I Want my 4.09 Spit FM's back.........

[Fenrir]

Originally Posted by TheGrunch

Well, I don't think DT did the *wrong* thing, per-se. The maximum economical cruising speed for the Spit IX (from the manual) is 170mph, so they clearly went toward making those long missions more comfortable.

"The recommended speed for maximum range is 170 m.p.h. (147 kts) I.A.S. if the aircraft is lightly loaded. At heavy loads, especially if the rear fuselage tanks are full this speed can be increased to 200 m.p.h. (172 kts) I.A.S. without incurring a serious loss of range."

"On aircraft not fitted with interconnected throttle and propeller
controls
(a) With the supercharger switch at MS fly at the maximum
obtainable boost (not exceeding + 7 lb./sq.in.) and obtain the
recommended speed by reducing r.p.m, as
required.

NOTE.— (i) R.p.m should not be reduced below a minimum of 1,800. At low altitudes, therefore, it may be necessary to reduce boost or the recommended speed will be exceeded.

(ii) As the boost falls at high altitudes it will not be possible to maintain the recommended speed in low gear, even at maximum continuous r.p.m, and full throttle. It will then he necessary to set the supercharger switch to AUTO. Boost will thus be restored and it will be possible to reduce r.p.m, again (as outlined in(a) above).
(iii) In both low and high gears r.p.m, which promote
rough running should be avoided. "

Roger that, however there's a massive difference between 170mph at 3000rpm and 170mph at the most economical rpm; around about 1800.

The torque effects will be VERY different.

If the current flight model is geared for this figure at 3000rpm then it's wrong - the best cruise settings for the Mk V (I don't have the data for the IX) were:

- 230mph IAS, +2.75lb, 1800rpm @ 2000ft = 35 gallons per hour
- 200mph IAS, -1.5lb, 1800rpm @ 10,000ft = 29 gallons per hour
- 250mph IAS, +3.75lb, 2000rpm @ 10,000ft = 42 gallons per hour
- 200mph IAS, -1.75lb, 2200rpm @ 20,000ft = 36 gallons per hour
- 230mph IAS, +1.5lb, 2400rpm @ 20,000ft = 46 gallons per hour
- 180mph IAS, -3.25lb, 2850rpm @ 30,000ft = 41 gallons per hour

It seems that a minimum of 200mph was recommended, with 230-250 preffered. This represents cruise over friendly/neutral territory; there's no way any self respecting Spitfire pilot wandered around over enemy ground at 170mph - combat cruise should be faster still than even these airspeeds I have given, and at higher revs and boost. I'll see what figures I can find for that, if any.

My point still stands which is - as you can see - even at these higher speeds we're not at full rpm so torque effects are reduced again, therefore, less need to correct for it in the airframe.

I suspect that TD have gotten their spits set up incorrectly on 2 counts:

1) a/c inherently trimmed at far too low a cruise speed

2) compounded by these being attained at much lower rpm in reality - therefore TD are correcting for too much torque at that airspeed.

Cheers.