Sunday, June 1, 2008
Settling With Power, Redefined
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.
Besides vertical descent, the problem also occurs in low-speed forward flight, which will be discussed next time.