Black 6 Thu 19th Apr update re Spit II

[JG5_emil]

Ivan/Banks
"up to the rated altitude this will increase boost (about 12lb./sq.in. at sea level)."

It looks to me that 12 Lb boost is only possible at or close to sea level and that the amount of boost decreases with altitude, which makes sense to me. Is this correct or wrong?

[fruitbat]

Originally Posted by JG5_emil

Ivan/Banks
"up to the rated altitude this will increase boost (about 12lb./sq.in. at sea level)."

It looks to me that 12 Lb boost is only possible at or close to sea level and that the amount of boost decreases with altitude, which makes sense to me. Is this correct or wrong?

To quote 'The Spitfire Story' by Alfred Price,

Re 100 octane fuel and boost,

Although this gave no improvement at or above the 16,500 foot full throttle height of the merlin II and III engines fitted to spitfires, below that altitude the maximum boost pressure could be raised from 6 1/4 pounds to 12 pounds without causing detonation in the cylinders; the resultant extra power increased the maximum speed by a useful 25 mph at sea level and 34 mph at 10,000 feet.

[41Sqn_Banks]

[ATAG_Bliss]

It'll be interesting to see how the new FMs fly. They can show us all the graphs they want, but I still think it's a little premature to argue over the picture until we've experienced new FMs 1st hand.

Good bit of info though.

[fruitbat]

Originally Posted by ATAG_Bliss

It'll be interesting to see how the new FMs fly. They can show us all the graphs they want, but I still think it's a little premature to argue over the picture until we've experienced new FMs 1st hand.

Good bit of info though.

Agreed, but i'm already resigned to seeing our Mk I spits on 6 1/4 boost and 87 octane fuel performance unfortunately.

[camber]

Originally Posted by JG5_emil

Ivan/Banks
"up to the rated altitude this will increase boost (about 12lb./sq.in. at sea level)."

It looks to me that 12 Lb boost is only possible at or close to sea level and that the amount of boost decreases with altitude, which makes sense to me. Is this correct or wrong?

Hi Emil, it is Fruitbat said, a bit of extra info:

The supercharger can deliver about +20psi at sea level with throttle valve fully open, so the boost controller quietly (without moving the pilot handle!) moves the throttle valve to give +12psi if the red cutout is on. When altitude has increased to where the supercharger maximum output is +12 psi, the boost controller cannot control any more (both the pilot's throttle handle and actual throttle valve are at 100%), and the boost will drop from then on.

This will happen a bit earlier than if the boost controller was on +6 1/4 boost, so the full throttle height is lower for +12psi than +6 1/4 psi. It is still >15000 feet though.

The gate system is different though because it gives a set throttle valve position...it will give the higher boost at takeoff, then decay quite rapidly to the lower controlled value (and stay there) as you climb. I think the system should act exactly the same if you are through the gate or not, above a few 1000 feet.

camber