Spits

[perttime]

At least according to Wikipedia articles, Miss Shilling's Orifice, Tilly Orifice, or Miss Tilly's Diaphragm was officially called the R.A.E. restrictor.

 

http://en.wikipedia.org/wiki/Beatrice_Shilling

[buster prop]

I'm surprised that when the Merlin fuel system was being designed no one thought of the effect of negative G. Probably not important for engines used in bombers but pretty important for a fighter though. Maybe the system was carried over from the racing seaplanes mentioned above which would never experience negative G. If they ever did the the pilot was in bad trouble!

[Erfolg]

Hmmm. I am not sure that it was so much an oversight, using carburetors.

 

The use of fuel injection by Mercedes on an aircraft engine being the innovation.

 

The more pertinent question, why was UK manufacturers so reluctant to introduce fuel injection. I guess it was about manufacturing capacity, and knowledge of how to successfully make the change, whilst maintaining production rates. I understand that the MB used a Bosch system, who had a lot of experience derived from diesel engines.

 

One of the drivers would have been the ability to achieve near stoichiometric combustion at all altitudes and positions. Basically the same reason we all now drive cars with it today.

[Alan Randall]

Rolls Royce knew about fuel injection but at the time chose to ignore it. When a supercharger sqeezes the air it heats it up and the heat reduces the effect of the oxygen content on the bang when the spark plug fires. By spraying in the fuel via the carb, it cools the charge air and restores its efficency. Later when two stage supercharges were introduced, the intercooler was introduced to cool the charge air between each stage.

 

The Germans used direct injection of fuel into the cylinders so the air from the supercharger was not cooled in the same way and thus lost some of its efficency, so they gained during violent manouvers but lost out to some extent on engine efficiency.

 

As stated above, Miss Shillings oriffice partly solved the problem but it was the Bendix (I think) carb that finally solved the problem.

 

The amazing thing about the Merlin was the amount of power that RR extracted from it's 27 lites  compared with the 109s Daimler Benze engine and the FW190's BMW radial which I think was about 42 lites.

 

Stanley Hookers 'Not Much and Engineer' is worth reading for its back ground on the Merlin development and also that of the jet.. Hooker was responsible for much of the Merlins power increases during the war through his supercharger developments, before moving on to the early jets.

 

Rolls Royce also avoided the turbo charger because by simply ejecting the exhaust backwards through the exhaust stubs, they found that they obtained the equiverlent of an extra 150 hp of propulsive power. The turbocharger would have robbed the exhaust of this power and also possibly introduced the effects of turbo lag when you slam the throttle open. The supercharger being driven by the engine would not suffer from this.

 

The shape of the Spitfires radiators also had a propulsive effect contributing to the Spitfires speed.

Edited By Alan Randall on 23/01/2012 22:55:02

Edited By Alan Randall on 23/01/2012 23:05:21

[David Bess]

Well you guys are well informed about the history, flight characteristics and the engines ,so as a novice , I have a question for you ;what is the weight to power ratio of a Merlin in a Spit ?and what power IC or electric would be equal or scale if you like to a 2kg model ?

[Erfolg]

I do think we have to be careful to think or believe that aircraft are unique and developed by brilliance alone.

 

The aspects of supercharging and inter cooling were concepts known to many engineers in that era. As with Formula ! designers, designers have watched ech other at every opportunity. Both ideas going back before the second world war, used in many industries. Just as Fuel Injection

 

The theoretical benefits of heat cycles are studied to this day in the subject area of "Heat Engines". Open cycle systems studied as closed loop.

 

By the end of the piston engine era, the advantages of Turbo Charging and fuel Injection were becoming the norm. From the German perspective, there lack of high temperature alloys and materials restricted there developments. At that time German metallurgists were amongst the world leaders in understanding materials. Shortages were the problem for them.

 

In case of the UK, I suspect it was also about resources as much as anything. In the case of the UK, it was not only effective use of manpower, but also that of manufacturing capacity and its effective deployment.

 

The idea of thrust from exhausts and radiators was also known, hence Naca cowls.

 

Just as on the grid at a GP, every opposition crashed aircraft would be carefully examined by engineers, pilots etc. Any new ideas, considered and rapidly incorporated if any merit can be seen.

[Steve Hargreaves - Moderator]

Which Merlin & which Spit do you mean David.....?

 

The original Spit weighed around 5,000lbs but by the end of the war it was over 9,000lbs & the original Merlin produced just over 1,000hp but was developed to around 1,700 with the Griffon producing even more.....

 

NB figures taken from ageing brain so might not be completely accurate......

[Ian Jones]

Posted by Ian Jones on 21/01/2012 01:09:49:
One icon still flying is ML407 which has a long and remarkable history behind it. I will be starting a thread in the full size aviation section as soon as I have finished putting some words to the photos....

... and here is said thread, hope it's useful.

[Simon Chaddock]

I think it is a bit of an overstatement to say RR ignored turbo charging.

 

They did test turbo chargers on many development engines but at the time it was the shortage of the high high temperature steels that ruled them out for mass production coupled with a simple lack of space in many of the fighter designs.

 

The Americans did have such steels available in quantity and used turbo charging on many of their engines but mainly on high altitude bombers where turbo charging works at its best..

 

The only single engine WWII turbo charged plane I can think of that was built in any numbers was the P47 Thunderbolt and that was big!

