Edit: rotor blades could break prior to reversing depending on materials used in construction. Descent rate conversion: Feet per Minute (FPM) / 100 roughly equals vertical knots
Welcome back! I'm Jacob and this video expands on my original Autorotations "The Basics" video ( • Autorotations (The Bas... ). If you haven't seen it, I recommend checking it out first because this video addresses some of the common questions left in the comments section of that video. Let's get to it!
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One of the most asked questions that seems to dumbfound the most people is: Why doesn't the rotor reverse in an autorotation? Here's why. First, in powered flight the rotor is already working against drag. Engine power overcomes drag and lift is created. For simplicity consider the airfoil diagram in the video represents all the blades of the helicopter. You'll have your lift, thrust, weight, and drag depicted accordingly. As pitch in the blade increases to produce lift, drag thereby increases. Thrust (provided by engine power) equals out drag to maintain a constant rotor RPM while it produces this lift. If the engine were to stop when doing this (now unpowered flight), thrust stops, and if the pitch angle isn't reduced, drag will rapidly decay rotor RPM. If you've ever trained engine failures in helicopters you know this all too well and it's abundantly clear how rotor RPM slows down if you don't reduce collective after cutting engine power. So we lower the collective to flatten pitch across the blades to a relatively neutral position. This allows the now upwards flow of air to continue turning the disk in the direction that it was already turning. It only does this because the airfoil enters a neutral position. If the pitch was never reduced, the rotor would slow to zero and eventually reverse. If you could enter a negative pitch in the autorotation you could increase rotor RPM in the autorotation.
Simply put: positive pitch slows/reverses rotor RPM. Neutral pitch maintains rotor RPM. Negative pitch increases rotor RPM. Since many helicopters can't do negative collective pitch, they're left to just the first 2 options. To survive the engine failure you must maintain rotor RPM and neutralize the pitch. Failure to do so would reverse the rotor. But not a properly executed auto. To clarify, the toro would only reverse if the pitch wasn't reduced.
Moving on, another question commonly asked is what would happen to the rotor in a turn in an autorotation? Simply put, anything that makes you fall faster will increase rotor RPM. If you're heavier, you fall faster. Rates of descent increase the more weight increases in helicopters. Density Altitude (DA): where the air is thinner, there is less air friction to slow you down. You fall faster and rotor RPM increases. Trim: if you're out of trim and aerodynamically less efficient, you fall faster. What about turns? By turning you're shifting your lift vector (or glide vector) which makes you fall faster and increase rotor RPM. This applies to both left and right turns but varies in severity. Think "left a little increase, right a rapid increase." This applies to counter clockwise rotor systems and is reversed for clockwise rotor systems. The is due to the same rotor efficiencies outlined in my Transient Torque Spikes video ( • Transient Torque Spike... ). One exception to this is forward or aft cyclic. Forward cyclic will slow the rotor RPM and increase rate of descent while aft cyclic will increase rotor RPM and decrease rate of descent.
Is better to autorotate with rotor within normal limits or allow it to stay high if its above limits? It depends. Remember my last video the regions of the rotor during an autorotation. There is the stall region, driving, and driven. The driving region harnesses up flow or air to maintain rotor RPM. The driven or lifting region is what affects the glide. Most authoritative descent charts are depicted to show an auto with rotor within limits. If you autorotate with excessively high rotor RPM, the driving region expands and the driven shrinks. This translates to higher rotor RPM and higher rates of descent. Higher rotor RPM autos result in more of a descent rate to arrest at the bottom flat phase. This usually extends out the distance of the flare to dissipate rotor rpm which can affect making a specific touchdown point. Ultimately autos are about survivability. You have roughly a 50% chance of surviving a 30 knot impact. (Descent rate conversion: Feet per Minute (FPM) / 100 roughly equals vertical knots). A high descent rate of 3000 FPM equals a 30 knot vertical descent. So think in terms of reducing vertical and forward airspeeds close to zero.
Lastly, what happens to the tail rotor? It's mechanically connected. If main rotor turns, so should tail rotor.