Airfoil selection for a flying plank, how to achieve statical equilibrium?

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oriol

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Thanks so much guys for the very illustrative responses!

I guess that at the end, the only way to tell if a plank can be at least, as good as a primary glider. Is building two RC aircraft with the same wing and see which one flies best.

At the end everything is a compromise; for every pro there is a con. NOw I realize how important is the prelimary design phase is. No formula can cure a bad idea.

The reason for my "obsession" with a flying plank is the will of attemptimg a <70Kg airplane. This is only a divertimento to play with formulas and have some fun learning aerodynamics.

Cheers,

Oriol
 

Norman

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Grand Junction, Colorado
I guess that at the end, the only way to tell if a plank can be at least, as good as a primary glider. Is building two RC aircraft with the same wing and see which one flies best.

That depends on what you mean by "best". A horizontal stabilizer gives you more pitch authority and much higher pitch damping than a plank at the cost of higher parasite drag. Control and stability of a plank can be adequate but that's about it... If you have a wing that is stable and controllable without a tail then adding a fuselage with a long tail arm will increase the controlability and dynamic stability but also vastly increase the parasite drag and reduce the span efficiency a bit (higher induced drag). This is because the fuselage accounts for 40 to 60% of the parasite drag of conventional airplanes and low wing airplanes typically have span efficiency of around 70% because of interference from the fuselage. The attached graph shows the parasite drag with and without a fuselage. Where the parasite line crosses the induced drag line is the best lift to drag ratio of a sailplane or best cruise speed of an airplane. As you can see just adding the parasite drag of a fuselage costs a lot. Best cruise speed is slower and power required is higher. So, all else being equal, the flying wing can be more efficient in cruise at the sacrifice of minimum landing speed* and aerobatics.

Oh yeah... *Since a plank controls pitch by removing camber from the aft part of the wing CLmax is very limited.
 

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Matt K

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I flew the Genesis 2 glider in Condor so that makes me an expert, right?

Due to its flying wing influences, the wing has wonderful performance at low speeds. That is, it has wonderful performance in a straight line at low speeds, because there is not enough rudder authority to coordinate the turns with any decent roll rate. In practice that means, when you catch a thermal, you will have to struggle struggle to center the lift, not only in the initial turn but also for readjustments. it's very frustrating.

this problem can be easily solved -- put the cg more to the front, no problem. This way you have nice directional stability and good rudder authority. Of course this glider is now competitively worthless, because the additional downforce has removed the only advantage which is low wing loading.

Luckily the glider has fantastic performance , and excellent glide ratio, because it is a very advanced design. If you take the tail and put it on the trailing edge, then it's naturally a lower moment arm and higher chord. For this reason, relative to the the other standard class gliders (which are even more fantastic and excellent), the Condor Genesis has relatively terrible performance at high speed, because the additional chord makes the wing a plank.

Well, at least the glider has excellent performance in weak conditions on ridge tasks. This is a great niche! A very niche niche for a very plane plane. Well, at least this one may be easier to trailer or something...?

Disclaimer: None of my observations are consistent what we have known all this time, that flying wings are 100% wing, whereas the other airplanes are only 50% wing or maybe 80% wing tops, and everybody knows that a wing which is 100% wing is just plane better!
 

raytol

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Dec 17, 2021
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I like the flying wing book written by Nickel and Wolfhardt. Goes through everything and offers solutions. Great design manual.
 

WonderousMountain

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Apr 10, 2010
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Bellingham, Wa
Pictured below is Fauvels 14% Tailess reflexed foil.
I am looking at this to figure out stabilizer airfoil for
Stubby Bipe. A flat section is needed to lay Stablets
down on & stow. Which will leave a small footprint
dissembled. No claim as to quick or easy.
R (1)_kindlephoto-3715393.png
My concern first is that Camber appears excessive,
I would be using a .36C movable surface. Vertical
will be styled after Souricette & Skybaby Surface. S
Area horizontl either 12.5 ft^2 or 15. No complaints
regarding Guppy musings, just a change.
 

Tiger Tim

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Thunder Bay
Plenty of CH-700 series airplanes have converted from airfoil tails to symmetrical. You sure you want to go the other way?
 

WonderousMountain

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Bellingham, Wa
Sadly no, the arguement I gave seems kinda lazy and there is not much for backup. Also, switching from symmetric doesn't make it any simpler. Think maybe the stablets could be attached as airfoil extensions stowed, rather than on top.
 
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