Bug 174 on 12lbs boost. Review please.

[JtD]

Originally Posted by 41Sqn_Stormcrow

12 lbs boost + 2270 rpm

2270rpm is the minimum to prevent engine damage, 3000 was the proper setting.

Ah thanks. I did not read minimum:

so

for take-off they say:

rpm in between 2270 and 3000 with max boost 12 boost either up to 1000 ft for extended time or up to 3 min.

[Kurfürst]

Originally Posted by 41Sqn_Banks

AP 1565B, Vol I, Section 8 was issued in August 1940 and mentions the "two different" systems. "6. The quadrant plate has a ... gate for the throttle lever in the take-off position". And in paragraph 7 mentions that the "red-painted thumb lever can be pushed forward in emergency".

They are the same. The throttle had a section for takeoff gate postion, and the "Red painted thumb lever" covered that position as safety switch, physically preventing the throttle to be pushed into the take off position until it was pushed forward.

Hence the confusion. The manual itself does not describe any two different systems.

[Kurfürst]

Originally Posted by JtD

Now how would they technically do that, make the boost of 12" drop off quickly instead of having it follow the outside pressure? What did the automatic boost control cut out do on the Spitfire II if not disable automatic boost control?

As I understand, the cut out as its name suggest disabled to automatic boost control and gave direct control to the pilot over the boost. Maximum forward position on the throttle was set to open the cross section for +12 lbs at SL, and naturally this fell off with altitude; the pilot would have to open even further to compensate, which was however physically impossible - the throttle could not be pushed even further, it was already in the end position.

This graph suggest (see boost falling) that +12 fell back to +9 within about 2500 feet. This would also suggest that using the boost cutout was quite useless above that altitude, since it gave you the same performance as not using it at all.

http://www.spitfireperformance.com/p7280-speed.jpg

Come to think of it, it seems to work exactly the same as the 109E's 1-minute rating, altough that latter was automated.

[41Sqn_Banks]

Originally Posted by Kurfürst

They are the same. The throttle had a section for takeoff gate postion, and the "Red painted thumb lever" covered that position as safety switch, physically preventing the throttle to be pushed into the take off position until it was pushed forward.

Hence the confusion. The manual itself does not describe any two different systems.

Please explain how the red painted thumb lever was able to physically prevent the throttle to advance into the gate position.

[Kwiatek]

Originally Posted by 41Sqn_Stormcrow

Ah thanks. I did not read minimum:

so

for take-off they say:

rpm in between 2270 and 3000 with max boost 12 boost either up to 1000 ft for extended time or up to 3 min.

Exacly +12 lbs up to 1000ft or up to 3 minutes. Merlin XII was initialy adopted for 100 Octan fuel not like Merlin III which required modification. Im sure that in SPitfire MK II was allowed for +12 lbs for emergency for short time from the begning. ( initialy up to 3 minutes later up to 5 minutes).

[Kurfürst]

Originally Posted by 41Sqn_Banks

Please explain how the red painted thumb lever was able to physically prevent the throttle to advance into the gate position.

In the override position...?

Use your imagination a bit where that little thing just above the "Push" sign goes when the red painted thumb lever is rotated back. Yup, it prevents the throttle from being pushed into the takeoff gate.

[41Sqn_Banks]

Originally Posted by Kurfürst

As I understand, the cut out as its name suggest disabled to automatic boost control and gave direct control to the pilot over the boost. Maximum forward position on the throttle was set to open the cross section for +12 lbs at SL, and naturally this fell off with altitude; the pilot would have to open even further to compensate, which was however physically impossible - the throttle could not be pushed even further, it was already in the end position.

This graph suggest (see boost falling) that +12 fell back to +9 within about 2500 feet. This would also suggest that using the boost cutout was quite useless above that altitude, since it gave you the same performance as not using it at all.

http://www.spitfireperformance.com/p7280-speed.jpg

Come to think of it, it seems to work exactly the same as the 109E's 1-minute rating, altough that latter was automated.

The "gate control" set a fixed throttle valve position. Indeed this was not adjusted by decreasing atmospheric pressure and thus would fall off with altitude as you describe.

The "boost control cut-out" did not set a fixed throttle valve position. The position was directly controlled by the pilot but limited the opening of the valve that +12 boost was not exceeded. Thus when the throttle was fully forward (not in the gate position) the opening would increase with decreasing atmospheric pressure until it is fully open (at FTH).

This is explained here: http://www.enginehistory.org/Piston/...erlinABC.shtml

[41Sqn_Banks]

Originally Posted by Kurfürst

In the override position...?

Use your imagination a bit where that little thing just above the "Push" sign goes when the red painted thumb lever is rotated back. Yup, it prevents the throttle from being pushed into the takeoff gate.

Both positions of the red painted lever are shown in the drawing. I painted red lines into the drawing to show that the lever doesn't obstacle the gate position in any of the two positions.

[JtD]

Originally Posted by Kurfürst

This graph suggest (see boost falling) that +12 fell back to +9 within about 2500 feet. This would also suggest that using the boost cutout was quite useless above that altitude, since it gave you the same performance as not using it at all.

This graph shows that 12lbs boost would be available below about 15k feet, 2.5k feet below the full throttle altitude of 9lbs boost. That's how it works with outside pressure. And that's what being made possible with the abc cut out, see 41Sqn_Banks link.