Maj. Devasish Mishra of the Indian Army writes to inquire about vortex ring state.
He is a helicopter flight instructor at the Combat Army Aviation Training School at Gandhinagar Airfield, Nashik Road, India. He flies the Hindustan Aeronautics Ltd.-produced versions of Eurocopter’s Alouette 3 (the Chetak) and Lama (the Cheetah), conducting ab initio training there. "I have a basic doubt regarding vortex ring state," he says, "which none of the publications seems to address.
"It is commonly understood that a helicopter would enter vortex ring state when it settles down in its own wake or when the rate of descent approaches the value of its induced flow," he goes on. "If this be so, when two similar helicopters are operating under similar conditions but different all-up (gross) weights, the helicopter with a lesser weight should enter vortex ring state earlier (that is, at a lesser rate of descent) since the induced flow is less. This, however, appears to be opposite of commonly held beliefs. Kindly clarify."
I presented an explanation of the vortex ring phenomenon in vertical descent for the main rotor in the July issue, which also discussed this problem with tail rotors. ("Tail-Rotor Vortex Ring State," July 2007, page 58). To recap that explanation, when a cigarette smoker launches a smoke ring, it propels itself through the air with a constant velocity that is proportional to the strength of its "circulation."
A hovering rotor makes its own smoke rings in the form of tip vortices, which propel themselves downward at a constant velocity proportional to the strength of their circulation. That can be related to the disc loading of the helicopter.
When the pilot chooses to descend vertically by reducing collective pitch and using partial power, he can reach a speed approaching that of the self-produced velocity of the vortices. This happens at descent rates of about 800 fpm for the Robinson Helicopter R22 with a disc loading of 2.5 psf to about 2,000 fpm for the Sikorsky Aircraft CH-53E, with a disc loading of 14.
Since the vortices are not descending any faster than the helicopter, they hang around, entangle with each other, and produce erratic angles of attack at the blade elements, resulting in lift changes and buffeting.
As far as I know, this explanation holds for vertical descent and should apply to two similar helicopters with different gross weights. The lighter one, with a smaller disc loading, should enter the vortex ring state at a lesser rate of descent than the heavier one. I, for one, do not agree with the "commonly held belief" to which the major refers. I would be interested in any experiences or measurements that would make me change my mind.
This flight condition of vertical descent with partial power should not be confused with autorotation in forward flight that was discussed in the October issue ("Gross Weight’s Effect on Autorotation," October 2007, page 44). In that case, the lighter helicopter will come down faster.