# Horsepower, RPM, and Supercharging



## Zipper730 (Jun 28, 2019)

I know engines have a range of RPM's they work at and I know mechanically driven superchargers are geared to the shaft and spin at a given gear ratio, ratios, or range of ratios; I also know that superchargers produce disproportional compression when their RPM is ramped up.

The thing is how do you end up producing full RPM without over-boosting the hell out of the engine? I know the throttle isn't all the way forward when you're taking off and is run progressively further forward until critical altitude is reached () and that sort of thing.

How does boost get varied independent of engine RPM. It's probably something I should have asked right away when joining, but it somehow never popped into my head.

The only real knowledge I have with reciprocal engines doesn't involve manifold pressure: It's not a measurement on cars, there's RPM, the gear you're in (if you don't have an automatic transmission), and your speed. In the US we don't have to get our license on a stick to be able to drive a stick, we can learn on anything and the DMV favors people to drive an automatic (you have less items to work against you for not shifting smoothly). Once you got the license, I actually did learn how to drive stick to a degree (meaning I drove my mother's older car, and sometimes acted as a designated driver because I don't drink), but the car I ended up with had an automatic transmission (the biggest difference is you have an extra pedal, and you shift at 2000-3000 RPM).

That all said, pushing the gas-pedal produces an increase in RPM: You'd figure pushing the throttle forward would increase boost and RPM.


----------



## wuzak (Jun 28, 2019)

Zipper730 said:


> I know engines have a range of RPM's they work at and I know mechanically driven superchargers are geared to the shaft and spin at a given gear ratio, ratios, or range of ratios; I also know that superchargers produce disproportional compression when their RPM is ramped up.
> 
> The thing is how do you end up producing full RPM without over-boosting the hell out of the engine? I know the throttle isn't all the way forward when you're taking off and is run progressively further forward until critical altitude is reached () and that sort of thing.
> 
> How does boost get varied independent of engine RPM. It's probably something I should have asked right away when joining, but it somehow never popped into my head.



I think you've answered your own question.

It's the throttle. It controls teh mass air flow through the supercharger and, consequently, the boost.




Zipper730 said:


> That all said, pushing the gas-pedal produces an increase in RPM: You'd figure pushing the throttle forward would increase boost and RPM.



That's not actually true. 

If you are driving along a flat road at 50mph and then the goes into a climb, you will have to increase the throttle to maintain speed, and for a fixed gear ratio (manual or auto with torque converter locked), rpm.

Opening the throttle admits more air and fuel into the engine to make more power and torque, but will only lead to an increase in speed and rpm if the power is in excess of the load. If the power is less than the load the rpm and speed will drop.


----------



## pbehn (Jun 28, 2019)

Don't they have a waste gate?


----------



## wuzak (Jun 28, 2019)

pbehn said:


> Don't they have a waste gate?



Turbochargers use a wastegate to adjust the amount of exhaust going through the turbine, which controls the speed and boost.

Mechanical superchargers don't have that.

Modern turbos can also have a blow-off valve, which releases excess boost after the compressor. They could do that with a mechanical supercharger, but I don't believe they did in WW2.


----------



## pbehn (Jun 28, 2019)

wuzak said:


> Turbochargers use a wastegate to adjust the amount of exhaust going through the turbine, which controls the speed and boost.
> 
> Mechanical superchargers don't have that.
> 
> Modern turbos can also have a blow-off valve, which releases excess boost after the compressor. They could do that with a mechanical supercharger, but I don't believe they did in WW2.


How did they set and main maximum boost, on say a Merlin?


----------



## wuzak (Jun 28, 2019)

Throttle plate on the carburetor.

Reactions: Agree Agree:
1 | Like List reactions


----------



## Shortround6 (Jun 28, 2019)

There was an air pressure capsule with piston the compared the pressure in the capsule to the outside air and was linked to the throttle plate (or equivalent) so that the throttle could only be fully opened when the pressure difference reached a certain value. as the pressure difference decreased (lower altitude) the mechanism slowly restricted the throttle opening until it reached a preset limit when the pressures were equal. 
That is the basic set up, there were often additional bits and pieces to allow overriding (WEP) throttle openings. 
Some early American aircraft had no such mechanical restriction and while it allowed the pilots to use much higher pressures in combat it also allowed for some dangerous overboosting in an emergency situation, pilot rams the throttle all the way forward and then gets occupied trying to to do an evasive maneuver or recover from a strange attitude and by the time he can look at the pressure gauge and adjust the throttle the engine has gone well over even any common WEP pressure.


