View Full Version : Great read about prop pitch, overeaving and turbo\superchargers!

07-15-2009, 12:41 AM
http://www.worldaccessnet.com/~delta6/ground/prop.htm (http://www.worldaccessnet.com/%7Edelta6/ground/prop.htm)

Prop Types
High-speed whirley-gigs

Everyone knows that God made aircraft with engines that went "bang" instead of "screeeeeeeeeeeeeech", and that the hum of a prop is the song of angels. Here is where you'll find explanations about the three most common prop types US aircraft used during the war. This information is somewhat technical, so unless you have an honest interest in them I'd advise against trying to understand it. For those who want to figure out how these things worked, I'm starting off with the Hamilton-Standard. It's a simple prop that is nearly bulletproof in design. More complex models are the Curtiss Electric and the Aeroproducts. The Curtiss used electricity, while the Aeroproducts was entirely self-contained.
The second page of this piece covers prop over-speed, back-driving, and how the prop actually controlled engine RPM. Below you'll find a short description of how each prop worked, along with any notes or quirks about it. This is not an attempt to make anyone an expert about propellers. It is here for those who want to know more about the business-end of any WW2 aircraft.

Hamilton Standard Hydromatic

Nearly anyone who has ever heard of aircraft knows about the famous Hamilton-Standard propeller. Regardless of the year or aircraft, the ol Ham just keeps flying. Many fighters and bombers used the Hamilton-Standard Hydromatic prop during WW2. And many of them still fly to this day. P-47 Thunderbolt, P-51 Mustang, F4U Corsair, F4F Wildcat, A-26 Invader, A-28 Hudson, B-17 Flying Fort, B-24 Liberator, B-25 Mitchell, B-29 Super Fort, C-47, and the TBF/M Avenger all used this marvelous prop. Pilots loved it for how well it worked; rarely did a Hamilton break, and when it did, the crews knew how to fix it. Mechanics loved it too, because you could tear one down and rebuild it in record time.
How the Hamilton works is painfully simple to explain. Oil is pressurized by an oil pump so it can flow throughout the engine, filters, and coolers without trouble. Usually this is around 50 PSI or so; enough that the oil gets squirted into all the nooks and crannies of the engine. Hamilton-Standard props required only an accessory oil line be run from the engine out to another pump just in front of the prop governor. This pump drives engine oil pressure up from 50 PSI to 200 PSI or more, enough to work the control plate in the prop dome. A prop governor is probably the simplest, and most critical, parts of any prop. It consists of two fly-weights that are gear-driven from the engine. As the governor speeds up, the fly-weights close somewhat, opening a valve and releasing oil. Now if the governor isn't spinning fast enough, it moves the valve and starts dumping more oil in. Which is also how it controls engine RPM. How you set that RPM is simplicity itself. On top of the fly-weights is a geared shaft that engages a small gear wheel. Between that and the fly-weights is a simple spring. Working the RPM control compresses, or relaxes, that spring via that geared shaft, which also changes how much pressure the fly-weights have to work against.
See, as the fly-weights rotate they are trying to smash that spring, while the spring is trying to shove the fly-weights back down. When the two are in balance, your RPM stays constant. If the spring is relaxed, you get low RPM; if it gets compressed, you get high RPM. Compress it too much and the governor goes too fast, which releases a small amount of oil to bring things back in line. Fairly simple. Now comes the fun part, the prop itself. Oil from the governor-controlled valve goes out to the prop dome. Inside the dome is a plate attached to a shaft running back to a bevel-geared cam just behind the prop dome. As oil comes in behind the plate it pushes the plate forward. This pulls the shaft forward, rotating the cam counter-clockwise. Since the butt-end of each prop blade also has a beveled gear that meets this cam, they get rotated counter-clockwise as well. Which is what sets high pitch (low RPM). Now oil is also on the other side of this plate, countering the force of oil that wants to push the plate forward. The oil in front of the plate is under normal engine pressure, or about 50 PSI. The oil behind the plate is coming from the governor's pump at around 200 PSI. When the governor is in balance, so is the oil pressure on both sides of the plate. Should you need high RPM (low blade pitch) oil pressure on the back of the plate is reduced slightly. This allows the plate to slide back, pushing the shaft backwards and rotating the beveled cam clockwise. As a result, the blades also rotate clockwise which sets low blade pitch. Because of this plate arrangement the Hamilton can't be feathered; there isn't enough travel room for the plate.
You can see why so many aircraft used it, and why so many people loved this prop. It was simple to make, easy to maintain, and unless the engine took a hit it almost always worked. The sole problem with the Hamilton is also what made it so successful; reliance on engine oil. In any event of oil pressure loss the prop had no means of control and would usually "run away." The blades would go to high RPM (full low pitch) while producing no thrust and a ton of drag. All the while the engine would red-line at max RPM or even higher, possibly damaging it. This, of course, requires full throttle or nearly full throttle in order to happen. You pilots out there will recognize this as why you can't get max RPM at idle. There isn't enough oil pressure among other things. Hamilton is in business today. Have you ever seen a DeHavilland Dash-8? It uses Hamilton-Standard Hydromatic props, albeit much more advanced models than the ones mounted on any P-47.
Some of the quirks involved in running an oil-based prop you can imagine. Until oil pressure comes up, usually once the engine is really running, you can't get full prop control. Some games accurately model this as a need for increased power before you can do a prop check. Other games don't bother with it at all. Aces High and Warbirds are two that do not require any run-up check, or any power increase beyond about 15" of manifold pressure to get almost full RPM. It's highly inaccurate, but it's also a combat game and not an attempt to build a very-accurate flight simulator. Although Aces High does model a slight pitch-change lag between flooring it and the RPM climbing to match. It isn't an accurate representation of the Hamilton prop because they do this with every prop. Another problem with hydraulic props is blowing oil. A loose line or worn seal can result in having several gallons of oil blown all over the aircraft. With the Hamilton oil loss is a real concern because it uses engine oil to operate. If you lose oil from the engine, you lose prop control as well.

