As a young boy, I was enthralled with the movie The Right Stuff, and enjoyed the book later in life too. One theme was the “steak diner and all the trimmings” for the pilot who would break the sound barrier. Interestingly, it was trimming that finally solved the problem of the X-1, and it was Chuck Yeager who went to the hardware store for the jack screws and linkage to allow the elevator to be trimmed. Once the X-1 was able to be properly trimmed in flight, Yeager was able to become “the fastest man alive.”
Staying in Neutral, discusses the idea of learning to fly at neutral trim, and you may want to take a few minutes reading that article before diving deeper into the concept of “…and all the trimmings.” Many pilots, including myself, take too long before mastering the concept of trim, and that, in this instructors humble opinion, is because we say things like “relieve the pressure off the yoke”, “the yoke is the primary flight control, and trim is a secondary system”, and “pitch first then trim.” Though I do not disagree with these sayings, they don’t tell the whole story nor do they provide meaningful flight exercises in the air.
For the sake of simplicity this article will only be discussing elevator or stabilator trim in typical modern light trainers.
Let’s imagine a Cessna 172M with no elevator trim. There would be no trim wheel, nor an elevator trim tab, just a solid airfoil controlled by the pulley system connected to the yoke. Would take off change? Not really, you would accelerate to lift off speed pull back on the yoke to break ground, release back pressure and the airplane will fly at a constant airspeed. It would continue to climb, and without other inputs from the pilot, except for managing the rudder and engine, the airplane would reach an altitude where it could no longer climb. Eventually the engine would run out of fuel and stop producing power, the propellor would stop producing thrust and the nose would fall off and the airplane would start picking up speed. With no control input except for coordinated rudder, the airplane would accelerate until the various airfoils stabilized the airplane at a constant descent. Once the pilot was close to the landing, back pressure on the yoke would raise the nose, slowing the aircraft down, until approach speed, then a little farther back on the flare and then fully back for a full stall landing.
Little extreme? Maybe, but if during preflight you ensured the trim tab was flush with the elevator you could run this experiment…though I would suggest taking a CFI with you.
Once the trim tab is properly set for take off, by visually inspecting its alignment in the walk around, leave it alone. After rotation, relax that back pressure. I guarantee you, that in a typical training load you will fly out at Vy, or pretty darn close. Once at altitude, pull the power out slowly as not to let the nose fall rapidly, until you have achieved straight and level flight. You should be at the fastest airspeed of the white arc, which, in a typical training load is pretty close to your Va. But why the change in airspeed? I thought once you trimmed an aircraft it stays at that speed? It has to do with the angle of attack of the wing. The greater the angle of attack, the slower the airspeed. During full power, the propellor is creating excess thrust and pitching the nose up, creating a greater AOA. When power is pulled, and thrust equals drag, the AOA is less, resulting in a higher airspeed at that trim setting.
Drastic and sudden power settings adversely effects trim airspeed by rapidly changing the AOA of the wing. It will take the airplane a while to stabilize to its trimmed airspeed. This exercise is not as dramatic as climbing to the service ceiling and running out of gas, and can be practiced at typical safe training altitudes. First, level off at neutral trim or what is also called take off trim. You should be around 85 knts hands free. Second, slowly add power, so not to let the nose rise quickly. You should begin to climb and airspeed will slowly decrease and stabilize when you reach full power. Third, slowly decrease power, allowing the nose to fall slowly. Once you pass your level speed, pull carb heat, and continue to slowly pull the power until you stabilize again in a descent. CAUTION: You will now be gaining airspeed. Fourth, slowly raise the nose with back elevator pressure, and when you approach 85 knts, come back in with power and level off.
