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C4 - GENERAL - Why does the helicopter go left and backwards on take off ?

This really is a question that crops up all the time, because new pilots quite reasonably expect a helicopter to lift off the ground going straight up.

Well the problem is down to the main blades spinning quite fast, and the tail rotor trying to stop the rest of the helicopter from spinning in the opposite direction (Remember Physics at School - To every action there is an equal and opposite reaction ?)  So with a large number of model helicopters using a clockwise rotation of the main blades, the tail rotor is trying to force the helicopter to NOT go anti clockwise, which is it's natural reaction.

Therefore, it is logical to realise that the tail rotor is actually trying to make the helicopter go clockwise, so it is pushing the tail to the left - with me so far ?  While you are on the ground, especially until the helicopters main blades are up to flying speed, the tail rotor is also not up to full speed, so needs more rudder input to stop the helicopter from rotating on the ground, especially if you are on a hard surface.

As soon as the helicopter lifts off the ground, all these forces act together with nothing to stop them, and because the tail is pushing left, the helicopter naturally goes left.

Going backwards is a little harder to describe, but it is mostly due to precession, which is what goes on with the main blades whenever the swash plate is not exactly level. Your first thought I am sure if you want the heli to go forwards is to push the elevator forward, and you would expect, if you could stop the blades instantly, to find the forward pointing blade with downwards pitch, and the backwards pointing blade with positive pitch to make the helicopter nose go down ?  Well, your wrong...........

In fact, due to the laws of physics, the position of the blades would be as described above, except that it occurs a full 90 degrees before you would expect it to happen.  Therefore, as the helicopter lifts off the ground, this precession tends to make the helicopter come backwards towards you.

The solution to all of this is quite simple, and most more experienced pilots do this without thinking about it, but just before take off, you push the cyclic stick a little way towards the 2pm position, which applies forward and right cyclic, correcting this tendency. Once the helicopter is fully airborne, you re-centre the cyclic as it will no longer be going left and back.

The best rule of all to remember however is to always look carefully at the main rotor disk position, and make sure it is ABSOLUTELY LEVEL with the horizon just before lift off.



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