Why is the Spitfire Mk IX still unstable??

[MiloMorai]

That is for a G-2. You have one for the K-4?

[JtD]

From what I have seen, yes.

Rgr on the first part. On the quoted second part I wonder - how was the forward limit on the Spitfire determined? In terms of MAC, the rearward limits on the 109 and on the Spitfire were close, so a difference in permissible range comes mainly from the forward limit. I don't recall having ever seen anything illustrating that for the Spitfire, in fact, 99% of tests I've seen dealing with permissible CoG ranges deal with the rearward limit.

[bongodriver]

From what I have seen, yes.

 

Surely you have the figures switched, even Crumpp has done the 'math' and shown the rear CG limit to be 9" on the Spit, it's actually 9.9" on the shown rear fuel tank tests.

[JtD]

Kurfürst was speaking about the range, not the most rearward position. Spitfire is like 3.5 to 7 or 5 to 8.5 or something depending on the model and details, so about 3.5. It's 40.4-62.9cm for the 109G-2trop in the sheet Kurfürst showed, so about 9.

[Crumpp]

Surely you have the figures switched, even Crumpp has done the 'math' and shown the rear CG limit to be 9" on the Spit, it's actually 9.9" on the shown rear fuel tank tests.

 

Increasing the area of the elevator/horizontal stabilizer is one of the design changes that allows for movement of the AC and subsequent increase in the stability margin. There is no details as to the design changes of the new elevator.

 

It is one of those design changes that allows for the CG limits to be moved.

 

Unfortunately, it just did not happen in the Spitfire. The rearward CG limit remained 9.0 inches aft of datum.

 

[Crumpp]

Kurfürst was speaking about the range, not the most rearward position. Spitfire is like 3.5 to 7 or 5 to 8.5 or something depending on the model and details, so about 3.5. It's 40.4-62.9cm for the 109G-2trop in the sheet Kurfürst showed, so about 9.

 

 

40.4cm to 62.9cm = 11.3 inches

 

That is a good CG range.

[bongodriver]

Increasing the area of the elevator/horizontal stabilizer is one of the design changes that allows for movement of the AC and subsequent increase in the stability margin. There is no details as to the design changes of the new elevator.

 

It is one of those design changes that allows for the CG limits to be moved.

 

Unfortunately, it just did not happen in the Spitfire. The rearward CG limit remained 9.0 inches aft of datum.

 

 

Not a big deal really, as your document shows the MkIX Spit was well within the range at full fighter load and even had capacity for some fuel in the rear tank.

[Cripple]

 

I've almost heard it all now, a stability 'margin' is not 'felt' the stability of the aircraft is what is felt, if the aircraft 'feels' stable then it actually is.

 

 

Ummmmmm... really? "If an aircraft feels stable (to a particularly pilot) then it is stable - QED"?

 

I think this is what I alluded to when I suggested thrashing out a definition of (In)Stability in my earlier post.

 

If we can take a step back from WW2 aircraft for a minute and look at the common bicycle. Is it "stable"?

a) "Yes. Of course it is, don't be silly. Children can ride one. I see people riding them every day and they don't fall off. Proof."

b) "No. The Bicycle is an inherently unstable machine and requires both sufficient velocity and a sufficiently experienced rider to dynamically balance the machine... risking over-controlling at slow speeds and Rider Induced Oscillations (remember those?) in the handlebars. Maths and stuff."

 

Which is correct? Erm, they both are... depending upon how you *choose* to define Stability.

[Kurfürst]

Surely you have the figures switched, even Crumpp has done the 'math' and shown the rear CG limit to be 9" on the Spit, it's actually 9.9" on the shown rear fuel tank tests.

 

No, the rear tank test reads to me that the rear CG was about 9.9" with the rear tanks half used and its still extremely difficult to fly in that condition and was unstable. requiring a very experienced pilot to limit acceleration with push on the stick. Emptying the rear tanks fully moves the CG forward by about 5.2", so this is about your normal CG position: roughly 4.7" from Datum.

 

IOWs, the IX with the rear tanks filled (CG ~10-12" aft datum )to just about any extent was WELL out of the rear limits (ca 6.5" aft datum) so its isn't much of a surprise that its behaving nasty.

 

Getting allowable CG ranges for Spits is a bit difficult, but its mentioned here and there and the normal CG was always about 4.5" from datum. For the XIV, its very specifically stated as

 

"Permissible C.G. range for all forms of flying...........from 3.5 inches to 6.5 inches aft of datum point."

