MH-65 Right Wing Down
 

OK, maybe not the correct term given the context but I'm told the US Coast Guard MH-65 flies with an inherent 2 degrees "right wing down" in forward flight and so, as part of an avionics update, they want a secondary flight instrument that can be configured to display straight and level in this condition. This may sound like a stupid question (but I'm no helicopter expert, so be gentle with me) - wouldn't that cause the aircraft to continually deviate from its heading? Or does the flight control system compensate for it in some way?

It's because the centre of thrust from the fenestron is lower than the rotor head. This causes the fuselage to "hang" slightly right wing low. Speak to any Dauphin pilot. It doesn't take long to get used to. If the rigging is correct, it shouldn't change heading.

EC120 has the same profile for the same reasons. Single pilot from RIGHT seat, at high power setting (hover / approach to land) it is very pronounced - feels like you should slide off the seat on occasions!


Less pronounced in forward flight at high speed, but you still notice it.


For helicopters, it is the disc that is flying and the machine simply hangs underneath; it goes along for the ride. Not the same as fixed-wing, where the cabin is joined to the wing so that cabin down = wing down = turn. So we can have "cabin down" on one side but the disc is level = no turn

It's called inherent sideslip and exists in all helos with a TR.

The TR (or the fin, or vertical stabs or combination thereof) has to always produce anti-torque force and so, with the 'wings' level, you will be slipping slightly sideways. See Shawn Coyle's book for details.

Think about TR drift in the hover and how that is corrected with lateral cyclic - then extend that principle into forward flight.

Thanks very much for the explanations Gents.

Regards, MSJ