What causes a helicopter to shudder when ETL is lost?

[slowrotor]

those nasty three letter words

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 !!!!!!

[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... ..http://www.pprune.org/forums/showthr...hreadid=169959

Regards

[Dash1B]

Hrm....

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"

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.

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...

[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 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'

thats as gindergarten as i can make it.

[[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.

[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!

[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 .... 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.

[[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.
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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