What causes a helicopter to shudder when ETL is lost?

[thecontroller]

What causes a helicopter to shudder when ETL is lost?

[Thomas coupling]

What???????????

[Banjo]

ETL ??? Oh God something else that I don't know what it is.

The only normal light shuddering I can think of with anything to do with those letters would be Translational Lift but not sure what the E would be...Entering maybe?

Answers on a postcard.

Hope someone can answer this as I am curious now

[MightyGem]

That's what I thought. Having done a quick search, I can only think that he means translational lift, although I can't say that I've ever noticed it coming to the hover. Only when transitioning from the hover to forward flight, with the onset of inflow roll and flapback. Although logically, I suppose you should feel it as you loose inflow roll and flapback at the other end.

I'm sure someone will come up with a more indepth explanation.

[bladewashout]

Definitely get the shudder transitioning from and to a hover in R22, around 25Kt IAS.

My guess would be that around the boundary where there is a significant change in relative airflow (more/less forward component), the blades have a less consistent flapping pattern and the disturbances manifest themselves as a shudder through the air frame.

No doubt someone will let me know if that's close...

BW

[jellycopter]

ETL = 'Effective Translational Lift' ; a term from across the pond I understand.

As for the shudder, i thought it was caused by vortex roll-under. Hence the shudder is greater when closeer to the ground but tends to be far less when flying a climbing transition.

Doesn't explain the shudder (and god didn't the ol' Wessex shudder!) when returning to the hover.

J

[thecontroller]

i mean what causes the aircraft to shudder at the end of a normal approach when effective translational lift is lost

[Fun Police]

in laymans terms, it is the change in airflow from up and over the disc (above translation the disc itself acts like a wing) to down and throught the disc where below translation the rotor blades are acting like individual wings.
i stand to be corrected.

[joedirt]

The loss of ETL causes the shudder.

[NickLappos]

The consumption of the shed vortex from the preceeding blades (it is called Blade Vortex Interaction) is what you feel when decelerating through translational lift.

Some helos also get the shudder when accelerating thru ETL, especially when the acceleration is made level, and not from a climb. Same cause of course.

From the first source below, "Under certain conditions of powered descent or vigorous manoeuvring, rotorcraft blades pass through the wake and trailed tip vortices from previous blades. This interaction of the rotor blade with the tip vortex of a preceding blade has been identified as a significant source of noise and vibration in rotorcraft."

Here is a bit on BVI:

http://www.aero.gla.ac.uk/Research/LowSpeedAero/Oldbvi.htm

http://halfdome.arc.nasa.gov/research/bvi.html

[Arm out the window]

Ahhh, the shed vortex; I knew it would be something simple like that.
When you transition, you're often flying past a hangar, and when you hit the turbulence coming off the building (the 'shed vortex') it buffets you around.
Dead easy, this aerodynamics!

[Steve76]

...OK!... so it does have a name eh?
Like 'Arm out the Window', I get "shed" vortex landing the Astar outside the hanger too...
Amazing, all this time I thought I was just a ****ty pilot....

[TheFlyingSquirrel]

Steve, is it true they called it the Astar over there because Yanks can't spell Sskwirral?

[joedirt]

You guys forgot that the man wanted a simple answer.

[Love Monkey]

Once I was flying sober.
That made me shudder.

German beer kills.

[Whirlybird]

Well, there I was, lying low because I didn't even understand the question, and no-one else from this side of the pond did either. :)

But it's reminded me of something. When I was in Russia back in 2002, you got a HUGE amount of shuddering when bringing the MI-8 to the hover. It was scary! :eek: I tried to ask why, but no-one spoke enough English to do any more than assure me it was quite normal. So, why particularly in the MI-8? Nick???

[widgeon]

A Star is cos squirrels are regarded as pests on this side of the pond ( Much like A stars LOL ).

[Heli-Ice]

A few things I was taught back then about ETL:

When landing, coming to a stop in the hover, ETL is lost under c.a. 25kts airspeed. The airflow over and under and all around changes and hits the helicopter from everywhere and everything starts to shake and shiver. The ****ty pilot thinks he is loosing control and makes a browny on his tail and overcontrols the heli when trying to stay out of the incoming shed!

