RPM questions

[Klam]

Having never flown a real plane, just these beautiful cartoon toys.
I have some questions of what the RPM function does in AH and RL.

I have the RPM mapped to an analogue wheel on my X52 throttle.  I use it to gain range when low on fuel.
A car engines RPM is controlled via the throttle.  In AH the engine increases RPM via the throttle but also the RPM function.

Sit on the runway and turn the RPM all the way down and push the throttle full up....only a small increase in RPM occurs.
Then turn the RPM up and the engine will max out (not wep).
Does the RPM function also alter fuel/air mix?
I read somewhere else that the RPM in AH also alters the prop pitch.

Might be a silly question but I know nothing of aircraft engines or how they are governed.

[morfiend]

 Klam,

  The rpm is the prop speed not the engine revolutions like you'd see in you car! You can reduce the rpm of the prop independantly from the engine rpm. The MAP is similar to throttle in a car so 35 in. of map would be like half throttle if 70 in. is max.

  I know it's not a complete explaination but I think it will help you understand.


   

[kvuo75]

Klam,

  The rpm is the prop speed not the engine revolutions like you'd see in you car! You can reduce the rpm of the prop independantly from the engine rpm. The MAP is similar to throttle in a car so 35 in. of map would be like half throttle if 70 in. is max.

  I know it's not a complete explaination but I think it will help you understand.


   

the rpm of the prop is not independent of the engine.  they are directly linked, but yes, the prop controls the rpm

[PuppetZ]

The way I understand it is when adjusting the RPM setting, you change the angle of the propeller blade, making them take a bigger "bite" in the air, thus making it harder to rotate and in turn reducing the engine crankshaft RPMs. A lower RPM at the same MP setting will save fuel because there are less explosion in a given time.

A car engines RPM is controlled via the throttle.  In AH the engine increases RPM via the throttle but also the RPM function.

Partially true. The propeller model in AH is a idealised model of a constant speed propeller. The way a constant speed prop works is by automaticaly adjusting blade pitch to maintain a consatant RPM.

Sit on the runway and turn the RPM all the way down and push the throttle full up....only a small increase in RPM occurs.
Then turn the RPM up and the engine will max out (not wep).

Again partially true. Take a F4U-1A out. Apply full brake and watch the MP and RPM setting. Slowly opening up throttle, you'll see the RPM increase until it max out at 2700RPM. The MP at this point is about 30". Continue to open throttle and MP will max out at 51" and RPM are still at 2700.

If you go back and re-read the 1st part of my post, you'll notice that I said increase blade angle so they "bite" more. What it does is increase the AOA of the blade. It increase the "lift" (or pull in this case) but at the same time it increase the drag the blades generate making them harder to turn. The max design speed for the engine being 2700, the blades adjust their "bite" to maintain that RPM, also providing more pull in the process.

Now maintaining full MP, lower the RPMs. The MP stay where it is down to the point where the supercharger is unable to provide enough compressed air to maintain your desired MP(remember that a supercharger is driven by the engine, so the lower the RPM, the lower the volume of compressed air).

As for fuel savings, a P&W R2800 18 cylinder produce 9 explosion per crankshaft rotation. @2700 RPM, that add up to 24300 explosion/minute(or a whooping 405/seconds!)
The same engine @2400 RPM it's 21600/Minute. At the same(or close to) MP setting, you can easily see where you save fuel.

The very same principle apply for all the planes in AH save for the WW1 planeset and the storch because they are equipped with a constant pitch propeller.

 

edited for clarity

[morfiend]

the rpm of the prop is not independent of the engine.  they are directly linked, but yes, the prop controls the rpm

  I didnt say it was independent of the engine,I said you can control it independent from the engine.   you could have 50 inches map and run 2300 rpm or you could run 2100 rpm.

  I was trying to simplify the explaination so I can see how you might have read it as such.


   

[Oldman731]

The way I understand it is when adjusting the RPM setting, you change the angle of the propeller blade, making them take a bigger "bite" in the air, thus making it harder to rotate and in turn reducing the engine crankshaft RPMs. 