 

I doubt the others were much different since MAC stayed at the very same place (except for the Mk I which has the rear cog at 5.4 but ranging with various modifications to 9) and because all of them seem to have a normal loaded CG position at exactly the midst of this range, at roughly 4.5" from datum. If anything, the sheer weight of the Griffon in the front would just allow more CG play in the rear.

Edited October 21, 2015 by Kurfürst

[bongodriver]

Ummmmmm... really? "If an aircraft feels stable (to a particularly pilot) then it is stable - QED"?

 

I think this is what I alluded to when I suggested thrashing out a definition of (In)Stability in my earlier post.

 

If we can take a step back from WW2 aircraft for a minute and look at the common bicycle. Is it "stable"?

a) "Yes. Of course it is, don't be silly. Children can ride one. I see people riding them every day and they don't fall off. Proof."

b) "No. The Bicycle is an inherently unstable machine and requires both sufficient velocity and a sufficiently experienced rider to dynamically balance the machine... risking over-controlling at slow speeds and Rider Induced Oscillations (remember those?) in the handlebars. Maths and stuff."

 

Which is correct? Erm, they both are... depending upon how you *choose* to define Stability.

 

A bicycle is unstable when standing still, they become stable when moving, balancing with feet off the ground on a stationary bike is difficult, riding a bike is easy, likewise flying unstable aircraft is very difficult to near impossible (we have computers to do that for us now), the Spitfire was famously easy to fly, 19 year olds with 10 hours on type were flying into combat and anybody with time on the type has said exactly that, it is simply not unstable.

[bongodriver]

No, the rear tank test reads to me that the rear CG was about 9.9" with the rear tanks half used and its still extremely difficult to fly in that condition and was unstable. requiring a very experienced pilot to limit acceleration with push on the stick. Emptying the rear tanks fully moves the CG forward by about 5.2", so this is about your normal CG position: roughly 4.7" from Datum..

 

Well no, what it actually said is it was not considered dangerous if flown by experienced pilots, you know, experienced as in they are regularly flying and have done so for some time, a bit like military pilots do really, at no point did I read any embellishments such as very experienced and extremely dangerous.

[Crumpp]

Ummmmmm... really? "If an aircraft feels stable (to a particularly pilot) then it is stable - QED"?

 

Which is exactly there was a need for stability and control standards. Most nations adopted them post war but only a few had some sort of standard during the war.

 

Opinion is not quantified without a standard. Even to the point of differing conclusions:

 

 

The history of the development of the Cooper-Harper handling qualities rating scale:

 

https://engineering.purdue.edu/~andrisan/Courses/AAE490A_S2010/Buffer/HCooper.pdf

[Crumpp]

A bicycle is unstable when standing still, they become stable when moving, balancing with feet off the ground on a stationary bike is difficult, riding a bike is easy, likewise flying unstable aircraft is very difficult to near impossible (we have computers to do that for us now), the Spitfire was famously easy to fly, 19 year olds with 10 hours on type were flying into combat and anybody with time on the type has said exactly that, it is simply not unstable.

 

A more stable aircraft would have improved their survival chances while they gained experience.

 

[Kurfürst]

Well no, what it actually said is it was not considered dangerous if flown by experienced pilots, you know, experienced as in they are regularly flying and have done so for some time, a bit like military pilots do really, at no point did I read any embellishments such as very experienced and extremely dangerous.

 

Well dangerous is perhaps not the right choice of word, so lets just stick to what being stated in the report with half the rear tanks emptied (5.3): unsatisfactory handling qualities, unsatisfactory as a fighter and the loading recommended only as an operational necessity, and requiring constant check of the acceleration build up - a trait of longitudinal instability.

 

In practice this means that right after pulling the stick (and thus initiating g-acceleration) you have to push it right back to keep the aircraft from increasing acceleration on its own ('tightening up') and eventually spin out of control if left unchecked.

[bongodriver]

Well dangerous is perhaps not the right choice of word, so lets just stick to what being stated in the report with half the rear tanks emptied (5.3): unsatisfactory handling qualities, unsatisfactory as a fighter and the loading recommended only as an operational necessity, and requiring constant check of the acceleration build up - a trait of longitudinal instability.

