Very low aspect ratio planes?

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rotax618

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This is a sketch of one of my more successful models, I found that at high alpha, the vortex would “break” on the forward centre LE as the angle of sweep was less, this caused the nose to lower giving stability. The double delta planform with the inboard LE sweep greater than the outboard is unstable, its great for fighter aircraft where neutral or negative longitudinal is required for radical manoeuvres but not so good in a homebuilt, as I pointer out before the SAAB Draken and its ilk exhibited an irrecoverable deep stall because of the double delta shape. I believe ther was a mod to the Dyke Delta to lessen the forward sweep.
1C9C8654-5705-428D-A941-4641ED6B3AAF.jpeg
 

Mavigogun

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... I found that at high alpha, the vortex would “break” on the forward centre LE as the angle of sweep was less, this caused the nose to lower giving stability.
Cool- essentially performing the same role as washout. Barnaby remarked that much (all?) of the FMX4‘s washout was unnecessary, and is absent in the FMX5/Talon Topper.
 

rotax618

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The way I see it, the secret to success with LAR is low wing loading. LAR aircraft, depending on planform can be very stable, can exhibit no stall break and can be spin resistant, the trade off is a higher induced drag. This increased induced drag can be alleviated by keeping the wing loading low, fortunately most LARs by their planform geometry have a large wing area for a given span.
 

berridos

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Do you have a source with plans and data on the Facet, for example to compare the wingloading, crosssectional cuts, of the Facet?
 

berridos

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The aerodynamic center on nurfluegler isnt on the quarter chord. Its nonlinear and lies at the center behind the quarter chord and at the tips in front. Pretty hard to caculate and identify the CG.
 
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berridos

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Ways to check it is to fabricate a catapult and launch it upside and downside adjusting ailerons for horizontal flight and the cross over of the curve is the neutral point.
On a car roof how should one hang up the model in order to find the neutral point?
 

Sockmonkey

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For clarity...
Thanks, but I'm thinking even more extreme. Like a very stretched oval. Tip vortexes would be crazy, but if the span is tiny enough compared to the diameter of the vortexes, wouldn't they meet each other in the center and cancel each other out?
On further thought, maybe some scalloping along the length in the form of little vortex generators so the lift distribution evens out fore and aft.
 

berridos

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vortex generators as far as i understand create turbulent air, not whirls. Eddys are orderly and create centrifugal forces increase velocity and therefor reduce pressure.
The elimination of washout is allowed in vortex lift according to previous comments (double delta), however in cruise, washout is the only feature that compensates for the moment that is created between the Cg and the neutralpoint in LAR, in case you are not using a simetrical profile.
If a simetrical foil is used, than the neutral point will be stable and the profile moment has not to be compensated by washout.
 
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Himat

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Stall speed doesn't give you a wing area unless you're constrained to one particular airfoil, or airfoil and flap combination.

There may be a graph helpful to you in one of Hoerner's books. Given your level of enthusiasm, I recommend both of them to you if they're within your budget and you don't have to pack light when moving.

Once you've got a fuselage involved, things may get a bit complicated for really short spans, especially if you take advantage of the long chords involved by burying things in the wing.
Wing area do not not quite, but pretty close set the stall speed. How much flap lift that is available depend pretty much on how complex the design is. There are differences in how much flap lift that is possible with different plan forms, but again complexity and weight set the limit.

There is also a trade of for high lift wing sections and the Reynolds number may limit the selection.

Edit, changed wording to be more pecice.
 
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cluttonfred

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OK, guys, let's not go too crazy. Yes, flaps matter, and yes, airfoils change, but if you are designing to LSA standards then your maximum 45 kt stall speed is *flaps up* and the real world performance of most common, conventional airfoils is pretty similar in terms of CL. Within those constraints, span + stall speed + weight *does* give you an approximate wing area. :p
 

Himat

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For a given span, lift, and speed, and equivalent lift distribution, the wing area doesn't affect actual induced drag, although the Cdi changes. Remember that you have to multiply by wing area to get the actual drag. Or, at least, that's what the theory we use for moderate and high aspect ratio wings that are straight says. Try the calculations and see what I mean.
Well, if we go back to:

Cdi = Cl^2/(pi*aspect ratio* efficiency factor)

Evaluated at constant span and equal speed the formula for induced drag can be rearranged to:

Cdi = k*Cl^2*AR

The induced drag is proportional to the lift coefficient squared. Then for a given span, lift and speed the induced drag is linked to wing area due to the lift coefficient. And as the wing area shrink with increasing aspect ratio the necessary lift coefficient rises, but linear.

The sum up is that both high span loading and high wing loading is bad for induced drag. There is three possible ways to reduce induced drag:

  • Increase wing span.
  • Increase wing area.
  • Fly faster.
 

Jimstix

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Using a vortex to increase the Cl requires high angle of attack and a big increase in power. Most light planes and all LSA’s are short of extra horsepower, so taking advantage of vortex lift is an invitation to visit the back side of the power curve.
 

rotax618

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It seems difficult for some to understand that at an AR around 2 and lower, there is no stall in the conventional sense and airfoil selection has very little to do with minimum speed, the three factors that determine an LARs minimum speed are planform, wing loading and available thrust.
 
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