High Speed Shaking

[Bewolf]

Ok, I am an old boom and zoomer back from the IL2 days. Nothing more satisfying to get rewarded for all that patience in climbing and restraining oneself to go into the furball, specially in aircraft that could be considered bricks otherwise.

However, there are a couple problems with this tactic lately. Preferring the BF110 currently, imho the aircraft with most character in CoD, boom and zooming is an absolute must as direct dogfighting is pure suicide. 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.

The 109 encounters this at around 600 kph.

Was this really this way? If it was, I will shut up, but I have a couple doubts about the nature of shaking encountered. Would be very nice if a more knowledgeable person could shed some light on that.

[Viper2000]

How high were you?

Since the latest patch doesn't fix the high altitude engine problems (which seem to be related to mixture strength), it's possible that you've got engine trouble; obviously this will be worse for a twin than a single. You might also have engine rpm problems. Obviously, anything engine related will cause yaw and roll in a twin.

I don't honestly know what VNE for the 109 and 110 would have been in 1940. However, in IL2 we've got used to pushing many aeroplanes way beyond their published VNE (e.g. I'd regularly exceed 850 km/h IAS+ in the Fw190, when the redline on the ASI in the cockpit was at 750 km/h), just as we also got used to being very casual about airframe g limits or indeed engine handling. So it's possible that VNE is now being enforced more strictly than before.

Doubtless Kurfurst knows the answer to the VNE question.

[Bewolf]

Hey Viper, I think you misunderstood my post. This shaking is not really engine dependent, even with idle thrust and prop pitch to idle this occurs when crossing the 500 kph mark in the 110. You may want to try it yourself to see what it is about. Push your nose down at, let's say, 3000 meters, if you are going full speed even 2000 meter and see what happens once crossing that speed mark. You lose direction control, making it near impossible to line up for shooting. The same happens to other planes long before reaching their maximum speed limit.

[Viper2000]

I tested it.

It's classic directional snaking behaviour. I don't know whether the 110 suffered from it at this speed, but it's not an unreasonable generic high speed (mis)behaviour; lots of aeroplanes from this period suffered it if pushed too fast.

PS - the 110 is a real killer! I got 4 Spitfires in the low level dogfight quick mission. Couldn't find the engine oil temperature though, which was annoying, and the engines died just as I got back to France...

[Bewolf]

I tested it.

It's classic directional snaking behaviour. I don't know whether the 110 suffered from it at this speed, but it's not an unreasonable generic high speed (mis)behaviour; lots of aeroplanes from this period suffered it if pushed too fast.

PS - the 110 is a real killer! I got 4 Spitfires in the low level dogfight quick mission. Couldn't find the engine oil temperature though, which was annoying, and the engines died just as I got back to France...

Yeah, try that against 4 Spits in Multiplayer ;)
Against everything else she is a killer par excellance. My fav bird here so far, shredders everything appearing in front of her nose, hehe.

Directional snaking, hm? Gotta see what google comes up with here.

Btw, you may want to check the engine nacelles next time you look for instruments in the 110

[Viper2000]

It's certainly no Fw190, but lots of relatively big guns in the nose and reasonable top speed should make it pretty effective against opponents with poor SA (who should be more plentiful now that there's a lot more systems management for people to worry about...).

As for snaking & other aspects of stability & control, try this:

http://history.nasa.gov/monograph12/ch4.htm

[Peril]

boom and zooming is an absolute must as direct dogfighting is pure suicide.

Mm, sorry dude not trying to be rude here but this has nothing to do with accuracy of the 110C4 in game, that's a 'want'. The 110 was not known as a 'good' fighter plane and was chewed up in real life BoB buy the RAF fighters. I think it's better if we just stick to the facts and try to prove the accuracy. Now having said that, I have some documents here that may help define the VNE, but they are in German.

I'll see what I can translate.

[Bewolf]

Mm, sorry dude not trying to be rude here but this has nothing to do with accuracy of the 110C4 in game, that's a 'want'. The 110 was not known as a 'good' fighter plane and was chewed up in real life BoB buy the RAF fighters. I think it's better if we just stick to the facts and try to prove the accuracy. Now having said that, I have some documents here that may help define the VNE, but they are in German.

I'll see what I can translate.

Then why are you? I consider "want" a perfectly legit motivation to check up on an observation that neither appeared in anything I read yet not experienced in another simulator. That does not mean it was not to there but it warrants enough reason to ask a question when personal flying style relies on it, don't you think? And it certainly does not justify to imply here I was actively seeking to change FM against realism for personal gain.

