Stability and Control characteristics of the Early Mark Spitfires

[robtek]

@ taildraggernut

ok, you've wrote at first NACA document, but you meant a Nasa-document about the Naca-document.
And you didn't flatter me, i meant sugarcoating, english is not my first language.
All those chaps with 10 h were those who did fly so carefully that they were outturned by 109's, maybe.

@ Al Schlageter
Of course light elevators ARE desirable, it's the combination with short stick travel which may cause problems for the less experienced.

[taildraggernut]

Originally Posted by robtek

@ taildraggernut

ok, you've wrote at first NACA document, but you meant a Nasa-document about the Naca-document.
And you didn't flatter me, i meant sugarcoating, english is not my first language.
All those chaps with 10 h were those who did fly so carefully that they were outturned by 109's, maybe.

@ Al Schlageter
Of course light elevators ARE desirable, it's the combination with short stick travel which may cause problems for the less experienced.

Does it matter much? does the fact it is a NASA document about a NACA document invalidate it? is it impossible to ascertain what I was refering to? I don't quite follow the need for that correction.

Sorry but english is my first and only language, I simply assumed you used such an uncommon word because you knew what it meant, I had no warning that you may not have understood the words you are using.

so none of the chaps survived an engagement with a 109? despite the fact....sorry anecdotal fact.....that new guys were told to turn as hard as they could in order to evade the 109?

with regards to short stick travel this is apparently another desireable quality, from another part of the same article, interstingly you will se that research was put into trying to make control forces 'light', now apparently this is what makes the Spitfire 'tricky', but what it actually did was compensate for the slight instability and made it more controlable.

The Quest for Reduced Control Forces

One of the most serious problems encountered by designers of military airplanes during WW II was keeping control forces desirably light while airplanes were being made with greatly increased weight, size, and speed. Flying qualities research had shown that maximum control forces should be kept below what a pilot could conveniently exert with one hand on the control stick or wheel. For ailerons, this force was about 30 pounds on a control stick or 80 pounds on a control wheel. Increasing the mechanical advantage of the pilot's controls was impossible because of the limited size of the cockpit and the lag in deflecting a control wheel more than plus or minus 90 degrees. Studies of aerodynamic balancing devices to reduce the aerodynamic moments on control surfaces became one of the main research objectives of wind tunnels involved in stability research.

Aerodynamic balance on most airplanes designed prior to WW II was usually accomplished by locating some control surface area ahead of the hinge line. Various arrangements of these balances are shown in figure 6.1. These balances had advantages and disadvantages from both mechanical and aerodynamic standpoints. In general, balances that were permanently located in the air stream were subject to icing that might jam the controls. Balances that broke the contour of the airfoil added drag. In addition to such practical considerations, balances had to be selected on the basis of the hinge moment parameters such as the variations of control surface hinge moment with angle of attack and with control deflection. These parameters had fundamental effects on the flying qualities. The effect on snaking oscillations of the variation of hinge moment with angle of attack has already been mentioned. To obtain light control forces, both of these parameters had to be reduced.

Theoretically, the control forces could be reduced to zero by reducing these hinge moment parameters to zero, but in practice this goal could not be attained. One problem was the nonlinearity of the hinge-moment variations. For example, a control surface that was properly balanced at low deflections might be overbalanced at large deflections. A second problem that limited the degree of aerodynamic balance on large and high-speed airplanes was the effect of small changes in contour due to manufacturing differences. These differences might be almost too small to detect, yet could cause quite large changes in the control forces. The Germans, in an effort to obtain very light aileron forces on the Me109 airplane, would test fly the airplane and try different sets of ailerons until one was found that would give forces in the desired range. The British, on testing the Spitfire, mentioned encountering "rogue" airplanes that had different characteristics from the standard airplanes, the reasons for which could not be detected.

As a result of these problems, a practical limit had to be set on the degree of aerodynamic balance, which was usually 25 to 30 percent of the forces produced by an unbalanced control surface. This degree of balance, however, was nowhere near what was required to provide desirable handling qualities on the largest or fastest airplanes. In some cases, forces would have to be reduced to about 2 to 4 percent of those of an unbalanced surface.

[Glider]

To get a feel for how fragile the SPitfire was the following is a posting I found which breaks the losses bytype.

remembering that 20,000+ Spts were built during WW2 the numbers are pretty low

thought I remembered reading an interview on this subject many years ago- and finally found it in a yellowed copy of Alfred Price's 'Spifire At War' (published 1974). It's germane to this discussion (as my teacher used to say) because the person being interviewed is none other than Mr Eric Newton who spent the war with the Air Accident Investigation Branch. He was still employed by them as an investigator in 1974- the time of the interview- so presumably still had the facts at his fingertips. This body was, and is, independent of the RAF.

Mr Newton was called in to investigate Spitfire crashes which could not be immediately attributed to pilot error (the same crashes which are detailed in Morgan and Shacklady). He says:

"Out of a total of 121 serious or major accidents to Spifires reported to us between the begining of 1941 and the end of the war, 68 involved structural failure in the air. Initially the most common reason for such failures, with 22 instances in 1941 and 1942, was aileron instability. The symptoms were not at all clear cut: the aircraft were usually diving at high speed when they simply fell to pieces. Only after one of the pilots had survived this traumatic experience and parachuted successfully were we able to find the cause. During his dive he saw both of his ailerons suddenly flip up, producing an extremely violent pitch- up which caused the wing to fail and the aircraft to break up. In collaboration with RAE we did a lot of tests and found that aileron up- float was made possible by stretch in the control cables; in those days tensioning was a hit or miss affair with no compensation for temperature. On our recommendation the RAF introduced a tensometer which ensured accurate tensioning of the controls; this, and the simultaneous introduction of metal surfaced ailerons ('42/'43), cured almost all the cases of aileron instability in the Spitfire.

