Boost (Hurricane 1 and Merlin II/III)
 

I've been wading through some old manuals as part of a private project looking at the Hurricane 1.

I understand MAP and CS props well enough on modern aeroplanes, but I'm struggling with how this maps to "Boost" as used on the Merlin engine. Can anybody explain what Boost actually is and is controlled - is it just an alternative term for MAP, or as I suspect something defined a bit differently?

At the same time, references to the early CS props on the Hurricane 1 / Merlin II combination refer to a "2 speed" prop, which suggests a CS prop with only two RPM settings - as opposed to the infinitely variable lever I'd see on a modern aeroplane. Is that correct, or is it more modern than I think, or even is it a 2-pitch (fine and coarse) prop such as is fitted on one or two modern motorgliders?

And if anybody knows the answers to those, I'm guessing that they might know the answer to my third question. At what point was the 2 blade fixed pitch prop finally withdrawn from the Hurricane fleet?

Cheers chaps.

G

Rubbish removed

Sorry, but boost and MAP are not the same.

MAP is the absolute pressure in the manifold, measured from zero datum ie a vacuum.

Boost is the pressure in the manifold above atmospheric.

The early systems on the Hurricane and Spifire had two blade angle settings only. The rpm in either was controlled by the throttle, unlike modern constant speed propellers.

From someone's book - cannot remember which - early prop was 2-speed - coarse and fine - with a lever in the cockpit - I have a feeling it was DRSB who crashed an aircraft on take-off - only to have lever point at him like an accusing finger as it was still in "coarse" whereas it should have been in "fine" for take-off

You are correct. He put it through a fence at the end of the airfield if I remember.

Those days have gone.

No they have not.

Try flying a WW2 warbird instead of a helicopter...........

"take zero boost as 14 ins of MAP. "

Wrong.

Zero boost is approx 30 inches of mercury. + 8 boost is therefore 8 X 2 plus 30, i.e. 48 in MAP

But basically zero boost is effectively idle power, and above that is linear-ish with increasing power presumably?

Does "boost" actually serve any real purpose then, since we seem to be talking about fixed pitch propellers - even if you have a choice of three fixed pitches?

G

Nope, zero boost isn't idle power, far from it. Zero boost is almost enough to tip a Spitfire or similar on it's nose while doing the run up to check mags. Zero boost would be a typical cruise power setting. Idle power would be off the scale, somewhere below -8 boost.

Zero boost is static atmospheric pressure; often referred to as "static boost" ie 29-30" MAP. They are both indications of the pressure in the inlet manifold and therefore are "the same" in a sense however, MAP uses absolute zero pressure as its datum and (usually) In Hg as the units, "boost" uses the prevailing atmospheric pressure as its datum and (usually) psi as its units.

Positive boost pressures are seen on a forced induction engine (Supercharged or turbocharged) and equate to MAP figures above 29"Hg.

Boost certainly serves a purpose as it allows you to set desired engine power at various RPMs (or with two speed VP (ie not CS) props). I have only known one aeroplane with a fixed pitch prop and a boost gauge. This was an engine which was supercharged to allow it to maintain sea level power output as the altitude increased; when you got high enough such that full throttle gave you zero boost that was effectively full throttle height; any further climb and you would start to lose power.

I am getting old. I mixed up lbs/inch2 with inches of mercury.
LM I did train on a supercharged radial engined aircraft and also a supercharged radial engined helicopter. Same engine, different gauges.

So in a nutshell - Boost, normally expressed in in.Hg, is MAP minus (MAP at a nominal cruise power setting). So idle is something like -8 Boost, and combat power was around +10 boost, expressed in in.Hg.

It was used, as MAP is, for setting power in general preference to using RPM. The Hurricane 1 did not have a constant speed prop - the earliest models had a fixed pitch prop, the later models had a variable-fixed pitch prop with a choice of two or three pitch settings. So a pitch setting was made, then the powerplant was controlled on the throttle, primarily by reference to boost pressure, rather than RPM.

Have I got that right?

G

Just about.

Wossa guage please?