----------



## wuzak (Jun 28, 2019)

You are describing automatic boost control?


----------



## Snowygrouch (Jun 28, 2019)

> The thing is how do you end up producing full RPM without over-boosting the hell out of the engine?



This is all very thoroughly described here:

https://youtu.be/Pm4SaAnPtYI  

...and with respect to some different throttling methods even more thoroughly described here:

https://youtu.be/HwALos8CwJ0

Reactions: Like Like:
1 | Like List reactions


----------



## pbehn (Jun 28, 2019)

I really do hope there was some means to control it, if not all I have read about max boost and WEP and max climb must be horth thit.


----------



## MIflyer (Jun 28, 2019)

There was an article about the RAF finding some Tomahawk I's still in their crates and deciding they may as well put them togther and use them for something, specfically for training bomber gunners. They put them toghter and stuck some relativley inexpericed pilots in them. The first one pushed in the thgrottle and went screaming off, impressed with the P-40's perfor,ance, then then ... "Bam!" The Allison blew its top from over boosting. In "Winkle" Brown's excellent descriptions of flying various airplanes he always notes how that with the early Allison's you had to be careful not to overboost; the Merlin the Brits were used to had a manifold pressure regulator.

I read where when the Japanese hit the PI, a P-40E pilot ran to his airplane, shoved in the throttle, and took off, desperately trying to build up some speed and altitude. Then he looked at his manifold pressure gauge and was distressed. It only showed something like 10 inches; obviously his engine had a bad problem. But as it turned out the gauge actaully was showing something like 70 inches. The needle had gone all the way around past the 60 inch mark and began a new circuit of the dial.


----------



## Zipper730 (Jun 28, 2019)

So basically, to summarize...

Superchargers designed for altitude are, effectively oversized
This takes more horsepower off the shaft to drive it
For them to break even (the pressure produced adds enough HP to compensate for the HP taken off to drive it), you'd over-boost the engine at sea-level
The engine runs innately with a horsepower reduction at low-altitude for the given manifold pressure
That said, as the altitude goes up, they can keep producing full manifold pressure well above what a naturally aspirated engine would do (or an engine with a sea-level supercharger).
All altitude superchargers are sea-level superchargers if the maximum manifold pressure could be raised high enough... (yeah, I'm taking that to the nth degree...)

Airflow is generally reduced via a butterfly-valve
The movement of the throttle adjusts the butterfly-valve
This gives enough air to the engine to maintain the right RPM, without over-boosting
The butterfly-valve effectively imposes additional losses because of the fact that it forces the supercharger to run at a higher pressure ratio (unsure I grasp)


----------



## Zipper730 (Jun 28, 2019)

As for the Swirl-Throttle set-up: This reminds me a little bit of a variable guide vane used on a jet-engine. It seems to reduce the throttling loss to nearly zero, which makes me wonder if the Russians were the first to think this up...


----------



## wuzak (Jun 29, 2019)

Zipper730 said:


> So basically, to summarize...
> 
> Superchargers designed for altitude are, effectively oversized



Yes. If the supercharger was designed to give the rated boost at sea level, then the engine would perform like a non-supercharged engine, but with more power. The exception is if the engine was used in conjunction with a turbo designed to provide sea level conditions up to its critical altitude.




Zipper730 said:


> This takes more horsepower off the shaft to drive it
> For them to break even (the pressure produced adds enough HP to compensate for the HP taken off to drive it), you'd over-boost the engine at sea-level



Yes, the supercharger will take more power to drive than a lower altitude rated supercharger because it is putting more work into the air.

Not sure what you are trying to say.




Zipper730 said:


> The engine runs innately with a horsepower reduction at low-altitude for the given manifold pressure
> That said, as the altitude goes up, they can keep producing full manifold pressure well above what a naturally aspirated engine would do (or an engine with a sea-level supercharger).



