Form and Function

Remember all those times that you carefully trimmed the airplane for hands-free flight at a certain airspeed, and a couple of minutes later it wasn't in trim anymore? Your instructor kept reminding you to trim the airplane. You kept thinking, "I just did. What am I doing wrong?" Maybe your instructor even took control of the airplane and trimmed it for you. As he slowly released the yoke, he looked at you from the right-seat pulpit. "See?" he said.

Yeah, you "saw," but it didn't look any different from the way you did it. So you took control of the airplane to continue with your lesson, and a few minutes later it was out of trim again. Apparently, you just were not the trimmer your instructor was. Whether your instructor blamed you or not, you probably felt that the trimming problem was your fault. Stop beating yourself up! The trim problem may not be a problem at all. The change may have been the result of your airplane's trim-speed band-the range of speeds at which an airplane will fly with the same trim setting.

The trim/airspeed relationship
Most of us were taught to stabilize the airplane in a certain condition-let's say 100 knots in straight-and-level flight-then trim the elevator until the pull or push force on the yoke disappeared. The yoke doesn't move as we trim out the yoke force. By holding the pressure on the yoke while we trim, we are also holding the elevator or stabilator at some deflection angle. It is this angle of deflection that determines the airplane's angle of attack (AOA).

AOA is the angle between the relative wind and the wing chord line. Holding the yoke further aft deflects the elevator more trailing-edge up, which causes the airplane to fly at a higher AOA. If the power setting is not changed, flying at a higher AOA results in a slower airspeed. Whether the airplane climbs or descends at this slower airspeed depends on the power setting, but the point is that the elevator deflection angle ultimately determines the airplane's airspeed. In shin-bone-to-knee-bone fashion, we have the pilot holding the yoke at some displaced position which maintains the elevator at some deflection angle which establishes a specific AOA between the relative wind and the wing chord line. The result is a particular airspeed.

If the deflection of the elevator changes, the result is a different airspeed. When you initially trim the airplane, it maintains a certain airspeed. If it no longer maintains that airspeed a few minutes later-assuming that you haven't retrimmed, changed power settings, or redistributed weight, then chances are that the elevator deflection has changed. The question, then, is how and why does this happen?

Friction
Suppose there's friction in the elevator control system. Friction is a force that must be overcome, along with the air load on the elevator, to move the control system. When you displace the yoke and change the elevator deflection, you are applying enough force to overcome the combination of friction and air load on the elevator. When you relax the force on the yoke and the air load on the deflected elevator is high enough, the air load overcomes the friction and the elevator deflection decreases until the force from the air load equals the friction force.

As an example, let's say that you trim the airplane for hands-free flight and then perform a 30-degree bank turn. During the turn, you displace the yoke aft a little to maintain altitude. As you roll out, you gradually relax your pull on the yoke until you are once again able to fly hands free. If there's enough friction in the system, the elevator and yoke may not return all the way to where they were before the turn. The result is slightly more trailing-edge up elevator deflection and a slower airspeed than you trimmed for before the turn.

At this point you might conclude that you never really had the airplane trimmed properly in the first place, but you probably did. It's friction that prevents the complete return of the elevator to its previously trimmed position. Try nudging the yoke forward a little. If the airplane reacts by eventually settling at a slightly faster airspeed when in hands-free level flight, then you can conclude that there is a friction band in the elevator control system that is affecting the trim speed.

So, how do you account for this friction band when trying to maintain a certain trim speed? Rather than adjusting the throttle and trimming all over again, you can simply reposition the yoke to the original displacement within the friction band. This sounds easy, but it can be time consuming and frustrating. You know that you have to push a little on the yoke to get back to your original trimmed airspeed-how much to push is the tricky part. If you don't push far enough, the airplane will still fly slower than the original speed. If you push too much, the friction can hold the elevator more trailing-edge down and the airplane will fly faster than the original speed. Finding the correct yoke position becomes a trial-and-error process because of the variety of hands-free control positions. Each of these positions results in a different airspeed, and the range of airspeeds that the airplane maintains while you are flying hands free is the trim-speed band mentioned earlier.

Sweating the small stuff
Before spending too much rental money on battling the friction band in an effort to trim to an exact airspeed, it might be a good idea to see how wide the trim-speed band is. You can try this at a safe altitude on a clear, calm day when you can see a distinct horizon. Trim the airplane for straight-and-level flight using a recommended cruise power setting. Note the indicated airspeed. Let's say it's 100 kts. Now, using only the yoke, slow the airplane and stabilize at 95 kts. You'll probably be climbing slightly. Watching the airplane's nose relative to the horizon (the real one, not the instrument), slowly relax your pull on the yoke. If the nose lowers as you relax the pull, the air load on the elevator is stronger than the friction, and the airplane is outside of the trim-speed band at 95 kts. Repeat the procedure at, say, 97 kts. If the nose does not lower when you relax your pull, the friction is at least as strong as the air load, and the airplane is inside the trim-speed band. You are now flying hands free at 97 kts, although you trimmed for 100 kts. You can do it again at 96 kts. If the nose lowers upon relaxation, you're outside the band, and the slow end of the trim-speed band is 97 kts. If the nose does not lower upon relaxation, you're inside the band, and you'll know that the slow end is 96 kts because the nose lowered at 95 kts.

Repeat the entire process, flying faster than the original trim speed of 100 kts. Don't adjust power or trim yet. Try 105 kts. If the nose does not rise when you relax your push on the yoke, you're still in the band and will have to try a faster speed. If it does rise, you're outside the band and can split the difference between 100 kts and 105 kts for the next point.

Let's say the trim-speed band from this test extends from 97 kts to 104 kts. That means the airplane will maintain any speed within that range hands free with the current trim and power settings. The speed it maintains depends on where you leave the control system within the friction band. In an airplane with this trim-speed band, trying to establish a trimmed airspeed around 100 kts to any greater accuracy than seven kts is at best a trial-and-error proposition. Yes, it can be accomplished, but it is more an exercise in persistence than in proper trimming technique.

Before you defensively pummel your instructor with an arsenal of trim-speed-band buzz words the next time he corrects you, you should know that the trim-speed band is usually small in general aviation airplanes. FAR 23.173 requires the trim-speed band to be within plus or minus 10 percent of the original trim speed. That's a hands-free range from 90 to 110 kts in our example.