Low rotor RPM blade stall

[cajunjack]

removed

Edited October 15, 2009 by cajunjack

[Eric Hunt]

If you were in ground effect, the induced flow is reduced, so you were able to hold the bird off the ground with a smaller pitch angle - though the required angle of attack should have been even bigger at 70% than for 80%. Obviously the angle of attack for a hover is related to weight, so you must have been light.

 

But i wouldn't do it again - your coning angle would have been huge, and you will damage the grips and head if you do it. Most of the strength in the head comes from the centripetal force (the reason a piece of string can cut through grass in a lawn edger) but if the RRPM is too small, the blades suffer bigger bending stresses.

[Inferno]

Ya, that just seems like a generally bad idea. Lowest I've ever seen was just above 80% thanks to a throttle chop at a very high power setting. Scared the hell out of me.

[r22butters]

Rolling the throttle down to 70%, chopping the throttle at a high power setting? Man, you guys are nuts!

[Inferno]

My instructor was proving a point about how quickly everything can go to hell. He made it, and then some. Gotta love old school pilots!

Rolling the throttle down to 70%, chopping the throttle at a high power setting? Man, you guys are nuts!

[lelebebbel]

Call me a "new school pilot", but here is one piece of advice for those who want don't want to die in a helicopter:

 

DO NOT MESS WITH LOW ROTOR RPM.

 

High power throttle chops in an R22 are suicidal, no matter how experienced the instructor is.

I flew with one of the most experienced heli pilots in all of Australia recently, he's had real engine failures and all kinds of stuff happen to him in over 13,000hrs of flying helicopters. But he told me that the closest he's come to dying in a helicopter was when a student f'd up after a throttle chop in a R22.

 

 

And by the way, you might also want to read your POH and look up what that read line on the rotor tach means. Hint: it's explained in the "limitations" section.

Edited October 3, 2009 by lelebebbel

[RkyMtnHI]

Hey all, super busy, but just couldn't let this one go by.

 

We talk so much about how we can fix flight schools to make them better and what they can do for us. Well, that is a two way street. You MUST consider that the ship you are flying costs a lot of money, maybe more than you will make in your lifetime. And, if you mess it up, in any way, it could shut the operation you are flying at down.. completely. At the least it's going to cost a ton of money to fix (a simple over speed which is easy to happen in a Robbie, could cost from 2K to 80k just in maintenance, not to mention the down time loss of revenue). The schools insurance could go up or be cancelled, and then YOU and everyone else suffers: the students, the instructors, the janitor, the aircraft owners, the school owners, all of their families... hopefully you begin to get the point. The POH was put together for many reasons, by people that get all of this and more. I haven't even gotten into the safety aspect but i hope you get that. People have died collecting that data. The limitations are put there for reasons, with safety margins but that doesn't mean you should go out and see if they are correct.. do that with your own helicopter on your own time with your own insurance. Chances are after you have worked hard enough to have the money to buy something that substantial you won't take chances with it, much less other peoples futures.

 

jmho,

 

dp

 

oh yeah, thanks for the reminder, i forgot that i was going to add a line to my interview questionnaire:

do you participate in any online forums and if so what is your online name"

[beckwith]

oh yeah, thanks for the reminder, i forgot that i was going to add a line to my interview questionnaire:

do you participate in any online forums and if so what is your online name"

 

Totally agree with you on the pilot responsibility front dp.

 

I have a problem with that question though, or two problems really, the first is selfish. I have been on forums since I was 14 with all sorts of topics ranging from religion to online gaming. I even administered one for a while. I have said some dumb stuff. I have been out of line here on this forum from time to time as well. I know you and I have gotten into it about facebook before and I see this as a similar kind of thing. Maybe you can ask someone if they are on a heli forum and for their screen name, but even that I get a little sketched about.

 

the second is more serious. I think it is dangerous to introduce the idea that you have to disclose an online identity in an interview. This thread is the perfect example of why. It is a good thing for us that this question was asked, both as a community and as an industry. Because it is a dangerous situation that hasn't come up in my training and I'm betting didn't come up in to many other peoples. I have learned many lessons about safety here on VR because of the semi-anonymous nature of these boards allowing people to ask honest questions even that potentially cast them in a poor light. If you and other employers take that away you may be making the people flying your helicopters less safety aware.

JMHO

 

Hope it is all going well over that side of the airport. see you around.

Edited October 3, 2009 by beckwith

[Miloe]

Something very interesting to see if you happen to think of it while you're at the Robbie safety course. Have one of their pilots demonstrate just how fast RRPM drops after a throttle chop and you don't lower the collective. It's a huge differnce between 19 and 25 inches in a R44. Makes you really pucker during those high power pulled take offs when you stop and think about it.