Curtiss Electric

Unlike the Hamilton, or the Aeroproducts prop below, this propeller was controlled with electricity. Stop laughing, it is no joke. Instead of oil moving a plate inside the prop dome, the Curtiss had an electric motor to control the works. It was a bit more complex than the Hamilton because of the electrical stuff. It was also a lot less reliable without a picky, and rather careful, mechanic on hand to maintain it. When it did work, it worked flawlessly. And controlling the the works required no high-pressure oil lines from either the engine or a stand-alone reservoir. As a matter of fact, once you set the RPM this propeller required no electricity at all! Only when you changed pitch settings was electricity required; otherwise you could kill power and not see one hiccup. With the Curtiss you had max RPM and pitch immediately when you applied power. A definite plus when operating from any short field.
Unlike the Hamilton, the Curtiss had a governor you could actually turn off. The P-47, for example, had a two position switch: Fixed Pitch and Auto. In the Fixed Pitch setting you were the prop governor! Once you picked a given pitch the prop simply held that pitch until you wanted to change it. Changing pitch with the governor off was done with an "increase/decrease" switch. In Auto the prop worked like any other and would hold a given RPM based on where you parked the prop control lever. Here's the kicker: on any Hamilton you'd be watching rpm on the cockpit gage, while on a Curtiss you were looking at prop pitch. You don't control RPM on a Curtiss, you control the blade pitch. The gage in the cockpit tells you what the RPM is doing. Confused yet? Most new pilots to a Curtiss were, especially when they tried understanding what was going on in that prop hub. When you set full low pitch a lot of things happened at once to get the prop moving. Electrical brushes (contacts) in the engine nose met up with slip rings on the butt-end of the prop housing. Roughly in the same location as the Hamilton's beveled cam. Power was sent down the brushes, where the slip rings picked it up, and that got the whole prop "live." Yes, the whole prop was electrified! That power also got an electric motor turning, which rotated two sets of planetary gears. A planetary gear is a large ring with three small gears rotating inside of it. The motor turned the first set of planetary gears, and those gears turned a second set. Which then turned a beveled gear that engaged the butt-end of each prop blade.
So instead of the Hamilton's oil push plate, the Curtiss had an electric motor with two sets of planetary gears that did the same job. To keep the prop blades where you put 'em, the motor had a friction brake in the very tip of the prop dome. Between that, and the 7,500:1 gearing ratio of the planetary gears, the prop was locked. Any desired change in pitch would send voltage to the prop, tripping the brake release, and then the motor would start cranking. It also had a rather curious quirk. When crews would do prop checks they used the battery. This would leave the battery drained down to the point it had just enough juice to trip the brake off, but not enough to get the electric motor turning. When this happened the prop would immediately slam to full low pitch (max RPM) and "run away." Crews were warned against using the battery for starts because changing the prop pitch would literally suck them dry. Contrary to popular belief, a generator doesn't run all the time in any WW2 aircraft. At low power there isn't enough speed to get it generating electricity. Which means you need either a putt-putt (lawnmower engine generator) or a huge battery. Every bomber had a putt-putt for operating basic electrical equipment on the ground. Fighters didn't have that though, they had a battery. Hence the reason prop checks were usually made during run-up. It was the only time when the generator was turning fast enough to crank out plenty of juice for the prop to be cycled.
Given that the electrical motors and wiring back then weren't all that great, you can imagine how frightful the crews were. The idea that your prop could suddenly run away because of wiring corrosion or rust on a contact isn't exactly comforting. Don't get the idea that it wasn't a reliable prop system. With proper care, and an eye for detail, a Curtiss could be kept running far longer than any hydraulic prop. Still, when it functioned, it was better than any Hamilton or Aeroproducts prop. You could directly set the pitch, or let the Auto setting handle that stuff for you. Pitch changes were precise and rather fast. Takeoff power gave you max RPM immediately. And you never had to worry if you might blow an oil line because the prop didn't need any oil. It got even better because you only needed electricity to change prop settings. Once the friction brake was locked it didn't need power to hold the pitch at a constant point. If battle damage or some sort of glitch stopped the generators from working, you didn't have to worry. A quick burp of the battery and you could make a fast change to whatever pitch or RPM setting you needed. Kick the battery off when you're done and there was minimal electrical drain.
Like the Hamilton, no computer simulation models the Curtiss to any accurate degree. Some model the almost instant application of full RPM on takeoff, but none model the sub-systems that made it a great prop. With a Hamilton or Aeroproducts you were always in Auto; the governor controlled everything. But with a Curtiss you were in command. You could set the exact pitch you wanted for takeoff, and then switch to Auto for the cruise out. Plus the system didn't require oil; both hydraulic props were known to blow oil if any seals or lines were even a little loose. All the Curtiss needed was a single electrical cable run from the wiring harness out to the brushes. Truly an amazing piece of hardware.

