Stability and Control characteristics of the Early Mark Spitfires

[NZtyphoon]

Originally Posted by Crumpp

Are we really gonna have this conversation??

Originally Posted by Crumpp

JtD, they flat out state the stick fixed longitudinal stability is unacceptable. That is the NACA, not me.

Why do you think bob-weights were added to the design to fix the longitudinal stability?

I can't help it if you're confused: the-reason-inertia-weights-were-introduced-was-because-Spitfire Vs-were-being-badly-loaded-at-an-operational-level-in-1942. As more operational equipment was introduced some squadrons were ignoring the loading diagrams. It had nothing to do with the NACA report.

[Crumpp]

it was statistically established that, as soon as the longitudinal stability of the Spitfire was thus brought under control

Why don't you read the Operating notes for any early mark Spitfire.

"longitudinal instability" is used often....

I am sure the Spitfire Mark V increase in weight and speed caused an unmistakeable as well as difficult to ignore increase in in-flight break ups.

[NZtyphoon]

Originally Posted by Crumpp

I am sure the Spitfire Mark V increase in weight and speed caused an unmistakeable as well as difficult to ignore increase in in-flight break ups.

Nope, you are quoting well out of context: what caused some break-ups was poor loading by the squadrons, as was explained by Jeffrey Quill:

Originally Posted by NZtyphoon

Quill
"In general configuration the Mk I and Mk II production aeroplanes were almost identical to the prototype and so there was no problem with their stability." (231-232)

"Therefore, for each sub-variant of the Mk V detailed instructions for the correct loading of the aircraft were issued to squadrons....However the importance of these loading instructions was not generally appreciated in squadrons and in the daily round of operational activity they tended to be disregarded." Once the bobweights had been introduced and, in later marks, the modified mass balances on the elevators...it was statistically established that, as soon as the longitudinal stability of the Spitfire was thus brought under control, the problem of the unexplained breakings-up of aircraft in mid-air,...'softly and suddenly vanished away'." (page 238 )

[JtD]

Originally Posted by Crumpp

Now let's see the instability.

Green:

You can find similar spots in turn time histories for nearly all WW2 aircraft tested this way. Or, you can pick another spot on the very same curve and will find different figures or pick the curves 16-18 and get different results again. If it was a plane quality, there'd be the same behaviour in every test they've done, but it isn't.

NACA evaluates the the behaviour:

"In turns at speeds high enough to prevent reaching maximum lift coefficient because of the excessive accelerations involved, the small static longitudinal stability of the Spitfire caused undue sensitivity of the normal acceleration to small movements of the stick. As shown by the time histories of high-speed turns (figs. 15 to 18), it was necessary for the pilot to pull back the stick and then ease it forward almost to its original position in order to enter a turn rapidly without overshooting the desired normal acceleration. Although this procedure appears to come naturally to a skillful pilot, flight records from other airplanes show that a turn may be entered rapidly and the desired normal acceleration may be held constant by a single rearward motion of the stick provided the static stability of an airplane is sufficiently large. By careful flying, the pilot was able to make smooth turns at high speed, as shown by figures 17 and 18. Ordinarily, however, small movements of the stick caused appreciable variations in the normal acceleration, as shown in figures 15 and 20."

So, what they are saying again is that there are large reactions to small stick travel. Not that the plane was unstable. The stick force gradient and the elevator angle gradient were both found to be positive, as I've said already.

[Crumpp]

So, what they are saying again is that there are large reactions to small stick travel. Not that the plane was unstable.

Yes, they definately say there are large aceleration changes for a small amount of elevator travel.

The other part is that they do say the airplane is unstable. Neutral or unstable......

At a neutral or unstable condition, the Spitfire is not able to hold a constant aceleration in a turn.

One can run the math on Cm and see that too.

flight records from other airplanes show that a turn may be entered rapidly and the desired normal acceleration may be held constant by a single rearward motion of the stick provided the static stability of an airplane is sufficiently large.

At forward CG, the static stability is sufficiently large. At normal and aft CG the static margin is neutral or unstable.

[bongodriver]

At forward CG, the static stability is sufficiently large. At normal and aft CG the static margin is neutral or unstable.

But this applies to 'all' aircraft, it seems pretty odd to criticise the Spitfire for something that is universal, this is exactly the same problem the Mustang has with a full fuselage tank.

to determine how much of a problem this is for the Spitfire a decent weight and balance schedule is needed, from that it can be calculated how much fuel burn is required to put the CoG forward.

[Sandstone]

Originally Posted by Crumpp

At a neutral or unstable condition, the Spitfire is not able to hold a constant aceleration in a turn.

But is perfectly capable of doing so when flown by a pilot of even limited experience.

[Sternjaeger II]

I always thought that the "breaking up in the air" of early Spits was mainly caused by design flaws on the tail section (like in the early Typhoons) which emerged in high speed dives.

[Crumpp]

But this applies to 'all' aircraft

No it does not.

It depends on the static margin. The static margin will move as the CG changes but most aircraft are designed to have positive stick fixed stability at the most rearward position.

In fact, that point defines the rear CG limit.

[IvanK]

So then Mr Crumpp exactly what is wrong stability wise with the IL2 CLOD Spitfire MI I or II Ver 1.06.17582+Hot Fix