Horsepower, RPM, and Supercharging

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Quick rundown on the P-38 engines

P-38 model.......................supercharger gear...............turbo type.............power at 25,000ft

up to the F.........................6.44..............................................B-2...............................1150hp
the F.....................................7.48..............................................B-2...............................1325hp
the G....................................7.48..............................................B-13.............................1325hp
The H-1...............................8.10..............................................B-13.............................1425hp
the H-5................................8.10..............................................B-33.............................1425hp
J & L.....................................8.10..............................................B-33..............................1425hp

The power ratings are the original planned ratings for military power. Some models had trouble with insufficient intercooling and insufficient radiator airflow and could not sustain the military ratings without overheating (depends on climate?) Later models got better and were allowed WEP ratings.

But in any case the increase in engine supercharger gear ratio was used to increase the power. However this change in gear ratio also coincided with stronger engine parts. Much like the engines in the H and above used the same basic engine as the P-40K which was rated at 1325hp for take-off but used the 8.80 gears.
 
Yes, the H model was limited by the elegant and low drag but less effective leading edge intercoolers - which had to be a pain to manufacture as well.

The new chin type air-to-air intercooler brought its own problems - excessive cooling that led to liquid rather than vaporized fuel being introduced to the cylinders under some circumstances. At the 9th Photo Recon they put baffles in the intercooler cooling air exhaust to cut down the airflow, since they tended to fly mainly long range high altitude missions where adjustments were not needed.

Lockheed could have really helped things out by using a liquid cooled intercooler placed back where the radiators were; although they had the example of Stanley Hooker's brilliant idea they chose not to do so.

By the way, the cooling radiator size did not change between the early and late models. They just introduced a lip to separate the boundary layer from the faster airflow and that improved cooling enough to take care of the greater horsepower.
 
MIflyer said:
I understand that with the BF-109, which used a fluid coupling drive that eliminated the specific supercharger gear shift points, at sea level the supercharger actually absorbed more power than it added
Click to expand...

You understand that from where exactly ? The slip coupling gave it about 100bhp MORE at sea level that it would otherwise have had if it had a gear driven supercharger from something like a Merlin-III.

In fact in the 601A the coupling absorbed 22hp at sea level (in the form of oil heating, not crank parasitic loss) - or if you prefer it in the US units from the
American wartime report where that was actually measured on the test-bench 1000 BTU/min, dropping to 30 BTU/min at rated altitude where slip
reached the minium value of about 2.5%

At sea level at max boost (1.42 atmospheres) its just about multiplying crank output by 1.42, i.e. the supercharger is contributing about 300hp to that
which the engine would be giving without a supercharger.
 
Snowygrouch said:
If the engine has a mechanical supercharger geared to the crankshaft, and has no swirl-throttle, it MUST show a "sawtooth" pattern (depending on how many gears it has, obviously one peak per gear, so basically 1 or 2 in nearly every case).
Click to expand...
That I grasp, but when I look at charts of World War II aircraft performance, more often than not shows a sawtooth shape for the speed portion; however the horsepower section (the F4U chart on WWII Aircraft Performance) often shows a straight line in BHP.

Basically, you end up with the following
  1. Speed
    • Speed goes up with altitude until critical altitude is reached; then it falls off
    • If another supercharger gearing is available, when the time to clutch occurs, speed starts going back up until critical altitude is reached
  2. BHP
    • The BHP usually appears straight until critical altitude is reached, at which point, it continues to go down until it becomes time to shift gears
    • BHP then stays straight again until critical altitude is reached, and then it falls off
They might not be showing the throttling loss, but the charts don't seem to indicate the sawtooth except in speed.

pbehn said:
If you create enough boost then combustion occurs as soon as fuel is introduced.
Click to expand...
And that's why it's a joke... you'd require some seriously high octane fuel, lots of water injection, and over-boost that would blow up any ordinary engine. It's just a thought experiment.
 
