Spit IIa

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The Spitfire was tested with around 5 lb stick for per g normal acceleration.

Again so everyone is clear. This should read:

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.