Powered lift aircraft that do not need to generate large lift coefficient at low speed make great use of LAR wings.Full disclosure – I love goofy, weird airplanes with bookshelfs to prove it. Low-aspect-ratio (LAR) wings just have no real place on non-goofy general aviation airplanes. Sure, there are structural advantages and “look what I built” pride of the builder. However, these are not compelling reasons to design a LAR light airplane.
Reasons you might want to have a LAR airplane include:
My guess is that you won’t be doing those things.
- Launch into space and reentry into the atmosphere (lifting bodies and shuttle orbiter)
- Fit into the bomb bay (XF-85) or on a MIR or TIR (JDAM kitted Mk 82’s)
- Fly faster than the speed of sound (gobs of delta-winged jets)
- You need improved body lift to maneuver (Standard Missile)
The opposite of LAR wings is HAR wings – like sailplanes and (wait for it) helicopters.
We all know that sailplanes are efficient (low induced and profile drag). What about helicopters? Long skinny, twisty, droopy HAR blades work much better than short, wide LAR blades having the same blade area. That’s because a key figure of merit in a helicopter rotor is disk area – not blade area. Helicopters barely fly with those HAR blades and will not work with the short, wide LAR blades.
I don’t wish to be a Debbie Downer on LAR wings, I just can’t find a use for them on light airplanes. Jimstix
Can you qualify that understanding by citing a reference? As I recall, the entire sharp leading edge conspires to spawn stable vortices within a set of conditions- not just the outboard/tip sections.According to my basic understanding of vortex lift, only the tip sections are producing vortex lift and therefor should be sharpened (Dyke Delta).
Yes. If you preclude using the advantage of having a high aspect ratio, then a high aspect ratio doesn't look so good. Similarly, if you require a long span, a low aspect ratio doesn't look so good.Is really HAR more efficient?
When people say a high aspect ratio wing is more efficient, they usually or often fall back and cite the classic induced drag coefficient:
Cdi = Cl^2/(pi*aspect ratio* efficiency factor)
And skip two underlying assumptions:
That is, wings with the same area are compared at a fixed speed. Now, instead of keeping area constant, keep the wing span constant. You will find that with fixed wing span and evaluated at the same speed, the Cdi rises with increasing aspect ratio. Then what do that tell us about aspect ratio and efficiency?
- Constant wing area.
- Evaluated at the same speed.
Within the speed range where most sport aircraft operate, there is no advantage to a high AR. There is an induced drag advantage to a greater span, which, coupled with a constant area, also produces a disadvantagous reduction in Reynolds number.If you preclude using the advantage of having a high aspect ratio, then a high aspect ratio doesn't look so good.