THERE'S A BIG DOG CALLED "FLUTTER" HIDING UP THERE

[Captain]

THERE'S A BIG DOG CALLED "FLUTTER" HIDING UP THERE

 

Below is a good reminder about the possible consequences of exceeding VNE .... which is what the glidingcommunity say may have been the reason for the problem described below. The glider was described as doing a low fast pass in front of the crowd on May 24th at the Swiss Championships.

 

It is sometimes easy to forget.

 

The below translation from German could be better.

 

 

_________________ Captain

 

 

[Guest ozzie]

this is again another good reason for fitting recovery systems.

 

please think about it.

 

 

[Ultralights]

i was under the impression that most if not all glider pilots competing in events wore a personal parachute?

 

 

[Guest Guest]

It's compulsory to wear a chute whenever you go cross country in a sailplane.

 

But a personal chute, or a recovery chute, doesn't help when you are as close to the ground as this was.

 

Easier to stay at or below VNE.

 

 

[Guest micgrace]

Hi

 

What a lot of glider pilots forget the wing loading can easily exceed structural limits, especially in competition. Plus flutter can be developed if, in competition use you diliberately tend towards an aft CG to decrease drag, increase speed etc (lifts nose slightly) but o so dangerous.

 

Parachute x-country mandatory. All gliders I know of fitted with canopy fast release of some type.

 

The only thing that irrates me in gliding is the winch/tow Apart from that, excellent.

 

Micgrace

 

 

[Ross]

Hi Crew

 

I would have thought the more important speed was manoeuvring speed. Now I am not considering flutter at all but from what I have heard is that flutter can happen and massive damage can occur almost before a pilot was aware it had happened but I think that occurs at or aboveVne and we can be in enough trouble long before we get near Vne from excessive g loads. So the following remarks are just based on the potential g loads that can occur on an aircraft.

 

The following calculations are based on assuming the aircraft wings are subject to turbulent air flow rather than laminar. High performance gliders have more of their flight envelope in the laminar region which changes the L/D equation drastically from what occurs in turbulent flow conditions.

 

Theroretically from what I remember learning in gliding the lift a normalwing can exert is proportional to the square of the true air speed of the unstalled wing.

 

So if a wing on the point of stall flying straight and level isat 40 knots TAS it is exerting 1 g on the aeroplane. That wing at 80 knots TAScould without stalling exert the square of the ratio of the speeds times 1 g on the aeroplanei.e. (80/40x80/40)= 4 gs.

 

So it could do a steep turn up to the point where it experiences 4 g at which point it would stall.

 

At 120 knots TAS we are talking 9 gs or potentially 9times theweight of the aeroplane without the protection of the wing being able to stall before that happens whether it be a glider or a RAA aircraft.

 

From what I understand anyone that flies an aeroplane at anything like near Vne would want to be doing it when the air was dead calm and use extremly small and gentle control inputs.

 

For a mythical plane for which I havethe owners manual various speeds inmy owners manual are:-

 

Clean stall power off is 52 KIAS read 1 g by my calculations

 

Vno Max structural cruising speed 104 KIAS read 4 g possible by my calculation

 

("do not exceed this speed except in smooth air and then only with caution")

 

Va Manoeuvring speed 104 KIAS read 4 g by my calculation ("Do not make full or abrupt control movements above this speed")

 

Vne Never exceed speed 131 KIAS read 6.34 g possible by my calculation ("Do not exceed this speed in any operation")

 

I think 6 g is close to or exceedsthe maximum acceleration that a fit human body could stand without blacking out.

 

The flight load factor quoted for this plane is +3.8 and -2.0

 

Using my calculations 3.8 g could be experienced at the square root of 3.8 times clean stall speed of 52=101.4 Knots and that is before any gusts etc are taken into account.

 

-2.0 g couldbe experienced at 73.5 Knots.

 

Note that these are the g forces that the plane can be subjected too at the posted speeds in still calm air.

