The Art of the Helicopter John Watkinson - Karatunov.net

94 The Art of the Helicopter
Fig. 3.26 H-force and Y-force. Conditions experienced by the blades in forward flight are asymmetrical and
this reflects as cyclic differences in blade drag that produce a force in the plane of the rotor. The rotor thrust
is not precisely at right angles to the tip path plane because of this force. H- and Y-forces are the components
of this force in the fore-and-aft and transverse directions. The effect is very small and not really noticeable to
the pilot.
accompanied by an increase in drag. As acceleration is the limiting factor, at low
relative velocities air can negotiate the blade at a larger angle of attack than at high
velocity. Figure 3.27(a) shows the stall limit diagram for a typical section. As the speed
of sound is approached, the allowable angle of attack becomes very small. Note that the
speed of sound falls with falling temperature and in very low temperature conditions
helicopters with high tip speed will suffer a performance loss.
In fast translational flight, the tips of the rotor blades will be encountering relative
airflow that alternately adds to and subtracts from the rotational speed. The pitch
angle will change sinusoidally owing to the use of cyclic feathering but the angle of
attack follows a more complex function owing to the effects of coning and inflow.
Figure 3.27(b) shows the angle of attack of a part of the blade near the tip plotted
against the relative airflow velocity. It will be seen that the plot is an elongated figure
of eight. At positions A and C the blade movement is transverse, whereas at B and D
the forward speed is subtracted from or added to the rotational velocity.
When the plot is superimposed on the stall limits of the airfoil section, it becomes
possible to predict where blade stall will occur. This has been done in Figure 3.28.
Figure 3.28(a) shows the conditions near the maximum airspeed. (b) shows that with
a heavily loaded machine, stall can occur on the retreating blade where it has very low
relative speed. The retreating blade needs to be feathered to a large angle of attack to
resist lift asymmetry and inflow roll. The peak angle of attack will be reached at about
285 ◦ and the length of stalled blade will be greatest here. The loss of lift around 285 ◦ is
converted by rotor phase lag to a pitch-up and roll. An attempt to correct the roll with
cyclic will accentuate the stall and loss of control will result. Figure 3.28(c) shows that
with a lightly loaded machine in straight and level flight or in a dive, advancing blade
compressibility stall will occur first at high forward speed.
Partial blade stall can also be provoked at moderate speeds by tightly banked turns
and if this is detected, the severity of the manoeuvre should be reduced immediately.
Blade stall of either type causes serious vibration and control difficulty before the
loss of lift becomes significant. Serious alternating stresses are set up and can cause
blade delamination.