I was just thinking about building a little somthing to play with, and the first problem I think I would run into is knowing where CG would be. Thinking more I think I remember hearing or reading that if you take the aircraft and hang it level by the rear fuse and forward spinner by a string a center hanging stirng would point at CG. The pic is an example of what im thinking. Red is the string.

Your diagram shows one way to measure where the CG of the model is located; HOWEVER, it does not tell you where the CG should be for stable flight.

I can't tell from your post, but somehow I get the impression that you are designing a plane? I have some simplified formulas for determining the Neutral Point (NP.) CG on a wing/stab plane typically needs to be in front of the NP. The NP is dependent upon many factors such as the MAC (mean aerodynamic chord) of the wing and stab as well as the distance or moment between the MAC of the wing and stab. Unfortunately, the formulas are at home and I'm not home now. If you are building a plane, post accordingly and I'll try to locate the equations when I get home.

Thanks for your reply John. I'm not designing a plane. What I am doing however is taking parts and putting together a plane. I have an old Solo Star wing that I have put onto a duraplane fuse. I have already made the adjustments to the fuse to accept the wing. This wing is much bigger than the wing that the duraplane fuse had. I also moved the landing gear forward to make it a tail dragger. I just want to be sure the CG is correct. It may not be a good flier when I'm done anyway, but I would like to be sure its stable CG wise.

Balance it acording the the wing chord. Where the CG was on the Solo Star (probably right under the main spar...) is the correct CG for your hybrid.

If it is your own design and you have no recommended CG, try to put the CG at 25-30% of the chord.

If the wing you are using is considerably larger than the original one that was on the fuse, you need to be careful as the model may not be spin stabilized in addition to you may have pitch stability issues even if the CG is set to the same location on the wing when it was on the original fuse. This is because the NP can move greatly based on stab design/location and a larger wing typically requires more yaw damping. To give an idea of how much NP/CG movement we are talking about, I have some indoor planes that need their CG at about 20% MAC to fly well, but my 2M pattern ship is balanced at 45% MAC.

I could provide more info on what may or may not need to be adjusted, but it would be easier if I had better info on both planes, such as wing chords, aspect, stab/fin dimensions, moments etc.

I'm sure your bash will probably fly without adjustments, but it's not hard to check all this and you will be rewarded with a good flying plane.

Thanks for your offer to help Jonh. I think what im going to do is use the old CG of the wing which is 3 3/4 from LE. I will attempt it and see how she handles. If it turns out to be unstable I will post what you need to get a CG for the overall plane. My main goal is to slow this dura plane down with this wing. I want to help a friend learn to fly, but I dont want to risk my Corsair.

In another thread there is a link to a CG Calculator. It is on Giestware's Aircraft Proving Grounds website. Here it is.

http://www.geistware.com/rcmodeling/cg_super_calc.htm


I've looked it over and I give it a thumbs up. It does however work kinda backwards for what you need, but it will work. Just insert your data, and pick where you think the CG should be. Check the static margin. Follow the instructions for getting the CG adjusted such that the static margin is in the 5% to 15% range. Keep moving the CG around by typing in different locations until your static margin is in the 5% to 15% range. This then will be your calculated CG. You can then use this CG figure to balance your real plane, i.e. balance your plane where the calculator indicated a CG with a static margin of 5% to 15%.

Thanks for the link John. I will give that a shot.

Here is another site that has a cg calculator you might want to try. http://www.palosrc.com/instructors/cg.htm

The Hybrid flew pretty good using the recommended CG of the wing. The first run it seemed to be a little heavy in the front (take off) I didn't want to move the gear forward so I drooped some weight off of the nose. After that and a little trim she flew like a real ugly trainer. Flew slow which is what I wanted, but also would pull off a decent loop. As far as a roll. Looked more like a cork screw not real tight. I didn't make it for that anyhow. Just wanted a nice slow flier. I will let the Corsair handle the rolls and loops. Just figured I would let ya know how it went. Thanks for the help.

Keith

Glad to hear it worked out okay.
Dennis-