[r22butters]

Inferno Posted Today, 02:11

My instructor was proving a point about how quickly everything can go to hell. He made it, and then some. Gotta love old school pilots!

 

They used to demonstrate Low-G Pushovers for the same reason. Some things are better left to theory.

[Inferno]

Listen, I agree completely. He was making a point, and in retrospect, it was a good one. You don't really know just how fast rrpm decays when pulling a high power setting. I had the collective down the instant I heard the horn, and the RPM still got damn close to 80%. Now imagine you were on takeoff on a day you needed a lot of power and you wern't sitting there ready to to initiate auto on a hair trigger. I for one am a better pilot because of that experience. I know how quickly it all can go wrong, and I'm ready for when it does.

 

Do I think this is a good lesson to teach student pilots? Hell no. I did this during advanced training learning about student f ups and recovery procedures. A student would glean nothing for it, and the instructor and student might die.

They used to demonstrate Low-G Pushovers for the same reason. Some things are better left to theory.

[Pohi]

I for one, applaud the op's efforts to raise the BS flag on the POH.

 

Now that this mythbuster has proven this one to be a load of crap, may I suggest the op's next test be the Robinson Safety Notice 23? Do the helicopter industry and the gene pool a favor and hit that one head on.

 

Thanks, and have a great day.

 

All kidding aside, some things a simple wtf check should prevent a person from doing.

[HelliBoy]

But i wouldn't do it again - your coning angle would have been huge, and you will damage the grips and head if you do it. Most of the strength in the head comes from the centripetal force (the reason a piece of string can cut through grass in a lawn edger) but if the RRPM is too small, the blades suffer bigger bending stresses.

 

How low did your RPM get on your last fulldown?

 

The blades and grips are designed to cone beyond blade stall. You're going to touch down because of a simple loss of lift before the blades truly stall anyways.

 

You're not the first person to do this either and has anyone ever heard of this 'damage' to the grips?

 

Im not advocating doing it because it doesnt really prove anything, and stalling in a hover is not going to be at the same RPM as in flight. It could seriously mislead a student into thinking the blades will stall at a lower RPM than they actually will.

[choppedair]

Let us know how your test on forward flight rpm blade stall works.

Sure we all have done full down autos and hover autos. It is a little different than directly ignoring a limitation to see what happens and sustaining a hover at 70% RPM.

 

Forward flight dynamics are different than a stationary hover, especially in ground effect. Blade stall could happen at different RPMs depending on several different factors. I suspect 80% is a fudge factor, normally intended to instill fear of a pilot to ensure that the limits are not approached. Apparently some people feel the need to push the limits to "find out". I think you would have felt stupid if you crashed the helicopter and had to explain that you were trying to get the blades to stall, and lost tail rotor authority or something.

 

Next time you think about testing something like this (or doing ANYthing in a helicopter), ask yourself if it is necessary, and how you are going to explain yourself if something goes wrong.

[cajunjack]

removed

Edited October 15, 2009 by cajunjack

[r22butters]

Just out of curiosity, did you ask your instructor why you could hover at 70%?

 

To be objective, the best place for the answer would probably be at the Robinson Safety Course, or you could just try e-mailing them anonymously, just say, "someone told you they did it".

 

By the way, don't be too hard on the forum, arguing is what makes this fun.

[lelebebbel]

I'm calling BS on the whole grip issue also. If that was true hovering autos would be too hard on the heli. And I've always taken the redline on the rotor rpm to mean that during an auto not to let it go lower than that %.

 

Thanks to the one guy who didn't pretend he has never done anything stupid. I think the most dangerous attitude is to think your perfect.

 

 

Call whatever you like. Hover autos ARE hard on the heli. And yet, in a hover auto the blades stall within a second and you're on the ground. Holding the helicopter in a hover below the red line is a different story. Take a look from the outside while someone else is doing a hover auto, and then look at the coning angle on a R22 holding a hover at 90%. The centrifugal force that stiffens the blades grows exponentially with rotor speed, so at 80% it is far, far less than even at 90%. So imagine the coning angle at 80%.

 

Even if you ignore the unnecessary stress on the blades, have you thought about what you would've done if the helicopter had entered an accelerating right spin while you were doing this? There is not much tail authority in such a high torque / low RPM situation, you must've been pretty close to the left pedal stop and also close to full throttle.

 

How would you recover from LTE at 80% RRPM in a low hover?

 

Guess this website isn't the place to ask a real question that I have if every response is just about how stupid you are to try something that was shown to you that you didn't really understand.

