The 109... questions

[Blackdog_kt]

Originally Posted by adonys

I've found out it's rather hard to exceed 400 IAS in Bf109E3, but sometimes you can do it.

Combat (full power settings - bold settings are your main goal):
- oil radiator almost closed 30% open (between 20-40% open, I'm modifying it all the time) - keep an eye on oil's temp all the time
- prop pitch anywhere around 90% (between 80-100%, I'm modifying it all the time, accordingly with oil rad modifications, and trying to keep the RPM in the desired area)
- throttle kept modified (accordingly with the above oil rad and prop pitch settings and trying to keep the RPM in the desired area)
- RPM around 2500 (between 2000 and 3000)
- speed over 400 IAS
- water rad fully open

Don't forget to lower/cut the the throttle in dives (and eventually ease your prop pitch too), and get it back once leveled.

Cruise (economy settings - bold settings are your main goal).
- water radiator fully opened
- oil radiator fully opened
- prop pitch 100% (between 90-100%)
- throttle around 45% (between 45-55%)
- RPM around 2000
- speed over 300 IAS
- manifold pressure as lower as you can manage it (to conserve fuel while keeping a reasonable speed)

Excuse me for saying so, but i tend to do cruise a bit different, lower the throttle and then drop the RPM. Reducing the pitch has the effect of raising the manifold pressure again, so i re-adjust the throttle which again affects the RPM (throttle affects RPM only on the German aircraft and on the RAF aircraft that don't have a constant speed prop, aircraft with a CSP keep the RPM steady on their own) and so on...but after a couple of back and forths it settles into a steady situation.

However, i use lower RPM (in fact i use the settings stated in the manual) with 1.2 Ata, since i think that RPM has a bigger effect on fuel consumption. For example, the Hurricane has a "cruise mode lock" on the propeller once you pull it fully back and it uses 1800 RPM or so for extra fuel economy.

Using 1.2 Ata, 2300 RPM and bit of trim i can easily cruise at 380-400 km/h with the water rads full open and the oil rads about 70% open.

The reason i'm using mid-range values for MP and RPM instead of high RPM with low MP is that every single aircraft checklist i've seen (both for combat and civilian planes) tells the pilots to use lower RPM in cruise to conserve fuel.

MP also has an effect in fuel consumption as the more air getting sucked into the engine the more fuel the metering valves/injection system/carburetors drop into he mix.

The thing is that using lower RPM lets you get more speed from the same amount of MP, as long as we're talking about cruise in level flight. After all, the reason RPM drops is because the prop is in coarse pitch and getting a bigger bite out of the air, making it more efficient for straight and level flight.
It's like going to 6th gear on a car when driving on a highway and dropping your RPM to 2500 without changing the amount of throttle you use: you won't accelerate/decelerate fast if you change how much you press the gas pedal, but you are sure getting where you want to go fast and with lower fuel burn, as long as you don't need fast responses from the engine.

Going low Ata with RPM is like doing the same speed in that car but instead of using 6th gear and 2500 RPM you use 4th gear and 6500 RPM: it's going to burn much more fuel and it's not even sure you'll get the same speed in the end.

The way it works is very different, but the end results are very similar so that it's easy to think of prop pitch and RPM like gears in a car. For example, whenever the car goes uphill we down-shift to increase our RPM and the engine's responsiveness, whenever the aircraft climbs we can go to high RPM to do the same.
During slow driving we use low gears that give us high RPM and fast response at low speeds, during the landing circuit we use fine pitch and high RPM for the same reason. Just like lifting your foot from the gas pedal will slow your car down via the gears if you are in low gear, using high RPM with low MP in an aircraft will do something similar. In this case however the braking is not only a function of the gearbox absorbing part of the energy, but mostly the propeller acting as an airbrake.

If you are on fine pitch/high RPM and the engine is not making enough power to turn that prop then it's getting turned by the air and windmills while at fine pitch (ie, the prop is almost flat, face-on to the incoming airflow)-->extra drag.

So, by using that analogy you can think of many useful scenarios.
What do you do to speed up in the dive faster or just let it pick up speed more efficiently? The same thing you'd do to speed up when driving a car downhill, progressively switch to higher gears/lower RPM while pressing the gas pedal or just go high gear and let it roll on its own. In our aircraft, this means coming back on the RPM a bit.

What if you want to go slow in the dive? Well, it's similar to what you'd do to keep your car from speeding up too much when driving downhill, use a lower gear/higher RPM while coming off the gas pedal. So, just chop the throttle and go to high RPM and the prop windmilling effect will give you the drag you need.

I'm not an expert or a real pilot and i certainly don't want to play it smart on you, i'm just surprised that your method is so different and that makes me want to compare notes.
You know, like a couple of pilots talking about "how to get that extra bit of performance" between sorties. Cheers