Ask My Sky Mom. Aviation wit and wisdom.

Hey Mom,

I’ve noticed that some airplanes have wing tips that droop a lot lower than other planes that are otherwise exactly the same and I’ve even heard them referred to as “drooped wing tips”.  So what’s up with that?  Why would you do that to your plane?

Thanks,

Feeling Snoopy About the Droopy

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Sup, Snoop?

Well, the answer should be obvious: when your airplane  is parked in a line with a bunch of similar looking airplanes, you’ll need to pick it out of the crowd somehow, right?  Or maybe not.  I guess the N# (tail number, registration number) will help with that little dilemma, too.  The other reason?  The REAL reason?  Well, what do you know about wing tip vortices? Let’s get back to Bernoulli.

Most airplane wings are designed with a higher camber (curve) on the top of the wing than on the bottom of the wing.  The airstream, otherwise known as the relative wind, separates at the leading edge of the wing and rejoins at the trailing edge.  Because the air has farther to travel over the top surface of the wing than it does over the wing’s bottom surface, the air flowing over the top has got to travel faster.  Our buddy Bernoulli worked out that the faster a fluid moves, the lower the pressure will be.  Air behaves like a fluid and so the upshot of all this is that we have lower pressure above the wing than we do below the wing.  Voila!  We have lift, people.

Right.  So, back to the droopy.  We all know that nature abhors a vacuum, even a partial one.  Air can’t help wanting to flow from where the pressure is higher to where the pressure is lower.  When given the choice, don’t we all?  The most efficient way for that pressure to equalize is for the air to come furling up from under the wing, up over the wingtips to where the pressure is lower.  It comes rushing up and around those wing tips creating wing tip vortices.  We’ll chat more about those at some later date-there’s an awful lot more that could (and should!) be said.  Check out this video.  You can see the wingtip vortices in the blowing snow as the airplane lands.

[youtube=http://www.youtube.com/watch?v=hDpyoghjZgE]

The only problem with this whole aerodynamic dance is that, well, the equalizing of pressure equals less lift.  Not good.  Not good at all.  We find it in our best interests to try to keep as much of the relatively high pressure air below the wing as possible for as long as possible to create the pressure differential we prefer.  Drooped wingtips are one method of trying to do this.  Another innovation along the same lines is winglets.  Different looking but same general idea.

Pretty neat when you think about it.  Aren’t you glad the engineers did?  Keep the pressure where it oughta be and keep the shiny side up,

Mom

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You’ve got questions?  I’ll find you an answer.  Email your aviation related question to mom@myskymom.com and check out myskymom.com to read the answers to questions previously posed. The more you know, the safer you’ll be.  Read on, and fly safe!

~ by myskymom on September 2, 2009.