Ye Olde Rocket Forum

Go Back   Ye Olde Rocket Forum > Work Bench > Rocket Boosted Gliders
User Name
Password
Auctions Register FAQ Members List Calendar Today's Posts Search Mark Forums Read


Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 09-06-2020, 05:21 PM
Rktman's Avatar
Rktman Rktman is offline
Mad for modrocs
 
Join Date: May 2019
Location: Chapel Hill, NC
Posts: 62
Default How to Determine Glider Neutral Point and CG?

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.
Attached Thumbnails
Click image for larger version

Name:  Icarus layout top view.jpg
Views: 18
Size:  26.4 KB  
__________________
“I would like to die on Mars. Just not on impact”. — Elon Musk

“Minds are like parachutes--they only function when open”. —Thomas Dewar
Reply With Quote
  #2  
Old 09-07-2020, 12:59 AM
Ez2cDave's Avatar
Ez2cDave Ez2cDave is offline
Master Modeler
 
Join Date: Mar 2005
Posts: 736
Default

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.
__________________
"Failure Is Not An Option . . . Adapt & Overcome ! "

NAR # 26128 L2
TRIPOLI # 517 L2 ( inactive, at present )
Reply With Quote
  #3  
Old 09-07-2020, 08:21 AM
GuyNoir's Avatar
GuyNoir GuyNoir is offline
NAR # 19250 - Life Member
 
Join Date: Sep 2008
Location: Woodstock, IL
Posts: 409
Default

This should give you a good start:

https://rcplanes.online/cg_calc.htm
__________________
A dark night in a city that knows how to keep its secrets. But high above the quiet streets on the 12th floor of the Acme Building, one man is still trying to find the answers to life's persistent questions. Guy Noir, Private Eye.
Reply With Quote
  #4  
Old 09-07-2020, 11:51 AM
Ez2cDave's Avatar
Ez2cDave Ez2cDave is offline
Master Modeler
 
Join Date: Mar 2005
Posts: 736
Default

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.
__________________
"Failure Is Not An Option . . . Adapt & Overcome ! "

NAR # 26128 L2
TRIPOLI # 517 L2 ( inactive, at present )
Reply With Quote
  #5  
Old 09-07-2020, 05:35 PM
Rktman's Avatar
Rktman Rktman is offline
Mad for modrocs
 
Join Date: May 2019
Location: Chapel Hill, NC
Posts: 62
Default

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.
__________________
“I would like to die on Mars. Just not on impact”. — Elon Musk

“Minds are like parachutes--they only function when open”. —Thomas Dewar

Last edited by Rktman : 09-08-2020 at 09:38 AM.
Reply With Quote
  #6  
Old 09-07-2020, 05:52 PM
Ez2cDave's Avatar
Ez2cDave Ez2cDave is offline
Master Modeler
 
Join Date: Mar 2005
Posts: 736
Default

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.
__________________
"Failure Is Not An Option . . . Adapt & Overcome ! "

NAR # 26128 L2
TRIPOLI # 517 L2 ( inactive, at present )
Reply With Quote
  #7  
Old 09-07-2020, 09:56 PM
olDave olDave is offline
Craftsman
 
Join Date: Sep 2015
Posts: 102
Default 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.
__________________
NAR 20602
used to be "powderburner" in another life
Reply With Quote
  #8  
Old 09-07-2020, 10:37 PM
Ez2cDave's Avatar
Ez2cDave Ez2cDave is offline
Master Modeler
 
Join Date: Mar 2005
Posts: 736
Default

Guppy Youngren, in the April, 1980 edition of the M.I.T. Rocketry Society Journal, addresses the Neutral Point.

Images below . . .

Dave F.
Attached Thumbnails
Click image for larger version

Name:  NP-1.JPG
Views: 11
Size:  148.6 KB  Click image for larger version

Name:  NP-2.JPG
Views: 11
Size:  135.3 KB  Click image for larger version

Name:  NP-3.JPG
Views: 11
Size:  166.0 KB  
__________________
"Failure Is Not An Option . . . Adapt & Overcome ! "

NAR # 26128 L2
TRIPOLI # 517 L2 ( inactive, at present )
Reply With Quote
  #9  
Old 09-08-2020, 09:14 AM
Rktman's Avatar
Rktman Rktman is offline
Mad for modrocs
 
Join Date: May 2019
Location: Chapel Hill, NC
Posts: 62
Default

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).
__________________
“I would like to die on Mars. Just not on impact”. — Elon Musk

“Minds are like parachutes--they only function when open”. —Thomas Dewar
Reply With Quote
  #10  
Old 09-08-2020, 09:33 AM
Rktman's Avatar
Rktman Rktman is offline
Mad for modrocs
 
Join Date: May 2019
Location: Chapel Hill, NC
Posts: 62
Default

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.
__________________
“I would like to die on Mars. Just not on impact”. — Elon Musk

“Minds are like parachutes--they only function when open”. —Thomas Dewar
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump



All times are GMT -5. The time now is 11:46 PM.


Powered by: vBulletin Version 3.0.7
Copyright ©2000 - 2020, Jelsoft Enterprises Ltd.
Ye Olde Rocket Shoppe © 1998-2020