View Full Version : More CEM from a B-17 pilot

10-13-2005, 07:14 AM
Now your Questions.

The B17F and older models had 4 levers to set the Manifold pressures (Waste gates) for manifold pressures above about 28 inches or so
inches. Less as you went up One could grab them all 4 in one hand and move them together but
to get a fine adjustment individually one at a time. It was no big deal nor difficult There were two pilots but it did take a bit of time.

With the waste gates open. All the exhaust gas going out the exhaust pipe. The supercharger would not be turning and the maximum manifold
pressure with the throttles wide open near sea level would be at the most around 28 inches. This would be from normal air pressure only. The same as an unsupercharged engine. There is no way a B17 F or G could get off the ground with a
full crew, 10 men, 8000 pounds of bombs and 2732 gallons of gasoline with only 28 inch of manifold pressure even on the Azores 3 mile runway at sea level.

The speed of the engines above a fast idle was set by the pilots from the cockpit. The Wright engines turned Hamilton constant speed full feathering 3 bladed propellers. Were controlled by some small flyweights in the governor that set on the nose section of the engine. Lets say the pilots set the governors to keep the engine at say 2300 RPMs or any other RPMs up to 2500. The governors would hold the engines at that speed unless the pilots reduced the fuel going to the engine with the 4 throttles so much the prop blades were in such a flat pitch they could go no farther to unload the engine and the engines would then slow down to a predetermined idle speed The throttle handles were so designed either pilot could grasp them all 4 or the two outboards, or the 2 inboards or any one of them with one hand very easily and very quickly. The throttles setting did NOT control the fuel mixture or the engine speeds except at
a fast idle and below. They did control along with the supercharger the power out put.

The governors controlled the engine speed by changing the pitch of the prop blades.(angle of attack) If they were set to say 2300 RPMs and the engines started to turn a little tiny bit faster than that like the nose dropped and
we started down just a bit or the pilots opened the throttles just a bit. The 2 little fly weights in the governor, due to centrifugal force, would move out just a bit this would direct engine oil to the prop dome where it would move a piston connected to the prop blades to increase the pitch (Angle of attack)
a little putting more load on the engine and slowing it back down to 2300 RPMs again. This little governor is constantly moving the little valve up and down to hold the speed exactly a 2300 or where ever else it is set. The engine
speed has nothing to do with the throttles or manifold pressure unless it is so low the prop blade have reached the limit of flat pitch travel and the engine will slow to an idle.

Now lets say we are sitting on the end of the runway, The manifold pressure (the little black door knob sitting on the front of the horizontal
instrument panel by the base of the throttle levers is set for 46 inches. We don;t
have that much as the engines are at idle and there is little exhaust gas to turn the supercharger. We are cleared for take off. The pilots points the bird down the runway and
kind of slowly advances the 4 throttle, He maintains direction down the center of the runway maybe with his brakes just a bit right at first. Then as he picks up a bit of speed he can stay down the runway with his two outboard engines. Now he has enough speed he can hold his direction with rudder alone, Feet on the rudder pedals. The 4 throttles are wide open The manifold pressure has reached 46 inches. The waste gate setting is holding it there The
governors are holding the engine speed at 2500 by changing the pitch of the prop blades,
Flatter if under speeding, bigger bite (of the Air) if overspeeding. The fuel air mix is set at full rich or auto rich. Our air speed reaches 105 mph. We gently pull the bird off, up the gear and flaps. Reach 120 air speed, pull the throttles back to 42 inch manifold pressure. We don't see this but when we reduced power the engines started to slow down. Real quick the little fly weighs started to come together. The valve moved. The piston in the prop domes moved the prop blades flattened out (less angle of
attack) the engine load was decreased The engines went back up to 2500. This all
happened so quickly and so automatic us 2 dumb pilots never noticed it or thought about it. Now we reset the governors to 2400 rpms. The spring
load on the little valve in the governor was changed and the little fly weights could do
their job in and out at 2400 rpm.

As we went up and got into our formation we continued to change power as above until we were down to about 2200 rpms held by the governors and whatever manifold pressure we needed to hold formation as long as it was in limits set by our rpm which we could change our manifold pressure as need up or down.wth the throttles. Coming home we might lower our engine speed 1900 or so put the mixture in auto lean. Our bombs and most of our gas was gone it was down hill so this saved fuel. so we get home with enough to land

As we went up we were in thinner and thinner air so the supercharger had to turn faster and faster to hold the manifold pressure we needed. We had no way to know how fast the turbo was turning so we just had to estimate. It had
no governor. Common sense told us if we were at 30,000 feet and trying to hold 46 inches it was way over speeding (over 33,000) RPM or so) and
possibly could fly apart I suppose this happened but not too often

I just stopped to read your questions. High blower and lo blower are terms for gear driven superchargers. We did not have them. They were driven from gears in the back of the engine
like a manual car transmission. They had 3 gears (or 4) Low,2nd & high The gear driven
supercharger transmission have 2 gears (low & High).

Our B17s had only one exhaust pipe on each engine that carried the exhaust out of all the 9 cylinders, about 6 inches before the rear end
of it going out was the Supercharger waste gate. If it was closed most all the exhaust went thru the turbo if it was open none of it did. It could be in between and more open or less open as needed. The other exhaust pipe you saw could
have been an air intake or a tube from an inter cooler that cooled intake air after it was compressed by the supercharger

Hope I have answered all you questions, If not throw them at me. IL try again.

10-13-2005, 07:23 AM
Yes I can help you with this one and I will. It will be a bit long and I may not have time to type it all to day so it might be in two or
three letters and go over until tomorrow.

