manifold pressure and rpms

[smoe]

Just an idea, are prop RPM's calculated using the number of blades?

I.E.
So, Kvuo75 would need to divide 1754 feet per second by the number of prop blades for the 3000 rpm calculation.


Another Example: A 4-cylinder, 4-stroke engine sharing the same shaft as a 2-blade prop (no gear in between).
In one 720 degree turn (for simplicity) on the shaft, the prop and engine revolutions would count as four (4) each?
Both prop and engine RPM = 4 / (time in minutes for a prop and engine shaft to spin two (2) complete revolutions, aka 720 degrees)

Basically this example is a 1:1 ratio, change the number of prop blades while keeping the same engine and the prop and engine RPM's are no longer 1:1.

[SIK1]

RPM is rpm regardless of the number of blades, or cylinders for that matter. RPM is the number of times the shaft rotates in a minute. Kvuo's calculation has more to do with the props diameter than it does with the number of blades. The larger the diameter the faster the tip must travel at a given rpm because it has to travel the same 360 degrees that the shaft does in the same amount of time while inscribing a larger circle.

[FLOOB]

The guy who started the thread was asking about AH planes, and you guys went off on real life planes confusing everyone. First earl says it's engine rpm then he says it's prop rpm, then hitech comes in and says that in every real plane (i'm assuming he meant real) he's been in the rpm guage indicates engine rpm.

So could someone please answer the poster's question. In AH warbirds what does the rpm guage indicate?

[earl1937]

The guy who started the thread was asking about AH planes, and you guys went off on real life planes confusing everyone. First earl says it's engine rpm then he says it's prop rpm, then hitech comes in and says that in every real plane (i'm assuming he meant real) he's been in the rpm guage indicates engine rpm.

So could someone please answer the poster's question. In AH warbirds what does the rpm guage indicate?

I did make a mistake to start with, but to answer you own question, next time you fly a B-17, after getting into cruise confg, pull your throttles back to 30 inches of MP and watch your tach and see what it does. Then go back to cruise power, 38 inches, then press your minus key on your key pad and watch your tach. I Think that will answer your question.

[hitech]

In AH rpm is engine rpm just like the real thing.

HiTech

[colmbo]

It seems we have a contradiction.  Who's right?

On real world airplanes the tach is showing engine RPM. 

[colmbo]

On aircraft with constant speed (variable pitch) props however, the RPM gauge generally shows prop RPM (which again may or may not be the same as engine RPM). This is just what I have seen on light civil aircraft which I have flown or ridden in.

That is absolutely not correct.  The tach shows engine RPM...that is how it's worked on all the aircraft I've flown   J3 cub to Cessna to T6 to Mustang to B-24.  Tach shows engine RPM.

[FLOOB]

Thank you. Earl I hope your wife is better soon.

[SIK1]

I did make a mistake to start with, but to answer you own question, next time you fly a B-17, after getting into cruise confg, pull your throttles back to 30 inches of MP and watch your tach and see what it does. Then go back to cruise power, 38 inches, then press your minus key on your key pad and watch your tach. I Think that will answer your question.

What exactly is this suppose to prove?

[colmbo]

What exactly is this suppose to prove?

I think he's showing that the prop will maintain RPM with a throttle reduction.

His second example should show a change in manifold pressure with reduction of RPM --- this doesn't always work correctly in AH.

R/L on the B-17, at normal inflight power settings, if you reduce RPM you will get a manifold pressure increase up until 7500 feet or so.  Above that altitude a reduction in RPM will  result in a reduction of manifold pressure.  The engines in the B-17 and B-24 have superchargers built into the engine case, pilot has no control of them.  At sea level you can get 44 inches MAP or more without using the turbochargers (normally aspirated engine would get around 27-29 inches MAP at SL).  At higher altitudes if you reduce RPM you are also reducing output from the supercharger, therefore you'll get a drop in manifold pressure with RPM reduction.

[earl1937]

What exactly is this suppose to prove?

There is no doubt that the "experts" say the tach shows engine RPM's and that is what I have always been taught and for the most part, I guess that is true, but, pull a B-25 or something with full feathering props out on the runway, feather both engines, then push your throttles up and see if you get any increase in RPM's on the tach! (he, he) If the "experts" are right, you would get an increase in RPM's, right? (Seriously, don't actually do that as you probably would blow some jugs off the crankcase). Helluva discussion though, isn't it?

[Puma44]

Ok. I get how a car,motorcycle and boat engines rev and provide power to a degree. but I would love to understand manifold pressure and rpm in regards to our AH warbirds. Earl? thanks in advance.

Just to clarify, are you asking about the theory of it (which has pretty much been discussed) or how to operate and manage manifold pressure and RPM in a realistic manner?

[morfiend]

 Speaking of manifold pressure,does anyone know what exactly ATA stands for?

  I was asked this question and wasn't sure of the exact meaning so I broke out the trusty google and found that even google didn't have a good answer.



   

[Scherf]

If that's German nomenclature, it means atmospheres.

So, 1.42 ATA is 1.42 times atmospheric pressure (at sea level, I believe, but caveat emptor on that).

[SirNuke]

they should have gone with 1455 hectopascals