Aeroproducts Hydraulic

The most complex propeller used by any aircraft, from any country, during the second world war. Pilots didn't like the reliability and mechanics absolutely hated working on it. Like the Hamilton, it used oil to control blade pitch. Unlike the Hamilton, it was entirely self-contained inside the prop case. Oil, governor, oil reservoir, control lines, slip ring, screw-jacks, splined drives and hydraulic pistons were all contained in the prop case or in the prop itself. It was highly compact, making for a small weight penalty and easy identification. If you ever see a WW2 aircraft that doesn't have a dome sticking off the prop, it's an Aeroproducts. The F8F Bearcat uses this type of prop to give one example.
Instead of a fly-weight driven governor, the Aeroproducts had a slip-ring contact-type governor. A control ring mounted on the engine nose was controlled by the prop lever inside the cockpit. Moving the lever caused this ring to rotate slightly left or right. It looked a lot like a planetary gear system, because the inside of this ring had teeth to mesh with three small geared screws (screw jacks). These jacks were screwed into a second ring that had a bevel to it. When the control ring rotated slightly, the screws also rotated, moving the governor ring forward or backward. On top of this ring was a small contact that looked a lot like a record player's arm. That arm was the governor, and it rode on the ring's bevel. Low RPM was set by moving this ring forward so the arm dropped down to the low point on the ring's bevel. High RPM was the opposite; you moved the plate back so the arm rode higher on the bevel ring. If that got you confused, just wait until we get to the pitch-change system. Like the Hamilton, this governor arm was attached to a valve that either dumped oil out or let more in.
Now just inside the control ring was a geared ring from the reduction gear case that meshed with a small hydraulic pump. This pump supplied oil from the reservoir under high pressure to work the prop itself. When the pilot commanded a pitch change the control ring rotated left or right, which also rotated the screws, moving the governor plate forward or backward. The governor contact arm rode on the governor ring's bevel, moving up or down when the ring moved forward or backward. That moved a small valve to allow oil into each prop butt. Inside the prop butt was a piston chamber that filled with oil. As pressure increased it pushed down on a piston that compressed a splined drive shaft. Under the prop butt was a gear which meshed with teeth on the inner ring of the prop butt. As the spline was pushed down it hit that small gear, causing the whole prop to rotate. The travel distance of those pistons was carefully computed to also act as a blade stop without feathering the prop. This allowed, in case of an emergency, for the prop to operate as a fixed-pitch model.

07-15-2009, 12:42 AM
http://www.worldaccessnet.com/...ta6/ground/prop1.htm (http://www.worldaccessnet.com/%7Edelta6/ground/prop1.htm)

How to use this stuff

The notes below contain information vital to the operation of any propeller-driven aircraft. Over-speed conditions, back driving, and how the prop sets engine RPM are all covered here. Most of it is explained in plain English, with the odd technical term thrown in to keep things clear. I'd suggest you read the Propellers page first to gain an understanding into how these things work. Once you understand how a given prop design works, you'll be able to really understand what is going on in the sections below. To find out more information, I'd suggest heading over to the AvWeb Columns listing for John Deakin's articles. He's written more about props, mixture, and engines than anyone else I know of.

Overspeed Condition

Also known as a "runaway" or "running wild" prop. How it happens depends entirely on the prop design. Why it happens is almost universal: a loss of prop control. When a prop runs away it means the prop blades have no forces keeping them fixed at a given angle. Due to a Centrifugal Twisting Moment (CTM) the prop always wants to go to full low pitch. This is why props need some sort of lock to keep them from twisting in their sockets on the prop hub. Runaway props happen when that locking mechanism, whatever it may be, fails to keep the blade locked down. The blades slam down to full low pitch or against the physical stop while the engine immediately redlines. Without any RPM control the engine is free to run as fast as it wants. What's worse is the centrifugal force applied to the prop blades. As the prop rotates the blades want to come out of the hub. The root cuffs holding it securely in place won't allow for that. But when a prop runs wild the engine is cranking the prop over so fast the forces wanting to rip the blades out approach infinity. This is when blade separation occurs. It can destroy the engine, the prop, and even the aircraft.
Should a runaway happen on takeoff, you should immediately kill the engine and try to get the aircraft stopped. If the prop runs away on a twin-engined aircraft, it's a simple matter to shut down the engine that's running away. Continuing flight entirely depends on whether or not you're above minimum single engine speed. Single-engined aircraft, like most fighters, can have a devil of a time with runaway props. Especially in flight. With the prop making more drag than anything else, your only choice is to shut down and belly it in. Twin-engined aircraft can kill the engine that's running wild and still expect to make it home. The key here is speed, both on your part and the sheer velocity of the prop. The faster you notice the runaway the faster you can get the engine killed. If you don't pick up on it quickly enough (a very hard thing not to notice) you may throw a blade.
Hamilton-Standard props have a fixed plate in the prop dome that limits blade travel. Which means the prop can't go into full feather when it runs away. Thankfully it can still be controlled with the throttle because of the fixed blade stop. Granted, it means the prop is now acting like a fixed-pitch model, but at least it can still be controlled somewhat. Unfortunately, when a Hamilton runs wild it means all of the engine oil has been lost as well. Without oil you may as well find a cozy spot to belly it in or just bail out. Either way, the engine will most likely seize up from a lack of lubrication at some point in time. Leaving you with a glider instead of a fighter.
Curtiss Electric props running wild are even worse. They have no fixed stop; all pitch changes are done with an electric motor running through planetary gears. This means it isn't easy to get the prop running away. At a reduction ratio of 7,500 to 1 it is incredibly hard to back-drive the electric motor by changing prop blade angles. Especially with the friction brake working. Once the friction brake is cut out, the motor burns out, or a short in the system crops up... that's all she wrote. The blades will immediately swing out to full feather and start ocelating in the blade sockets. This ocelation doesn't last long, just a second or two, but it does create one hell of a vibration. The engine is now running away madly, the prop is creating zilch for thrust, and those big flat blades are making tons of drag. If some kind engineer had placed a physical stop inside the prop gears, you could still get home. But with the feathering requirement no stop exists, so you're stuck.
Aeroproducts props can run wild like any other. Though it does take a lot to get the prop into a condition like this. With the oil reservoir being mounted within the prop casing there's no danger of an engine hit disabling prop control. Usually what would happen is the nose-end of the aircraft would take a hit that damaged either the prop or the governor system. Either one might cause a runaway, and like the Hamilton this one does have a stop. The travel limit of those pistons inside each prop butt also acts as a blade stop. So you can still get home when this prop runs wild, though it'll act like a fixed-pitch model. Controlling the RPM with the throttle is more of a throw-back to the 1920's, but it works!
The only cure for a runaway prop is to kill the engine at once.


