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Unknown c.g for two planes

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Old 11-14-2018, 06:17 AM
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riadh
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Default Unknown c.g for two planes

I have two planes that I need to balance and determine the c.g
1- 70 inch high wing trainer that goes by the name TRI 60 by Super Flying Model . Their website does not give any details for this plane and they list it as discontinued..
2- 72 inch wing span scale Thunderbolt of unknown maker . The plane looks like almost all other thunderbolts .grey in color with checkered black and white cowl .
I have read that a good start would be to use the point 1/3 wing cord from the leading edge . Assuming that this would be a good approximation do I need to include the control surfaces (flaps ,ailerons) in the measurement of the wing cord ?
Thanks .
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Old 11-14-2018, 09:10 AM
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Always include the control surfaces in any calculations you do for any of the flying surfaces.

If you want to err on the side of caution I'd say more like 28 to 30% instead of 1/3 which would be 33%. But that doesn't work unless you find the Mean Aerodynamic Chord for a wing such as the somewhat odd shaped P-47 wing shape. If you went with even 28% of the root chord the shape is somewhat swept forward due to the trailing edge shape.

There's two things you can do. First is to run both models through an online CG calculator like THIS ONE. For the trainer it likely has a constant chord wing. So simple as pie. For the P-47 to get it more accurate I'd split the semi elliptical wing into two parts with an inner and outer trapezoid shapes that closely approximate the elliptical outline. Then input the data for these two closely equivalent panels into the "two panel" wing option you can link too from that page.

Another option for the P-47 is to look up what folks used for the balance point location for other brands and sizes of P-47. The shape is the same so the % of the chord to the balance will be the same regardless of size.
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Old 11-21-2018, 03:42 PM
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Sorry for the late feedback. Yes thanks a lot for the information. Went along with 30% of chord for the trainer which proved to be spot on . I have not tried to fly the thunderbolt yet and I am looking for other makes to check what they recommend for the c.g. I have noticed that it is actually different from one make to the other although having the same wingspan .
I will investigate further .
Thanks once again.
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Old 11-22-2018, 04:27 AM
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From a set of plans for an old Ziroli design (55 inch wing), the CG is right at the 28% mark of the Root rib.

Being a little nose heavy for first flights is a whole lot better than being a little tail heavy.
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Old 11-22-2018, 05:43 AM
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I would start with seeing how the planes balance on the wing spar for both planes and weight them to balance out there. at worst, with no added weight, this might result with a slightly nose heavy plane ,....way better than tail heavy,.....you never want tail heavy. fly them, trim the planes out for level flight, then take a look at the trimmed position of the elevator and balance the plane accordingly. IOW, if you needed a bunch of up trim at the elevator for level flight, the balance is nose heavy and vise-verse. although it is good to know a static balance point for obvious reasons, the truth is in the testing.....it's all about how the plane flies "around" it's C/G. starting out balanced on the spar is always a safe bet because the wing spar is there to carry the main weight and stresses of the plane in flight and the control surfaces work to maneuver the plane that point.
another factor is that a high wing plane will have the G/G slightly farther back and a low wing plane will have the C/G slightly farther forward to compensate for the drag of the wing's position in relation to the C/G. this is where adjusting the balance by the amount of trim the plane needed in initial flight comes into play.
another method would be to balance the plane decidedly nose heavy, fly it and then adjust the balance to eliminate the trim input needed to attain level flight and good (even) up and down control input response.
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Old 11-22-2018, 08:06 AM
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I would like at add that one should also know where the wing and stab incidences are prior to a maiden flight. The elevator trim is certainly affected by CG but also by incidence and engine thrustline. Knowing the starting point on everything really helps sort out the airplane during the first few flights.
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Old 11-22-2018, 09:38 PM
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good point speedracer, after read your post, I remembered my buddy's dad shimming the leading edge of the wing up on one of his planes he just built, because the because the elevator was showing some down-trim after the initial flight. I agree it's good to know what the incidence should be before that first flight, yet,....most planes will fly as long as incidence is close to what it should be and you really don't know if the plane's incidence is right, even if you have it set to the plan, until you make that first flight,,,,,and then,...even if it is initially right and the plane shows it could be changed,.... it is what it is..... and changing it (deviating from spec, or maybe actually getting to spec) is warranted if flight shows it needs to be adjusted.
but bottom line you are right, incidence should be considered when making trim changes because that is sometimes all it takes.. I have a t-tail mid-wing "Rivits" looking sport flyer that was flying fine, but the tail looked slightly low all the time and a small shim on the leading edge of the horizontal stab ( a screw-on mounting at the top of the vertical stab on this plane) is all it took to bring the tail up nicely. the plane looked kind if silly flying around with the tail feathers slightly sagging,...sort of "lazy looking", until I gave the elevator a little down trim. after it was on the ground, three thicknesses of business card under the leading edge of the horizontal stab and a linkage adjustment back to neutral, is all it took to bring the tail up where it should be.
I didn't think that was too bad for a first shot at a t-tail plane !. it is a plan blow up to .60 size scratch build from a "Sumpin Else" 1/2A pylon plane with a few other mods..... I moved the wing up to shoulder height, used a simple Clark-Y airfoil on it with no dihedral and thinned out the expanded fuselage a bit, to make it look a little better proportionally (the plan expansion made the fuselage look fat for a plane that size). I built it to be my "second plane" and an aileron trainer after re-learning on a "Spadett" trainer, when I started back flying after being away for some 45 years.
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