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Airplane trimming - climbs on full throttle, falls on partly throttle

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Airplane trimming - climbs on full throttle, falls on partly throttle

Old 05-25-2021, 11:34 PM
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MrRover75
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Default Airplane trimming - climbs on full throttle, falls on partly throttle

Hi all, and greetings from Norway!

Pretty new to this forum, but have been building and flying since the mid -90`s, mostly with that times conventional .40 size glow models built from kits. Have had a longer "break" for 14 years now and are starting to get many of my old models back up in the air together with a few new ones.
I recently got hold of a TT Sport Flyer 40L ARF which was put together with an OS 46FX and had its maiden flight a few days ago. This model has a bad tendency to really climb/ pulls its nose up on full throttle and I had to apply a good amount of down elevator trim to get a level flight. On the other hand, the nose drops pretty much when pulling the throttle back to 1/2 - 1/3 and more up elevator trim are needed to get a level flight. I have to admit that this tendency is seen on most of my models to a degree, but not as much as on this model.
So then my question: What is the best way to trim away this tendency? The CG is set spot on where the manual states it should be. My first thought is that more down thrust to the engine is needed to compensate for this. Am I right, or does the CG also affects this tendency? I tried to add Throttle -> elevator trim to the radio, but it does not work that smooth as there is always some "lag" between the throttle output and throttle position, making the model "jump" when pulling down from full throttle before achieving level flight. Not to happy with this model at the moment, but guess the bad tendency should be solved by proper trimming? Appreciate all input on this issue
Old 05-26-2021, 03:46 AM
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BarracudaHockey
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Yep, try a washer under the upper engine mount bolts between the firewall and the engine mount
Old 05-26-2021, 10:59 AM
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scale only 4 me
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I'd put an incidence meter on it also,, see if you have some extreme difference between the wing an tail

That said,,, you can probably add some throttle/elevator mixing to help minimize the dipping and climbing

good luck
Old 05-26-2021, 07:03 PM
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speedracerntrixie
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Odds are that it is too nose heavy and the up trim used to compensate is creating the issue.
Old 05-26-2021, 08:33 PM
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There are several advanced methods for fully trimming a plane (I like the "Peter Goldsmith" method). You can google it to get all the details. But for a quick, rough setup:

1. Fly at about 3/4 throttle or above. Trim it so it stays straight and level.
2. Pull and climb at a 45 deg angle. Roll to inverted in the climb. Do NOT touch elevator or throttle. While upside down, if it pitches down, it is nose heavy. If up, tail heavy. Repeat the test several times to verify the behavior. A bit nose heavy is fine. Serious 3D guys will want it to neither climb or dive in this test. Adjust CG, and test again, until you get it to just slowly pitch nose down.
3. Now try your high/low speed testing. If it still behaves as you noted, shim the engine to give more down thrust.
4. If that helps, test for CG again. The change in thrust can mask the CG effect a bit, so reverify and correct again if needed. If you did have to correct, try your fast/slow test again.

Purists will note that some of what you describe is normal, since speed generates lift, and one then has to trim to compensate. But what you describe seems excessive. Hence the shims.
If you have to go more than a couple of washers, as advised above, check the relation of the incidence of the wing to that of the tail. Something there may be way off.

Last edited by tedsander; 05-26-2021 at 08:37 PM.
Old 05-27-2021, 12:05 AM
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MrRover75
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Originally Posted by tedsander View Post
There are several advanced methods for fully trimming a plane (I like the "Peter Goldsmith" method). You can google it to get all the details. But for a quick, rough setup:

1. Fly at about 3/4 throttle or above. Trim it so it stays straight and level.
2. Pull and climb at a 45 deg angle. Roll to inverted in the climb. Do NOT touch elevator or throttle. While upside down, if it pitches down, it is nose heavy. If up, tail heavy. Repeat the test several times to verify the behavior. A bit nose heavy is fine. Serious 3D guys will want it to neither climb or dive in this test. Adjust CG, and test again, until you get it to just slowly pitch nose down.
3. Now try your high/low speed testing. If it still behaves as you noted, shim the engine to give more down thrust.
4. If that helps, test for CG again. The change in thrust can mask the CG effect a bit, so reverify and correct again if needed. If you did have to correct, try your fast/slow test again.

Purists will note that some of what you describe is normal, since speed generates lift, and one then has to trim to compensate. But what you describe seems excessive. Hence the shims.
If you have to go more than a couple of washers, as advised above, check the relation of the incidence of the wing to that of the tail. Something there may be way off.
Thanks a lot! This seems to be a very reasonable approach to this issue. I will do some checking and trimming at the next opportunity and report back

Just posting a picture of this "classic" It was also available as a kit from Global models back then if I remember correctly:



Old 05-28-2021, 07:40 PM
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Desertlakesflying
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That wing alone is going to cause taht
Old 05-29-2021, 05:47 AM
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speedracerntrixie
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Originally Posted by Desertlakesflying View Post
That wing alone is going to cause taht



Unless you are aware that it has a flat bottom airfoil I don't see how you could make that statement from the limited information in the thread.
Old Yesterday, 02:27 AM
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MrRover75
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Hi,

Have not had time to look into this issue until now. I removed the cowl and engine and used a digital angel measurement device and checked the motor mount against the horizontal stab. 0degree difference between those two, so this indicates that this model is not set up with any downthrust at all from the factory. I will make some tappered shims to put under the engine mounting lugs, giving it approx 2 degree of downthrust. Think that will more or less solve the issue.
Old Yesterday, 03:53 AM
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speedracerntrixie
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Maybe, you need to consider CG, wing incidence and airfoil type as well. All these things influence pitch trim at any given speed.
Old Yesterday, 04:45 AM
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Originally Posted by MrRover75 View Post
Hi,

Have not had time to look into this issue until now. I removed the cowl and engine and used a digital angel measurement device and checked the motor mount against the horizontal stab. 0degree difference between those two, so this indicates that this model is not set up with any downthrust at all from the factory. I will make some tappered shims to put under the engine mounting lugs, giving it approx 2 degree of downthrust. Think that will more or less solve the issue.
Check your CG FIRST by flight testing, before assuming the factory made a setup error.

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