Autorotation - Stretching the glide

[HumblePilot]

All pilots know that lift is related to the square of airflow-speed. So basically a high RRPM should mean more lift and low RRPM should mean less lift .... However every helicopter pilot knows that during an Autorotation a low RRPM, within green range of course, is going to stretch the glide.

 

I have some ideas of what is behind all of this, but would be interested to know if somebody could give me a consistant explaination to this phenomena?

 

... looking forwards to good contributions

[captkirkyota]

All pilots know that lift is related to the square of airflow-speed. So basically a high RRPM should mean more lift and low RRPM should mean less lift .... However every helicopter pilot knows that during an Autorotation a low RRPM, within green range of course, is going to stretch the glide.

 

I have some ideas of what is behind all of this, but would be interested to know if somebody could give me a consistant explaination to this phenomena?

 

... looking forwards to good contributions

 

When air is driving the rotors, the more air going thru it means faster falling but higher rpm's, slower rpm's means less air flow due to increased blade pitch and thus more lift affect and slower fall rate. That is how I understand it in the shortest way to say it, assuming you know what I'm trying to say and we do not need to be so stringent with exact terminology. Correct?

[HumblePilot]

Thanks for your answer... I often got this explaination. I find it however not quite satisfying and think that it might need a little more depth to get this explaination straight.

 

 

In order to be a little more precise, I will reformulate my question:

A helicopter can maintain a level flight because lift is equal to gravity force (m x G) and the engine is producing enough power to keep the rotor turning. Now when your engine fails you don't have the power available anymore to get the lift required to maintain level flight. By entering an autorotation the main rotor will produce lift powered by the AR-airflow. Not quite as much power as during powered flight, but enough to get the helicopter safely down. Now by reducing RRPM during an autorotation you are at first sight reducing lift (squared!). But obvously something "magic" is happening that makes it worthwhile to reduce RRPM. What is the "magic" ?

Edited August 19, 2007 by HumblePilot

[captkirkyota]

Thanks for your answer... I often got this explaination. I find it however not quite satisfying and think that it might need a little more depth to get this explaination straight.

In order to be a little more precise, I will reformulate my question:

A helicopter can maintain a level flight because lift is equal to gravity force (m x G) and the engine is producing enough power to keep the rotor turning. Now when your engine fails you don't have the power available anymore to get the lift required to maintain level flight. By entering an autorotation the main rotor will produce lift powered by the AR-airflow. Not quite as much power as during powered flight, but enough to get the helicopter safely down. Now by reducing RRPM during an autorotation you are at first sight reducing lift (squared!). But obvously something "magic" is happening that makes it worthwhile to reduce RRPM. What is the "magic" ?

You are creating more drag ,falling through the sky slower, thus staying aloft a little longer.

You are creating drag by increasing the angle of attack and thus that produces an amount of lift, which only slows the rate of decent, coupled with the forward momentum, which will also add the effective translational lift. So yes your Rotor R's are going to slow down, the only reason they don't during powered flight is you have power to add to overcome their desire to slow down.

So when all those factors combine you get the "magic" lift that extends the forward glide. Remember for this to work there must be forward movment, hover auto's are only good for so high before it turns into a hover plop.

Don't think of the lift you get as an actual lifting up, but rather it is more that the mass that air molecules take up, cause resistance to your fall, not enough to lift you up and not fall, but enough to slow the process. Like if you jump off a diving board into a pool, you can slow your decent down to the bottom of the pool with how you manipulate your body parts thru the water to increase or decrease drag, but unless you, the object are more buoyant than the water, you will sink to the bottom, how quickly depends on how much drag and lifting affect your body creates as it moves thru the water.

Again though, if I'm not spot on, anyone feel free to refine/correct me, I know what it is, but knowing how to say it and type it without being a 87k words essay is another thing.

Edited August 19, 2007 by captkirkyota

[Gomer Pylot]

Lift does not depend on airspeed alone. Angle of attack is just as important.

[captkirkyota]

Lift does not depend on airspeed alone. Angle of attack is just as important.

