Tuesday, January 1, 2008
Vortex Ring State and Gross Weight
Ray W. Prouty
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

Sunday, June 1, 2008
Settling With Power, Redefined
Ray Prouty
The recent letter from CW4 Steven Kersting reminded me that the term "settling with power" is somewhat misleading ("VRS vs.
Settling With Power, February 2008, page 7). Power has little to do with the phenomenon.
As I discussed in the July 2007 issue, in a vertical descent, tip vortices are being generated that self-activate themselves to go
down at a speed that is determined by the rotor disc loading. At some descent speed, the helicopter is coming down at the same
speed as the tip vortices and so the rotor gets entangled with them. It is now in the vortex ring state.
Not only does the local environment become chaotic, making the thrust fluctuate, but the average thrust decreases. This is due
to the surrounding tip vortices inducing some of the wake to make a U-turn and come back down through the rotor, as shown in
the photo (at right) taken from one frame of a smoke movie. At a constant pitch, this reduces the angles of attack on the blade
elements.
The magnitude of the loss is shown on the plot of thrust versus rate of vertical descent. This is based on model tests conducted
on a "long track" where the air stands still and the model moves through it, unlike a wind tunnel in which the model stands still
while air moves past it. (The results are the same.) Although the test results were presented in coefficient form, I have
converted them to represent a helicopter that hovers at 4,000 lb with a collective pitch of 12 deg.
The effects on the plot have also been observed in a wind tunnel test simulating vertical descent by using a remote control on
collective pitch to hold thrust constant as the tunnel speed was increased. Initially, the required pitch went down, but at the
speed for the vortex ring, it had to be increased.
As the rate of descent slowly increases from hover, the pilot must initially reduce the collective to maintain thrust equal to
weight. If, in this initial phase, the rate inadvertently increases, the thrust increases and the helicopter returns to the rate it had
before the disturbance. Thus, we can say that initially the thrust is stable with descent rate. For this helicopter, that
characteristic changes above 500 fpm as it enters the vortex ring state. Beyond this point, if there is an inadvertent increase in
the rate, the thrust decreases and the helicopter comes down even faster. Thus, it is unstable. If the pilot just hangs on, the
helicopter will all by itself increase its rate of descent until it reaches a region of stability above 1,500 fpm on its way toward
vertical autorotation. Even if, while in the unstable region, the pilot increases collective, he will only get a transient increase in
thrust that will do him little good since the lines for higher collectives are also unstable. Down he comes!
In a wind tunnel or on a long track, it is possible to acquire steady test data in the unstable region because the model cannot
respond to changes of thrust. That is not true in the flight of an actual helicopter. The instability makes it impossible to acquire
steady data in the vortex ring state. The pilot would be moving his collective up and down in a vain chase of any steady
condition at which to take data.
What about power?
As you can see, power did not enter into this discussion. It does come in, as a secondary consideration, in those wind tunnel
tests where the thrust is held constant with collective. In the steady conditions for vortex ring, the downward inflow shown in the
smoke-movie photo simulates a rate of climb and consequently the power required goes up. In flight, since no steady point can
be obtained, this effect would generally not be observable.
So our vocabulary should change. Instead of "power settling," I would rather refer to the phenomenon as "thrust instability."
This situation involving the vortex ring state should not be confused with that other "power settling" condition illustrated by
flying to the top of a mountain in forward flight but finding there isn’t enough engine power to hover and so settling as the rotor
rpm drifts into the red zone. I would call this "running out of power" and so we have eliminated "power settling" from our
vocabulary.