High Speed Shaking

[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/wwwDOTda...1CBC682E928D2A

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

[Bewolf]

Originally Posted by Sternjaeger II

check out this video at 6.07

http://www.youtube.com/user/wwwDOTda...1CBC682E928D2A

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]

Originally Posted by 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.

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]

Originally Posted by Viper2000

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.