How Prop Pitch Really Works and Why It is Important
If there is one topic that constantly pops up in IL2 forums, it is that of prop pitch and how to adjust it. Any thread in which this subject appears is often filled by well meaning posters who only help to confuse the matter. I finally got tired of reading bad explanations and decided to write something which I hope will make Prop Pitch crystal clear to any IL2 flier who reads this. So lets get to it:
To understand prop pitch correctly, we need to learn about the 'powerband' of the aircraft engine. Lets take as an example in this discussion, the F4U-D Corsair which has a big honking Pratt and Whitney R-2800-8W engine. As the pistons of that engine cycle up and down they turn the crankshaft which spins the propeller, which in turn generates thrust. Normally at full throttle, that propeller will spin around at 2700 revolutions per minute (RPMs). This 2700 rpm is normal and healthy for the engine. In fact the engine generates it maximum thrust (and thus level flight speed) at 2700 rpms. If I lower the rpms to say, 2300, I will no longer be getting maximum thrust (and hence speed). Now, you would think, well if 2700 rpms gives me a lot of thrust, then surely, 3200 rpms will give me even more thrust (and hence, speed). Actually, this is not true. If the rpms of the engine raise above 2700 rpms to 3200 rpms, it is exactly like trying to drive a car with manual transmission in 2nd gear at 50 mph. My engine efficiency will actually fall off. So trying to fly a Corsair at 3200 rpms will actually give you less thrust than flying it at 2700 rpms (the exact reason will be at the end of this article). From this, you can set up a nice little graph with rpms on the X axis and thrust on the Y axis. And what you would see is a nice curving line that peaks at 2700 rpms. This is known as a RPM to Thrust diagram (also similar to a RPM to torque plot). So we can say that the engine works best when the propeller is spinning at about 2600 to 2800 revolutions per minute. This is known as the 'powerband' of the Corsair's engine. Not all planes have the same powerband and some may for example give maximum thrust at 3300 rpms while another engine may work best at 2400 rpms. Also the Corsair has a fairly narrow powerband from 2600 to 2800 rpms but other airframes may have a larger powerband from say 2500 to 3000 rpms (on an X-Y plot, the powerband appears to flatten).
The next thing we need to take note of is how the propeller reacts at a fixed throttle level. Lets say I keep my throttle fully maxed at exactly 100 percent (which is how most noobs in IL2 fly anyway) and I do not change it at all. Now, what will happen to the propeller with my throttle fixed ? Well, if you watch your RPM gauge carefully, you will see that as you dive, your propeller RPMs will increase and as you climb, they will decrease. Why is this ? It has to do with the inertia of your airframe and the flow of air over the prop. In a dive you have a gravity assist and the air is hitting the prop faster and the opposite is true as you climb and the speed of the air striking the prop is slower.
And so what happens if you fly many shallow long dives and your RPMs continue to hang at 3000 instead of optimal 2700 for the Corsair ? First off, you will not be getting maximum thrust and second, the extra 300 rpms per minute will cause engine overheating. In fact continually riding above the powerband only produces extra heat and given enough time will kill your engine. Likewise, continually riding around below the powerband means your engine stays cooler but you wont be producing maximum thrust when needed in combat.
So how do we adjust the propeller to give maximum thrust no matter if we are diving or climbing ? With Prop Pitch of course. We are going to use our ability to govern prop pitch to keep the engine RPMs in that nice narrow power band from 2600 to 2800 RPMs. So how does Prop Pitch accomplish this ? It adjusts the angle at which the blades of the propeller 'bite' into the air. If the angle is increased the propeller will bite more deeply into the air and for one given revolution will push more air. But at the same time, that deeper bite of air also increases resistance which means the propeller slows down. So if I dive without prop pitch, my rpms may quickly move above 3000 and I will actually lose thrust while overheating my engine. With prop pitch, I enter a combat dive and I will move prop pitch down to 80% (this is confusing because the pitch of the blades is increasing but in IL2 by the game's nomenclature, you are lowering prop pitch), and I will get an acceleration in speed and my rpms will be closer to 2700. So I am picking up a quick boost of speed, keeping my engine cooler and maintaining my RPMs in the powerband (and keeping maximum thrust). I am also slowly building kinetic energy over any opponent who is not using prop pitch (or applying it incorrectly). Likewise if I go into a combat climb, my RPMs will start to drop off so I will raise move prop pitch up to 100% so that the blades of the propeller bite less deeply and my RPMs will move back up to 2700. Of course as I approach the apex of my climb and start to stall out, there will be nothing more I can do. Prop pitch is already raised to its full 100% and there is simply not enough air moving over the prop now (because of gravity and the inertia of the airframe) to generate any more thrust. (Although, some people are advocates of quickly lowering the prop pitch down to say 70% in that last two seconds to help stabilize the stalling characteristics at the apex of the climb, but this depends on airframe used and stall characteristics)
Now you can see the main tenet of prop pitch in IL2: Prop pitch used correctly acts as a governor for engine RPMs to keep you in the powerband.