Pretty good explanation... ei...? :D

Here's what actually happens: The shuddering is caused by the tail rotor going into turbulent air caused by the main rotor downwash as the heli slows down.

[delta3]

Vibrations come from the main rotor blades interfering with vortices of preceding blades (cfr Nicks references). At slower speeds these vortices are not shed backwards. Gordon Leismans book 'Principles of helicopter aerodynamics', shows several nice drawings about this at different speeds (Section Rotor Wakes and Tip Vortices, p 426 onwards)

The difference in behaviour/strenght of this vibrations between different types and even different helos of the same type, are also influenced by different resonant behaviours of the main rotors, this can also depend on the balance of the blades.


Delta3

[Alan.Devins]

was wondering what that was yesterday when doing circuits.

:ok:

[slowrotor]

It is interesting that several ppruners did not know what ETL was .

This happens quite often for me. It is frustrating trying to understand these wonderfull postings when you are missing the meaning of one or more of these annoying acronyms. The better magazines define the acronym with the first usage so its a simple matter to go back and check. It would be nice if that could be done by ppruners as well. Remember the readers are of all ages and experience levels from different countries. Please define your acronyms.

Last month I was reading a multi-part post about LTE. I had no idea what LTE could be, but it sounded alot like ETL. Finally someone gave a clue and I deduced the answer.
ETL= effective translational lift
LTE= loss of tail rotor effectiveness

[Lightning_Boy]

Also not forgetting what an ELT is !!!!!! :ok:

[T4.5]

"This happens quite often for me. It is frustrating trying to understand these wonderfull postings when you are missing the meaning of one or more of these annoying acronyms"

I agree with you Slowrotor !!

I wonder what W.A.F.L.O.B means...:confused: ..http://www.pprune.org/forums/showthread.php?s=&threadid=169959

Regards

[Dash1B]

I'm just a wee student pilot but...

I thought the ETL transition shudder is due to dissimilar angles of attack (and corresponding differences in lift and drag) between the fore and aft sections of the rotor disk, also known as the transverse flow effect.

Furthermore, if it were BVI then wouldn't the pilot would also hear pronounced blade slap? I don't remember hearing any slap while transitioning through ETL.

-1B

[Bravo73]

Is that a serious query, T4.5?

Cos if it is, I've never actually used it but it might stand for: "What A F**king Load Of B*llocks" :E

Would make sense if it did!


Regards,

B73


P.S. Whilst on the subject, 'PMSL' has got to be one of my favourites!

[NickLappos]

Dash1B said, "I thought..."

Don't you mean, "I was told...."

The gobbledigook you have spouted is really a poor description of BVI.

BVI is not slap, unless significant air velocity is involved, so that some transsonic flow (the actual slap) is encountered.

[genocchio]

My two cents….

The reason we get the vibrations is due to Transverse Flow Effect… or In Flow Roll for those of you on the other side of the pond. :D

In forward flight, air passing through the rear portion of the rotor disk has a greater downwash angle than air passing through the forward portion. This is due to that air being accelerated for a longer period of time as it travels to the rear of the rotor system.

The downward flow at the rear of the rotor disk causes a reduced angle of attack, resulting in less lift. Increased angle of attack and more lift is produced at the front portion of the disk because airflow is more horizontal. These differences between the fore and aft parts of the rotor disk are called transverse flow effect.

They cause unequal drag in the fore and aft parts of the disk resulting in vibrations that are easily recognizable by the pilot. The vibrations are more noticeable for most helicopters between 10 and 20 knots.

So, what does this mean to us pilot's? Well, the result is the tendency for the helicopter to roll slightly to the Right as it accelerates through approximately 20 knots or if the headwind is approximately 20 knots. (Assuming a counterclockwise main rotor rotation, reverse for a clockwise rotation).

You can recognize transverse flow effect because of increased vibrations of the helicopter at airspeeds just below effective translational lift (ETL) on takeoff and just passing through ETL during landing.

To counteract transverse flow effect, a cyclic input will be needed to correct the rolling tendency.

Hope this helps ya controller... :ok:

[Dash1B]

Mr. Lappos,

Yes, the gobbledegook is per my instructors, and my assigned reading.