Zipper730 said:
And that's why it's a joke... you'd require some seriously high octane fuel, lots of water injection, and over-boost that would blow up any ordinary engine. It's just a thought experiment.
Click to expand...
I think you missed my point, many people realised you could cut out the piston and crankshaft middle man.
 
MIflyer said:
I understand that with the BF-109, which used a fluid coupling drive that eliminated the specific supercharger gear shift points, at sea level the supercharger actually absorbed more power than it added.
Click to expand...

To add to Mr. Douglas, there are two things going on with the DB supercharger, the first is, Like Mr. Douglas said, the coupling is wasting power in heat. The efficiency of the coupling itself was sometimes questioned but we seem to have the answer.

The 2nd thing is how much power the actual supercharger was taking to drive. Someone may know the actual figures but in this case we can fall back on the fact that that power to drive a supercharger impeller (of a particular supercharger) pretty much varies with the square of the speed of the impeller. At a 7.0:1 gear ratio the supercharger will take roughly half the power to drive than it would with a 10 gear ratio (7x7=49 and 10 x 10=100) so on the 109 the supercharger took roughly 1/2 (I believe the fluid drive minimum speed was just a bit over 7 to 1?)
than it did at high altitude with the fluid drive at minimum slip (just over 10 to 1 ratio?) while the drive took 22hp, As Mr Douglas has stated, cramming 42% more air (and no aircraft engine without a supercharger ever came close to 100% volumetric efficiency) means 42% more power which far exceeds the drive requirements of the supercharger setup in tota at sea level.
 
Last edited by a moderator:
I'm wondering if the Russians were the first to think up the idea of the swirl throttle arrangement? This seems like it would be a straight forward answer, but sometimes ideas are thought up by loads of people, but only one person ends up actually getting around to using it.
 

Try to get a ride in a recip with a constant speed prop. Failing that, download the POH or dash one for such an airplane. Study a little. The answers to your questions are pretty elementary.
Throttle controls manifold pressure. More throttle= higher manifold pressure.
Prop governor controls RPM. You can set the RPM anywhere within limits by moving a lever.
Each engine has limits to how much manifold pressure you can apply at a given RPM w/o running the risk of damaging the engine.
All you need do is keep the split within the limits found in your handy POH or dash 1. Forget automatic transmissions, DMVs, etc.
Learning a little zoomie terminology might help you understand. Boost and MP are not exact synonyms. You can't boost a normally aspirated engine. You can apply excessive MP for a given rpm in a normally aspirated engine.
 
Zipper730, post: 1485484, How does boost get varied independent of engine RPM?

tommayer said:
Throttle controls manifold pressure. More throttle= higher manifold pressure.
Prop governor controls RPM. You can set the RPM anywhere within limits by moving a lever.
Click to expand...
Jees, 8 days and two pages worth of esoteric supercharger drive info, and somebody finally gets around to answering Zip's basic question. Gotta walk before ya can run. Nice job, Tommayer!
Cheers,
Wes
 
Wes,

Thanks. Am sitting around trying to avoid working on my CFI renewal on line, and this is a nice diversion. Did not mean to sound condescending. Zipper, if you're still tuned in I forgot to say supercharging has nothing to do with the matter, per se. It's perfectly possible on a supercharged engine to be operating without any boost, boost being MP in excess of ambient air pressure, and still apply more MP at a given RPM than the manufacturer says is safe.
As all the erudite (some of it is even correct) stuff in posts above might indicate to you, mechanical supercharging and turbocharging are subjects with some pretty esoteric aspects. Since you sound like a really young IT guy, I recommend the videos on mechanical supercharging and turboing on Greg's airplanes and automobiles on youtube. Don't know him, but he sounds like he's actually held a stick and watched a manifold pressure gauge himself. His videos on WWII techie matters are pretty easy to understand but not blatantly simple minded. Anyway, for your purposes, it does not matter if the engine is normally aspirated, has a turbo, has a single speed centrifical supercharger (aka a blower), has a turbo shoving air into a single speed blower, has a 2 speed blower, or a 2 stage and 2 speed blower, nor does it matter if you're at sea level or seeing how fast an F8F-2 will really go at FL 210--throttle controls manifold pressure; more throttle increases manifold pressure; the prop control tells the prop how fast to spin regardless of how much manifold pressure is being applied. You as heir to the Black Sheep or Adolf G or whoever turns you on need only to consult the POH/dash 1 to find out what MP is OK with what RPM. The tach and the MP gauge will have markings that reinforce your memory. Hang in there. Old airplanes are not that complicated to fly, they just bite hard if you don't understand them.
 
