Re: objects on wing tips

From:         greg@saltydog.dpsi.com (Gregory R. Travis)
Organization: Data Parallel Systems, Inc
Date:         10 Dec 92 00:52:11 PST
References:   1 2 3 4
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In <airliners.1992.104@ohare.Chicago.COM> dowlatir@cu1.crl.aecl.ca.crl.aecl.ca (Ramin Dowlati) writes:

>Q1. What is the technical name for these flap-like things?

Winglets.

>Q2. Are they mobile or fixed?

I've always seen them fixed.

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

No.

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

They have.  Look at the new Learjets or, heck, any Cessna after about
1974.


As I understand it, the winglets serve to prevent "spillage" of air
from the high-pressure area under the wing to the low-pressure area
above it.

In flight, there is a considerable difference in air pressure between the
lower side of the wing and the upper side.  Nature, being as it is, finds
this situation abhorrent and constantly tries to find ways to equalize the
air pressure between the two surfaces.  One way of doing this is to allow air
to spill out of the underside of the wing at the tip and curl upward to the
upper side of the wing.  

Unfortunately for the airline bean-counters and aerodynamicists, this
spillage of air creates strong vortices - miniature tornados lying
horizontally along the axis of the fuselage.  The vortices themselves and
the loss of pressure from the lower side of the wing both contribute to drag
and loss of lift & wing efficiency.

The "winglets" prevent the spillage up to the upper wing surface via a
physical wall.  The air escapes from the bottom surface and tries to
curl up to the upper surface and is stopped by the winglet.

There are several different winglet designs.  As I mentioned, Cessna and others
have been doing it for some time with drooped wingtips.  One can
put the barrier so that it is either hanging DOWN (Cessna) from the wing, or
so that it protrudes ABOVE the wing (Boeing) - the net effect is, roughly,
the same.

Likewise, an airplane in ground-effect flies much more efficiently because
the proximity of the wing to the ground prevents the full formation of the
wingtip vorticies and thus air spillage to the top of the wing is greatly
reduced.

The big issue is in constructing a winglet that recovers more lost lift than it
effectively destroys with added drag.

It's interesting that Airbus, I believe, is intending to offer a device which
would trail from each wingtip and employ a propeller of some sort.  The
propeller would hook to a electrical generator and produce power from the
wingtip vortices.

greg

--
Gregory Reed Travis				D  P  S  I
Data Parallel Systems Incorporated   greg@dpsi.com (For MX mailers only!)
Bloomington, IN			     greg@indiana.edu (For the others)