[Edit: Removed the repeat of info from earlier post]

I am also puzzled about your statement regarding blade slap. The following suggests high forward airspeed and transonic shock formation exacerbates, but is not required for, blade slap (pulled these from the Internet):

"The first type of [helicopter] impulsive noise, high-speed impulsive (HSI) noise, is caused by compressibility effects associated with high-speed blade motion. This HSI noise is generally associated with advancing tip Mach numbers above 0.85 and with the appearance of shock waves in the flow field around the rotor. A second type of impulsive noise comes from the interaction of the rotor blades with their vortical wake systems. Blade-vortex interaction (BVI)...commonly occurs when helicopters are descending for landings. During descent, the advancing tip Mach numbers are usually subcritical." - A Comparison of Computational Aeroacoustic Prediction Methods For Transonic Rotor Noise, NASA Langley Research Center.

"For a typical helicopter, blade slap occurs during partial power descents or when a blade intersects its own vortex system or that of another blade. When this happens, the blade experiences rapid changes in angle of attack. Either or both phenomenon generates sound. Blade slap is also generated during high speed level flight due to a shock formation being created on the advancing blade tip." - FAA Advisory Circular 91-66

"A helicopter main rotor generates primarily low frequency noise and, in certain operating regimes, high amplitude low-to-mid-frequency noise modulated at the blade passage frequency...such as Blade Vortex Interaction (BVI) noise and High Speed Impulsive (HSI) noise, become dominant in specific operating regimes, namely in descents and at high forward airspeeds, respectively...BVI noise is the impulsive sound emitted when a rotating blade's aerodynamic forces rapidly fluctuate during an interaction with vortices shed from the blade tip. The severity of main rotor noise can also increase during en-route operations due to compressibility effects under high-speed flow conditions..." -Revolutionary Concepts for Helicopter Noise Reduction — S.I.L.E.N.T. Program, Bell Helicopter Textron Inc.

If ETL transition vibrations are in fact due to BVI, it is still not clear to me why there is no associated blade slap.

-1B

[[email protected]]

When will you guys ever learn to listen to Nick, his credentials are impeccable and his knowledge profound - not only that but he has been a TP for many years. If you want to see the difference between the storybook Principles of Flight given in flight schools all over the world and the real aerodynamic explanation then borrow a copy of the books they use at TP school but be prepared for heavy duty maths. P of F is a simplified explanation of what happens - not a scientific proof.

Vibration during the onset or loss of translational lift is due to BVI as the blades bump into the vortex shed by the one in front. There is no blade slap because the leading edge of the blade hits the vortex - blade slap occurs in manoeuvres where the vortex strikes the blade from underneath.

Don't confuse transverse flow (inflow roll) with translational lift they are not the same. Flapback makes the disc tilt backwards and transverse flow makes the disc tilt down laterally towards the advancing side - translational lift makes the aircraft vibrate and climb. The only problem is separating these effects as they are all most noticeable during the transition to forward flight.

[212man]

Probably the next best thing to Crab's suggestion (to read the books the TPS' use, but not readily available) would be to buy 'Helicopter test and Evaluation' by Cooke and Fitzpatrick; what a superb volume, written by two ETPS instructors.

[Quick Release]

did anyone really read the original question "controller asked"

for those who got side tracked "Crab" explaned your blade slap for you :ok: follow his ref: and read, it may help you get your CPL-H, you couldnt possibly already have it right?...right?? (kidding)

The first question wanted to know why it "SHUDDERED" when comming to the hover, When a helo is in the hover, given nil wind, the blade works quite hard due to induced flow, as the helo moves Fwd the blade is moving into un- accelerated air... think of a boat with a heavey ass end getting up onto the "plane" right? a rotor moving into ETL is the same, there is a huge step it needs to get over, how do you think the Pwr required curve gets its shape? so the same for arriving back into the hover only a little more aggrivated by ROD and attitude, as the fwd speed stops making life easy for the blade it starts to sink a little more of coarse you pull more collective increasing the shudder, until the induced flow stablizes into a nice down wash.

Taking off you are lowering the lever a little as you make the transition to stop an early climb as you accelerate and so your easing the effects of TL, or are you one of these surfers that just stop the climb with attitude and think 'yep thats just one of those things':rolleyes:

thats as gindergarten as i can make it. :ok:

[[email protected]]

Quick release - did you either spell check or read your post before you submitted it - it is so kindergarden that it is total bo**ocks.

