The Four Forces of Flight - Thrust
Thrust opposes drag. The engine creates thrust and moves the plane forward. (Gravity provides the thrust for a glider.) The engines push air back with the same force that the air moves the plane forward; this thrust force-pair is always equal and opposite according to Newton's 3rd Law. When the plane flies level at constant velocity, thrust equals drag.
Propellors
A Propeller is a type of fan which transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and air or water is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's principle and Newton's third law.
Aircraft propellers convert rotary motion from piston engines or turboprops to provide propulsive force. They may be fixed or variable pitch.
Wooden Propellers: Wooden propellers were used almost exclusively on personal and business aircraft prior to World War II .A wood propeller is not cut from a solid block but is built up of a number of separate layers of carefully selected .any types of wood have been used in making propellers, but the most satisfactory are yellow birch, sugar maple, black cherry, and black walnut. The use of lamination of wood will reduce the tendency for propeller to warp. For standard one-piece wood propellers, from five to nine separate wood laminations about 3/4 in. thick are used.
Metal Propellers: During 1940, solid steel propellers were made for military use. Modern propellers are fabricated from high-strength, heat-treated, aluminum alloy by forging a single bar of aluminum alloy to the required shape. Metal propellers are now extensively used in the construction of propellers for all type of aircraft. The general appearance of the metal propeller is similar to the wood propeller, except that the sections are generally thinner.
The most modern propeller designs use high-technology composite materials.
The British still occasionally refer to the propeller as the "airscrew", a term that accurately reflects the basic design and function of the propeller, which dates back as far as 1493, when Leonardo Da Vinci proposed the concept of a “helical screw” to power a machine vertically into the air. While it was not implemented until the 19th century, the propeller uses that principle to provide propulsion through the air, much like a threaded screw advances through a solid medium, but with some notable exceptions, primarily related to the loss of forward movement because the medium is not solid. Nonetheless, the propeller is similar to a screw in some common features:
1) The pitch of a propeller is the theoretical distance the propeller would move forward in one revolution (similar to a screw) and conceptually is the same as the pitch of a screw, namely the distance between threads if the propeller were a continuous helix. For this reason, propellers will frequently be stamped with a designation such as "D 2550/P2610". This means that the diameter (in this case length of propeller or thickness of a screw) is 2.550 meters, and the pitch is 2.610 meters, so that in a mathematical sense, one revolution of this propeller would move it forward a distance of 2.610 meters. (Technically, the propeller is more of a double helix, in order to combine the two blades into one, but the principle is still that of a screw.)
2) The angle of the blade changes along the radius, so that close to the hub, the angle is very steep and at the tip of the blade it is much more shallow.
3) Just as screws come in left hand and right hand threads, propellers have the same designation. When facing the airflow if the top of the propeller moves to the right, it is designated "Right Hand" and if to the left it is "Left Hand". (As viewed from the front a right hand propeller turns counterclockwise and a left hand propeller turns clockwise.) Propellers will frequently be stamped as "RH" or "LH" to reflect this design feature.
The cross section of any propeller will demonstrate that the forward traveling surface is convex, while the trailing surface is either flat or slightly concave. This is similar to the basic design of most aircraft wings. The propeller gains efficiency by using this same airfoil concept, and it is important to recognize that virtually all propellers have a "front", curved surface, and a "back" flat surface. Whether the propeller is designed as a "pusher" application or a "tractor" application, the features will be the same, and it is not possible to determine which of those were intended by the features of the propeller airfoil alone.
Propellers have blueprints which are used to manufacture them to exact specifications determined by the designer. Also for this reason, their “drawing number”, referring to the blueprint that defines their dimensions and shape, identifies older propellers. Newer propellers usually have a "model number", but this number simply refers back to a blueprint drawing as well. The blade outline, the cross sectional detail, and the pitch are all specific elements on a numbered drawing.
A propeller blade is twisted. The blade angle changes from the hub to the tip with the greatest angle of incidence, or highest pitch, at the hub and the smallest at the tip (see figures above). The reason for the twist is to produce uniform lift from the hub to the tip. As the blade rotates, there is a difference in the actual speed of the various portions of the blade.