This is a correct explanation of RPM.  The problem is that there is no free lunch.  If you reduce your RPM at the same manifold pressure setting you're putting a greater strain on the motor, which is struggling to do more work without additional power, and which will either go to sleep or reach out and slap your face if you don't change things.  So you need balance.  The engine manufacturers give you tables with options, but essentially you can have lower RPMs with higher MPAs, or higher RPMs with lower MPAs.  One of those combinations will work best for your purpose, balancing engine temperatures with fuel consumption.

- oldman

[Klam]

Thanks for the explanations....I think I get most of it.

So the the pitch of the prop blades are altered to change the AoA.  I'm guessing now that the bite would be less at sea level than at at 20,000 ft where the air is thinner.
The prop speed is constant.  Just the AoA will change with alt or air density.  The pilot in RL relies on an automatic mechanism to look after this.

So at max throttle the engine will still only top out at the RPM set by some sort of governor?

The fuel/air mixture would also change with air density/temp/alt and this setting is also linked to the RPM function in AH?  I read somewhere that it was.

I'd be interested to know what the poor guy had to deal with in RL when a fight could start at 25k and end up at sea level.

Thanks again for the posts. 

[earl1937]

 

Having never flown a real plane, just these beautiful cartoon toys.
I have some questions of what the RPM function does in AH and RL.

I have the RPM mapped to an analogue wheel on my X52 throttle.  I use it to gain range when low on fuel.
A car engines RPM is controlled via the throttle.  In AH the engine increases RPM via the throttle but also the RPM function.

Sit on the runway and turn the RPM all the way down and push the throttle full up....only a small increase in RPM occurs.
Then turn the RPM up and the engine will max out (not wep).
Does the RPM function also alter fuel/air mix?
I read somewhere else that the RPM in AH also alters the prop pitch.

Might be a silly question but I know nothing of aircraft engines or how they are governed.

None of the answers I have read to this thread are correct totally! #1- The RPM gauge shows the RPM of the "crank shaft" of the engine. #2 The prop speed is always less that engine RPM, effected by a gearing reduction in the "Nose Cone" of the engine. These engines are known as "constant speed" prop aircraft. NOW I am only talking of WW2 aircraft and "piston" powered aircraft. Some of the newer engines such as turbine type engines, do in fact have a faster prop speed than the engine! But, as far as aircraft in this game, generally speaking, if your manifold pressure is set at one value and you reduce your engine RPM, you "reduce" your fuel flow to the engine because of the lower RPM. Aces High has the E6B function which does other things beside helping you calibrate your bomb sight. There are usually 3 to 4 different settings, which allows you to get the most "time aloft" from the aircraft which you are in.
Now, if you consider a "fixed" pitch prop aircraft such as the "Storch", the prop and engine RPM are the SAME. You can get engine over speed easily in the Storch, which will result in engine denotation and a failed engine, then sir, your are a glider!

[hitech]

Klam, You are correct, your prop/rpm/pitch control (these are just different names for the same thing) adjust what is in essence a fly ball governor hooked to a oil valve.  As the prop turns faster then the setting of the governor, the governor releases oil one way to make the prop take a bigger bite, I.E. increased AOA and hence the prop slows down to the preset RPM. And visa versa, if it is going to slow oil is sent to the prop to decrease RPM. (Note the oil to or from the prop can be different on different systems, also some planes had electrical controlled instead of oil controlled).

As a plane goes faster the prop must take a bigger bite to maintain RPM. With fixed pitch prop systems like in WWI, as you start going faster the engine increase RPM, at some point you must back off on the throttle (I.E. lower torque) to not over rev the engine.

Power = RPM X Torque. Hence lowering your RPM will lower Horse Power in the engine. (note torque RPM curves vary but for any plane engine peek power is always at max RPM setting of the governor).

As earl1937 states prop RMP is slower the engine RPM on most WWII planes, this is not always true for slower less powerful aircraft many times they are 1-1. But the tach always shows engine RPM, the prop and engine are always in a fixed gear ratio to each other.

The throttle is no different then the throttle on normal gas engines. It simply opens a butterfly valve that allows more air into the engine and , fuel flow is also increased but the method may vary as to how it maintains the same approximate fuel /air mixture.

So as you increase throttle, more air and fuel flow into the engine. This produces more torque to the prop, the increased torque accelerates the engine RPM which is then lowered by the governor increasing the pitch of the prop back to the original RPM.

HiTech