[Quick Release]

thats gratitude for giving u the thumbs up Crab, so i have a typo' with the G' u should spell check yours mate there is no D" after Kinder /kindergarten dude. The guy wanted a simple explanation to a shudder comming through / out of ETL, simple desturbed air situation and a rotor blade having a hard time of it.

then ya get all the crappin on about inflow / flapback / slappin blades etc: what a joke.:yuk:

[Lightning_Boy]

I thought it was something simple like the difference in lift between the front and aft of the disk, causing a high speed oscillation resulting in vibrations.

But there again, I'm just a simple guy! :confused:

[Captain Pheremone]

Hey Quick Release....!!!

I think sum guys have infilllttrated this forum attemptin 2 turn it into a pissing competition. The only way sum guys look good is by making others look bad.

Don't let them fase u, we appreshiate yaw coments so carry on.

Big Z.

[Quick Release]

Thanks Captain:ok: .... guys it was only my 2 cents worth giving someone a simplistic idea that there is a rapid change in efficiency in the rotor blade/disk when moving Fwd through ETL in those first stages and so the same comming back,nothing more. i wasnt throwing in any X+Y*3= what ever.
Cant be bothered spell checking either, didnt think it was compulsery. i will leave it there and sorry for the spelling errors in this one i hope it doesnt upset too many people.:D :ok:

[[email protected]]

QuickRelease - Iam afraid I failed to see how your post was giving me the thumbs up - it seemed to me like you were taking the piss.
I don't see how talking about boats with heavy asses getting up onto the plane or the blades working hard or easily is going to help explain the vibration as the helo passes through ETL.
You can't dumb down the subject entirely or it is totally meaningless.

In a still wind, free air hover the air enters the disc at 90 degrees ie from directly above it so the inflow angle is very high giving a high pitch angle on the blade but a low AoA. If you could blow 20 kts of wind across the disc, the air would meet the disc from an angle instead of straight down so the inflow angle would decrease giving an increase of AoA for the same pitch angle. This is the main benefit of Translational lift although it is reduced slightly by the need to tilt the disc forwards to maintain ground position since the 20 kts of wind would push the fuselage downwind.

If you look at the downwash pattern in a free air still wind hover and you could see the tip vortices, you would be looking at a helix for each blade as the vortex for each tip is shed downwards. As the aircraft moves forwards (or you blow 20 kts across the disc again), the angle that the vortex is shed moves closer to the horizontal until at the onset of ETL, the blades are bumping into the vortex shed by the preceeding blade and that gives the vibration (shudder). It is most noticeable in ground effect because the vortices get pushed outwards by the ground as they cannot go down any further - this gives a roll-up vortex which is strong and persistent and the disc must penetrate this vortex in order to get ETL. This is the reason that the power requirement actually goes up just before the onset of ETL and not because you fall off the non-existent ground cushion.

For the return to the hover, the decelerative (nose-up) attitude needed to slow the aircraft moves the Inflow angle (in a severe flare the inflow will be coming from the underside of the disc)giving an increase in AoA and masking the real power required to hover. As the benefits of the flare are lost and forward speed reduces, the inflow angle moves towards the vertical requiring an increase in pitch angle to produce the AoA and thus generates the powerful tip vortices again which the poor old blades must battle through until the forward speed is slow enough to allow them to be shed downwards again. This is why it takes more power to come to the hover than it does to hover in a steady state.

[Quick Release]

As the benefits of the flare are lost and forward speed reduces, the inflow angle moves towards the vertical requiring an increase in pitch angle to produce the AoA and thus generates the powerful tip vortices again which the poor old blades must battle through until the forward speed is slow enough to allow them to be shed downwards again. This is why it takes more power to come to the hover than it does to hover in a steady state.
-----------------------------------------------------------------------------------
this is a true stament and i totaly agree, i wasnt taking the piss my grammer is just bad i guess, i did dumb it down because thats how i read the question, i dropped all the aeroD" and tried to paint an image in the mind of ELT. Any how better stop there B4 i dig another hole. cheers

[JimL]

Crab,

Now that's what I call a simple explanation.

Jim

[Dash1B]

Regarding BVI in ETL transition but why no blade slap:

Upon further reading, it appears the magnitude of the resultant noise from BVI depends on, among other things, the blade-vortex miss distance, strength of the encountered vortex, and the velocity vector of vortex incidence to the blade.

