Actual operation of the Merlin boost regulator

[Sutts]

Originally Posted by 41Sqn_Banks

The Merlin XII used in the Spitfire II has higher engine limits because of certain changes (e.g. it uses a different coolant, 70/30% water/glycol instead of 100% glycol). For example climb boost is +9 instead of +6 1/4.

Thanks 41Sqn_Banks, my understanding is that is only with 100 octane though - correct?

[Sutts]

Originally Posted by camber

Lane, that is a nice diagram showing the cable connection from red tab to the boost cutout override (no wonder it is invisible in flight sim cockpits considering where it sits!) The mechanical connections go to the control bar on the other side of the firewall that connects to the differential gear (and then on to the real carby throttle) but this is not shown.

Sutts, I would think that on the Merlin XII in Spit II rated for 100 octane, the highest boost allowed by the boost regulator setup (boost cut off red tab = OFF) would be +9psi. But with the variable aneroid in the boost controller (an improvement available by the Merlin III), the pilot can advance throttle to any boost under +9psi and the system will maintain it there as you climb. So if you have filled up with 87 fuel after landing at a station without 100 octane, you must be careful only to pull 7psi on the hand throttle. Otherwise the engine will probably let you know!

If you push the red tab (boost cut off = ON) you have access to +12psi (provided altitude is low enough that the supercharger can actually achieve it). With 87 fuel this would also be a very bad idea but nothing would stop you from doing it (except a desire to keep being a fighter pilot).

The thing that still confuses me is that the Spit II also has a throttle gate that allows "take off boost" of +12.5psi that is only supposed to be for taking off from small fields or overloaded. I'm not sure why both systems (boost cut-off red tab, throttle gate) need to be there at the same time.

Thanks for this camber. I was assuming the manual was talking about 2 versions of the Spit II - 87 and 100 octane capable but of course you are correct....the +7 boost is simply a limit that the pilot must be aware of when filling up with 87 octane.

In terms of maintaining boost in a climb automatically, my understanding is that would be the case if the pilot had pushed the throttle to its max position before the climb. In this case the boost limiter would kick in and the excess boost would gradually be applied as altitude increased until a point where excess boost was no longer available (10000ft?), at which point boost would start tailing off.

If, however, the pilot only advanced the throttle as far as required to select the climbing boost value and no further...then the boost limiter would not be in operation and boost would NOT be maintained as he climbed to higher altitudes. Do you agree?

Can I also seek confirmation regarding the boost cutout switch please? From what I've read it seems that the switch does not physically allow the throttle to move any further forward - like in the game. It simply disables the boost control, immediately removing the boost limit for the currently selected throttle position. So, if the throttle is full forward at the time the switch is flicked, the boost will surge immediately to +12 (depending on altitude of course) without any need to touch the throttle lever. Is that correct please?

Cheers

[41Sqn_Banks]

Originally Posted by Sutts

In terms of maintaining boost in a climb automatically, my understanding is that would be the case if the pilot had pushed the throttle to its max position before the climb. In this case the boost limiter would kick in and the excess boost would gradually be applied as altitude increased until a point where excess boost was no longer available (10000ft?), at which point boost would start tailing off.

If, however, the pilot only advanced the throttle as far as required to select the climbing boost value and no further...then the boost limiter would not be in operation and boost would NOT be maintained as he climbed to higher altitudes. Do you agree?

In case of a (early) fixed datum type boost control you are correct. However the variable datum type boost control does maintain the selected boost until full throttle height even if the throttle is not at the gate.

[lane]

Originally Posted by camber

Lane, that is a nice diagram showing the cable connection from red tab to the boost cutout override (no wonder it is invisible in flight sim cockpits considering where it sits!) The mechanical connections go to the control bar on the other side of the firewall that connects to the differential gear (and then on to the real carby throttle) but this is not shown.

Acknowledged camber. It should be noted that the control arrangement for the boost cut-out is a bit different in the Hurricane I:

[41Sqn_Banks]

Originally Posted by camber

The thing that still confuses me is that the Spit II also has a throttle gate that allows "take off boost" of +12.5psi that is only supposed to be for taking off from small fields or overloaded. I'm not sure why both systems (boost cut-off red tab, throttle gate) need to be there at the same time.

AP 2095 Pilot's Notes General (1st Edition, 1941) describes this. See attachment.

[Sutts]

Originally Posted by 41Sqn_Banks

AP 2095 Pilot's Notes General (1st Edition, 1941) describes this. See attachment.

Thanks for the extra info 41Sqn_Banks, great stuff. I feel enlightened.

[wedgehead383]

so according to the boost altitude diagramm the overboost function is only possible till 12250 feet are reached- above that altitude the pressure drops constantly and can not be raised e.g. theres no difference in engine power / thermic load between overboost mode and regular full throttle ???

[41Sqn_Banks]

Originally Posted by wedgehead383

so according to the boost altitude diagramm the overboost function is only possible till 12250 feet are reached- above that altitude the pressure drops constantly and can not be raised e.g. theres no difference in engine power / thermic load between overboost mode and regular full throttle ???

Correct. With boost control cut-out the throttle would behave different because the variable datum boost control is bypassed but otherwhise not difference.

[Versuch]

I know its an old thread but here is the boost control unit.