Heli lateral CofG

[whyisitsohard]

Ive been looking through the AFE version of the pilot's manual whilst on a nice Christmas day Lanzerote and the graphs for the CG are intriguing.

Can anyone tell me why the lateral balance graph is not symetrical? It would appear that I can load the aircraft differently on the left from the right. Why is this? Is it typical of all helicopters?

David

[21lefthand]

In an R44 the main fuel tank is on the left which off sets the load when flying on your own

[lelebebbel]

One reason could be the effect of tail rotor thrust in a hover, or "translating tendency". The main rotor thrust has to be angled to compensate for this (either by rigging of the cyclic control, angle of the MR mast, or simply by manual cyclic input), which reduces the amount of available cyclic in that direction.

In forward flight, all single rotor helicopters have to compensate for disymmetry of lift due to the difference in airspeed of the retreating and advancing blade, which also affects the amount of available cyclic towards the retreating side. I'm not an expert, but I'd imagine that this could also have an effect on the allowable CG envelope.


The position of the fuel tank affects the CG, but not the CG _limitations_

[deeper]

you are right lelelebel, your not:E

[lelebebbel]

thank you for that comment. Very helpful.

[whyisitsohard]

Thanks guys. Seems to add up. And I omitted to say, it was an R22 manual!
david

[jeepys]

The TR Thrust does play a big part. For example in a heli whose blades rotate counter-clockwise the torque reaction provides a rotation clockwise which the tail rotor must be able to thrust against. In a R22 for example this gives a sideways thrust to the right. Tail rotor drift some people call it. As you pick the heli up into the hover in order to prevent immediate sideways flight the cyclic must be positioned to the left. This can often reflect the reason why the heli may hover left skid low. You therefore have less available cyclic input to the left which would affect the flight envelope.

But that's my take on it. Somebody else may have a better explanation. Maybe Igor.

[Graviman]

whyisitsohard,

The simplest answer is that the cyclic does not move in a straight line with speed. Instead it normally follows a shallow lateral "S" movement, due to various non-linear and non-symmetrical effects, which the designer tries to minimise. Mostly it's due to the complex way the air flows into and out of the rotor, compared to the ideal streamtube.

[KNIEVEL77]

Jeepys,

Do you fancy writing a training manual for me.......at last an explanation I can understand......why do most manuals expect you to have a degree in physics to understand the damn things........or an I just thick?????

K77.

[[email protected]]

If your lateral C of G is to the left of datum, it helps to counter tail rotor drift as you have to use less left cyclic to tilt the disc to the required attitude since the Cof G is trying to roll the fuselage left already. If the C of G is to the right of datum, you would have to use left cyclic to oppose the rolling moment and then more left cyclic to counter the TR drift. (all this on a counter-clockwise rotating rotor - it is the opposite for French Helos!)

As far as I am aware, lateral C of G is only an issue in the hover and low speed flight envelope.

Cyclic lateral migration as you increase forward speed is to counter inflow roll - where the dissymmetry of lift wants to roll the aircraft towards the advancing side of the disc (right on an R22).

[Shawn Coyle]

There could be any number of reasons for the asymmetric CG envelope. Stresses in the mast, control margin with a side wind, ability to land on sloping ground - unless Frank Robinson lifts the veil on the reasons we'll never really know.