Thrust is the force that moves an airplane forward through
the air. A propeller spinning or a jet engine expelling hot
air produce thrust. Your hand provides thrust when it throws
a paper airplane. According to the scientist Sir Isaac Newton,
"If an object is not moving, it will not start moving
by itself." In other words, something needs to push the
plane forward in order for it to fly. Depending on how heavy
or big the plane is, the pilot will need more thrust to overcome
its "drag" or weight.
The Wright brothers wanted to find an engine for their new
glider so they wrote to all the top manufacturers for help.
Unfortunately, all of the available engines that were powerful
enough were also extremely heavy (about 100 pounds per horsepower!).
Orville and Wilbur feared that the weight of the engine might
crash their plane. So they asked Charlie Tate, their mechanic
from the bicycle shop, to design a small lightweight engine.
It took the whole winter for Charlie to get it working but
when he did the Wright brothers were very happy with his results.