Turning the engine with the prop? It is entirely possible, but requires a very specific set of circumstances in order to occur. Diving at very low power settings for a length of time can cause it. So can running the RPM back so low that the air is moving faster than the prop wash. The typical scenario goes something like this. You're buzzing along at 20,000 feet and decide to land by using a somewhat steep descent. So you dial the RPM up for max drag to slow you down and chop power to begin your descent. Normally, an approach is flown at around 700 feet/minute of decent rate and several miles out. But you, deciding that a short and fast descent is best, cut power and nose it over. With the engine producing almost no horsepower the prop can't really bite into the airflow. So, because the airflow is moving faster than the prop wash, the airflow over the prop starts turning it with more force than the engine is. The load on the crankshaft shifts to the aft crank bearing while the prop starts turning the engine.
Meanwhile, all those crankshaft throws have a piston on them and the space between the two is getting loaded the wrong way. A few seconds of this isn't bad, especially if you keep the aircraft level or descending only slightly. A short dive in this condition probably won't hurt anything either. You're coming in to land, not diving in on an enemy fighter. A few minutes of this can start to cause serious problems. As you continue the descent you notice the RPM starts climbing. Now the governor senses this and it tries dialing pitch back to maintain your pre-set RPM level. Unfortunately the governor can't correct the problem because the aircraft isn't slowing down. The governor drives the pitch control right down to the stop, still to no effect. Roughly 30 seconds after this began, the main crank bearing has super-heated all the oil around it. Since the oil is too hot it becomes almost like gum, and without oil for lubrication metal-to-metal contact happens. The event itself is catastrophic; the crank jerks to a halt as the crank bearings melt down and hit the crankcase bearings. This causes the prop to really bark to a stop, giving the entire aircraft a torque jerk hard enough to wake up a narcoleptic.
The alert readers will say "Wait a minnit! The Curtiss doesn't have a stop!" Well, you are correct, sir! The Curtiss doesn't make an engine immune to this condition, but it does allow for a much larger pitch range. Which means instead of having a sudden load shift on the crank bearing that keeps increasing, the Curtiss can keep the loads roughly constant. If there's too much load on the engine, the governor on a Curtiss can call for full feather. That would terminate any problem right there from back-driving, but it would also let the engine run wild. Never mind the mess of drag it would make! You might be tempted to say the Aeroproducts can do the same thing; it can't. Instead of using one big plate (ala Ham-Stan) the Aeroproducts uses four smaller hydraulic pistons. Those have a fixed range of motion, which acts just like a stop only on an individual scale for each blade. In addition they also require high oil pressure to work, just like the Hamilton. Thankfully you can't blow the oil lines on either prop, as they both have a poppet valve to release excess pressure.
How can you avoid this? Real pilots would say the smart thing: "Don't descend with such low power." By pulling some power you keep the prop "loaded" and prevent this from happening. Or, you could do a very smart thing, and simply avoid low power/high RPM descents completely. Only a sudden shift in crank loads or prolonged back-driving can cause harm. Leaving the power at idle on final while in a semi-steep descent won't kill anything. As long as you do it for short periods of time, anyway.

How RPM stays put

This is incredibly simple to figure out. Keep in mind that engines can't produce unlimited power, and everything below is based on an engine running at a given power setting. As the prop rotates it keeps a near constant torque load on the engine. When the prop's torque load and the engine's torque load are equal, RPM is balanced. The engine is putting out enough torque to spin the prop, and the prop's torque from meeting airflow matches it exactly. When the prop torque load increases from a blade pitch increase, it slows down the engine. See, the engine wasn't producing enough power to spin the prop with such a large surface area now meeting the air. This is why high blade pitch equals low RPM; the large surface area of the prop meeting the air requires more power to get it cranking. Which results in the engine slowing down some while sending more torque to the prop. The opposite is also true: When you reduce the blade pitch angle you suddenly make the prop blades more streamlined, which makes the RPM rise. RPM rises because the engine was putting out enough force to turn the prop with a lot of torque. But when all that drag went away, the engine speed increased from a lack of high torque load. Hence the reason why low blade pitch equals high RPM.
On the ground full RPM isn't available for a multitude of reasons. First, you're not pulling enough power just sitting on the ramp at idle. The prop's torque load is rather large and the engine isn't putting out enough torque to spin it up to full RPM. Plus hydraulic props don't have enough oil pressure built up to change the blade angle that much. In a way, a prop is a lot like a drill bit. If you just pull the trigger and let the drill whir it's turning over at max RPM, but with almost no torque load. Now stick that same drill bit into a block of wood and see what happens. Surprised? The drill slows down because you're putting more of a torque load on the engine. When the torque load on the engine from the bit hitting wood and the force the engine exerts on the bit are equal, you have balanced RPM. The exact same thing happens when a prop blade meets the air. Higher blade pitch means more force is required to turn the prop over; all that surface area being whirled around creates a load of drag. Yet if you reduce the blade pitch to make it more streamlined, you reduce that surface area and decrease the power required to turn it. And, since the engine is putting out a fixed amount of power, it speeds up. And to think they figured this out back in the 1920's!

07-15-2009, 12:43 AM
http://www.worldaccessnet.com/~delta6/ground/lop.htm (http://www.worldaccessnet.com/%7Edelta6/ground/lop.htm)

Long Range Ops
Making your *** uncomfortably numb

Today, ladies and gents, you'll learn about that red knob. No, not the one that says "Kill Dweeb", the mixture knob. You can do all sorts of fun stuff with this thing. Not the least of which is making your butt go numb by flying eight-hour missions. Marine and Army Air Force pilots did this over the Pacific, so why can't we? Well, if your chosen sim doesn't have any mixture control then you can skip this one. Neither Warbirds nor Aces High have a mixture control. Targetware does, and so does Il-2. All you need now is a EGT gage and we can play!

