Why does my helicopter lean in the hover?

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helicharter
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Post Posted: Mon May 07, 2007 7:43 pm
Post subject: Why does my helicopter lean in the hover?
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Why does my helicopter lean in the hover?


Why does my helicopter lean in the hover?
Ah well if you own a helicopter with a tail rotor system then you may have noticed that when it’s hovering that one skid sits lower than the other.
Now there’s a reason for this and no it’s not because you looking at it sideways Wink . It is caused by the tail rotor and is because of a thing called tail rotor drift.
Here is an old picture of my T-rex 450 were you can see it leaning to the right in the hover:

What is tail rotor drift?
Ok tail rotor is drift is caused from the reaction of the tail rotor thrust. Think about it like this because of (good old Newton strikes again) Newton’s 3rd law: For ever action there is an equal and opposite reaction.

To make the main rotor spin the motor has to produce power this is normally referred to as torque now on the ground this is fine. But the moment you leave the ground the helicopter would spin at the same speed as the head only in the opposite direction.

So you have to fit a tail rotor the tail generates thrust 90 degrees to the main rotor to stop the helicopter from spinning. The thrust needed is dependent on the torque being generated from the main rotor. So the helicopter is sitting in the hover all nice and stable (easier said than done Rolling Eyes ) if you applied Newton’s 3rd law to it the thrust stopping the helicopter spinning, is also thrusting the helicopter sideways because there has to be a reaction so says Newton!

So on a helicopter with a clockwise turning head (looking from the top) which most RC helicopters are. Without the tail rotor the helicopter would spin anticlockwise so the tail rotor thrust is pushed out to the right to stop it spinning this means from that the reaction is for the helicopter to want move to the left so you can’t call it a hover if the helicopter keeps moving to the left. So you tilt the head to the right then the drift stops but this means the helicopter is tilting with the right skid low there is nothing you can do to stop this real helicopter suffers from the problem too. There are ways they used to reduce this the most common why is to left the tail rotor up to the same height as the main rotor, because if you think about it having the tail below the main rotor disc means it’s not directly in line with the torque point so it’s having to twist or tilt the helicopter to stop the spinning. It’s not hurting the helicopter putting the tail rotor lower and the problems with weight and complex gearbox’s makes it not worth bringing the tail up on an RC helicopter and some real helicopters as well!
In the pictures below you'll see some real helicopters one with the tail rotor at the same height has the main and one with it lower than the main rotor.
This is a S76 as you can see it has the tail rotor at the same height as the main rotor.

This is a A109 as you can see it has the tail rotor below the height of the main rotor.


Why does my helicopter tilt more with training gear on?
Well that is simple when you think about it training skids upset the lift from the main rotor so you need more power to lift the helicopter more power = more torque. So the tail rotor has to produce more thrust, means more thrust, means more drift = more tilt.
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Peter
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