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Autorotation Question


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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?

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One factor is just how high the rpm gets. If the rpm is high enough to be an overspeed, then that's no fun for the pocketbook. High rpm might be nice for the extra energy at the bottom, and depending on the situation, some might say screw the rotor, do what it takes to go home at the end of the day.

 

But, that really has nothing to do with adverse affects during the auto, that's more about bad affects when the helicopter gets to the shop

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One factor is just how high the rpm gets. If the rpm is high enough to be an overspeed, then that's no fun for the pocketbook. High rpm might be nice for the extra energy at the bottom, and depending on the situation, some might say screw the rotor, do what it takes to go home at the end of the day.

 

But, that really has nothing to do with adverse affects during the auto, that's more about bad affects when the helicopter gets to the shop

 

I hate to split hairs, but it's possible to overspeed some helicopters so bad that you can actually lose a component or have a bearing literally blow up. Don't go over the recommended RPM limit in the auto. There is an allowable range for a reason. A minor overspeed might only do very minimal damage. Many minor overspeeds can do significant cumulative damage, and a large overspeed can completely destroy components. Seen an R22 main hub bearing litterally fall to pieces when it was removed from the helicopter for an inspection. Pohi is right, more RPM is nothing but good for the autorotation itself, but it can be not so good for the aircraft. Keep this in mind when PRACTICING. When the real thing happens, just make sure you get it on the ground in a survivable manner.

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Increased rpm will give you an increased rate of descent.

Max glide or min descent rate may be given as a specific airspeed w/ rotor rpm at less than 100%.

You're using more potential energy to spin the rotor faster.

 

Good point here too. Do you need glide distance? You will have to sacrifice some of that rotor RPM. If you need RPM, you will have to sacrifice some glide distance. You just have to make it work.

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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.

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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.

 

It's possible that your CFI is doing this because he is afraid of being near the overspeed range. Do you fly R22s? It sounds like it. When practicing autos we would take it as close to the allowable max RRPM as we could in that flare--why? Precisely what you were saying: it gives you that much more energy when you need it as you pull in collective. However, many CFIs I am sure are probably uncomfortable with getting so close to this limit, especially in the early stages of teaching autorotation. Again, I don't know your CFI so this is just an assumption, but it's the only reason I can think of.

 

The chief pilot at the school I teach at would take it as high as he could allowably get in the flare when we did autos. He has over 20,000 hours in the R22 and is more than comfortable with it. As a new CFI myself, having taught just a few autos, it's something I have to flesh out. For now, I tell them "keep it in the green." That will go up with experience on my part and the student's.

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I think nr in the steady state around 100% is going to give you, as a student, a little room to give aft cyclic and build nr up to the allowable limit. If you set your glide up at 109 you're left with less room for error as you transition into the flare. R22's will gain/lose nr quickly...

If you're set up at 95% you're going to have to have less energy for that hover auto at the end (unless you flare aggressively). I think the real disadvantage to a steady state auto at 109 is the increased rate of descent you'll have to arrest at the bottom.

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I think nr in the steady state around 100% is going to give you, as a student, a little room to give aft cyclic and build nr up to the allowable limit. If you set your glide up at 109 you're left with less room for error as you transition into the flare. R22's will gain/lose nr quickly...

If you're set up at 95% you're going to have to have less energy for that hover auto at the end (unless you flare aggressively). I think the real disadvantage to a steady state auto at 109 is the increased rate of descent you'll have to arrest at the bottom.

 

I *think* (but am not sure) that he is referring mainly to the flare--that his instructor is still maintaining "keep it at 100%" rather than allowing it to go higher.

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My bird is allowed 90-107% in an auto with a 107-125% 5 sec transient.

 

Many times I will enter an SEF, bottom out at 90, and be clearing 105% with an audio alarm going off by the time I setup the auto and do my callouts. Let alone a 180 auto to the right. I have many time pulled pitch and let it sit there to keep rpm at 99 or 100. I do let it do what it wants on the decel, though. I have a generous transient and I want as much energy as I can get.

 

I will say that I gain quite a bit of distance with 95% vs. 100% with the same A/S. I had that demo'd by an IP. It was pretty impressive.

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It may help to think of practice autorotations in two different ways.

 

Simulated engine fail: goal is to maneuver to a spot that would afford some potential for a safe landing. I would accept a "bad" auto to a good spot vs a "good" auto to a bad spot.

 

Practice Autorotation: Goal is to practice maintaining rpm and airspeed as defined by the PTS.

 

Focusing on keeping a specific RPM through the whole maneuver/procedure is a little bit of a waste, however, if the POH says use between 90-110% nr, it's reasonable to tell your student to just split the difference, ie 100%.

 

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V15x- I'm not a student, I fly Hawks in the Army but we still practice Autos (not touchdown, but that's not important here). However, regardless of the airframe, the instructor, or the instructee having 5 or 5,000 hours I've always seen it taught the same way. You have a good point about it being so you don't exceed the limit during the flare at the end.

 

Lindsey- I've heard the same thing as your instructor from time to time from other pilots that they think the gain in RPM would be beneficial, but I haven't tried it, not have I seen it written anywhere that this is true. And I am talking about during the descent not the decel.

 

 

 

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Oooh, sorry. No offense taken I hope!

 

None taken, I don't have all the answers.

 

So, let's take a vote. Who thinks an increase in rotor RPM over 100 (but within limits) will

 

A- help out at the bottom for more cushion

 

B- increase rate of descent too much and make the touchdown faster

 

C- make no difference at all

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It may help to think of practice autorotations in two different ways.

