Not long after the helicopter pilot leaves hover to go into forward flight, he will recognize a need to manipulate the controls if he is to stay on his or her intended flight path. The reason is found in the rapid change of flow conditions at the rotor disc.
A rotor produces thrust by inducing a downward velocity in the air going past it. The amount of velocity required depends on the "mass flow." When the mass flow is small, the rotor has to work harder and produce a larger downward induced velocity than is required for the same thrust when the mass flow is large.
In hover, the mass flow is small, being made up only of the air flowing downward at the hover induced velocity. But, in forward flight, the air coming horizontally at the rotor increases the effective mass flow and allows the induced velocity to decrease (Figure 8-1).
Whether in hover or forward flight, a rotor producing a constant thrust will have about the same average angle of attack at its blade elements. In hover, the collective pitch must be high enough to provide this angle of attack while compensating for the high downward velocity. If the collective pitch is left at the hover position as the helicopter goes into forward flight, it will overcompensate for the continually decreasing induced velocity, the average angle of attack will increase, thrust will be more than the gross weight of the aircraft, and the helicopter will begin to climb.
It will increase its rate of climb until the relative velocity coming down due to climb makes up for the decreased downward induced velocity. At this point, the thrust will again be equal to the gross weight (with perhaps some difference caused by increased aerodynamic download on the airframe), and the helicopter will be in a steady climb.
This tendency to automatically go into a climb first becomes noticeable at forward speeds as low as 10 or 20 kt and is known as "translational lift" — or somethings "transitional lift." It can be very helpful in getting heavily loaded helicopters into the air. Translational lift is simply the result of the collective pitch being in an out-of-trim position for level flight.
If the pilot does not want to climb during the transition to forward flight, he has two options. During a slow acceleration, he can decrease the collective pitch to keep the average angle of attack of the blade elements constant as the induced velocity decreases.
Figure 8-2 shows the the decrease will not go on indefinitely. At some speed, the forward tilt of the rotor required to overcome the parasite drag of the helicopter will increase the "inflow velocity" through the rotor, even as the induced velocity continues to decrease. At about the speed for minimum power (50 to 80 kt depending on the helicopter), the required collective pitch for level flight will also be a minimum and will begin to rise as speed is increased.
The second option may be used during a fast acceleration by leaving the collective in the hover position but tilting the helicopter nose down with he cyclic pitch so that vertical component of the ever-increasing thrust is supporting the gross weight, while the horizontal component accelerates the helicopter along the flight path (Figure 8-3).
This goes on until the collective required is again equal to the hover value. At that speed, the helicopter will stop accelerating and will settle into steady, level flight. To go faster, the pilot will have to increase the collective. In actual operation, a combination of the two options is generally used.
The other thing the pilot will notice as he or she goes from hover to forward flight is that the helicopter has an apparent desire to roll down toward its advancing blade (to the right on American helicopters). This is caused not by the change in the average induced velocity but by the change in its distribution over the disc.
The induced velocity can be thought of as being generated by the action of the whirlpool-like vortices left by each blade tip. In hover, these tip vortices spiral down, forming a cylinder and inducing an equally distributed downward flow around the rotor.
In forward flight, however, the vortices trail out behind the rotor and are thus much more influential in affecting the flow at the back of the disc than at the front. Wind-tunnel aberrations have shown that at moderately fast forward speeds, the induced velocity at the front of the disc is essentially zero, whereas at the back it is twice the average value.
If the pilot holds the lateral cyclic pitch at its hover value as he goes into forward flight, the blade over the nose will soon be subjected to the decrease in induced velocity. This will increase its angle of attack (Figure 8-4), its lift will increase, and, because of the quarter-revolution delay in response, it will flap up on the treating side.
Conversely, the local angle of attack on the blade over the tail will decrease. This will lead to flapping down on the advancing side. The flapping produces a rolling moment similar to what would be caused by a side wind — and so it is called a "transverse-flow" effect, even though its root cause is a longitudinal change in the flow pattern.
The fly straight and level, the pilot must use left stick (with American rotors) to compensate for the change in the induced velocity distribution to prevent the roll. The lateral stick displacement required (Figure 8-5) is highest at low forward speeds, quickly peaks and then fades almost as quickly. The French have a phrase for the shape of the resulting plot. They call it "boss du manche" (the hill of the stick).
As speed is increased still further, another effect comes important; that of coning, which increases the angle of attack on the blade over the nose and decreases it on the blade over the tail. To compensate for this also requires left stick.