Understanding Auto Rotations in Helicopters

Welcome back to Helicopter Lessons in 10 Minutes or Less. I'm Jacob and this video expands on my original auto rotations video. If you haven't seen it, check it out first. I'll put the link above and in the description, but this video will address some of the common questions left in the comment section of that video as well as expand into some further auto rotation considerations. Let's get to it. So one of the most common questions that seems to dumbfound the most amount of people is why doesn't the rotor reverse in an auto rotation? Great question, and here's why. First, in powered flight, the rotor is already working against drag. In this case, the engine power overcomes drag. Sorry, that's a little squished there. So engine power overcomes drag, and lift is created. So for simplicity, we'll just draw a basic airfoil diagram here. Above it, we have lifting force. In front, we have thrust. Below it, we have weight. And behind it, we have drag. All right, for simplicity, once again, consider this diagram represents all the blades of the helicopter. So as pitch increases in this airfoil, it produces lift. But as it produces lift, drag is also increasing. So how do we overcome that? Well, thrust in the form of engine power equals out. out drag to maintain a constant rotor rpm so that this rotor can keep turning we can keep producing lift next if the engine were to stop or we enter unpowered flight thrust stops and if the pitch angle isn't reduced drag will rapidly decay rotor rpm so so engine stops that equates to rotor decay 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 the collective after cutting engine power. So we lower the collective to flatten the pitch across the engine. the blades to a relatively neutral position. This allows the upwards flow of air to continue turning the disc 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. So when you're talking about why does the rotor not reverse, it's because if you don't execute a proper auto rotation, if you keep the pitch in, it will decay the rotor RPM and eventually reverse. If you execute the auto properly, it should not reverse. So if you could enter a negative pitch situation in the auto rotation, you could actually increase rotor RPM or RPM. But simply put, positive pitch is going to slow. or reverse the rotor in an auto rotation. Neutral pitch is going to maintain the rotor RPM in an auto rotation. And negative pitch, if you had that option, is going to accelerate rotor RPM. Now, since many helicopters can't do negative pitch or negative collective pitch, they're left to just the first two options. Now, to survive an engine failure, you must maintain rotor RPM. and neutralize the pitch. Failure to do so would reverse or slow down the rotor, reverse the rotor, make it pretty much useless, but not a properly executed auto. So to clarify, the rotor would only decrease or only reverse if pitch wasn't reduced in the auto. Now moving on to the next question commonly asked is what happens to the rotor if you turn in an auto rotation? Well, simply put, anything that makes you fall faster is going to increase your rotor RPM. So think about this. If you want things to be faster... If you fall faster, you're probably going to have higher rotor as well. Anything that makes you fall faster makes you have higher rotor RPM. So a few things that can go into this. One is if you're heavier, so things like weight. If you are heavier, you fall faster, rate of descents go up, the heavier the aircraft, the heavier the helicopter. Next is high density altitude. If you're high up in the, say, mountains or something like that where the air is thinner, there's less air friction to... to slow you down so you fall faster. Rotor RPM is faster. Next is going to be trim. If you are out of trim, if you are not in an aerodynamically efficient profile, you fall faster because you're not efficient, and that's going to make a rotor RPM increase. Now what about turns? Well once again if I'm doing turns I'm gonna fall faster because I'm shifting that lift vector or in an auto it's more like a glide vector in this case from a near vertical to slightly offset which is going to make you fall faster and increase rotor rpm but it's gonna be slightly different if you're going left versus right. One way to memorize this is if you turn left it's going to be a little increase and rotor RPM, whereas if you do right it's going to be a rapid increase in rotor RPM. Now the reason that this happened is pretty much the same rotor efficiencies outlined in my transient torque spikes video based on where the pitch increase is made. One part of clarity for this one is this is for a counterclockwise rotor, so reverse this if you're operating in a clockwise rotating rotor. Now an exception to this rule going to be forward and aft cyclic. Forward cyclic will slow the rotor and increase the rate of descent while aft cyclic will increase the rotor rpm and decrease the rate of descent. So once again dropping rotor and aft cyclic will increase rotor. All right, another question asked is, is it better to auto rotate with a rotor within normal rotor limits or allow it to stay high if it's already above? So in this question, NR above or in limits, which is better? Well, that just depends, just like all the other things in flight. Remember from the last video that I outlined the autorotative regions of the rotor, the stall, the driving, and the driven, and it's going to look something like this. So you got A, B, C. A is going to be the stall. Not much going on here other than stalling. B is going to be the driven, or sorry, the driving region, and C is going to be the driven region. The driving region harnesses the upflow of air to maintain rotor RPM. The driven or the lifting region is what affects the glide. Now, most autorotative descent charts are depicted to show an auto with a rotor within limits. But if you autorotate with excessively high rotor RPM, the driving region expands and the driven region shrinks. So these regions aren't fixed. They can expand and contract based on speed of the rotor. This expansion of the driving region... translates to higher rotor rpm and higher rates of descent. So high rotor is going to equal high rates of descent. So how is this information important or useful? Well higher rotor rpm results in more of a rate of descent Which is going to be more that you have to rest in the bottom of the auto Compared to a normal rotor rpm auto this usually extends out the distance of the flare because you have to dissipate all the energy of The rotor so it comes down to judging available touchdown point that you're aiming for Ultimately autos are all about survivability. So you have roughly a 50% survival rate of living through a 30 knot impact of an object. That's just based on the body's ability to survive an impact. So descent rates can be roughly converted by multiplying our feet per minute times 100. So I'll write that formula out here. Feet per minute times 100 is roughly going to be the knots of descent rate. Keep in mind 30 is where you're you got about a 50% chance of surviving. So if I'm doing an auto with a high rotor and it results in a 3000 feet per minute rate of descent, that equals a 30 knot vertical descent. 50% chance of death on impact. So when you're thinking about your autos, think about not just the terms of forward velocity, but also vertical velocity. Wanting to get both of those less than 30 knots or preferably as close to zero as possible to make them as survivable as possible. Last question is what's going on? with the tail rotor in an auto rotation. Do you still have control? Is it still turning? Well, the tail rotor is still, or it should be, mechanically linked to the main rotor in the form of the drivetrain. So if the main rotor is within... limits, the tail rotor should be within limits. If the main rotor is high, the tail rotor is high. If the main rotor is low, the tail rotor is low. So that should be no change to the authority as long as you're keeping the rotor within limits. But that's about all the time I have for this video. Leave your comments and questions below and if we get enough feedback on that one, I'll look to push out a third auto rotations video. Thanks again for watching. As always, I'm Jacob. This is Helicopter Lessons in 10 Minutes West. Safe flying.

