Stability and Control characteristics of the Early Mark Spitfires

[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.

[MiG-3U]

The basic airframe dimensions of the Spitfires were unchanged up to the Spitfire IX except the nose section and radiator configuration. The loading table is attached, some parts rewritten for clarity. It's for the Spitfire Ia and Ib but the CoG locations are exactly the same for the Spitfire V (as refered in the right corner of the table).

The exact values of the CoG location are following at accuracy of two decimals (verified from drawings and RM 2525), these are values by RAE and slightly different as given by NZtyphoon because mean aerodynamic chord is at different position, datum line being the same:

The lenght of the mean aerodynamical chord is 78.54"
The datum line is 18.65" behind the leading edge of the mean aerodynamical chord
The datum line is 23.60" behind the leading at the wing root
The CoG of the NACA tested Spitfire V was 31.40" behind the leading edge at the wing root and 7.80" behind the datum line.

[NZtyphoon]

Originally Posted by Crumpp

...and the steps the RAE took to fix the longitudinal stability in later Marks.

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

Please prove that the RAE had no belief in weight and balance, and explain how the RAE took steps to fix so-called longitudinal instability when they had no belief in weight and balance.

Originally Posted by Crumpp

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.

NACA did not have any drawings for the Spitfire and specifically state that their measurements might have been in error - without a weight and loading diagram of the specific aircraft tested there is no way to know whether the aircraft was teetering on instability because it was loaded beyond the usual tolerances.

To claim that this report proves all early Spitfires were unstable is a stretch, particularly when the Supermarine chief test pilot Jeffrey Quill, states in his book that the longitudinal stability of the Spitfire I was okay.

[NZtyphoon]

Originally Posted by Crumpp

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.

Completely wrong, neither individual CG drawings, nor weight and balance sheets were issued with the Pilot's Notes - fighter pilots especially were rarely allocated their own aircraft and had to take what was made available. Pilot's Notes General, Section 7:


Each and every aircraft type used by the RAF and FAA had generic cg/w&b sheets printed, which had fixed fore and aft limits, beyond which the flight qualities started to suffer: Lancaster cg drawings:



Loading diagram up to L7532...


L7533 on...


Careful study shows the cg limits fore and aft were identical, despite different equipment and loadings - the airframe was the same, so the limits stayed the same - those fore and aft limits for ALL early Marks of Merlin engined Spitfires were identical, Mk I to Mk V and were not changed until the modified elevators with larger mass balances were introduced.

It was the responsibility of the groundcrew to ensure that the cg limits were adhered to. The only crews that needed to know the position of the cg were bomber crews with their large disposable loads and multiple crew positions

This is how the cg was calculated:







Relatively small changes in equipment weight and equipment position could still make a big difference to the final cg - a few kg a few inches aft of the rearmost cg position could upset the handling of an aircraft;

NACA made it quite clear that their calculations for the Spitfire may well have been in error - until Crumpp can prove that NACA had calculated the cg position correctly, according to early Spitfire cg data charts, the report needs to be viewed with some suspicion.

Originally Posted by Crumpp

Now let's get back to the NACA report so there is a better understanding of the issue.

We will look at a condition of flight essential to a dogfighter. The ability to make abrupt turns.

The pilot must be able to precisely control the amount of acceleration he loads on the aircraft.
Doing that in an early Mark Spitfire was difficult and something only a skillful pilot could perform.

Where did you get this nonsense? Show me accounts of even trainee pilots who found manoeuvring the early marks of Spitfire difficult - I repeat Supermarine's Chief Test Pilot Jeffrey Quill stated categorically that there was no problem with the longitudinal stability of early Mk I & II Spitfires.