Location: On the Rump of Pendle Hill Lancashire UK
I will ask her if she wants to appear, she was a very good secretary and became a fountain of knowledge on the Merlin and the Griffon 58 (incl Props) that adorned my reception office, so much that I only needed to speak to vistors if they wanted to see me,..
She was a well bult Lancashire Lass with a very good sense of humour!!
I know that MAP is measured on an aneroid device - in other words a barometer that measures absolute pressure. It happens to be calibrated just like a US barometer, in inches of mercury. Which means it will (when shut down) vary from day to day, just like a mercury barometer as weather patterns change.
It will similarly read low at a high-altitude airfield, e.g about 24" at 5000', vs nominally 29.92" at sea level.
But the BOOST gage - am I correct in perceiving it as a "PSIG" instrument, as opposed to an absolute pressure device? If so, with the engine shut down, it will read zero boost REGARDLESS OF ALTITUDE or weather patterns.
Am I correct here, or is the boost gauge also an aneroid device? As I re-read Lightning Mate, I believe he and I agree that boost is a "PSIG" parameter.
A boost pressure gauge, in spite of its name, differs fundamentally from the fuel pressure gauges, oil pressure gauges, and other pressure-measuring instruments used in an aircraft. Like an altimeter, it records absolute pressure, and not merely pressure above atmospheric. In fact, the American name of "manifold pressure gauge " is more correct, the gauge giving no indication of the extent to which the supercharger has boosted atmospheric pressure. The orthodox British system of boost gauge calibration ignores this fundamental fact, pressures being given in pounds per square inch above or below an absolute pressure of 14.7 pounds per square inch. The latter pressure, which is the mean sea level atmospheric pressure, is arbitrarily known as "zero boost."
The fact that the boost/MP gauge reads ambient pressure prior to engine start is used by some manufactures as a power check on the engine run up before take off. An example from one particular aircraft.
Note MP prior to engine start. Run Up - Adjust throttle to field barometric pressure as read on MP gauge before starting engine For propeller type XXX check for 2150 +-50 RPM For propeller type YYY check for 2250 +-50 RPM
Note If RPM is too low for given MP, engine is not developing sufficient power and should be checked before flight.
Maybe this will help Ghengis. I had lots of hours as flying instructor on Lincolns. For take off we would open the power on all engines to zero boost (30 inches MP approx) on the brakes. If every thing looked OK on engine instruments we would release the brakes and open the throttles leading with the port outer a bit more than the others to help directional control until the rudders became effective. Then we selected plus 12 boost and check the RPM was 3000. +12 boost was about 54" MP.
If the runway was short (in those days we did not have take off charts and we hoped we would reach lift off speed by the end of the runway), we would go through a "gate" on the throttle quadrant to obtain maximum power which in the RR Merlin 102 was +18 boost or 66"MP. . At that power setting the noise of the engines in the cockpit was ear shattering and painful. It was just as bad in the Mustang which I flew.
We rarely used +18 boost as it greatly reduced the life of the engine. We wore cloth flying helmets which gave no noise protection and to minimize the possibility of damage to the hearing the technique used by some copilots was to hold one hand pressed hard over the ear nearest to the nearest engine (for the copilot that was the starboard inner known as No 3).
The captain had no choice since his left hand was on the control wheel and his right hand had all four throttle levers. As soon as possible after lift off we would pull back the throttles to +12 then at 500 feet during flap retraction we would reduce further to +9 boost and 2650 RPM. At +18 boost the glycol coolant temperatures would heat up very quickly which was another reason to reduce power from +18 as soon as possible after lift off. Normal cruise in the Lincolns we flew (Mk 31 or Long Nose version) was +6 boost and 2400rpm.
In the circuit we used +4 boost and 2400 RPM then increase to 2650 RPM late downwind. Turning base we set zero boost - which was descent power - and all being well maintained zero boost until late final when, depending on speed, we would gradually reduce power towards minus 4 boost ensuring all throttles were fully closed during the round out. It was then you could hear the characteristic "popping and crackling" noise of the Merlin at idle power. If on three engines downwind with one prop feathered (happened a lot in the tropics) then power was set at +7 boost and 2650 RPM which held the speed nicely.
PS. The noise of the four Merlins in the Lincoln caused a problem one night at Townsville. There was a zoo close to base leg for runway 02 and the zoo had a tall monkey cage. It was the bloody cage that was tall not the monkeys We were doing circuits with new pilots on a still dark night and I understand the monkeys had gone to sleep high on the perches of their cage. The copilot increased the RPM just prior to turning base and the harmonics caused by the increase in RPM and atmospheric conditions must have caused unusual sound waves to affect several monkeys who apparently involuntarily released their hand grips on the perch and fell to their deaths on the concrete floor.
The zoo owner protested to our CO who had little sympathy although he later suggested the zoo owner install a flashing strobe on the top of the cage. That was done eventually and we were instructed to make night circuits wider in order to avoid flying directly over the zoo monkey cage.
I suppose that Lightning Mate's explanation of the boost as a relative pressure gauge showing plus or minus relative to current ambient pressure might useful and might be the way it functions in some applications. I have flown several engines with boost pressure gauges (including a Centaurus) and I have never seen one which worked this way. I have only ever seen boost gauges which indicate the pressure in the inlet manifold relative to standard pressure. The boost with the engine stopped will vary according to the atmospheric pressure of the day and one sets static boost (ie the same as the reading with the engine stopped) during the run up to check the reference RMP as described by Brian Abraham.
Don't think the Lightning had boost gauges and I wonder who sets the questions for LM's professional exams! Perhaps he could be specific, which aircraft has a boost gauge which shows zero regardless of the ambient conditions?
No need for embarrassment Barit. It is a commentary though, that in this age of the jet we have forgotten the basics of what the pilot of old knew ie care, feeding and workings of the recip. From my ground school commercial pilot notes (when the big recip was on it's last legs airline wise, but still plentiful in the military - C121, C97, S2, A26, A1, T28, and more.
Perhaps I should add that the above course notes were written by C. S. Hames, who joined the RAF as an engine apprentice in 1938, on completion of that course remustered as aircrew, trained in the USA, flew operations over Europe during WWII, was part of the Japan occupation force and subsequently graduated from CFS as an instructor.
Last edited by Brian Abraham; 21st Feb 2012 at 03:36.
I can see in your example above where the confusion as to a boost gauge being an absolute pressure device or a pressure differential indicator comes from.
The last description of the diaphram boost guage makes no mention of the evacuated part of the assembly. Therefore by definition if one side of the diaphram is connected to the inlet manifold and the other side to an open case then when the engine is at rest the gauge will read Zero; at any altitude.