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This question already has an answer here:

I do know that the blades on a propeller generate lift to provide a force pushing back on the air based on newton's law the 2nd. What I am having trouble understanding is how the blades generate thrust by cutting through the air? I know that helicopter hover because they push air downwards, but on a propeller the angle of the blades is much different than of a helicopters blades. Also I don't understand a zero lift configuration. I am having a lot of trouble understanding how helicopters push air downwards, as you can see in this picture

enter image description here

The airfoil doesn't look at all like it can push air downwards. I hope you guys can understand my confusion with this subject. Also I have looked this up on google and was provided answers that were way too confusing for me to understand, so can you give me an answer that is easy to understand. Thanks.

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marked as duplicate by fooot, Ralph J, mins, Federico, kevin Sep 23 '15 at 6:17

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ I'm really sorry for the bad question. I just want to get rid of this confusion I have about these 2 subjects. $\endgroup$ – Ethan Sep 23 '15 at 2:10
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    $\begingroup$ Or the second time you asked the question. Can you explain how this question is different? $\endgroup$ – fooot Sep 23 '15 at 2:18
  • $\begingroup$ @fooot Its more about the blades on the propeller. I am having a little bit confusion about this question. I have reviewed my other questions that have similar questions, but the answers confused me. I sometimes have these random confusions and start asking lots of questions about a certain subject. $\endgroup$ – Ethan Sep 23 '15 at 2:21
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    $\begingroup$ Note that for a helicopter, the collective control changes the pitch of the rotor blades (kind of like a variable pitch prop on a fixed wing aircraft, but more complicated). Your image shows the blade pitch in a neutral position. $\endgroup$ – Greg Hewgill Sep 23 '15 at 2:45
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    $\begingroup$ As @GregHewgill mentioned, the blades angle of attack is variable, what confuses you is that the image shows them in a neutral configuration that would not generate lift. The working angle is similar to what you can see on a fixed wing aircraft. Peter's answer and links clarify this. $\endgroup$ – mins Sep 23 '15 at 5:01
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Propeller blades or rotor blades are like wings: They move through the air, and when they have the right angle of attack, they bend the air back- (in case of a propeller) or downwards (in case of the helicopter). On a wing, this causes downwash, on a propeller this causes prop blast. Both are really the same. Air gets a kick in a direction orthogonal to the movement of the airfoil, be it on a wing, a propeller or a rotor.

To make sure the angle of attack is right, a helicopter rotor uses a mechanism called collective pitch control to vary the angle of attack on all blades simultaneously. In addition, the rotor airfoil is carefully trimmed to have no chordwise shift in the center of pressure over angle of attack. Note that the rotor blade in the picture attached to your question has a small vane at the trailing edge: This is bent slightly upwards to trim the whole rotor such that it will not be twisted by aerodynamic loads.

Please do yourself a favor and follow all the links to other answers here on Aviation SE (it's only three links), and read them carefully. This should help to cure your confusion.

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    $\begingroup$ Just to hammer home the point: In older helicopters such as the one pictured in the question, the angle of attack (i.e. twist) of the rotor blade is controlled by a long lever (like a typical car brake lever) in the pilot's left hand. The pilot lifts the lever to increase the angle of attack of the rotor blade. When the pilot has landed, he lets go of the collective lever, it's normal position is down - in this position the rotor blades can spin without producing lift. This is what you want when waiting on the ground with the engine running. $\endgroup$ – RedGrittyBrick Sep 23 '15 at 9:58
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    $\begingroup$ @RedGrittyBrick ... and during auto-rotation. $\endgroup$ – Simon Sep 23 '15 at 11:30

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