Why do people say this about trim?

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Lindberg

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Lindberg
I've read, more times than I can count, claims similar to this: "If you trim for a speed and let go of the yoke, your plane will keep flying at that speed, regardless of your power setting."

It isn't true in three airplanes I've tested. And based on how most elevator trim on light planes works, I don't understand how it could be true. But I'm in no aeronauticist.

If you've tested this and it worked or didn't (I tried three power settings, full, partial, and idle), please post your results. And if you can explain why this is or isn't true, let's put this to rest.
 
Lindberg said:
I've read, more times than I can count, claims similar to this: "If you trim for a speed and let go of the yoke, your plane will keep flying at that speed, regardless of your power setting."

It isn't true in three airplanes I've tested. And based on how most elevator trim on light planes works, I don't understand how it could be true. But I'm in no aeronauticist.

If you've tested this and it worked or didn't (I tried three power settings, full, partial, and idle), please post your results. And if you can explain why this is or isn't true, let's put this to rest.
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It’s true in my experience. You have to wait for the airplane to reach equilibrium though, not accelerating or decelerating though. And that means also not holding altitude.


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Keep in mind that propwash affects the elevator somewhat beyond the airflow from moving through the air. If you’re stable at crises power and reduce to idle, you’ll probably see a slightly faster airspeed when it stabilizes due to the loss of additional airflow from the prop.
 
Weekend Warrior said:
Interesting. I’m not doubting your results @Lindberg , but it’s always been the case in my plane: trim for airspeed, power for altitude.
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What he said. If you're trimmed in cruise power and add power? You should climb at essentially the same airspeed you trimmed for. Reduce power to original and you should level out on speed.
 
It's true in both airplanes I've owned...a 172 & 182, both straight-tails.
 
Weekend Warrior said:
Interesting. I’m not doubting your results @Lindberg , but it’s always been the case in my plane: trim for airspeed, power for altitude.
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What type? Was not true in my tests in a 170, Cub, or Citabria. In order to avoid phugoid oscillations, after changing power, I pitched to stabilize at the original speed before releasing the controls.
 
Not even close to true in my ride. Trim for Vy is almost the same as trim for cruise. And, when you pull power, the nose is going down...
 
Lindberg said:
I've read, more times than I can count, claims similar to this: "If you trim for a speed and let go of the yoke, your plane will keep flying at that speed, regardless of your power setting."

It isn't true in three airplanes I've tested. And based on how most elevator trim on light planes works, I don't understand how it could be true. But I'm in no aeronauticist.

If you've tested this and it worked or didn't (I tried three power settings, full, partial, and idle), please post your results. And if you can explain why this is or isn't true, let's put this to rest.
Click to expand...
I find it to be true in C-150, 152, 172, and Piper Cherokee.
If one increases power, the plane climbs at the speed it was trimmed for in level flight. Decreasing power causes a descent at that speed.
The C-150 and C-152 are light on the elevator, but trimming them still aids in keeping straight and level flight.
Doing steep turns, I'll roll into a bank and pull two turns of up trim in a C-172, and I'll back them out when returning to level flight- makes them a lot easier for me.
@Lindberg , what plane are you using?
 
It's true, in general, after some oscillations as others have pointed out. But depending on the aircraft, there will be larger or smaller deviations. Changes in prop blast can affect stick forces and the trim tab's authority. If the thrust line is not in line with the aircraft's center of drag there will be effects (it's definitely not true with a high mounted engine like flying boats or many ultralights).

It also should work better with forward CG positions, since we're talking about longitudinal stability here, which decreases as the CG moves aft.

If your plane doesn't hold a speed with trim, try it while holding the stick/yoke in exactly the same position (easy with a hoke, wrap your hand around the shaft against were it exits the panel).
 
