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Help Finding CG on a F-20

Old 10-28-2013, 02:48 AM
  #1  
CHELLIE
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Build Review Help Finding CG on a F-20

Hi Everyone Know what, Tapered wings have always thrown me for a loop trying to find the CG, i have used CG calculators before, only to have the CG way off and tail heavy, I Dont know if its me not using them right, or the CG calculators not being correct here is what I have. I am looking for a 25% MAC, Thank You In Advance, Chellie

http://adamone.rchomepage.com/cg_calc.htm

Main Wing

Wing Root Cord (A) 16"
Wing Tip Cord (B) 5"
Wing Sweep Distance (S) 11"
Wing Half Span (Y) 16.75"

Stabilizer

Stab Root Cord (AA) 8.75"
Stab Tip Cord (BB) 3"
Stab Sweep Distance (SS) 5.75"
Stab Half Span (YY) 10"

Distance from Both Leading edges (D) 20.50"

I Entered a Static margin of 15%

I am thinking that Maybe 6" Back from the LE will give me approx 25% Root Cord / MAC from the LE, But I am not Sure

Last edited by CHELLIE; 10-28-2013 at 03:57 AM.
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Old 10-28-2013, 03:02 AM
  #2  
earthsciteach
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Hey, Chellie. The F-20 is pretty much a modified delta wing with a tail, right? As a SWAG, find the centroid of the wing planform. It should be a bit back from 1/3 the distance from the trailing edge forward to the vertex of the triangle that is the wing.

Look at that point as the center of lift. The cog should be forward of that by a couple of % of chord. If I am babbling, let me know and I'll try to do a better job of explaining.
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Old 10-28-2013, 03:11 AM
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CHELLIE
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Originally Posted by earthsciteach View Post
Hey, Chellie. The F-20 is pretty much a modified delta wing with a tail, right? As a SWAG, find the centroid of the wing planform. It should be a bit back from 1/3 the distance from the trailing edge forward to the vertex of the triangle that is the wing.

Look at that point as the center of lift. The cog should be forward of that by a couple of % of chord. If I am babbling, let me know and I'll try to do a better job of explaining.
Hi I get what your saying My Plane also has a Flat bottom, that I am trying to take into consideration, that why a 25% mac or 25% back from the leading edge, to make it a tad nose heavy, better a little nose heavy than tail heavy but like i said before, tapered wings throw me for a loop big time
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Old 10-28-2013, 03:14 AM
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Originally Posted by CHELLIE View Post
Hi Everyone Know what, Tapered wings have always thrown me for a loop trying to find the CG, i have used CG calculators before, only to have the CG way off and tail heavy, I Dont know if its me not using them right, or the CG calculators not being correct here is what I have. I am looking for a 25% MAC, Thank You In Advance, Chellie

http://adamone.rchomepage.com/cg_calc.htm

Main Wing

Wing Root Cord (A) 16"
Wing Tip Cord (B) 5"
Wing Sweep Distance (S) 11"
Wing Half Span (Y) 16.75"

Stabilizer

Stab Root Cord (AA) 8.75"
Stab Tip Cord (BB) 3"
Stab Sweep Distance (SS) 5.75"
Stab Half Span (YY) 10"

Distance from Both Leading edges (D) 20.50"

I Entered a Static margin of 15%

I am thinking that Maybe 6" Back from the LE will give me approx 25% Root Cord from the LE, But I am not Sure
Actually closer to 7" back from LE of wing root. (6.86" according to the calc you listed) At 6" you'll be close to 22% static margin (a little high according to your link). You'll know when it gets in the air.....
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Old 10-28-2013, 04:20 AM
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flying wing/delta starting CG is 15% MAC.
but the F-20 isn't even close to a flying wing.

Count the area of the fuselage from TE of the wing forward to the intakes for the engines as part of the wing. The fuselage of the F-20 contributes to the aircraft's lifting surface. If you don't include that block of area forward of the wings a part of the lifting surface the CG will be too far back using the normal 30% MAC for a conventional aircraft design.
This is potentially going to push the CG almost up to where the wing LE meets the fuselage.

Make a smaller "chuck glider" (overhead and profile view flat plates) and test the CG. Apply about 3 deg of up elevator. Where that glides passably will be acceptable for test flying the RC model.
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Old 10-28-2013, 04:57 AM
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Originally Posted by fhhuber View Post
flying wing/delta starting CG is 15% MAC.
but the F-20 isn't even close to a flying wing.

