Stability and Control characteristics of the Early Mark Spitfires

[Crumpp]

If it is defined that the only stability and control engineers at that time were in the US (specifically associated with NACA), and they formulated standards which the Spitfire failed, then the Spitfire failed...as defined in this rather narrow question.

Gates was not NACA and neither was the RAE when they published the Operating Notes.

Spitfire is not a good example of the value of the advances in stability and control.

It is not meant to be a good example of advances in stability and control. The thread is meant to point out the measureable and definable characteristics so that they can be modeled for the game.

[camber]

Originally Posted by Crumpp

Gates was not NACA and neither was the RAE when they published the Operating Notes.

I read the notes. Do you consider that the warnings against misuse are exceptional for the period, or exceptional compared to later WWII aircraft with better stability characteristics?

Originally Posted by Crumpp

It is not meant to be a good example of advances in stability and control. The thread is meant to point out the measureable and definable characteristics so that they can be modeled for the game.

That is a good idea and worth pursuing, but there is an unfortunate snag. Not only are people using different control hardware to control the same virtual aircraft, they have the option to tune the response between the physical control deflection and the virtual control surface deflection with nonlinear curves. This ability is not under the umbrella of the flight sim software itself. Some people have simulated control surface loading (FFB), some do not, and again the user can quietly do their own stability modifications to make their plane handle differently to what the devs attempt to program.

camber

[CaptainDoggles]

Originally Posted by camber

That is a good idea and worth pursuing, but there is an unfortunate snag. Not only are people using different control hardware to control the same virtual aircraft, they have the option to tune the response between the physical control deflection and the virtual control surface deflection with nonlinear curves. This ability is not under the umbrella of the flight sim software itself. Some people have simulated control surface loading (FFB), some do not, and again the user can quietly do their own stability modifications to make their plane handle differently to what the devs attempt to program.

Yes and no. The user can dampen their inputs to the aircraft (e.g. very flat curve around the center), but the user cannot affect the aircraft's response to said inputs. If the aircraft has a tendency to diverge from equilibrium, then it will still do so regardless of what the user's stick curve looks like. A high-wing monoplane like the Storch will still be very stable in the roll axis due to the keel effect. An aircraft with a lot of anhedral is still going to be largely unstable about the roll axis.

Keeping in mind of course that real control columns have a much greater throw than your average consumer-level HOTAS.

[MiG-3U]

Originally Posted by Crumpp

It was actually addressed in the Spitfire Mk V but the longitudinal instability existed from the beginning.

The longitudinal instability existed only if the CoG was in the aft positions as pointed out in the A&AEE report:

(v) Stability - The aircraft is laterally stable at all speeds except in the immediate vicinity of the stall when it is unstable. The aircraft is directionally stable engine 'OFF' and 'ON' at all speeds, but on the climb this is difficult to assess owing to insufficient rudder bias. Longitudinally, the aircraft is stable with centre of gravity forward, but is unstable with centre of gravity normal and aft with engine 'OFF' and 'ON'. Longitudinal stability records are attached

Note that early CoG limits are 5.8" to 8.6" aft the datum point. The revised limits are 5.4" to 7.9" for DeHavilland prop without bob weight (7.5" for Rotol prop).

Originally Posted by Crumpp

They published all the warnings and characteristics of the Longitudinal instability in the Operating Notes.

These warning can be found only from the operating notes of the Spitfire II with Rotol prop (most CoG sensitive combination) before the revised CoG limits and bob weights (which were needed only if CoG was too far aft as was case in the NACA tested Spitfire V).

Originally Posted by Crumpp

It is kind of hard to argue that the NACA was incapable of performing a simple weight and balance when the RAE fixed the same issue and published warnings in the Operating Notes. The truth is they just did not know what the NACA was talking about as the stability and control criteria was classified at the time and Gates had not completed his visit.

1. RAE criticized NACA static longitudinal stability test and for a good reason. Tests were done only at one position of CoG and that position was aft the revised limits.

