Stability and Control characteristics of the Early Mark Spitfires

[JtD]

Originally Posted by robtek

The instability in the NACA report is, that a constant stick input, say one inch back with 6lbs pressure, led to a ever increasing g-load, and that was the malus for the Spitfire controls.

No, there's nothing like that in the NACA reports. Neither in pull ups (figure 12), nor in turns (figure 21). The curves clearly show progressive elevator angle (stick travel) and progressive forces, even though the stick travel is small.

[Crumpp]

No, there's nothing like that in the NACA reports.

Yes it is what the NACA report says.

ONLY the abrupt pull ups are stick free.

The instability in the NACA report is, that a constant stick input, say one inch back with 6lbs pressure, led to a ever increasing g-load, and that was the malus for the Spitfire controls.

Correct. The abrupt turns and all other conditions are stick fixed. The NACA did have to modify their force gauge to allow the pilot to adjust the force holding the stick fixed so as not to exceed the airframe limitations. He could move it to keep from damaging the airframe.

That is all explained in the report.

[Crumpp]

As for the other qualities evaluated by NACA, be it good or bad, it sums up that there's nothing critical.

JtD, they flat out state the stick fixed longitudinal stability is unacceptable. That is the NACA, not me.

Why do you think bob-weights were added to the design to fix the longitudinal stability?

Because they just felt like changing something? Maybe they were bored and had nothing else to do?

Or maybe, just maybe, there was a real engineering issue with the longitudinal stability that required a real engineering solution? Nahhhh!!!

[JtD]

NACA shows progressive stick travel and elevator force over lift coefficient and normal load. You are directly contradicting NACA. I'd appreciate if you could point out the chart or table you are basing your conclusion on.

Additionally - the Spitfire stability "failed to meet an accepted requirement". This does not make the stability unacceptable. It may seem to be the same for people unfamiliar with testing and test reports, but it is not. In this case the difference is evident from the fact that the Spitfire was accepted into service with 30+ air forces worldwide, among them the USAAF. This would not have happened had the stability been unacceptable.

[ACE-OF-ACES]

Originally Posted by JtD

Additionally - the Spitfire stability "failed to meet an accepted requirement". This does not make the stability unacceptable. It may seem to be the same for people unfamiliar with testing and test reports, but it is not. In this case the difference is evident from the fact that the Spitfire was accepted into service with 30+ air forces worldwide, among them the USAAF. This would not have happened had the stability been unacceptable.

Agreed 100%

[bongodriver]

NACA shows progressive stick travel and elevator force over lift coefficient and normal load. You are directly contradicting NACA. I'd appreciate if you could point out the chart or table you are basing your conclusion on.

Additionally - the Spitfire stability "failed to meet an accepted requirement". This does not make the stability unacceptable. It may seem to be the same for people unfamiliar with testing and test reports, but it is not. In this case the difference is evident from the fact that the Spitfire was accepted into service with 30+ air forces worldwide, among them the USAAF. This would not have happened had the stability been unacceptable.

+1

it's like adopting a standard where everything has to be purple and then testing something which is green and surprise surprise it fails the purple standard.

This was a test on a MkV, nothing in the test mentions a similarity with earlier models, and the MkV had a different weight and balance.
Was it the Americans that advised the addition of the bob weight?
I believe some RAF units asked for it to be removed so they could have the original flying qualities back, doesn't sound like the original ones were so bad in that case.

This whole thread has been about making a mountain out of a molehill.

[Glider]

In my reply to my question about any examples of any aircraft of any type in any airforce that was stable enought to meet your requirements and your reply

Originally Posted by Crumpp

Most aircraft are not positive statically stable and negative dynamically stable stick free. It is an unacceptable characteristic.

It is a situation where the the aircraft moves toward the trim condition but increases the force on the axis of motion so that it overshoots the mark. The key is our force is increasing with each oscillation. This causes the motion over time to grow larger and the velocity along the axis of motion to increase. Eventually, the forces will overload the aircraft and it will destroy itself.

I take it that your reply is a very long way of saying no, you have no examples.

The words you have written are as far as I can ideological rubbish and would depend on the pilot being stupid enough to overcompensate with every oscillation. Nothing to do with examples which I take it you cannot supply.

[NZtyphoon]

Originally Posted by Crumpp

Why do you think bob-weights were added to the design to fix the longitudinal stability?

Because they just felt like changing something? Maybe they were bored and had nothing else to do?

Or maybe, just maybe, there was a real engineering issue with the longitudinal stability that required a real engineering solution?

Quill devotes an entire chapter to longitudinal stability (pages 229-241 Murray 1983) in his book "Spitfire a Test Pilot's Story" in which he goes into detail about the problems involving several Spitfire Vs which broke up in 1942; Quill describes the problem of Spitfire Vs breaking up; he then goes on to describe the solutions which were a): to ensure that when new equipment was added that the loading was kept within limits and b):the design of bobweights, which were added to the elevator circuit, as well as the modified elevators fitted to later Spitfire marks. After the bob-weights were fitted, and the loading sorted out, the problem disappeared.


Quill
"In general configuration the Mk I and Mk II production aeroplanes were almost identical to the prototype and so there was no problem with their stability." (231-232)

"The Mk III Spitfire did not go into production, but the success of the bobweight experiment in curing its instability...opened up the possibility of its use for later marks of Spitfire....which was just as well as we had to...respond to a nasty situation which developed in 1942.
The Mk V aircraft was...in full service with Fighter Command and,...a fair amount of additional operational equipment had gradually crept into the aircraft, most of it stowed within the fuselage. The aftmost acceptable position for the aircraft's centre of gravity had been fixed in the mormal course of flight testing by the firm and by the A & AEE....Any rearward movement of the centre of gravity in service, for whatever reason, would begin to destabilise the aircraft. Therefore, for each sub-variant of the Mk V detailed instructions for the correct loading of the aircraft were issued to squadrons....However the importance of these loading instructions was not generally appreciated in squadrons and in the daily round of operational activity they tended to be disregarded." (pages 232-233 -Quill goes on to describe 65 Sqn's Spitfire Vbs which were found to be dangerously unstable)

"There was thus a real chance that, as of that moment, in almost every squadron in the Command Spitfires were flying in a dangerous state of instability....Up to that time there had been a distressing and increasing incidence of total structural failure of Spitfires in the air, which was causing great comcern in the MAP and especially at Supermarine." (pages234-235) Once the bobweights had been introduced and, in later marks, the modified mass balances on the elevators...it was statistically established that, as soon as the longitudinal stability of the Spitfire was thus brought under control, the problem of the unexplained breakings-up of aircraft in mid-air,...'softly and suddenly vanished away'." (page 238 )



So the bob-or inertia weights were introduced in 1942 to help solve bad loading or overloading of Spitfire Vs at Squadron level - it had nothing to do with NACA's report.

[Crumpp]

NACA shows progressive stick travel and elevator force over lift coefficient and normal load.

I have to figure the slope anyway for the bugtracker. Might as well do it now.

Yellow:

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.

Green:
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.

[Crumpp]

So the bob-or inertia weights were introduced in 1942 to help solve bad loading or overloading of Spitfire Vs at Squadron level - it had nothing to do with NACA's report.

Are we really gonna have this conversation??