Spitfire IXE inverted (BUG?)

[FTC_Cule]

If you fly a WWII airplane (of which all the ones available in the sim do not have symmetrical aerofoils) and fly inverted, the aircraft's lift acts downward, bringing the airplane towards the ground and hence one will require pushing the stick forward to increase angle of attack, counter the downforce and maintain altitude, albeit red-ing out slightly.

 

Put the spitfire IXe inverted (i've tried merlin 66 only), and something interesting happens: When flying level, the plane will maintain altitude without a touch of the controls, and what's more bizarre, if you push your stick forward to elevate oneself while inverted, the plane will immediately respond and will massively keep increasing the angle of attack by itself up to a point where it's climbing at around a 5-10º angle, inverted, and of course the pilot is in a redout before more data can be obtained. I cannot make up my mind how a NACA 2213 can generate this behaviour?

 

Notice that by increasing angle of attack in the above context I mean what would be pitching down (decreasing AoA) in an upright configuration, but I thought it would make it less clear if we started to talk into negative values throughout.

 

Testing was conducted on standard cloudy weather, spring setting.

[JtD]

The Spitfire had a very low longitudinal stability, and it is possible that in some scenarios it is unstable. Which would make it increase angle of attack by itself after a disturbance, such as a slight input from the elevators. So what you describe is not necessarily unrealistic. I'd blame the small tail surfaces, not the 2200 series airfoil.

[PainGod85]

14 minutes ago, JtD said:

The Spitfire had a very low longitudinal stability, and it is possible that in some scenarios it is unstable. Which would make it increase angle of attack by itself after a disturbance, such as a slight input from the elevators. So what you describe is not necessarily unrealistic. I'd blame the small tail surfaces, not the 2200 series airfoil.

 

Has it occurred to you that what you just wrote is using an entirely different set of physics than what the OP described?

 

Just thought I'd point that out.

[FTC_Cule]

The (dare I say) negative stability of some aspects of the spitfire surely would explain some of the flight model behaviours that are not related to this particular issue and the very sensitive controls, but it does not explain - in fact it almost contradicts - that if you put a IXe on inverted flight it will suprisingly maintain itself flying straight for as long as the carburettor will take it (haven't tested max inverted flight time duration). And it's not about spending some time to find the sweet spot while inverted that flies well - it's like a magnet, do a half roll, either harsh or slowly and it comfortably sets itself in this awkward stable flight without problems.

 

In fact, it flies much better (from stability point of view, as-is right now in 3.003) inverted than upright.

[JtD]

Why would neutral longitudinal stability in your opinion almost contradict the behaviour? You're flying this thing on a computer, there is no disturbance until you move the stick. So it doesn't change pitch until after you move the stick, and then it changes pitch significantly.

 

Maybe it should roll off, the dihedral being negative, but then I don't know if that's not offset by the aircraft being a shoulder decker now.

[unreasonable]

8 hours ago, II/JG54_Cule said:

The (dare I say) negative stability of some aspects of the spitfire surely would explain some of the flight model behaviours that are not related to this particular issue and the very sensitive controls, but it does not explain - in fact it almost contradicts - that if you put a IXe on inverted flight it will suprisingly maintain itself flying straight for as long as the carburettor will take it (haven't tested max inverted flight time duration). And it's not about spending some time to find the sweet spot while inverted that flies well - it's like a magnet, do a half roll, either harsh or slowly and it comfortably sets itself in this awkward stable flight without problems.

 

In fact, it flies much better (from stability point of view, as-is right now in 3.003) inverted than upright.

 

Not sure that it is really flying exactly maintaining altitude, since you only have a few seconds before the engine starts to hiccup and it is pretty hard to tell. But having just tested this myself I agree that compared to other aircraft there is not a noticeable diving effect when you turn inverted when trimmed for level flight. Although I did not find the resulting flight to be unusually stable.  I think you have spotted something interesting happening.

 

Before assuming that this is a bug, however, it should be noted that the Spitfire has a comparatively low wing loading: say 65% of contemporary 109s.  Therefore, assuming a similar wing CL, it requires a comparatively low AoA to maintain level flight: i.e. maintain enough lift to equal it's weight.  So not only is this true in normal flight but also in inverted flight.

 

For instance: suppose for plane A with a higher wing loading the required AoA (trimmed for hands off level flight) is 3 degrees.  For inverted flight it is -3 degrees. (Not exactly...)

When you invert the AoA has to change 6% to maintain level flight - so you either do this by pushing the nose towards the sky or you will start to descend...

 

Suppose for a Spitfire the AoA required for level flight is only 2 degrees - hence when you invert the required change is only 4 degrees. 

 

So I suspect that the Spitfire should feel as though there is less need to push the nose towards the sky than for most other planes, although someone who knows what they are talking about may be able to debunk this theory very easily.   Whether that - if true -  is enough to fully explain the observed behaviour I expect not - there may well be something else. [edit: Does the Spitfire have a larger proportion of effective over total AoA for instance? ]

 

[edit - Forum has merged my supplementary post:] Having just tested the clipped wing version in the same way, I suggest that the OP does too. Interesting.

Edited May 25, 2018 by unreasonable

[FTC_Cule]

I have flown inverted for almost 40s and there's no hickups at all. (Spring map, on the deck, Realism difficulty)

[303_Bies]

I've checked Spitfire Mk.Vb also.

It flies reversed in a very similar way.

cheers

[PainGod85]

17 hours ago, JtD said:

Why would neutral longitudinal stability in your opinion almost contradict the behaviour? You're flying this thing on a computer, there is no disturbance until you move the stick. So it doesn't change pitch until after you move the stick, and then it changes pitch significantly.

 

Maybe it should roll off, the dihedral being negative, but then I don't know if that's not offset by the aircraft being a shoulder decker now.

 

Longitudinal stability has very little to do with the fact the airfoil isn't meant to sustain negative alpha. It should experience near complete flow separation once the pilot pushes the stick away from himself while flying inverted, certainly before control is lost due to red-out.

[unreasonable]

9 hours ago, II/JG54_Cule said:

I have flown inverted for almost 40s and there's no hickups at all. (Spring map, on the deck, Realism difficulty)

 

That is odd  - I got erratic rpms after a few seconds:  I was not counting. Have you tried the identical set up with the clipped wing version yet? I would be interested to see what happens.

 

When I tried I got entirely expected behaviour: ie a diving tendency requiring a push forwards on the stick.

[unreasonable]

Here is brief video with some music - it illustrates the rpm irregularity, and the difference made by clipped wings. It is a bit rough....

 

https://www.youtube.com/edit?o=U&video_id=4ap9FPgVAn4

Edited May 26, 2018 by unreasonable