 

 

spsssst.... hey.....

 

>>You are creating drag by increasing the angle of attack and thus that produces an amount of lift

>>coupled with the forward momentum

>>which will also add the effective translational lift.

>>So when all those factors combine you get the "magic" lift that extends the forward glide.

[Linc]

Lower RRPM means that God has to give you more time to allow you to contemplate your self-induced, reduced margin of error. It's in some holy book, I swear!

[Gomer Pylot]

spsssst.... hey.....

 

>>You are creating drag by increasing the angle of attack and thus that produces an amount of lift<<

>>coupled with the forward momentum<<

>>which will also add the effective translational lift.<<

>>So when all those factors combine you get the "magic" lift that extends the forward glide.<<

If you insist on all those words, OK.

[Witch]

Huh?

[FauxZ]

If you want to get all scientific about it, you can think in terms of energy. We have Kinetic and Potential energy. We have to use energy to keep the rotor spinning. Kinetic energy is 1/2 mass x velocity squared, so forward speed (the rotors also have KE, but we won't worry about that for right now)... If you want to speed up the rotor, you need to transfer some of that KE from your forward airspeed into the rotor... Aft cyclic... Now, imagine we're holding at a steady forward airspeed, we still have to use some energy to keep the rotors spinning, so we'll use Potential energy or Mass x height...

 

Now, spinning the rotors at 100% takes Y amount of energy, and if we don't change airspeed, we simply convert altitude into KE in the rotors. If you don't want to spin the rotor as fast, you don't have to transfer as much energy, so at 90% you simply don't have to sacrifice as many feet per second to keep the rpm stable..

 

This is an exceptionally simple description which, like most physics demonstrations, assumes we live in a perfectly balanced world.

[Goldy]

OK, obviously it's important to know how to extend your glide, it could make a real difference in an emergency. However, have you ever shot an auto at normal rpm/speed and then try it again from the same spot using max glide rpm and speed ? Hmmm....yes, it makes a difference, but not as much as I wish it did !!

 

Whats your best guess in an R22 ? I didnt measure it, but it felt like a difference of about 75-100 feet from a 500 foot AGL auto. Then again, its tough to know exactly the same spot to start the auto at, so maybe its just me.

[captkirkyota]

OK, obviously it's important to know how to extend your glide, it could make a real difference in an emergency. However, have you ever shot an auto at normal rpm/speed and then try it again from the same spot using max glide rpm and speed ? Hmmm....yes, it makes a difference, but not as much as I wish it did !!

 

Whats your best guess in an R22 ? I didnt measure it, but it felt like a difference of about 75-100 feet from a 500 foot AGL auto. Then again, its tough to know exactly the same spot to start the auto at, so maybe its just me.

 

I was having difficulty knowing where the 22 would place and thus starting point was always a guess. I had an instructor tell me that on most decent wx days with non to light head winds, the tops of the trim strings at or just before your desired touchdown spot works as a pretty good reference, the bottoms of them for a bit more wind, say..... 10 knots-ish or more, for straight in auto's. Tried it and sure enough my placement got much more refined. Now for a 90 or 180, well duh.

Edited August 21, 2007 by captkirkyota

[HumblePilot]

OK folks... I would be glad if we could get back to the subject.... By going through my books I found an article of R.W. Prouty in "Helicopter aerodynamics" that seems to answer my question.

 

The answer that R.W. Prouty gives is that by lowering RRPM during an autorotation your are getting a better Lift/Drag ratio than with high RRPM.

 

My way of seeing thing is that if you are getting a better Lift/Drag ratio with a lowered RRPM compared to higher RRPM scenario, it also means that you need a smaller airflow to go through the rotor disc in order to produce the required lift to maintain a steady state autorotation (constant ROD, constant IAS).... less airflow means also a smaller ROD .... smaller ROD means more time in the air ..... more time in the air means also that you can fly longer .... if you can fly longer then you can go further.... and further means that you are stretching your glide.

