High Speed Shaking

[Sternjaeger II]

Originally Posted by Kurfurst

750 kph IAS for 109 (as on all models up to 109K, which increased it to 850), 650 kph IAS on 110.

Different limits were not specified for higher altitudes, ie. it did not decrease with altitude, at least not in the manual (nor did it in 1940 British manuals - although it was only due limited knowledge of Mach effects at the time in both cases, so in reality it was certainly less at altitude). When flutter problems were started to be encountered and people on both sides of the Channel become more aware of it, they tended to decrease VNE at altitude, but left max. VNE near SL the same..

The thing that is odd about the 110 snaking is though that it occurs at low altitudes just as well, at near VNE speeds. If it would be high altitude, I'd understand that its some Mach-related effect.. but its like as if the rudder would be living a life on its own.

Which kinda makes me thing that there are two possibilities:

1. COD has a new hard coded VNE structural dive failure - I have seen similiar on the 109, albeit in much more hairy dives during intentional 'dumb flying', and it sets in earlier because the 110 is limited to 650

2. Engines (props) of the 110 run wildly asyncronized in dive, and this tears the plane apart.

3. Some game controller induced bug with new patch. Curiously, I did have similiar problem with the 2nd beta patch installed, for some reason, my view kept jumping off in the cocpit from time to time without using the HAT switch on the jstick..


What I found annoying though if 1, is the reason, that while no amount of dumb flying (pulling waaay too many Gs with the stick) seem to be able to break the plane, so stupid pilots would not suffer, we may have a rather odd limitation built in by the possible 1. case, which prevents you from even approaching the limits the manufacturers deemed safe yet..!

Kurfurst, flutter is an aerodynamic effect, as such is more pronounced where the air is denser, that is to say at low altitude.
Bear in mind that VNEs are not flutter speeds, they're redundant by a certain percentage (Viperpedia might help us here) and do actually vary according to altitude (modern planes have the so called "barbers' pole" that indicated the KIAS VNE according to altitude).

[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.

[Peril]

Originally Posted by Bewolf

That said, I am curious what you come up with

I came up with exactly what K said (he also posted on the other 110 thread). Seems we have the same documents, I think I got them from him for our Targetware 110 project?

650kmh IAS.

In the sim engines worked on the high speed shaking effect was based on the VNE figure. It was coded by the builder and entered in the data table, presumption is that this is the same with CoD? There was a margin implied where the shaking started OVER this figure and increased with speed in a linear way. Unless this is another form of DM effect (G limit, compression effect etc) which needs to be determined.

However, once crossing 500 kph (which actually is close to the max speed of some AC) the aircraft not just shakes, but changes directions and twist around like being caught in a Tornado. Forget about having the time or stability to actually take aim. This also means that 500 kph is the max you can take into zooming again, not a lot really and gone in no time.

This would indicate there is an issue of some kind with the 110 FM, just where it lies and how to correct it is for the 1C FM guru to determine.

[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..

[Kurfurst]

Originally Posted by Peril

I came up with exactly what K said (he also posted on the other 110 thread). Seems we have the same documents, I think I got them from him for our Targetware 110 project? 650kmh IAS.

I have some English language 110C manual, presumably a translation of the original German short manual for 110C. It follows the same pattern as the 109E Kurzbescreibung of December 1939..

[Bewolf]

Originally Posted by Peril

I came up with exactly what K said (he also posted on the other 110 thread). Seems we have the same documents, I think I got them from him for our Targetware 110 project?

650kmh IAS.

In the sim engines worked on the high speed shaking effect was based on the VNE figure. It was coded by the builder and entered in the data table, presumption is that this is the same with CoD? There was a margin implied where the shaking started OVER this figure and increased with speed in a linear way. Unless this is another form of DM effect (G limit, compression effect etc) which needs to be determined.



This would indicate there is an issue of some kind with the 110 FM, just where it lies and how to correct it is for the 1C FM guru to determine.

Thanks for the effort! Same applies to Kurfürst.

And yeah, I am not disputing the 650 value at all, that is what I have been reading over the years as well.
But I do get that right that everybody is a bit on the speculation side on the issue of crossing 500kph?

[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...