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CG vs weight

squishy

Pirate ParkFlyer
#2
The CG of an airplane is far different than the proper CG for an airplane, try to be specific, because the CG could be anywhere, a proper CG is a specific point that will allow it to fly stable and balanced...it's the difference between flying and not flying...

When the weight is further from the proper CG point it is harder for the airplane to change velocity quickly. The closer the weight is to the proper CG the faster that weight can rotate around that point and change velocity. It is not as important as finding the proper CG for stable flight, as a beginner it is not something to worry very much about. Beginners usually need stable and slower reacting airplanes, not fast acting ones...besides the difference is very slight on most designs...A good rule of thumb is to simply pack all the electronics near the proper CG point, this is about all you can do with most designs as the electronics are the only things you really have an option to move around. Proper CG is almost always achieved by battery placement because it is the heaviest object on the airplane. If you wish to place the battery closer to the proper CG then you must counter balance it with lighter weight objects out on the fringe. So make sure you take into account the mass of the objects, if you place lightweight things near the edges it's not as much of an impact as placing heavier objects there, get it?
 
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earthsciteach

Moderator
Moderator
#3
The center of gravity is the single point at which the plane will balance. Think of it as the weight of the entire plane concentrated at that point. It is the relationship of the location of the cg to the location of the center of lift that is important when designing a plane. The center of lift can be thought of as the "anti-cg," as it is the single location (or line) where the sum of the force of lift can be represented as a single force. The center of gravity should be slightly forward of the center of lift for a stable airplane and this is usually between 1/4 to 1/3 the wing chord (distance from leading edge to trailing edge) of the wing. The center of lift is somewhere closer to 1/2 the wing chord back from the leading edge.

Let me know if you are unsure about any of this!
 

Attachments

#4
I got it. I have a wing that I built. I crashed it and crashed it and crashed it. The nose got messed up so I thought I would rebuild it. After rebuilding in my mentor told me that I have added way too much weight to the front. So I suggested we add more weight to the back and a bigger motor. He said it still would not fly that the CG is all messed up. That it would be better to just build a new one but I feel like I've put my life and soul into building and fixing this one. Not even get to fly it.
 

earthsciteach

Moderator
Moderator
#5
Flying wings are very picky about cg location. You can use this cg locater to determine where the cg should be located based on the layout of the flying wing. Once you know where the cg should be located, you can arrange the components (motor, battery, etc) so that it balances at that point. Its best to start with the cg 15-20% of the mean aerodynamic chord (MAC). The MAC is just the average wing chord length for tapered, swept wings.

Can you describe the problems your wing had?
 
#6
no matter what I did it would crash nose down. when I built it I marked the CG. I thought it was the battery so we moved the battery back and still no help. So after crashing it 1000 times it got messed up. With the new nose even more nose heavy. I think I am going to make a new wing with a electronic plate. IMAG0583.jpg IMAG0584.jpg
 
#7
keeping the electronics close to the cg maybe even stacking them side-by-side over the CG including the servos. That way if I crash a million times I can build a new wing and take off the electronic plate and put on the new wing really easy.
 
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earthsciteach

Moderator
Moderator
#8
From your sketch, it looks like the battery, along with all of the other components, are behind the stop on a KF airfoil wing. Is this correct?

If you post or send my the following info, I can help you with the cg.
1. Wingspan
2. root wing chord
3. tip wing chord
4. angle of wing sweep or the length of a line from the tip of the nose perpendicular to a line drawn from the leading edge wingtip
5. weight of battery, motor, motor mount or model numbers so I can look them up.

With those, I can tell you correct cg location and where to place the components to achieve it. Or, at least get really close.
 

aiidanwings

Senior Member
#9
I got it. I have a wing that I built. I crashed it and crashed it and crashed it. The nose got messed up so I thought I would rebuild it. After rebuilding in my mentor told me that I have added way too much weight to the front. So I suggested we add more weight to the back and a bigger motor. He said it still would not fly that the CG is all messed up. That it would be better to just build a new one but I feel like I've put my life and soul into building and fixing this one. Not even get to fly it.

Don't despair! you have put this energy not into a failure, but a great learning experience. I have built many things in life that didn't work on the first try. but the second, or third, or fourteenth try it did. Failure is giving up, wining is stepping over failure to success.
 
#11
As Thomas Edison once said, “I have not failed. I've just found 10,000 ways that won't work.” Learn what you did wrong on this one and build another.
 
#13
thanks guys. I wish the power pod have the battery and the servos in it. so when you crash the FT Versa Wing that way you can just pull it off and put it on a new FT Versa Wing. I guess I will have to make a all in one power rob that will work on a wing.