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Horizontal Stab Qusetion

Old 01-12-2008, 04:12 PM
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ElectricMoose
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Default Horizontal Stab Qusetion

I recently designed and built an airplane that is loosely based on a Cessna 140 and after several flights, of which only one was successful, I started to look for and repair minor design flaws on the airplane. The most recent one I have found is when I rig the airplane on my Vanessa rig for level flight I noticed the horizontal stabilizer is 3 degrees below being level. So my question, is the stab supposed to be level with the wing or is it normal for it to be a fews degrees off from the wing? I looked at the 3 views that I drew the plane from but it is really hard to tell on that particular picture. Any help will be much appreciated.
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Old 01-12-2008, 04:27 PM
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Murocflyer
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Yes, typically it needs to be on the same line at the wing. Although slight corrections can be made with the elevator.

This is sometimes called angle of horizontal incidence although I heard that might be a misnomer.

Frank
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Old 01-12-2008, 05:46 PM
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Murocflyer
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After a little more research, I found that the relationship of the horizontal to the wing is also called dacalage.

You can read more about it here. I think that is the misnomer I was thinking about.

http://www.fatlion.com/sailplanes/decalage.html

Frank

Last edited by Murocflyer; 01-12-2008 at 09:43 PM. Reason: Grammar
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Old 01-12-2008, 06:26 PM
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Sky Sharkster
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Default Wing And Tail Angle?

Hello ElectricMoose, Welcome to Wattflyer!
Frank has provided a good link, that should help with the trimming. Generally there is a slight difference of angles between the wing and horizontal stabilizer, correctly called "Decalage". If you drew a datum line along the side of the fuselage, either the wing trailing edge would be lower or the stab trailing edge would be higher. Either will result in the aircraft climbing slightly under power.
Each separate angle (ie; the angle of the wing relative to the datum line, or the stab relative to the datum line) is an "Angle of incidence".
The resulting angle that the aircraft will fly at (faces the airflow, or relative to the ground) is called the "Angle of Attack".
So, if the model has 2-3 degrees of decalage and is flying incorrectly, there may be another problem. The most common culprit is center of gravity, this would result in the model (if the C.G. is too far rearward) being unstable, easy to stall and having a climb-and-dive (porpoising) flight path.
If you post a description of the flight problem, we may be able to narrow it down further. It is possible that the decalage is too low or high for that particular airfoil or design.
Good Luck!
Ron
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Old 01-12-2008, 07:34 PM
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ElectricMoose
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In the beginning there were many problems the biggest being "CG". After that catastrophe is when I researched the Vanessa rig and built one. Once the CG was fixed I was able to fly a little better but the ailerons gave little to no affect when they were used and it seemed as though the aircraft wanted to climb until it stalled out. The main wing has 3 degrees of up incidence from aft to the forward of the wing and the horizontal stab has 3 degrees of down incidence from aft to forward on it. The las flight before I tore it down to parade rest for a complete overhaul the flight were completely uncontrollable and it was slow to respond to any input other than elevator to which it would either shoot straight up or down depending on what input I gave it. After the final crash I inspected it and found that the elevator had a lot of play on one side and the rudder was incredibly loose as well. I'm not sure if this happened during the flight or not but I'm positive it had at least some affect on the flight. I'm now rebuilding using monocote hinges to try and get a tighter fit of these surfaces and that is when I noticed the angle difference. All parts of the plane were drawn from 3 views so I would assume that the angles are correct, but I have been wrong a lot in the past.
Thanks for the link Murocflyer I'm going to go there and do some research of my own on this subject.
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Old 01-12-2008, 08:17 PM
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After reading the information given at the link Murocflyer provided I went out and re-measured the wing and the stab and discovered that if I were to put the two parallel it would resemble this ,< , kinda like a pac-man mouth. If I read and understood the information correctly I think this could be one of the main culprits in my airplanes wacky flight characteristics. It seems as though the stab was pushing the aft of the plane down which caused the main wing to have an increased incidence and want to climb dramatically. I think I may have figured it out but please let me know if I may have missed something.
Again thank you for all your help.
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Old 01-12-2008, 08:27 PM
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pd1
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ElectricMoose, It looks like you've got it.
The set up you describe will cause the plane to climb, a lot.

For simplicity use Ron's method, draw a line down the fuselage for a reference.

Make the horizontal stab parallel to that line.
Make the wing also parallel to the line and make the thrust slightly down.

This should get you in the ball park.
You might need to adjust the wing a little, but you should be a lot closer.

The plane should trim for level flight at a power setting of your choice.
You might have to adjust wing incidence or CG for that.

Good luck,

Paul
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