Mar 14, 2007, 01:54 AM
Registered User
Livermore, CA
Joined Sep 2004
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Discussion
Cord Line / Thrust line

I've addressed it on some of my other threads and thought I'd post the pic for others to see.
As you can see, the Cord Line goes from the very tip, to the very end of the wing cord. For a medium to fast Plane or Wing, the motor shaft is best if its in line with this invisable line. If the motor is above it and a pusher, it will tend to push the nose down and the visa-versa with the motor is below this line.
So if the motor is above this line, you can raise the back of the motor just a bit, to compensate for it being above the Cord Line. That pushes down on the rear of the wing just a bit, to keep the nose from being pushed down. Butch

# Images

Last edited by Butch777; Mar 14, 2007 at 02:47 AM.
Mar 14, 2007, 01:05 PM
Joined Aug 2006
2,422 Posts
That's only exactly true in limited circumstances. Ideally the thrust line should pass through both the center of mass and the center of drag. If your plane has a symmetrical airfoil and no dihedral then both of those points may indeed be on the chord line. Cambered airfoil sections always have their aerodynamic center (and thus the center of drag) above the chord. If the thrust line misses one of these points it will produce a moment (torque) equal to the offset distance times the thrust. This moment is why you have trim changes with power settings. Depending on which point is producing the greater moment the effect on the plane will be different. If the moment is produced by the AC it will always be present while power is on. The moment about the CG is only present during acceleration. Since you can trim out the moment about the AC it's better to have the thrust line go through the center of mass if you can only hit one.

# Images

 Feb 07, 2008, 09:48 AM Registered User Kingsland, Texas USA Joined Aug 2000 2,466 Posts Norm, do you have an estimation of the needed downthrust?
Feb 07, 2008, 12:13 PM
Joined Aug 2006
2,422 Posts
Sorry, no, no estimate but a method. Loop some string around the propelor blade roots and hang it up. The string line will point at the center of mass so if the string is parallel to the motor shaft the thrust line goes through the center of mass. If the thrust line passes above the center of mass the plane will pitch down during acceleration This is true with the motor at either end of the airplane. The angle of up or down thrust depends on how high and how far away the motor is from the CG. Say your motor is 1/2” above the trailing edge and 8” back from the CG. With a fairly low CG you would need 4.5 degrees of down thrust to avoid having the nose pitch down during acceleration but things like fuselages and landing gear move the center of mass down a bit so every plane is different.

--Norm

# Images

Last edited by nmasters; Feb 07, 2008 at 12:59 PM.
 Feb 07, 2008, 01:17 PM Team Cobra!!! Ssssssss! United States, NE, Doniphan Joined Feb 2005 7,919 Posts Cool, I like the "method". Very true, although, I never thought of doing it quite like that. It should be quite helpful for me later on. Nice tip. Laine. Latest blog entry: We have wingtips...
 Feb 07, 2008, 02:02 PM resU deretsigeR Dublin Ca Joined Aug 2004 7,787 Posts I agree great explination, will have to test it on a few of my dialed wings to see the visual and to test my calibrated eye.. But it will definatly take some of the guess work out.. Thanks
 Feb 07, 2008, 02:40 PM internet gadfly Colorado Joined Aug 2006 2,422 Posts You're welcome. I wish I could say it was my idea but it wasn't. I ran across a verbal description on the 3D forum. I do think my sketch makes it a lot easier to understand though. One word of caution though. If your 'wing weighs more than 60lbs you could damage the thrust bearings so make a dummy motor with a hole where the shaft fits on the propelor. Viola! Now your plane's a fancy plumbob.
 Oct 13, 2011, 01:02 PM Registered User United States, NY, New York Joined Dec 2009 561 Posts thanks for the information. we go technical
Dec 15, 2013, 04:11 PM
Registered User
Joined Oct 2010
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Quote:
 Originally Posted by nmasters You're welcome. I wish I could say it was my idea but it wasn't. I ran across a verbal description on the 3D forum. I do think my sketch makes it a lot easier to understand though. One word of caution though. If your 'wing weighs more than 60lbs you could damage the thrust bearings so make a dummy motor with a hole where the shaft fits on the propelor. Viola! Now your plane's a fancy plumbob.
Hi, since you are quite familiar with this issue i would like to ask a question......
On this plane with the default line the plane when i give throttle it pitches down and needs constant trim up to fly level (Blue LINE)
In your opinion in order to achieve a better situation which way do you think i should change the angle? Towards the red line or the green line?
Thanks