The BVI in the ETL transition regime is usually not "strong" enough to generate an audible slap, but nevertheless strong enough to vibrate the ship.

Perhaps some of the experienced folks out there (who can manage a civil response without sounding like an arrogant condescending b@stard) can let me know if this is about right?

-1B

[NickLappos]

Dash1B,

Speaking as one arrogant condescending Bastard, I can assure you that you have finally come to where I was several pages back. Thanks, next time try to get there before you write the return post!

Quick Release, The post I made answering the original question had the exact answer, as well as providing two research web sites with illustrations and data. How did that miss your exactling eye?

Gentlemen, I find it amusing when a question is asked, it is then anserwed, and then several student pilots read their text books back to refute the answer. Specifically, when Crab or JimL or any of the truly experienced experts who frequent this site post an answer, they do so not by reading the "Dick and Jane" texts that students get, but by being in that rare group who actually did the work that helped write those texts. I will flatter myself and say that I am in that group. Call that arrogant if you wish. Or call that informed. Point of view, I think.

[Quick Release]

Nicklappos

I do stand corrected, no i didnt read many posts and did miss the important ones (obveousley), i only offered an enalergy instead, be it a bad one. I wll call it a learning curve re: posting and will only offer technical fact in future. Now can we move on?
:O

[Dash1B]

Mr. Lappos:

Telling a student who is simply trying reconcile your explanation with the flight school textbook answer that he is "spouting gobbledegook" is condescending and unconstructive.

Every student reserves the right to question and not accept answers on blind faith, even those coming from household names with monolithic credentials.

You made two points:

1. ETL transition vibrations are due to BVI. Thank you for setting us straight, unfortunate that the textbook is so misleading.

2. There is no blade slap because the velocities aren't high enough for transsonic shock formation. While true, this did not answer the question, there are still several other mechanisms present to generate blade slap at subsonic velocities. Mr. Crab helped shed some light on this by mentioning the angle dependency, thank you Mr. Crab.

Anyway, my name-calling was equally unconstructive, and I retract it.

-1B

[NickLappos]

Dash1B,

Thanks for that, your points were valid, and questions are always instructive!
Keep digging, please, just watch how you swing that shovel.

As a clarification, most BVI does involve increased noise. The classic blade "slap" that one hears when a Huey passes overhead is a transonic effect, while the muffled blade slap that one sometimes hear (hack writers sometimes say the "clatter of rotors") is just the subsonic flow disturbence. One helo I flew had a nagging muttering noise at 40 knots while at aft CG, the product of the BVI. Forward CG was fine, and if the aircraft was acelerating, the noise disappeared.

[Dash1B]

Mr. Lappos:

Thanks for the further clarification. Sorry for crossing swords with you. We are fortunate that someone with your wealth of experience frequents these boards. I'd say Mr. Controller got his money's worth!

-1B

[Barotrauma]

Trying to find a conclusive answer as to the cause of the vibrations felt when moving through ETL.
They have been described as "vortices blowing off the blades" and " one blade traveling through clean air whilst the other blades are still in dirty air" Both seem rather simple answers to what I am sure is a more complex aerodynamic issue

Anyone able to enlighten me?

Barotrauma

[deeper]

Very simple, not complex at all.

Don't worry about it and it will go away.:ok:

[ConwayB]

Hi there,

I think this subject has been discussed in another forum with 45 replies... so hopefully one of them will strike a chord (no pun intended) with you.

I'm not sure how to post a hyperlink to the thread but go to the search function and type in 'vibrations' and 'ETL' and it will take you right there. (16 July 2005, I think)

(Maybe someone more computer savvy can post the link.)

Cheers
CB

[JimL]

Here it is.

http://www.pprune.org/forums/showthread.php?s=&threadid=174972&perpage=15&highlight=vibration%20AND%20ETL&pagenumber=1

Jim

[Simon853]

Telling a student who is simply trying reconcile your explanation with the flight school textbook answer that he is "spouting gobbledegook" is condescending and unconstructive.


As a student myself, I would suggest it's also quite confusing. I've heard first hand one high-time instructor tell me things like "Wagtendonk is just plain wrong". So what's the student to do when the industry experts often disagree themselves?

Si