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-   -   How to Determine Glider Neutral Point and CG? (http://www.oldrocketforum.com/showthread.php?t=18810)

Rktman 09-06-2020 04:21 PM

How to Determine Glider Neutral Point and CG?
 
1 Attachment(s)
Need some help guys! The biggest problem is not the math involved, but the fact that every article, formula and online calculator I've come across assumes you have rectangular wings and stabs. Mine are ellipsoid. Actually they're half ellipsoid; the TE curves upward at the tip to meet a straight leading edge (imagine the wings of a British Spitfire or Apogee’s Cirrus Breeze glider except that the leading edge is straight). See sketch below.
The stab is the same shape. All my gliders use this planform and I don't want to abandon it for various reasons.

The formulas and calculators all require you to input a tip chord measurement for both the wings and stab. I have no idea what measurement to use, since the LE and TE come to a point at the tip.

I came across an article by Guppy Youngren in a 1980 issue of the MIT Rocket Society Journal that details how to determine NP and CG, and part of his formula includes how to do that for elliptical wings (where both the LE AND TE are curved). Close…but frustratingly my wings have a STRAIGHT leading edge.

So that's the problem I’ve been struggling with. I’m hoping you knowledgeable and experienced glider enthusiasts can help me out on this one.

Ez2cDave 09-06-2020 11:59 PM

Quote:
Originally Posted by Rktman
Need some help guys! The biggest problem is not the math involved, but the fact that every article, formula and online calculator I've come across assumes you have rectangular wings and stabs. Mine are ellipsoid. Actually they're half ellipsoid; the TE curves upward at the tip to meet a straight leading edge (imagine the wings of a British Spitfire or Apogee’s Cirrus Breeze glider except that the leading edge is straight). See sketch below.
The stab is the same shape. All my gliders use this planform and I don't want to abandon it for various reasons.

The formulas and calculators all require you to input a tip chord measurement for both the wings and stab. I have no idea what measurement to use, since the LE and TE come to a point at the tip.

I came across an article by Guppy Youngren in a 1980 issue of the MIT Rocket Society Journal that details how to determine NP and CG, and part of his formula includes how to do that for elliptical wings (where both the LE AND TE are curved). Close…but frustratingly my wings have a STRAIGHT leading edge.

So that's the problem I’ve been struggling with. I’m hoping you knowledgeable and experienced glider enthusiasts can help me out on this one.


Eric,

What if you were to invert the wing planform and add it to the existing planform, creating an "imaginary" elliptical planform, "do the math", and then divide by 2 ?

It MIGHT be necessary to, "theoretically double", the wingspan, also.

Just some theories . . .

Dave F.

GuyNoir 09-07-2020 07:21 AM

This should give you a good start:

https://rcplanes.online/cg_calc.htm

Ez2cDave 09-07-2020 10:51 AM

Quote:
Originally Posted by GuyNoir
This should give you a good start:

https://rcplanes.online/cg_calc.htm


Eric,

This online calculator appears to be more comprehensive . . .

http://holdfastmac.asn.au/technical-articles/supercalc

Dave F.

Rktman 09-07-2020 04:35 PM

Quote:
Originally Posted by Ez2cDave
Eric,

What if you were to invert the wing planform and add it to the existing planform, creating an "imaginary" elliptical planform, "do the math", and then divide by 2 ?

It MIGHT be necessary to, "theoretically double", the wingspan, also.

Just some theories . . .

Dave F.

Seems logical...but I guess math formulas must have a logic all their own. Doubling the platform and dividing by 2 gives me a wing aerodynamic center of 1/8" back from the LE. Way off.

So odd that with all the elliptical winged BGs and RGs and model planes that have been designed, there is no calculator that will take elliptical tip chords into account, only square tip chords.

Ez2cDave 09-07-2020 04:52 PM

Quote:
Originally Posted by Rktman
Seems logical...but I guess math formulas must have a logic all their own. Doubling the platform and dividing by 2 gives me a wing aerodynamic center of 1/8" back from the LE. Way off.

So odd that with all the elliptical winged BGs and RGs and model planes that have been designed, there is no calculator that will take elliptical tip chords into account, on square tip chords.


Eric,

Did you try doubling the span ?

Dave F.

olDave 09-07-2020 08:56 PM

Rktman, I have a more basic qstn----
 
Is there some reason that you are fixated on this particular planform? I mean, do you just like the looks, or did someone steer you onto this as being the "best" way to go? Not trying to be ugly here, just wondering why.

The fabled elliptical wing planform is a somewhat clumsy way to approximate an elliptical spanwise lift distribution to strive for minimum induced drag on a real aircraft. The same spanwise distribution can be achieved with a constant-chord wing planform by varying the airfoil section along the span, or can be quite closely approximated by using a trapezoidal planform with taper ratios of 0.10 to 0.30. Several ways to get there--

And all that theoretical aerodynamic stuff goes right out the window when you are working with model rocketry stuff at very very low Reynolds numbers. A flat plank (no airfoil at all) with a simple planform shape works just as well. Think in terms of insect wings.

Ez2cDave 09-07-2020 09:37 PM

3 Attachment(s)
Guppy Youngren, in the April, 1980 edition of the M.I.T. Rocketry Society Journal, addresses the Neutral Point.

Images below . . .

Dave F.

Rktman 09-08-2020 08:14 AM

Quote:
Originally Posted by Ez2cDave
Eric,

Did you try doubling the span ?

Dave F.

No because that portion of the calculation is to determine the aerodynamic center of the wing, which doesn't change with span (except with a swept wing I think, which mine is not).

Rktman 09-08-2020 08:33 AM

Quote:
Originally Posted by olDave
Is there some reason that you are fixated on this particular planform? I mean, do you just like the looks, or did someone steer you onto this as being the "best" way to go? Not trying to be ugly here, just wondering why.

The fabled elliptical wing planform is a somewhat clumsy way to approximate an elliptical spanwise lift distribution to strive for minimum induced drag on a real aircraft. The same spanwise distribution can be achieved with a constant-chord wing planform by varying the airfoil section along the span, or can be quite closely approximated by using a trapezoidal planform with taper ratios of 0.10 to 0.30. Several ways to get there--

And all that theoretical aerodynamic stuff goes right out the window when you are working with model rocketry stuff at very very low Reynolds numbers. A flat plank (no airfoil at all) with a simple planform shape works just as well. Think in terms of insect wings.

I chose it primarily to reduce tip vortex drag, and because I've constantly encountered comments that the Spitfire's planform was an extremely efficient one for subsonic aircraft (yes, I realize that the Spitfire wing is actually made up of two slightly different ellipses, but I didn't want a carbon copy of it). And I admit I like the looks of a Spitfire-type wing and it's efficiency.

It's beginning to look like it's more trouble than it's worth though, and I may just abandon my shape and use a truly elliptical planform, or give in and use a plain rectangular planform with square tip chords, which all the online calculators appear to support.


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