“Let go of the controls and see what happens.”

For an eager student pilot looking to learn to control the airplane, most of the tasks of even basic airplane control seem like hard work. There’s so much to think about, and constant corrections and control inputs. Even just straight and level flight can be challenging. First the plane wants to climb – then the plane wants to descend. I found out fairly early in my training that even when heavy inputs aren’t needed on the controls, it’s easy to tense up and not realize the pressure, or lack of pressure, that you are placing on the controls. “Let go of the controls and see what happens,” my instructor would say. More often than not, I was holding some sort of pressure on the yoke. The lesson? Trim the control pressures away. Aside from minor corrections, a properly trimmed airplane nearly flies itself hands off. So what is trim and how does it work?

The Trim Tab
In many light training aircraft, the airplane is outfitted with elevator trim. The trim tab is a small surface on the elevator that can move up or down through a small push rod controlled by the pilot with the trim wheel. It’s almost like a small flap that can be extended from the elevator surface of the tail. If the elevator is being held in a certain position, say in a climb, the trim wheel can be rotated to extend the trim tab into the relative wind. The force of the air against the trim tab provides a constant force on the elevator, which allows the pilot to relax the force on the yoke. When the right amount of trim tab surface is used for a particular configuration, the elevator maintains its position without the pilot having to hold it there.

X-Plane 11 Airfoillabs Cessna 172

The trim tab is the small rectangular surface on the right hand side of this airplane’s elevator.  You can see the push rod used to adjust it over on the left of the tab.

 

The Trim Wheel
In the cockpit, the trim wheel is used to position the trim tab. The wheel is labeled “nose down” at the top and “nose up” at the bottom. The effect of rotating the wheel towards “nose down” is exactly that. Assuming you were in level cruise with no control forces on the yoke, rotating the trim wheel towards “nose down” would deflect the trim tab upwards, deflecting the elevator down, and the nose of the airplane would drop. Rotate the trim wheel towards “nose up” and the opposite happens. The trim tab deflects downward, deflecting the elevator upward, and the result is that the airplane nose raises into a climb.

X-Plane 11 Airfoillabs Cessna 172

The amount of deflection towards the nose up or nose down position can be seen with the small horizontal line (next to the word “take”), seen currently in a fairly nose down position.  Setting the trim line to the arrow, or to the “takeoff” position, is normally done before takeoff.

 

Don’t Fly The Trim
It’s important not to confuse controlling the elevator with trim as a means to flying the airplane. If, for instance, you are cruising and wish to descend, simply rotating the trim wheel towards nose down (applying nose down trim) is not the way to initiate the descent. Sure, it would technically work, but it’s a bad practice. The proper use of trim involves first changing the pitch of the airplane with the yoke. Once the pitch is as desired – in this case, a descent has been established – then the trim wheel is rotated to relieve the pressures being put on the yoke. In the case of the descent, the pilot might be pushing the yoke, so applying some nose down trim would relieve that pushing pressure, allowing the desired descent without any control pressure needed.

X-Plane 11 Airfoillabs Cessna 172

This is a good bit of nose up trim.  Normal flight doesn’t usually require this much, but I used it to exaggerate the tab for viewing purposes.

 

When To Adjust Trim
Once the airplane is properly trimmed for a given configuration, there are still small corrections needed on the yoke. Much like when you drive down the road, even on a straightaway, you are contantly making minute corrections back and forth on the steering wheel. It’s not really possible to just hold the wheel perfectly still without drifting one way or the other. In the same way, the trim should be used to relieve control pressure for a given configuration of pitch and power, but then the minor corrections to maintain control should be made on the yoke. When power or pitch is adjusted, expect to have forces applied to the yoke to maintain that new configuration. In that case, use the trim wheel to relieve those forces.

X-Plane 11 Airfoillabs Cessna 172

Some airplanes have electric trim controls on the yoke as seen here next to the red “Push To Talk” button.

 

Check Yourself
Even after adjusting the trim, it’s easy to subconsciously apply pressure to the yoke. It makes me think of when my car needs an alignment. I’ll drive down the road, applying a bit of left pressure to my right pulling car without even thinking about it. In the airplane, after you’ve trimmed it out, take your fingers just a couple inches off the yoke, briefly, and see what happens. If the airplane begins to change its pitch at all, you clearly still have some pressure that needs to be trimmed out. The “hands off” check is a great way tell what sort of unnecessary pressures you are applying. As a student pilot, my instructor checked me with it to serve as a demonstration of how much harder I was working than I needed to be on controlling the airplane.

To see the trim in action, along with an exciting random engine failure, check out this week’s episode of Bucket List Flights: Flying To Oshkosh below.  Visit the discussion on the X-Plane.org Flight School as well!

 

 

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4 Comments

  1. KP

    Trim demystified. Great article.

    Reply
    • Clay

      Thanks KP! I’m all about trimming out the demystification!

      Reply
  2. Reid Columbia

    Great article Clay!!

    Reply
    • Clay

      Thanks, Reid! I can hear you saying “let go of the controls and see what happens,” followed by me releasing the controls, the airplane pitching dramatically, I sigh, and reach for the trim.

      Reply

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