ETL: simple question that requires a simple answer
 

What causes a helicopter to shudder when ETL is lost?

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

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

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.

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

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

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

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.

The loss of ETL causes the shudder.

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/L...ero/Oldbvi.htm

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

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!

...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....

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

You guys forgot that the man wanted a simple answer.

Udder Shudder
 

Once I was flying sober.
That made me shudder.

German beer kills.

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???

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

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.

Vibration when going out of ETL
 

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

was wondering what that was yesterday when doing circuits.

:ok: