wide wingspan and good lift to drag ratios

[Tony]

Why is it gliders have such long slender wings? Would longer chords
give as effective lift to drag ratios at typical sailplane speeds, is
the wingspan just a more effective way of finding small thermals? My
first thought was a long wing would induce more drag at the many edges,
but clearly that's wrong.

[john smith]

In article om>,
"Tony" > wrote:

> Why is it gliders have such long slender wings? Would longer chords
> give as effective lift to drag ratios at typical sailplane speeds, is
> the wingspan just a more effective way of finding small thermals? My
> first thought was a long wing would induce more drag at the many edges,
> but clearly that's wrong.

What you are referring to is aspect ratio, narrow cord and long wings.

[Jose]

> Why is it gliders have such long slender wings? Would longer chords
> give as effective lift to drag ratios at typical sailplane speeds, is
> the wingspan just a more effective way of finding small thermals? My
> first thought was a long wing would induce more drag at the many edges,
> but clearly that's wrong.

Here's a picturesque way to look at it (which is sure to generate some
heat): the wing works by throwing air down. This forces air up
elsewhere, and the airplane rides on the updraft while it's throwing the
air back down. Other people may say the same thing as "the wing causes
high pressure beneath, and low pressure above... and a vortex is
formed..."; it's the same thing ultimately.

But, the air from below that is trying to get above can either do so by
scurrying out in =front= of the wing as an updraft (where the wing will
meet it and ride the wave), or out the side (making wingtip vortices,
where the energy used to squeeze the air is then lost as far as the wing
is concerned). Only the updrafts in front of the wing help the wing
stay up. The wingtip vortices are lost energy.

So, the longer the span, the greater the ratio of "useful" updraft to
"useless" updraft.

Jose
--
Money: what you need when you run out of brains.
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[Tony]

I suspected most "lift" or at least delta momentum was generated near
the leading edge, so having a long chord didn't contribute much at low
airspeeds.

I do appreciate the effects of the burble or vortex at the wing ends
and the advantages of winglets, at least at higher air speeds.

[Orval Fairbairn]

In article . com>,
"Tony" > wrote:

> I suspected most "lift" or at least delta momentum was generated near
> the leading edge, so having a long chord didn't contribute much at low
> airspeeds.
>
> I do appreciate the effects of the burble or vortex at the wing ends
> and the advantages of winglets, at least at higher air speeds.


High aspect ratio reduces induced drag, which helps aircraft flying at
slow indicated airspeed.

The formula is: CDi = CL**2/(pi*A*e), where:

CDi = induced drag coefficient
CL = lift coefficient
pi = 3.141759..........
A = aspect ratio = wingspan**2/wing area
e = wing shape efficiency (elliptical lift distribution is best;
constant chord is least)

[Tony]

If I'm reading this right induced lift for constant area is inversely
proportional to span, and goes up linearly with area.

At more or less constant chord, area is linear with span, so the idea
would be to have chord go down as span goes up. I'd guess e starts
changing as the wing grows too slender.

That pretty much gives me what I wanted to know. I'd guess higher order
terms have to come into play with increasing airspeed.

Thanks

Tony

[Orval Fairbairn]

In article . com>,
"Tony" > wrote:

> If I'm reading this right induced lift for constant area is inversely
> proportional to span, and goes up linearly with area.
>
> At more or less constant chord, area is linear with span, so the idea
> would be to have chord go down as span goes up. I'd guess e starts
> changing as the wing grows too slender.
>
> That pretty much gives me what I wanted to know. I'd guess higher order
> terms have to come into play with increasing airspeed.
>
> Thanks
>
> Tony

Only partially correct. Induced drag goes up inversely with the square
of span, and directly with area.

The efficiency factor, e, is a lift distribution (wing shape) factor,
being, theoretically, 1.0 for a wing with elliptical lift distribution
and reducing in magnitude for tapered distributions and constant chord.

Bear in mind, e can be tricked into higher efficiencies by varying the
angle of incidence or changing the airfoil shape from root to tip, so it
*may* not represent an elliptical planform.

Flutter problems may arise with wings that have long spans and narrow
chord, so everything you do is a compromise.