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Thread: Aircraft Instrumentation - Engine Instruments

  1. #1

    Default Aircraft Instrumentation - Engine Instruments

    Aircraft Instrumentation – Engine Instruments

    This discussion will deal with engine instruments that are unique to aircraft. The main differences being aircraft that are liquid cooled and those that are air cooled, and aircraft that are equipped with a fixed pitch propeller and those that have a constant speed propeller. The other difference to be discussed will be the difference between a turbo supercharged airplane and one that is supercharger.

    Liquid cooled vs air cooled engines

    The P-40 engine is liquid cooled and as the name would imply the engine is cooled much the same way any modern day automobile would be cooled. The engine is surrounded by a coolant jacket that circulates coolant around the engine drawing off the heat. The coolant is then routed through a radiator that cools the coolant and re-circulates it around the engine. By adjusting the position of the radiator door opening you can regulate the amount of cooling provided to the engine. That big nose inlet, that gives the P-40 its distinctive look, is the coolant radiator. If you look at the underside of the airplane, just ahead of the leading edge of the wing, you will see the doors that operate allowing you to open or close them to regulate engine cooling. On the P-51 you will notice the radiator door on the bottom of the airplane aft of the trailing edge of the wing. Instrumentation used to monitor the temperature of the engine is primarily the coolant temp gauge. Oil temperature, to a lesser degree, is also used to monitor engine temperatures.

    On an air cooled engine, the P-47 for example, the engine is cooled by air passing through and around the vanes on each cylinder. By opening and closing the cowl flaps, that’s the ring of little doors that surround the aft portion of the engine cowling, you regulate the amount of air passing through the engine cowling thus regulating the amount of engine cooling. The system is very simple yet very effective. Instrumentation used to monitor the temperature of an air cooled engine is primarily the cylinder head temperature (CHT), and of almost equal importance is the oil temperature. Oil in an air cooled engine is usually measured in gallons not quarts. In addition to lubricating the engine, the oil draws off much of the engine heat and cools the engine by passing through an oil radiator. Many an air cooled radial engine pilot has stopped for fuel and jokingly told the attendant to “top the oil and check the gas”.

    When you compare the two different cooling systems, some advantages and disadvantages become apparent. Consider the amount of plumbing and hardware, which adds additional weight, in a liquid cooled engine. Also consider what a single bullet can do to the cooling system of a liquid cooled airplane. Normally the liquid cooled airplane has less frontal area which creates less drag. The P-40 being an exception with that big radiator located under the engine. However, compare the frontal area of the P-47 and the P-51 and you will see the difference.

    Fixed pitch propeller vs Constant speed propeller


    First of all you have to understand that a propeller is an airfoil, just like the wing is an airfoil, and subject to the same aerodynamic principles of any airfoil. I will discuss loss of power with altitude when I discuss turbo and super charging, but for the time being let’s assume we are flying a normally aspirated engine.

    Each propeller blade has a twist and thickness that change from the root of the blade to the tip of the blade. Because the blade is rotating, the speed of rotation is different along its length and the twist and thickness are designed to provide an even distribution of thrust from the blades of the propeller. Think of the twist of the blade as the angle-of-attach or pitch, because that is what it is. In its simplest form, when the fixed-pitch propeller is attached to an engine, the pitch of the blades is designed to allow the engine to rev up to the maximum allowable rpm for that engine. On takeoff, when you open the throttle, the engine will accelerate to red line as indicated on the tachometer, producing the rated horse power. As you start a climb you will notice the rpm drops as the propeller pulls the airplane upward. Think in terms of an automobile that starts to climb a hill. After completing a climb and leveling off, the fixed pitch engine will start to accelerate back up to the red line just like the car would when leveling out on a flat road after its hill climb. At this point you would pull the throttle back to establish whatever engine rpm is appropriate for your flight, the throttle having control of the engine rpm. Lowering the nose of the airplane will cause the engine rpm to increase, as it would in a car going downhill, in order to keep the engine from exceeding the red you would have to pull the throttle back.

    To sum it all up, the throttle controls engine rpm and engine rpm changes with the pitch attitude of the airplane. The only instrument we have to indicate the power output of the engine is the tachometer.

    As you will see, a constant speed propeller has many advantages over its fixed pitch counter part. The blades of the constant speed propeller actually rotate within the hub of the propeller. They rotate in unison so when the pitch of one blade is changed, the pitch of all blades is changed. Depending on the application, the propeller is designed with a low pitch setting that when attached to an engine allows that engine to rev up to its maximum rpm which in turn provides maximum rated horsepower. As you apply power on takeoff, the engine will accelerate to its maximum rpm, usually well before you have advanced the throttle all the way. At that point the blades on the propeller start to rotate increasing the pitch angle. As with any airfoil that increase in pitch, (angle-of-attack) increases thrust, (lift) which increases drag, which slows the engine rpm. A governor is used to adjust the pitch on the blades to keep them from exceeding the engine red line.

    After takeoff with a constant speed propeller, you will notice the propeller rpm remains the same during the climb. The propeller governor is adjusting the pitch on the propeller blades allowing the engine to maintain its maximum rpm. Because rpm is horsepower you are able to maintain maximum horsepower throughout the climb, thus maintaining maximum performance. Because the throttle is not directly controlling the propeller rpm, another engine instrument must provide the pilot with information on engine power. The manifold pressure (MP) gauge provides that information.



