barka

Is there anyone out there who's heard of a condition called "fan flap" on high by-pass ratio engines? My boyfriend is an engineer, and he says that there is no such thing!!!!!!!!!

I'm sure that I've read about this somewhere - any clues?

Mad (Flt) Scientist

barka, I suspect that you mean "fan flutter" - which means he's technically right, but being a bit pedantic.

All airfoils/wings, including fan blades, have aeroelastic behaviour. That is, they must be considered to be non-rigid items (they bend under load) and the interaction of the bending under load must be considered with the change to the load caused by the bending.

Aeroelasticity is usually a bigger concern with the aircraft wing - as this is the most flexible of the various wing-like items we must consider. But fan blades, especially in the larger bypass engines, are pretty big, and are attached to the disc with relatively small root sections. So if the blade were to start to flap then it might be that the aerodynamic forces were powerful enough to overcome the structural resistance to bending, and the blade might fail.

In order to reduce the amount of blade deflection and vibration, manufacturers used to add 'snubbers' to the blades at about half their length. These are really parts of a ring around the engine, with the section added to each blade, so that the neighbouring blades stiffen each other. More modern engines/blades, which use more advanced blade materials, don't use these (they cause penalties to the aerodynamic efficiency of the blade).

Hope that helps.

By the way, what kind of engineer?

Golden Rivet

never heard the term before.

eng123

Nor me.Any one heard of it?

HOVIS

If one adds a N and Y to the first word all seems clear.

I think someone has been winding you up.

Dr Illitout

I remember a FinAir engineer refering to a MD-80's Fan air valve as a "Fanney air valve". Mind you as his 1st language was Finnish and he was doing a course on the MD-80 in English he can be forgiven. Perhaps the boy friend's mother tongue is not English, or is he just taking the...........
Rgds Dr I
P.S. been flying recently Hovis or is the back playing up?!!!!!!

TURIN

Dr I, it was a fanny Hair valve if memory serves.

Right question, wrong target. Dr I. I Will be up and away tomorrow above Mam Tor if the thermals kick off as predicted. Bring your camera>

HOVIS

Huh???????

Dr Illitout

Sorry Hovis, got you mixed up with another "Northener"!!!. No offence ment.
Rgds Dr.I.

ronster

Maybe he ment...."Fanny flap" or "Flappy Fanny"....you can normally find a few around the Galley areas of most aircraft.

rwm

On the RR Spey we used to get vibrations that were cause by the fan blades not floating in the hub properly. We would spray them with various diferent solvents to try to find the one that would work best at cleaning out the roots. I think the last thing I remember useing was a mix of LPS 2 and White Spirits.

mrcabbage

RWM and Mad (flt) Scientist you are absolutely correct.In the RAF, we were taught the terms 'first phase fan flutter' to describe the initial vibration you hear on engine start up before self sustaining speed is reached. Can't remember it being mentioned on any other licence courses though. You hear it particularly well on the old Nimrod and Buccaneer Speys and extremely well on the Tristar RB211s. Just because some of you guys were not told about the condition it does not mean it doesn't exist! The blade dampers fitted to the root shrouding on CFM 56 engines as well as the mid-span shrouds help to disguise the condition. Its really about fine tuning the lenghts, widths and shapes of the fans......

BoeingMEL

From a seriously ill ex-737 driver...thanks for your wit and humour on this topic guys.... you've started my day with a laugh for the first time in months. Bless you all! BM

HOVIS

Mrcabbage, barka was asking about fan flap not flutter. Everyone knows about fan flutter... I lie awake at night dreaming about fan flutter... OK, that is not strictly true. I am not an anorak I am a free man!

Ok rant over.

BoeingMEL do get well soon, we aim to please, the rest is just a side effect.

asheng

Could be worse you could lie awake all night dreaming of "Fanny Flaps"

Exhausting though!