Why does a flat clover-like shape fall slower when it is rotating?

Why does a flat clover-like shape fall slower when it is rotating?


1 Answers

askIITians Faculty 747 Points
8 years ago
Air has momentum. Put another way, it takes air some time to get out of the way.

When the blades are rotating fast enough, they approximate a disk for the purpose of air resistance.

You can prove this is not a propeller effect by pitching the blades as in a propeller. When dropped, the propeller will start rotating so as to "screw" itself thru the air downwards. However, the object will drop noticably slower once it is spinning. If it were only a propeller issue, it should drop faster once the propeller is rotating in the direction to make the object go down. The reason it doesn't is because the propeller blades together approximating a disk adds much more air resistance than the bare blades by themselves.

There have been aircraft built on this principle. Look up something called a auto-copter or auto-gyro. These use freely-rotating blades to form a disk-shaped wing. They look a lot like helicopters, but in a helicopter the rotor is powered and the propeller effect used to create lift (at least when hovering or vertical flight). In a auto-copter, somethine else, usually a traditional pushing or pulling propeller, is powered, but the vertical-axis rotor is free spinning.

This effect is also exploited to land a helicopter when the engine dies. When the engine is powering the rotor, the blades are pitched so that the air is pushed down. You can think of the blades trying to "screw" upwards thru the air. When the rotor becomes free-turning, they have to be pitched the other way to "screw" dowwards thru the air so that the downward motion of the craft causes the rotor to keep spinning.

There is a optimum pitch angle for the blades. Too little, and there won't be enough torque to keep the rotor spinning. Too much, and the "gearing" is too low such that there isn't enough spin per unit drop. Close to the ground, the blades are pitched upwards again. The momentum of the spinning blade is harvested to actually provide a propeller push upwards, or at least a nearly flat pitch. The momentum is spent quickly, so it takes a lot of skill to do this at just the right time.

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