B-17 Bomber's Engines and Propellers Explained

Let's go ahead and review the B-17 Bomber's engines. The Bomber's engines were produced by a company called Curtis Wright, and the engines were called the Wright Cyclone R-1820-97. The engines were turbocharged, supercharged, and intercooled. The Wright Cyclones were radial engines, so you can tell a radial engine from a liquid-cooled engine by looking at the engine cowling. So this is a cowling, this big round fairing at the front of the engine. So if it's big and round like this, then that's generally a radial engine. A radial engine is characterized by the orientation of the piston. So the piston cylinders that we see here, there's nine of them, they're all spidering out from a central hub, and that's why the R is in the designation of the engine. So R represents it's a radial engine. The 1820 in this designation represents the displacement in cubic inches. So this is an 1,820 cubic inch displaced engine. They're also 1,200 horsepower. The engines are turbocharged, supercharged, and intercooled. The turbocharged and supercharged system is there because these engines are gonna be operating, they're aspirated engines, they're gonna be operating at altitudes much greater than sea level. So the 1,200 horsepower, that's great at sea level, but when you get at higher altitudes, the atmospheric characteristics, the air density, air pressure, and actually air temperature change dramatically. And you're not gonna be able to hold 1,200 horsepower due to the density changes of the air. So what we wanna do is take the outside air coming in at, say, typical cruise combat altitude for a B-17 is anywhere from 24 to 30,000 feet. So if we're up at 30,000 feet, we wanna take that outside air and we wanna compress it. And the compressing mechanism is going to be the turbocharged and supercharged system. And that's great when we turbocharge the air. The kind of problem we have is that the air, because we're applying a mechanical force to it, it's going to heat up at the same time. And we always want cold air going into an engine like this. So at that point, we're gonna run it through an air radiator called an intercooler. So now the air is going to be more efficient when it enters the engine. So the fuel system is carbureted, and these engines are actually air-cooled. And you can see the cooling fins on the engine cylinders here. Back in World War II, you had decisions to make of what kind of engine you want to provide thrust for your airplane, and you had liquid-cooled versus air-cooled, a lot of discussion on this. But roughly, an air-cooled engine is more battle damage resistant, which was more of a premium for bombers. Some fighters needed higher performance and they wanted to go with a liquid-cooled engine. So if you look at the P-51 Mustang, that's powered by a liquid-cooled, either the Allison or the Rolls-Royce Merlin engine. The problem with a liquid-cooled engine, they're not as battle damage resistant. So if you get a breach in the cooling system or radiator, the engine might seize up in five or so minutes. So that would not be good. Just a little bit of battle damage could make that airplane inoperative. So the fuel system is either 100 octane or higher gasoline, and the pistons are connected directly to a crankshaft. Now, from the crankshaft to the propeller shaft, we actually have a gear reduction of 56%. So the propeller is spinning at 56% the speed of the crankshaft, and we do that for a couple reasons. First off, if the crankshaft and propeller shaft rotate at the same speed, if there's any unbalance on that propeller, then you're gonna set up a lot of harmonics and dynamics. So that's probably not a good thing. Also, we wanna keep the tip speed of the propeller. We wanna maximize the efficiency of the engine, so it's gonna be rotating at a certain speed. But we don't want the tip speed of the propeller to hit Mach one. Then we set up certain aerodynamic drags on the propeller. So Mach one is the speed of sound, and at sea level, that's about 750 miles an hour. When you go up in altitude, that's gonna drop to about 650 miles an hour, something like that. The propellers are produced by Hamilton Standard, 11 feet, seven inches in diameter. There are three blade, and they are variable pitch. So the pilot has the ability to take this propeller, and there's gearing in the hub, and actually rotate it. So we can bite into the wing, bite into the wind, more or less, depending on the atmospheric characteristics and the efficiency of the engine. So we can rotate each one of these blades for that reason. Also, we have full feathering capability. So if we had a situation where the engine was shut down, the engine could, or the engine seized, then what's gonna happen is, is that these blades produce a lot of drag. So we wanna rotate these blades so that they're not biting into the wind at all. The other reason is that the air load on these blades is quite strong, and we can start backspinning this propeller. Now, the propeller shaft will start to spin, and since it's connected directly to the crankshaft with this gear reduction, the pistons will start to move in those cylinders. Since