Autorotation Question

[iChris]

This is what's taught in the sort of aerodynamics 101 we all receive in flight training but it's not necessarily true. 100% will provide an acceptable level of lift throughout all combinations of GW, DA and airspeed the aircraft is rated for but will not usually be the most efficient RPM except during one combination of GW, DA, and airspeed. Variable rotor aircraft take advantage of this. For a certain profile increasing or decreasing rotor RPM will actually result in greater lift for an equivalent torque. Understanding of these aerodynamics is usually beyond what we receive as pilots but is certainly understood by engineers. From my understanding variable rotors typically decrease RPM during cruise and increase at hover and low airspeed (though not necessarily since GW plays a part).

 

To clarify to all. The question isn't on how to do an auto. If you do any auto within the manufacturers parameters it will be successful every time. It is a thought experiment to better understand aerodynamics.

 

You're correct there is an optimum autorotative rotor speed that results in the most blade elements working at their best angles of attack and maximum lift-to-drag ratios. That best angle is related to what is called the “blade-loading coefficient”.

 

Many helicopters are designed to operate at lower angles under normal conditions to allow for emergency conditions. In autorotation at low gross weight and altitude lowering the rotor speed can increase efficiency. However, in some cases the opposite is true.

 

Most rotors are said to be at their peak efficiency in forward flight when the “blade-loading coefficient” is about .08. If the normal value is less than this, slowing the rotor will give better autorotative performance and if the coefficient is already above the optimum, the rotor speed should be increased.

 

Lowering the rotor speed or setting it to some set value (like 100%) does not always relate to increased rotor efficiency. That’s another general statement that doesn’t fit all the aerodynamics. As shown below, at high gross weight and/or altitude a higher rotor speed would somewhat increase rotor efficiency.

 

This seems to be more a discussion on aerodynamics than a talk on any autorotation training exercise. CFR Part 27 gives a general outline on how those rotor RPM numbers in your RFM were determined. Some of you maybe surprised.

 

27.33 - Main rotor speed and pitch limits

27.71 – Autorotation performance

27.1509 – Rotor speed

 

Edited July 23, 2012 by iChris

[CharyouTree]

To make it more clear, I am not talking about exceeding the structural limits. For instance in the aircraft I fly rotor speed is allowed to be between 90 and 110 in autorotation. If it is, than no limit is exceeded, no inspection is done, no parts are damaged. I am told, though I haven't seen it written that exactly 100% is preferred and we add collective during autorotation to maintain that.

 

I understand that collective application would expand your driven range and result in a slightly slower rate of descent but I see no reason why this wouldn't be preferred at the touchdown point when rate of descent is most critical, and therefore an increase in RPM would either be preferred or make no difference. And if 100 is better than 109 than why wouldn't 95 be better than 100?

 

So why is it important to be at 100 and not better to be at 109? Or 95?

 

We concern ourselves with this during training Auto's. I ask because I'm not sure if it's just because during training we want to ensure we don't exceed limits. However, in an actual emergency I'm not sure I'm going to be looking at that gauge in the middle of trying to find a landing spot and getting airspeed and everything set and I'm not sure it's smart to be paying attention to it in that situation so I'm curious.

 

First of all, I don't know your level of experience, and I don't want you to come under the impression that I'm talking down to you, BUT... I'm going to go IP on you right now... That's in the -10. Para 9-12.

 

Secondly, the MTPs are supposed to set (or rather, have the mechanics set) the pitch change links to be at 100% with collective full down. (During the auto-rotational RPM check). If it's less than 100%, they adjust the linkages so you get less angle of incidence at 0 collective. If higher, they adjust so you have a higher AOI. This changes based on season/location/DA, so it could be off, but it should be adjusted regularly as they do full test flights for other things. (Or if someone came back from practicing autos and wrote up some droop at flat pitch.)

 

Ok...so, higher RPMR gives us more energy at the bottom, right? But it also gives us a higher rate of descent. So, we end up taking that additional energy, and having to use it to counteract the higher ROD we got from not just increasing the collective during the descent.

[Bobsyouruncle]

 

 

First of all, I don't know your level of experience, and I don't want you to come under the impression that I'm talking down to you, BUT... I'm going to go IP on you right now... That's in the -10. Para 9-12.

 

Secondly, the MTPs are supposed to set (or rather, have the mechanics set) the pitch change links to be at 100% with collective full down. (During the auto-rotational RPM check). If it's less than 100%, they adjust the linkages so you get less angle of incidence at 0 collective. If higher, they adjust so you have a higher AOI. This changes based on season/location/DA, so it could be off, but it should be adjusted regularly as they do full test flights for other things. (Or if someone came back from practicing autos and wrote up some droop at flat pitch.)

 

Ok...so, higher RPMR gives us more energy at the bottom, right? But it also gives us a higher rate of descent. So, we end up taking that additional energy, and having to use it to counteract the higher ROD we got from not just increasing the collective during the descent.

 

Chris- thanks, that's what I was looking for. I haven't seen those charts anywhere.

