Stability and Control characteristics of the Early Mark Spitfires

[NZtyphoon]

Originally Posted by Crumpp

This is the second or third time in this same thread the same argument has arisen.

The instability existed in all early mark Spitfires at normal and aft CG until it was fixed with the inertial weights.

It is a function of the tail design and elevator, static margin, and fuselage length.

The Operating Notes are full of warnings about it. It was not limited to one propeller or a specific load.

Prove that you are not making a worst-case scenario out of just two documents: Prove - with documentation - that the Spitfire had such bad longitudinal stability characteristics that it affected its abilities in general flight and in combat and, above all PROVE that this can be replicated in a flight sim made for PCs.

[Crumpp]

Here is a little experiment you can do at home, NzTyphoon.

Make a paper airplane. Toss it.....

See how stable it flys.

Now add a paperclip to the nose and throw it again.

Which is more stable?

[Crumpp]

Prove - with documentation

ok

The Stability and control characteristics of the Early Mark Spitfires:

Now let's look at the Spitfire in an abrupt pull out as measured by the NACA.



First thing to notice is the stick forces. There are light but acceptable in abrupt pull outs. While very steep, the slope of the curve matches our acceleration curve and the controls float without overcoming the inherent stability of the design. The steepness of the curve tells us the pilot is able to very rapidly load the airframe. In fact, the NACA had to make allowance in their stick fixed measurements to prevent damage to the aircraft from acceleration because of the rapid onset the controls allowed.

However, if we look at the acceleration curve we see an abrupt change and not the desirable smooth curve. This points to the stability characteristics contributing to the rapid fluctuations in acceleration that the aircraft exhibits under other conditions.

Next we will get into the unacceptable longitudinal stability characteristics of the design.

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. All aircraft performance depends on velocity. In order to get maximum performance out of the aircraft above maneuvering speed, Va, he needs to be able to make a 6 G turn and not exceed that load factor to prevent damage to the airframe. Below Va, the pilot needs to control the acceleration so that he does not stall the aircraft making the abrupt maneuver as well being able to maintain a maximum performance turn.

Doing that in an early Mark Spitfire was difficult and something only a skillful pilot could perform.

First the NACA report. Abrupt 180 degree turns were conducted at various entry speeds to gauge the level of control the pilot had in maintaining steady accelerations. The turns were also done to the stall point in order to gauge the behavior and amount of control.

"In turns at speeds high enough to prevent reaching maximum lift co-efficient" means turns above Va.





"By careful flying" a pilot can hold a steady acceleration. That agrees with the Operating Notes warning for the pilot to brace himself against the cockpit to get better control when making turns.

Now let's look at the measured results.



Here we see in a rapid left turn performed at 223 mph the test pilot is unable to hold constant acceleration on the airframe. Very small variations in stick movement and stick force changes of 1-3lbs results in large fluctuations in acceleration.

Taking two point we can compare the slope of the curves of stick input to acceleration over time.

For the intital pull up:

Acceleration over time 3.5G-(-.5G) divided by 4.5s-3.5s = m
m = 4

Stick force over time: (19lbs - 0lbs) divided 5lbs/G all divide by 4.5s-3.5s = m
m = 3.8

*The slopes should match and they are close enough.* +However, our stick force grows at a slower rate than our acceleration.+ This is the initial input of the pilot.

Now let's see the instability.

Stick force over time 15lbs-15lbs divided by 5lbs/G all divided by 6.8s-5.5s = m
m = 0

Of course m = 0, our stick is held fixed by the force measurement equipment

Acceleration over time 4.2G-3.2G divided by 6.8s-5.5s = m
m = .76

So, while our stick remains fixed, the aircraft continues to accelerate on its own. As the nature of instability, there is no correlation stick force input and acceleration.

Now, our pilot in this case only input force to reach 3.5G. In a stable airplane, we should see the aircraft dampen all subsequent accelerations which means the aircraft would not exceed 3.5G without control input.

In this case, the instability or divergent oscillation a 4.2G acceleration with stick fixed slightly below the stick force required to produce a 3.5G acceleration.