[buster prop]

Another famous WW2 fighter which used turbocharging was the P38 Lightning, although that was a twin. Interestingly, its engines (Allison V1710) were also used in the early Mustang with supercharging. As we know, that plane lacked high altitude performance until re-engined with Merlin 61 licence built by Packard, with a Stanley Hooker designed supercharger. The Allison was about the the same size as the Merlin and with turbocharging went on to be a successful engine. A good read on this subject is 'Piston Aero Engines by Bill Gunson.

[Erfolg]

Simon

 

I thought the B17 was Turbo charged.

 

As a general discussion

 

With respect to RR, it easy to with hindsight to have little perspective of the difficulties at that general era.

 

RR were not good at the time at mass producing aero engines. Putting the statement into perspective, the company did not produce many hundreds of any engine type per year prior to WW2. In WW2, the massed produced units were manufactured mainly by Ford, Packard etc. The Merlin engine came out of the quest for speed in the Schneider trophy, the engine a consequence of the experience gained.

 

The application of supercharging, being the first step to ultimately 3 stage supercharging. As has been highlighted the engines weight and size grew, with the increase in power as time past. Towards the end of WW2, RR believed that the Merlin had reached the limit of useful development, Hence the even larger Griffon.

 

Non of these developments came trouble free. There were real issues with propellers, number of blades, contra rotating, that were problematical to RR, Vickers, Rotol.

 

You will also have read of planes that never reached thier potential due to RR lack of resources to develop other troublesome engines.

 

I suspect that RR never developed a Turbo engine, with fuel injection, because of resources. Nimonic was certainly available in the UK. The lack of Nickel in quantity was an issue for the German turbo charged ambitions (and jet engines), that is why they started to consider ceramic impellers.

 

I think we should consider that RR at the commencement of WW2 was a small company, with limited resources, to manufacture, develop engines, not the world leader it is today.

 

With respect to the Spitfire, we should recognise that all the extra power came with a higher fuel consumption. I have since read (this week) that the driver for the Spiteful was extra range, as as an escort fighter, or reconnaissance, it was compromised from the ideal set by the Mustang.

[Steve Hargreaves - Moderator]

I have a very interesting article about the RR Crecy engine...a V12 supercharged sleeve valve 2 stroke that may have used turbo charging & was thought to be capable of around 5000BHP....

 

It appeared in another magazine so I can't reproduce it but if anyone would like a copy feel free to PM me......its not a brilliant copy but quite readable.....

[Erfolg]

Steve

 

There is a description in Wikipedia . It does mention 5000 hp, by extrapolation from a single cylinder. As a V12, the output appeared to be about 2,000 hp. Reading between the lines, fuel consumption appeared to be the Achilles heel.

 

As with all planes to date, the problem for the Spitfire was that as an aircraft, it had reached the end of its potential for development, by about 1944.

 

Perhaps what may be surprising, is that Supermarine did not seem to have the design and political ability to develop replacement aircraft. Was it a policy of Vickers or other issues?

 

Re-acquainting myself with the early history of the Spitfire, was very interesting, in that the Spitfire story acknowledges the contribution of the Schneider Trophy races, yet rarely expresses the linage. The SB 6b, can be argued owes a lot of its genes to the Curtis Racers and subsequent refinements by many manufacturers, as they wrestled with issues such as inline engines, radial, biplane, monoplane configuration. Starting with Curtis the batten being picked up by Piaggio, Machi, Sovia, Short, Gloster aircraft, all having a lot in common with Mitchell's later efforts, something akin to current F1 cars.

[Steve Hargreaves - Moderator]

That's the one Erfolg.....

 

I suspect that extracting 5000hp would have melted something......

[Simon Chaddock]

buster prop

The Allison in the Mustang was mechanically super charged, like the Merlin, but as you say the its single stage design did not favour the high altitudes used in the European theatre.

The ultimate RR two stage, 3 speed, inter cooled set up was a remarkably effective and compact package.

Erfolg

Yes, the B17, like nearly all American bombers, was turbo supercharged but I was referring to single engine fighters.

The turbo charger on the P47 was behind the pilot so the hot exhaust and the compressed inlet air was ducted under his seat and the fuselage fuel tank!

[Erfolg]

Simon

 

When I think about the Merlin, in many respects I am reminded of a process which seemed to ellude many UK car manufacturers in the 60's and 70's. That is a stright foreward design concept that was continually improved and developed, "Incremntal Development". How I despaired that my BMC and later Leyland cars all included the same design faults, Ball Joints, that wore out about every MOT. CV joints, which, apparently had some strange cone, which either could not be undone, or came loose etc, etc.

 

Although not radical, the Merlin, was broadly as capable as any German engine at any point in time.

 

Although not knowledgeable with respect the DB 601, the approach seems different, being a right first time design, not being substantially different at the end of the engines history to the beginning. When more power was required, the solution being to bore it out, to increase capacity, then becoming the 603 and 605.

 

Both approaches appear to have worked. Although getting something right first time, takes some doing, in my opinion.

 

I also think that the two approaches to ever higher altitudes is interesting. RR, taking the route of multiple super chargers and clutches. DB using a fluid flywheel to control supercharger speed, via a altitude sensor and control system. Both approaches seem to have worked. I would expect that a paper or book has been published which compares the merits of the two approaches and their limitations.