Messrs. Bernoulli and Newton have compounded theories which have caused airmen no need of trouble. These scholars have each expressed laws, without which, airplanes would not fly. However, at a certain point their theories crash head-on with each other--and airplanes don't fly.

According to the Bernoulli theorem, when moving air strikes an obstruction which splits the parcel of air, both parcels go around the obstruction, arriving at the other side at the same time. Even though it is farther around one side than the other.

Obviously to make this possible, one parcel of air has to speed up in order to travel the greater distance in the same elapsed time. Now, says Mr. Bernoulli, the faster-moving parcel of air has stretched itself, presto, it is thinner. It exerts less pressure upon a nearby solid surface than its slower moving counterpart!

If you have ever looked closely at an airplane wing, the bottom side of the wing is flat. The top side is rounded into a curve. It is a greater distance from the leading edge of the wing to trailing edge of the wing, if you go the high road, over the top, than if you go the low road, straight across the bottom.

 

Part of the lift of an airfoil comes from the fact that the parcel of air is split by the wing moving through it. Half goes underneath, straight from the leading edge to trailing edge. Half goes over the top, clinging smoothly to the upper surface, travels farther. Therefore though both parcels are pressing against the surface of the wing, the bottom parcel is pressing with, for example, five pounds pressure, while the faster-moving top parcel is pressing with only two pounds pressure, creating a three-pound pressure differential straight up.

Simple, isn't it? You have lift. Additional lift is created by the angle of attack of the wing. That is, it is striking the air at an angle, pointed upward and is deflected, according to part of Mr. Newton's theory, in an upward direction. You have more lift!

So our airplane flies. That is, it flies when sufficient flying speed into this wind is reached. Without sufficient movement to create Mr. Bernoulli's kind of lift, and Mr. Newton's brand of pressure, the wing stalls, the plane falls.

Strangely enough, however, it is not always too little speed that causes the airplane to stall. Sometimes too much speed will do the same trick, much to the amazement of some unwary pilot. He falls even though he has plenty of flying speed, and he wonders why.

This is the very dangerous and deadly high-speed stall, which is now accounting for a great many of our serious accidents.

It is the direct clash between the Bernoulli theorem and Newton's law of motion that causes the high-speed stall. Let me say now that it does not happen in straight and level flying, nor during reasonable flying maneuvers.

The high-speed stall occurs when some sudden and abrupt change in direction of the aircraft is brought about by the pilot. Generally, in a high-speed, steeply-banked turn, or in a pull-out after a dive or in competing aerobatics.

Here is what happens:

The air is moving smoothly at sufficient flying speed around both sides of the wing, creating lift. Suddenly the pilot changes direction of the airplane. Wham! Mr. Newton's law of motion says the object (the airplane) will tend to continue in the same line of direction. The wing then presents its big, flat surface, to the air.

Even though traveling at high speed, it "mushes" through the air and causes a "burble" on the upper or back side of the wing instead of a free flow of air. Mr. Bernoulli quits lifting and the airplane stalls at high speed!

If you are moving your hand sideways or knifed through water at a high speed, the water flows smoothly around the streamlined edges. Now, without slowing down the hand, suddenly turn the flat side of your hand in the direction of travel. What happens? The water churns and boils behind your hand as it moves through the water.

That is exactly what happens to the air flowing around an airplane wing, when the wing suddenly and abruptly changes direction. The air burbles. It does not lift. The airplane crashes.

Of all the accidents which occur to aircraft, there is less excuse for the high-speed stall than any other. For the high-speed stall is the direct result of some maneuver the pilot shouldn't be doing anyhow.

A smoothly-operated airplane at sufficient flying speed but no more, makes normal turns and banks, gliding gradually and climbing gently, will fly and will not fall. "Always maintain flying speed" --but--"slow your airplane down if traveling at an excessively high rate of speed before attempting any abrupt maneuver." You'll live longer.

A good pilot doesn't fly by the seat of his pants, but according to the theory of flight. At least old pilots do.

On the Airlines
Here's an interesting fact: a recent survey shows that one-fourth of the passengers flying United Airlines were taking their first airplane ride. Nearly one-third of them were women. Forty-eight percent were over 40 years old. Forty-two percent had incomes under $5,000. Seventy-five percent were on business and twenty-three percent were flying for pleasure.

Warsaw Daily Times Fri. Nov. 7, 1947

Back | Next