1

I wanted to understand a part in ‘Take Off” where the study starts to explain “… the airspeed for best rate of Climb.” I am a little confused about the (how to) of pushing the wheel forward to relieve forward pressure (nose down trim) -or- Rotate the trim-wheel back towards you to relieve back pressure (nose up trim). Is the motion in opposite of when pushed down, nose up trim and vice versa to that motion?

Hope I am explaining it articulately and its comprehensible.

TaliaJ
  • 11
  • 1
  • 3
  • Some aircraft don't have trim "wheels", for example the Comanche has a handle on the ceiling you turn. Others may have a wheel but most of the time are actuated electrically via a switch on the control yoke. I'm also not sure what study you are referring to. Unfortunately your question is hard to follow so it isn't clear what you are asking. – Ron Beyer Feb 07 '18 at 23:38
  • Thank you, my question should no longer be on hold because I was able to b e answered by the last two responses. Thank you. : ) – TaliaJ Feb 08 '18 at 18:49

2 Answers2

3

There are two pitch inputs* in the cockpit: the yoke and the trim wheel. The effects add up - you can achieve the same pitch up moment by the yoke only, the trim wheel only, or a combination of the two. The pitch up or pitch down motion of the airplane is always the sum of the two.

*That is not a very correct statement, but for now let's put things that way for easier understanding.

The difference between the yoke and the trim wheel is that once you let go of the yoke, it returns to neutral. However the trim wheel stays where it is.

So let's say you want 2 units of "pitch up force" (and let's ignore what the actual unit in physics is, it is just a number) in a certain flying situation. You pull on the yoke until you reach the desired pitch. Great. However if you let go of the yoke, you will loose that pitch up force and the aircraft will start to pitch down. That is not what we want - we want the airplane to hold this pitch attitude, stable, for the next 10 minutes. If we move the trim wheel from the neutral position to 0.5 units up, the desired amount of pulling on the yoke is reduced - we still need to pull back on the yoke, but not as much - we only need 1.5 units of pitch up on the yoke because the other 0.5 units is provided by the trim wheel. This is what "relieve back pressure" is referring to. If you move the trim wheel to 2 units up, you can now completely let go of the yoke, and the aircraft is stable.

Is the motion in opposite of when pushed down, nose up trim and vice versa to that motion?

I think you might be confusing the pitch input with the movement of the trim tab on the elevator, assuming that your textbook mentions it. On most general aircrafts, a trim tab is used for pitch trim control. A trim tab is essentially "an elevator of the elevator". The elevator needs to move up for the airplane to pitch up. But the trim tab needs to move down for the airplane to pitch up.

Do not confuse how the trim is achieved aerodynamically, with what you input in the cockpit. The trim tab is just one of many ways to control pitch trim. Airliners, for example, do not have a trim tab. However, the principal of operating the trim wheel is the same. It is just a large aircraft uses a different method to hold a stable pitch attitude.

kevin
  • 39,731
  • 17
  • 148
  • 278
  • I read the OP's "opposite motion" as referring to the old joystick gaming misunderstanding of "push the stick up to go down" as opposed to "push the stick forward to tip the plane forward" - not to do with the physical operation of the trim tabs. – IanF1 Feb 08 '18 at 06:31
  • Understood where you explained 'pitch up force' Kevin. – TaliaJ Feb 08 '18 at 18:13
  • But I think the clarification of what the motion I was relating to, is the joystick gaming method of what the commenter [IanF1] said. Which is not what I want to do, I know the cockpit is not a gaming device - but reading it game me the idea of that what the motion might be? Maybe even the book was unclear. – TaliaJ Feb 08 '18 at 18:18
2

I up voted Kevin's excellent answer. I'm wondering, though, if

I am a little confused about the (how to) of pushing the wheel forward to relieve forward pressure (nose down trim) -or- Rotate the trim-wheel back towards you to relieve back pressure (nose up trim). Is the motion in opposite of when pushed down, nose up trim and vice versa to that motion?

is in reference to the relative motion of the aft edge or top edge of the trim wheel.

Considering the trim wheel as a disk, it is typically mounted vertically such that it's plane of rotation is parallel to the aircraft's longitudinal axis and it's axis of rotation is parallel to the aircraft's lateral axis.

enter image description here

In the image below taken from Wikipedia of a Cessna 172 trim wheel, you see that the disk (to the left of the mic) is recessed such that the only part of it protruding is the aft part with the edge having little bumps.

To trim nose up, you would rotate the wheel such that the movement of the bumps would be downward. To trim nose down, you would rotate the wheel such that the movement of the bumps would be upward.

Putting it another way, if the wheel was recessed in a center console between the pilots seats such that only it's top was showing, to trim nose up, you would rotate the wheel such that the movement of the bumps would be toward the back of the airplane. To trim nose down, the movement of the bumps would be toward the front of the airplane.

My apologies if this has been too pedantic.

Terry
  • 39,137
  • 5
  • 109
  • 184
  • Thank you Terry! its understood, via the image portrayed. Reading it and actually seeing it with explaining is very vivid. I am studying the cockpit and flight of a Cessna 152 however I will be actually flying the Cessna 172 (I hope its a smooth transition). – TaliaJ Feb 08 '18 at 18:01