Author Topic: RPM and MAP (Read 946 times)

F4UDOA · « **on:** January 31, 2003, 01:53:21 PM »

It was just pointed out much to my surprise that we actually have seperate RPM control by using the +/- keys.

My question is this.

Why on earth is the manifold pressure mapped to the throttle instead of RPM? RPM after all is a function of the throttle not manifold pressure.

In fact I'm not sure what the effect of reducing MAP without reducing RPM should be.

Can this mapping be changed? It doesn't seem possible.

ra · « **Reply #1 on:** January 31, 2003, 02:02:46 PM »

Quote

Why on earth is the manifold pressure mapped to the throttle instead of RPM? RPM after all is a function of the throttle not manifold pressure.

You are wrong about this. Throttle controls engine (MAP), prop control is for prop pitch, which is set for a desired RPM.

ra

Batz · « **Reply #2 on:** January 31, 2003, 02:30:11 PM »

watch some of those training films at Zenos drive in to see what ra means.

He has f4u and f6f film there.

But you can map your rpm to your throttle if ya want, it will be "wrong" though.

I have rpm mapped to the throttle on my ch fighter stick.

Throttle is on is my ch pro throttle. The only time I touch rpm is to decrease drag when my eng is knocked out. From some quick tests it doesnt seem that rpm effects fuel usage.

F4UDOA · « **Reply #3 on:** January 31, 2003, 03:07:00 PM »

Huh??

RA, I know the throttle controls the MAP. I would rather have it control RPM however since RPM has more direct control over the engine. We don't have prop pitch control here. Then again I could be wrong.

hazed- · « **Reply #4 on:** January 31, 2003, 03:25:35 PM »

I think in AH prop pitch is mapped so that if you reduce RPM you pitch the props to a more aerodynamic position,

At least thats what i thought.

Terror · « **Reply #5 on:** January 31, 2003, 04:01:25 PM »

I believe all the planes have "Constant Speed Props". So prop RPM is NOT a function of manifold pressure (ie. throttle setting). The prop uses blade pitch to control RPM.

The prop rpm setting in AH is very important during idle or engine out operation. A high rpm setting creates ALOT more drag than the minimal setting. By reducing RPM in a glide, the glide distance is extended considerably.

From what I have read about AH, reduced prop rpm settings during powered flight does nothing but waste fuel. In real life, both rpm AND power setting are very important for all phases of flight. Particularly for effeciency of cruise flight.

Terror

J_A_B · « **Reply #6 on:** January 31, 2003, 04:08:51 PM »

The throttle controls MAP. This is now it works in actual WW2 fighters, which is why it works that way in AH. RPM is not throttle and doesn't affect engine power output as much as MAP does.

RPM is controlled separately from the throttle in an airplane. In the system modeled in AH it's essentially used to control prop pitch and fuel economy over long distances. RPM controls prop pitch in AH because AH models the US system--the pitch of the prop automatically adjusts itself for whatever RPM you set. Generally, as you lower RPM the prop will move more towards a fine pitch.

In other words, in AH (and a lot of real WW2 fighters), RPM is your prop pitch control. Technically this is somewhat backwards for some airplanes (like 109's) in which you manually set prop pitch, but that's just a nitpick.

J_A_B

salem · « **Reply #7 on:** January 31, 2003, 05:58:18 PM »

My 2c....

By WW2, most (military?) aircraft had constant speed units (CSU's) linked to the prop. The pilot set a desired speed and forgot about it. The idea behind this was that the engine was allowed to run at a relatively efficient speed regardless of power requirement. A modern interpretation might be that the engine had a 'peaky' torque curve. Moving the thottle changes the power output of the engine. Rather than spinning the prop faster (or slower), the CSU increases the pitch of the blades to absorb the extra power.

From what I understand, a reduced prop pitch (hence rpm) for a given throttle setting should reduce fuel consumption. Although I have yet to test this empirically in AH, it is certainly what was recommended by the RAF for fuel economy.

