Fw 190A

[Debonair]

http://www.shockwaveproductions.com/store/fw190/tactical_trials.htm
is where i read, it seems to be a transcrpition of a similar document...

[joeblogs]

Remember these guys are measuring sustained rates of climb not the max output of the engine for a short period. It's not uncommon to require the pilot to hold back on engine RPM in a sustained climb as this is the worst situation in terms of engine overheating. We've had many long threads on that point already.

Other guys have posted documents on the boost systems using 96 octane gas. They get 1.42 ata at higher RPM, but the engine is only allowed to be run this way for a few minutes at most.

-Blogs

Originally posted by Bruno
Kutscha (MiloMorai) posted this scan on another forum. It's consistent with what I have read in that the British tested Faber's A-3 at 1.35 ata max.

British test of Faber's A-3:

 

[Charge]

Just saw Gripen's pic.

I wonder about the missing dates and why has the text been crossed over at some stage (now showing as white lines)?

"BTW even the early models of this engine were rated at 2,700 RPM. There are ratings for 87 and 92 octane but they don't seem to matter much, at least at sea level."

Higher RPM would probably need a change in transmission to keep the propellor blades in acceptable speed region. I just think that it is beneficial to increase RPM only if the supercharger output exceeds the ability of the engine to breathe sufficiently at a lower RPM, ie. Bigger blower output -> increase RPM. With this I'm making a wild guess that the BMW was actually under charged to begin with, and only more octanes (enabling more boost) were required to increase the engine perfomance. So even at increased boost pressures the engine was able to breathe sufficiently without the need to increase RPM which would in turn cause reliability issues.

-C+

PS. BTW I do not cease to wonder these guys popping out of nowhere and asking something pretty much technical that has been under debate at some stage. Why must people hide behind fabricated handles? Who are you "Zoom"?

[joeblogs]

Like just about all WW2 engines, the BMW801 had reduction gearing. It also had a constant speed propeller. With the FW190, it had a single controller for manifold pressure, RPM, and propeller control, making the plane simple to fly. The mechanical supercharger on the BMW801 was geared to the crankshaft so it ran faster as engine RPM increased. True it would lose some efficiency at higher speeds, but not so much to negate the benefit of higher engine RPM.

-Blogs

Originally posted by Charge
Just saw Gripen's pic.

...Higher RPM would probably need a change in transmission to keep the propellor blades in acceptable speed region. I just think that it is beneficial to increase RPM only if the supercharger output exceeds the ability of the engine to breathe sufficiently at a lower RPM...,
 

[Glasses]

Originally posted by joeblogs
Like just about all WW2 engines, the BMW801 had reduction gearing. It also had a constant speed propeller. With the FW190, it had a single controller for manifold pressure, RPM, and propeller control, making the plane simple to fly. The mechanical supercharger on the BMW801 was geared to the crankshaft so it ran faster as engine RPM increased. True it would lose some efficiency at higher speeds, but not so much to negate the benefit of higher engine RPM.

-Blogs

And Mixture for altitudes.

[Charge]

Well, the RPM increase is not always a very straightforward way to increase power. Increased RMP means more air moving through the engine but also head/valve design/ piston speeds can have effect on how much air is reasonable to expect to flow through the engine. It really depends on so many things that if e.g.  Merlin benefits from increasing RPM the case is not necessarily similar for a stock radial BMW801 without significant modifications to engine head or supercharger gearing. Also the cylinder diameters and piston movement range have effect in what way more power is reasonable to tune out of the engine without significantly reducing its reliability.

-C+

[gripen]

Originally posted by Charge

I wonder about the missing dates and why has the text been crossed over at some stage (now showing as white lines)?

The title and the date are below, it's from reel 8054 in NASM (partially unreadable). Obiviously that limited rating part was crossed over once the  higher rating was allowed. No idea when that happened, no one has posted documentation on that yet.

Faber's plane was captured 23rd June ie before the higher ratings were allowed. RAE tested it's performance using these limited ratings (max 1,35ata 2450rpm) while AFDU tactical trials were done using higher ratings (1,42ata 2700rpm). Apparently they just did not knew about the limitation in AFDU.

gripen

[Gunzo]

Originally posted by Bruno
Kutscha (MiloMorai) posted this scan on another forum. It's consistent with what I have read in that the British tested Faber's A-3 at 1.35 ata max.