 

Charyoutree- my -10 says to maintain rotor RPM within the power off limits which isn't the same as saying 100% is the best RPM. It goes onto say that 110kts and 100% rotor gives us the maximum glide, that approximately 100% RPM R provides a good rate of descent and that an increase in rotor RPM over 100% can be beneficial by using the inertial energy for the landing. All useful information for landing a blackhawk but not an explanation on the relationship of varied rotor rpm vs pitch.

 

I'm not trying to reinvent the way Blackhawks or any other aircraft are auto'd. I became curious after reading about many of the modern helicopters and the new UAV helicopter use varying rotor RPM and how the new EC-225 increased its RPM R when it didn't have single engine capability to provide more potential energy in the event it lost an engine and had to ditch. Got me thinking about if we used more rotor RPM what would the effect on the cushion be? If we used less would would the effect be? Where would the ideal compromise between the two be? And why?

[ridethisbike]

What kind of builds can one expect with the following parameters set?

What is a "build"? You ask about builds in plural, in what areas do you expect "builds"?

 

A/S: 65 KIAS

RRPM: 101-104%

Rate of Descent: 1600 FPM

 

 

What if it's the same A/S and RRPM but the ROD is 2000 fpm? Can I expect a bigger build with the higher descent rate?

Expect a "build" during what part of an autorotation? Is it stabilized at these parameters? What airframe and rotor system are we asking about? R22, S300, R44, B206, etc. Is there a wind component to consider?

 

Sorry, didn't realize the question was so vague. The helicopter in question is an R22, and the build I'm referring to is in the flare. Yes, stabilized at those parameters during the descent. I'm just looking more for a general answer to this. I do realize wind components can change things dramatically and left winds out for that very reason, but for the sake of argument, assume head wind @ 5 kts.

[CharyouTree]

 

 

?

[Mikemv]

Sorry, didn't realize the question was so vague. The helicopter in question is an R22, and the build I'm referring to is in the flare. Yes, stabilized at those parameters during the descent. I'm just looking more for a general answer to this. I do realize wind components can change things dramatically and left winds out for that very reason, but for the sake of argument, assume head wind @ 5 kts.

 

RTB,

 

The change in RRPM will be determined by the amount of flare and DA in both cases, GW is the same in the given parameters. The problem will be decreasing the ROD but can be best accomplished by checking the RRPM at the upper limit to prevent an overspeed and put/keep the brakes on as the flare continues. When you level you should have the RRPM at or just below the upper limit for maximum use in cushioning the landing.

 

I feel the rate of change in each case with the same exact flare will be different with the 2K rate causing a faster increase towards the upper limit and require more pitch application to check RRPM & prevent overspeed.

 

On a side note, 2K ROD seems high in a R22 but in an actual engine failure and for discussion it is established at the point you stated.

 

Mike

[C of G]

My $.02

 

I think it can also be a workload issue. If I find a pilot overworking the controls in an auto, I'll strongly suggest that the RRPM be set at mid to low 90's during a stabilized, straight in portion of the autorotation, which sets up a few things, primarily wiggle room. Should turns be required, the collective doesn't need to be adjusted as the rotor increases, and correspondingly it doesn't need to be readjusted as you stop the turn. Also, when entering the deceleration portion or flare, do you need to raise the collective to prevent an overspeed? If so, why go into that decel portion with such high RRPM? It adds inertia and unnecessary airframe momentum at the bottom and adds to the interference of controls at that point as some pilots will overly rely on the collective to stop the forward momentum. That should be the cyclic's job. The collective is left to cushion. So, when flaring, going in with moderate RRPM, the rotor may build, and you can maintain the RRPM by lowering the collective a bit in the flare, giving more cushion at the bottom.

 

Those charts come from Ray Prouty's articles in rotor and wing (I believe, as it's been a while since I've looked at them). They were collected and printed as "Helicopter aerodynamics" "More Helicopter Aerodynamics" and "Even More Helicopter Aerodynamics" Recently (past 3-4 years) they were reprinted in a single bound collection. I forget the printers name, but they were associated with Shawn Coyle and his seminars. Eagle Eye solutions, I believe. Good group. Interesting article as well on how GW effects ROD. The opposite of what you would expect. (Coyle had a good one on local velocity and pedal turns as well, resulting in a climb in a left pedal turn. Good stuff you get you thinking)

[cbl2799]

During autorotation in some aircraft the rotor RPM will rise with collective full down. I'm told that this will negatively effect the autorotation and therefore we apply some collective during autorotation to maintain 100%.

 

My question is, how does this negatively effect autorotation? As long as the rise is within the structural limits of the rotor I would think the gain in stored energy would be beneficial during decel.

 

There lots of information on the effect on glide vs descent rate for airspeeds but I've seen nothing on rotor RPM effectiveness at different speeds and I can't really work out in my head why autorotational performance would be negatively affected by an increase in RPM.

 

Anybody know?

 

Like previously stated, keeping rotor RPM within limits is essential for a successful auto. But, overspeeding can be a really bad & expensive thing during flight training & currency.

 

That being said, if I were in a no-BS engine out scenario and I was forced to auto, I'd make damn sure to keep the RPM's on the upper band of limitations (especially with a low-inertia rotor system), then do what I had to do to land safely, even if it means accidently overspeeding the system. I'd rather deal with extra paperwork and walk away, instead of becoming a statistic!