Originally Posted by JtD
Blacking out takes time, damaging the airframe does not. There's no reasonable way to exceed the airframes limit in a sustained manoeuvre, however, in a sudden pull out, or due to buffeting with elbows not fixated, it may happen.
Put it as you want, personally I'd not agree with "dangerously low" control forces as propagated by some, nor do I agree that it was a none issue. The Spitfire II pilot notes had the paragraphs added in early 1940, after at least two fatal accidents due to wing failure had occurred and several Spitfires went to maintenance with bend wings. It wasn't a that rare thing to happen, but certainly Spitfires weren't falling from the sky as some here seem to believe. An extra warning was deemed necessary, as pilots transferring to Spitfire equipped units simply were not used to a feature like that. WRT elevator forces, I'd say that the Spitfires elevator forces were not sufficiently high to prevent accidents. Hence they were getting some attention.
Once more about longitudinal stability - the about neutral static stability of the Spitfire gave a lot of pitch reaction to little elevator movement. However, as opposed to what some claim here, the airframe would not self destruct given a bit of elevator input, as would be the case in a statically unstable aircraft. It was not necessary to pull back the stick to get a pitch up, and then push the stick forward to maintain controlled flight. A low static stability means that an aircraft needs a lot of time to settle around a new condition. This in turn means, that if you want a strong initial reaction, you'll have to pull back the stick like you would in a more stable aircraft, however, you'd end up at a lot more g. Or, you can pull back more gently and end up at the same g as a more stable aircraft, but you'd have a slower initial reaction. This leads to the pull - push routine an experience pilot would employ while putting the Spitfire through manoeuvres. Large deflection for a quick initial reaction, eased forward for moderate final loads. This is not critical at all, as the stick forces increase as the g load builds up. In a normally loaded Spitfire I, the pilot would still have to exert near 50 lbs of force to damage the airframe.
On to dynamic stability - short period oscillations were sufficiently damped by the Spitfire. Long period oscillations were not. This was however, typical for aircraft of that era, and opposed to what one poster claims, fighter aircraft of that time in general were no hands off aircraft. In fact, the Spitfire stability in long period oscillation is above average from what I've seen, much better than say a Hurricane. One of NACA's chief test engineers states that all fighter aircraft they tested were dynamically unstable in long period oscillations. They do not matter much to the pilot while flying, where they do matter is if the pilot's unconscious, and regains consciousness only to find himself in a steep spiral dive he can't get out of.
I'd recommend that instead of going over the same Spitfire chart time and again, try finding charts for other aircraft. This would certainly help to get the proper perspective, and maybe even an overall more accurate FM for many planes, not just the Spitfire. The focus of some on that plane is worrying.
|