 

In practice this means that right after pulling the stick (and thus initiating g-acceleration) you have to push it right back to keep the aircraft from increasing acceleration on its own ('tightening up') and eventually spin out of control if left unchecked.

 

Correct, a condition that any aircraft will suffer with a CG to the rear.

[MiloMorai]

How many Spitfire IX/XVI actually flew combat missions with the rear fuselage tank full of fuel?

[bongodriver]

A more stable aircraft would have improved their survival chances while they gained experience.

 

 

They did just fine, saying the Spitfire was not as good a gun platform than a Hurricane does not necessarily mean it was not a good gun platform.

[Cripple]

A bicycle is unstable when standing still, they become stable when moving, balancing with feet off the ground on a stationary bike is difficult, riding a bike is easy, likewise flying unstable aircraft is very difficult to near impossible (we have computers to do that for us now), the Spitfire was famously easy to fly, 19 year olds with 10 hours on type were flying into combat and anybody with time on the type has said exactly that, it is simply not unstable.

 

"Riding a bicycle is easy" - try tell that to a five year old, or someone who's never been on one... Instead it is a learned skill that becomes second-nature.

 

Which is rather like flying or indeed flying the Spitfire. The crux of this debate seems to be whether the Spitfire IX was inherently Stable/Unstable, not how easy it was to learn to fly one (which is a factor of *how* stable/forgiving an aircraft is, rather than a litmus test of absolute Stability/Instability).

 

(As an aside, I watched a documentary on the Mig21 last night. From this is seemed that the introduction of computer-assisted control was due to the increase in aircraft speed. The need for an aircraft that was stable at Mach 2+ lead to an aircraft that became unstable at sub-Mach speeds. That does not mean the aircraft was Unstable per se - in fact it was very stable at the speeds it was expected to engage the enemy, but landing again would be a bitch. :P Yes, I am aware that later fighter jets were designed to be Unstable throughout their flight envelope in order in increase manoeuvrability, but that's beyond the scope of this discussion.)

[bongodriver]

I'm sure a 5 year old would struggle if you gave them the keys to a spit too.

[Cripple]

I'm sure a 5 year old would struggle if you gave them the keys to a spit too.

 

Which isn't the point, is it? And certainly doesn't help the argument that the Spit was inherently stable.

 

The thing is, I agree with the majority of your argument - everything I have read seems to indicate that contemporary pilots considered the Spitfire to be a relatively easy aircraft to fly.

 

However I'd want to ask "compared to what?".

[bongodriver]

Which isn't the point, is it? And certainly doesn't help the argument that the Spit was inherently stable.

 

The thing is, I agree with the majority of your argument - everything I have read seems to indicate that contemporary pilots considered the Spitfire to be a relatively easy aircraft to fly.

 

However I'd want to ask "compared to what?".

 

OK, people who regularly fly spitfires tend to have experience on some of it's contemporary aircraft, there is one comparison for you.

I didn't quite see you bicycle argument as helping the point either for what it's worth.

[JtD]

No response JtD?

 

To what - that you can't tell the difference between the position of the CoG relative to the datum and the position of the LE of the MAC relative to the LE of the chord at the root or that you don't know how much an inch is in cm?

[Crumpp]

However I'd want to ask "compared to what?".

 

Not compared to any modern example of the Spitfire. None of the modern flying examples of the Spitfire are allowed to have the same CG limits as an RAF service Spitfire.

 

Modern flying examples are restricted to a much narrower CG range due to the stability of the design.

 

Your bicycle comparison is actually spot on.

 

Taking pilot anecdotes from pilots of the early 1940's in context, we find they had very different experience on what constitutes pleasant to fly compared with a modern day Cessna driver.

 

An experienced pilot in the early 1940's had little experience in stable aircraft. Most of the designs up until the late 1930's did not have the speed or the weight required to make stability and control engineering very critical at all.

 

Many airplanes were unstable and some had some pathological handling qualities.

 

part:

“Once in the air...the pilot is faced with almost total control disharmony.

The Camel is mildly unstable in pitch and considerably unstable in yaw, and

both elevator and rudder are extremely light and sensitive.. .the ailerons are in

direct and quite awe-inspiring contrast. The Camel.. .has four enormous...barn

doors [for ailerons] which require an equally enormous force to be moved quickly.