Yes, for a throttle and geared drive system like the Merlin and V-1710. May be different for a supercharger with variable speed drive.

Yes, that's the whole point of supercharging.




Zipper730 said:


> All altitude superchargers are sea-level superchargers if the maximum manifold pressure could be raised high enough... (yeah, I'm taking that to the nth degree...)



Um, what?

A sea-level supercharger is one where the critical altitude or full throttle height is sea-level. An altitude supercharger is where the critical altitude or full throttle height is above seas level, at some nominal altitude.

If we use the British term, boost is the pressure above sea-level atmospheric pressure. Thus +18psi is 18psi above standard sea-level pressure no matter at what altitude the engine is. Maximum boost at maximum rpm or max continuous rpm (3,000rpm and 2,850rpm, respectively, for most Merlins) gives positive boost up to very high altitudes.

Cruising settings, as for most economical, use low rpm and boost, which can go slightly negative boost at altitude. 




Zipper730 said:


> So basically, to summarize...
> 
> Airflow is generally reduced via a butterfly-valve
> The movement of the throttle adjusts the butterfly-valve
> ...



This is a supercharger map for a modern centrifugal supercharger (like those used in most supercharge WW2 engines).





Vortech Superchargers 

The bottom axis is mass air flow 
The vertical axis is pressure ratio.
The curved lines left to right are constant compressor rpm.
The curved lines down to up (plus the loops) are lines of constant efficiency.

Depending where you start on the map, decreasing mass flow can increase or decrease the pressure ratio.

For WW2 engine superchargers it can only have reduced teh pressure ratio, otherwise boost would have been higher.

If anybody has a good map of a Merlin, or any other, supercharger, that would be good.

[Note: modern superchargers and turbocharger maps for car applications will often use volumetric air flow rather than mass air flow]


----------



## Snowygrouch (Jun 29, 2019)

wuzak said:


> For WW2 engine superchargers it can only have reduced teh pressure ratio, otherwise boost would have been higher.



Hmmm try not to confuse the pressure RATIO of the compressor with the subsequent ABSOLUTE pressure available to the engine, a slightly *higher* pressure ratio
acting on a considerably lower ABSOLUTE inlet pressure (caused by inlet throttling at constant compressor speed) can provide *lower* ABSOLUTE boost pressure.

Your request to see WW2 compressor maps is satisfied in the 2nd video I posted here:



and...here



Sadly the audio and video quality is rubbish - but luckily the actual information is not. Sorry I dont know why its decided to put huge
thumbnails of my face this time when it didnt before... anyway.


----------



## wuzak (Jun 29, 2019)

Snowygrouch said:


> Hmmm try not to confuse the pressure RATIO of the compressor with the subsequent ABSOLUTE pressure available to the engine, a slightly *higher* pressure ratio
> acting on a considerably lower ABSOLUTE inlet pressure (caused by inlet throttling at constant compressor speed) can provide *lower* ABSOLUTE boost pressure.



Yeah, forgot about the pressure loss across the throttle.


----------



## Zipper730 (Jun 29, 2019)

wuzak said:


> Yes. If the supercharger was designed to give the rated boost at sea level, then the engine would perform like a non-supercharged engine, but with more power. The exception is if the engine was used in conjunction with a turbo designed to provide sea level conditions up to its critical altitude.


Understood


> Not sure what you are trying to say.


If I had a sea-level supercharger, it would take off a certain amount of horsepower to drive it, but it would, presumably boost the pressure enough to raise horsepower beyond the amount that the driving of the impeller took off.

If the engine could be boosted hard enough, eventually you'd hit that point, the key word is "if you could boost the engine that much". My father was fond of saying "if the Queen had balls, she'd be King".


> Um, what?
> 
> A sea-level supercharger is one where the critical altitude or full throttle height is sea-level. An altitude supercharger is where the critical altitude or full throttle height is above seas level, at some nominal altitude.


It's a joke. It was just taking the idea of boost to it's logical conclusion.

