Thrust Instability In Forward Flight
My last column discussed the phenomena involving the vortex ring state in vertical descent in what is commonly known as "power settling" ("Settling With Power, Redefined" June 2008, page 48). In it, I suggested eliminating that term and instead referring to the phenomena as "thrust instability."
Pilots are taught that they can escape the clutches of the tip vortices by putting the helicopter into forward flight. It turns out, however, that the instability prevails until forward speed is significant. This means that you can get into trouble in steep descents at low forward-flight speeds.
This was brought to the attention of the vertical flight community by the April 2000 crash of a Bell Helicopter/Boeing V-22 Osprey. It was making a steep landing approach at about 40 kt when the pilot lost control. Subsequent NASA wind tunnel tests of a model of the V-22 rotor showed what probably caused the problem.
The model was mounted on a turntable that could be rotated 90 deg to represent flow angles from level flight to vertical descent. What was being changed was the rotor angle of attack, but since in helicopter mode the Osprey would be coming down with its rotors essentially level, we equate the test angle of attack to the descent angle. Tests done at tunnel speeds from 20 to 60 kt represented speeds along the flight path. The data was presented in coefficient form, but I have converted it to a V-22 rotor lifting 23,000 lb in level flight. The result (Figure 1) is a plot of thrust vs. descent angle at 40 kt along the flight path at three values of collective pitch (measured at the 75-percent radius station). The initial, level-flight condition is at Point A.
The same characteristic that was evident in vertical descent in the previous column exists when going from Point A to Point B. The collective pitch has to be reduced to maintain a constant thrust. While flying in this region, an inadvertent increase in the descent angle will increase the thrust and the aircraft will automatically return to the angle before the disturbance, thus showing stability.
This changes above 30 deg, where the situation becomes unstable as the vortex ring state develops. Here, with an inadvertent increase in descent angle producing a decrease in thrust, the machine will come down even steeper. As in vertical descent, the decrease in thrust is due to the re-ingestion of some of the wake due to the influence of the nearby tip vortices. This reduces the angles of attack of the blade elements. (This point, 30 deg, was also the critical angle for other speeds tested.)
Although the model representing a V-22 rotor had tapered and highly twisted blades, another test shows that simpler rotors have the same characteristics. A fuselage with a tail rotor with constant-chord, untwisted blades and fixed collective pitch was mounted on a turntable in a low-speed wind tunnel so that it could be tested at zero sideslip (corresponding to forward flight) all the way to 90 deg of sideslip (corresponding to left sideward flight). Up until about 30 deg, the tail rotor thrust increased, but beyond that it decreased just as it had on the V-22 rotor model. Thus it can be concluded that any rotor will have these same characteristics in the vortex ring state.
For a tilt-rotor aircraft in the helicopter mode, we could make the same scenario for descent in the vortex ring state if both rotors were acting the same. It is probable, however, that they will not act the same because of thrust fluctuations that are always associated with the vortex ring state. If, for some reason, a small roll to the right starts, the right rotor drops, its descent angle increases and it loses thrust in the process. The left rotor, on the other hand, rises and regains some of the thrust that it had lost. This is another unstable situation in which the roll rate will just keep increasing in a divergent manner unless the pilot stops it with his roll control. But since the situation is unstable, he might go into a divergent left roll. It can be represented by a one-degree-of-freedom system with negative damping — a very difficult situation to control!
Figure 1 shows that at 40 kt, the region of trouble is at a descent angle of about 40 deg. Extensive flight tests on the V-22 have verified this. In Figure 2, solid dots represent the conditions where the V-22 wanted to "roll off" all by itself. The four points at 40 kt and a 40 deg descent angle correspond to the critical region on Figure 1.
Test pilots have found that by tilting the engine nacelles forward, they can get out of this interesting situation, but the main result of the tests was to define a safe flight envelope within which they never would get into trouble.
This an abbreviated version of my article on the same subject in the Summer 2001 issue of Vertiflite, the magazine of the American Helicopter Society.