This article discusses how the main rotor collective control is rigged for autorotation. The pitch angle of the blades on the main rotor are adjusted to attain specific rotor speeds (NR) during autorotation at the lowest collective setting. The intent of this adjustment is to keep the autorotational NR in a desired range. We will see that this depends on the target operating environment of the helicopter (specifically air density). First, a short review of relevant properties of autorotation.
Before discussing the rigging, let’s cover a few relevant points about autorotation. Herein we will always be talking about steady state autorotation at a fixed airspeed, say 60 KTAS. In this flight condition, the air rising though the rotor (due to the aircraft descent rate) keeps the rotor turning at a fixed speed (NR), even without engine power. This NR (and descent rate) increases with (1) lower collective blade angle, (2) larger aircraft weight or (3) lower air density. For instance, a lighter helicopter (with less fuel, passengers and/or cargo) needs a lower collective blade angle to achieve the same NR as a heavier helicopter.
One of the pilot’s top priorities in autorotation is to keep NR within a specific range, typically close to 100. Pilot’s achieve this by moving the collective – if the rotor speed drops too much they lower the collective, and if it rises too much they increase collective. The pilot must be able to maintain this NR range whether flying a fully loaded, heavy aircraft or an empty, light aircraft. Furthermore, he/she must be capable of doing this in the most extreme low- or high-density altitude (air density). This is where the collective rigging comes to play. We’ll see below that rigging provides lower blade angles for aircraft operating at lower density altitudes (higher air density) and vice versa.
Before rigging the collective, a minimum operational density altitude - DA0 – must be established. This is done by checking the historical climate for the region(s) in which the helicopter will operate. Knowing DA0, the chart below specifies NR as a function of (1) gross weight (GW) and (2) the operating density altitude minus DA0 (the “delta density altitude”). These NR values are associated with steady descent at a specified airspeed - 60KTAS here. A pilot will fly the helicopter (at a certain weight) at 0 collective and idle engine(s) and record NR versus density altitude. If the NR is above (right) of the target in this chart, the blade angles will be increased by a prescribed amount. If the NR is too low, the blade angles will be decreased by a prescribed amount. The pilot and maintainer may repeat this process a few times until the helicopter matches the chart.
As you should expect from the “autorotation review” section above, target NR values are larger when a helicopter is more heavily loaded (larger GW). Furthermore, as the density altitude increases (air density decreases), NR increases at the same aircraft weight. Both things follow automatically from the physics.
Consider two helicopters A and B (same model, using the plot provided above) expected to fly in regions with minimum density altitudes of -5000’ and -1000’ respectively. If both aircraft are calibrated at the same time, at 3000’ density altitude and max GW, then what NR should the aircraft be calibrated to?
Helicopter A is at a delta density altitude of 8000’, and B at 4000’. Hence A will be rigged to achieve about 103.5 NR (where the silver -max GW - line crosses the horizontal line at 8000’) and B will be rigged to achieve about 101 NR (where the silver line crosses the horizontal line at 4000’). This makes sense in that A will need a relatively larger NR (relatively lower blade pitch angles) to achieve sufficient NR at lower density altitudes (higher air densities) where the NR will otherwise be smaller.