Stability and Control characteristics of the Early Mark Spitfires

[CaptainDoggles]

Originally Posted by NZtyphoon

Crumpp wasted countless hours nitpicking the 100 Octane threads with minute, forensic examination of every single little detail - his contention, that the early marks of Spitfire had longitudinal stability problems which needs to be replicated by this game, needs to be proven to the same level that he demanded for 100 Octane fuel; nothing less should do.

That 100-octane thread was monumentally stupid on all sides. It just does not matter, even remotely, what percentage of Spitfires were on 100 vs how many were on 87. Both should appear in the game, and both now do appear in the game (performance problems notwithstanding).

Vendettas aside, the sheet that Lane posted looks interesting.

I'm not 100% clear on what those graphs are supposed to be representing, but if we look at #4 for example, it shows the airspeed diverging wildly from equilibrium, which I would assume is due to the aircraft doing the rollercoaster "porpoise" motion.

A stable aircraft should return to equilibrium, not diverge from it.

[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:











Tommorrow I think we can discuss game behaviors to ask for in the bugtracker.

[Crumpp]

Lenght of aerodynamical mean chord (MAC): 78.54"

The rest is simple math:
Aft limit behind datum line at MAC: 26.7036" - 18.65" = 8.05"
Datum line behind leading edge at wing root: 31.656" - 8.05" = 23.6024"
NACA CoG behind datum line: 31.4" - 23.6024" = 7.7976"
NACA CoG location at MAC behind leading edge: 18.65" + 7.7976" = 26.4476"
NACA CoG % at MAC: 26.4476" / 78.54 * 100 = 33.6741%

[Robo.]

Originally Posted by Crumpp

Tommorrow I think we can discuss game behaviors to ask for in the bugtracker.

I wonder how exactly would you like to model jaming pilots elbow against his body.

My opinion is (flying all available fighter airplane in the game) that it's the Hurricane and Bf 109 elevator is too light even at higher speeds rather than Spitfire elevator being not light enough. Generally I like how game calculates forces on the stick and how they increase with the increasing airspeed, it just needs some fine tuning and obviously structural G limits modelled.

I believe there already is a bugtracker issue raised regarding structural G limits somewhere, will confirm.

The only problem I see at the moment (1.07) is that they have changed something on the Spitfire FM and it is nearly impossible to get the plane into a high speed stall. Before that, iirc, it was a plane matching the description much better - you had to be careful not to bring it too close to the stall, you had to be more careful with the the elevator than now in 1.07. Have you noticed the same thing Crumpp?

[Robo.]

Looking at the bugtracker, I can't seem to find the 'structural G limits' issue, but I am sure it has been discussed. This would be a most welcome feature, but very difficult to model reasonably - the virtual pilot is not getting the kind of feedback like the real pilot did. This is already a problem in old Il-2, but the new features enhanced the gameplay in a great way already.

Reading through the other bugtracker issue (re: Merlin incorrect power settings)

http://www.il2bugtracker.com/issues/370#change-1216

I found Crumpp posted following comment:

He has the engine settings wrong for the Merlin III at 100 Octane.

The correct settings by the Operating Notes are:

+12lbs 2850 rpm *below* 20,000 feet
+12lbs 3000 rpm above 20,000 feet

There is not much to discuss. I just was not clear in my post.

interpreting the hard data in most extraordinary way. I believe you've made a mistake in there and I suggest you're more careful with your bugtracker activities, because devs seem to actually read that from time to time and your views are often wrong and misleading. Thank you.

[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.