So, How Fast does My Prop Have to Go...?
| In the first article we saw
that it was not difficult to determine the lift that would be generated by a particular
size of wing. In the second article we saw how to determine the drag that would be
generated by the various parts of our plane.
By separating the level flight forces in into vertical and horizontal components the problem was simplified. It's often said that one reason it took man so long to conquer the air was the fact that he did not separate the forces of lift and thrust.
By trying to emulate the flight of birds, man was unable to develop a flying machine. But this might not be so.
After all, how much technology is involved in a hang glider? And birds dont have engines.
An Ancient Mystery
Over 2000 years ago, in the Nazca desert which lies directly on the equator, the Nazca Indians may well have been the first people to fly.
Giant drawings on the desert floor, of perfectly executed insects and animals can only be seen from the air. Pottery
from nearby grave sites display winged men and the burial cloth rivals the
tightest weave of any modern machine.
| Well, maybe not the first. Long
before that sea gulls plied the shoreline cliffs taking advantage of rising air currents
and hawks circle for hours using thermals with nary a flap of wing.
The Nazca secret is forever lost in the remote and desolate desert and we can only ponder the enigmatic drawings that remain.
The age of modern flight began with the Wright Brothers. They were the first to separate thrust from lift.
Lift and Drag
Following their lead we successfully computed the lift of any wing for any weight. Next we tackled the drag that was produced by the forward motion of the plane. It was complicated, but we simplified the total drag by adding up the drag of the parts.
In order for the airplane to fly, this drag force pulling the airplane back, must be balanced by the thrust pulling it forward as shown in fig. 1.
We have shown several values in fig. 2 to show how the drag increases with the aircraft speed.
Thats why it takes a lot more power (a bigger engine) or a lot less drag to make the plane go faster. You either increase power or decrease drag.
A 40 size engine produces about 1.2 shaft horsepower at 16,000 RPM.
Thrust is a force the same as drag and is also measured in pounds. The thrust at the speed we want to fly must be equal to the drag of the plane at that same speed.
Now, enter, the propeller.
This little gadget is simply a device for converting the engine power into thrust. It is like an auto transmission.
Cont. on Page 5