Complex Engine Management / Oil on window ?

[jrg]

Hi Ivan, check this link: http://www.shockwaveproductions.com/store/solo/bf109/ scroll down the page to find the engine limitations characteristics (they're for the 109e-4 but ok for the E-3).


There is 3 kinds of designs: Fixed pitch propeller, constant speed propeller, and variable pitch propeller.

- In the first one the blades of the propeller have a fixed angle, so increasing speed or throttle increase RPM. (eg some spits, the Tiger Moth)

- The CSP: (eg: some spits)
With a constant speed propeller the CSU (constant speed unit) adjust the angle of the blades in order to keep a certain RPM, if your speed increase or decrease or if you change the throttle position, the blades angles will be automatically adjusted by the mechanism to keep the rpm at the determined speed (I simplify, if you want more explanations don't bother to ask). So for example if you set your rpm to 3000, the rpm will not change with your speed (simplified again^^).

- The VPP: (eg. the 109)
In a variable pitch propeller (yes the CSP is also a variable pitch prop, but here there is no CSU, I hope I'm not too confusing, tell me if needed. The bf109 uses a VPP) you adjust directly the angle of the blades, the rpm will stay constant only if your speed and the throttle settings are stable (and i think altitude too). If you accelerate, the rpm will increase, because the angle of the blades will not adjust itself, (imagine that you are blowing with differents strenghts onto a small windmill like the ones for children) so you have to reduce the lever (which in fact increase the angle of the blades) to reduce rpm. If you are slowing down, you must reduce the angle (pushing the lever) to keep your desired RPM, wheras a CSP (constant speed propeller) will adjust the angle to maintain the same RPM on it's own.

The settings for cruise dive, military power (see the link) are a combination of RPM and Manifold pressure/Boost (the gauge in ATA). Thoses settings are defined by the manufacturer of the engine as the best settings for the purpose. For example if you want the max power see the charts and set your MP to 1.3ata and your rpm at 2400. Increasing the rpm in this case above 2400rpm will only shorten engine life and reduce speed and acceleration, because 2400 is already the best combination for this task.

Don't think too mutch about cars because the comparison can be confusing, yes gears in cars and propeller pitch in aviation serves the same function (make an efficient use of the engine), but they work differenly, in the car you have only 5gears maybe, but in the plane you can adjust rpm with a full range between 0 to 2400 for the 109 (above 2500 is like the redline in your car.)

When you dive the procedure in the 109 was to throttle down and reduce the prop pitch at minimum, (see the manual in the il2 COD folder) take care of the temps, they could drop sharply.

Generally speking, a variable pitch prop is used to find the best thrust to drag ratio, because a propeller cause a lot of drag (i should say a huge amount of drag).

Finally if you don't have complete tables of performances for a plane try at least to find the max admissible rpm (besst power) on a document or internet and use the procedure described below to find a suitable cruise setting combination:

For cruising adjust the throttle to attain your desired speed (eg 200mph) and then reduce the rpm until you can no longer maintain this speed, the lowest rpm at which you can maintain the desired speed will be the best setting. When reducing rpm (if your manifold pressure is not on max but on a reasonable setting for cruise) your plane will accelerate, this is because the thrust to drag ratio will be better, so in order to keep the speed at eg. 200mph you will have to reduce the throttle. Once the trhrottle is reduced repeat the operations as long as you can maintain your desired speed. Here you see what's the use of the prop pitch = making the best use of the engine power, reducing consumption and wear that's great don't you think! .

A last thing, the normal procedure is to increase rpm before increasing throttle, and to reduce throttle before decreasing rpm. This is to avoid detonations in the engine.