For example if I could maintain 52.8" MAP to 5900 feet with one stage of supercharging; then with two stages in low gear, you'd be able to go to 14000 feet, with less horsepower, but overall, more speed as you'd be chopping through thinner air; should one be able to spin those two stages in high gear, possibly something like 23800 feet would be possible on the existing manifold pressure, with even less horsepower (yes this is lifted off the F4U, though the figures might be a tiny bit off here and there). Should I increase the maximum manifold pressure, the critical altitude would go down because I can't produce that pressure at higher altitudes in the thinner air, but it can be produced in thicker air.

That said, if the engine could be boosted hard enough, eventually you would reach some comically absurd manifold pressure that would be impossible to achieve at any altitude but sea level because the air is too thin at any other altitude (Yeah, I know, lame).


> This is a supercharger map for a modern centrifugal supercharger (like those used in most supercharge WW2 engines)


What I find strange is that many engine charts often seem to list BHP as a straight line right on up to the critical altitude rather than revealing the pumping loss. I'm not sure why airplane performance graphs don't show that kind of thing -- you'd think it'd be important.


> Depending where you start on the map, decreasing mass flow can increase or decrease the pressure ratio.


I was working along what Calum posted, he said something about raising the pressure ratio.



Snowygrouch said:


> Hmmm try not to confuse the pressure RATIO of the compressor with the subsequent ABSOLUTE pressure available to the engine, a slightly *higher* pressure ratio acting on a considerably lower ABSOLUTE inlet pressure (caused by inlet throttling at constant compressor speed) can provide *lower* ABSOLUTE boost pressure.


I have a hunch I'm probably totally misinterpreting this (and I haven't really slept in almost 24 hours), but the butterfly valve closing, does that produce an area change like a bell-mouth? If so I could imagine that an undersized bell mouth would produce a pressure gain but not enough air...


> Sadly the audio and video quality is rubbish


I can understand you just fine


----------



## Snowygrouch (Jun 29, 2019)

Zipper730 said:


> What I find strange is that many engine charts often seem to list BHP as a straight line right on up to the critical altitude rather than revealing the pumping loss. I'm not sure why airplane performance graphs don't show that kind of thing -- you'd think it'd be important.



Depends which system their using.....

A turbocharged plane can alter the compressor shaft speed to provide the correct boost level without throttling (by using a wastegate) - similary
any engine with a swirl-throttle has an ALMOST flat BPH/altitude graph because the throttling losses (even with constant speed compressor geared
to the crank) are so low.

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).


----------



## MIflyer (Jun 29, 2019)

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. One BF-109 pilot said he was sneaking up behind a couple of P-38's at low altitude when they spotted him, poured on the coal, and pulled away from him at a rate he found to be absolutely incredible, "They just disappeared!"

An A-36A in the Med shot down an FW-190 that was being ferried to an operational unit. When the German pilot met the American who got him he asked, "How in the world did you catch me?" Well, the A-36A best performance was set up for about 5000 ft; there probably was not much it could not catch down low. 

On early P-38's the turbos were known to come apart at high boost. They added a shield on each side to protect the pilot from turbo shrapnel and changed the V-1710 supercharger gear ratio to get more pressure out of the mechanical supercharger. What this change did to performance I do not know.


----------



## pbehn (Jun 29, 2019)

Zipper730 said:


> That said, if the engine could be boosted hard enough, eventually you would reach some comically absurd manifold pressure that would be impossible to achieve at any altitude but sea level because the air is too thin at any other altitude (Yeah, I know, lame).


If you create enough boost then combustion occurs as soon as fuel is introduced. You can then dispense with the cylinders and other stuff and concentrate on refining the development of your jet engine.


----------



## Shortround6 (Jun 29, 2019)

MIflyer said:


> On early P-38's the turbos were known to come apart at high boost. They added a shield on each side to protect the pilot from turbo shrapnel and changed the V-1710 supercharger gear ratio to get more pressure out of the mechanical supercharger. What this change did to performance I do not know.




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.

Reactions: Like Like:
1 | Like List reactions


----------



## MIflyer (Jun 29, 2019)

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.


----------



## Snowygrouch (Jun 29, 2019)

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



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.

Reactions: Like Like:
2 | Like List reactions


----------



## MIflyer (Jun 29, 2019)

Article by Bill Gunston.