(1) Air has weight. a colume of air one foot square at the bottom and extending up about 3 or so miles would weigh over one half ton. Because of this weight air pressure at sea level will average about 14.7 pounds per square inch

Us scientists , pilots and weather people do not use pounds but "Inches of Mercury" to measure air pressure. If we got a glass tube about a
yard long and sealed one end of it and laid it down in a pan of mercury until it was
COMPLETELY full of mercury and then leaving the open end submerged in the mercury raise the sealed end to straight up at near sea level the
mercury in the tube would stand to about 28 to 32 inches. Where ever it was could then say the
barometric pressure was so many inches.

What is pushing the mercury up in the tube, is the pressure of the air pushing down on the surface of our pan of mercury..In the top of our
tube there is a complete vacuum (No air) so nothing is pushing down in our tube. (We
could use water in the tube but mercury is 13.6 times as heavy as water so a water tube would have to be about 14 feet tall. It would hit the
ceiling in most rooms.

(2) A gasoline engine Burns a mixture of asoline and air in a ratio of about 12 to 15 pounds of air to one pound of gasoline. When the piston
goes down on the intake stroke it reduces the air pressure in the combustion chamber considerably. The intake valve opens and the air pressure outside pushes air through the carburetor, through the intake manifold and into the combustion chamber. As the air goes through the carburetor it picks up the gasoline in a
spray, vaporizes it in the hot manifold and goes through the open intake valve. Near the bottom of the intake stroke the intake valve closes and the fuel air mixture is compressed as the piston moves up to top dead center were it is ignited by a spark plug and the burning fuel air mixture greatly increases the pressure pushing the piston down and through a system of levers, push
rods and crankshaft supply the power of the engine.

(3)About one half of the earths air is below 18,000 feet. As we go up there is less and less air above us so the air pressures gets less and
less. Hence there is less and less pressure in the air to push it through the intake system. Most unsupercharged engines cannot put out enough power for the aircraft it is powering can get above 8 to 15000 feet or so, depending on its load. If we had 28 inches at sea level we would have only 14 inches at 18,000 feet.

(4) A Supercharger is an air pump. On aircraft it is gear driven or exhaust driven. on B17s and B24s it is a blower powered by a turbine rotor driven by exhaust gases. There is a valve in the exhaust pipe, called a waste gate that looks
very much like the choke in a carburetor. When this gate is open all the exhaust goes out the exhaust pipe. AS this gate is closed it diverts the exhaust gases through the turbine that turns the blower that blows air into the intake manifold. The more the gate is closed the more exhaust goes through the turbine making
it turn faster, making the blower turn faster and blowing more air into the intake manifold

In the before the G model there was four levers beside the four throttle levers in the cockpit and four manifold pressure gauges on the instrument panel. One gauge and one lever for each engine. When the pilots wanted to change
power they could adjust the engine speed by adjusting the prop governors with another
4 levers. One for each engine. They could see how fast the engines were turning with a gage on the instrument panel one for each engine. They
could adjust the manifold pressure with 4 throttle levers. One for each engine/If
the throttle levers could not give them the power they wanted they could adjust the
waste gates with the 4 levers. One for each super charger.

It was important for the engines to be synchronized usually. Turning at the same speed, manifold pressures, & fuel air mixtures These had to be coordinated so if you changed one they all needed to be adjusted accordingly. This was no big deal when just boring holes is the sky, but flying close formation and in combat when all these levers had to changed and adjusted
frequently and one at a time and quickly it could get a bit hectic. The G model removed the four levers for the waste gates and put in
one knob about the size of a small door knob and when one turned it, it adjust all four waste gates to the same manifold pressure with one small twist of the one knob. It was a nice improvement.

For your information we used 46 inches of manifold pressure and 2500 RPMs for take off. 42 inches and 2400 for fast climb. 2300 and 36
inches for maximum cruse. 2200 and 32 inches for normal cruse. and 2000 and 28 inches and below for maximum fuel economy. We were supposed to use the first two for take off and fast climb for 5 minutes only but some time we had to cheat a bit if we lost an engine or 2 or some other war emergency. That is why they could get
1500 hours out of these engines in civilian use and we were lucky to get 450 hours out of them.

The superchargers took that thin air and compressed it so we had the capacity to get up to around 35 to 40 thousand feet. We didn't because many problems appear up that high in un- pressurized planes included but not limited to
the bends. which are very dangerous to ones health' The air is so thin your blood will start boiling and you couldn't cook a pot of beans if you cooked them for a week without a pressure cooker.
Best Wishes,

10-13-2005, 08:04 AM
Good stuff, thanks for posting them, keep 'em coming http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif
Allso please pass on our thanks to the gentleman who has taken the time to explain all of this in a very easy to understand manner.

10-13-2005, 01:50 PM
Phenominal read. Excellent!

10-13-2005, 05:50 PM
Yes, very interesting read! http://forums.ubi.com/images/smilies/25.gif

10-13-2005, 07:00 PM

10-13-2005, 07:29 PM
I assume that by his saying their was no gear driven supercharger he really meant there was no supercharger control in he cockpit. Their were single, two and three speed superchargers on R-1820s (all were supercharged one of these ways) as well as direct and gear drive models. The B-17 used a gear drive propshaft, single speed supercharger model with a supplemental turbo. The turbo maintained sea-level pressure or so ahead of the turbo so a multi-speed supercharger wasn't necessary. I believe none of the B-17s flying today have functioning turbos. The two I've worked on don't even have the intake trunk between the turbo and the carb.