Mixture: what it does

Mixture controls how much fuel gets dumped into each cylinder, just as the throttle controls how much air goes in. For takeoff you want all the power you can get, so you go full up on everything. Full power, full mixture, full RPM. Ahh, but did you know there's a reason you run the mixture full rich on takeoff? No, it isn't because it gives the most power. It's because you need fuel for cooling. What? No, it's not a joke! The more fuel you dump into each cylinder the longer it takes to completely burn, so you actually end up running cooler. Not only does this keep the engine cool it also prevents detonation. Unfortunately you also torch off whole mess of fuel just to keep the engine from being roasted. Pilots didn't know this back then, and most don't now.
Detonation happens when you pull the mixture back to what's called "40 rich of peak". EGT is short for exhaust gas temp, or how hot the engine exhaust is. Pulling mixture back to the 40 rich reading means the spark plug is lighting off the fuel dangerously close to top-dead-center on each piston. Instead of getting a hard shove, that piston is now being beaten to death with a sledgehammer. Unchecked, detonation can blow off cylinder heads or really destroy your pistons. Most pilots simply park the mixture control high enough so the EGT gage reads 50 on the far side of peak (or 100 rich of peak). The "Auto: Rich" setting for almost every US aircraft automatically held the mixture around this point. It gave max power for combat and takeoff, but also roasted a ton of fuel off. Seeing a fuel flow of 150 gallons an hour or more wasn't uncommon.
For cruise flight you can pull the throttle back to a more conservative setting, and pop the mixture back to "Auto: Lean." Since you're pulling less than 65% of the engine's total power output, you don't have to worry about detonation. Plus putting it back to this setting burns less fuel. The less fuel wasted on cooling the farther you can go. Reducing RPM is also a good thing; you'll burn a little less fuel and let the prop grind along. So what is this "rich of peak" stuff? Glad you asked...

Peak: Rich and Lean

You need an EGT gage to see this one. All it has is a needle that rises and falls based on how hot or cold the EGT is. If you push the mixture forward you're making it richer, so pushing it past peak it called "rich of peak." Pulling the red knob back is making the mixture leaner, so pulling it past peak is called "lean of peak." Here's an example: You've just taken off and want to set up regular cruise power. So you pull the RPM back from 2,700 to 2,400, reduce throttle from 54" to 42", and pull the mixture back until you see the needle peak. Once you figure out where peak is, you rather quickly set it up for 100 or so rich of peak. This is done by looking for where the needle peaks and pushing the mixture forward (richer) until you get 100 less than peak. Right now you're probably burning off somewhere near 200 gallons an hour, possibly more. Throwing any WW2 aircraft into Auto Rich will do the same thing. Want to go a little farther? Ok, try this...
Pull power back to 35" and pull the revs back to 2,250. Now pull the mixture back into the Auto Lean position. This pops the mixture back to somewhere around peak EGT, but since you're pulling less than 65% power you're okay. Detonation can't occur at low power settings. Right now that fuel flow gage is probably saying that you're cooking off 170 gallons an hour or so. Want to go a lot farther? Ok, try this. Leave everything right where it is and pull the mixture back until you see it peak, and keep right on pulling until you see 75 less than peak. You're now running at 65% of rated power, on the lean side of the peak EGT. Which means you're very far away from detonation, all of the fuel-air mixture is burning, no fuel is being wasted on cooling, and your engine is burning much cleaner. Fuel flow at this setting is probably around 150 gallons an hour, possibly even a bit less than that. Instead of torching off gobs of fuel by running rich of peak, you're now burning 15-20% less gas while the engine is putting out only 5% less power.
As you can imagine, you aren't going to go all that fast when running lean of peak (LOP). For high speeds you need Auto Rich and all the engine controls shoved into the firewall. This is intended for very long flights over huge distances. Remember the P-38's that intercepted Admiral Isoroku Yamamoto over Bougainville? They ran leak of peak all the way from Guadalcanal to bag him that morning. It was a six hour mission; not because they were going so slow, but because they had that far to fly. Running with the usual power settings would have put them somewhere off Bougainville with dry tanks. P-47N's operating in the Pacific also figured out lean of peak, and with such huge fuel loads they could run for almost ten hours! Let's see any P-51 jockey over Germany beat that! Aircraft fitted with turbosuperchargers were fantastic at running LOP. At high altitudes you have a higher ground speed and burn a lot less fuel. Throw LOP operation into the mix and you can see how flying at 25,000 feet would let you run almost forever.

07-15-2009, 12:44 AM
http://www.worldaccessnet.com/...ta6/ground/super.htm (http://www.worldaccessnet.com/%7Edelta6/ground/super.htm)

Turbos & Supers
Horsepower to spare

Supercharging began during the 1920's as a way to break altitude records. Not being slow when it came to faster aircraft, the military quickly caught on. And right behind them were the engine manufacturers. Pratt & Whitney, Wright, and Allison all packed superchargers on their engines. Pilots and mechanics alike called them blowers. Turbosupercharging (turbos for short) came along in the 1930's along with two-speed two-stage blowers. Using a variable speed supercharger and a governor to keep the speed set right, they allowed full sea-level power clear up to 30,000 feet or more. At such high altitudes engine power was usually half or a quarter of normal. With these new toys, aircraft got a whole lot faster!