Count the area of the fuselage from TE of the wing forward to the intakes for the engines as part of the wing. The fuselage of the F-20 contributes to the aircraft's lifting surface. If you don't include that block of area forward of the wings a part of the lifting surface the CG will be too far back using the normal 30% MAC for a conventional aircraft design.
This is potentially going to push the CG almost up to where the wing LE meets the fuselage.

Make a smaller "chuck glider" (overhead and profile view flat plates) and test the CG. Apply about 3 deg of up elevator. Where that glides passably will be acceptable for test flying the RC model.
Great idea I think a profile test glider will keep me safe, Thank You, Chellie
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Old 10-28-2013, 05:05 AM
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This pic might give me a little idea of to where the CG should be at or close to it.

http://www.rcuniverse.com/forum/elec...igershark.html

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Old 10-28-2013, 02:42 PM
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Originally Posted by CHELLIE View Post
Hi Everyone Know what, Tapered wings have always thrown me for a loop trying to find the CG, i have used CG calculators before, only to have the CG way off and tail heavy, I Dont know if its me not using them right, or the CG calculators not being correct here is what I have. I am looking for a 25% MAC, Thank You In Advance, Chellie

http://adamone.rchomepage.com/cg_calc.htm

Main Wing.............
That particular calculator ALWAYS results in a pitch sensitive CG in my experience. I would start with 25% MAC as the most rearward CG, maybe 20% for the maiden unless the WL is very high. "Nose heavy flies poorly, tail heavy flies once!"

Great looking model BTW.
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Old 10-28-2013, 10:51 PM
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CHELLIE
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Originally Posted by pmullen503 View Post
That particular calculator ALWAYS results in a pitch sensitive CG in my experience. I would start with 25% MAC as the most rearward CG, maybe 20% for the maiden unless the WL is very high. "Nose heavy flies poorly, tail heavy flies once!"

Great looking model BTW.
Thank you for the reply, Thats what i kind of figured that the Calculator is not correct, and giving me a tail heavy plane, I think i might have to make a chuck glider and see where the cg needs to be at, Take care, Chellie
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Old 11-19-2013, 03:33 PM
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fhhuber is right. Those CG calculators are really good for fairly "conventional" designs, but as soon as you add in funky factors (like parts of the fuselage that act as lifting bodies, "wake interference" from canards that are "too" close to the main wing, conventional tailplane that is "too" close to main wing so that it gets the main wing's "wake", etc.) they can be wildly inaccurate. I've done a couple of scratch builds that were considerably off the beaten path and the only way I could get the CG right was through trial and error - the CG calculator was just plain misleading.
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Old 11-19-2013, 03:48 PM
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Originally Posted by CHELLIE View Post
This pic might give me a little idea of to where the CG should be at or close to it.

http://www.rcuniverse.com/forum/elec...igershark.html

That is one smart F20 !!

Nigel
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Old 11-19-2013, 04:03 PM
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My calc gives safe recc'd as 7.63" and max rearward as 8.23" back from LE joint to fuselage ...

But to be honest - I don't trust many of these calcs !!

Nigel
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Old 11-19-2013, 07:33 PM
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Originally Posted by solentlife View Post
My calc gives safe recc'd as 7.63" and max rearward as 8.23" back from LE joint to fuselage ...

But to be honest - I don't trust many of these calcs !!

Nigel
The calc is only as good as the info its given.

When the fuselage becomes a significant contributor of lift then you have to figure out what portion of the fuselage to put into the equations. Most CG calculator can't deal with the strange shapes and you can do better by drawing the top view on graph paper and counting squares to get the correct proportion of squares ahead of CG vs total squares for the style of wing and stabilizers.
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Old 11-19-2013, 07:47 PM
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Originally Posted by fhhuber View Post
The calc is only as good as the info its given.

When the fuselage becomes a significant contributor of lift then you have to figure out what portion of the fuselage to put into the equations. Most CG calculator can't deal with the strange shapes and you can do better by drawing the top view on graph paper and counting squares to get the correct proportion of squares ahead of CG vs total squares for the style of wing and stabilizers.
Very true ... when I rebuilt the smashed up F16 pal gave me - imagine the trouble getting that CoG with such a lifting body.

But amazingly the calc I used was not far out. So I used the same one here.

I can't upload it here as it's a spreadsheet and the PC I'm on is a crappy Win8 Company job.

Nigel
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