Stability and Control characteristics of the Early Mark Spitfires

[robtek]

The necessary Stick movement (elevator) to induce a 3 g load at cruise speed was three quarters of an inch in the Spitfire, afaik, very easy to get unintended reactions there if your arm isn't completely fixated.

[winny]

Originally Posted by robtek

The necessary Stick movement (elevator) to induce a 3 g load at cruise speed was three quarters of an inch in the Spitfire, afaik, very easy to get unintended reactions there if your arm isn't completely fixated.

To be fair, there are loads of references by pilot's to having to either wedge their elbows into the side walls or into their own stomachs to steady themselves.
Quite a few mention going 2 handed. They adapted.

As in most cases in WW2, the pilot's coped with the quirks of their machines and got the best out of them ( the good ones at least ).

[robtek]

Originally Posted by winny

To be fair, there are loads of references by pilot's to having to either wedge their elbows into the side walls or into their own stomachs to steady themselves.
Quite a few mention going 2 handed. They adapted.

As in most cases in WW2, the pilot's coped with the quirks of their machines and got the best out of them ( the good ones at least ).

Exactly, one of the quirks of the Spit was the extreme easy elevator, great for experts, more difficult for beginners; The difference to planes with "normal" handling should be in the game.
Same for the very heavy elevator at very high speeds (>600 km/h) in the 109, i.e.

[Crumpp]

Great document Lane!!

It gives us a measurement of the divergence and the slope of the Cm increase.

Exactly, one of the quirks of the Spit was the extreme easy elevator

Not only that, it is unstable. That means your coefficient of moment increases each cycle instead of decreasing.

So if you pull a 6 G turn and did nothing except hold the stick fixed, the next oscillation will exceed 6G and continue to increase with each cycle until the airframe is destroyed.

That is the reason why "flick" maneuvers were not allowed in it.

Let's summarize the behavior that occur at normal and aft CG positions and categorize them to be implemented in the game. Then we can build a list for the bug tracker. These are all at NORMAL CG. If the game models a shifting CG then they increase in severity at aft CG positions.

The take away is:

1. The large accelerations change for very little elevator movement.
2. The very rapid rate at which the pilot was able to load the airframe to 5G's.
3. The equally rapid rate at which the airframe unloaded down to 2G's when the pre-stall buffet was encountered. In 1 second, the aircraft went from 5G's to 2G's due to buffet losses. This means a rapid decay in turn rate resulted.
4. The violence of the pre-stall buffet combined with the longitudinal stability and control caused large fluctuations in the accelerations on the aircraft.
5. The violent accelerated stall behavior resulting in spin/loss of height

1. The large accelerations change for very little elevator movement.

1. The Spitfire should be twitchy and unstable gun platform. IIRC, in IL2, people used to complain about the "twitchy" behavior or the Corsair and P-51's so I am sure it is within the games engine to model a twitchy airplane.

It should take very careful and small stick movements to get the gun sight on target. That means a Spitfire pilot will require more skill to hit a maneuvering target than he would need in a stable platform such as the Hurricane.

2. Above Va, large or abrupt elevator control can more easily exceed the airframe limitations of 6G for damage. Currently, it is impossible to stall the Spitfire in a turn or a dive. The reality is it requires careful flying so as NOT to induce an accelerated stall or exceed the airframe limitations.

3. In the turn, the violent buffet is a double edged sword. There is no such thing as a free lunch especially in physics. In the NACA measured results, encountering the buffet represents a change in available angle of bank. The airplane goes from 78.5 degrees of bank to 60 degrees of bank in one second.

****5G @ 147.73KIAS:

ROT = 1091*tan(78.5) divided by 147.73 KIAS = 36.2 degrees a second

****2G @ 141.647 KIAS:

ROT = 1091*tan(60) divided by 141.647 = 13.34 degrees a second

As a quick ballpark using IAS to get an idea of the scope of the effect on turn performance, we see the rate of turn drop from 36.2 degrees a second to 13.34 degrees a second. That means our time to complete a 360 degree turn changes from 10 seconds to 27 seconds!!!

As the Operating Notes relate, you do not want to turn any airplane in the buffet. Energy cannot be created or destroyed, all the energy that was being used to achieve an instantaneous performance rate of turn of 36.2 degrees a second from our ballpark went to warn the pilot of an impending stall, taking the aircraft right down well inside its sustainable performance envelope of 13.34 degrees a second.