----------



## Zipper730 (Jun 29, 2019)

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).


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

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

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.


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.


----------



## pbehn (Jun 29, 2019)

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.


I think you missed my point, many people realised you could cut out the piston and crankshaft middle man.


----------



## Zipper730 (Jun 29, 2019)

pbehn said:


> I think you missed my point, many people realised you could cut out the piston and crankshaft middle man.


Well with a turbocharger at least...


----------



## Shortround6 (Jun 29, 2019)

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.



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.

Reactions: Like Like:
2 | Like List reactions


----------



## Zipper730 (Jul 1, 2019)

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.


----------



## mad_max (Jul 4, 2019)

From what I read the German Jumo engineers copied the swirl throttle from the Russian Am Series of engines.

Here's one piece about it.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030068451.pdf

The fliud drive of the DB series is not much different than an old automatic transmission. There's a loss of efficiency because of slippage to take into account.

Reactions: Like Like:
1 | Like List reactions


----------



## tommayer (Jul 4, 2019)

Zipper730 said:


> I know engines have a range of RPM's they work at and I know mechanically driven superchargers are geared to the shaft and spin at a given gear ratio, ratios, or range of ratios; I also know that superchargers produce disproportional compression when their RPM is ramped up.
> 
> The thing is how do you end up producing full RPM without over-boosting the hell out of the engine? I know the throttle isn't all the way forward when you're taking off and is run progressively further forward until critical altitude is reached () and that sort of thing.
> 
> ...


 
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.

Reactions: Bacon Bacon:
1 | Like List reactions


----------



## XBe02Drvr (Jul 4, 2019)

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.


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

Reactions: Agree Agree:
1 | Like List reactions


----------



## tommayer (Jul 4, 2019)

XBe02Drvr said:


> Zipper730, post: 1485484, How does boost get varied independent of engine RPM?
> 
> 
> 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!
> ...


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.

Reactions: Like Like:
1 | Agree Agree:
1 | Winner Winner:
1 | Like List reactions


----------



## XBe02Drvr (Jul 4, 2019)

tommayer said:


> Am sitting around trying to avoid working on my CFI renewal on line.


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


----------



## tommayer (Jul 4, 2019)

XBe02Drvr said:


> 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


XBe02Drvr said:


> 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


 
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

Reactions: Like Like:
1 | Like List reactions


----------



## 11bwmech (Jul 4, 2019)

XBe02Drvr said:


> Zipper730, post: 1485484, How does boost get varied independent of engine RPM?
> 
> 
> 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!
> ...


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!


----------



## XBe02Drvr (Jul 4, 2019)

tommayer said:


> Who'd you fly for?


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


----------



## Snowygrouch (Jul 5, 2019)

XBe02Drvr said:


> Zipper730, post: 1485484, How does boost get varied independent of engine RPM?
> 
> 
> 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!
> ...



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....

Reactions: Funny Funny:
1 | Like List reactions


----------



## XBe02Drvr (Jul 5, 2019)

Snowygrouch said:


> Does mean you have to actually sit through the videos though....


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


----------



## Snowygrouch (Jul 5, 2019)

XBe02Drvr said:


> very long, poorly executed video. Calum needs to learn a bit about videography



Feel free to list a suggested improved story-board, and a list of recommended equiptment.


----------



## XBe02Drvr (Jul 5, 2019)

Snowygrouch said:


> Feel free to list a suggested improved story-board, and a list of recommended equiptment.