 

But something interesting that Prouty also mentions is that best Lift/Drag ratio is not always achieved by lowering RRPM during an autorotation. At high altitudes and high gross weight it might need an RRPM increase above 100% to achieve an Lift/Drag-ration optimum.

Edited August 21, 2007 by HumblePilot

[Linc]

Humblepilot,

 

Look at the driven, driving, and no-lift areas during autorotation and consider what happens when you reduce RRPM. There's a relationship there that Gomer is talking about.

 

L/D ratio. (shakes head) That is so un-pilot-like. I prefer terms like thing-a-ma-bobs and gee-whos-e-whuts-its, like all the REAL pilots!

[Eric Hunt]

Humble said:

"less airflow means also a smaller ROD .... smaller ROD means more time in the air ..... more time in the air means also that you can fly longer .... if you can fly longer then you can go further."

 

Well, by that way of thinking, slow the RRPM to zero and you stay up there forever.

 

The real benefit of a better L/D ratio is changing the angle of the descent. Look at a triangle, with the vertical side being your vertical rate of descent. The flat part on the bottom is the distance you travel, and the angled part is your airspeed to get there. If you want to get a greater distance travelled, you need to increase your airspeed and change the angle at the top by altering your L/D ratio.

 

So, pull the revs down, poke the nose down for more airspeed, and away you go. But at the bottom, you have more airspeed to wash off and less RRPM toplay with. Needs some careful judgment with the flare and lowering the lever to regain revs before the last big suck.

[HumblePilot]

Humble said:

"less airflow means also a smaller ROD .... smaller ROD means more time in the air ..... more time in the air means also that you can fly longer .... if you can fly longer then you can go further."

 

Well, by that way of thinking, slow the RRPM to zero and you stay up there forever.

 

Just to put things straight ... RRPM remains within the "Power off" green range and it is just the question whether you choose the high or low end of the green range.

It is also obvious that going below the green range is asking for big trouble resp. the blades will stall.

 

Again I would like to stay on the subject "stretching the glide" and the reasons of this effect. How and when to stretch the glide is another subject.

Edited August 22, 2007 by HumblePilot

[HumblePilot]

Humblepilot,

 

Look at the driven, driving, and no-lift areas during autorotation and consider what happens when you reduce RRPM. There's a relationship there that Gomer is talking about.

 

Every book talks about these ranges.... but however I don't see the chain of arguments that explain that a lower green range RRPM will in the most cases stretch the autorotation glide. Perhaps it just needs more depth..... I looking forwards to a cool answer.

 

 

L/D ratio. (shakes head) That is so un-pilot-like. I prefer terms like thing-a-ma-bobs and gee-whos-e-whuts-its, like all the REAL pilots!

 

To be honest I don't get your point here.

[Gomer Pylot]

To put it as simply as possible, you're just adding pitch, which gives more lift, and thus increases glide distance or decreases rate of descent, depending on the airspeed you select. It works just like it would work if the engine were running and supplying power, as far as the pilot's viewpoint is concerned. Nothing really changes in autorotation as far as the pilot is concerned, except that you can't pull enough collective to stop the descent, and you're descending.

 

As for the bottom quote, as Foghorn Leghorn would say, "That's... I say, that's a joke, son".

[flyby_heli]

Lift formula: C_L x P x S x V² x 1/2

 

C_L= Angle of attack

V²= Velocity square (Airspeed and RPM velocity)

 

The extra lift produced by a higher angle of attack (C_L) is greater than the extra drag produced by a higher angle of attack plus the loss of lift due to a lower RRPM (part of your V²). The net effect is more lift, which means a lower descent rate, thus a longer glide in the autorotation for a given airspeed.

[HumblePilot]

Lift formula: C_L x P x S x V² x 1/2

 

C_L= Angle of attack

V²= Velocity square (Airspeed and RPM velocity)

 

The extra lift produced by a higher angle of attack (C_L) is greater than the extra drag produced by a higher angle of attack plus the loss of lift due to a lower RRPM (part of your V²). The net effect is more lift, which means a lower descent rate, thus a longer glide in the autorotation for a given airspeed.

 

Thanks ... I really like this type of answer .... short, clear and well phrased.