Your question was answered in the very first reply to your post. In addition to that, other pilots here have told you very clearly that you should not be exceeding limitations in the POH, and they have also explained why. Of course we've all done stupid things. People are trying to give you some good advice here, don't confuse that with a personal attack against you.

Edited October 4, 2009 by lelebebbel

[RockyMountainPilot]

Airfoils do not stall at a specific airspeed or rotor rpm. They stall at a specific angle of attack. What RPM does that AOA occur? Any RPM. You can stall the blades at 100% RPM or higher. The blades can also produce lift right down to 1 RPM. I can take you up in an airplane with an listed stall speed of 50 knots and stall it at 80 knots or fly it down to 0 airspeed and never stall it. It is all a matter of load factor.

 

If you are at 70% in a hover and you are by yourself, you have a pretty low load factor. You will run out of engine power trying to maintain a hover before you stall the blades in a hover. If you are descending, you have an upward flow through the blades and the blades are trying to decelerate the helicopter increasing the load factor. This upward flown of air combines with the rotational relative wind to increase the AOA. If you imagine a blade at 0 pitch and no RPM falling, it would have an AOA of 90 degrees.

 

When you do touchdown autos, have someone watch the RPM gauge on touchdown. It can get down to 60, 50, even 40% and the blades are still producing a lot of lift. You eventually run out of collective and the blades are still producing lift. This is because the load factor is right around 1 and there is no upward flow to increase the AOA. At this point, the flow is downward which is why you can pull full pitch and still not stall the blades.

Edited October 4, 2009 by RockyMountainPilot

[HelliBoy]

Lelebebbel- since you're such an expert on blade grip failure could you please provide me with some data to support your claim? I've never seen nor heard of grip or blade failure from running rpms below the redline in a low hover.

 

Also, if your hover autos are hard on the aircraft I would contest you're not doing them right. There are machines out there that have had thousands and thousands of hoverautos performed on them. Are they unsafe aircraft?

 

People have been doing this demonstration for many years (I dont because I think its pointless) and your grip and blade failure scenario has never materialized. Good question Cajunjack.

 

The point of my post is just because you dont do something doesnt mean it is wrong or dangerous.

[MN Heli Flyer]

By the way, don't be too hard on the forum, arguing is what makes this fun.

 

 

I don"t agree. I am here to share with others, not to argue with them. I took a few months off from VR because of this. I appreciate helpful ideas and suggestions but the criticism can be done elsewhere.

 

I think we can all learn from other peoples experiences and that is what is unique about this site. Thank you VR for the opportunity to learn from others!!!

[choppedair]

We have all done stupid things and made mistakes as pilots. The important thing is to learn from them.

We are not calling you stupid or saying that none of us have done anything stupid, but (most of us) are suggesting that practicing hovering at 70% isn't a good idea. Several reasons why were provided to you, both as to why they may stall at RPM other than the manual provides, and why it isn't a great idea to do it.

 

We have all shown students that the helicopter will hover below 101% RRPM, so they can understand low rpm, it's correlation of required manifold pressure, and see that the helicopter isn't going to fall out of the sky when practicing low rpm recovery. Reduced RPM is also used to for stuck left pedal. However when we demonstrate this we don't keep reducing the RPM to satisfy our curiosity about when the blades stall.

 

Something I said in my last post is something I heard my previous boss say a lot. I ask myself these questions frequently when I am flying, and asked to do something questionable by a passenger.

"Do I need to be here, do I need to be doing this, and/or how am I going to explain this if something goes wrong?"

[crashed_05]

Something I said in my last post is something I heard my previous boss say a lot. I ask myself these questions frequently when I am flying, and asked to do something questionable by a passenger.

"Do I need to be here, do I need to be doing this, and/or how am I going to explain this if something goes wrong?"

 

Very good questions to ask yourself.

[Eric Hunt]

How low did your RPM get on your last fulldown?

 

Well, they sure as heck didn't get to 70% while I was still airborne!

 

The end of an auto is a descent to the ground, just reducing a rate of descent to something tolerable, so you are pulling less pitch than if you were attempting to hover.

[RockyMountainPilot]

Well, they sure as heck didn't get to 70% while I was still airborne!

 

The end of an auto is a descent to the ground, just reducing a rate of descent to something tolerable, so you are pulling less pitch than if you were attempting to hover.

 

Not an instructor?

[Eric Hunt]

Yeah, I could have phrased that better.

 

Of course the lever ends up under your armpit with lots of pitch, but what I MEANT to say was that you are not trying to develop enough LIFT to stay off the ground. You are trying to soften the bump.

 

And yes, been instructing since 1976.