Re: objects on wing tips

From: (Paul Raveling)
Organization: Unify Corporation (Sacramento)
Date:         11 Dec 92 17:42:33 PST
References:   1 2 3 4
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In article <airliners.1992.104@ohare.Chicago.COM>, (Ramin Dowlati) writes:
> I have a few questions for any of you passenger airplane gurus.
> Several years ago, the aeropspace industry introduced vertical
> flap-like things on the ends of their airplane wings. I've
> only noticed these on 'larger' planes such Airbus, 747-400 and
> MD-11.
> Q1. What is the technical name for these flap-like things?

	Winglets.  The most common name usage was "Whitcomb winglets"
	until they became fairly common.  Many would say that Whitcomb
	was their inventor, but watching the tip feathers of large
	soaring birds suggests that Mother Nature should get some credit.

> Q2. Are they mobile or fixed?


> Q3. Do they only serve to stabilize the flight?

	No.  Their purpose is to reduce induced drag, which they do
	by reducing circulation in wingtip vortices.

> Q4. Why haven't they appeared on smaller aircraft?

	They have.  In fact recent times have brought some controversy
	to competitive soaring, about whether winglets should count
	for measuring sailplane wingspan.  This application uses winglets
	on airframes that weigh a few hundred pounds and carry one person,
	sometimes with about 1/4 inch of headroom for a pilot who's
	already laid out almost flat on his [or her] back.

> Q5. The ones I saw on the Airbus were shaped like a 'V'
>     and symmetric with the wing tip, ie. one leg of the 'V'
>     was above the wing and the other pointed below the wing.
>     Whereas the ones on the 747-400 looked like extensions
>     of the actually wing, but bent 90 degrees upwards.
>     Why the difference?

	The 'V' form sounds like the classic Whitcomb design.  Sailplanes
	don't use the downward-pointing winglet because ground clearance
	at the wingtips won't allow it.  The same might be true of many
	airliners, with variations.  This is just a guess:  Many need
	clearance under the wings for servicing vehicles, such as fuel
	trucks.  Having "hanging" winglets would increase the rate of
	ground damage.  Some might also have a ground clearance problem
	for landing with an engine out, where designers usually plan
	for the certification limit of 5 degrees of bank (plus rudder
	of course) to compensate for asymmetric thrust.  Finally,
	the extra winglet might require enough extra structure to
	negate most of the aerodynamic benefit it would produce.

Paul Raveling