# Images

Dec 15, 2013, 09:31 PM
Joined Aug 2006
2,422 Posts
Quote:
 Originally Posted by exteng In your opinion in order to achieve a better situation which way do you think i should change the angle? Towards the red line or the green line? Thanks
Neither. Trim change with power settings with pylon mounted engines and low CG can only be fixed with the horizontal stabilizer. If you could move the horizontal stabilizer up into the propwash it would trim itself but that model looks too flimsy to just move it without doing some significant strengthening to the tail boom and the vertical fin.

--Norm

Dec 16, 2013, 12:17 AM
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Joined Oct 2010
82 Posts
Quote:
 Originally Posted by nmasters Neither. Trim change with power settings with pylon mounted engines and low CG can only be fixed with the horizontal stabilizer. If you could move the horizontal stabilizer up into the propwash it would trim itself but that model looks too flimsy to just move it without doing some significant strengthening to the tail boom and the vertical fin. --Norm
What if i change the motor angle like on the second of the drawings......Then without having to change the horizontal stabilizer the prop wash will hit the stabilizer (causing trim up) and act as trim up.......or not?
 Dec 16, 2013, 01:29 AM internet gadfly Colorado Joined Aug 2006 2,422 Posts Draw what you're thinking of on paper and measure the angle between the thrust line and the chord of the stabilizer. If it's more then 8 or 9 degrees your tail will be stalled and your plane won't fly at all. The way to fix this is to make the thrust line parallel to the wing chord (or pretty close (less than 8 degrees)) and raise the stab up to get it into the propwash. You can strengthen your aft fuselage with tape. Aluminum tape is available from many hardware stores and not only will it stiffen up the tail better than other tapes but it looks good if you don't wrinkle it
Dec 16, 2013, 02:38 AM
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Joined Oct 2010
82 Posts
Quote:
 Originally Posted by nmasters Draw what you're thinking of on paper and measure the angle between the thrust line and the chord of the stabilizer. If it's more then 8 or 9 degrees your tail will be stalled and your plane won't fly at all. The way to fix this is to make the thrust line parallel to the wing chord (or pretty close (less than 8 degrees)) and raise the stab up to get it into the propwash. You can strengthen your aft fuselage with tape. Aluminum tape is available from many hardware stores and not only will it stiffen up the tail better than other tapes but it looks good if you don't wrinkle it
I was thinking of this or at least put ther thrust line in parallel to the wing chord.......
What will hapen if i try the one on the drawing or put the thrust line parallel to the wing chord?

# Images

Dec 16, 2013, 03:12 PM
Joined Aug 2006
2,422 Posts
Quote:
 Originally Posted by exteng On this plane with the default line the plane when i give throttle it pitches down
It is normal for airplanes with high thrust lines to pitch down during acceleration.
Quote:
 and needs constant trim up to fly level
This is also normal. Moving the H-stab into the propwash just helps to make the trim adjustment automatic. If you put the stab in the propwash make sure that the its span is greater than the diameter of the propeller. If you feel that you don't have enough pitch authority under full power you may need a bigger stabilizer. Try making the stabilizer you have a bit bigger where it is before you totally redesign the empenage. Also put a turbulator strip on the bottom of the stabilizer.
Dec 16, 2013, 04:03 PM
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Joined Oct 2010
82 Posts
Quote:
 Originally Posted by nmasters It is normal for airplanes with high thrust lines to pitch down during acceleration. This is also normal. Moving the H-stab into the propwash just helps to make the trim adjustment automatic. If you put the stab in the propwash make sure that the its span is greater than the diameter of the propeller. If you feel that you don't have enough pitch authority under full power you may need a bigger stabilizer. Try making the stabilizer you have a bit bigger where it is before you totally redesign the empenage. Also put a turbulator strip on the bottom of the stabilizer.
Ok i did not get it......
Check out the first pic and the second one: With the new angle didnt i just put the H stab into the prop wash ? Or not? Cause i didn't understand it very well.......
In the second scenario how do you think the plane will behave?.....