[ed] As mentioned above, the throttle in a WW2 aircraft almost invariably was linked to the boost (MAP) control. In this way it is fundamentally different from a car engine, but makes sense when you think about it.

SFRT - Frenchy · « **Reply #8 on:** February 01, 2003, 12:31:33 AM »

F4UDOA,

See it like a car, throttle is the accelerator and the gear box is the RPM of the prop.

The manifold (throttle) will control how wide you open the body throttle meaning how much Fuel/Air you put into your cylinders. The more you put, the faster your crankshaft will turn (Engine RPM if you may).

The RPM lever will adjust the blade angle and allow you to take a bigger bite of air. You always want the manifold to be lower than the RPM setting.

If you run a high manifold and low RPM, you have the engine turning like crazy and the propeller trying to take those huge bites of air ... the stress on the crankshaft will be such that it will just brake.

The only time you can run high manifold and low RPM is if you have a gear box between the crankshaft and your propeller ... to absorb the stress.

As far as setting your RPM, when you climb or take off, you want high propeller RPM, so you can create as much thrust as you can. It's like when u are riding a bike cvlimbing a hill, you pedal very fast "effortless" and sloly climb.
In cruise, you can take a bigger bite and save on fuel/noise. Like with your bike when you cruise, pedaling slowly but moving fast.

The RPM will have it's limits. It's not always because you set high RPM that you will have more performances. If your prop is turning too fast, the tips will actually reach the speed of sound and become ineficient. Even general aviations can do it, the very noisy ones on take off ususally, it's pilot stupidity/ignorance mostly. If they would actually lower their RPM, they would climb better.

Would all that make sense to you F4U?

Oh BTW, didn't German aircrafts had "automatic MAN/RPM" linkages?

Glasses · « **Reply #9 on:** February 01, 2003, 12:45:29 AM »

Kommandogerant Yes.,Although the 109 had something similar that controled the RPM also ,however i don't know how it is called.

Batz · « **Reply #10 on:** February 01, 2003, 07:06:56 AM »

Sorry F4u but rpm controls in AH effect prop pitch as JAB said.

Quote

From what I understand, a reduced prop pitch (hence rpm) for a given throttle setting should reduce fuel consumption. Although I have yet to test this empirically in AH, it is certainly what was recommended by the RAF for fuel economy.

This is my understanding as well, but like I said from the quick tests I have done rpm has no impact on fuel comsumption.

-ammo- · « **Reply #11 on:** February 01, 2003, 08:02:06 AM »

Actually, adjusting your prop RPM in AH does affect fuel consumption considerably. At least I can say this for the P-47 models.

-ammo- · « **Reply #12 on:** February 01, 2003, 08:19:06 AM »

just ran a test. Used offline madoe and for expedience, set fuel burn rate at maximum allowable (10.0). Used a P-47D11 with 25% fuel and otherwise clean. Took off from a1 under full power. As soon as I was wheels up I brought the MAP back to 40 inches for both runs. First run I left the prop RPM at max (2700 RPM). The AC ran out of fuel at 2 minutes and 10 seconds. Next run, same AC and conditions, but I dropped the prop RPM to 2100. This time the AC ran out of fuel at 3 minutes and 6 seconds.

I don't know how all the other AC are affected by the RPM controls, but I do know the P-51 behaves similarly.

214thCavalier · « **Reply #13 on:** February 01, 2003, 12:07:30 PM »

Having tested the F6F-5 in the past I can say prop pitch definitely affects fuel consumption.

F6F-5 fuel tests

Chart 3 and 4 will show comparison with diff MAP and RPM. Constants were alt, loadout and cruise speed.

F4UDOA · « **Reply #14 on:** February 01, 2003, 04:13:50 PM »

Great information,

But all I really wanted to know is if I could map RPM to my throttle control with or instead of MAP.

I just think for response it is easier to have RPM adjusted via throttle control. I thought this was more realistic but it seems not to be.

Has anyone changed there RPM or throttle mapping?