Other questions people have, are how much prop pitch should I apply and for how long ? That is more difficult because prop pitch is one of the few 'messed up' flight characteristics of IL2. With complex engine management (CEM) enabled, the engine will more accurately model real life but prop pitch still does not respond as it would in a real airframe (or as powerfully either). Generally speaking you will apply lower prop pitch in a dive until you see your RPMs go back down into the powerband. And in a combat climb, you will raise prop pitch until you see the RPMs climb back up into the powerband (and hold them there until stalling starts when possible). Typically, for example with the Corsair, you will only need to lower PP to about 70% for one to three seconds in a dive before you move back down into the powerband. And in a combat climb, you can raise prop pitch up to 100% and keep it there.
And what about cruising to a combat area. For this, it is probably a good idea to keep PP in the 90% range. If you keep it lower at say 70% like some recommend, and then you get bounced or surprised, you are too far from the powerband to get back up to optimal thrust in time to ward off attacks. However, at 90%, you are still keeping your engine cool and can move right back up into the powerband within 3 seconds or so, if an enemy surprises you.
Hopefully Prop Pitch is getting much clearer now. So why did I take time to even go thru this stuff ?
Well, what if I told you I can defeat almost any.... and I do mean almost any open cockpit flier on Hyperlobby fairly easily ?
You would surely say, yeah right, …... this dude is full of shit. He been toking on the ganja weed too long.
But think about it for a minute. What is it that open cockpit guys cant see (when they are flying without the cockpit) ? …. you guessed it.... BINGO!!! ... No RPM gauge ! They have no idea at what RPM their engine is at during combat because their cockpit is turned off. They can tell from the engine sounds if they are about in the right range but this is still not precise enough. They cant tell if they are in the powerband or not, they just have to take their best guess (In fact many could care less, and I would guess, only about 5% of open pit guys even know how to use PP in combat). So lets say, my adversary and I start out in a duel with F4U-D Corsairs at equal altitudes and speeds in an open pit server. As my opponent switches off his cockpit to have a wider view, I keep mine on. We then pass on the first merge and I carefully watch my RPM gauge and keep myself in the powerband by adjusting PP when needed. As he is busy chasing me and calling me a coward for not turn fighting with him, I am slowly making passes on him and building kinetic energy in each pass, since I am flying much closer to the powerband than he is. At the end of each pass I transform this gained kinetic energy into potential energy as I climb. Within three to five passes, I have pulled above him by 700 meters all due to flying in the powerband while he did not. And now the fight is over..... he just does not realize it yet. I now switch my cockpit off as well to get a clearer view, and start to remove his wings. Unless I make a bad mistake and give up my altitude advantage the fight is already over.
I also use the same tactic often against closed cockpit and full real pilots. The better full real pilots know how to read their instruments and are also flying in the powerband so I have to make other adjustments but you get the picture of why this Prop Pitch stuff can become so important. Even over other good full real guys, I can often build an advantage in altitude and potential energy within the first few minutes of the fight and then it is over for them.
This is one reason I advocate strongly for any flier transitioning to closed cockpit. Because closed pit forces you to become aware of, or take into account many factors that open pit guys totally disregard. Once you start to factor these complexities in, going back to lesser opponents (even in a full real server) becomes easy.
Hope this helps you guys out who may be confused on the prop pitch stuff.
The beauty of the Universe can best be understood by learning the language that Mother Nature speaks in, …... mathematics.
(The answer to why a prop moving at 3200 rpms can generate less thrust than the exact same prop moving at say, 2700 rpms has to do with the Navier Stokes equations and wind shear turbulence over the propellor, which is more detailed than I want to go into here because the mathematics will confuse many)
(As another note, for any RPM to Thrust plots where there is only one global maximum (which is true for single engine prop fighters), where the partial derivative PT/PRPM = 0 is the optimal powerband)