Everything you always wanted to ask...

[John Howling Mouse]

Thanks.  I forgot that the initial charge is a more contained explosion than the full-blown rocket motor which takes over a moment later.

Now, is anyone here intimately familiar with superchargers, esp. those in certain WW2 a/c operated by the USA (I'm not saying "Thunderbolts" specifically, buy you may imagine where I'm heading with this...)

[deathjester]

Wasn't there something in this topic by JonCarrFarrelly posted a wee while ago? And what do you need to know?

[kitnut617]

Thanks.  I forgot that the initial charge is a more contained explosion than the full-blown rocket motor which takes over a moment later.

Now, is anyone here intimately familiar with superchargers, esp. those in certain WW2 a/c operated by the USA (I'm not saying "Thunderbolts" specifically, buy you may imagine where I'm heading with this...)

I believe (and someone correct me if I'm wrong) that the P-47 had both a supercharger and a turbocharger, it was powered by an R-2800 which had an in-built supercharger like in the photo further back in this thread.

[jcf]

Now, is anyone here intimately familiar with superchargers, esp. those in certain WW2 a/c operated by the USA (I'm not saying "Thunderbolts" specifically, buy you may imagine where I'm heading with this...)

What are you looking for?

Jon

[John Howling Mouse]

I looked it up and a supercharger is the same thing as what we now call a turbocharger, so it makes more sense to me know.

In my Tamiya T-bolt, there is a handy little illustration of the ducting for the supercharger, the general location of the intercooler is shown but the text explanation was a little confusing (to me).

Cowling intake air is brought all the way back to the "supercharger turbine" where it is compressed.  Okay, good.  Then they write "The turbine used engine exhaust gas to run."  It further goes on to describe "The compressed air, which was brought to a high temperature passed then through the intercooler to be cooled down prior [to] entering [the] engine carburetor via air ducts placed on each side of the cockpit." 

I'm not understanding why the intercooler was necessary to cool down already compressed air, which I thought is normally cold to begin with (I'm guessing I'm wrong about that).  I was also wondering why they used engine exhaust gas as opposed to a more mechanical means to operate the supercharger turbine.   

[Edit: just found out why:  "Positive displacement superchargers may absorb as much as a third of the total crankshaft power of the engine, and in many applications are less efficient than turbochargers."]

[kitnut617]

Barry, your missing a word in your description, a turbocharger is more correctly called a Turbine Supercharger, it just designates what turns it.  Generally though when someone says 'supercharger' they're talking about a mechanically driven unit, whereas when they say 'turbocharger' they're talking about an exhaust driven unit.  But as I said, the R-2800 engine in a Thunderbolt had both, so the air inlet would feed a certain amount of air straight to the supercharger (probable a single stage) via the carburetor, and the rest would be ducted back to the turbochager. 

As to the intercooler, this is needed to cool the compressed air, if you feel your air brush compressor it will be hot when you run it, this is because when you compress air it gets hot.  Now recipicating engines run better on cold air as the air is more dense, which produces more power when mixed with a fuel (an engineer can probably give you the exact termology for all this) so this is why the air passes through an intercooler.  On two stage superchargers, the air passes through the first stage and is compressed to a certain psi, it then passes through an intercooler, then enters the second stage of the supercharger which compresses it even more, this then is usually passed through another intercooler before entering the combustion chamber.  When the cpmpressed air passes through the intercooler it still stays compressed.

HTHs

Robert

[kitnut617]

There's another engine designation that involves super chargers, these are called compound engines.  These are where engine builders started to harness the wasted energy when passed through the turbochargers.  What they did was to 'slave the turbine shaft to a variable speed gearbox that was connected to the crankshaft.  This offset some of the energy lost by the mecanical supercharger and also they used the exhaust thrust to argument forward motion too.

In one of my reference books called Rolls Royce and the Crecy it says that about one pound of thrust is the equivelent to 1 hp.  A Merlin engine gave of about 150 odd pounds of thrust from the exhaust (which is why you see different exhaust nozzles of the different Merlin powered aircraft, they were utilizing the thrust better) so this amounted to about 150 free hp.  This was very benificial when flying at height because the engine would lose power the higher it got.   In the Crecy engine which was a two stroke, RR took this even further.  Two stokes have a habit of not burning the whole charge as it passes through the combustion chamber so some raw fuel came out of the exhaust, RR' idea was to add more raw fuel into the turbine and then ignite it, a very primative afterburner.

[kitnut617]

Here's a very good photo of a restored and cut-away R-2800 engine like what was found in the P-47.  The supercharger is all in blue;

http://www.midwaysaircraft.org/images/90%20%20Second%20L%20H%20side%20of%20engine.jpg

[Hobbes]

JHM: what you're thinking of is the air from a compressed air bottle, which feels cold. What happens there is that the compressed air is at room temperature, and the expansion causes it to cool down (see http://en.wikipedia.org/wiki/Ideal_gas_law).
When you compress air, it will heat up. The air in a turbocharged or supercharged engine can get so hot that the air/fuel mixture ignites prematurely (IIRC Rolls Royce Griffons are prone to backfires because of this). Also, hot air is less dense than cold air, so you can mix in less fuel and you get less power from hot air.