It's what the engine innards start to do if you don't keep the needles in the green.

G

wossa a gauge please?

bit like a plum

gs

Lightening Mait got a dishunary for his birfday

i'm like honured innit i meen like fings ere ar like good innit

fred is like well drifting like innit

Boost pressure and manifold air pressure are two different names for exactly the same thing. American engine manufacturers favored MAP and the British favoured boost. Only the scaling on the gauges/gages differed. If you put a boost gauge and a MAP gage next to each other at sea level on an ISA standard day the boost gauge needle would point at 0 boost on the scale and the MAP needle to a shade below 30"Hg.

The early Merlins weren't fitted with constant speed propellors, they were fixed pitch. Later propellors could be selected to either of two fixed pitches any time that the engine was running. RPM varied as a function of engine power and TAS. If a take-off was attempted in coarse pitch the RPM, and thus engine power, would initially be kept low by the very high torque load on the engine but they would increase as the aircraft accelerated. If that Hurricane had had a much longer runway it would have got airborne eventually just as the Schneider Trophy seaplanes did. Incidentally, the Schneider Trophy racers, with their very coarse fixed pitch props, used to start their take-off runs at 90 degrees to their intended take-off direction just to allow for that torque.

.................:ugh:

Hi Genghis,

we are getting close but not quite there yet, re-read my second paragraph.

"Zero boost is static atmospheric pressure; often referred to as "static boost" ie 29-30" MAP. They are both indications of the pressure in the inlet manifold and therefore are "the same" in a sense however, MAP uses absolute zero pressure as its datum and (usually) In Hg as the units, "boost" uses the prevailing atmospheric pressure as its datum and (usually) psi as its units."

The usual units for boost is psi and for MAP is In Hg. Think Fahrenheit and Celsius; two systems for measuring temperature but each with a different datum point and measured in units of different increments. Two different systems of measuring the same thing.

Happy landings

3 Point

Could somebody explain what (a) boost cutout is and when it might be used. Thanks.

You can call a boost gauge a manifold pressure gauge/indicator but you can't always call a manifold pressure gauge a boost gauge. For that, the engine needs to be super/turbocharged.
Many normally-aspirated light aircraft have manifold pressure gauges.

I will try again. The gauges do the same thing. They measure the pressure in the inlet manifold and convey the result to the cockpit in either lbs per square inch or inches of mercury. With everything stationary both instruments will read the ambient pressure.
The checks for both are the same. You wind your altimeter to Zero and if the subscale is lower than standard 1013.2 then the boost or MAP gauge will read either either below Zero or below 29.92 ins. With an inches subscale they will read the same.
The ratio of boost to MAP is 2.036, or 2 for short; so for every pound of boost pushed into the engine a MAP gauge would increase by 2 ins and when the engine goes into suction mode, vice versa.
An old check on the Hercules used to be a climb at Zero boost and see what height the supercharger coudn't maintain boost any more.

A "boost cutout" was more accurately a boost regulator cutout. The boost regulator was part of the linkage between the throttle lever and the actual throttle valve, and limited the throttle valve opening in order to avoid over-boosting the engine in normal operations. In an emergency the boost cutout lever could be pulled and an extra couple of pounds of boost gained.

It also slowly opened the throttle, without moving the throttle lever, as the aircraft climbed in order maintain the selected manifold air pressure until you reached full throttle height.

Of course the term may have found other meanings since then.

I've slowly come round to the conclusion that the Americans were right; manifold air pressure is a much more useful term than boost pressure.

Not so my friend.

The MAP gauge will, but the boost gauge will read zero - hardly the same thing.

I use this illustration, drawn by myself, for instructional purposes. It shows what each gauge will read with the engine shut down at sea level:

http://i636.photobucket.com/albums/u...9/boostMAP.jpg

I agree, my friend. At 1013.2 at sea level. OR 1013.2 ambient pressure at ANY height.
I can still remember my checks from 1960 and checking that the boost gauge agreed with the QFE was one of them.

Anyway they don't agree, if the boost gauge is at Zero then the MAP should be at 29.9 ins Hg.

