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#1
10-19-2011, 06:46 AM
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What kind of verification do you have for these sources Crummp? As far as I know you've written that stuff on an old typewriter and scanned it. Besides, when it says 'failed to meet requirements' - whose?! What requirements? For all I know the Spitfire fails to meet requirements for a heavy lift wide body! Context man, for pitys sake.
Besides, if what you infer is correct we'd have seen spitfires and DC-3s - or more accurately, there constituent parts - scattered all over the landscape because every single one was an inherently dangerous saftey hazard. Take a look how many survive into the modern day and are flown regularly and aerobatted reguularly without incident. Look at the war record of these a/c. Since when on either type is it apparent that they were falling out of the sky in pieces with a methodical regularity? Do I have to point out that the pictoral example of a structural failure that you provide IS A BLOODY Mk. FIVE again. Gimme strength! Besides which where on that photo/drawing does it show that this breakup was caused by excessive g due to instability? Oh that's right, it doesnt. It could have been faulty construction, metal fatigue, flutter, any number of causes. You just assume that it's down to some inherent flaw with Spitfires stability because you've got your axe to grind. As for your quote on the Mk. II that buffeting can cause large variation in stick travel and g - wow, revelation. Any one who's read into the spitfire knows how sensitive the elevators were. At what what point does it say ANYWHERE in that text that the a/c is longitudinally unstable or prone to taking itself to pieces in that text? It does not. You're extrapolating, badly while your at it, tying it in with other flawed and irrelevant data. The simple fact is your opinion extrapolated from text book teachings do not correlate with the historical record from a massive amount of disparate sources. And your one textbook evidence - whose validity I suspect - is not only being qouted without context - again WHAT & WHOSE requirements - but upon re-reading them it even agrees with me - NOTE the passage that you underlined 'the small static longitudinal stability', It says small. It does not say none. It says the stick was very sensitive to movement in pitch. It does not say Spitfires were falling apart all over the sky. AT NO POINT DOES IT SAY THAT A SPITFIRE IS DANGEROUSLY AND INHERENTLY UNSTABLE. Last edited by Fenrir; 10-19-2011 at 07:01 AM. 
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#2
10-19-2011, 06:53 AM
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Sorry just had to laugh; read the first line of your text, then the last line of the text under the 3rd image.:
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Last edited by Fenrir; 10-19-2011 at 06:56 AM.
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#3
10-19-2011, 07:58 AM
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 I find it interesting, but I do not think it Crumpp prove that the Spitfire was a bad aircraft, which is obviously not true. But there was not a perfect plane, perhaps, as some would like to believe. The Spitfire is a legend (in a good sense of the word), and not wonder if something like these test results, opinions have never enjoyed great popularity.
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#4
10-19-2011, 10:02 AM
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Fenrir comments are way too much aggressive to be relevant.
Nothing that Crumpp has says is a non-sense unless over interpreted by the reader. It fit actually many well known aero principles. By the way Fenir, the marginal mkV was just the most mass produced Spit variant ever.
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#5
10-19-2011, 10:37 AM
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The longitudinal instability was not corrected until the Mk V with the installation of bob weights to increase the stick force per G. Bob weights certainly help the pilot to maintain better control of his accelerations but they did not fix the actual problem of insufficient vertical and horizontal stabilizer area. That too was fixed in later marques as stability and control matured greatly as a science during the war. At the time the Spitfire was designed, the United Kingdom did not have a standard and there was no such thing as a stability and control engineer. It just was not that big a deal at the low speeds of open cockpit biplanes common before the war. As speeds and power increased though, it became very important. Quote:
Think about what the NACA says on the stick travel. You only have 3/4 of an inch of travel to run the wing from a CL of .3 to CLmax. The minimum standard was 4 inches. It is no wonder the Operating Notes suggest the pilot brace himself on the cockpit walls to control the aircraft. Imagine trying to land on a gusty day getting tossed around the cockpit with only 3/4 of an inch movement between controlled flight and a stall spin accident.
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#6
10-19-2011, 02:15 PM
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It would be unstable if it was <=0 lb/g. It wasn't. P-39 was less stable, with down to 2 lb/g at the most rearward CG allowed.
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#7
10-19-2011, 04:02 PM
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 Do you mean inverted ctrl ? Unstable means either that you have a too variable force to pull/push per deg of pitch (ideally it would hve been linear) or that you encounter a zone were the stick forces are reversed (but not negative). For ex the WWI Camel had a degree of reverse ctrl were you needed to push on he stick to raise the nose further up. Last edited by TomcatViP; 10-19-2011 at 04:04 PM.
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#8
10-19-2011, 07:21 PM
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The Spitfire was tested with around 5 lb stick force per g normal acceleration. Last edited by JtD; 10-20-2011 at 05:18 AM.
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#9
10-20-2011, 12:58 AM
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The Spitfire exhibited a stick for per g of 5lbs under the tested conditions. Stick Force per G is not stick force nor is it something that was applied by the tester during the test. It is something that was measured and can be calculated in the design phase for a condition of flight and CG position. It is the force required to reach 1G increment in acceleration. It represents the slope of the stick force gradient. It is a function of the hinge moments and stability margin. It is also a function of dynamic pressure and varies with altitude and condition. IIRC, in the case of the NACA test, the stick force at CLmax was ~22lbs. How does that stack up? Sounds like such light controls would be wonderful, huh? Not at all.... To put it in perspective, the FAA dictates minimum control force to reach maximum airframe g limits. Maximum limits is not structural failure. An aerobatic aircraft catagory is rated for a maximum of 6G's for example. A quick formula to ballpark the minimum control force is weight of the aircraft divided by 140. 7500lbs/140 = 53lbs 53lbs would be considered the minimum control force the pilot should experience at a 6G acceleration. You can begin to see why the NACA classified the Spitfire as unacceptable. Now let's get a ballpark figure for how long it would take our pilot to stall the aircraft with the Spitfires acceleration gradient. We will fudge it with known NACA measurements that are considerably higher than the Spitfires measured 5lbs per G. One of things engineers had to do when stability and control became a science was determine what the parameters were for a pilot to move the controls. According to the NACA, at 33lbs of stick force, the slowest rate of pull they recorded was 33 inches per second and the fastest rate 80 inches per second. With mental distractions, this rate dropped to 22 inches per second for the minimum recorded value. Time = Distance / Rate Time = .75 in divided by 22 in/sec Time = 0.034 seconds to move the stick from cruise CL of .3 to CLmax and stall at the minimum recorded value. The average pilot with the lower Stick Forces of the Spitfire could do it literally in the blink of an eye. Last edited by Crumpp; 10-20-2011 at 01:01 AM.
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#10
10-20-2011, 01:04 AM
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Do you have sources for those numbers?
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