The next most serious cause of structural failure in the Spitfire was pilots overstressing the airframe. She was extremely responsive on the controls and one must remember that in those days there was no accelerometer to tell the pilot how close he was to the limit. So it was not difficult to exceed the aircraft's 10G ultimate stress factor (what was the 109's?- Berkshire) during combat or when pulling out from a high speed dive; during the war we were able to put down 46 major accidents to this cause, though undoubtedly there were many other occasions when it happened and we did not see the wreckage. Incidentally, if there was a structural failure in the Spitfire it was almost inevitably the wing that went; the fuselage was far less likely to fail first (the same for most low wing monoplane fighters?-except the Typhoon?- Berkshire).

I once asked a very senior RAF officer why the accelerometer- technically a simple instrument- was not introduced during the war. He replied that he was sure it would have an adverse effect on the fighting spirit of the pilots (same was said re the parachute in WW1!- Berkshire).

Whether that would have been so I cannot say. But I do know that when they finally introduced the accelerometer into service in the Hunter in 1954, and began educating the pilots on structural limitations and the dangers of overstressing, accidents to this cause virtually ceased.

After structural failure the next largest category of accidents proved on investigation to have followed loss of control by the pilot (36 cases). Of these 20 occured in cloud and could be put down to pilot error; one must remember that in the rush to get pilots operational instrument training was not up to peacetime standards. A further 13 accidents were shown to have been caused by oxygen starvation; the oxygen system had been used incorrectly with the result that the pilot had passed out and the aircraft had crashed. As a result of our investigations the system was modified to make it easier to operate.

The remaining 3 accidents in the loss of control category were initiated by the pilot pulling excessive G and blacking himself out.

Engine failures and fires contributed a further 17 accidents, and the remainder could be put down under the 'miscellaneous' heading (long story here about fuel leaks and explosions on the ground- Berkshire)

As I have mentioned we investigated a total of 121 Spitfire accidents during the war. The causes did not always fit simply into neat categories mentioned above. For example, a pilot might lose control in cloud and his aircraft then broke up in the ensuing dive due to aileron instability- in that case the accident would have been listed under two categories. There were one or two accidents caused by the light- weight plastic bucket seats fitted to some batches of Spitfires. The trouble was they were not strong enough and if there was a heavy pilot who pulled a bit of G they tended to collapse- on to the elevator control runs which ran underneath. We soon had that type of seat replaced.

In the nature of my work I tend to concentrate on an aircraft's failings and ignore its good points; but how safe was the Spitfire? I think the figures speak for themselves; a total of more than 22,000 were built, and we were called in on only 130 occasions- and in not all of those was the Spitfire at fault. If one considers that she was not a simple trainer built for ease of handling, there can be no doubt that the Spifire was a remarkably safe little aircraft."

To summarise:
There were 121 Spitfire crash investigations between 1941 and May 1945 involving serious structural failure:
22 aileron instability
46 pilot overstressed airframe
20 pilot error in cloud
13 misuse of oxygen system- pilot error
3 pilot blacked out
17 engine failure/fire

[TomcatViP]

The doc is quite interesting and sum-up pretty well what we know abt the plane but it ends in horrendous and hair rising conclusion.

It shld say that out of 133 accidents investigated 126 were from plane malfunction. And what does it says : out of 20000+ units only 126 crashed from plane malfunction !!!

This guy shld hve worked for insurance companies.

[robtek]

@ taildraggernut

do you earnestly propose that having only a three quarter inch useable stick travel out of about 20 inches possible stick travel is desireable?????

Just to remember that three quarters of a inch stick travel is needed to pull the spit in a stall at cruise speed and above.

Eventally add bumpy air and you'll find a lot of unwanted pilot induced oscillations!

[taildraggernut]

Originally Posted by robtek

@ taildraggernut

do you earnestly propose that having only a three quarter inch useable stick travel out of about 20 inches possible stick travel is desireable?????

Just to remember that three quarters of a inch stick travel is needed to pull the spit in a stall at cruise speed and above.

Eventally add bumpy air and you'll find a lot of unwanted pilot induced oscillations!

how did they ever manage to fly in formation if the aircraft needed to be wrestled with so much? don't tell me...all the footage we ever see of spitfires flying was done on smooth days?.......they were flying reeeealy sloooowly?

Seriously the desparation is really showing now, I can't believe this isn't getting embarrassing for you chaps.

[JtD]

The P-39 has less stick travel and a lighter elevator and was still accepted into service.

Did the Spitfire I also show static longitudinal instability when it was equipped with a different propeller?

[Crumpp]

121 Spitfire crash investigations

That does not even scratch the surface on the number of pilots who damaged the aircraft but managed to make it home.

There were stories of Spit wings in huge piles at maintenance units as fractured wings were replaced after sharp pullouts at low levels - something that the more solid Hawker products were not affected by.

http://members.madasafish.com/~d_hod...wker-Vspit.htm

[taildraggernut]

That does not even scratch the surface on the number of pilots who damaged the aircraft but managed to make it home.

and thats a bad thing?

There were stories of Spit wings in huge piles at maintenance units as fractured wings were replaced after sharp pullouts at low levels - something that the more solid Hawker products were not affected by.

Can we get at least one thing sorted here, is annecdotal evidence valid or not?

[41Sqn_Banks]

Originally Posted by taildraggernut

and thats a bad thing?

No, because the damage to the engine by using more than 400 hp was much more troublesome