Blowing hot air

A supercharger is a really simple mechanism. It consists of a wheel with many vanes radiating out from the center section. These vanes are straight-bladed to prevent them from cracking or breaking under high loads. Mounted between the intake manifold and the carburetor, they supplied a pressurized mix of fuel and air to the engine. By using a pressurized fuel-air charge they delayed power loss brought on by climbing into thinner air. They were either run directly off the crankshaft (direct drive) or were geared off of it to increase blower speed. Blowers also came in five flavors. Some aircraft, like the early Wildcat, had a simple single-speed single-stage blower. It allowed full rated power up to around 12-15,000 feet, after which power fell off rather fast. A single-stage, two-speed model on the FM-2 or the P-40F allowed the pilot to "shift gears" on the blower by putting it in "high blower." Instead of spinning at five times crankshaft speed, it was now whirling at eight times crank speed. Which meant a setting of 2,500 RPM on the engine would have the blower running at 20,000 RPM. It helped, but not nearly enough to hit the high altitudes required for long flights. After reaching 18,000 feet the engine would start to starve for air.
Two-speed two-stage blowers had three different types. The Merlin engine had a two-speed two-stage blower with both blowers running on the same shaft. This had the effect of rapidly compressing the fuel-air charge, but couldn't be directly blown into the engine. At such high compression rates the fuel-air temperature was hideously high, and it needed to be cooled off. So Rolls-Royce installed an "aftercooler" consisting of a radiator placed just in front of the intake manifold. Glycol was pumped from the aircraft radiator under the belly, up to the engine, where it hit a splitter. That splitter ran it down to the aftercooler's radiator, and then back up to the engine's hot coolant lines. From there the coolant ran back down to the radiator under the belly. It worked; the fuel-air charge was cooled to the point it could be blown into the engine and not cause detonation. This allowed a maximum altitude of 30,000 feet to be reached with minimal power loss. To boot, it was entirely automatic in operation.
Another type of two-speed two-stage model was the type the US used on the F6F, F4U, and F4F-4. It consisted of a single-speed single-stage blower running off the engine crank between the carburetor and the intake manifold. Like the single-speed single-stage above, it constantly ran at a 5:1 ratio off the crankshaft. A second two-speed blower was piped directly into the carburetor, with an intercooler between the auxiliary blower and the carb. Takeoff was done using so-called "neutral blower" since the auxiliary blower was not running. When the pilot needed more power he shifted into "low" blower, which used a clutch to drive the auxiliary blower off the engine crank. Air went through the auxiliary blower, passed through an intercooler (air-to-air radiator), and then got dumped into the carb. The single-speed blower then compressed fuel with the pre-compressed air charge and sent it on to the engine. If a pilot needed even more power, he hit "high" blower, which kicked the aux blower into high gear. Low blower was used between 5 and 18,000 feet, while high blower was used from 18,000 feet on up.
Only one aircraft used a variable-speed two-stage blower; the P-63 King Cobra. It had, like most other aircraft, a single-speed single-stage blower running off the crankshaft. But just in front of it was a variable-speed blower that had a real mess of gears to pick from. At 7,000 feet it cut in, and was entirely automatic in operation. As the air got thinner the blower would shift gears into a higher ratio, allowing smooth operation clear up to 25,000 feet. The P-63's airspeed vs. altitude curve resembles a backwards C because of this unit.

Infinite speed control

Turbosuperchargers are a true marvel of operation. They take exhaust gas to crank a blower at infinitely variable speed, allowing full rated power beyond 35,000 feet. At such high altitudes an aircraft has a much higher ground speed, not to mention a longer range. Fighting at those altitudes requires a lot of horsepower to be generated, which the turbo does very well. As an example, I'll use the greatest high-altitude fighter of the war: the P-47.
It starts with air entering a duct under the engine that runs clear back to the tail. Air goes back to the turbo where it gets pressurized, just like in a blower, and passes through an intercooler. An intercooler is a two-chambered box where outside air and compressed air pass by each other, exchanging heat. This cools off the compressed air enough so it can be directly dumped into the engine and not cause problems. As this compressed air charge runs through the carb and towards the engine, it gets mixed with fuel. Then it's compressed again by a single-speed single-stage blower. The engine exhaust is routed past a waste-gate that controls exhaust back pressure to regulate the turbo speed. Too much pressure means the turbo is spinning too fast, and the waste-gate opens to dump more exhaust overboard. If the turbo needs to spin faster, the waste-gate closes a bit to increase both exhaust pressure and turbo speed. As the exhaust hits the turbo it contacts a wheel with little "buckets" on it. That spins up both the exhaust wheel and the blower right above it. Both the blower and exhaust wheel sit on the same shaft, which is how a turbo regulates both manifold pressure and its own speed.

When a pilot was running at cruise power and suddenly needed an extra kick, he could slam the throttle forward. This action closed the waste-gate, which in turn dramatically raised manifold pressure. As a result the pilot had a whole lot of power available at any altitude. With such a variable speed, the turbo could give a little or a lot of additional air depending on the altitude. Controlling this beast were two things: the throttle, and the governor. The throttle opened or closed the waste-gate by a series of push-rods and a control unit. The governor worked with oil pressure; too much pressure meant the turbo was turning a tad too fast, so it would crack the waste-gate open. By lowering exhaust pressure the turbo slowed down, which reduced oil pressure. Not enough oil pressure meant the turbo had to really get moving, so the governor closed the waste-gate. That increased exhaust pressure and got the turbo spinning faster. In all it was an ingenious system of giving an engine maximum air charge to create maximum power at very high altitudes.

07-15-2009, 12:45 AM
http://www.worldaccessnet.com/~delta6/ground/aero.htm (http://www.worldaccessnet.com/%7Edelta6/ground/aero.htm)

Aerodynamics of Takeoff
Make it stop!

Looking for the right answer to the proverbial question "what makes the airplane go crazy on takeoff"? Well, you've got the right page. Too many people believe it's only torque that makes the plane go ape when you punch it. In reality, that's only one third the story. Yup, there's three other factors involved.


Have a drill handy? Good, it makes explaining torque really easy. Torque is a measure of force, usually in foot-pounds, that something produces. In our case, an engine with many cylinders and around 1,000 horsepower. What happens when you shove the drill bit into wood? The bit bores a hole in the block, but what does the drill do? Yup, it tries twisting right out of your hand. This is torque. See, the drill is applying the same amount of torque to the bit as it does to your hand. Without you holding it there, the drill would simply spin in place with the bit jammed into the block. Funny thing, a prop on any aircraft acts the same way.
On takeoff, or with a rapid application of power, the engine tries twisting right out of its mounts. Like the drill, though, it is rather anchored; to the airframe. The wings, fuselage, and tail surfaces act just like you hand with the drill. They hold the whole works in roughly one place while the engine tries spinning the fuselage faster than a carnival ride. When you're in the air in a Bf-109, punch WEP. It'll suddenly rock hard to the left; that's torque! On the ground it tries to drive the left gear leg into the pavement, in the air it tries twisting the motor mounts into a pretzel. Thankfully, motor mounts and gear legs are stronger than the engine's maximum torque force, so they don't get bent.
This does, however, explain why the aircraft suddenly dips to one side and starts tracking left (or right). With one gear leg partially compressed, and the other partially extended, it's like one side of a car getting light when you blitz through a turn. But it isn't responsible for for the aircraft trying to zig-zag all over the place. Ohhh no, that's called P-factor.