To a first approximation this is true in every light single I have flown, including various Cessna, Piper, and Grumman models. Indeed, it is the basis of basic IFR attitude flying. For example, trim to 90 kt level flight, retard the throttle 200-300 rpm and descend 500 rpm at the same airspeed. I would suspect at power extremes there might be a slight variation of trimmed airspeed depending on the particular dynamics of prop wash on trim surfaces for a particular airframe.
 
Cap'n Jack said:
I find it to be true in C-150, 152, 172, and Piper Cherokee.
If one increases power, the plane climbs at the speed it was trimmed for in level flight. Decreasing power causes a descent at that speed.
The C-150 and C-152 are light on the elevator, but trimming them still aids in keeping straight and level flight.
Doing steep turns, I'll roll into a bank and pull two turns of up trim in a C-172, and I'll back them out when returning to level flight- makes them a lot easier for me.
@Lindberg , what plane are you using?
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Cessna 170, Citabria, and Cub
 
Lindberg said:
I've read, more times than I can count, claims similar to this: "If you trim for a speed and let go of the yoke, your plane will keep flying at that speed, regardless of your power setting."

It isn't true in three airplanes I've tested. And based on how most elevator trim on light planes works, I don't understand how it could be true. But I'm in no aeronauticist.

If you've tested this and it worked or didn't (I tried three power settings, full, partial, and idle), please post your results. And if you can explain why this is or isn't true, let's put this to rest.
Click to expand...

Don't know "how" to explain how the physic work...but I sure as heck can trim out my 182 to fly hands off in climb, straight and level, and descents.

I set my AP with heading bug, set power, then trim to maintain speed once stabilized, hands off in any phase of flight. Easy peasy.

Any adjustment are made by hand then reset trim, you can not adjust with trim alone if that is what you are testing otherwise you will be in never ending chase of desired settings.
 
Lindberg said:
I've read, more times than I can count, claims similar to this: "If you trim for a speed and let go of the yoke, your plane will keep flying at that speed, regardless of your power setting."

It isn't true in three airplanes I've tested. And based on how most elevator trim on light planes works, I don't understand how it could be true. But I'm in no aeronauticist.

If you've tested this and it worked or didn't (I tried three power settings, full, partial, and idle), please post your results. And if you can explain why this is or isn't true, let's put this to rest.
Click to expand...

The speed will remain very close unless you apply a large power change. When you add or reduce power you also add or reduce a portion of the thrust acting on the vertical stabilizer.
Attempt your experiment with <50 RPM power changes and let the plane stabilize.
 
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It's not true in the sense that there is some law of physics that says the airspeed will remain identical regardless of power setting. But it's true to enough extent to give a student pilot an idea of how trim and power affect the airplane.
 
Capt. Geoffrey Thorpe said:
Not even close to true in my ride. Trim for Vy is almost the same as trim for cruise. And, when you pull power, the nose is going down...
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The nose should go down. And when the plane stabilizes out it will maintain trimmed speed. That’s the way it’s been for all the planes I have flown.


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"Trim." Hmmmm, don't have trim on my airplane... just a piece of beer can screwed to an elevator.

Ron Wanttaja
 
The whole "pitch for air speed, power for altitude" that was always annoying to me as well, there's nothing that says you can't fly at 2000 ft or 5000 ft or 10,000 ft at the same power and RPM setting.. and when you were landing the airplane you're obviously going to have to make pitch and power changes.. dumb mnemonics like that in my opinion take away from the actual understanding of flying an airplane

Climb, level off, and as the plane accelerates trim away the pressure in the control stick until things reach an equilibrium
 
dmspilot said:
It's not true in the sense that there is some law of physics that says the airspeed will remain identical regardless of power setting. But it's true to enough extent to give a student pilot an idea of how trim and power affect the airplane.
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Actually it is kinda true in a law of physics sense...


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timwinters said:
It's true in both airplanes I've owned...a 172 & 182, both straight-tails.
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Even with full flaps? I agree in normal trim, but put in 40 degrees of flaps and then add full throttle for a go around and the 182 I flew would pitch up violently and slow quite a bit. I never let it go long enough to see how much.

the added airflow of that big engine at full power over the horizontal stabilizer makes it more effective.
 
