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Unless you're a professional weight-lifter, trim tabs are one of the best things on your airplane. So what are trim tabs? They're a secondary flight control surface that help you reduce (or eliminate) the need to place pressure on the yoke or rudder to keep your airplane flying straight and level.
There are four main types of trim tabs:
So now that you know the four types, let's look at how each of them work.
The plain old trim tab is one of the most common types of tabs used in small single-engine airplanes. A trim tab is attached to the trailing edge of an elevator, and it's operated by moving a small control wheel in the cockpit.
The operation is pretty simple: roll the wheel in the nose up position, and the tab moves down. Roll the wheel in the nose down direction, and the tab moves up. Check out the animation below:
When you move the trim tab up or down, it sticks out into the free air stream, and deflects the elevator in the opposite direction. So even though it may seem 'backwards' to move the tab down to make the nose of the plane go up, seeing the tab in action helps it all make sense.
Some aircraft have very heavy control loads, especially at high speeds. That's where balance tabs come in handy. Balance tabs look like trim tabs, but they have one major difference: balance tabs are attached to the control surface linkage, so when the control surface is moved in one direction, the balance tab moves in the opposite direction.
By moving the balance tab in the opposite direction, the control load on your yoke is significantly reduced, making your airplane easier to fly.
Antiservo tabs are similar to balance tabs, but they move in the opposite direction. For example, when your elevator or stabilator moves up, the antiservo tab moves in the same direction.
So why would you want your tab to move in the same direction as your elevator/stabilator? In small aircraft, it increases the control feel, and helps prevent you from over-controlling your aircraft's pitch. One of the most popular examples of the antiservo tab is on the Piper Cherokee. Without it, the plane would be much easier to pitch up and down, but it would also be easy to over-control, and possibly overstress the airframe..
The fourth and final tab is the ground adjustable tab. If you've flown a training airplane, there's a good chance it had one of these on the rudder. Ground adjustable tabs are just that: only adjustable on the ground. So how do you adjust it? By bending it left or right, preferably between two solid surfaces, like blocks of wood. But before you run out and start adjusting the tab yourself, it's a good idea to see what your POH or mechanic recommends first.
The ground tab is used to keep your plane flying coordinated in level flight. And since it's a trial and error system, it might take a few flights of adjusting, flying, and more adjusting. But once you've got it set, you're good to go.
If they're used right, trim tabs can act like an autopilot, allowing you to take your hands off the yoke in almost every phase of flight. After all, you have enough going on in the cockpit - why not make one thing in your life easier?
Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at firstname.lastname@example.org.