I heard from a ground instructor that full nose up trim will yield best glide in a 152 and 172.

As I understand best L/D, a lighter loaded plane will require a slower glide than the same craft loaded more heavily.

Am I correct in thinking that the best glide in the POH is an average glide for an average load?

Was my ground instructor correct? Those extra feet may be useful one day!

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    $\begingroup$ I'm not sure that it does, in the 172 when practicing engine-outs I would have to trim nose down to achieve best glide. $\endgroup$ – Ron Beyer Aug 30 '16 at 17:15
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    $\begingroup$ I don't think the POH refers to "average glide" and "average load". I think almost all of the POH is written for Standard Temp & Pressure (STP) and max-weight. $\endgroup$ – abelenky Aug 30 '16 at 17:42
  • $\begingroup$ Interesting to hear the POH best L/D is for max load, presumably then a slower speed is would yield a better glide distance if well under max load. I wonder where my ground instructor got her information from! It was an aside from a class sometime ago, so cant follow it up. Perhaps this is something I need to experiment with using a GPS and a hard deck! $\endgroup$ – Tom Aug 30 '16 at 18:30

That ground instructor is giving you bad information. You can not make a blanket statement like "full nose up trim" and expect it to be valid for any and all situations.

You are correct that the best L/D speed will yield the best glide ratio and that the speed will vary with weight.

The best trim position is the one that will maintain the best glide speed hands off. Any other trim position will require an additional control input to fight the trim and that input will cause additional drag.

So no, full nose up trim will not yield the best glide unless by some remarkable coincidence that is also the trim position for hands off best glide speed.

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    $\begingroup$ From my personal experience, I believe that full up trim will be close to Vbg, but not exactly. In an emergency or high-workload situation, it may be pretty good to just set full trim, and not worry about being off 2 or 3 kts. $\endgroup$ – abelenky Aug 30 '16 at 17:40
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    $\begingroup$ But what if the aircraft is loaded so the C of G is at the rear limit? Would you still blindly dial in full nose up trim and hope for the best? $\endgroup$ – Mike Sowsun Aug 30 '16 at 17:52
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    $\begingroup$ Common sense always applies. I mean in a typical situation. I would not normally fly near max Arm limits. $\endgroup$ – abelenky Aug 30 '16 at 17:53
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    $\begingroup$ Flying properly trimmed is more efficient in aircraft in aircraft with trimable stabiliser. But C152 and C172 have just trim tabs, so it does not apply. $\endgroup$ – Jan Hudec Aug 30 '16 at 21:47

Your instructor was correct in that full—or nearly full—nose up elevator trim in the Cessna 172 will give you roughly best glide speed. This is a rule-of-thumb and must be understood as such.

Variances in aircraft CG and even trim rigging render it impossible to make statements such as "X trim setting will give Y speed." And we should note that the best glide speed varies by weight. The Cessna POH or AFM will only give best glide speed for max gross weight.

However, within these limitations, trimming close to full up will get you close. This can be a very helpful rule-of-thumb to get your aircraft quickly trimmed very close to where it should be before fine tuning the trim setting, such that you can glide hands off or with minimal elevator input. This can be very helpful to minimize task saturation when dealing with an emergency, simulated or otherwise.

Now, best glide speed, it should be noted, has a fair margin of error, both positive and negative, in which a change in airspeed will result in minimal change in glide distance. The L/D curve is such that a positive or negative speed variance from best glide speed will give minimal change in lift and drag, generally within a margin of a few knots for an aircraft such as these light Cessnas. For more on this and a visual representation of the L/D curve, see this answer to What is the definition of “best L/D”?. Practically speaking, if you trim full nose up and have the aircraft trimmed hands off to within a few knots of best glide, you are probably doing well. Don't use this as an excuse to be lazy, but don't allow yourself to become fixated either.

I should note that while I have a lot of experience in the 172, I have never flown a 150 0r 152 so I cannot speak to those.


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