Manifold pressure limits on fixed pitch Merlins

[Lumps]

Does anyone know how the pilots of early Hurricanes and Spitfires fitted with fixed pitch propellers avoided 'over boost' (detonation from too higher MP at too low RPM) at for instance, take-off? They seem to have quite coarse pitch, and I assume one could not just firewall the throttle as the RPM would be so low. Or could you? Seeing as the supercharger is RPM dependant.

[Onceapilot]

Lumps, you are correct that the very coarse fixed pitch prop would hold back the RPM at low speed, and so reduce the supercharger output, but the Merlin was fitted with a boost controller which throttled the supercharger to prevent excessive manifold pressure. This could be overridden in emergency, with risk of engine damage/failure. Unfortunately, I do not know if the override completely removed control of boost or, if it allowed a higher controlled level of boost. The latter would seem more likely as the amount of extra boost available at low level was considerably more than rated boost.
Cheers

[Lumps]

Thanks OAP, makes sense.

[aerobat77]

thats an interesting question !

are you sure that there were merlins with a fixed pitch prop ? this would give the disadvantage of not reaching full rpm,s at lets say take off and with a mechanical supercharger also not full boost / power.

i believed that at least a mechanical pitch setting was available first and later replaced by a constant speed prop.

if indeed there were fixed pitch props on the first series i quess there was like written above some self regulation due to lower supercharger speed at lower rpms and so naturally lower boost.

i also quess the very first versions were not that very high boosted variants.

what i know is the fact UK was supplied quickly in the war ( i believe 1940) with high octane fuel from USA which should improve the situation.

hopefully somebody has more detailed informations !

cheers

[Dick Whittingham]

The early Hurricanes had the Merlin C with a two blade wooden fixed pitch prop. Later aircraft with the Merlin II or III upgraded to a metal 3 blade two position variable pitch prop and then to a constant speed prop

If you can find it see the film of an S6 with a RR "R" type and a fixed pitch wooden prop set for very high speed skipping sideways as it struggles to get airborne

[Lumps]

I'd like to go a bit further - as it was actually a picture of the S.6 that sparked the question - these fixed pitch props were good for over 400mph so must have been turning very slowly at take-off speeds, could one have even opened the throttle to atmospheric pressure (0 lbs boost, 30" whichever you like) without bad things happening in the combustion chamber / drivetrain etc?

[Onceapilot]

I might be able to find my S6 info later but, the reality was about 1000bhp at take-off for the 1929 S6. Revs were low due to coarse prop and I believe they probably used up to full throttle to get some boost at this low rpm.
As an aside, Rolls Royce quickly realised the efficiency of high boost with low revs and second World War Merlin handling for long range cruise was throttle fully open but revs held down to 1700 with coarse prop via the speed governor.
Cheers
PS, For some great video of S6 etc, with sound! Search Youtube "1929 schneider trophy".
PPS, Just checked the data for the 1929 Rolls-Royce "R" engine, they were getting about 1100bhp at 2200rpm for take-off using a coarse race prop. I am sure that was with full throttle, boost not recorded but, I guess around zero-boost.

[Lumps]

Cheers Ill have a look

[Brian Abraham]

The two blade mahogony wooden props on the Spitfire had a diameter of 10' 8" and a pitch of 24' 6". Jeffrey Quill stated the prop was designed to produce 3,000RPM at full throttle (6.25lb boost) at 18,000 feet. Static RPM at full throttle was about 1,900.

[Lumps]

Interesting. Why then did it take until the end of the war for RR engines (Griffon) to reliably produce what the R type was putting out a decade before?

[Onceapilot]

Well Lumps, The R engines were short life and effectively needed overhaul after their 1 hour at full throttle race use. The fuel was not available for general use either.
The technical development that gave the ability to produce similar power in a service engine involved huge advances in materials, mechanical engineering, supercharging, sparkplug technology and fuel technology.
Good reading to be had in the RR Heritage books.

[Brian Abraham]

A good summation here Rolls-Royce R - Wikipedia, the free encyclopedia

As Onceapilot mentions, special fuels were concocted to control detonation. The original engine was rated at 1900 horsepower (at the then unheard of 55 inches Hg boost and 2900 rpm) running on 78% benzole, 22% Romanian petrol and 2 cubic cm of tetraethyl lead per gallon. A Rolls Royce 1927 patent for a ram induction intake to the supercharher also contributed 10% in power.