 

Simulated engine fail: goal is to maneuver to a spot that would afford some potential for a safe landing. I would accept a "bad" auto to a good spot vs a "good" auto to a bad spot.

 

This is "Totally Backwards"!!!!! You always want to make a good auto to either a good or bad spot and NEVER make a bad auto to a good spot as this can and will kill you. Bad autos are never acceptable.

 

Practice Autorotation: Goal is to practice maintaining rpm and airspeed as defined by the PTS.

Goal here is to accomplish all aerodynamic transitions from powered flight to autorotative flight and establish a stabilzed rate of descent. Begin flare at the proper height to build rotor rpm without an overspeed, reduce rate of descent and reduce ground speed, level and cushion! The PTS is a minimum standard and you should be better than that. The RFM sets limits not the PTS.

 

Focusing on keeping a specific RPM through the whole maneuver/procedure is a little bit of a waste,(this is exactly what you want to do to maintain a stabilized rate of descent which gives you Situational Awareness to make the spot) however, if the POH says use between 90-110% nr, it's reasonable to tell your student to just split the difference, ie 100%.

 

The Rotor Tach is the most important instrument/indicator in the helicopter during an autorotation and should be checked with a good scan.

 

If you are not at MGW and having to increase collective to keep the rotor RPM from reaching the upper limit when entering an auto then the pitch links need to be adjusted. The pitch links should be set so the collective can be maintained on the bottom except during turns to allow for continued use during flare & maximum pitch application for touchdown.

 

Be advised that a new AC is coming out shortly on autorotation training. CFIs should know and teach the three aerodynamic transitions to autorotative flight and the three factors that will control where the rotor RPM/ROD will stabilize for a given airspeed/attitude. Over speeds are not acceptable and the collective should be used to control the rotor rpm and not allow the rate of descent to increase because of increased rpm. Use the controls to fly the helicopter! Do not accept less, it will hurt you.

 

Mike

Edited by Mikemv
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Mike,

I like the points you make.

 

To be fair, a perfectly set up autorotative approach into a house is something that I feel is less acceptable than an auto to a hard level surface that is on the edges of the rfm limitations. Its been my experience that students tend to fixate on nr and forget or overlook the real goal, a safe landing. Perhaps I should substitute "sloppy" for "bad". It may be an issue of semantics.

 

As far as the PTS being the minimum standard, that depends on the stage of training. I think the pvt PTS is a pretty tough standard for a pre-solo student.

 

My point about maintaining a constant nr was directed at those that felt/thought you should not allow nr to move during the entire maneuver, not just the steady state portion.

 

 

 

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I would liken 100% rpm during the auto to l/d max in a fixed wing. You get the best rotor efficiency there. And like others have said, within the limits you can trade off variables in your auto to achieve the proper end result.

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

 

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?

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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?

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Mike,

I like the points you make.

 

To be fair, a perfectly set up autorotative approach into a house is something that I feel is less acceptable than an auto to a hard level surface that is on the edges of the rfm limitations.

I certainly agree with this statement. On the edges but within the RFM limitations would not be a "Bad" auto but one flown by a pilot with skills in controlling and using learned skill sets to make the desired spot.

 

Its been my experience that students tend to fixate on nr and forget or overlook the real goal, a safe landing. Perhaps I should substitute "sloppy" for "bad". It may be an issue of semantics. Some pilots have been killed by trying to stretch an auto to a great spot by using rotor RPM and getting to the spot with RRPM well below the lower limit, the helo being uncontrollable, rolling on its side prior to touchdown and burning, killing all on boad. Bad auto to a Great spot.

 

As far as the PTS being the minimum standard, that depends on the stage of training. I think the pvt PTS is a pretty tough standard for a pre-solo student. The PTS is only meant to be used for the Practical test for an Applicant that a CFI has recommended them for, stating they are prepared to take the test. A pre-solo student is learning maneuvers and EPs and not held to PTS. A student ready to solo should at least have an understanding of what is desired in an auto. They should know the entry procedure and be able to establish a stabilized auto and taught to fly the helo all the way to the ground as best as possible at this stage.

 

My point about maintaining a constant nr was directed at those that felt/thought you should not allow nr to move during the entire maneuver, not just the steady state portion. Once the Nr or RRPM is stabilized a pilot can then make judgment calls on using RRPM/Nr as desired to reduce ROD or extend the glide or prevent an overspeed. Flying the rotor system (they are different) is something to learn from a stabilized ROD/RPM to establish skill sets.

 

Thanks for your reply and I agree and liked your response. My reply is for the mentoring of those learning from VR. As a member of the JHSIT training work group, we are trying to improve autorotational training and understanding by pilots. This includes better "training the trainers" and upgrading the CFI PTS. It has been a short coming of the industry that we are addressing to reduce the autorotational training accidents which is the major portion of GA helicopter training accidents.

 

Mike

Edited by Mikemv
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I would liken 100% rpm during the auto to l/d max in a fixed wing. You get the best rotor efficiency there. And like others have said, within the limits you can trade off variables in your auto to achieve the proper end result.

 

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).

 

This is really what's at the heart of the question since the touchdown on an auto is cushioned by converting the potential energy left in the collective pitch and rotor rpm into lift. By increasing collective during descent you will certainly slow your rate of descent on an auto but there will be less collective left for cushion at the end. Since it is understood that 100% rotor is not always the most efficient speed we can deduce that some combination of rotor and pitch will be better than others and that during the cushion it's unlikely that 100% is the best speed throughout all parameters.

 

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.

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