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Just passed a helicopter on my way to work. We have read in some detail how an airplane gets forward thrust and lift by deflecting air. How does a helicopter with horizontal fans achieve that ?

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Please edit the part "an airplane get forward thrust and lift by deflecting air". We need to understand what you know to helpfully answer the question, and the degree to which this reflects typing errors versus gaps in understanding is indecipherable. – Alan Rominger Jul 1 '11 at 17:20
up vote 9 down vote accepted

In general, the forward thruswt is achieved by tilting the entire helicopter forward. This converts some of the lift produced by the main rotor into a forward component of force. The same is true for turns.

Yes, the swashplate may be used to create a thrust imbalance; but this imbalance does not provide any horizontal forces. It merely creates a force imbalance which tips the helo slightly, so that a component of the upward force of the main rotor gets converted into a lateral force.

Take a look at any helo moving with any kind of rapid forward velocity and you will see that the entire craft is tipped noticably forward. Same with sharp turns: the helo tilts noticably in the direction of the turn.

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The same is true for airplanes to a certain degree. The way airplanes turn is by adjusting the various elements on the wing and the tail to chage the roll of the aircraft, and then the "lift" provided by the wings are no longer vertical, with the horizontal components allowing the plane to turn. Similarly, the swashplate of the main rotor on a heli is used to adjust the pitch and roll of the aircraft, with the tail rotor used to adjust the yaw. – Willie Wong Jul 2 '11 at 11:44
Excellent point Willie. I was going to add that and decided not to clutter up the post by going into airplanes; but your point is germaine. – Vintage Jul 6 '11 at 17:29

The fans are not fixed to be always horizontal. The pilot is able to control the deflection angle of each fan, and this deflection angle causes air to be pushed in one preferential horizontal direction which gives thrust.

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To add to lurscher's post: watch this which shows how by adjusting the angles of attack of individual fan blades you can preferentially generate more lift on one side than the other. – Willie Wong Jul 1 '11 at 18:25
thanks for video ! – New Horizon Jul 1 '11 at 20:44
This is misleading. The deflection angle does not directly push air horizontally. It creates a lift imbalance, torquing the entire assembly (with a ~90 degree phase lag as per the cross product). Horizontal thrust is the result of the plane of the blades not being horizontal. – Chris White Apr 23 '14 at 7:08

protected by Qmechanic Apr 23 '14 at 10:06

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