Let's get to it. So one of the most common questions that seems to dumbfound the most amount of people is why doesn't the rotor reverse in an auto rotation? Great question, and here's why.

First, in powered flight, the rotor is already working against drag. In this case, the engine power overcomes drag. Sorry, that's a little squished there.

So engine power overcomes drag, and lift is created. So for simplicity, we'll just draw a basic airfoil diagram here. Above it, we have lifting force.

In front, we have thrust. Below it, we have weight. And behind it, we have drag. All right, for simplicity, once again, consider this diagram represents all the blades of the helicopter. So as pitch increases in this airfoil, it produces lift.

But as it produces lift, drag is also increasing. So how do we overcome that? Well, thrust in the form of engine power equals out.

out drag to maintain a constant rotor rpm so that this rotor can keep turning we can keep producing lift next if the engine were to stop or we enter unpowered flight thrust stops and if the pitch angle isn't reduced drag will rapidly decay rotor rpm so so engine stops that equates to rotor decay 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 the collective after cutting engine power. So we lower the collective to flatten the pitch across the engine. the blades to a relatively neutral position. This allows the upwards flow of air to continue turning the disc 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. So when you're talking about why does the rotor not reverse, it's because if you don't execute a proper auto rotation, if you keep the pitch in, it will decay the rotor RPM and eventually reverse. If you execute the auto properly, it should not reverse. So if you could enter a negative pitch situation in the auto rotation, you could actually increase rotor RPM or RPM.

But simply put, positive pitch is going to slow. or reverse the rotor in an auto rotation. Neutral pitch is going to maintain the rotor RPM in an auto rotation. And negative pitch, if you had that option, is going to accelerate rotor RPM.

Now, since many helicopters can't do negative pitch or negative collective pitch, they're left to just the first two options. Now, to survive an engine failure, you must maintain rotor RPM. and neutralize the pitch. Failure to do so would reverse or slow down the rotor, reverse the rotor, make it pretty much useless, but not a properly executed auto.

So to clarify, the rotor would only decrease or only reverse if pitch wasn't reduced in the auto. Now moving on to the next question commonly asked is what happens to the rotor if you turn in an auto rotation? Well, simply put, anything that makes you fall faster is going to increase your rotor RPM. So think about this.

