Takeoff Trim

[Jaybird180]

The C172 has a mark (or notch) for takeoff position for the elevator trim tab. I noticed that this setting is right around the landing approach speed (within about quarter inch). I'm not sure if it's the plane I'm flying, but unless I pull, it will happily do a high-speed taxi. I'm wondering if I should 'hedge' the trim for takeoff.

 

[poadeleted20]

I think you want to need a bit of "pull" to get the nose off the ground. Otherwise, as the plane accelerates after liftoff, you'll need to make a pretty quick nose-down trim change to keep the plane from trying to continue to pitch up past normal climb speed. If you don't need Arnold Schwartenegger arms to get the nose off the runway at normal rotation speed, and if after liftoff the stick back force zeroes out around Vx-Vy without any further trim input, you've probably got the takeoff trim setting just right.

 

[tonycondon]

on the 172's i've flown i've learned that i prefer a little more nose up trim than the mark for takeoff. i suspect this is because i typically fly at a fairly forward CG. Just two people and full fuel. Although when I've done trips with the baggage loaded and worked to get the CG as far aft as possible then the takeoff mark or even a little nose down from there worked well

 

[The Old Man]

The reason for the 2 marks being so close together is that trim really sets a speed at which the plane is aerodynamically in balance. IOW, left to its own devices (hands off), it will nose up or down on its own to maintain that speed.
You can verify this by getting trimmed out at cruise and then changing the powersetting. If you increase power, the nose will come up and the airplane will climb with the AS still at the original value. Reduce power and the nose will drop and the plane will find balance again at cruise speed, but in a descent.
That being said, the approach speed and Vy (normal climb speed) are very close together (Just a few knots at most). Therefore, the marks are also close together.
As an aside, the initial approach speed on my aircraft is Vy. Since I am the only one that flys it, the only change in trim required for takeoff is the changes in W&B since the last landing.

 

[denverpilot]

There's another technique for Cessnas that works well.

Find where your aircraft trims out for Vy (or Vx if you like) and put the trim there for takeoff.

You'll probably need tiny bit of forward pressure to keep from lifting off too soon in ground-effect, but if something distracting happens right after takeoff, you'll know the aircraft is going to fly the trim speed.

I don't like where the indicator for takeoff is on our 182. Too much back pressure required to get the nose wheel off the ground and the aircraft immediately accelerates past Vy.

Trimming for Vx will have the aircraft trying hard to pop off too soon and requires too much forward pressure for my taste, I'd rather pull from Vy to Vx since you can't see a dang thing over the nose of our Robertson STOL equipped 182 at Vx and you typically just don't need to do do it.

If you count the number of "full" top-to-bottom or bottom-to-top turns required from either the full-up or full-down stops and the mechanism is working right, the indicator can be broken and you're still at the right place. Also works as a double-check that your indicator hasn't jumped one of the "record player" grooves in a Cessna trim wheel. The elevator trim jackscrew mechanism is a lot more reliable than the indicator.

I've gotten in at least one rental where the trim indicator was way out of whack. I knew by this method.

So... I go to the stop and then count, during pre-flight, and note if the indicator is correct.

 

[skidoo]

There's another technique for Cessnas that works well.

snip...

I don't like where the indicator for takeoff is on our 182. Too much back pressure required to get the nose wheel off the ground and the aircraft immediately accelerates past Vy.

...snip

That is interesting... On my 182, I find I have to trim down about 1/4"-1/2" from the mark. Otherwise, I'm pushing heavy on the yoke after takeoff. And, I'm usually near forward cg!

I'll have to go and check the count, etc... How many counts from full down to the take-off trim mark on yours?

 

[denverpilot]

That is interesting... On my 182, I find I have to trim down about 1/4"-1/2" from the mark. Otherwise, I'm pushing heavy on the yoke after takeoff. And, I'm usually near forward cg!

I'll have to go and check the count, etc... How many counts from full down to the take-off trim mark on yours?

I'll check when she's back from Annual. We can compare.

 

[Doggtyred]

The reason for the 2 marks being so close together is that trim really sets a speed at which the plane is aerodynamically in balance. IOW, left to its own devices (hands off), it will nose up or down on its own to maintain that speed......

Excellent pearl for students that isn't always apparent....

 

[TMetzinger]

And that position indicator is probably "dead-on balls accurate" - to quote Mona Lisa Vito, only when it leaves the factory. Any subsequent cable stretching, re-stringing, re-rigging, etc can put it off a bit. This is less true for other types that use pushrods or jackscrews for trim.

I agree with Ron that you should have to exert some (not a lot) effort to rotate the airplane.

 

[denverpilot]

I'll have to go and check the count, etc... How many counts from full down to the take-off trim mark on yours?

I haven't forgotten about this, but I did when we went flying with three of us to do the CPA Poker Run. I meant to check it, and kept getting distracted with other stuff...

I sent myself a note in e-mail though, and it's been sitting in my inbox bugging me.

 

[skidoo]

I haven't forgotten about this, but I did when we went flying with three of us to do the CPA Poker Run. I meant to check it, and kept getting distracted with other stuff...

I sent myself a note in e-mail though, and it's been sitting in my inbox bugging me.

I checked mine last time I went up. I recall that it was too many turns (like 10 or more) to make that a good method for me. So, I am sticking with the mark and reference from there. I failed to make a note of exactly how many turns, and now I forgot. I hope to check it again and record it...

 

[denverpilot]

What I actually like to do is pull the elevator up and look out the back window to find neutral and then two "top to bottom" turns up from there.

But this thread has me wanting to get an accurate count for specific airspeeds for my aircraft both with and without Flaps-10, so I'm planning on finding out where those are on my marks and by counting turns.

I'll share what I find, but it's probably going to be slightly different for each individual aircraft...

 

[Greg Bockelman]

Ok, what no one has specifically mentioned, (Well, Tony alluded to it in his post #3) is that trim setting is CG specific. IOW, the trim setting for a certain speed at a forward CG is going to be different than the trim setting for an aft CG at the same airspeed. In other words, you need to experiment with different CG locations to see what settings work in those cases.