Well, to start with, more diffuse and brighter lighting would make the board easier to read, along with broader, more contrasty markers with more vibrant colors. This isn't rocket science. Two or three cheap clip lamps with daylight LED bulbs and thin, whispy white muslin or cheesecloth draped over the front, strategically aimed, can do a lot for short money. The goal is multi source light and shadow minimization.
The audio leaves a lot to be desired. It sounds like a cheap consumer grade video camera's built in mike. A shotgun mike mounted on the camera or on a stand in front of the lecturer, but out of the picture, would help. A "studio room" with sound dampening(even some heavy drapes or rolls of fiberglass) would also help if you can manage it.
When editing, it pays to address the audio level in the finished product, as these videos were a little on the low side. I've seen many student videos where truly stunning camera work was marred by too loud, too soft, distorted, or inconsistent audio.
When you've got your final edited product, I would suggest "test flying" it on as many different platforms as you can access. A production that looks good on your typical editing computer may not look or sound so good on an Android, an iPhone, a tablet or other device.
I hesitate to recommend specific models of equipment, as I've been retired now for four years, and the marketplace has changed. The school I worked for couldn't afford to outfit an entire class with professional grade equipment, so I used to buy "prosumer" stuff, and had good luck with Panasonic. If I was going professional, I would probably go with Canon or Sony, but not without a lot of research first. Until you get into top grade professional work, it's really less about the equipment you use and more about what you do with it. I've seen stunning videos shot by teenagers on Flip cams and carefully edited, and sloppy stuff done by rich kids who "borrowed" their parents' professional equipment and brought it to school to show off.
Well, my apologies for getting so deep into lecture mode. Hope this helps.
Cheers,
Wes


----------



## tommayer (Jul 5, 2019)

tommayer said:


> best, Tom
> 
> 
> Wes,
> ...


----------



## tommayer (Jul 5, 2019)

XBe02Drvr said:


> 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.
> ...



Wes,
Just read your reply, 7/5. 
Sounds like you had a neat career.
I was a college English teacher who worked merely to eat and to pay for airplane parts. Back in the dark ages I did some instructing, but found I hate flying with people who don't know how to fly. Now confine myself to BFRs for people I like who have airplanes I like. Spent ten years making a video of Mexican bush flying. Nobody wanted to see it but the guys in it, and they're mostly dead now. One of the places I went to regularly was Chapo's home town, though I did not know that until years later. I spray mangoes for a friend sometimes. 
Staggerwing. Have had the thing since 1970. Got it cause I was working off a rocky, hump-backed 2300 foot strip at 6700 msl & high OATs. I wanted a C-185, but, back then, the old Beech was cheaper. Found out via experience it is better at high DAs too. Fun factoid. Manual from WWII says a GB-1-2 (the Navy version of the D-17S, P&W R-985 AN1 or 3) needs only 50 feet more on T/O to clear 50 feet at a DA of 6000 than at SL. Ground run at 6000 is a shade longer, but the crate climbs a shade better up high. 
They are fun to fly. Feel a lot like straight tail Bonanzas or T-34s, only they have better rudders. You'd think you were back in your 34, but suddenly had enough power and enough rudder to really hold the stops in a point roll. 
Visibility on the ground sucks, as you'd imagine. Touchdowns and roll outs are easy, if you have educated feet. One oddity: they have a 15:1 glide ratio clean. Drop the gear and flaps and you need almost 25 inches to hold 90-100 IAS in level flight. Pull power on the backside of the curve with flaps + gear extended, and you've turned a near sailplane into a free-falling elevator. I check people out by making them treat 100 IAS as if it were a blue line. This raises eyebrows, since the thing actually stalls at 50-55 IAS, so 100 IAS is a lot higher than 1.3-1.5 Vso. But as soon as you break the glide it's as if you'd run into a pillow, amazing deceleration is instantaneous. Best ROC is also 90-100, at which speeds torque is no problem if you need to go round. 
Full span ailerons give you pretty good roll rate, but acro involves some muscle. They'll loop from cruise, though of course that's something I've never tried. They'll carry anything. I'd tell you true stories, but some Fed might read this. I have invented my own CAFE race to specs I know will make a Beech 17 the winner, 1) T/O from muddy strip of 1500 feet at 5000 msl, 50 foot cement barrier at 1750 feet. 2) You get extra points for each SOB besides the pilot. The Beech will have 3 fatties or four skinnies in addition to me. 3) course is 500 nm. 4) MEA for some of it is 15K 5) big penalty for any TAS under 160 K 6) Destination also muddy, unimproved, less than 2000 feet usable, and 5000 msl or higher. 7) You are disqualified if you don't land with 1.5 hours of fuel remaining. 8) W&B must be within factory limits. 9) No Porters or C-130s or turbine Gooney birds allowed.
If you ever come to northern NM or southern Sinaloa, let me know.

best, Tom


----------



## tommayer (Jul 5, 2019)

11bwmech said:


> 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!