On superchargers vs. turbochargers:
turbochargers are more efficient, because they use energy from the exhaust that would otherwise be lost. The drawbacks of a turbocharger are:
-it gets very hot (the exhaust manifold of a turbocharged engine can glow red-hot), so you get the same problem as with early jet engines: you need exotic alloys that can withstand these temperatures. Superchargers don't run as hot.
-it takes time for the turbo to spool up. This "turbo lag" is more of a problem in cars than aircraft, though. Superchargers deliver their boost immediately.
Also, I seem to recall turbos are easier to get huge amounts of power from.

[John Howling Mouse]

Here's a very good photo of a restored and cut-away R-2800 engine like what was found in the P-47.  The supercharger is all in blue;

http://www.midwaysaircraft.org/images/90%20%20Second%20L%20H%20side%20of%20engine.jpg

On this graphic, they show the "supercharger" as being all the way in the tail.  Kinda makes me wonder if they could have stowed this assembly all much closer to the engine with an extended cowl and some basic modifications there instead of all the extra ducting to the tail and back.  Even if it was a CoG limitation, they could have balanced the extended cowl and its additional weight with another fueltank where the real-world's T-bolt had the supercharger and intercooler. 

Hmmm...hmmm...hmmm a P-47Z OmniBolt variant which differs from the complexities of the contra-bladed XP-72 Ultrabolt but still could have competed with the Mustang (I've often read that the only limiting factor on the P-47 as a high-altitude bomber escort wasn't in its maneuverability but in its range, which was a bit inferior to the Mustang's)

[kitnut617]

Here's a very good photo of a restored and cut-away R-2800 engine like what was found in the P-47.  The supercharger is all in blue;

http://www.midwaysaircraft.org/images/90%20%20Second%20L%20H%20side%20of%20engine.jpg

On this graphic, they show the "supercharger" as being all the way in the tail.  Kinda makes me wonder if they could have stowed this assembly all much closer to the engine with an extended cowl and some basic modifications there instead of all the extra ducting to the tail and back.  Even if it was a CoG limitation, they could have balanced the extended cowl and its additional weight with another fueltank where the real-world's T-bolt had the supercharger and intercooler. 

Hmmm...hmmm...hmmm a P-47Z OmniBolt variant which differs from the complexities of the contra-bladed XP-72 Ultrabolt but still could have competed with the Mustang (I've often read that the only limiting factor on the P-47 as a high-altitude bomber escort wasn't in its maneuverability but in its range, which was a bit inferior to the Mustang's)

Yes that's right, it's a turbine supercharger which is called for short turbocharger nowadays.  The engine still had a mechanically driven supercharger too incorporated into the back of the engine like in the link I added in the last post.  In a turbocharger there's two turbines, one on each end of a short shaft. the whole contraption is spun by the energy of the exhaust pulses aimed at one of the turbines (these are centrifuge turbines BTW) usually the bottom one, this then spins the top turbine which is actually a compressor like what you see in blue on the back of the engine in that link, the fresh air is sucked into the top turbine, compressed, then sent back up to the front of the aircraft (still under pressure) where it passes through the carb then gets re-compressed in the mechanical supercharger.  It's basically a two stage supercharger.

Robert

[jcf]

An airflow graphic.



Baz, there is no room to forward mount a turbo-supercharger of the size used on the P-47.
Most of the fuselage depth on the P-47 is because of the turbo installation.

Turbo-superchargers are almost always remotely mounted.
B-24 installation:


The ducting for two-stage mechanical blower systems (engine-mounted supercharger + second-stage auxiliary blower) installations can take up a lot of territory.
P-61 installation:


F6F installation:


The Corsair also used an R-2800 with two-stage mechanical supercharging:

The installed length of R-2800s used on the F4U series grew from 88.47" for the F4U-1/2/3, to 93.75" for the F4U-4 then to an ultimate length of 98.5" for the F4U-5.

The R-2800-21 on the P-47D was 75.72" long.

The two-stage R-2800-10,-10W as used on the F6F series and P-61A was 88.47" long installed and would be a good choice of engine for your slimmed down T-bolts,
not as powerful as the P-47 engine with turbo-supercharger at higher altitudes, but not too shabby all around.

Jon

[B777LR]

Has anybody on this forum built a BAC 3-11 yet?

Has anybody on this forum flown onboard a military aircraft as passenger or crewmember? (I know Zac has flown Tiger Moths, but anyone else?)

[jcf]

Has anybody on this forum flown onboard a military aircraft as passenger or crewmember? (I know Zac has flown Tiger Moths, but anyone else?)

Does a B-17 count? 

Jon

[Ed S]

Has anybody on this forum flown onboard a military aircraft as passenger or crewmember? (I know Zac has flown Tiger Moths, but anyone else?)

I have flown in a number of different ones.  Although it was over 25 years ago. Why do you ask?

Ed