Salty said:
Even with full flaps? I agree in normal trim, but put in 40 degrees of flaps and then add full throttle for a go around and the 182 I flew would pitch up violently and slow quite a bit. I never let it go long enough to see how much.
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It’s not supposed to work if you add or subtract power. It’s for a given stabilized configuration...


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gsengle said:
It’s not supposed to work if you add or subtract power. It’s for a given stabilized configuration...


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Huh? The whole question was “regardless of power setting”. Again, huh?
 
Salty said:
Huh? The whole question was “regardless of power setting”. Again, huh?
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Then it’s never true lol


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gsengle said:
Then it’s never true lol


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It’s true regardless of power setting as long as you don’t change the power setting is more accurate


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Tantalum said:
The whole "pitch for air speed, power for altitude" that was always annoying to me as well, there's nothing that says you can't fly at 2000 ft or 5000 ft or 10,000 ft at the same power and RPM setting...
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Pitch for airspeed, power for altitude change (rate of climb). Rate of climb equals excess power divided by aircraft weight.

gsengle said:
Then it’s never true lol
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It's true, approximately, for most conventionally configured aircraft in most situations, after the initial oscillation damps out.
 
Dana said:
Pitch for airspeed, power for altitude change (rate of climb). Rate of climb equals excess power divided by aircraft weight.


It's true, approximately, for most conventionally configured aircraft in most situations, after the initial oscillation damps out.
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My point is it’s never true if you change power setting... see above...


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Dana said:
Pitch for airspeed, power for altitude change (rate of climb). Rate of climb equals excess power divided by aircraft weight.
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I understand the concept but still find it kind of dumb and not a great way to teach someone. If I want the airplane to go higher I pitch up, if I want the airplane to go lower I pitch down and if I want to go level I pitch roughly straight ahead.. if I want to go faster I give it more throttle, if I want to go slower I give it less throttle.. obviously the power is not unlimited so pitch is a factor, for example climbing out at full power I will pitch for 120 knots.. but I'm still pitching up in the primary purpose of gaining altitude, not to maintain a set airspeed
 
^same thing with descent. I don't understand when I fly with people and the first thing they do to descend is cut the power, why not just push the nose down and stay out of the yellow arc if the air is rough? It's a little bumpy out I'll reduce power to stay out of the yellow and ask for a 500 to 700 foot per minute to send
 
Some of it will depend on the CG location. An aft CG requires little downforce on the stab and so the trim tab deflection will be minimal (tab up, elevator down for cruise). Reducing power in that case won't get as much of a nose drop and the speed might decay a little. Take that to an extreme---a CG a little aft of CL, and you can see that a power reduction will have the nose rising and speed falling off dangerously. The tail is lifting in that case. That sort of thing was outlawed a long, long time ago. Some of the WWI fighters had lifting tails, and it led to a lot of unrecoverable spins and crashes.
 
So let's get geeky.
  1. Angle of attack controls airspeed (angle of attack corresponds to pitch only in straight and level flight; in every other situation, "pitch" is the wrong word).
  2. At any given weight and angle of attack, there's a specific amount of power that will result in level flight. A power surplus will result in a climb (trading power for altitude); a power deficit will result in a descent (trading altitude for power).
  3. In a conventional, non-T-tail single, power changes will influence your pre-trimmed angle of attack because of changes in the airflow over the vstab: adding power will usually slow you down by a few knots, while reducing power will usually speed you up (the opposite of what one might expect).
  4. In a turn, you need more power to stay level, because the plane is borrowing some of the existing power to change direction. Your choice is to add more power (if you have it), or to increase your angle of attack (which reduces the power requirement, but only to a point).
Not nearly as snappy as "pitch for speed, power for altitude" (or vice-versa)—which are both dangerous oversimplifications—but understanding these nuances will give a new student pilot a better chance of dying of old age.
 