 

If there are strong thermals or gusts or wind shear etc the air speeds can increase dramatically at any initial IAS with a subsequent dramatic increase in possible g loads on the airframe.

 

One dayin a glider winch launch over the airfield at Leeton I experienced continuous wind shear and saw the air speed jump from about 60 knots to about 80 knots. This was at about 500 feet AGL

 

Because the glider is in a very steep angle of attack when winch launching and carrying a lot of extra weight (up to part of the weight of possibly a mile of cable which the pilot cannot feel) it is normally flown with a weak link in the cable at what must be close to the stall (with the extra weight) to help reduce the possibility ofsudden gust loads.

 

So the pilot's defense is to get the nose over to reduce the angle of attack, signal the winch driver to slow down,to reduce the load on the wings on this particular aircraft and release the cable if the winch driver has not reduced power. Hopefully the winch driver will sense the extra load and reduce the speed and power applied to the winch wire before the speed increases too much. It is always best to have an experienced pilot as a winch driver.

 

Sometimes, with a wide awake winch man, the winch might actually be stopped and the glider be "kited" up by the pilot and the cable released by the pilot when he is high enough.

 

The speed changealso did the opposite with a large drop in air speed as I came in to land and it kept thisup for a couple of hours. A bit later onanother pilot had a cable break, surprise surprise. A glider pilot would immediately put the nose over with any slow down in speed even without a cable break.

 

Immediately gentle nose over - about 5 seconds available to do this before a stall

 

at the same time release what is left of the cable if any- it usually would self release

 

-get to a safe air speed before doing anything else - there is no height to play with

 

if sufficient space ahead

 

- full dive brakes

 

-land straight ahead,

 

still about 400 metres left after stopping. It is nice to have a big paddock!

 

Apologies on the length of these comments. It is bed time anyway!

 

Contradictory views,better explanation or commentson these points arewelcome. Probably rules applicable to gliding have changed since I stopped doing it but the engineering does not often change.

 

Regards

 

Ross

 

 

[Guest micgrace]

Hi Ross

 

I'd like to add, the weak link where I did some gliding was placed just foward of the nose so when it broke (heavy handed winch pilot usually) there was just a short 6" section of cable left.

 

A break happened to me at 6' Slightly heavy landing as glider hadn't reached flying speed.

 

The real problem of manouvering speed comes into play esp gliders due to very long wing. I'm not going into rotational kinematics theory to explain. (few do)

 

As for flutter, an easy way to see the effects are to follow someone on the highway pulling a load covered with a tarp.

 

Below a certain speed, just gentle movement of tarp. Above that voilent and very noisy flapping easily overheard over road noise.

 

Basically when the natural vibration frequency of the device (aerilon etc) matches the altered frequency (take air as a "wave" ) of the relative airspeed flutter occurs.

 

This does NOT mean high speed =flutter at any speed it can occur. But most likely at higner speed.

 

How do you control it? 2 ways, raise the natural frequency (stiffen aerlion etc, more hinge points, reduce unsupported area) or by mass balancing (dampen out the oscillations)

 

Micgrace

 

 

[Captain]

What a lot of glider pilots forget the wing loading can easily exceed structural limits, especially in competition.

There has been some further SPECULATION on one of the Gliding Forums re this accident, which varies from the original Flutter speculation.

 

This one has been attributed to the boss of the Swiss Gliding Federation ... but please treat it just as internet unofficial speculation at this time ... and I quote the other post as below ..... HOWEVER flaps going suddenly from positive to negative has been an issue in OZ toorecently:

 

'According to an unofficial report, the pilot passed the finish line [presumably of a competition] with his Ventus CT with high speed. The flaps seemed to be in a positive position. Then, the flaps slipped immediately in a negative position. This created a big up and down movement of the wings and the right wing broke away. The picture shows the glider 2 seconds before the impact on ground, 50 m away from the spectators. No failure of the glider's structure was detected up to now. [i have deleted here an opinion as to cause. CN] The pilot (69) had over 2000 hours on gliders and he participated several times in regional or national contests.