Stability and Control characteristics of the Early Mark Spitfires

[CaptainDoggles]

Originally Posted by Fenrir

The NACA test discovered what they discovered - I can't argue with their findings, FOR ONE PARTICULAR AIRCRAFT. However I cannot agree that these are representative of the breed.

What is it about the tested aircraft that makes it not a representative sample of the other aircraft?

[NZtyphoon]

Originally Posted by CaptainDoggles

What is it about the tested aircraft that makes it not a representative sample of the other aircraft?

There are some awkward little phrases in the NACA test viz:



Now, until Crumpp, or anyone else, can prove beyond reasonable doubt that NACA got their cg calculations right there is a question mark over the longitudinal stability of this Spitfire VA as tested.

[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.

[NZtyphoon]

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.

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.

[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?

[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).

[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.