AERODYNAMICS
Boys and girls, do you know that we are all surrounded? Well we are! We are surrounded all the time we are living on earth – by air pressure. All of us, suffers every day by plowing through this sea of air pressure at the rate of 14.7 pounds per square inch on each square inch of our bodies.Why don’t we get squashed you ask? Simply because the air pressure inside of us is the same as outside.
But wait! This air pressure can be used to our advantage. What for? To fly of course. How many of you have flown on an airplane? How many have seen a story on TV or the movies about airplanes?
Notice that there are four forces acting on an airplane:
Weight: As the airplane sits at the gate loading passengers you may have noticed that the attendants outside the airplane are loading fuel and luggage on the plane. Also, more attendants are loading food back in the galley kitchen (only on some flights ). All of this adds up to the total weight of the airplane and may amount to several thousand pounds. All of this weight is directed down toward the ground. How are we going to get this weight up in the air? Why, by the next two forces Lift and Thrust.
Thrust: As the airplane is backed away from the gate the pilot will start the engines. These may consist of two or four jet or propeller engines. These engines are very powerful, and will be more than five times the horsepower of an ordinary car engine. Jet engines work by injecting outside air in the front of the engine, adding fuel, and ejecting the air out the back. The thrust is caused by the hot air going out the back pushing the plane forward. A propeller operates by the engine driving the propeller around a shaft. The propeller blade is not flat, but is set at an angle so that the wind the propeller causes drives the plane forward. This is very similar to the way a cooling fan blows air.
Lift: Now the airplane has taxied to the beginning of the main runway. The pilot revs up the engines to maximum power and releases the brake. The airplane starts to roll down the runway faster and faster. When the pilot thinks the speed is fast enough, he will say “rotate”. What does this mean? When the pilot says “rotate” he pulls back on the control wheel and the nose of the airplane goes up. When this happens a peculiar thing happens. Notice that this wing section is more curved on the top than the bottom. So that when the wing is inclined like this shows, the air pressure on the bottom of the wing begins to increase. And since the curve on top of the wing is more than the bottom, the air slides over it faster which decreases the pressure. This difference in pressure causes the airplane to go up! That’s right, all of that weight on the airplane is being lifted up by that simple difference in air pressure! Doesn’t that make you feel a lot safer?
Drag: Oh, by the way, there is another force on the airplane we forgot to mention. This force is called drag and it is caused by the friction acting on the skin of the airplane, the shape of the airplane, and the airplane speed. This force is working against the thrust and the lift to try to drag the airplane down. Does this explain why airplane surfaces are smooth and streamlined? As the airplane speed increases this force becomes greater and greater until the airplane approaches the speed of sound it will become almost too great. This is why airplanes the travel very fast have to be designed a lot differently than those that travel slower. How many here have seen a film called “The Aviator”? In that picture Howard Hughes designed a plane that had polished surfaces and all the rivets heads were flush (even with) the surface. Why did he do that?
Now we are at 30,000 feet and the pilot announces that we are at cruising altitude. The plane levels off and seems to slow down.
What happened to the four forces at “cruising” altitude? Did the plane really slow down?
Now here we are at 30,000 feet and -oh oh! The engines all stop! (This usually never happens) What will happen to the airplane? Does it go straight down and crash?
No, at this point the plane becomes a glider. What force is now missing on the glider?
Notice, even though the thrust force is gone, the other three forces are still active. So the airplane will not crash right away but can “glide” down through the air for a while. If there is an airfield nearby the pilot might be able to land there. Eventually, though, the weight and drag forces will overcome the lift force and the airplane will have to land, or crash.
We hope you have learned something about Aerodynamics from our discussion today, thank you.
