Go Back  RCU Forums > RC Airplanes > Tips & Techniques
Reload this Page >

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

Notices
Tips & Techniques Want to share a tip or special technique you have either in the workshop or at the flying field or race track? Post it right here!

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

Old 05-25-2021, 11:34 PM
  #1  
MrRover75
Junior Member
Thread Starter
 
Join Date: May 2021
Posts: 17
Likes: 0
Received 1 Like on 1 Post
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
  #2  
BarracudaHockey
My Feedback: (11)
 
BarracudaHockey's Avatar
 
Join Date: Jul 2003
Location: Jacksonville, FL
Posts: 25,045
Received 133 Likes on 111 Posts
Default

Yep, try a washer under the upper engine mount bolts between the firewall and the engine mount
Old 05-26-2021, 10:59 AM
  #3  
scale only 4 me
My Feedback: (158)
 
scale only 4 me's Avatar
 
Join Date: Jan 2002
Location: Avon Lake, OH
Posts: 10,124
Received 27 Likes on 25 Posts
Default

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
  #4  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 8,233
Received 84 Likes on 80 Posts
Default

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
  #5  
tedsander
 
Join Date: Jul 2004
Location: White Bear lake, MN
Posts: 511
Likes: 0
Received 37 Likes on 35 Posts
Default

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
  #6  
MrRover75
Junior Member
Thread Starter
 
Join Date: May 2021
Posts: 17
Likes: 0
Received 1 Like on 1 Post
Default

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
  #7  
Desertlakesflying
My Feedback: (28)
 
Join Date: Jul 2005
Location: Sun Valley, NV
Posts: 2,840
Received 36 Likes on 32 Posts
Default

That wing alone is going to cause taht
Old 05-29-2021, 05:47 AM
  #8  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 8,233
Received 84 Likes on 80 Posts
Default

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 06-17-2021, 02:27 AM
  #9  
MrRover75
Junior Member
Thread Starter
 
Join Date: May 2021
Posts: 17
Likes: 0
Received 1 Like on 1 Post
Default

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 06-17-2021, 03:53 AM
  #10  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 8,233
Received 84 Likes on 80 Posts
Default

Maybe, you need to consider CG, wing incidence and airfoil type as well. All these things influence pitch trim at any given speed.
Old 06-17-2021, 04:45 AM
  #11  
tedsander
 
Join Date: Jul 2004
Location: White Bear lake, MN
Posts: 511
Likes: 0
Received 37 Likes on 35 Posts
Default

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.
Old 06-21-2021, 01:49 PM
  #12  
jaka54
Member
 
jaka54's Avatar
 
Join Date: Apr 2021
Posts: 47
Likes: 0
Received 1 Like on 1 Post
Default

Hi!
Always strive for as rear forward Cof G as possible and at the same time have as little elevator throw as possible.
And always trim the airplane at full power,flying straight forward!
When you throttle down the plane will of course nose down slightly as speed decreases.
To check for C of G ... at full power, straight ahead, do a thight pylon turn to the left just using aileron and elevator... if the plane will nose down slightly in the turn , the plane is nose heavy and you should alter the C of G slightly rearward!
Old 06-21-2021, 07:32 PM
  #13  
tedsander
 
Join Date: Jul 2004
Location: White Bear lake, MN
Posts: 511
Likes: 0
Received 37 Likes on 35 Posts
Default

Originally Posted by jaka54 View Post
Hi!
Always strive for as rear forward Cof G as possible and at the same time have as little elevator throw as possible.
And always trim the airplane at full power,flying straight forward!
When you throttle down the plane will of course nose down slightly as speed decreases.
To check for C of G ... at full power, straight ahead, do a thight pylon turn to the left just using aileron and elevator... if the plane will nose down slightly in the turn , the plane is nose heavy and you should alter the C of G slightly rearward!
Well...having nose heavy a slight bit is far better than accidentally too far back. The old saw "Nose heavy planes may fly poorly, but tail heavy only fly once" is more true than not. But trimming is an iterative process - squeeze the balloon, and something pops out somewhere. So it can be a lot of try and fly to get everything as tuned in relation to each other as the plane will let you. This plane will also get tail heavy as fuel is burned off. So very fine tuning would mandate that he work toward evaluating CG near the end of the tank. But that is something to worry about after he's reduced the other undesirable behaviors, and not at the beginning of his journey. So a little nose heavy is safer while he works out his main issue of pitch changes with speed variation. And doesn't mask problems by being too "pitchy" that can happen when tail heavy. Or snaps on landing flares...Full on aerobatics - yes, get it back as much as flight testing will allow. But this isn't a full on Pattern/IMAC/3D plane. It's respectable, but not a pure bred. Reduce the issues, then cycle back through everything reducing even more, until its as good as patience allows.
We can disagree about full power S&L for the same reasons - this plane may always have a bit of pitch change with change in speed, due to design. 3/4 throttle will find an initial medium as a baseline to again get to his first goal - see if thrust changes can moderate the problem. Later, he can redo things when starting from full throttle to see if it can be made even better.
Finally - the bank and yank method to check CG can work. I use it with pilots that are still rough on a little flying inverted. But it depends on getting the wings exactly vertical to the ground, and not leading or lagging on application of elevator. Errors there can mask the effect due to the lift vector having a bit of up or down direction. So to really confirm the climb/dive is CG and not pilot induced, it has to be repeated a lot. Preferably both to the right and the left, upwind and downwind. The climb and roll method takes out a lot of that uncertainty. Yes, still has to be repeated to confirm, but nowhere near as much.

Here's a link to one version of the Goldsmith method - he lays out compelling logic about how to take a structured approach to trimming: Goldsmith Trimming Article.pdf
Old 06-22-2021, 03:34 AM
  #14  
jaka54
Member
 
jaka54's Avatar
 
Join Date: Apr 2021
Posts: 47
Likes: 0
Received 1 Like on 1 Post
Default

Hi!
Why I recommended the tight pylon turn as a means of checking the CG is because I have flown pylon racing for many years, and there it is very obvious when a plane is nose heavy as it dives slightly in a 90 degree bank.
Old 06-22-2021, 05:19 AM
  #15  
tedsander
 
Join Date: Jul 2004
Location: White Bear lake, MN
Posts: 511
Likes: 0
Received 37 Likes on 35 Posts
Default

Originally Posted by jaka54 View Post
Hi!
Why I recommended the tight pylon turn as a means of checking the CG is because I have flown pylon racing for many years, and there it is very obvious when a plane is nose heavy as it dives slightly in a 90 degree bank.
Yep, I agree. It can be a very valid way to test. It just has it's own peculiarities that the pilot needs to take into account when using.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright 2021 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.