Lindberg said:
I've read, more times than I can count, claims similar to this: "If you trim for a speed and let go of the yoke, your plane will keep flying at that speed, regardless of your power setting."

It isn't true in three airplanes I've tested. And based on how most elevator trim on light planes works, I don't understand how it could be true. But I'm in no aeronauticist.

If you've tested this and it worked or didn't (I tried three power settings, full, partial, and idle), please post your results. And if you can explain why this is or isn't true, let's put this to rest.
Click to expand...
Try stabilising your airspeed first using the yoke, and only then trimming to remove pressure. If you lead with the trim wheel (or electric trim), you'll set up frustrating phugoid oscillations that can last a very long time (you'll think you're level, but then 20 seconds later the nose will slowly start rising or falling again unless you use the yoke to prevent it).

This is a mistake many new pilots (and some experienced ones who should know better) often make.
 
Lindberg said:
What type? Was not true in my tests in a 170, Cub, or Citabria. In order to avoid phugoid oscillations, after changing power, I pitched to stabilize at the original speed before releasing the controls.
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Warrior. In fact, I find it much easier to trim for level flight by reducing power to approximately cruise rpm, trimming to something close to level, then trim for 0 climb/descent with slight power changes. My fuel flow meter helps with this.
 
Tantalum said:
I understand the concept but still find it kind of dumb and not a great way to teach someone. If I want the airplane to go higher I pitch up, if I want the airplane to go lower I pitch down and if I want to go level I pitch roughly straight ahead.. if I want to go faster I give it more throttle, if I want to go slower I give it less throttle.. obviously the power is not unlimited so pitch is a factor, for example climbing out at full power I will pitch for 120 knots.. but I'm still pitching up in the primary purpose of gaining altitude, not to maintain a set airspeed
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Nose up to go slower and nose down to go faster. This rule always applies no matter the power setting.


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Peter Anderson said:
The nose should go down. And when the plane stabilizes out it will maintain trimmed speed. That’s the way it’s been for all the planes I have flown.
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You have never flown my airplane. Speed for a given trim setting varies with power. A lot.
 
Peter Anderson said:
Nose up to go slower and nose down to go faster. This rule always applies no matter the power setting.


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This implies no regard for desired altitude or glide slope. When right seat as safety I've watched people go all up and down and all over the place on a glide slope when learning instrument because they've been taught this ludicrous thing of managing your altitude with the power setting..

Why not just point the plane where you want it to go?

"Power for altitude" is completely asinine.. it's like telling people that wearing a sweater will make it cold outside

Changing pitch will primarily change your altitude, and obviously impact your speed as well if no adjustments are made the power setting..
 
Depends on the airplane. My Cherokee could have been easily flown without changes in trim. The Mooney is a trim monster, and has huge control forces, especially once the landing flaps come in.

As folks are saying, the aircraft will oscillate in pitch once a trim setting is reached. A bit of patience is needed to allow the aircraft to stabilize in pitch.
 
Lindberg said:
Why do people say this about trim?
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so people with a one-dimensional mind who cannot manage both their right and left hands doing two different dynamic things can stay (somewhat) ahead of the plane..

I know that's #aggressive.. but I can't think of any other reason
 
Tantalum said:
I understand the concept but still find it kind of dumb and not a great way to teach someone. If I want the airplane to go higher I pitch up, if I want the airplane to go lower I pitch down and if I want to go level I pitch roughly straight ahead.. if I want to go faster I give it more throttle, if I want to go slower I give it less throttle.. obviously the power is not unlimited so pitch is a factor, for example climbing out at full power I will pitch for 120 knots.. but I'm still pitching up in the primary purpose of gaining altitude, not to maintain a set airspeed
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The approach end of the runway would be littered with crashed airplanes if we taught pull back to go up. Stalls occur from the pilot misusing the elevator.
 
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