Spit IIa

[JtD]

Originally Posted by Fenrir

...What requirements?...

The Spitfire was at about 5 lb/g, requirements were around 8 lb/g. So it was too light on the elevator.

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.

[TomcatViP]

Originally Posted by JtD

It would be unstable if it was <=0 lb/g. It wasn't.

I don't get you ?! Negative mass in a Spit ? Is that in concordance with the black mass theory ?

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.

[JtD]

Originally Posted by TomcatViP

I don't get you ?! Negative mass in a Spit ? Is that in concordance with the black mass theory ?

Stick force. Thought that would be clear from the context, but reading again it is not. Sorry.

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

[Crumpp]

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.

[CaptainDoggles]

Do you have sources for those numbers?

[Crumpp]

You can look in the FAR.

http://rgl.faa.gov/Regulatory_and_Gu...4!OpenDocument

The stick rates comes from:

NACA RB No. L4E31

ORIGINALLY ISSUED

May 1944 as Restricted Bulletin L4E31

MAXIMUM RATES OF CONTROL
FROM GROUND TESTS

By De E. Beeler

[Crumpp]

As for the original premise of this discussion, the effect of a hard buffet for stall warning on turn performance:

Aircraft with shallow stick force-per-g gradients
can feel dramatically sensitive if your muscle
memory expects greater forces. Even
experienced aerobatic pilots stepping up to
higher performance aerobatic aircraft usually
find themselves pulling too hard, detaching the
boundary layer, and buffeting the
aircraft—especially in the excitement of
aerobatic competition. This is seen from the
ground as an abrupt flattening in the arc of a
loop, and from the cockpit as a sudden g-break.

http://www.flightlab.net/Flightlab.n...u%232BA152.pdf

In the absence of boundary layer devices, buffeting will increase the radius and decrease the rate of a turn. The harder the buffet and larger the buffet zone, the more dramatic the result.

Stall warning is another engineering trade off. If you produce an airplane with large amount of stall warning, it will not achieve best rate of turn at 2D CLmax. The less buffet with smaller buffet zone and less stall warning, the closer to 2D CLmax the aircraft can achieve best rate of turn.

[CaptainDoggles]

Originally Posted by Crumpp

You can look in the FAR.

http://rgl.faa.gov/Regulatory_and_Gu...4!OpenDocument

The stick rates comes from:

NACA RB No. L4E31

ORIGINALLY ISSUED

May 1944 as Restricted Bulletin L4E31

MAXIMUM RATES OF CONTROL
FROM GROUND TESTS

By De E. Beeler

Thanks

[RAF74_Winger]

Originally Posted by Crumpp

53lbs would be considered the minimum control force the pilot should experience at a 6G acceleration.

Originally Posted by Crumpp

7500lbs/140 = 53lbs

Selective reading, again. The linked document actually states this: (2) For stick controls, W/140 (where W is the maximum weight) or 15 pounds, whichever is greater, except that it need not be greater than 35 pounds.

The spit Mk1 MTOW is 5,844 lb BTW, gives me 42 lbs according to the FAA formula for chimp-proof civil aircraft.

Originally Posted by Crumpp

IIRC, in the case of the NACA test, the stick force at CLmax was ~22lbs.

At what speed? What was the resulting acceleration? You're only telling half the story.

W.

[IvanK]

NACA did not classify the Spitfire as Unacceptable what it actually said was .."therefore failed to meet the accepted requirements" (NACA's referenced requirements ... nobody else's) and to a specific item. If you read the various NACA reports in their entirety you don't come away with the impression that the Spitfire was a POS from a handling point of view.



They also said with respect to being able to rapidly pull to Clmax without the risk of stalling:



Something most Fighter pilots would consider a highly desirable characteristic.

CRUMPP you said above:

"As for the original premise of this discussion, the effect of a hard buffet for stall warning on turn performance:"

The premise of the discussion was NOT flying in Hard Buffet at all ! but on the very first indication i.e. The "Buzz" or the "Nibble" or the "Burble" ... what ever you want to call it. In a previous post you erroneously said the Buzz and Buffet I described was in fact the stickshaker going off even though in these aeroplanes no stickshaker system was fitted, you also told me that it was only valid technique in FBW aircraft ... even though we were talking about coventional cable/pushrod flight control systems ! You fail to accept that flying on the Buzz was/is a technique practised by Fighter pilots the world over and examples provided in this thread from at my count by 4 independent people/references ... by those that have actually used the technique....including a Spitfire pilot from the Battle Of Britain.