That said, I am curious what you come up with

[Sternjaeger II]

I haven't tried the game yet, but it sounds like good old flutter to me.
http://en.wikipedia.org/wiki/Aeroelasticity#Flutter

[Kurfurst]

I don't honestly know what VNE for the 109 and 110 would have been in 1940. However, in IL2 we've got used to pushing many aeroplanes way beyond their published VNE (e.g. I'd regularly exceed 850 km/h IAS+ in the Fw190, when the redline on the ASI in the cockpit was at 750 km/h), just as we also got used to being very casual about airframe g limits or indeed engine handling. So it's possible that VNE is now being enforced more strictly than before.

Doubtless Kurfurst knows the answer to the VNE question.

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..!

[Sternjaeger II]

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]

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]

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]

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]

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]

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 :-P ).

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=kQI3AWpTWhM&feature=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=qpJBvQXQC2M&NR=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.

[robtek]

Well, here is my FIRST youtube video.
http://www.youtube.com/watch?v=YhMSpMGLYk0

[Viper2000]

I did some more testing.

It certainly has the symptoms of snaking; you can see that the problem starts with un-commanded rudder movements.

In fact, I sometimes come out of the "manoeuvre" with the rudder jammed hard over to one side; so we've got over-balance as well.

Looking in the cockpit, there is also aileron buzz - the stick is vibrating from left to right pretty fast.

This might be caused by the snaking, or it might be a separate event.

It's probably covered by the "flutter" tickbox in the realism settings, but it's not exactly what I'd call classical flutter. It's a control system problem.

I suppose I should investigate what other aeroplanes do in high speed dives too...

[robtek]

Yes, the rudder was jammed to one side, quite impossible when there is someone standing on the rudder-pedals.
The rudder in rl becames pretty stiff at higher speeds (GA -> around 140 - to 160 kts).
There could be powers greater then the leg muscles of the pilot BUT that would have been written down in pilots notes or the manual of the 110 (be aware that if you exceed ......).
There is nothing in the literature!
So for me that is the possybility of a aerodyamic instability wrongfully applied to a stable plane.

[Sternjaeger II]

yep, sounds like a buggy rendition of flutter. Interesting stuff though, although I doubt wings will wobble. It's a shame they pay so much attention to so many details (some of them a bit redundant maybe) and then they don't take into account aeroelasticity..

[Sternjaeger II]

check out this video at 6.07

http://www.youtube.com/user/wwwDOTdalsgaardDOTeu#p/c/981CBC682E928D2A

u see the rudder and wingtips shaking? Is that what you're talking about?

[Bewolf]

check out this video at 6.07

http://www.youtube.com/user/wwwDOTdalsgaardDOTeu#p/c/981CBC682E928D2A

u see the rudder and wingtips shaking? Is that what you're talking about?

I think these are the initial symptoms just after crossing 500 kph, its gets much worse with increased speed up to 650 kph.

[Viper2000]

Yes, the rudder was jammed to one side, quite impossible when there is someone standing on the rudder-pedals.
The rudder in rl becames pretty stiff at higher speeds (GA -> around 140 - to 160 kts).
There could be powers greater then the leg muscles of the pilot BUT that would have been written down in pilots notes or the manual of the 110 (be aware that if you exceed ......).
There is nothing in the literature!
So for me that is the possybility of a aerodyamic instability wrongfully applied to a stable plane.

Rudder overbalance can easily exceed pilot strength. Control hinge moments can scale extremely fast.

I agree that you would expect to see something in the manual about it. But quite often manuals from this period can be rather oblique.

For example, the Mustang's manual prohibits aerobatics with a full rear tank, but it doesn't tell you that this is because derivative of stick force per g with respect to g is negative, such that above something like 2 g you need a push force to stop the g from increasing.

The attitude seems to have been that the pilot's job is to do and die, not reason why.

Therefore, I'd be more inclined to look for a placard in the manual (do not fly above x IAS at low altitude). Or possibly even just a statement that tracking manoeuvres and formation flying are difficult at speeds higher than x.

The other place to look is flight test reports.

The way the aeroplane misbehaves is more consistent with snaking than anything else; it's relatively low frequency. There is also aileron buzz, which is on the edge of flutter.

However, we don't have what I would call true flutter due to aeroelasticity of the main flying surfaces (the model probably assumes them to be perfectly rigid; most flight sim models do).

Real flutter doesn't generally cause the aeroplane to change direction much; the frequency tends to be too high. It also generally happens so fast that the pilot's experience is more like:

Flying...
Flying faster...
Flying faster...
BANG!
Oh dear! What just happened?
Why is the aeroplane doing that?
I need to get out!
I can't get out - too much g, too much dynamic pressure!
...
Ashes to ashes, dust to dust...

Ejection seats are wonderful things.

Control surface flutter is a somewhat different animal because the stiffness of the system is more variable, the balance of the surfaces is more variable, and the pilot has an influence over the way in which the system is damped.