P-factor is otherwise known as "propeller factor" and refers to a somewhat interesting side-effect of that over-sized fan out front. Asymmetrical (not even) airflow is generated by anything with a propeller and its *** on the ground. Look at any taildragger just sitting there on the ramp. Note how the nose is sitting at a positive angle while the wheels are level. Yeah, I know it's obvious, but this is important. As the aircraft travels forward along the runway, the prop wash won't be even. Air strikes the lower prop blades first because they stick out more, generating more thrust down there than up top. With the engine spinning right, torque smashing down on the left gear leg, and the prop wash moving right, the aircraft wants to go left.
Lose you? Newton's Third Law of Motion: for every action there is an equal and opposite reaction. With the uneven prop wash going right, the right side of the aircraft is effectively "unloaded." Throw the left-side torque into things and it's just like steering a car left by moving big weights side to side. As a result, the aircraft goes left. Though you should know P-factor isn't nearly as strong as...

Spiral prop wash

I know you've seen the photos of spectacular propwash vortices. There's this wisp of a contrail thread flying back off the prop tips and over the wings or fuselage. Looks pretty cool, doesn't it? It's also the main reason why your aircraft jerks left when you stomp on the gas. Since the prop is spinning to generate all that wind, the wind is spinning too. It's getting smacked by a prop blade and moved in a circle. An object in motion tends to stay in motion, so the prop wash spins right on over the aircraft. Even at idle.
Now look on back at the tail feathers, especially the vertical stabilizer. It sticks up into the prop wash and gets smacked on the left side by this high-pressure spiral of air. Set a pen on your desk and push against the left side, back near the clicker-end. The nose of your pen went left, the same side you pushed against. Oddly enough, the prop wash does the same thing with the tail and nose of any prop-driven plane. With the prop wash smacking the left side of the tail, the nose goes left. So you need to punt in some right rudder or get hired as the world's fastest lawnmower! Between the spiral prop wash, torque, and P-factor you can pretty much explain why the aircraft zings off to the left. Torque compressed one shock and unloads the other, just like when a car turns. P-factor gets things moving while spiral prop wash really kicks the tail around.

Gyroscopic Precession

Uhhh, yeah, that thing. Ever play with a gyroscope as a kid? Well, that's what the prop is. One big gyro spinning at around half of the engine speed. Did I mention the prop weighs at least 300 pounds? For Cessna-type jobs it weighs a lot, for old warbirds is weighs even more. The 15-footers on a C-46 Commando tip the scales at roughly a ton and a half each. Hell of a gyro! Anyway, this is what makes the aircraft slide out when you pitch up. Fly a P-51B in Aces High and you'll see it the instant you pitch up at low speed. It does this because gyro precession applies the force 90 from the direction you're pulling the aircraft in. In the case of nearly aircraft, this results in a slight (or hard) yaw. Pull up in a P-51B and you'll watch the ***-end slide out to the left. This is from gyro precession. Pulling any aircraft up in a three-point attitude from the runway will result in the same thing. The tail will slide out to the left usually as precession from the prop kicks in.
Which is about as complex as I can get on the subject. Aerodynamics is a complicated subject, especially when you start talking precession. Right about now a geometry major is probably trying to wrap his head around this one while the aerospace engineer is laughing himself silly. Still, it's the best explanation I could come up with.

Countering this stuff

Use the RUDDER! Gyro precession, spiral prop wash, and P-factor all get countered by stepping on those pedals on the floor. In flight, on takeoff, or even landing you can counter those three terror twins by stomping on the rudder pedal. Torque is a rolling moment, not a yawing moment, so you sling the stick over just a skosh to the right. In every case (even the 109) you can stop torque from barrel rolling the aircraft by adding a little aileron trim. That'll stop the plane from doing barrel rolls unless you tell it to.

07-15-2009, 07:17 AM
http://forums.ubi.com/images/smilies/11.gif http://forums.ubi.com/images/smilies/25.gif http://forums.ubi.com/images/smilies/clap.gif http://forums.ubi.com/images/smilies/metal.gif http://forums.ubi.com/groupee_common/emoticons/icon_cool.gif

I can't thank you enough for this! This stuff interests me so much, and i feel a lot smarter just from learning all of this (particularly the section on the propellers, something i'd always been curious about). I also found the part "back driving" very interesting, something that obviously isn't modelled, but will come in handy if i ever get to fly a 2000hp warbird! Last thing you want is your crankshaft running dry...

Now i'm wondering if mixture in il2 actually changes fuel consumption....I suddenly have the urge to fly FSX for the CEM and clickable cockpits http://forums.ubi.com/images/smilies/shady.gif

Seriously, thanks!

I'll also add the link to the page your taking this from, there's some interesting stuff there...

http://www.wa-net.com/~delta6/ (http://www.wa-net.com/%7Edelta6/)

07-15-2009, 08:35 AM
Originally posted by danjama:
Now i'm wondering if mixture in il2 actually changes fuel consumption.... Very unlikely.
Messing with mixture doesn't change the speed (if you are within limits, i.e. if you do not see the trailing smoke from engine due to bad mix settings). Therefore, most logical thing to conclude http://dammitja.net/lj/opp/smileyvulcan.gif is that it doesn't change the fuel consumption.

