Here's a question - about the Spitfire boost cut out

[92 Sqn. Philstyle (QJ-P)]

Hi chaps,

As most red pilots know, the boost cut-out on the Spitfires (and hurris) can only be used sparingly at high engine RPM. At full-fine pitch , you get somewhere between a minute and three minute's use before she blows. Now, I understand this limitation to be historical, no problem there.

However, I note that when the engine RPM is quite low (say 2400 to 2600) the cut out can be enabled for some lengthy periods of time. I've never read any documentation indicating that this was possible. From the various pilots reports and books I've come across, it always seemed as though the cut-out had limited use, irrespective of rpm.
What say you people? Does anybody here know more about this?

Thanks

[92 Sqn. Folmar (QJ-F)]

According to the Spit IIa pilots operating hand book it is suggested that it only be used for no more than 5 mins at 3000 rpm.

[92 Sqn. Philstyle (QJ-P)]

According to the Spit IIa pilots operating hand book it is suggested that it only be used for no more than two mins at any rpm.

Yeah, that's what I suspected.
Does it explicitly mention rpm in the boost cut-out section of the 2a Handbook?

[92 Sqn. Folmar (QJ-F)]

Yeah, that's what I suspected.
Does it explicitly mention rpm in the boost cut-out section of the 2a Handbook?

Not that I can find...

***Correction*** On page 16 of the IIa P.O.H, it states that the max Combat limit is 5 mins a 3000 RPM with a boost of +12, +7 in the 87 octane versions.
All information is from the Spit IIa P.O.H. from Zenos warbird videos.com
RPM Boost LB/Sq. In Temp. (Clnt.) Degrees C (Oil)
Max. Take-off to 1000 ft. 3000 +12.5 - -
Max. Climbing 1 HR limit. 2850 +9 125 90
Max. Rich Cont. 2650 +7 105 90
Max. Weak Cont. 2650 +4 105 90
Combat 5 min limit. 3000 +12 135 105

Footnote: Please be aware that all settings for radiator except for Climb are at minimum drag settings.

[92 Sqn. Philstyle (QJ-P)]

Not that I can find...

Hmm.. so we can't be sure that's it's not currently working as it should.
Let's keep hunting...

[92 Sqn. Folmar (QJ-F)]

P i fixed some stuff from my previous post see above.

[92 Sqn. Philstyle (QJ-P)]

P i fixed some stuff from my previous post see above.

Brilliant. Looks like it is modelled fairly well already then.
Cheers.

[92 Sqn. Folmar (QJ-F)]

Brilliant. Looks like it is modelled fairly well already then.
Cheers.

Yeah it's not to far off from what I remember. I'll be doing some testing with it when I'm back up and running.

[Roblex]

Funny you should post this as on Wednesday evening I engaged boost cut out and reduced the throttle then put it back to max because I was trying to catch someone. I did not reduce it again for at least 30 minutes and the engine and oil temps both stayed at optimum the whole time. That was a 100octane Spit. Odd really when you consider the number of times I have cooked the engine without using boost cut-out.
The fact I was flying at max speed with revs set to 2500 will have helped but not to that degree.

[92 Sqn. Philstyle (QJ-P)]

The fact I was flying at max speed with revs set to 2500 will have helped but not to that degree.

I've found that once under 2700rpm, the boost cut out seems to be operable almost indefinitely, provided you don;t get low on speed, adn teh radiator stays open.