The Wright brothers used a falling mass attached to their airplane by ropes and

Question

The Wright brothers used a falling mass attached to their airplane by ropes and pulleys to accelerate their plane down a rail (as shown). According to notes from Wilbur Wright for their initial test flight “A 600 lb. weight dropped 16-1/2 feet, pulled the aeroplane forward at a good clip” (http://www.thewrightbrothers.org/1904.html). The airplane (plus pilot) had a weight of 745 pounds. For this analysis you can neglect any effects from the mass of the pulleys and rope.

3. Of course the falling mass alone did not accelerate the airplane, it also had an engine driving a pair of propellers providing constant thrust. These propellers push the air backwards with a force of magnitude F. How big is the forward force on the airplane due to these propellers? What causes it? Explain.

Explanation / Answer

Newton's third law of motion is in action here. It states that for every action, there is an equal and opposite reaction. If rotating propellers throw air backwards with a force magnitude of F, then as per newton's law

F = -F ....... It means an equal force will be applied by the air with same magnitude but in opposite direction. The negative sign indicates the opposite direction.

In general, the tip of the propeller rotates at higher speed than the hub. This high speed propeller when comes in contact with air then due to its curved surface, most of the air is pushed backwards at high speed and some air is pushed downwards also. This pushed air does its reaction on the aircraft's body and move it in forward direction. Higher the rpm, higher will be the forward force. That's the reason, In commercial aircrafts with propeller, the speed(rpm) is extremely high in order to provide sufficient forward thrust.

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