything trying to turn while moving will feel it, including airplanes.

We need some force to counteract this centrifugal force. If we tried to keep the wings level and turn just using the rudder, the plane's fuselage would act like a very poor wing and generate enough sideways lift to eventually pull the airplane around to a different heading. It would be an extremely draggy, slow and inefficient way to steer the airplane. Fuselages are designed to enclose and streamline the internal components and to hold the tail in the correct position relative to the wing. They are not designed to make lift, either upwards or sideways.

The best lift-producer on the entire airplane is the wing. Generally speaking, the other components should act in a supporting role and let the wing do the job of making whatever lift is required. This includes sideways lift to counteract centrifugal force in a turn. We do this by tilting the wing to one side ("banking" the plane), so that some of the wing's lift is directed sideways.

For a turn, we deflect one aileron up and the other down. This increases the lift on one side and decreases it on the other, causing the plane to start rolling towards the "up" aileron (actually it's more complex than that, but this explanation is adequate for this discussion). Note that "up" in this case means relative to the cockpit, not the ground, the ailerons roll the plane the same direction for the same control deflections regardless of whether the plane is right side up or upside down. The airplane will continue to roll to steeper and steeper bank angles as long as the ailerons are deflected. If you hold them deflected long enough, the plane will roll completely over inverted and back to right side up again. This is essentially how airshow pilots do slow rolls.

However, we are just interested in turning the plane in flight today, not in doing aerobatics, so we center the ailerons when the plane has rolled to a comfortable bank angle for our turn, maybe about a 30 degree bank angle. After we've reached the initial bank angle, we use small aileron inputs in either direction as as required to keep the bank at our desired bank angle.

As the airplane tilts further and further, more and more of the wing's lift is directed to one side. This starts to pull the airplane sideways and the airplane starts to turn, causing centrifugal force that balances the sideways lift. Since some of the lift is now being used to turn, there isn't as much holding the airplane up. We need to either speed up or pull the nose up a little to make enough additional total lift so that the portion that's still upwards is equal to the plane's weight. If the pilot does nothing about this, the plane will compensate by starting to descend, which causes the airspeed to increase and thereby increases the total lift. The other option is for the pilot to pull the nose up a little, increasing the wing's angle of attack and making more lift that way. The only down side of that approach is that there is only so much lift a wing can make, and if you increase the angle of attack too far, the wing will "stall". This term has nothing to do with the engine, it means that the airflow will stop following the wing's upper surface, but will go swirling off in big, turbulent eddies, and stop making lift. Because the wing has to make more lift in a turn than in level flight, the stall airspeed in a turn is higher than in level flight.

One other quirk of ailerons: a "down" aileron makes more drag than an "up" aileron. This causes what's referred to as "adverse yaw". If we try to roll the airplane to the left with just ailerons alone, the plane will roll to the left, but the nose will yaw to the right because of the extra drag of the "down" aileron on the right wing. We compensate by using just enough left rudder to counteract the adverse yaw, and keep the fuselage lined up with the airflow. On the ground we steer with the rudder, but in flight we steer with the ailerons and just use the rudder to compensate for adverse yaw.

So, let's recap so far: We used the ailerons to roll the airplane to a 30 bank angle (and rudder to compensate for adverse yaw), then centered the ailerons and rudder except for small corrections to keep the bank angle at 30 degrees. We pulled in a little "up" elevator to keep the airplane's altitude constant. The airplane is now in a nice, steady turn. The sideways portion of the lift is exactly equal to the centrifugal force, the upward portion of the wing's lift is exactly equal to the plane's weight. In the cockpit the pilot feels a little heavier, but because all the forces are in perfect balance it feels like the airplane is in level flight! Our sense of balance generated by the fluid sloshing around in the semicircular canals in our inner ears cannot tell the difference between centrifugal force and gravity, and therefore cannot tell the difference between level flight and a perfectly coordinated turn. BTW, this is why we have to use gyroscope instruments like the artificial horizon and the turn-and-bank indicator to fly inside of clouds.

So we're turning, turning, and now our plane is coming around to our desired new course heading. We now use opposite aileron (and the appropriate amount of rudder to compensate for adverse yaw) to roll the plane back to level flight. As the turn stops and the centrifugal force from it goes away, we return the elevator to neutral since we don't need to make extra lift anymore. Our plane is now back in level flight, but on its new course, and we've centered the ailerons and rudder except for small corrections as required to keep the wings level. We use small amounts of elevator trim and/or throttle to keep our airspeed and altitude constant.

So what about airplanes that have no ailerons, such as many R/C models? In those, the wing has enough "dihedral" (typically anywhere from about 8 to 15 degrees on each side for a "no ailerons" model, quite a bit more than in an aircraft that has ailerons) to eliminate the need for ailerons. When you apply rudder, the plane yaws to one side. Because of the dihedral, the wing that's yawed forwards sees a greater angle of attack (in other, grossly simplified terms, the airflow tends to catch more "under" that wing), while the wing that's yawed aft sees a lower angle of attack (the air tends to catch it more on top). This increases the lift on the yawed-forward wing and decreases the lift on the yawed-aft wing, creating the same effect as ailerons. It's not as coordinated as proper use of ailerons and rudder together, and the control response from this method tends to be slower and less crisp and precise, but the airplane's control system is dramatically simpler to build and operate. For planes where simplicity of construction and operation, and/or light weight are essential, this can be a good option.

Don Stackhouse
DJ Aerotech