Next let's look at the pilots ability to control the accelerations in the pre-stall buffet.



Here we see the pilot was able to load the airframe to 5G's in 1 second to reach the pre-stall buffet 3 times. The smooth positive sloped portion of the curve represents the aircraft flying while accelerations are increasing. The top of the acceleration curve represents the pre-stall buffet. The bottom of the curve represents the stall point.

The amount of stick travel as measured by the NACA was not acceptable.




Next let's look at the opinion of Stability and Control Engineers on the Early Mark Spitfires.









There is no doubt that the Air Ministry was aware of the longitudinal instability of the early mark Spitfires.

Just some of the many references to the Longitudinal instability found in all of the early Mark Spitfires.

Spitfire Mk I Operating Notes, July 1940:

[NZtyphoon]

Originally Posted by Crumpp

Here is a little experiment you can do at home, NzTyphoon.

Make a paper airplane. Toss it.....

See how stable it flys.

Now add a paperclip to the nose and throw it again.

Which is more stable?

Go make your own paper aeroplanes: The NACA report concentrates on one aircraft which, in spite of your claims, may not have been correctly loaded:

Tony had been test flying correctly loaded Mk Vs with us at Supermarine and he knew very well the difference between a stable and unstable aeroplane.

Supermarine's Chief test pilot knew more about flying the Spitfire and its capabilities and characteristics than NACA and took he took urgent action when he realised that there was a problem with badly loaded Mk Vs.



You can interpret documents any way you like - fact is that there are other valid opinions which show that longitudinal stability only became a real issue when the loading instructions were ignored or misunderstood at an operational level. As for the excerpts from your book

the [stable] Hurricane, Typhoon and Tempest were highly manoeuverable and were greatly superior gun platforms to the skittish Spitfire.

This also falls down as a source because quite clearly some basic research is missing because both the typhoon and Tempest PNs state they were slightly unstable longitudinally, and only the Hurricane could turn inside the Spitfire... as for gun platform:



The pilot's notes were read in conjunction with the Pilot's Notes General: Note that the PNG carry similar warnings to those in the Spitfire PNs see (ii) which applied to all aircraft



You have not demonstrated that you are not applying a worst-case interpretation to both the NACA flight trials and PNs.

Nor have you explained how you propose to alter the flight characteristics of a computer based flight sim to accurately replicate this so-called instability considering the plethora of different set-ups used by players.

[MiG-3U]

Well, here we go again, the same stuff with flawed interpretation posted again and again.

Over and Out

[Al Schlageter]

Originally Posted by MiG-3U

Well, here we go again, the same stuff with flawed interpretation posted again and again.

Over and Out

Just like the 100 octane threads. Why does myopic tunnel vision come to mind?

[NZtyphoon]

Originally Posted by MiG-3U

Well, here we go again, the same stuff with flawed interpretation posted again and again.

Over and Out

At the very least putting a worst-case scenario on two documents while claiming that these alone are definitive proof of the Spitfire's bad longitudinal stability - when another NACA document on the Spitfire's stall characteristics was presented it was dismissed as being irrelevant:



It will be truly interesting to see how the proposed bug-tracker will define the "problem" and how it proposes to alter the Spitfire's flight characteristics to cater for a flight sim in which different equipment is used and tuned by individual players...

[IvanK]

Its been put in on bug tracker:

http://www.il2bugtracker.com/issues/415

Alas no proof or in game tests in the bug tracker entry to indicate CLOD Spit MKI is indeed flawed in the Sim.

[Glider]

A lot of energy is being spent on the Mk V which most would agree had an issue that was resolved by bob weights.
There seems to be no real evidence that a problem existed in the Mk I or II which were the versions used in the BOB

[Crumpp]

There seems to be no real evidence that a problem existed in the Mk I or II which were the versions used in the BOB

LOL...Really??

Spitfire Mk I Operating Notes, July 1940:

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'.

http://www.spitfireperformance.com/k9787-fuel.html