07-15-2009, 08:57 AM
Originally posted by DKoor:
Therefore, most logical thing to conclude...

The most logical thing is not to conclude anything before actually testing.

07-15-2009, 09:06 AM
It doesn't.
Nice reads, thanks for posting!

07-15-2009, 09:07 AM
Originally posted by DKoor:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by danjama:
Now i'm wondering if mixture in il2 actually changes fuel consumption.... Very unlikely.
Messing with mixture doesn't change the speed (if you are within limits, i.e. if you do not see the trailing smoke from engine due to bad mix settings). Therefore, most logical thing to conclude http://dammitja.net/lj/opp/smileyvulcan.gif is that it doesn't change the fuel consumption. </div></BLOCKQUOTE>

Yea this is what i thought anyway, oh well...

07-15-2009, 09:23 AM
Originally posted by idonno:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by DKoor:
Therefore, most logical thing to conclude...

The most logical thing is not to conclude anything before actually testing. </div></BLOCKQUOTE>You are right in a sense that if it can be checked then why not.
I'll just cut it...
IL-2 test conditions, LaGG-3S35 test results after 5 mins of level flight.
Starting fuel - 130
40% mix - 102,9
120% mix - 102,8
Fuel consumption is virtually same.
Speed is same, 526kph.

BTW LaGG is quite a consumer http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif .

07-15-2009, 09:33 AM

Just kidding thanks for this, it's very helpful bud.

07-15-2009, 09:42 AM
mixture can help with tempurature too.

Not sure if its modeled in game, but if you're burning too lean then there is a tendency for the engine to run hotter.
Sometimes using 120% on deck helps with this, bt i don't notice it much in game, but as for the effects of burning too rich, that seems to be more noticeable.

07-15-2009, 09:47 AM
Originally posted by danjama:
Now i'm wondering if mixture in il2 actually changes fuel consumption.... in some planes it does. for sure. it is common use in virtual racing, if a course is too long for 25% fuel.

07-15-2009, 04:02 PM
Since this is greatly misunderstood and relevant info I have to.....Bumpinsky!

07-15-2009, 04:20 PM
I think anybody here with a great interest in propeller driven airplanes (isnt that all of us?) should read this!

07-15-2009, 04:47 PM
Agreed, should be a sticky~!

07-15-2009, 06:02 PM
Thank you sir for this excellent, and much needed, post!

I hope that when SoW appears that realistic prop pitch, and it's effects, are modeled.

07-15-2009, 06:10 PM
For those interested, much great stuff also available at Aircraft Engine Historical Society (http://www.enginehistory.org/).

07-15-2009, 06:28 PM
Originally posted by R_Target:
For those interested, much great stuff also available at Aircraft Engine Historical Society (http://www.enginehistory.org/).

http://forums.ubi.com/images/smilies/clap.gif http://forums.ubi.com/images/smilies/11.gif

07-15-2009, 06:39 PM
Taken from that same website above, i'd like to post this. I think it just about sums up my http://forums.ubi.com/images/smilies/inlove.gif loving feelings for warbirds and piston engines:

The Unlimiteds go flashing through the racecourse, engines howling, air shearing, heat waves streaming. Four hundred eighty miles an hour is 8 miles a minute, and the elite racers take about 70 seconds to cover the 9.1 mile Reno course. If you could take a souped P-51 racer flying the circuit at Reno, slow time down, and examine just one second, what would you find?

In that one second, the V-12 Rolls-Royce Merlin engine would have gone through 60 revolutions, with each of the 48 valves slamming open and closed 30 times. The twenty four spark plugs have fired 720 times. Each piston has traveled a total of 60 feet in linear distance at an average speed of 41 miles per hour, with the direction of movement reversing 180o after every 6 inches. Three hundred and sixty power pulses have been transmitted to the crankshaft, making 360 sonic booms as the exhaust gas is expelled from the cylinder with a velocity exceeding the speed of sound. The water pump impeller has spun 90 revolutions, sending 4 gallons of coolant surging through the engine and radiators. The oil pumps have forced 47 fluid ounces, roughly one-third gallon, of oil through the engine, oil cooler, and oil tank, scavenging heat and lubricating the flailing machinery. The supercharger rotor has completed 348 revolutions, it's rim spinning at Mach 1, forcing 4.2 pounds or 55 ft3 of ambient air into the combustion chambers under 3 atmospheres of boost pressure. Around 9 fluid ounces of high octane aviation fuel, 7843 BTU's worth of energy, has been injected into the carburetor along with 5.3 fluid ounces of methanol/water anti-detonant injection fluid. Perhaps 1/8 fluid ounce of engine oil has been either combusted or blown overboard via the crankcase breather tube. Over 1.65 million foot pounds of work have been done, the equivalent of lifting a station wagon to the top of the Statue of Liberty.

In that one second, the hard-running Merlin has turned the propeller through 25 complete revolutions, with each of the blade tips having arced through a distance of 884 feet at a rotational velocity of 0.8 Mach. Fifteen fluid ounces of spray bar water has been atomized and spread across the face of the radiator to accelerate the transfer of waste heat from the cooling system to the atmosphere.

In that one second, the aircraft itself has traveled 704 feet, close to 1/8 mile, or roughly 1.5% of a single lap. The pilot's heart has taken 1.5 beats, pumping 5.4 fluid ounces of blood through his body at a peak pressure of 4.7 inches of mercury over ambient pressure. Our pilot happened to inspire during our measured second, inhaling approximately 30 cubic inches (0.5 liter) of oxygen from the on-board system, and 2.4 million, yes million, new red blood cells have been formed in the pilot's bone marrow.

In just one second, an amazing sequence of events have taken place beneath those polished cowlings and visored helmets. It's the world's fastest motorsport. Don't blink!

07-15-2009, 06:55 PM
This also gets me going http://forums.ubi.com/images/smilies/heart.gif :


H type 24 cylinder napier sabre engine

07-16-2009, 02:38 PM