Pitch stability...

[Anatoli-Kagari9]

Noticed that with a trim setting of near neutral, if I let go of the stick, the Spitfire initiates a dive never recovering ?

[NeilWillis]

The fix is to trim the aircraft. Neutral trim isn't the same thing as trim depends on aircraft weight, engine settings, and air speed.

[Whisper]

Not yet discovered the bird completely, but so far, for take off / landing loop training, my procedure is -1 trim for take off, then cruise settings and +1/+1.5 trim for cruising, that I keep during landing run.

Not done it yet for combat settings.

[Anatoli-Kagari9]

The fix is to trim the aircraft. Neutral trim isn't the same thing as trim depends on aircraft weight, engine settings, and air speed.

 

Thx Neil,

 

I am well aware of that, since I fly for real too :-), but what I find strange is that even starting from altitude, it will never recover if you push the stick and then let it return to neutral.

 

The aircraft enters a dive, and actually diverges, with increasing negative pitch.

[escaner]

The aircraft enters a dive, and actually diverges, with increasing negative pitch.

 

That means that the aircraft is statically unstable.

Much has been written about the Spitfire stability, but I haven't followed it, so can't say whether it is correct or not.

[humptydumpty]

maybe it has RSS

[Sporg]

Quotes from Chuck's guide thread:

I noticed that with a trim setting near neutral for pitch, if I push the stick and return it to neutral, the aircraft initiates a dive of which it never recovers, actually increasing the dive angle as it goes down at increasing rates... ?

I think that the Spitfire, like many WWII fighters, had marginal longitudinal dynamic stability, probably also depending on c.g. position, throttle setting, etc. So a divergent phugoid seems a possibility.

If you read the book "Spitfire: A Test Pilot's Story" by Jeffrey Quill, you will see that during most of the Spitfire development they fought with the longitudinal stability.

 

For instance they could not put much of a fuel tank in the body because it would move the CoG too much aft.

(That's part of the reason there was no long distance version other than the unarmed PR versions.)

 

They also had a problem with Mk Vs disintegrating in the air, seemingly without any cause.

When they looked into the issue, they found that small changes like repairs or installation of new equipment could move the CoG enough for the aircraft to become unstable.

First fix (with old type elevators) was to put a bob weight in the stick mechanism. That stopped the crashes.

Second fix (used in the Mk IX and onward) was to make longer horns on the elevators.

 

He mentions that they only completely fixed the problem near the end of the war: There was no time earlier to implement bigger changes that might impede production.

 

It should be said, the production Spitfires were not unstable, only perhaps sensitive.

His description is from a test and engineering perspective.

 

But according to that description, it would make sense if the Mk IX has a little bit marginal stability.

 

Anyway, the book is a very recommended read, if you want to learn some things about the background of the Spitfire development and production. :)

[Anatoli-Kagari9]

Thx Sporg :thumbup:

Edited December 21, 2016 by jcomm

[NeilWillis]

Thx Neil,

 

I am well aware of that, since I fly for real too :-), but what I find strange is that even starting from altitude, it will never recover if you push the stick and then let it return to neutral.

 

The aircraft enters a dive, and actually diverges, with increasing negative pitch.

 

Sorry, I wasn't trying to teach granny to duck eggs. It was just the wording of what you said - "neutral trim" - I read that as "trim set neutral accoring to the trimming gauge" and not "trimmed for level flight".

 

Just a question of semantics, but to correct a problem needs everyone to be on the same page. I'm not being awkward for the sake of it here.

 

Longitudinal borderline instability and borderline instability in general is a desirable trait in high performance fighter, the Spitfirest has her vices. It probably took a lot of work to get it to misbehave "just so" rather than being neutral and docile by staying level when trimmed correctly.

[Anatoli-Kagari9]

Sorry, I wasn't trying to teach granny to duck eggs. It was just the wording of what you said - "neutral trim" - I read that as "trim set neutral accoring to the trimming gauge" and not "trimmed for level flight".

 

Just a question of semantics, but to correct a problem needs everyone to be on the same page. I'm not being awkward for the sake of it here.

 

Longitudinal borderline instability and borderline instability in general is a desirable trait in high performance fighter, the Spitfirest has her vices. It probably took a lot of work to get it to misbehave "just so" rather than being neutral and docile by staying level when trimmed correctly.

 

Thx Neil,

 

it is probably a feature more than a quirk with the FDM indeed...

 

I begin to learn better how to handle the Spit the more I take it to a ride :-)

[Tomsk]

I begin to learn better how to handle the Spit the more I take it to a ride :-)

 

I feel like that, and I've gotten to like it and feel at home in it quite quickly. Yes it's a bit of a handful on the ground, but you get used to it. It's also lovely in the air, what was said of the Spit is that it was "a perfect lady. She had no vices. She was beautifully responsive.", I feel that the DCS Spitfire captures that (IMO showing just how good the DCS planes are). It's not that the Spitfire doesn't stall or wobble, it's that you always feel in control ... it never runs away from you.

[Yo-Yo]

Noticed that with a trim setting of near neutral, if I let go of the stick, the Spitfire initiates a dive never recovering ?

 

Yes, Mk IX was described as positively stable with STICK-FREE and close to neutral in STICK-FIXED conditions. As sims use stick-fixed approach (with a light flavour of trim changes for non-FFB devices) you can see what you see.

Additionally, any light sneeze can affect your test: joystick free play, yaw angle changing during the diving (via P-factor mainly), AoA and Mach number changing, because the neutral margin varies with them.

The plane can be stable within one AoA range and unstable within the other.

 

But you can see that in the cruise flight the properly trimmed plane requires not so much efforts to keep it in lane... :)

 

By the way, you can see the very noticable changes in Bf-109 behaviour with 100% of fuel and full rear tank and, for example 15% fuel load - from very pronounced instability at full power (the required stick position at low speed is MORE FORWARD than at high speed!), neutral - a idle and high positive stability with 15% of fuel (normal stick tendency vs speed).

Edited December 21, 2016 by Yo-Yo

[Anatoli-Kagari9]

Yes, Mk IX was described as positively stable with STICK-FREE and close to neutral in STICK-FIXED conditions. As sims use stick-fixed approach (with a light flavour of trim changes for non-FFB devices) you can see what you see.

Additionally, any light sneeze can affect your test: joystick free play, yaw angle changing during the diving (via P-factor mainly), AoA and Mach number changing, because the neutral margin varies with them.

The plane can be stable within one AoA range and unstable within the other.

 

But you can see that in the cruise flight the properly trimmed plane requires not so much efforts to keep it in lane... :)

 

By the way, you can see the very noticable changes in Bf-109 behaviour with 100% of fuel and full rear tank and, for example 15% fuel load - from very pronounced instability at full power (the required stick position at low speed is MORE FORWARD than at high speed!), neutral - a idle and high positive stability with 15% of fuel (normal stick tendency vs speed).

 

Yep, DCS by Yo-Yo :-)

 

Detail and accuracy always taken to the possible limits, within the restrictions imposed by the hardware and absence of the "seat-of-pants" feedback :-)

 

Kudos for yet another great module !