Increasing altitude will cause both instruments to reduce. That is the whole reason for a supercharger; to maintain inlet manifold pressure at that altitude.

Not so.

A supercharger will hold a set boost up to full throttle altitude, after which it will start to reduce.

It is possible to hold MAP in a climb, but impractible. With a constant speed propeller without a turbocharger it can be done by using reduced power (rpm & MAP) in the initial climb. As altitude increases the throttle may be progressively opened, which will hold the MAP until the throttle is fully open.

Perhaps this discussion would be better by PM. :)

Genghis,

I used to have a full rebuilt Merlin in my reception office when I owned a factory with assorted offices, the Merlin I had was a two stage two speed Supercharger type, just below the main air tube going into the intake manifold (looking from the rear) was a quadrant of levers that would have been connected by rods and bell cranks back to the cockpit controls, but it seemed that the second stage S/Charger would be activated by the same pushrod that activated the first stage (or speed), the same quadrant of levers were all connected as well to two cranked arms to either side of the mopping bucket sized Carburettor. So it always seemed to me that just one lever would operate the functions of stepping up and giving extra motion lotion, and more HP.
All these levers and bell cranks also were connected to a water injection system as well as the a small fist sized fitting at the front of the engine that I imagine was also connected with the propeller speed.

Who ever perfected that system possibly went mad.

I had many people come to see the Merlin, or possibly my well endowed Secretary but no one ever did satisfy my question of how it was all controlled, most were old Flyers who came to touch and remember things in past years.with a few philistines who wanted to know if it would fit into a car!!.

My regards

Peter R-B
Lancashire

They don't build them like that anymore.

Wish I had one in my office. :sad:

They're both reading ambient pressure L M, it's only the numbers on the dial that are different.

My boost gauge would show zero at any ambient pressure because it's a relative or differential pressure gauge.

OK gents, I'll give up here and continue to get my students through professional examinations. :)

Lancman - Thanks for the Boost Cutout explanation.

lightning mate, your pic is correct
29 92 and 0 are the same, depending on which side of the pond your from, after 0 the manifolds more pressurized than ambient, but only if the props in inc rpm.

gs


ps this one will go for ages, someone will allways argue.

I have been pondering the implications of both instruments.

Climbing to critical altitude, whilst maintaining constant MAP (i.e. absolute pressure), the back pressure on the exhaust system is decreasing, as is crankcase pressure and thus the force on the bottom of the pistons.

At the same time, ambient temperature is dropping, for greater air density at the same MAP.

I read these two factors as a gradual increase in power during the climb.

On the other hand, a Boost "gage" (Yankee spelling) would tend to even out the first of these factors. I've never flown a Boost engine - maybe someone can expand on this. :E

From a RAAF (1942) ground school training publication,

Of course, we colonials may not understand the complexities of machinery, but to me that says a boost gauge is exactly the same as a MP gauge. The boost gauge will indicate ambient pressure with the engine shut down.

Re boost cut out. The Lancastrian (transport Lancaster) engine lists a maximum of +14 boost, but with the boost cut out +18 was available. +18 was only to be used on take off if necessary, and if so used, boost was to be reduced to +14 as soon as the +14 take off safety speed (140 knots) was reached.

LM
"Wish I had one in my office."

Which one? The rebuilt Merlin or the well endowed secretary!

In a previous job I had 18 Merlins (a mix of 35s and 500s) on my inventory. Starting up 4 of those was really quite satisfying.

http://www.sae.org/aeromag/techupdat...images/12b.jpg

Something wonderfully appropriate about using 4 Merlins to test Typhoon...

G

Office Decoration circa 1994
 

Peter R-B

I don't know if this will answer your questions but have a look at

R-R Merlin Automatic Boost Control

for a discussion of how Merlin Boost Control works.

There is a good discussion of how and why this manifold pressure business is so confusing in the piece by Jerry Wells in the Torque Meter magazine / fall 05

Quote:

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Office Decoration circa 1994

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Wheressa secretary!!!!!!!

(carefully not responding to ill-informed garbage)