British test of Faber's A-3:

Hi Bruno

That looks as though it came from a book. Very detailed data, kinda book I'd like to get, did MiloMorai post a source?  

Can you direct me to the other forum so I can check?

You have any leads on the source?

Gunzo

[MiloMorai]

Gunzo,

it is from a Fw190 book by Swanborough/Green published by Arco in 1976. ISBN 0-668-04001-7

It was in the chapter (16pgs) of the British technical description of Faber's 190.

[joeblogs]

The effect of RPM on output is nearly linear. It is not quite so due to the loss in supercharger efficiency at higher speeds and additional friction losses in the engine.

-Blogs

Originally posted by Charge
Well, the RPM increase is not always a very straightforward way to increase power. Increased RMP means more air moving through the engine but also head/valve design/ piston speeds can have effect on how much air is reasonable to expect to flow through the engine. It really depends on so many things that if e.g.  Merlin benefits from increasing RPM the case is not necessarily similar for a stock radial BMW801 without significant modifications to engine head or supercharger gearing. Also the cylinder diameters and piston movement range have effect in what way more power is reasonable to tune out of the engine without significantly reducing its reliability.

-C+

[Debonair]

according to what i've read the problem at too high RPMs is valve float, which is why the sleeve value pistons were supposed to have been such an improvement, but i guess that if they really were, they would have been adopted universally.  since they weren't, i'd have to assume sleeve valves aint so great

[joeblogs]

True, but in terms of gross output, these are second order effects.

-Blogs

Originally posted by Debonair
according to what i've read the problem at too high RPMs is valve float, which is why the sleeve value pistons were supposed to have been such an improvement, but i guess that if they really were, they would have been adopted universally.  since they weren't, i'd have to assume sleeve valves aint so great

[Charge]

"The effect of RPM on output is nearly linear."

At least on paper. In practice the basic engine design determines the maximum reasonable RPM -even if the reliability issues are ignored.


By this I mean that after a certain point the flow is so restricted that no matter how much you increase the RPM the flow doesn't really increase and the output remains the same or even starts to drop. AFAIK, that is...

PS. I wonder what a WW2 aero engine power/torque chart looks like...

-C+

[joeblogs]

No one is saying you can increase RPM without limit.

In fact, it's harder to build an engine to manage higher RPMs than higher manifold pressure because the mechanical strain imposed is not linear, it rises at something like the square or cube of RPM. This limit typically kicks in before the limiting effect (efficiency) of the supercharger and even the "breathability" of the engine, assuming the engine is well designed. Many engines of this era had variable valve timing.

The other key limiting factor to RPM is heat rejection. The cooling system can only throw off so much heat per second, and higher RPM and manifold pressure increases the flow of heat that must be dissipated. That is why RPM is often held back from the maximum in a sustained climb where there is not enough air flow across the cylinders (or radiator) to reject enough heat from the engine.

Unlike car engines, aircraft engines have a pretty flat torque curve. They are designed to be used across a broader range of RPMs than an engine in a car, which typically has a sweet spot around 3,000-4,000 RPM.

There's a very nice series of articles on A/C engine perfomance in a column called Pelican Perch on the web. A lot of this stuff is discussed there.

-blogs

Originally posted by Charge
"The effect of RPM on output is nearly linear."

At least on paper. In practice the basic engine design determines the maximum reasonable RPM -even if the reliability issues are ignored.


By this I mean that after a certain point the flow is so restricted that no matter how much you increase the RPM the flow doesn't really increase and the output remains the same or even starts to drop. AFAIK, that is...

PS. I wonder what a WW2 aero engine power/torque chart looks like...

-C+

[Charge]

"Pelican Perch on the web. A lot of this stuff is discussed there."

Yup, a great place. Deakin seems to know his stuff.
Lots of good info about engines. Not to mention those Hurri and Zero articles, too.

I may be wrong because I was talking about non-charged engines. As you said the thermal issues may well be the biggest problem in charged engines and they may in turn induce severe knocking issues which quickly lead into a mechanical failure.

"They are designed to be used across a broader range of RPMs than an engine in a car, which typically has a sweet spot around 3,000-4,000 RPM."

Actually I thought it was another way around. I have always thought that the a/c engine is nearly always used at its most optimal RPM and the head and cam timing/design try to peak max torque and max power at that point, where as the car engine has to provide enough torque first and highest power only on higher RPMs.

I don't know so much of this stuff that I could argue about it. Just some assumpitions I've had for awhile.

-C+