The longitudinal stability characteristics of the Spitfire requires skilled flying to achieve a maximum performance turn. In a stable airplane, the pilot would have a much easier time keeping the aircraft at the maximum rate of turn velocity and a less violent buffet would have subsequently reduced effects on the turn performance.

4. Below Va, the Longitudinal instability of the Spitfire make it more difficult for the average pilot to prevent an acelerated stall or overcontrol the aircraft by pulling deeper into the buffet zone.

I think the fact that you have to return the stick almost to neutral after entering a high g turn (>3 g) to prevent oversteering in a Spitfire should be in game, also the very sensible elevator with large reaction for small inputs and the roll rate as documented.
This will be a problem for ham-handed pilots, but a delight for the virtuosos, as it was in RL.
I don't see that as "porking" the Spit further, but to give it the characteristics that made it famous.
Every aircraft in CoD should reflect its pro's and con's as they where documented then.

Exactly. That is our goal to recreate the flying qualities of all of these aircraft.

In this thread we have focused primarily on the Longitudinal stability. Most of the Spitfires issues stem from that.

For example, the heavy lateral control forces would not be an issue if the control forces were equal on all axes. The control harmony was poor in the Spitfire and Gimpy raises a good point:

well personally I would expect a much lower Roll rate at speed, and a very sensitive Unbalanced Elevator (unbalanced meaning it needs much more roll input than pitch).

The pilots ability to apply lateral control would be reduced by the longitudinal control characteristics.


*****Not a silly argument on actual turn performance, just a quick ballpark so readers understand the importance and general effect of encountering the buffet on turn performance.

[Crumpp]

the British, with the likes of William Lanchester, were pioneers in laying down scientific principles for aeronautics,

You are confused about this whole subject. Of course, there were some brilliant British engineers.

What does he have to with an adopted measureable standard for stability and control????

You understand, an engineer in the United States or German designing a fighter could go look to see the measured characteristics that he must meet.

Gates was the British engineer who tried to shoulder that task of getting the Air Ministry to adopt a measureable standard. He eventually achieved it in the post war.

[taildraggernut]

So why was the Mustang III considered longitudinaly unstable too?

[winny]

So, now all we need is the same data for a MK I and II.
Because, correct me if I'm wrong, they didn't test them at NACA.

I'm pretty sure that if I came on here saying that the 109 E was wrong, and used an F or G's data I'd get laughed out of here. Especially by you.

So I'd like some hard data on the same phenomenon in I's and II's, please.

Thanks.

Edit: and bear in mind that the spitfire in lanes docs is around 300lb lighter than a spit in BoB trim.

[JtD]

It was designed for a load factor of 10, not 6. Calculation showed wing to be the weakest point, it was tested and met specification. Specification was changed to 12 for later marks.

[NZtyphoon]

Originally Posted by Crumpp

You are confused about this whole subject. Of course, there were some brilliant British engineers.

What does he have to with an adopted measureable standard for stability and control????

You understand, an engineer in the United States or German designing a fighter could go look to see the measured characteristics that he must meet.

Gates was the British engineer who tried to shoulder that task of getting the Air Ministry to adopt a measureable standard. He eventually achieved it in the post war.

Wrong again, Lanchester's work on aerodynamics, as well as other British academics, provided a basis for the adopted measurable standards for stability & control worldwide - the British did indeed have such standards, and adopted them well before NACA; to claim that only the Americans and Germans had such standards is farcical. Read the articles and do some historical research of your own before making such claims. What work, for example, did the National Physical Laboratory in Britain do during WW1? http://www.npl.co.uk/about/history/

The Duplex wind tunnel was completed in 1919. It had a cross-section of 2 m by 4 m.

During the first world war, activity in aerodynamics expanded dramatically and NPL made major contributions to advances in theoretical and practical aspects of the stability of aeroplanes, airships, kite balloons and parachutes. Techniques had been developed for testing scale models of wings, ailerons, propellers and of complete models of aeroplanes in wind tunnels.

[taildraggernut]

Originally Posted by Crumpp

That is the reason why "flick" maneuvers were not allowed in it.

Flick manouvres were permitted in the Mk2 (from pilots' notes) at slow speeds, there are several other aircraft including the P-40 which were prohibited from 'flick' rolls and intentional spinning.