For the 1931 race they were looking for an output of 2300 horsepower. The failure of crankshafts, con rods, exhaust valves and high oil consumption had to be overcome. Oil was being consumed at a rate of 112 gallons per hour which was solved by improved piston ring design to control blow by. Exhaust valve problems were solved with the introduction of sodium filled valves. For this race the fuel was 70% benzole, 20% Californian petrol, 10% methanol and tetraethyl lead.

After this race an attempt was made on the world speed record with the engine boosted to 70 inches Hg and running on a mixture of 60% methanol, 30% benzole, 10% acetone plus tetraethyl lead, At this point the engine was running at its limit, even stretching the cylinder hold down bolts.

[Dick Whittingham]

So, not a case of "why did it take so long", rather "didn't they do well"

And Griffons were in service with several aircraft well before the end of the war. Note that RR had the new Eagle on the testbeds, that flew in the prototype Wyvern - a 24cyl H in the 3000hp range.

Dick

[Xray4277]

Originally Posted by Onceapilot

Lumps, you are correct that the very coarse fixed pitch prop would hold back the RPM at low speed, and so reduce the supercharger output, but the Merlin was fitted with a boost controller which throttled the supercharger to prevent excessive manifold pressure. This could be overridden in emergency, with risk of engine damage/failure. Unfortunately, I do not know if the override completely removed control of boost or, if it allowed a higher controlled level of boost. The latter would seem more likely as the amount of extra boost available at low level was considerably more than rated boost.
Cheers

There is an excellent explanation of how the Merlin boost control works here

The use of the term 'throttle lever' on a Merlin (and other similarly controlled engines) is a a bit of a misnomer, since it does not have complete authority over throttle position. It's more of a 'power lever' working in conjunction with the propeller pitch (rpm) lever.

[Centaurus]

As an aside, Rolls Royce quickly realised the efficiency of high boost with low revs and second World War Merlin handling for long range cruise was throttle fully open but revs held down to 1700 with coarse prop via the speed governor.

I was unaware of the claimed second world war RR Merlin long range cruise policy. We certainly did not use those power settings when I flew Mustangs and Lincolns with RR engines in 1953
That policy might have reduced fuel consumption but was the cause of many engine failures at our squadron of RAAF Lincoln Mk 31 Long Nose equipped with RR Merlin 102 engines. We were a Maritime squadron.
To explain. The Pilots Notes for type recommended +6 boost and 2400 RPM for cruise. Mustangs and Lincolns.

Then an RAF exchange pilot arrived at the squadron in Townsville, Australia circa 1957. He had flown Shackletons in the RAF where low RPM and relatively high boost was engine handling policy. It might have worked on the Shack RR engines but was a disaster when used on the RR Merlins 102.

Following his advice on how to get better endurance for long range maritime operations at the low RPM /high boost combination, squadron crews were instructed to change from +6 /2400 RPM regime to +7 boost/ 1700 RPM. Fuel consumption improved markedly and everyone though we were on to a good thing. Except this writer Centaurus who was the squadron QFI.

The RAF chap out-ranked me as I was but a lowly Flying Officer and the RAF chap (a really pleasant well meaning bloke) had the ear of the CO. I felt more research was needed rather than word of mouth from someone who had vast maritime experience but on a different type of aircraft with a different RR design.

Within a few weeks we had the first engine failure on the RR Mk 102 followed by several more in the next few months. I suspected it was an engine handling problem as it seemed quite a coincidence the cluster of engine failures occurred at the new power settings introduced on the RR Merlin Mk 102's.

RAAF HQ started to ask questions and sent an engineering officer to investigate. He was a Flight Lieutenant and previously an NCO engine fitter on Lincolns at the squadron about five years earlier. By coincidence I had been an NCO pilot on Lincolns when he was there. A small world.

I was able to fill him in on the new policy of low RPM /high boost cruise introduced by the RAF former Shackleton pilot and upon which I had my reservations. However the HQ investigating officer was given short thrift by the squadron leader flight commander who was firmly of the belief that high boost and low RPM was the best engine handling technique for long range ops. If it was good for the Shackleton then it must be good for the Lincoln albeit slightly different engines. That was his theory, anyway.

The RAAF HQ investigating officer reported back to his Group Captain boss at HQ that in his opinion the engine failures were caused by poor engine handling. A rocket was sent to the squadron by HQ, directing the low RPM/high boost technique not to be used and to stick with the original power settings published in Pilots Notes Lincoln. The engines failures ceased and we went back to normal engine handling figures.