High Speed Shaking

[RAF74_Winger]

Kurfurst, flutter is an aerodynamic effect, as such is more pronounced where the air is denser, that is to say at low altitude.

Wouldn't the damping ratio be increased at lower altitudes with denser air, leading to greater risk of instability/flutter higher up?

I don't know that, just asking the question.

W.

[Sternjaeger II]

Originally Posted by RAF74_Winger

Wouldn't the damping ratio be increased at lower altitudes with denser air, leading to greater risk of instability/flutter higher up?

I don't know that, just asking the question.

W.

Flutter will occur at any altitude, the higher you go, the higher the IAS. Consequently your VNE would increase the higher you go, I am not sure whether they went as far as giving different VNEs according to altitude in those days though..

[Viper2000]

Originally Posted by RAF74_Winger

Wouldn't the damping ratio be increased at lower altitudes with denser air, leading to greater risk of instability/flutter higher up?

I don't know that, just asking the question.

W.

That's my understanding.

VNE of quite a lot of gliders decreases at high altitude for flutter avoidance (clearly gliders with a <150 KIAS VNE at sea level aren't bothered about compressibility ).

But I'm pretty certain that this behaviour isn't meant to be flutter, because you do it repeatedly without recourse to the refly button.

Some people fear flutter because they do not understand it; others fear it because they do.

I can't remember who coined that one, but it's certainly true; I've only seen it once IRL, and it ripped the aeroplane affected into bits in 3 cycles and probably 0.5 s. Thankfully it was unmanned, so it was just a case of picking up the bits. But it was a truly awe inspiring sight (and sound - first the aerodynamic buzzing, and then the structural failure, followed by almost total silence as the bits descended at terminal velocity). Unless you've had that sort of experience, you almost can't comprehend just how impressive the forces involved really are. Everybody who saw it had to pick their jaws up off the floor...

[Sternjaeger II]

hang on hang on, there's a bit of confusion here..

Flutter is one thing, compressibility is another thing, and VNE is still another thing..

Example of flutter (start from 1.30)

http://www.youtube.com/watch?v=aZQlmxrmjXQ

what you can see from the video is that the oscillation of the control surface triggers the flutter, but in theory you could "stay in flutter" like in this video

http://www.youtube.com/watch?v=kQI3A...eature=related

and then slow down.. although it's definitely an unhealthy hobby..

if you push beyond the flutter speed you will surely have a fracture.

The VNE would sit right before the aileron excitation.

The important aspect which I don't think they simulated in CoD either (but I hope I'm wrong) is that the aeroplane structures are elastic, and as such flex, deform, fold and break.

[Sternjaeger II]

another interesting video

http://www.youtube.com/watch?v=qpJBv...1&feature=fvwp

There's an ancient (lol) sim called "Fighter Squadron: The Screamin' Demons Over Europe" which had a fantastic aeroelastic FM: it simulated flexibility, flutter and breaking in an incredible way.

[Viper2000]

If you accelerate very gently then you can get into a situation where small amplitude oscillations aren't damped but large ones are, such that bits wobble without falling off.

However, the window between a pilot-detectable wobble and rapid spontaneous disassembly of the aeroplane tends to be pretty small, which is why flutter testing is done with mechanical or pyrotechnic exciters and heavily instrumented aeroplanes in a very careful programme, such that you can plot the declining damping on a graph and put sensible placards in the manual without actually going there in flight.

Since people reach 500 km/h IAS in the 110 when diving, usually because they're chasing something or being chased, the chances are that if the problem was flutter then it would be fatal.

This would then be a clear modelling error because flutter below VNE = new pilot's notes & many heads from both the airframer and the customer's test organisation presented to top brass on silver platters at very high speed.

The general character of the behaviour as I have experienced it in flight is of divergent directional snaking, as explained in the link I posted earlier.

It's basically a yaw problem, with roll due to coupling.

It makes sense that this would be a problem for the 110 due to the relatively complicated rudder control run, which would likely be subject to cable stretch, friction, backlash etc. simply due to its geometry.

However, research is clearly needed to find out if this was a real problem in service.

Does anybody have a copy of Wings of the Luftwaffe lying around? Mine's at home...

[robtek]

I am absolutely shure that this kind of shaking would have been mentioned in the literature about the 110.
There is not one line in all the texts about the 110 where something like that is mentioned.
Thats all the proof i need to know that this shaking is a error in the FM.

[Sternjaeger II]

Every plane will do that,regardless of what it is or what the manual says. Physics are the same for all

Having said this,it would be really useful is someone of u guys could post a video of the problem.