In Depth : Trim Systems

This section explains how to fly using the new FS Force trim system, and how it is different from the old FSX trim system.  But first, a brief discussion of how trim works on a real airplane.  This discussion applies to planes with an elevator / trim tab combination.  Some planes have a stabilator, and they utilize a different trim system.  This discussion also does not apply to fly-by-wire aircraft.

Trim On A Real Airplane

An uninformed person might think that in order to fly an airplane at a constant pitch attitude, you simply position the yoke so that the elevator is perfectly aligned with the stabilizer, let go of the yoke, and the plane will continue along, just like a car.  But of course we know that is not true.  The amount of lift being generated by the wings varies with airspeed, (among other things), so if you just neutralize the elevator, the plane will likely start to climb or descend. At higher airspeeds, for example,  the airplane will likely climb if you let go of the yoke, so the pilot will have to push the yoke forward to keep it level.  This will require some force, since the elevator will be pushing into the flow of air moving past the stabilizer.  To relieve the pilot of the need to constantly push (or pull) on the flight yoke, a trim tab is placed on the trailing edge of the elevator in order to help keep the elevator at a given position.  The trim tab is much smaller than the elevator, but since it is placed at the trailing edge of the elevator it has more leverage and can therefore keep the elevator in a given position.

Below is an illustration of what you might see when an airplane is being operated at high cruising speed.  Some down elevator is being applied (the yoke is being pushed forward) to keep the airplane from climbing.  The trim tab has been adjusted to apply an opposing force to keep the elevator in that position, without the pilot having to apply any pressure.  With the airplane trimmed properly, the pilot can essentially now fly "hands-off".  

In this picture, the trim tab is in the up position, which is actually referred to as "nose-down" trim, because it causes the airplane to nose down.

What would all this feel like from the pilot's perspective?  Imagine a scenario where the airplane is being flown at regular cruise speed with the airplane perfectly trimmed.  The pilot is essentially flying hands off.  Now the pilot wants to increase speed, so the throttle setting is advanced and the airplane gradually speeds up.  As it does it will naturally begin to climb, so the pilot compensates by gradually pushing forward on the yoke to keep the nose down.  Once the desired airspeed has been reached, the pilot adjusts the throttle setting to maintain that airspeed.  But, the pilot is now having to constantly push on the yoke to keep the airplane in the proper pitch attitude.  So, the pilot puts in some "nose-down" trim, which causes the trim tab to move up, putting pressure on the elevator to move down.  In the cockpit, the pilot will notice that it now requires less pressure to hold the elevator in the same position.  The pilot will keep adjusting trim until NO pressure is required to keep the elevator in the desired position.  The airplane is now perfectly trimmed.

FSX TRIM SYSTEM

Now that we understand how a real trim system works, let's look at the FSX trim system.  (Everything in this discussion applies to earlier version of FS as well).  The key thing to understand is that the FSX trim system is designed for mechanical joysticks (non force feedback).  With a mechanical joystick the only place where you can experience no resistance (the feeling of being "in-trim") is at the physical center of the joystick motion.  But let's say you're flying along at cruise speed and you've got the stick pushed forward to keep from climbing.  Because it is a mechanical joystick, you're having to push against the springs, which simulates the feeling of being out of trim.  Now let's say you want to trim the aircraft.  Ideally we would like to be able to continue to hold the joystick in the same position and gradually apply nose down trim until we no longer have to exert any pressure, but that's impossible with a mechanical joystick, since the only way to "feel" in trim is to somehow get the joystick back to center.  But that is going to make the aircraft go into a climb!  So what Microsoft did was devise a very clever system such that when you make trim changes, the flight simulator is actually shortening or lengthening the "virtual linkage" between your joystick and the elevator, to allow you to slowly bring the joystick to center while maintaining your current pitch attitude (or at least, trying to!). In other words, when you apply nose-down trim, what's actually happening (in the flight model) is that the elevator is moving down, causing the aircraft to pitch down, and vice versa for nose-up trim.  So while you may visually see the trim tab moving in FS, what you're really doing, in terms of the flight dynamics, is moving the elevator.  There is an interesting side-effect to this : by holding the joystick steady and making trim changes (under the FSX system) you can actually climb and descend the aircraft!!  Of course you cannot do that in a real aircraft.  

FS FORCE TRIM SYSTEM

So how is the FS Force trim system different?  All kinds of interesting things are possible once you throw a force feedback joystick into the mix. First of all, the centering force is provided by servos, not springs.  Additionally, using DirectX, you can vary the strength of the centering force, as well as adjust the "center position" of the centering force (the place where the joystick wants to come to rest).  The center of the centering force (the place of no resistance) can be programmatically positioned anywhere along the 2 axis of the joystick.  That means you can experience the feeling of being in trim (no resistance) with the joystick pushed forward (for instance, at high speeds), or with it pulled back into your lap (at slow speeds), exactly like you would in a real plane.  But sadly Microsoft never bothered to modified the simulator to take advantage of this.  That is why FS Force intercepts your trim commands and prevents them from reaching FSX.  When you make trim changes, FS Force slowly moves the center position of the centering force until it coincides with the current physical position of the joystick.  The "sweet spot" comes to you, rather than you having to go find it.