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Mar 16, 2003, 09:40 PM
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Help! Can't figure out what to do.


I have built a foam flying wing with a geared GWS 300B mounted on the rear. The first 2 flights ended in crashes. The first was the worst. I could feel what I thought was the front end pulling downward and sure enough the plane went down hard and broke off the front end. I don't know if you can see in the picture I have attached that the motor angle is adjustable by loosening a screw and nut. So after the first flight I jacked the motor up in the rear some. ( It was already tilted up some) Second flight like the first went down but not as hard. It still broke the nose off just not as many pieces this time. I had the balance point at 4.375" inches and then moved it all the way back to 4.875". I also have have some reflex (1/4" to 3/8") on the ailerons. Plus I have jacked the motor shaft up to a high angle. When I hold it up side down at the balance point, the front end pulls upward therefore downward when right side up. It does fly but even with full up trim it still goes downward! I have to hold the stick to keep the plane going up. Am I missing something? More reflex? My CG is at 22% back from the front. I don't go back more, do I? Here are the specs. Sweep is 7.375", 1/2 span is 15.625". root chord is 9" and tip chord is 6" and RTF weight is about 13 ounces. Any suggestions? RCNutt
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Mar 16, 2003, 09:44 PM
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OOps, picture didn't come thru.


I'll try to post it again.
Mar 16, 2003, 09:56 PM
Team 30 Micro EDF
Mike Taylor's Avatar
It doesn't matter if you hold it upright or upside down - the plane should be level when held at the balancee point. You said the tail went down, so it is tail heavy...
Mar 17, 2003, 12:23 AM
Gravity sucks.
mrittinger's Avatar
Flying wings use a different formulae for determining the CG. ...
In an effort to find out if its the power doing it or the CG/trim, toss it with NO POWER first. Get a decent glide before trying to fly with power.
Wings are notoriously finicky about CG, so it might take some tinkering.
The tail going down first does not always mean tail heavy.You may have way too much reflex. Start with 1/8" or so...
Mark
Mar 17, 2003, 12:29 AM
Single-task at best...
tim hooper's Avatar
I'm no expert, but I'd guess that your thrustline is way too high (compared to all the flying wings that I've seen) which will tend to push the nose down no matter what you do adjustment-wise.

tim
Mar 17, 2003, 06:57 AM
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flying wing thrustline


Hi Tim. You may be right about the thrustline being too high. I had thought of that being the cause also. I kinda wanted to put it up a little higher to keep the prop from stricking the ground but this may be causing the problem of the nose pulling downward when the motor runs. I've got room to lower it a 1/2 to 5/8" so I guess I'll try that. mrittinger's idea of glide testing it with the power off is a good idea. The only problem I have is, around here is there are no weeds to throw it into. Plus I think the problem only occurs when the power is on because the balance point has been moved back and forth with little or no change being evident I'll post again after I try lowering the motor. Thanx for responding RCNutt
Mar 17, 2003, 08:53 AM
two suggestions. first if the the plane is pulling down (without motor) and it is not pitch sensitive,take the CG back. on such a swept wing the normal 25% wont work because it is too much forward.
second, when you lower the thrust line (by raising the prop-the thrust line is the line along which the motor+prop create the thrust) it goes beneath the C.G. and creates torque that raises the nose.

hope it helps,
Avi

p.s. as Mark said, start with just gliding it!
Mar 17, 2003, 09:32 AM
Unfortunately you will have to go further behind in the root chord with the CG as the aerodynamic center of the wing is really behind that (25 % of the root coord is a good rule of thumb for constant chord and straight leading edge wings.

Do the next:

1. Find the aerodynamic center of one wing, by (facing the airplane from behind):
1.1 Designing the full scale wing planform (in a larger paper, or whatsoevere you have at end)
1.2 Add the root chord lengt to the top and to lower point of the tip chord (lets call it "new tip line")
1.3 Add the tip chord lenght to the top and to the lower point of the root chord (new root line).
1.4 Join the top of the new tip line with the lower point of hte new chord line.
1.5 Join the top of the new chord line with the lower point of the new tip line
1.5 The intersection point is the aerodynamic center of the wing.

2. Draw the "Mean" Chord and workout the CG

2.1 On the intersection (aerodynamic center of the wing) point, draw a chord line corresponding to that point (that's the wing statistical "mean" chord).
Choose your CG using this "mean" chord - for instance, you can begin with 25% + (static margin) 10% of this mean chord, and draw a perpendicular line that passes throuhg that point, until it reaches the root chord. This intersection point will be your CG on the root chord.


This explanation would be much easier with a sketch, but I can't do it know (and I don't know how to upload it).

PT
Last edited by PTLdom; Mar 17, 2003 at 10:06 AM.
Mar 17, 2003, 01:45 PM
I do appologise for the terrible design work:
Mar 17, 2003, 06:23 PM
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CG


Unfortunately I'll have to glue it back together again before I can try anything. I lowered the motor and tried it again and it went downward again and broke the nose off in several pieces. It is getting really rough looking and heavier I'm sure because of all the glue. I guess I'll glue it back together again and try your method of finding the CG. Thanx for the suggestion. RCNutt


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