A newbies impression of the 109 and spit

[WTE_Galway]

Quote from page 46 of a book by Morgan & Shacklady taken from this discussion:

http://forum.keypublishing.com/showthread.php?t=75816


The Air Ministry was not completely satisfied with the spin recovery of the Spitfire, and at a meeting on 17 January 1938, chaired by Air Cdr. Verney, Supermarine persuaded those in attendance that no modifications be made to production aircraft apart from the addition of an anti-spin parachute. For the Air Ministry Verney said that based upon model tests at Farnborough production aircraft could not be passed for spinning even with a tail parachute. Supermarine then pointed out that Jeffrey Quill had made sixteen successful spins of eight turns in the prototype. After more discussion the DTD agreed to accept the Supermarine proposal and that the first 20 production models should be fitted with the tail parachute and undergo further spinning trials. He, DTD, would be satisfied with recovery at 15,000 ft. When the first production Mk 1 Spitfire, K9787, was completed at the beginning of May 1938 an anti-spin parachute was duly fitted and the aircraft made its first flight from Eastleigh, piloted by Quill, on 14th of the same month .

[Crumpp]

Originally Posted by JtD

In case some folks can't be bothered to read the NACA tests, I'll post a part to put the "unacceptable longitudinal dynamic stability" into proper perspective.
Eventually, the longitudinal stability was not unacceptable. The Spitfire was accepted into service with about 20+ military air forces, was built in 20000 examples, and is still being flown today. Unacceptable longitudinal stability would mean acceptance into 0 air forces, and a production of a handful of examples, and none would be cleared for flying today.
However, it is true that the Spitfire did not meet all the requirements set by NACA in "Requirements for Satisfactory Flying Qualities of Airplanes". Other planes that failed to meet all the requirements were for instance the P-39 or the XP-51.

The DC-3 also had longitudinal stability and control issues. The fact both the Spitfire and DC-3 had long careers is not an excuse to dismiss flying quality requirements. It is perfectly reasonable to assume that both of these fine airplanes would have been even better had they been more longitudinally stable under all flight conditions. Nobody has ever run definitive, statistically valid experiments on the value of good flying qualities in terms of accident reduction or military success. Common sense prevails and the entire world has since adopted stability and control standards to reduce accidents and increase air to air combat effectiveness.

[JtD]

Originally Posted by Crumpp

Right, I did the bold.

In that case, maybe you can next time avoid putting your words in my mouth by using proper formatting.

On the contents you added, I won't disagree with what you've said regarding the stick fixed and stick free stability testing, as it is absolutely right. You should, however, keep in mind that I try to explain things in a way that the concept can be understood by anyone interested, not just those with a suitable education or years of experience in the field. In my opinion, it is easier to understand "no trim change with constant elevator when speed changes" than to understand a description of an initial reaction to displacement.

However, I disagree with

The longitudinal dynamic stability (Long Period Oscillations) was neutral or negative as recorded by the NACA.

, because NACA says:
regarding dynamic stability:

only the short-period oscillation is dealt with here

and regarding neutral or negative stability which they recorded:

static longitudinal stability

.
To sum it up, NACA did not record long period oscillations for the Spitfire and the assessment of neutral or negative stability was made for static longitudinal stability.

[JtD]

Static stability is the aircrafts initial reaction to displacement.

Your words. It is my understanding that this is exactly what NACA assessed and what they found to be neutral or unstable, only that they used a different method for testing.
NACA didn't have a problem with (short period) oscillations over time, the Spitfire would dampen any (short period) oscillations within a cycle or two.
NACA did have a problem with the fact that a bit of extra elevator at any speed, if not reduced, would change the AoA for good, meaning the plane would not come back to a more level attitude even when speed was reduced.

Looking at the easy to understand diagrams CaptainDoggles linked, neutral static stability appears to be exactly the problem NACA had with the Spitfire.

I see you deleted your last post, but it might still help if I leave this one up to make sure we all use the same terminology.

[Crumpp]

To sum it up, NACA did not record long period oscillations for the Spitfire and the assessment of neutral or negative stability was made for static longitudinal stability.

They are talking about static stability. Wow, shows you how much things have changed and how new stability and control was as a science during WWII.

