Stability and Control characteristics of the Early Mark Spitfires

[MiG-3U]

Originally Posted by CaptainDoggles

You know Crumpp's right about expressing CG as a percentage of MAC. The Datum point doesn't have to be in the same spot for the results to be valid. That's why it's called a datum point.

There is only one accurate reference point for the CoG in the NACA report, the distance of the CoG from the leading edge at the wing root and that is given as 31.4". NACA admits that their measurements for MAC maybe in error and we can easily see that there is error because in the RM2535 the 34% CoG location at the RAE measured MAC is also given same way as distance from the leading edge at the wing root and the value is 2.638' which is 31.656". Even with these values only we can estimate that the real CoG location was about 33.7% at the MAC given by RAE instead 31.4% claimed by NACA (and NACA admited that their value might be wrong).

As we know accurate reference point at the wing root and dimensions for MAC used by RAE and A&AEE and datum line, we can also easily calculate these.

Lenght of the MAC measured by RAE and A&AEE is 78.54" (or 6,54') and position 31.4" behind leading edge at root is 26.4476" at MAC and that means that CoG was at position 33.6741% in the NACA tests using RAE and A&AEE dimensions.

However, British documentation gives CoG values usually as distance from the datum line so we need to make NACA CoG location comparable with these. And that is easy because we know that the datum line is 18.65" behind leading edge at the MAC:

26.4476" - 18.65" = 7.7976"

And this value, 7.8" aft datum line, is comparable with the other sources like A&AEE and RAE tests and loading instructions.

Over and out.

[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.

[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.