Thanks to HeavyCavalrySgt for this - Grab a cuppa and have a read, helped me get my head around prop pitch.
Engine management is complex, partly because it it is not standardized in WW2. Some planes have a greater degree of automation than others, some engines have complexities that others don’t have like superchargers. In the real world, unless you are Kermit Weeks, you learn how to fly a couple of aircraft (hopefully) well, and spend a long time memorizing things like power settings and emergency procedures, testing yourself and being tested on those items.
In a very general sense, in a fixed pitch airplane, power is controlled by the throttle. You control the engine, the prop speed is controlled by the engine directly. Props aren’t good at every environment though. most of them are compromises. A prop that gives you a good cruise speed (because it takes big bites out of the air each revolution) isn’t going to be great at climbing. If you have a climb prop, it is going to move less air per revolution because it is designed with more revolutions in mind so you pick a high throttle setting, and climb out cheerfully off the runway with no drama or interrupting the kids playing catch off the departure end of the runway. Then at cruise you go slower and burn more gas.
Think of someone with a multi-speed bike. Starting out or going up a hill, they might want a lower gear to spread the work they have to do out over more time. RPMS go up, the bicyclist peddles each revolution with less effort, covering less ground with each rotation, but it is good for getting up the hill. On level ground again, speed increases, and it doesn’t take as much effort to keep the bike going now that it already has some speed. A higher gear is appropriate, with slower RPM and covering more ground in each revolution.
Props have the same sort of effect (did you see my whiny story the other day about getting stuck in an airplane over the Sierras with a cruise prop on it? I took over with a density altitude of 8200 feet; it was not a good time.)
With any form of controllable pitch prop, you are pretty much giving yourself gears.
Heavy payload, hot day, high altitude airport and a lot of climbing to do to get over those mountains? You decrease the pitch and now you have a climb prop. At altitude, ready to go places? Increase the pitch to slow the engine down and now you have a fuel efficient cruiser.
Early ones were adjustable on the ground. Someone physically turned the prop blades (hopefully the same amount!) for a particular situation before engine start. Then there were different systems to allow pilots to control the prop pitch manually or automatically, directly or indirectly, in the air.starting in the early 20s. It can be done mechanically, electrically or hydraulically. That is another reason for confusion: different airplanes have different controls.
The one that was demo’d in the LaGG is not completely different from current systems. The prop is still connected to the engine more or less directly but the pilot basically picks an RPM that he wants the prop to run at based on what he is going to be doing. I’m climbing? I want the prop turning lots of RPM. I’m in cruise? Let’s slow the prop down and take nice big fuel efficient bites of air. The adjustment of the prop happens without any more pilot interaction beyond the selection of the RPM. When the pilot adds throttle, there is a control unit (generally) of some sort that says “Hey wait - the prop is going faster than it should!” and it increases the pitch on the blade to slow the prop - and engine - down again. If the pilot yanks the power out the control unit flattens the blades to try to maintain RPM by reducing the amount of work the engine has to do each RPM. Below or, in scary cases, above a certain RPM the prop physically can’t adjust the pitch anymore to stay at the selected RPM. We saw that in the video when the LaGG was at idle throttle, the blades couldn’t reduce the pitch enough to maintain whatever prop setting was selected which is fine and normal. The other situation usually happens when something other than the engine is causing the prop to speed up, usually a full throttle dive. Not long after that, the engine starts making expensive noises.
Now there are basically two controls and indicators for the pilot to be mindful of. The RPM primarily shows what the prop is doing and we talked about how that works.
The other control and indicator is the throttle and the manifold pressure gauge. The manifold pressure tells us how hard the engine is working. The more MP, the harder the engine is working. Race pilots this weekend are thinking a lot about how much MP they can safely get in Reno and for how long.
For mere mortals, we have charts that recommend specific RPMs and specific MPs for different situations. For a climb in a Cessna Cardinal RG, I remember “25 and 25” or, 2500 RPM on the prop, 25 inches of mercury on the throttle. The reality is that I live at a high elevation, and many times I can’t get that much power because the air isn’t dense enough – I never get full power except on a chilly day.
That is kind of the problem pilots were having a lot by WW2. You get to altitude and now your engine isn’t making anywhere near as much juice. In the Cardinal, at 12,000 on a ‘standard day’ (a largely mythical meteorological benchmark) the absolute most I can get out of the engine is ~59% of its rated power. That’s fine if I am just going somewhere. I can cruise at about 145 knots or so (166 mph) and do it pretty efficiently. If I had to mix it up with that guy in the Piper Warrior I am going to be sad I am not making more power, because I am going to be wasting energy trying to out-turn him and get on his tail, and I am going to want to apply a lot of acceleration, but I will be sluggish instead.
So designers started sticking turbos and superchargers or both on engines. This was probably an expensive time for folks that needed to fix engines and an exciting time for the pilots that flew them. Both work by pressurizing the air coming into the engine to (at first) offset the loss of atmospheric pressure at altitude, then to enhance it. Some WW2 warbirds could achieve between 45 and 50 inches of mercury in the manifold! The problem is, with high manifold pressure the engine is working hard and when something breaks it does it dramatically. Sorta like when a dragster tries to launch its manifold into low earth orbit.
So those systems are also generally often variable, much like the prop is. You decide how much pressure you want to pump into the engine at any given time. At the extreme upper limit you have emergency settings where you aren’t supposed to operate the engine for long, or you may find that it is no longer available to make any power at all. The B-29, for example, had an emergency setting (it was a dial labelled 1-10). The manual says that you can use that 10 setting only for emergencies, and not for more than 2 minutes or bad things happen. no suggestions on how to limit emergencies to 2 minutes or less.
The other control some airplanes have is a mixture control. This is just to change the fuel-air mixture the enters the cylinders to something like optimal. Too much gas (or not enough oxygen) and you have a mixture that is too rich, and you are wasting gas. Too lean and you have a mixture that burns hot (because gas helps cool the cylinders) and can seriously break stuff - melt holes in pistons, for example. I have been there and done that. Or, more likely, you can cause deposits to form which would otherwise have burned and now your exhaust valve is stuck. That’s no good, because now that cylinder is making much less power.
The three things you generally need to know is what power settings to use – these are documented in the aircraft manuals - and how to achieve those settings. In modern aircraft when you add power, you set the mixture, set the prop, then set the power. Decreasing power is the opposite. The third is what power settings not to use. The Cardinal doesn’t want to be run at below 10 inches of manifold pressure between 1700 and 2400 RPM, for example. It tends to make the mixture in the cylinders detonate rather than burn, and that is not going to last long before I have to practice my emergency landing skills.
Some combat help ~
The propeller control typical sets RPM - I’m talking about constant speed systems here - and they typically are not very smart systems, generally being driven at that time by simple hydraulic pressure. They don’t really care why engine RPM is going up or down, they just try to compensate to get back to the selected RPM.
In a long dive (or maybe a dive bombing attack? I have not received instruction on those so I am guessing here) you might pull power, then pull the prop back to keep the engine from overspeeding. The rest of the time in combat, you probably want something at or close to the climb setting because acceleration is what you want unless you have to chase down someone who just remembered an appointment elsewhere and is trying to exit the fight. Did you notice how the plane accelerated and decelerated dramatically in formation with the AI plane when the prop control was tweaked in the last livestream?
You probably would mess with the prop control when you needed to in combat but for the most part I suspect your attention would be elsewhere - set it and forget it unless you really are changing your strategies like trying to get away from that swarm of FWs that just came out of the clouds.
http://forum.il2sturmovik.com/topic/1314-help-me-crack-mysteries-cem/