Spitfires and Longitudinal Stability

[PRB]

Read a book by Jeffery Quill called Spitfire, A Test Pilot’s Story. Anybody interested in the history of the Spitfire must read this book. And if you’re thinking about making an FS flight model for a Spitfire, this book is Required Reading!
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Seems the Spitfire had a very narrow margin (fore and aft) of longitudinal stability. Primarily affected by the location of the CG, an unstable aircraft will continue pitching up or down, at an increasing rate, when elevator control is applied, and will tend to tighten into turns by itself (:icon_eek:), if not counteracted with opposite elevator, a potentially dangerous situation indeed! Later production models, with bigger props and more propeller blades, aggravated the problem because, as Quill put it, “the blades of a propeller are aerodynamically equivalent to a fixed horizontal surface equal to their projected area. This is tantamount to having foreplane acting in reverse sense to the fixed tailplane, and is therefore essentially destabilizing.” RAF pilots initially, and wrongly, attributed this instability to insufficient elevator authority. It made “common sense”, of course, because the plane was difficult to control in pitch. As the CG shifted during flight, as fuel burned off, the problem got worse. Also, in operation, additional equipment was added with the result that some operational squadrons were taking off in Spitfires with CGs outside the Supermarine’s published CG limitations, making then dangerously unstable.
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This book is great. All this gets me wondering about FS planes and about how real we really want these things to fly!

[tracyq144]

I can also recommend a book by Leo Mckinstry, Spitfire, that traces the development of the Spitfire, and includes anecdotes from pilots. While almost all pilots are effusive in their praise of the Spit, there is also a subchapter on how "they were not perfect."

While the book is probably 75% to 80% about the politics and manufacturing side of the Spitfire, there are enough insights into the flight model and improvements through the various Marks to make it a worthwhile read for anyone interested in this airplane.

I must also say the the A2A version seems to capture the idiosyncrasies of this plane, as related in this book almost perfectly. Yes, the early Marks were notoriously prone to overheating. :)

[PRB]

Thanks Tracy. I will look for that book. I've read a lot here, since the release of the A2A Spitfire, of the overheating issues. I don't remember Quill spending a lot of words on this problem. Have to go back and read it again.

[fliger747]

The Spitfire was once described as riding a light and firey horse. I had one once, the horse. All you had to do was think where you wanted to go.... don't sneeze.... Modern fighters achieve some of their manuverability from neutral stability, augmented into a flyable system by various stability enhancing go betweens, up to a full synthetic fly by wire system. American practice prefered about 10 lbs/G of stick force. I have flown planes with the above mentioned neutral/negative stability. Early marks of the Spit generally had high marks for handeling. To sucessfully fly such planes, low control friction is necessary. Including it the sim pilots setup.

Cheers: T

[tracyq144]

I've read a lot here, since the release of the A2A Spitfire, of the overheating issues

I agree, there doesn't seem to be much about this in the published literature, but there are a few very specific references to the problem in the above mentioned book.

"As soon as the engine started, pilots were desperate to get into the air because of the risk of overheating. Geoffrey Page, who was later shot down and badly burnt in combat, found this was his primary concern on his first flight:

A trickle of sweat ran down my forehead. Suddenly the powerful engine coughed loudly, blew a short stream of purply-white smoke into a small cloud and roared into life. Remembering that I had little time to spare before the temperature reached the danger mark of 110 (degrees), I waved my hands across my face. The waiting airman quickly ducked under the wing and pulled away the restraining chocks. Glancing down, I was alarmed to see that the glycol coolant temperature had risen from zero to 70 (degrees). Releasing the brake, I eased the throttle open and the surge of power carried the aircraft rapidly over the grass."

[hairyspin]

Hmmm. It depends what literature you read, but a novice Spitfire pilot, especially on the early marks, had a lot to take care of on his first flight. Trained on air-cooled Tiger Moths, then air-cooled Harvards, monitoring the coolant temperature on a liquid-cooled thoroughbred was yet another thing to look after along with everything else. With familiarity that would become second nature.

Boiling the coolant was fairly easy to do with full power at lowish airspeeds, especially on the earlier marks which had just one radiator unit to deal with coolant and intercooler heat loads. The later marks shared the intercooler heat load with the oil cooler in an enlarged port wing radiator unit.

Jeffrey Quill managed to boil the coolant in a Mk.I at 25,000ft - the vapour then froze on his windscreen. Oooer, missus!

Re stability and CG limits, Mustangs on long-range escort missions often took off with the CG aft of the official limit because the aft fuel tank was full: this tank was used first and the handling improved greatly as it emptied. This was typical of wartime practice, peacetime runs by different rules.

Unlike the photo-recon Typhoon which "just didn't feel right". Many weeks later the weight and CG were checked - eeek! Some heavy armour had been removed from the front, but the corresponding ballast was still in the tail...

But as far as Spitfire FMs go, the nearer to the real thing, the better imo.

[SkippyBing]

Reading through 'Up in Harm's Way' by R M Crosley it sounds like the Seafire exacerbated the stability problem by fitting a tailhook and HF radio, to the extent they had to fit weights to the elevator control runs in some of the later versions. Which didn't always help as they could make it impossible to pull out of a dive...

[fliger747]

I think "Winkle" Browns comment "Where needs must, the Devil drives" is appropriate, though he was refering to the F4U.

The engine had to be warm enough to accept takoff power, but not so much to overheat! In the quest for speed, cooling drag was reduced by small radiators.... not always the best compromise. Mustang used a good system that was an improvemet and allegedly gave a little thrust! Of course no one was aware of the area rule at the time.

Cheers: T