tommayer said:
Am sitting around trying to avoid working on my CFI renewal on line.
Click to expand...
On line huh? Must be nice to be able to do it from home like that. I used to have to go to a seminar every year. As the only guy in a 141 school with instrument and multi instructor tickets, I got stuck with chief instructor duties and wound up "supervising" guys twice my age who had taught me in earlier days. Not the happiest scenario.
Liked your post on RPM/MAP.
Cheers,
Wes
 


best, Tom

Wes,
Just finished proving I can read runway markings right 10 times out of 10. Feel all tingly with self satisfaction.
I have been doing the CFI drudgery on line for years. A matter of sheer necessity. I think AOPA is the best of several I've tried. I live in the boondocks in NM some of the year, in Mexico most of the year. Nobody holds seminars in rural Nayarit. AOPA online eFirc charges $128.00 (I think). Whatever it is, it's much cheaper than flying my Staggerwing to PHX or ABQ. No matter how you do it, it gets old, no? I HATE this new ACS stuff.
Where are you? Sometimes I can still arrange rides in a 47 working strips in the sierra that make anything in Alaska look like LAX. For a few years one had an 1820 on one side and an 1830-94 on the other. Kind of fun to fly something where the throttles are split 6 inches in cruise.
Who'd you fly for?

best, Tom
 
Times have sure changed when I went to A&E school (1951), I was told the purpose of the supercharger wa to bulls-t the carburetor. That is, to make it think it was at sea level. Never knew it was such a complicated subject. Keep at it, guys. Someone may learn something, although probably not I. I just know that the turbos on the B-36 ruined my hearing in the days before ear plugs. In the early days, they pulled 64" Hg on the ground during run up. What a scream!
 
tommayer said:
Who'd you fly for?
Click to expand...
Navy Key West Flying Club, Mansfield Aviation, Vermont Flying Service, Scan America Corp, Sugarbush Soaring, Brockway Air/Metro Air Northeast, and Wiggins Airways. A "checkered resume", you might say. All little stuff, nothing bigger than a Beech 1900.
Which engine does your Staggerwing have? I've flown a bunch of different Beeches, but all were of the "modern" variety. Staggerwings make me drool. My first complex was a T34 at the Navy club, in the days of young and foolish. That plane kept me alive through many a stupid (but educational) escapade.
Do you instruct for a school, an FBO, or on your own? A lot, or a little from time time, just for grins?
Back in the 80s, there was a local cancelled check and UPS feeder operator who had a small fleet of C47s done up in D-Day color schemes. One 1820 and three 1830s. They came into MPV twice a day for UPS, I knew most of the guys who flew them, but the promised ride never materialized. I eventually flew that UPS route for Wiggins in a Be99.
Cheers,
Wes
 

I posted two videos explaining exactly how that all works within the context of supercharging on 28th, same day the question was posted. Does mean you have to actually sit through the videos though....
 
Snowygrouch said:
Does mean you have to actually sit through the videos though....
Click to expand...
Thanks for the heads up. Finally sat all the way through the second video, and found the only clear explanation of the constant speed propeller near the end of that very long, poorly executed video. Calum needs to learn a bit about videography. The info is great, but the poor video and audio makes it hard to absorb.
Cheers,
Wes
 
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