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Help Please on CG for Extra strange anomoly

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Help Please on CG for Extra strange anomoly

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Old 12-19-2018, 07:39 PM
  #1  
coleboom
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Default Help Please on CG for Extra strange anomoly

I purchased this plane a couple of wears ago. It had a ZDZ 40 on it and I sold it to a buddy, who got the engine running but not well... He put the 2 NHIM 4 cells up front... he took it out flying several times but just didn't like the way it flew very well.. The last time this plane was airborn I flew it... I remember thinking it didn't fly so bad but not really any specifics.

Later I bought it back from him.... I have since installed a 55 cc engine on front... before started the engine change out process I found where it balanced Fuselage only, and marked the spot. By the time I took out the big muffler and moved the batteries back I had the same balance point as before...
Wanting to make sure. I found instructions to find the MAC. My buddy and I marked this point measured the 30 percent of the cord at the mac. and proceeded to lift the plane...

IT is nose heavy like crazy.... It just about balances ON the mac mark we put on using this method

https://www.airfieldmodels.com/infor...P6_FUsjwswfU-E





Notice where the Triangles of covering converge at the Fus... that is just about where it balances now. this is about 2 inches back from where the predicted CG is
Here is a pic of the bottom the forward most mark just in back of where the red starts is the predicted CG the long black broken line is still in front of where it actually banances by maybe an inch or so.

we put 2 123 blocks on top of the engine mount 4 lbs and 12 oz... That made it balance... This is a 12 lb airplane..


Any input? I believe that this plane was tail heavy by a lot... but it flew well before.. I am reluctant to put 5 lbs on the front of a 12 lb airplane...

Thinking about adding maybe a lb or so and then try flying it... I am just dumbfounded Is it possible that this plane flies great with the sg THAT Aft?

Last edited by coleboom; 12-19-2018 at 08:02 PM.
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Old 12-19-2018, 07:42 PM
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Here are a couple of pics.. as you can see the tail section is quite beefy... but I find no weight (lead) to remove and again 12 lbs with engine and batteries unless I ad 4 to get this cg




h

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Old 12-20-2018, 08:57 AM
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Where that triangle graphic meets on the wings meet some the fuselage would be a safe CG to test fly and then adjust to what you find after flying the airplane. My first guess on identification was Colombo Anderson but due to being all wood and built up changes the origin. It could be a Chip Hyde ARF. He did a bit of a copy of the CA Extra.
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Old 12-21-2018, 06:00 PM
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I'm with SRT. If you are flying something similar now, you should be able to handle the plane fine no matter which of the lines is correct, as long as you are balanced somewhere between them. You may like the way it handles, you may not, but you'll know a lot more about it once it's been flown.

If something like this seems a bit beyond your abilities/confidence level, do you have somebody you could ask to make this test flight for you?
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Old 12-22-2018, 10:59 AM
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Originally Posted by coleboom View Post
I purchased this plane a couple of wears ago.
.....
IT is nose heavy like crazy.... It just about balances ON the mac mark we put on using this method
....
Any input? I believe that this plane was tail heavy by a lot... but it flew well before.. I am reluctant to put 5 lbs on the front of a 12 lb airplane...

Thinking about adding maybe a lb or so and then try flying it... I am just dumbfounded Is it possible that this plane flies great with the sg THAT Aft?
I believe that the calculations on the link you have posted are not correct.
I would use the on-line calculators shown in the stcky thread by the top of this forum.
Additional ones:
https://rcplanes.online/cg_calc.htm

http://chrusion.com/BJ7/SuperCalc7.html

http://www.theampeer.org/cg/cg.html

I also recommend a Vanessa rig for finding the actual location of the CG:

Vanessa c.g. Rig: How to.

Also, if the incidences of wing and tail are way off, the plane could still fly with a CG not per calculations.
Please, see this old thread:
My plane has up trim in it, should I make it more tail heavy?


Last edited by Lnewqban; 12-22-2018 at 11:21 AM.
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Old 12-22-2018, 11:41 AM
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You said the plane flys well, take it up do an in flight C/G test. Do a 45 deg. upline pull power to about 1/2 roll inverted and see what the plane does. If it pulls upward prob. tail heavy, stays 45 deg. prob. just a little tail heavy, pulls down quickly, too nose heavy. What you want is just a slow downward arc. The airplane wants what it wants regardless of what the calcs. say. That's why we test fly. Of course it also depends on your style of flying. you might want it slightly tail heavy or nose heavy.Just adjust till your happy with it.
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Old 12-25-2018, 06:51 AM
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The issue could be that you are checking the CG with the plane upright by balancing it on that spot on the wings. Many aerobatic planes are top heavy when you lift them that way, which will throw off your measurements significantly. I had a Sukhoi that I balanced that way. It would never quite settle level, but I measured at the spot where it would teeter back and forth. It turned out to be scary tail heavy, almost costing me the plane. I then built a Vanessa CG balancer and found that I was 7% further aft than I thought I was. I rebalanced it using the Vanessa rig, and the plane flew beautifully.
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