Static instability is horrible in an airplane. Seriously...the FAA and ICAO would send you back to the drawing board if you were seeking certification.

http://www.law.cornell.edu/cfr/text/14/23.173

The longitudinal dynamic stability (Long Period Oscillations) was neutral or negative as recorded by the NACA.

Poor choice of words on my part. Long period Oscillation has specific meaning and it should read:

The longitudinal dynamic stability (Oscillations over time) was neutral or negative as recorded by the NACA.

[Crumpp]

Looking at the easy to understand diagrams CaptainDoggles linked, neutral static stability is exactly the problem NACA had with the Spitfire

Good diagrams and I have some others that will make it clearer for you all.

That diagram would make you think the elevator is held constant but it is not.

Keep in mind when that report was written there were not any standards of the day. It is not like testing processes or airworthiness. It was a very new science that was not covered in convention. In the 1980's there was even a "counter-revolution" in stability and control engineering.

[Kurfürst]

Originally Posted by winny

The weight was built into the elevator circuit, in simple terms it was used to counteract the effect of light elevators and high g forces. The weight was effected by the g forces so if there was a high load the elevators needed more force to move them.

It was fitted to spitfires to solve the problem of spitfires breaking up when pulling out of a high speed dive, which was a known, and big problem at the time.

Because the elevators were so light pilots were pulling out of the dives and over stressing the airframe. There were loads of examples of this happening.

So basically the more g that was pulled the harder the elevators became to move. As far as I know it had nothing to do with any instability.

As I understand and from RAE papers the installations of bob weights to the elevator and longitudal instability were related. To my best understanding - and do correct me if I am wrong - instability means that if you pull the controls (in whatever direction), the aircraft will not only change its roll/pitch/yaw to the extent of control movement, but also keep increasing it on its own, as if there were some kind of inertia/acceleration going on. This was noted on Spitfire Vs by the British.

By adding the bob weights and making the controls progressively harder to move for greater deflections, it made this increased acceleration problem more difficult to encounter.. It did not cure the instability itself, which was an inherent aerodynamic feature of the design, but made it harder for the pilot to make it happen.

[Kurfürst]

Originally Posted by WTE_Galway

Quote from page 46 of a book by Morgan & Shacklady taken from this discussion:

http://forum.keypublishing.com/showthread.php?t=75816


The Air Ministry was not completely satisfied with the spin recovery of the Spitfire, and at a meeting on 17 January 1938, chaired by Air Cdr. Verney, Supermarine persuaded those in attendance that no modifications be made to production aircraft apart from the addition of an anti-spin parachute. For the Air Ministry Verney said that based upon model tests at Farnborough production aircraft could not be passed for spinning even with a tail parachute. Supermarine then pointed out that Jeffrey Quill had made sixteen successful spins of eight turns in the prototype. After more discussion the DTD agreed to accept the Supermarine proposal and that the first 20 production models should be fitted with the tail parachute and undergo further spinning trials. He, DTD, would be satisfied with recovery at 15,000 ft. When the first production Mk 1 Spitfire, K9787, was completed at the beginning of May 1938 an anti-spin parachute was duly fitted and the aircraft made its first flight from Eastleigh, piloted by Quill, on 14th of the same month .

Its interesting to note that the Bf 109 won the fighter tender against its Heinkel rival due to the excellent spinning and stability characteristics much desired by the Imperial Air Ministry. Green notes, that the Commission ultimately ruled in favour of the Bf 109 because of the Messerschmitt test pilot's demonstration of the 109's capabilities during a series of spins, dives, flick rolls and tight turns, throughout which the pilot was in complete control of the aircraft.

[bolox]

while not strictly related to the spit/109 debate, the fitting of elevator bob weights wasn't unique

http://p51h.home.comcast.net/~p51h/sig/TO/01-60-90.pdf

[Crumpp]

while not strictly related to the spit/109 debate, the fitting of elevator bob weights wasn't unique

Stability and Control issues were not unique in WWII era aircraft. The science was very young and there were no standards in place. Most if not all of them had some sort of issue.

It is the area WWII fighters show the most variation in performance and is just as important to their fighting abilities as the aerodynamics.

Germany was the only combatant to have standards when the war started. The United States had standards by the time it entered the war as well. Everyone else did not adopt any defined standards until after the war.