    The MP gauge is used to measure the atmosphere within the intake manifold of the engine. When the engine is not running, with the airplane sitting on the ground at sea level, the MP gauge will indicate sea level pressure or roughly 30 inches of mercury (Hg). Since the atmosphere loses approximately 1 in. Hg. for ever 1,000 ft of altitude gained this same airplane sitting at an airport that was 5,000 ft above sea level would be indicating 25 in. Hg. So with the normally aspirated engine you will never get a higher manifold pressure than when at sea level or 30 in. Hg.

    Turbosupercharging vs Supercharging

    To increase an engine’s horsepower, manufacturers have developed supercharger and turbosupercharger systems that compress the intake air to increase its density. Airplanes with these systems have a manifold pressure gauge, which displays manifold absolute pressure (MAP) within the engine’s intake manifold.

    As a normally aspirated aircraft climbs it loses manifold pressure and eventually reaches an altitude where the MAP is insufficient to produce a climb. That altitude limit is the aircraft’s service ceiling, and it is directly affected by the engine’s ability to produce power. If the induction air entering the engine is pressurized, or boosted, by either a supercharger or a turbosupercharger, the aircraft’s service ceiling can be increased.

    Superchargers

    A supercharger is an engine-driven air pump or compressor that increases manifold pressure and forces the fuel/air mixture into the cylinders. The higher the manifold pressure, the more dense the fuel/air mixture, and the more power an engine can produce. With a normally aspirated engine, it is not possible to have manifold pressure higher than the existing atmospheric pressure at which it is flying. A supercharger is capable of boosting manifold pressure well above 30 in. Hg.

    A supercharger is driven by the engine through a gear train at one speed, two speeds, or variable speeds. In addition, superchargers can have one or more stages. Each stage provides an increase in pressure. Therefore, superchargers may be classified as single stage, two stage, or multistage, depending on the number of times compression occurs.

    Some of the large radial engines developed during World War II have a single-stage, two-speed supercharger. With this type of supercharger, a single impeller may be operated at two speeds. The low impeller speed is often referred to as the low blower setting, while the high impeller speed is called the high blower setting. On engines equipped with a two-speed supercharger, a lever or switch in the cockpit activates an oil-operated clutch that switches from one speed to the other.

    Turbosuperchargers

    The most efficient method of increasing horsepower in a reciprocating engine is by use of a turbosupercharger, or turbocharger, as it is usually called. A drawback of gear-driven superchargers is that they use a large amount of the engine’s power output for the amount of power increase they produce. This problem is avoided with a turbocharger, because turbochargers are powered by an engine’s exhaust gases. This means a turbocharger recovers energy from hot exhaust gases that would otherwise be lost.

    Turbochargers increase the pressure of the engine’s induction air, which allows the engine to develop sea level or greater horsepower at higher altitudes. A turbocharger is comprised of two main elements — a turbine and a compressor. The compressor section houses an impeller that turns at a high rate of speed. As induction air is drawn across the impeller blades, the impeller accelerates the air, allowing a large volume of air to be drawn into the compressor housing. The impeller’s action subsequently produces high-pressure, high-density air, which is delivered to the engine. To turn the impeller, the engine’s exhaust gases are used to drive a turbine wheel that is mounted on the opposite end of the impeller’s drive shaft. By directing different amounts of exhaust gases to flow over the turbine, more energy can be extracted, causing the impeller to deliver more compressed air to the engine.

    In summary, as good as the simulation is, it doesn’t represent engine indications totally accurate. It does reflect the overall conditions that the pilot would encounter while flying a high performance single-seat fighter. If you get anything out of this discussion just remember how to use your engine controls and instruments to help nurse a crippled airplane home. An over heating engine can be cooled by readjusting your cowl/radiator doors. You can also reduce power and increase airspeed to bring down a high temperature.

    If you have a runaway prop, it won’t take long for the engine to self destruct. What is a runaway prop you ask, it’s when a well place round or debris for a strafing run disables your propeller governor. All of a sudden the rpm on you engine blows by the red line and seems uncontrollable. However, you do have a way of controlling that runaway prop. What has happened is with the loss of the governor, the propeller has gone to low pitch. Low pitch is low angle-of-attack and low drag hence the over speed. You now are flying a fixed pitch propeller. The engine rpm on a fixed pitch propeller is controlled by throttle and if you want to save that engine, to get you home, you must pull the power back a bring the rpm back to a reasonable setting. Unless you have a bandit on your tail I would suggest you take the shortest route home, because you are out of the fight.


    "The most important branch of aviation is pursuit, which fights for and gains control of the air"
    U.S. Brigadier General William Mitchell

  2. #2

    Default Prop-Pitch

    RJ,

    Just to add to your explanations here's a link to a good discussion about prop-pitch.

    http://www.mission4today.com/index.p...ewtopic&t=6692

    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.

    Jay

    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)
    Trapper
    Some see the glass as half empty, some see it as half full. I see a glass that's too big.

  3. #3

    Default Re: Aircraft Instrumentation - Engine Instruments

    Trapper,

    When you have time on your hands, take one of the fighters up and climb and dive at moderate angle with the prop pitch set a 3000 or 2700 which is appropriate and tell me if the engine rpm change when you do that.

    RJ


    "The most important branch of aviation is pursuit, which fights for and gains control of the air"
    U.S. Brigadier General William Mitchell

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