And when you have moved them, the wing section is so degraded... that the

roll response is very slow indeed... . At the same time, aileron drag is quite

staggering. If you take your feet off the rudder bar and bank to the left, the

Camel will instantly yaw sharply to the right and keep going... .”

 

Pilots had to learn to fly them by reference to pitch attitude, as force feel was unrealiable. In flying the Jenny, there was a distinct tendency for pilots to overcontrol in pitch. The saying was, “Don’t fly it; drive it.” The Jenny was flown by the “wind-in-face”

technique for directional control. The Morse was even more unstable, requiring a

forward push on the stick in turns and exhibiting a tendency for overcontrolling

with the rudder. Landings were not difficult because of high drag and a short

landing roll, but might require a push instead of a pull on the stick during the

flare and touchdown

 

At 45 mph, tailplane lift overcomes the combined power of foreplane and

elevator and the machine is now intent on a downward outside loop. This

actually happened in the old days, and the aviator who was not strapped in fell

into the underside of the top wing. The machine completed a half outside loop,

stalled inverted in the climb and entered an uncontrolled inverted falling leaf

with stopped engine.. .The only good thing about the whole story is that the Boxkite fluttered down and disintegrated, but so slowly that the aviator was

completely unharmed.”

 

Gen. Doolittle relates one incident where he was making a simulated

pylon turn at 4,000 ft and, during the entry, sufficient sideslip developed that

the aircraft executed a double snaproll.

 

At any time, if the angle of-attack were changed, the aircraft would tend to diverge. It was found that a hard bang on the stick laterally would adjust the wing alignment and place it backin balance. One must marvel at the approach a test pilot must take toward his airplane that would enable him to find that kind of solution to a problem

 

The need to quantify pilot opinion was realized rather late. The engineers simply found themselves at the mercy of the test pilot's ego.

 

However, many of the handling qualities improvements were obtained by “cut and try” methods, and not all pilot evaluations were conducted with expertise and objectivity. For example, one contract test pilot was known to guard against revealing his kneepad observations to the engineers but insisted on interpreting his observations himself and conveying only what he thought the solution or “fix” might be. This is a rather extreme

example of poor communications between test pilot and engineer. Engineers and designers who were misled by such tactics developed an appreciation for the type of test pilot who objectively and accurately reported his observations before attempting to recommend solutions.

 

The airplane designs of the late 1930’s began to reflect benefits from an increased understanding of static stability and, to a much lesser extent, dynamic

stability, emanating largely from NACA flight research.

 

That is why the NACA made deliberate effort to professionalize the test pilot. Part of the process was to expose them to flying a variety of aircraft. They would do so with close communication with the engineers to improve their understanding of stability and control.

 

Deliberate efforts were made to provide opportunities for test pilots during this and following periods, to fly and evaluate a variety of different aircraft; this farsighted policy was invaluable in enabling them to develop objectivity and overcome the biases which occur when background and experience are limited.

 

Test pilots were being transformed from the "most macho guy at the aerodrome" to a precision instrument and valuable member of the design team.

 

These data are of interest in showing the qualitative (rather than quantitative)

nature of initial attempts to formalize the use of subjective pilot assessments

as a means of developing handling qualities criteria

 

As a result of the combined contributions of flight data documentation and

assessment, wind tunnel studies, and theoretical analysis, the first comprehensive

sets of military flying qualities specifications were issued by the Navy Bureau

of Aeronautics [17] in 1942 as NAVAER SR-119 and by the U.S. Army

Air Force in 1943 as AAF-C-1815.

 

They confirmed the fact, noted by Allen [13] that “flight testing was becoming a more exact science, combining accurate quantitative data with the pilot’s qualitative report.”

 

https://engineering.purdue.edu/~andrisan/Courses/AAE490A_S2010/Buffer/HCooper.pdf

 

Gates and I agree the RAE would have greatly benefited from a similar set of standards and approach during the war.

 

As it was, the RAE's "flying qualities" program was based on the opinion of a few ego's and at the mercy of that experience.

[Crumpp]

To what - that you can't tell the difference between the position of the CoG relative to the datum and the position of the LE of the MAC relative to the LE of the chord at the root or that you don't know how much an inch is in cm?

 

BTW, It is not hard to convert the NACA MAC to RAE arm and moment.

[Yo-Yo]

What's an epic battle! And most of the warriors for stability already fly fully loaded 109K that is unstable at low speed with full power.