Stability and Control characteristics of the Early Mark Spitfires

[NZtyphoon]

Originally Posted by Crumpp

Just some of the many references to the Longitudinal instability found in all of the early Mark Spitfires.

Spitfire Mk I Operatings Notes, July 1940:

Here are some good examples of pages designed to be used by trainee pilots and which are conservative in their assessments: for example the "violent pre-stall buffet" is a feature commented upon favourably by most Spitfire pilots who cite this feature as being a good warning device announcing that a stall was imminent, and it is something which was deliberately designed into the Spitfire by Mitchell.

How many aircraft need to be treated with care in bumpy conditions and high-g? All aircraft, except those that are particularly stable, need care when experiencing bumpy conditions under high-g loading, so there's nothing different about having such a warning in a Pilot's Notes. The "Pilot's Notes General" are specific about flying in bumpy conditions:

6. Flying in Bumpy Air.
(i) "Bumpy" air imposes g on the airframe and the effect of either horizontal or vertical variations of the wind on the airframe is proportional to the speed at which it is flying.
(ii) Speed should be restricted when flying in or near heavy cloud formations (especially cumulo-nimbus)...
(iii) As the effect of bumps may be added to g imposed by manœuvres, g due to manœvres should be kept to lower limits in rough weather.

In a high speed fighter pilots need to be careful in bumpy air - so what? Jeffrey Quill's comments about the elevators are interesting (to be posted later).

[robtek]

NZTyphoon,
bumpy conditions, where a pilot is moved around in the cockpit despite harness, are not so difficult to master if the pilot has to use some force to move the elevator.
The problem arises when minimal stick forces AND minimal stick movements are resulting in major changes.

[TomcatViP]

To get an actual idea, it's juts like having the joystick on a slippy surface. Each time you pull or push the stick, the base move. Obviously there the phenomena is reversed but roughly it's the same.

[NZtyphoon]

Originally Posted by robtek

NZTyphoon,
bumpy conditions, where a pilot is moved around in the cockpit despite harness, are not so difficult to master if the pilot has to use some force to move the elevator.
The problem arises when minimal stick forces AND minimal stick movements are resulting in major changes.

The notes specifically talk about the pilot jerking the stick while manoeuvring with high-g in bumpy conditions - that does not sound like minimal stick movement or forces.

Crumpp is putting a worst-case scenario on the "buffeting", a feature which many pilots have praised as a pre-stall warning, and on comments about the pilot accidentally jerking at the stick in adverse conditions; in other threads he has gone as far as to claim that early Spitfires were longitudinally unstable and dangerous to fly - as I have said before, he needs to back up such claims with solid evidence, in the same way he demanded that others provide 100% evidence for 100 Octane use. Why should we expect anything else?

[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.