If you want things to be faster... If you fall faster, you're probably going to have higher rotor as well. Anything that makes you fall faster makes you have higher rotor RPM. So a few things that can go into this.

One is if you're heavier, so things like weight. If you are heavier, you fall faster, rate of descents go up, the heavier the aircraft, the heavier the helicopter. Next is high density altitude.

If you're high up in the, say, mountains or something like that where the air is thinner, there's less air friction to... to slow you down so you fall faster. Rotor RPM is faster. Next is going to be trim.

If you are out of trim, if you are not in an aerodynamically efficient profile, you fall faster because you're not efficient, and that's going to make a rotor RPM increase. Now what about turns? Well once again if I'm doing turns I'm gonna fall faster because I'm shifting that lift vector or in an auto it's more like a glide vector in this case from a near vertical to slightly offset which is going to make you fall faster and increase rotor rpm but it's gonna be slightly different if you're going left versus right. One way to memorize this is if you turn left it's going to be a little increase and rotor RPM, whereas if you do right it's going to be a rapid increase in rotor RPM. Now the reason that this happened is pretty much the same rotor efficiencies outlined in my transient torque spikes video based on where the pitch increase is made.

One part of clarity for this one is this is for a counterclockwise rotor, so reverse this if you're operating in a clockwise rotating rotor. Now an exception to this rule going to be forward and aft cyclic. Forward cyclic will slow the rotor and increase the rate of descent while aft cyclic will increase the rotor rpm and decrease the rate of descent.

So once again dropping rotor and aft cyclic will increase rotor. All right, another question asked is, is it better to auto rotate with a rotor within normal rotor limits or allow it to stay high if it's already above? So in this question, NR above or in limits, which is better? Well, that just depends, just like all the other things in flight. Remember from the last video that I outlined the autorotative regions of the rotor, the stall, the driving, and the driven, and it's going to look something like this.

So you got A, B, C. A is going to be the stall. Not much going on here other than stalling. B is going to be the driven, or sorry, the driving region, and C is going to be the driven region.

The driving region harnesses the upflow of air to maintain rotor RPM. The driven or the lifting region is what affects the glide. Now, most autorotative descent charts are depicted to show an auto with a rotor within limits. But if you autorotate with excessively high rotor RPM, the driving region expands and the driven region shrinks. So these regions aren't fixed.

They can expand and contract based on speed of the rotor. This expansion of the driving region... translates to higher rotor rpm and higher rates of descent. So high rotor is going to equal high rates of descent.

So how is this information important or useful? Well higher rotor rpm results in more of a rate of descent Which is going to be more that you have to rest in the bottom of the auto Compared to a normal rotor rpm auto this usually extends out the distance of the flare because you have to dissipate all the energy of The rotor so it comes down to judging available touchdown point that you're aiming for Ultimately autos are all about survivability. So you have roughly a 50% survival rate of living through a 30 knot impact of an object. That's just based on the body's ability to survive an impact.

So descent rates can be roughly converted by multiplying our feet per minute times 100. So I'll write that formula out here. Feet per minute times 100 is roughly going to be the knots of descent rate. Keep in mind 30 is where you're you got about a 50% chance of surviving. So if I'm doing an auto with a high rotor and it results in a 3000 feet per minute rate of descent, that equals a 30 knot vertical descent. 50% chance of death on impact.

So when you're thinking about your autos, think about not just the terms of forward velocity, but also vertical velocity. Wanting to get both of those less than 30 knots or preferably as close to zero as possible to make them as survivable as possible. Last question is what's going on? with the tail rotor in an auto rotation.

Do you still have control? Is it still turning? Well, the tail rotor is still, or it should be, mechanically linked to the main rotor in the form of the drivetrain. So if the main rotor is within...

limits, the tail rotor should be within limits. If the main rotor is high, the tail rotor is high. If the main rotor is low, the tail rotor is low. So that should be no change to the authority as long as you're keeping the rotor within limits. But that's about all the time I have for this video.

Leave your comments and questions below and if we get enough feedback on that one, I'll look to push out a third auto rotations video. Thanks again for watching. As always, I'm Jacob.

This is Helicopter Lessons in 10 Minutes West. Safe flying.

Transcript for:Understanding Auto Rotations in Helicopters

Transcript for:
Understanding Auto Rotations in Helicopters