Nose pitches up with damaged horizontal stabilizer

[sturmkraehe]

Can somebody explain to me why the nose (of Spitties) pitches up when the horizontal stabilizer gets hit?

 

Generally the horizontal stabilizer of a classic configuration (horizontal stabilizer at the rear) has to generate negative lift afaik in order to compensate for the nose-down pitch moment induced by the wing lift (centre of gravity usually a little before pressure point). So when rear surface is damaged one should think that less downward lift is produced by rear horizontal stabilizer making the wing lift the predominant lift force that since behind the centre of gravity would then induce a pitch-down moment. That's what I would expect.

 

But ingame I observe always pitch up when my tail gets damaged. It happens in the Spits and I don't know if this is also like that in other planes.

 

Can somebody explain why this is behaving like that ingame and what are your opinions about that?

[ZachariasX]

The Merlin Spits cruise with a prominent elevator down position. This is not reflected in game, but it is a relatively tail heavy aircraft and behaves like such. I cannot make statement on what the sim makes from that in extreme conditions, but I see nothing implausible. All in the „maybe“ department.

[sturmkraehe]

That would mean that the CoP of the wing is before CoG. That is not impossible but pretty uncommon for classic aircraft architectures. Usually one has to add negative lift at the tail via that horizontal stabilizer to compensate for the pitch-down impact of the wing. This is of course an undesirable side-effect of stabilizing the aircraft as one reduces lift. If the Spitties have the same effect with positive lift that is pretty exotic.

 

If Merlin-Spitties fly with such elevator position, what is its responsivity towards pitch-down maneuvers? As with such position in cruise wouldn't be the authority for further down-tilting be more limited and hence the pitch-down moment?

 

This nose up behaviour I also find in the Spit14. Other planes: have to test.

[ZachariasX]

2 minutes ago, sturmkraehe said:

what is its responsivity towards pitch-down maneuvers?

Immediate.

[sturmkraehe]

My mistake, I was not clear. I meant how large is the pitch-down moment that can be generated with the remaining elevator deflection angle? If quite a lot of downward deflection is already eaten up by cruise elevator position I'd say maximum pitchdown moment would be less than maximum pitchup moment.

[JtD]

You can break or stall the Spitfire both with nose down and nose up input, the elevator is totally sufficient to cover the range. Because it is very sensitive.

 

The Spitfire is not unique with the downwards elevator deflection, the Hurricane does the same. With 0° trim on the horizontal stab, the Bf109 also requires an elevator push input. If I looked further, I could probably name a few more, it was not that uncommon.

[Holtzauge]

Some time ago I posted a figure showing the elevator angle for trim for the Windex 1100 motorized glider with different alfa and flap deflections (Should still be in this forum somewhere). The Windex 1100 is not an unstable aircraft but you will still see a positive lift on the tail in some cases even thought it has a slightly stick back (negative elevator angle) due to the combined effects of alfa and downwash. So it's not quite so simple that a stable aircraft always has a negative lift on the tail and consequently quite possible to get a pitch up effect if you lose tail surface on some stable aircraft under certain flight conditions.

[6./ZG26_Klaus_Mann]

The Spitfire is built around Neutral Static Stability more than any other WWII Fighter. To that End it has CoG very close to CoL and Vertical Stabilizer Trimmed Tailheavy.

The Result is that the Aircraft only changes Pitch Attitude if the Elevator is moved, without making Attempts to right itself, or only very slowly, with Elevator Trim not behaving linearly as on a normal Plane, but every Trim Setting drawing a completely different Graph on a Trim-Reaction-Force Diagram.

 

https://www.boldmethod.com/learn-to-fly/aerodynamics/3-types-of-static-and-dynamic-stability-in-aircraft/

 

You can also make the German Fighters behave that Way if you Trim them Tailheavy and don't mind having to constantly push forward.

Edited May 13, 2021 by 6./ZG26_Klaus_Mann

[ZachariasX]

6 hours ago, sturmkraehe said:

My mistake, I was not clear. I meant how large is the pitch-down moment that can be generated with the remaining elevator deflection angle? If quite a lot of downward deflection is already eaten up by cruise elevator position I'd say maximum pitchdown moment would be less than maximum pitchup moment.

Forward travel is less than backward travel, as preceived neutral position on the ground moves forward considerably, same as for instance in an ASK-21 glider at 120 km/h. But you still have plenty of stick travel as you need little deflection for most of your needs. But I wouldn't recomment doing an outward loop.

[sturmkraehe]

On a side note: It happens not unfrequently that when receiving damage the pitch control becomes all wobbly and when looking at the outside model: NOT A SINGLE damage on the horizontal stabilizer or elevator. 

 

My impression: The optical dmg system is not equal the value-reflected damage model.

 

Also I think that the impact of a few small holes on the entire aerodynamic is modelled pretty strongly.

[unreasonable]

Just now, sturmkraehe said:

On a side note: It happens not unfrequently that when receiving damage the pitch control becomes all wobbly and when looking at the outside model: NOT A SINGLE damage on the horizontal stabilizer or elevator. 

 

My impression: The optical dmg system is not equal the value-reflected damage model.

 

Also I think that the impact of a few small holes on the entire aerodynamic is modelled pretty strongly.

 

Might it also be possible that the "wobbliness" is due to damage on the wing? And that this is reasonable?

[sturmkraehe]

Can't imagine how. I may have some loss in lift, maybe there is a MINISCULE shift in CoL but since the lever arm between CoL and CoG is very small, this should have a highly negligible impact when one has a healthy elevator authority.

 

If the hypothesis that you put forward was the case the holes in the wing would have a disproportial impact on the Lift/AoA slope making it much steeper. Can't think of a physical phenomena that could have this effect. But please do not hesitate to let me know if you know such an effect. I am always happy to learn and this I mean like I say it.

 

By the way: I really think the effects of the holes are to my thinking overmodelled - assuming that there is a real connection between the graphical depiction of holes and the implemented damage to the FM. I also have sometimes a very limited number of small holes on the lateral stabilizer leaving, I'd generously say 99.5% of the aerodynamic shape of it intact, but the plane loses most of its directional stability. 

Edited May 14, 2021 by sturmkraehe

[unreasonable]

I am just speculating - with the Spitfires having such weak stability to start with.

 

On the DM the graphical connection between visible damage and actual effect is weak, in that there is a set number of layers of surface damage for each hit box, but someone recently managed to show that the additional drag on a hit box was a function of the actual number of hits, not stepped like the visual damage.  For all that, it is not that weak: the hits and graphical overlay are still by hit box. 

 

If you get an internal damage RNG determined effect, you might not know about it, in the case of a spar, but possibly there is an intermediate damage state in the case of control systems, which also might account for wobbliness.

 

TBH I doubt that we can answer many of these questions with much certainty. 

 

 

[6./ZG26_5tuka]

Until DVD is incooperated for all aircraft the visible damage model remains vague in it's depiction of hits using predrawn damage textures. DVD will place damage decals in accordance to type of shell, calibre and impact location to give a better impression of tge actual damage state.

Edited June 9, 2021 by 6./ZG26_5tuka