the cooling system, the oil cooling system's not activated, we could either start a fire or seize up the engine and dramatically damage the engine. So we want the ability to rotate these propellers so they don't spin. So you might see in movies where we have an engine out, and you'll just see a propeller in a static state, and that propeller would have been feathered. The propellers also have an anti-icing system. It's supplied by a 20-gallon reservoir of anti-icing fluid. Now, we don't really see it here, but right at the hub, you'd see a little nozzle, and that nozzle can spray an anti-icing fluid, which is typically glycol, and that'll smear along the propeller's upper leading-edge surface, and that'll keep ice from forming on the propeller, which would reduce its efficiency. Let's continue our discussion of the B-17, and let's take a look at the engine airflow diagram. So if we take a look at the leading edge of the wing of the B-17, we'll notice that each engine has three VAM air scoops. We have a, we'll call them supercharger intake, intercooler intake, and not shown on this diagram, we actually have an oil cooler. So let's follow the air as it makes its way from the supercharger intake all the way into the piston. Along the way, it's going to be acted upon by different systems. So we remember that the B-17s cruise at a altitude of 24,000 to 30,000 feet in combat. The true air speed is about 240 miles an hour. So this air entering is going to be reduced in pressure, reduced in density, and reduced in temperature. Temperature is gonna be about minus 50, 60 degrees Fahrenheit, and it's coming in at 240 miles an hour. It's gonna make its way down these cavities, make these turns, and the first system that it's going to contact is this device, and this is the turbocharger. The turbocharger is shaft-driven. It's got an impeller up here, which the air is going to hit, and it's going to be spun around. This is traveling at 22,000 RPM, and the intent of this impeller is to increase the pressure of the air, increase its density. Unfortunately, as a consequence of this, it's actually going to increase its temperature. Now, the temperature increase is not because we're adding fuel or anything in here. It's just that we're applying mechanical work to the outside atmospheric air. Now the air is going to transition through this cavity, and the second device that the air is going to hit is this box-shaped system called the intercooler. The intent of the intercooler is to try to maintain that pressure and density, yet we wanna reduce the air's temperature, and that's because with a piston-powered engine, you always want the air going into the engine to be as cold as possible. So how are we gonna cool that air? The cooling medium is going to be the outside air, and that's what this intercooler intake is for. So again, you got a ram air scoop on the leading edge of the wing. Air temperature is minus 50 degrees, and it is going to bleed off the heat of the air that has just come off of the turbine. Now the air, as it goes through from this intercooler intake, it's gotta go somewhere. So it's actually going to snake its way through the wing box, and on the upper wing surface of the B-17, you'll see these long, slotted louvers, and that's where this air is going to go. So the air at this point has been compressed. Density increase and temperature increase. At this point, the air is still compressed. It's still dense, but it's been cooled down. Now we can't maintain the actual pressure and density, but pretty close. We're gonna actually increase that one more time. So we follow the air through this cavity, and then we go into the carburetor. The carburetor is going to add the 100 octane fuel at the correct fuel to air ratio. Then the air is going to make its way down here, and this is the crankshaft of the engine. And recall that the crankshaft, there's a gear reduction to go to the propeller shaft. So this is producing the 1,200 horsepower for that propeller. But before the air goes into this piston, we are going to compress it again, and this is a supercharger. So the supercharger is shaft-driven. You can't turn it on and off. It just is always on. The turbocharger, we can turn on and off. We can bypass it if we're at lower altitude and we don't really need to compress this air. So the air is going to be compressed again. Now it does heat up a little bit, but it's a tolerable level. Then it's gonna make its way past this valve into the cylinder, which is going to provide the power for the shaft. If we take a look at the exhaust, all of the exhaust on the B-17 is collected in what's called a manifold ring, which is around the engine cowling. So this is a much thicker exhaust pipe here. And this air is actually very heated and it is at high pressure. So instead of this air going directly out of the tailpipe, which is over here, why don't we collect some of that energy? And that's actually done by closing off the tailpipe with what's called a waste gate. The pilot has the ability to close this off. Now the air has another path to travel, and that is through this turbine wheel, which will span the impeller of the turbocharger.