2. Operational testing and handbooks of the aircraft were made by A&AEE, not by RAE.

Here is the direct link to the document by Gates:

http://aerade.cranfield.ac.uk/ara/dl...rc/rm/2677.pdf

See the page 9. The Spitfire K.9796 was tested at CoG 7" aft the datum point and that is still quite aft given that the range was from 5.4" to 7.9" (revised limits without bobweight and with DeHavilland prop).

Interesting comparison can be made to the Mohawk AX.882 which was tested at CoG 21" behind datum point, rather nose heavy given the range being 19" to 26". And despite forward CoG, the stick force for pull out was about the same as in the case K.9796.

[TomcatViP]

Originally Posted by MiG-3U

Interesting comparison can be made to the Mohawk AX.882 which was tested at CoG 21" behind datum point, rather nose heavy given the range being 19" to 26". And despite forward CoG, the stick force for pull out was about the same as in the case K.9796.

Thx for the link.

The comparison with the Hurri values is more interesting IMOHO

[NZtyphoon]

Here is the NACA Report on Control Characteristics of Spitfire VA specifically stating that the CG of the Spitfire was estimated:



Crumpp can argue black and blue that NACA accurately calculated the cg properly - the report specifically states this was not the case:

Because no accurate drawings of the Spitfire were available, the calculated location of the mean aerodynamic chord may be somewhat in error....The center-of-gravity location with full military load is not known....center-of-gravity location 31.1 inches behind the leading edge of the wing.

and a Spitfire I CG diagram:

Datum point 19.5 in aft of wing leading edge
Maximum aft location of cg was 7.6 in (MiG-3U 7.9 to 8.6 in) aft of datum point, 19.5 in aft of the wing leading edge = 27.1 in aft of leading edge (up to 28.1 in) - NACA calculations = 31.1 in aft of leading edge, enough to make a difference in the longitudinal stability (slightly tail heavy).

[Crumpp]

and a Spitfire I CG diagram:

You are using the wrong variant weight and balance as well as being an actual W/B for a specific aircraft.

There is not any need though to go into any kind of depth in researching this....

The Operating Notes for the type clearly warn the operator of the characteristics the NACA discovered.

[Crumpp]

These warning can be found only from the operating notes of the Spitfire II with Rotol prop

No they exist in the Spitfire Mk I as well.

[NZtyphoon]

Originally Posted by Crumpp

You are using the wrong variant weight and balance as well as being an actual W/B for a specific aircraft.

Really? Then please show us your definitive cg drawings for all Spitfire Mk Is.

Originally Posted by Crumpp

There is not any need though to go into any kind of depth in researching this....

In your opinion - IMO this thread is based on a flawed premise which requires detailed research to point out where it is flawed.

Please explain why the NACA report specifically states that their calculations may be in error.

Originally Posted by Crumpp

The Operating Notes for the type clearly warn the operator of the characteristics the NACA discovered.

Clarify how an aircraft tested with the cg further aft than specifications can possibly emulate the control characteristics properly.

[Crumpp]

All the evidence for the Spitfire Longitundinal instability will be posted in this thread, that includes the Operating Notes, Gates conclusions, the NACA results, stability and control engineering opinion, and the steps the RAE took to fix the longitudinal stability in later Marks.

Really? Then please show us your definitive cg drawings for all Spitfire Mk Is.

The weight and balance is a sideline that the RAE did not even believe.

You don't need comprehensive drawings to do a weight and balance. I will explain the process and how it works both for a type AND the individual aircraft later in detail with documents.

In short, like anything that comes off an assembly line has variation. CG limits is no different and there is a range of acceptable limits for the empty weight CG for the type.

A weight and balance is done when the aircraft is complete and the empty weight CG is estabilished. It must be within that tolerance range for the type but the empty weight CG will be specific to the individual aircraft.

That empty weight CG for that specific aircraft then has its specific range for foward and aft limits based on its authorized configurations. That is why the weight and balance is part of the Pilot's Handbook for that aircraft. It is required documentation and just like the Handbook, propeller logs, engine logs, and airframe logbooks follows the aircraft throughout its life.

The minimum equipment you need to do an accurate weight and balance on any aircraft is a tape measure, plumb bob, string, scales, chaulk, and pen/paper.

The NACA used percentage MAC. Once you know the percentage MAC range, you get all the data from the tape measure and scales for the individual aircraft.