Ensure you are back at a safe altitude. Find Straight and level flight at 85 knots, or at the flap operating limit. Pull the power to 1500 RPMs, and while holding back pressure on the yoke to keep the nose from falling, add 10 degrees of flap. (Sound familiar?) While not touching trim, release back pressure, and allow the nose to come down. What is the new stabilized airspeed? Now add 20 degrees…what airspeed? Add full flaps…what airspeed? Remember, flaps changes the camber of the wing, which changes the angle of attack. Flaps slows the airplane and creates a steeper angle of descent while preventing an increase in airspeed. Now, while at full flaps and neutral trim, add full power to recover, by raising flaps to 20 degrees, and slowly take up the rest. You should find it to be a more controllable recovery.
And here lies one of the fallacies that is taught to students… “after flaps, trim.” If you trim for a slower airspeed, and add more flaps, you will become even slower and will be in an out of trim position for a go around. More on that later.
There are two airspeeds that pilots must find without hesitation nor delay. Maximum Flaps Operating Speed (bottom of the white arc or is it the top of the white arc…arghhh I am a Language Specialist and this is confusing…so lets call it the start…wait that doesn’t work either…ah Vfe) and the recommended “Engine Out Speed”, which we wrongly call Best Glide, however, thats for a heated discussion another day. Those are the two airspeeds that are critical and need to be found automatically. For landings, you need to be in the flaps operating range. How many of use where taught 90-100 knots downwind, when abeam the numbers, pull power, carb heat on, lift the nose, drop 10 degrees of flap, and put in two back rolls of trim.? And it’s that imprecise trimming that keeps pilots from having consistent landings, and the fact many hold airspeed with yoke pressure, because they are constantly out of trim for their airspeed.
What is “out of trim?” An aircraft is out of trim when a pilot releases the yoke and there is a pitch change. Five Things to Change in Your Pilot Training talks about how pilots must lose their “death grips” and learn to fly hands free. Having a heavy hand on the yoke prevents proper trimming. This coupled with teachings like, “trim and power when you add flaps” creates for an out of trim condition during landings. Unfortunately, pilots get away with this bad technique on most landings, however, an out of trim position on a go around can be disastrous. Let’s examine.
A pilot is fast on downwind, as the trim is forward of neutral, so the pilots pulls up on the elevator to slow to the white arc. Adding two long turns of nose up trim and now the trim is behind neutral. The next addition of flaps and the nose whats to rise. So the pilots pushes forward on the yoke to keep the nose down, but then adds more back trim. The descent steepens, so more power is added. As the nose comes up aggressively, more forward pressure on the yoke is needed. As the pilot approaches the runway, power is reduced rapidly, dropping the nose, as airspeed increases, the pilots pulls up the nose and puts in even more back trim. Unstable, and more than likely fast, the pilots salvages the landing by landing long. However, on the wrong day given the same scenario, the pilots must execute a late go around. Power is jammed in rapidly, because of the excess thrust, coupled with the increase lift of flaps, and an aft of neutral trim setting, the nose violently raises, and ….. a Loss of Control Accident.
The second airspeed, Best Glide, or more accurately the published Engine Out Speed, is found in a typical 172 by putting in full nose up trim. However, there is some concerns with this technique. Let’s examin. A pilot loses the engine at altitude in cruise flight, so full back trim is added. The airplane will fly stable and at a safe airspeed to allow the pilot to work the problem. If the engine can not be restarted, the pilot commits to landing. Now on short final with full back trim, the pilot drops all the flaps to land at the slowest possible airspeed. The aircraft is now slower, and the glide is now shortened. In a startle response, the pilot tries to stretch the glide by raising the nose and … a Loss of Control Accident.
Proper trimming is more important than most think.
It is recommend that an airplane is stabilized by 500 feet or 1000 feet for an IFR approach. What is stabilized? Well, in the most simple terms, you should be able to release the yoke and there would be no pitch change. Go arounds are best made early, and not as a result of a startle response. You will find that a properly trimmed airplane behave better on a go around than an out of trim aircraft.
So the next time you go flying, think of Chuck Yeager and make sure you have all the trimmings.