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I build this propeller-powered car (in image). The propeller pushes air backwards and the car moves forward.

I observed something weird. While I place my palm behind the propeller (about 2-3 cm from it) such that the air pushed by the propeller hit my palm, the car moves significantly faster.

My palm was placed facing the propeller's rear side and was parallel to the rotational plane of the propeller. My palm was not in contact with the car. Please refer to the image I have attached. (P.S. it is not a perfect illustration, however, it will give you an idea about the car)

Can anyone please help me with why this happens?

enter image description here

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You're increasing the pressure of the air on the back of your car, hence forcing it to accelerate!

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  • $\begingroup$ Hey Jorge Casajus. Thanks for answering. Could you please explain in detail what actually happens by increasing the pressure behind the car? Does the increase in air pressure itself push the car forward? Is it the air that's pushing the car? $\endgroup$
    – Shibu Nagendran
    Commented Feb 16, 2021 at 11:26
  • $\begingroup$ Sure. I guess it's easier to visualize if you picture the particles of the air as being ejected from the windmill, then "bouncing" in your hand and "pushing" the car. At a microscopic level, that's how pressure works. In a more formal sense, the pressure of a gas is defined as the exchange of momentum between the particles of the gas and the wall of the recipien it's contained in. In this case, the presence of your hand allows for pressure to build and exert force on your car. $\endgroup$
    – Jorge Casajus
    Commented Feb 16, 2021 at 13:17
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This is why helicopters hovering close to the ground in ground effect require less power to do so than when they are hovering high off the ground. A "bubble" of compressed air piles up in the space between the ground and the rotor blades and pushes back on the rotor blades, increasing the amount of lift. The same thing happens when you put your hand behind the propeller of your car model. Search on "ground effect" for a more complete explanation.

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In fact, you have just discovered the physics behind the oft-debated DDWFTTW (Dead downwind faster than the wind) vehicle. The increased back-pressure at the propellor forces the vehicle forward in response. (There are related videos that will show up at that youtube page)

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