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Design => Stunt design => Topic started by: Larry Renger on February 22, 2011, 09:57:10 AM
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The usual measure of performance potential that is used is Weight/Wing area. Sadly that comparison only works when you compare relatively identical models. There is a better way! mw~
Cubic wing loading takes into account the difference in size and configurations between different designs and model sizes. Ron St.Jean published an article on this in MAN a gazillion years ago, and I have followed up on it myself.
The essence is very simple: k = weight / ( wing area * winspan )
A couple of examples: Alan Brickhayus' Scepter 500 k = 38/(51.4*500) = .00148 Standard wingloading is = 38/500 =.076 oz/sq.in.
Baby Pathfinder k = 10/(35*236) = .00121 Standard wingloading is = 10/236 =.042 oz/sq.in.
Standard wingloading implies that the Baby Pathfinder should fly incredibly better than the Scepter, whereas the performance is really pretty similar. Cubic wingloading gives a much more accurate picture for comparison. :!
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I can't see what the physical significance of that is. I guess it's not as bad as "nose moment". You might consider wing loading / line length. That would give something that's proportional to the min. loop radius as a solid angle.
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I can't see what the physical significance of that is. I guess it's not as bad as "nose moment". You might consider wing loading / line length. That would give something that's proportional to the min. loop radius as a solid angle.
And as I recall we have a perfectly good method of determining scale effects.
I still haven't gotten over the idea that the Baby Pathifinder flies about the same as a Sceptor.
Brett
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The basis for this is that the turning radius is proportional to the size of the model. Also that the speed is proportional to the model. When you start there, the derivation is exact and the formula comes out to W/A3/2. There are a variety of ways to calculate that, but (wing area*wingspan) seems to have the best correlation when considering a variety of designs. If anyone is really interested, I'll dig back through my files and come up with the rigorous derivation and the whole article I wrote.
Ron St.Jean's article only dealt with models of the same design scaled up and down. Mine tried to generalize it to handle multiple designs.
However, reality trumps theory EVERY time! When you put the plane in the air, you find out what really works!
And yes, the Baby Pathfinder will turn a better pattern than the Sceptor, but only under the most ideal weather conditions. That's why you fly the "big ones"! I, personally whupped the big'uns in Phoenix with my .061 Sky Sport a couple of years ago. But I couldn't count on doing it in less than ideal weather.
Also, Brett, I would be fascinated to learn your own scaling techniques! #^
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I am interested. Dig if it's convenient, please.
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Model Airplane News, December 1997, page 78. "3-D Wingloadings: a Better Way to Scale Models"
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Also, Brett, I would be fascinated to learn your own scaling techniques! #^
I think Reynolds numbers are sufficient to predict scale effects.
Brett
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Reynolds numbers will let you sort out differences in airfoil performance, but doesn't deal with the need to live with the "square:cube" factors in making a small model fly in the same manner as a big one.
The key is that the small model should fly the same number of wingspans forward in the same time as the big one, and also turn in the same number of wingspans as the big one. If you only want the little one to fly at the same speed and turning radius as the big model, that is an entirely different criteria.
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Hi Larry,
Just trying to understand this so don't jump down my throat.....
It seems to be that it unduly favours hi aspect ratio wings to the extent you would see some "Very" good figures for a sailplane as an example. It you keep the wing area and weight static and just increase the wingspan (While reducing the chord) the numbers just get better and better. How does it work for Bi/Tri planes? Double/Triple span?
Thanks in advance
TTFN
John.
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Actually, the numbers DO get better and better for sailplanes until you run into reduced Reynolds numbers decreasing the airfoil efficiency.
Another version of the formula is k2 =Weight/wing area3/2 This one is probably better for biplane and triplane comparisons.
The whole problem is so complex that nothing is going to give you an exact answer in the real world. But the cubic wingloading concept is a heck of a lot closer than straight Weight/Wing Area.
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HI Larry,
When I have heard of "cubic" wing loading, it had brought a picture to my mind (never looked into it, there's the problem) that it was taking a number relative to the cubic displacement of the wing. A factor of the airfoil thickness, and wing area (and would be tough for me to measure! LOL!!). In other words a cubic "displacement". I wouldn't even know what math formulas to begin to use for that. What you have shown me is a factor of wing area and wingspan. I wasn't aware.
Bill
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There have been attempts to use Wingspan x Mean Aerodynamic Chord x Airfoil thickness at the MAC . Correlation to actual performance is better with the Area x Wingspan version and much simpler to calculate. Just keep in mind that although theoretically proveable with simple aerodynamics formulas, it leaves out a whole bunch of side effects. As such, it is only a guideline, but a lot better one than simple Weight/Wing Area. :!
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The version that was shown to me he called wing volume.
Wing volume = span X MAC X 1/2 of airfoil thickness.
The 1/2 of airfoil thickness is not always accurate but close enough to do the job...and much simpler.
This is measure of how much 'air' is displaced by the wing and is valid way of comparison.
Have always planed to use this method and never have.. HB~> LL~
David