Very low aspect ratio planes?

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lr27

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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.
To get the ACTUAL induced drag, you multiply the Cdi by the wing area and the dynamic pressure.
 

Himat

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To get the ACTUAL induced drag, you multiply the Cdi by the wing area and the dynamic pressure.
Yes, but

Cl = M * g/(0.5*Rho*V^2*S)

Decrease S, wing area and necessary Cl rises linear with reduced area.
Cdi on the other hand then increase with Cl^2.
That Cl squared term show that decreasing Cl reduce Cdi more than increasing AR. Same for the V, speed term that is squared. Fly faster and induced drag is reduced.
 

Vigilant1

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....the three factors that determine an LARs minimum speed are planform, wing loading and available thrust.
And weight (assuming we want to maintain level flight at "minimum speed.")
 

Speedboat100

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It can bearely sustain itself in the air. Rotation far to early.
Urquiola, are you sure its 2,49m? i read 2,60 or 2,55. Its an important design parameter in order to trailer my delta, and make the wing extension as tiny as posible.
It might need that ground effect phase in order to gain speed.
 

berridos

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That planform has an eliptical lift distribution and still benefits from vortex lift. Pretty optimal in all aspects.
 

Vigilant1

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-- Whatever its other advantages may be, when compared to a similar moderate/high AR design, a low aspect ratio design generally has more drag, and thus requires more thrust, to produce lift required for safe low speed climb.
-- Whatever other advantages it may have, a ducted fan requires more HP to produce the same thrust as a well designed open prop.

So, whatever other advantages it may have, choosing a ducted fan to propel a low AR design would be a decision that might be questionable.
 

berridos

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I agree, i would prefer that design as a tractor for our use, on top reducing the tractor setup the induced drag.
Airbus, obviously opted for a pusher version (see their new development)
Its somehow currently the academic ideal concept.
It fills the central hole in the lift distribution.
 
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BJC

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-- Whatever its other advantages may be, when compared to a similar moderate/high AR design, a low aspect ratio design generally has more drag, and thus requires more thrust, to produce lift required for safe low speed climb.
-- Whatever other advantages it may have, a ducted fan requires more HP to produce the same thrust as a well designed open prop.

So, whatever other advantages it may have, choosing a ducted fan to propel a low AR design would be a decision that might be questionable.
Case in point: Pushy Galore, a relatively low AR pusher, originally had a ducted propeller. The duct was eliminated to make it faster and to increase the RoC.



243167E6-B0BA-4F4F-A669-CF75413FBC28.jpeg

BJC
 

rotax618

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Johan’s aircraft would probably be unstable at high alpha, the straight outer wings will stall while the forward inner section with more sweep would be unstalled. Best to build a model to test the planform first.
 

lr27

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Yes, but

Cl = M * g/(0.5*Rho*V^2*S)

Decrease S, wing area and necessary Cl rises linear with reduced area.
Cdi on the other hand then increase with Cl^2.
That Cl squared term show that decreasing Cl reduce Cdi more than increasing AR. Same for the V, speed term that is squared. Fly faster and induced drag is reduced.
The AR term is one thing. Multiplying by the wing area is another If you pick some example numbers and calculate carefully, you will see that the area drops out and the only dimension left is the span.
 

danmoser

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-- Whatever its other advantages may be, when compared to a similar moderate/high AR design, a low aspect ratio design generally has more drag, and thus requires more thrust, to produce lift required for safe low speed climb.
Such a broad generality can lead to the wrong conclusion.
It's true that the lower aspect ratio design has higher induced drag over the entire speed range, assuming wing loading held constant,.
But note that the difference in total drag is negligible at higher speeds because that is where parasitic drag is dominant, and parasitic drag can actually be lower on the lower aspect ratio design due to two main factors:
1. Internalization of structural elements (struts, wires, etc.) can allow the lower aspect ratio deign to achieve a lower parasitic drag coefficient and therefore lower parasitic drag at higher speeds.
2. Longer chord results in a higher Reynolds number, enabling a lower parasitic drag coefficient.

Also, lower aspect ratio designs can achieve a lower structural weight and greater usable internal volume, especially at the wing root/fuselage junction, which can enable reduced wing area and/or weight for a given payload weight and/or volume.
Or you could use the structural weight savings to install a bigger engine, giving you a higher climb rate & cruise speed.

So a proper low aspect ratio design can give you a light, roomy aircraft that cruises efficiently at high speed, and can do steep approaches, but such potential benefits are not automatic.
Exploiting any of the potential advantages of low aspect designs requires some diligent engineering.
Simply modifying an existing design by lowering its wing aspect ratio won't provide many benefits, if any..
 
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Vigilant1

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So a proper low aspect ratio design can give you a light, roomy aircraft that cruises efficiently at high speed . . .
True, as Dr Wainfan's Facetmobile demonstrates. Do everything right and a low aspect ratio design can give satisfactory cruise performance. Do everything very right by minimizing weight and the climb performance might be comparable to a low- performance acft of "medium" AR.
 

erkki67

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5AFD0111-354A-4C21-B8C7-4B2FCA29B81A.jpeg
as a low aspect ratio bird, this contribution remain as my personal favorite one. The trailing edge might be a bit larger to incorporate 2 Elevons .
if the airfoil would allow a facet style, why not?!

and if not, it doesn’t matter either.
 

Johan Fleischer

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Johan’s aircraft would probably be unstable at high alpha, the straight outer wings will stall while the forward inner section with more sweep would be unstalled. Best to build a model to test the planform first.
Eh, I don't know if it's meant to be an aircraft at all. I found it on a Pinterest page with wing-in-ground crafts, AKA ekranoplans.:eek: :oops: 😄
 

berridos

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Deltas produce the washout with several elevons along the wingspan and deltaas can also have flaps if the inboard flaps are compensated by the outboard flaps with sweep. Also induced lift brakes work fine.
Find here an example of such a mixer.


Wish someone could explain me how this marvel works))
 

Jimstix

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The Convair B-58, F-102, and F-106 all had a type of washout that they called "conical camber" along the leading edges. It is difficult to describe, but easy to see if you stand at the tip and look along the leading edge toward the root. The underside of the leading edge forms the surface of a cone.
 

BJC

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