Additionally, the control surfaces can deflect through relatively large amplitudes without failing.

This means that you've probably got a little more chance of noticing the problem early, when it's a sort of incipient buzz, and slowing down before something falls off.

In any case, losing an aileron isn't likely to immediately kill you; the real danger is that the control surfaces might excite true flutter in the main aerofoils; clearly losing a wing will kill you unless you can get out of the aeroplane, which is likely to be very difficult due to the centrifugal forces and high dynamic pressures involved, unless you have a bang seat.

Snaking is caused by the fact that very small amounts of rudder float can lead to quite large changes in hinge moment, which means that the aeroplane can end up going from side to side for no obvious reason.

It isn't necessarily a safety issue, provided that the oscillation is damped. However, it makes it very difficult to track a target for a gun solution.

This problem afflicted various types well into the jet age; Me262, Meteor, Vampire etc all had it to varying degrees at high speeds below their VNE and MMO.

As with the whole thorny subject of tactical Mach numbers, there is a difference between what you can do with an aeroplane defensively, and what you can do with it offensively.

The majority of the limits in the Pilot's Notes tend to tell you what you can do defensively (i.e. before something other than enemy action is likely to endanger the aeroplane).

I suppose that the best way to describe it is to say that the flight envelope defines risk. If you keep your IAS nicely inside what is now known as "the green arc", you're very safe. If you go into the yellow arc then you're still pretty safe, but you have to take care to stay within your g limit, and remember that the stress calculations are based upon the assumption that one control is applied at a time. So rolling pullouts are a caveat emptor manoeuvre. You also don't want to fly this fast close to a CB because a serious gust might break the aeroplane.

If you fly up to VNE then you've got no more than 1/3rd control deflection in one axis at a time. You really don't want to hit a big gust.

However, exceeding VNE won't instantly break the aeroplane.

The limit is there to say that going faster starts to increase your risk exposure.

So if you're flying around subject to no threat whatsoever, then you'd want to stay inside the published envelope.

If you're under some finite level of threat then you might decide to exceed the published limits by a small amount, until the incremental risk of pushing the airframe harder matches the decremental risk from enemy action.

In the limit case, if the threat from the enemy is exceedingly severe then you might as well completely ignore the limits, because it makes no difference whether you are killed by enemy fire or structural failure; you're still dead.

Limits only have significance if you are likely to want to use the aeroplane again.

Flying offensively is a different kettle of fish. It's much harder to accurately aim your aeroplane at the target than it is to avoid somebody else's fire.

Therefore offensive flying requires far greater accuracy and precision of control. Failure to achieve this high standard of control really doesn't affect the safety of the aeroplane, and if you can't get a gun solution because of snaking or some other control defect then this doesn't preclude you from disengaging, going home and shooting a line about it in the bar.

For this reason, there is a tendency for non-safety related aircraft issues not to make it into the Notes. Instead, this sort of information often gets passed around via word of mouth, along with information about what funny noises from the airframe are normal, how much oil leaking from the cowling is too much etc..

Personally my feeling is that the current misbehaviour of the 110 above 500 km/h IAS is excessive, but equally I don't think that this sort of thing is black & white for the reasons outlined above.

[Lixma]

In fact, I sometimes come out of the "manoeuvre" with the rudder jammed hard over to one side; so we've got over-balance as well.
Yeah, the control surfaces stick in their last known position as you transition from the 'flutter-zone' to normal regimes.

[Peril]

We also need to consider that what you see is removed from what we feel in a sim. You can have effects triggered in a FM that need not be related to what we see, the two are quite separate. You can put 3D wings on backwards and it makes no difference to how it flies. Any direct link from what you 'SEE' in game may not be a prime indicator of the problem.

FM is the key trigger of effects but unfortunately you can't see a FM. eg. A 3D tail missing in a Spitfire does not mean the 'FM' tail is missing and so it flies accordingly (a bug seen recently).

[TomcatViP]

What ever we said to how and why, this shld not happens before max level speed. 500kph is bellow that speed for the 110.

Max recommended dive speed for most of the 109 was 750. In CoD diving faster than 600kph tend to render aiming impossible.

As a spit might fly for extended period of time at 480 at med alt , it simply negated the Boom&Zoom capability of the 109. I had even difficuties diving on Blenheim bombers flying in formation :eek:

IMHO this effect (twisting/shaking) is a wonderful add regarding old IL2 Mach 2 divers but it occurs without any transition at much too low speed.

I think devs shld tune the curve for the effect magnitude. Something hve turned wild in the process

[Buzpilot]

Interesting video about aeroelasticity here.

http://video.google.com/videoplay?docid=6786323708915748347#