Snowygrouch said:


> 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....



Again--just open the throttle if you want a bigger explosion in each jug. The prop governor will keep the RPM the same. Get out of your head the idea that a C/S prop responds like a car gear box when you goose or retard the throttle. It does NOT.


----------



## XBe02Drvr (Jul 5, 2019)

tommayer said:


> I did some instructing, but found I hate flying with people who don't know how to fly.


I always enjoyed flying with people who, by the time I was done with them DID know how to fly. Have ridden through some unintentional aerobatics perpetrated by ham handed students but due to my time early on in the T34 and 150 Acrobat, didn't have any problem with it. Almost none of my students soloed without first demonstrating spin recoveries. My first two or three did slip through unspun, but then I cracked down and started sending "spinaphobes" to other instructors.
Love that 985. The local duster outfit back in the day had two W670 Stearmans and one 985. The Continentals would shake, rattle, and roll at idle, while the 985 would just tick over quietly, steady as a rock. Duster pilots were unwelcome in the terminal building, so if they needed to make a comfort stop, they would land in the grass, climb out, and visit the bushes. The 670s had to shut down and restart, but that 985 would just sit there ticking over and waiting for her master's return. They made a point of doing this in a spot visible from the restaurant windows.


tommayer said:


> If you ever come to northern NM or southern Sinaloa, let me know.


Thanks, Tom, but not likely. Vermont summers are too hot for me as it is. We drove through ABQ on the old Route 66, on our way to LA when I was a kid, and I liked to have died. If you're ever in New England, give me a shout. I don't fly any more (eyes too bad), so I've turned into a water rat.
Cheers,
Wes


----------



## Sid327 (Sep 18, 2019)

Definitely recommend tommayer's post #33

about recommending this site:

Greg's Airplanes and Automobiles


----------



## Zipper730 (Feb 1, 2020)

Snowygrouch


So the problem with the butterfly valve and throttling losses are that...

You have a lower absolute inlet pressure due to throttling
The restricted airflow is forcing the compressor to work harder, and getting nothing out of it like if I was trying to breathe with a pillow in my face?
Does the higher pressure ratio produced have anything to do with the angle of the airflow relative to the blade? I'm just curious because the swirl throttle looks like the variable stator vanes seen on General Electric's J79...


----------



## tomo pauk (Feb 2, 2020)

Snowygrouch said:


> 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.
> 
> ...



To come back a bit here.
Would this not be a more fair comparison: Merlin X (gear-driven supercharger) vs. DB 601A?


----------



## Snowygrouch (Feb 4, 2020)

Zipper730 said:


> Snowygrouch
> 
> 
> So the problem with the butterfly valve and throttling losses are that...
> ...



It is very difficult to answer that fully without lots of maths - which I already made a video on, but...

1) Yes the butterfly (or barell) throttle work by reducing the area of the intake, since the engine still wants to ingest a certain volume of air, 
it will then give you a higher speed flow through the throttle, which is then at lower pressure (bernoulli).

2) The restricted flow does NOT make the compressor work harder, it uses less power because there is less mass of air flowing through, 
HOWEVER.... it would use EVEN LESS power, if you simply reduced the speed of the supercharger, instead of closing the throttle. The difference
in a 1000hp class aero engine is easily 100 to 150bhp at sea level (where the throttle is at its most closed position).

The swirl throttle does (eventually) reduce the flow area when its virtually shut, but its not really made to work like that, by spinning the air entering 
the compressor, it basically fools the supercharger into acting as if it IS spinning more slowly, because the spinning air off the 
vanes, effectively means that relative to the air, it IS spinning more slowly.

Variable stator vanes like the J79 are made to stop the air stalling when the compressor is working "off-design-point", not as a throttle.

However, a swirl throttle, can also be used to help the airflow into the supercharger valves come in at a more favourable angle when its running
at very low speeds, which can also extend the operational range of the supercharger before it stalls (meaning it will be able to run at 
slower speeds and still work).

You can do lots of things with variable vanes, but in a Jumo213 or AM35/38, its being used primarily as a throttle.

Reactions: Like Like:
2 | Like List reactions


----------

