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The Art of the Helicopter John Watkinson - Karatunov.net

The Art of the Helicopter John Watkinson - Karatunov.net

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80 <strong>The</strong> <strong>Art</strong> <strong>of</strong> <strong>the</strong> <strong>Helicopter</strong><br />

upwards, <strong>the</strong> acceleration has a rearward component. <strong>The</strong> reaction to this is forward,<br />

so <strong>the</strong> rotor can be driven against pr<strong>of</strong>ile drag and transmission losses. At a suitable<br />

setting <strong>of</strong> <strong>the</strong> collective pitch, <strong>the</strong> overall reaction on <strong>the</strong> blades is brought to <strong>the</strong> vertical<br />

and so <strong>the</strong>y will continue to turn at <strong>the</strong> same speed and provide lift. Note that in order<br />

to produce lift <strong>the</strong> blade must have a positive angle <strong>of</strong> attack with respect to RAFand<br />

so it is common for <strong>the</strong> blade to have positive pitch in autorotation.<br />

<strong>The</strong> machine is moving steadily downwards using potential energy to overcome pr<strong>of</strong>ile<br />

drag. In fact <strong>the</strong> rotor is taking momentum from <strong>the</strong> air so that induced power<br />

becomes negative and provides <strong>the</strong> pr<strong>of</strong>ile power needed. <strong>The</strong> strict definition <strong>of</strong> autorotation<br />

is <strong>the</strong> requirement for zero shaft torque. Where a rotor actually delivers shaft<br />

power it is said to be windmilling or in <strong>the</strong> windmill brake state. A real helicopter<br />

in autorotation is technically windmilling because <strong>the</strong> shaft torque is not zero. In <strong>the</strong><br />

absence <strong>of</strong> <strong>the</strong> engine, some shaft torque is needed to turn <strong>the</strong> tail rotor and any o<strong>the</strong>r<br />

important items such as hydraulic pumps.<br />

<strong>The</strong> speed <strong>of</strong> a blade element is proportional to radius. A fur<strong>the</strong>r complication is that<br />

most rotor blades have twist to improve <strong>the</strong> hover figure <strong>of</strong> merit, but in autorotation<br />

<strong>the</strong> inflow is reversed and <strong>the</strong> twist is <strong>the</strong> opposite <strong>of</strong> what is needed. It can be seen<br />

that on moving from tip to hub, <strong>the</strong> RAFdecreases in speed but <strong>the</strong> angle <strong>of</strong> attack<br />

increases. As a result <strong>the</strong> central part <strong>of</strong> <strong>the</strong> blade, shown at Figure 3.13(a) is being<br />

driven by <strong>the</strong> inflow and is windmilling. Figure 3.13(b) shows that <strong>the</strong> tips have a small<br />

angle <strong>of</strong> attack and high airspeed. Pr<strong>of</strong>ile drag dominates and L/D is poor, with <strong>the</strong><br />

total blade reaction being behind <strong>the</strong> vertical. This area <strong>of</strong> <strong>the</strong> blade is producing lift<br />

and induced drag as if it was in <strong>the</strong> power-on condition. Figure 3.13(c) shows that<br />

<strong>the</strong> innermost parts <strong>of</strong> <strong>the</strong> blades are stalled as <strong>the</strong>y have a high angle <strong>of</strong> attack and<br />

a low airspeed. <strong>The</strong> power needed to drive <strong>the</strong> tips and <strong>the</strong> inner parts <strong>of</strong> <strong>the</strong> blade<br />

comes from <strong>the</strong> windmilling <strong>of</strong> <strong>the</strong> centre parts. Only a small section <strong>of</strong> each blade at<br />

<strong>the</strong> boundary between <strong>the</strong> driven and driving regions is technically autorotating with<br />

a vertical total blade reaction. Thus what is meant in practice by autorotation is that<br />

<strong>the</strong> entire rotor has zero <strong>net</strong> torque. <strong>The</strong> entire rotor can also be considered to have an<br />

L/D ratio and in autorotation <strong>the</strong> slowest rate <strong>of</strong> descent will be where this is used.<br />

In practice <strong>the</strong> loss <strong>of</strong> performance in autorotation due to twisted blades is surprisingly<br />

small. One reason for this is that <strong>the</strong> pr<strong>of</strong>ile drag is determined only by <strong>the</strong> rotor<br />

speed and is largely unaffected by local angle <strong>of</strong> attack. Ano<strong>the</strong>r is that real blades<br />

don’t have anything like ideal taper and also have root cut-out so <strong>the</strong> stalled inner part<br />

<strong>of</strong> <strong>the</strong> rotor doesn’t absorb as much power as might be expected.<br />

In autorotation <strong>the</strong>re is still a pressure step across <strong>the</strong> rotor, but <strong>the</strong> inflow is now<br />

from below. Figure 3.13(d) shows that as <strong>the</strong> rotor is taking momentum from <strong>the</strong> air,<br />

<strong>the</strong> slipstream from <strong>the</strong> rotor is slower than <strong>the</strong> inflow. <strong>The</strong> pressure gradient along a<br />

streamline is rising and so <strong>the</strong> wake expands.<br />

In autorotation <strong>the</strong> best rate <strong>of</strong> descent will be where <strong>the</strong> entire rotor (not a blade<br />

section) operates at L/Dmax.. <strong>The</strong> highest RRPM will also be obtained at L/Dmax.,giving<br />

<strong>the</strong> greatest stored energy for landing. Figure 3.14 shows an autorotation diagram<br />

in which <strong>the</strong> vertical axis is drag over lift and <strong>the</strong> horizontal axis is angle <strong>of</strong> attack α.<br />

It will be seen from <strong>the</strong> figure that, at an idealized blade radius, <strong>the</strong> ratio <strong>of</strong> drag to lift<br />

determines <strong>the</strong> angle ϕ which must also be <strong>the</strong> angle <strong>of</strong> RAFif <strong>the</strong> blade reaction is to be<br />

vertical. If <strong>the</strong> pitch angle is θ, <strong>the</strong>n ϕ = α−θ. <strong>The</strong> function α−θ is shown on <strong>the</strong> graph<br />

to be a straight sloping line. <strong>The</strong> lowest pr<strong>of</strong>ile drag will be at <strong>the</strong> bottom <strong>of</strong> <strong>the</strong> drag<br />

‘bucket’ and <strong>the</strong> pitch angle to achieve that is where <strong>the</strong> line <strong>of</strong> α−θ intersects <strong>the</strong> α axis.<br />

Most conventional helicopters will be rigged so that <strong>the</strong> minimum pitch stop on <strong>the</strong><br />

collective lever is at about this value. Thus operation in autorotation is in <strong>the</strong> region to<br />

<strong>the</strong> right <strong>of</strong> <strong>the</strong> intersection, but not so far as <strong>the</strong> position <strong>of</strong> <strong>the</strong> dotted line where rotor

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