Thoughts From the Front Porch

Typical Wing & Propeller

Ever wonder why an airplane wing cross-section looks like a teardrop sliced down the middle?  Wouldn’t a nice sharp edge have less drag? Well, the birds have always known the answers to those questions.  We learned to fly from watching them.  In fact, without the birds as models, it’s doubtful we would have ever realized it possible.

If you stick a flat wooden board out a car window and tilt the front edge up slightly, it will rise due to air ramming into its bottom surface, same as a kite or a surfboard.  Given an engine with tremendous energy and a wide enough plank, one could actually make a flying machine that works that way, but it would be slow, hard to maneuver, and very costly to operate.  And, this ram-air force would be a poor substitute for the aerodynamic lift used by birds to fly so effortlessly.  To achieve true lift, our wing must be something other than a flat surface.

Atmospheric pressure is all around us, shoving everything equally with a force of about fourteen pounds PER SQUARE INCH.  Furthermore, air, unlike water and most other fluids, can be stretched.  Real flight became possible when some true genius finally realized that if you slightly reduce the ambient (surrounding) pressure on one side of something, the pressure on the direct opposite side will push toward the weaker pressure side.  Lay this surface horizontally and, Bingo! we have aerodynamic lift.  But how do we make the air on one side weaker?  We stretch it.

Look closely at a wing.  When the fat front edge is moved rapidly through air,  a curious thing happens.  Some of the oncoming air passes normally under the flat part of the wing and exerts normal ambient pressure, while the rest rolls over the top and slides rearward along the tapered part.  A curved line is longer than a straight line, which means air following this curve will be forced to speed up to try to rejoin the other air passing under the wing.

This in effect stretches the airflow on top, which makes it thinner, which reduces the ambient pressure it would normally exert if it were not moving.  And there we have it; a pressure differential that is capable of lifting almost anything.

While most planes fly slightly nose-high so that the wing benefits a little from ram-pressure underneath, it is theoretically possible to fly with the bottom surface absolutely level.  And, lift increases very quickly the faster you go, as per the Inverse Square Law we discussed recently.  Various wing shapes fly better at different speeds but, generally speaking, if you double the airspeed you get four times the lift, but it also takes about four times the energy to do it.

The real lifting power of a wing is realized when you do a bit of simple arithmetic.      It doesn’t take much pressure differential at all.  For instance, say we have a wing measuring 34′ x 5′, pretty much typical for a small plane. That’s about 170 square feet of wing.

Now let’s say the curvature of said wing generates an average of .05 pounds per square inch difference between its upper and lower airflows.  [Of course a wing will generate various pressure drops over different parts of its upper surface, so we'll just average them out.]  That’s only five one-hundredths of a pound, folks, but it’s per SQUARE INCH.

Doing the numbers, it works out to about seven pounds per square foot of lift, all derived from less than an ounce of difference per square inch.  If we apply this lift to our 170 square foot wing, we find that it can lift about 1200 pounds.  And that’s with the flat part of the wing level.

Tilt the wing up a bit so that ram air adds its force to the bottom pressure while also effectively making the top curvature even deeper, and the lift quickly amplifies.  If we really want to hoist some weight, merely increase the airspeed and the lift will obey the aforementioned Inverse Square Law, thus greatly increasing its lifting power.

Bear in mind the figures I’ve given are for small planes, the kind you see around any country airport, but the principle is the same for multi-engine jets capable of taking off with a weight of more than a million pounds.

Did you know that an airplane propeller is also a wing?  Take a close look at one next time you’re around a small plane.  The blades have the same general cross section as a wing.  In effect, a propeller “lifts” the plane forward through the air.  That kind of power from such a small, thin wing would seem impossible until you realize those blades are moving several hundred miles per hour through the airstream, at a much steeper tilt-angle than the wing itself.  In fact, on some planes the propeller tips actually travel faster than the speed of sound.  Good old Inverse Square Law; where would we be without it?

One thing for sure; we wouldn’t be flying.

Views From Benny Hill is a series by Jerry Smith

Views From Benny Hill