I'll just pick one point out.

 

"The net effect is more lift"

I thought that during a steady state autorotation (ROD remains constant) lift is always the same independent of RRPM or Airspeed, namely m x G.

So if lift is always the same independent of IAS or RRPM how can you be getting "more" lift?

Edited August 25, 2007 by HumblePilot

[nsdqjr]

Thanks ... I really like this type of answer .... short, clear and well phrased.

I'll just pick one point out.

I thought that during a steady state autorotation (ROD remains constant) lift is always the same independent of RRPM or Airspeed, namely m x G.

So if lift is always the same independent of IAS or RRPM how can you be getting "more" lift?

 

I've read this post for the last few days without responding but I can't keep my mouth shut anymore.

 

Humblepilot, change your screen name. I've read a few of your posts and it's obvious that you're pretty impressed with yourself. SEVERAL people have answered your question in SEVERAL different ways, yet you continue to argue the point.

 

Considering how smart you attempt to appear, this is an absolutely ridiculous question, and is basically an issue of "cart before the horse". Here's how it goes, without all of the technical equations:

 

1. Auto = power off, hopefully we don't disagree so far....

2. All things being equal, No power + pitch in the blades = reduction of RRPM (to varying degrees)

3. You can use the Collective to stretch an auto if necessary. Increasing Collective pitch increases drag which decreases RRPM. Less RRPM doesn't create more lift as you seem to believe, but rather it is a product of increased pitch during an Auto. Let's get that cart behind the horse, shall we?

 

Anytime you raise the Collective in a helicopter the RRPM is going to want to reduce. Uber technical understanding of any principle is really great, but sometimes all you need is Helicopter 101.

[HumblePilot]

Dear nsdqjr

 

Thanks for your contribution. Even yours is welcome too.... and sometimes I take a provocative position... but that is the fun of discussing and exchanging point of views.... So please don't get upset.

 

I have attached two articles (RW Prouty and R.J. Pegg, NASA) that address this subject. I found the second article yesterday on the internet. It goes into great depth without getting complicated. Both of them are not the newest but I believe that they still hold the case.

 

Both articles basically state that during an autorotation the L/D-ratio is better at lower RRPM. So in my terms of seeing things the rotor system is just more efficient in using the AR-airflow to produce the required amount of lift to maintain a steady state.

 

So the first part of Flyby_Heli's answer is consistant with the L/D-ratio statement (it is just formulated another way). The second part (see the quote below) however needs in my opinion a little tuning. What do you think Flyby_heli?

 

The net effect is more lift, which means a lower descent rate, thus a longer glide in the autorotation for a given airspeed.

NASA_StretchingTheGlide.pdf

Prouty_StretchingTheGlide.pdf

Edited August 27, 2007 by HumblePilot

[flyby_heli]

After reading through the two articles I have a few comments.

 

First of all: My hat is off to all the test pilots out there who put themselves through these tests to provide all others with better understanding of what is happening aerodynamically.

Second: Don't go try that stuff in an R-22

 

Third: The last part of my last post read: "the net effect is more lift, which means a lower descent rate, thus a longer glide in the autorotation for a given airspeed", if you want me to rephrase that i would say something like "the net effect is a better L/D ratio (more increase in lift than in drag), which means................"

It still says the same, but in other words.

[HumblePilot]

First of all: My hat is off to all the test pilots out there who put themselves through these tests to provide all others with better understanding of what is happening aerodynamically.

Second: Don't go try that stuff in an R-22

 

I agree 100%.... leave that kind of stuff to test pilots.

 

 

Third: The last part of my last post read: "the net effect is more lift, which means a lower descent rate, thus a longer glide in the autorotation for a given airspeed", if you want me to rephrase that i would say something like "the net effect is a better L/D ratio (more increase in lift than in drag), which means................"

It still says the same, but in other words.

 

I think I can live with that formulation .... I know .... I am beeing a little picky

 

To everybody participating .... thanks for the contributions.

[Linc]

I take all my contributions back. I'm taking my ball, too!!