The Controllable-Pitch Propeller
As aircraft engines advanced in the 1920s, it became obvious that the key to getting full performance potential out of any engine was a propeller whose pitch could be changed in flight. In the United States, Frank Caldwell, the federal government’s chief propeller engineer (1917-1928) joined the Hamilton Standard Propeller Corporation in 1929 to develop a controllable-pitch propeller. His first design was a ground-adjustable metal propeller that allowed the mechanics/pilot to preset the propeller pitch for the desired efficiency of the aircraft while in flight. Later, Caldwell designed a hydraulically actuated two-position design that provided efficiency at both takeoff and cruise, the two main operating regimes for the airplane.
Performance tests of the latter design revealed that Caldwell's invention maximized the performance of revolutionary aircraft such as the Boeing Model 247 and the Douglas DC-2. By the mid-1930s, controllable-pitch, variable-speed propellers were being manufactured worldwide. They contributed to the success of the early modern airliners.
The National Aeronautics Association recognized Caldwell and Hamilton Standard for their achievement by awarding them the 1933 Collier Trophy.
Airplanes are equipped with one of two types of propellers. The first is the fixed-pitch propeller, and the other is the controllable-pitch/constant-speed propeller.
The Fixed-Pitch Propeller
In this type of propeller the blades are mounted directly onto the hub in a position determined and "fixed" by the manufacturer and cannot be changed by the pilot -- hence the name.
There are two types of fixed-pitch propellers; the climb propeller and the cruise propeller. Whether the airplane has a climb or cruise propeller installed depends upon its intended use.
The Climb Propeller -
This propeller has a lower pitch, therefore less drag. This results in the capability of higher rpm and more horsepower being developed by the engine. This increases performance during takeoffs and climbs, but decreases performance during cruising flight.
The Cruise Propeller -
This propeller features a higher pitch, therefore more drag than that of the climb propeller. This results in lower rpm and less horsepower capability. This decreases performance during takeoffs and climbs, but increases efficiency during cruising flight.
The Variable-Pitch Propeller
There are two types of Variable-Pitch Propellers, ‘Adjustable’ and ‘Controllable’.
A mechanic can change the pitch of the Adjustable Propeller only on the ground in order to serve a particular purpose – power or speed (i.e., ‘Climb’ or ‘Cruise’).
With a Controllable Pitch Propeller, the pilot can change the pitch of the propeller in flight or while operating the engine by mean of a pitch changing mechanism that is hydraulically operated. In this type of propeller the blades are mounted separately on the hub, each on one axis of rotation, allowing a change of pitch in the blades. This arrangement allows the pilot to change the pitch on the blades in flight; therefore, they are referred to as controllable-pitch propellers. The number of pitch positions at which the propeller can be set may be limited, such as a two-position propeller with only high or low pitch available. Many other propellers, however, are variable pitch, and can be adjusted to any pitch angle between a minimum and maximum pitch setting.
An airplane equipped with a controllable-pitch propeller has two controls:
(1) A throttle control which controls the power output of the engine which is registered on the manifold pressure gauge.
(2) A propeller control which regulates the engine rpm and in turn the propeller rpm. The rpm is registered on the tachometer.
The pilot can set the throttle control and propeller control at any desired manifold pressure and rpm setting within the engine operating limitation. Within a given power setting, when using a constant-speed propeller, the pilot can set the propeller control to a given rpm and the propeller governor will automatically change the pitch (blade angle) to counteract any tendency for the engine to vary from this rpm. For example, if manifold pressure or engine power is increased, the propeller governor automatically increases the pitch of the blade (more propeller drag) to maintain the same rpm.
A controllable-pitch propeller permits the pilot to select the blade angle that will result in the most efficient performance for a particular flight condition. A low blade angle or decreased pitch, reduces the propeller drag and allows more engine power for takeoffs. After airspeed is attained during cruising flight, the propeller blade is changed to a higher angle or increased pitch. Consequently, the blade takes a larger bite of air at a lower power setting, and therefore increases the efficiency of the flight. This process is similar to shifting gears in an automobile from low gear to high gear.
Constant-Speed Propellers
In modern aircraft, a version of the controllable-pitch propeller is used where pitch control is achieved automatically. These are referred to as Constant-Speed Propellers. As power requirements vary, the pitch automatically changes, keeping the engine and the propeller operating at a constant rpm. If the rpm rate increases, as in a dive, a governor on the hydraulic system changes the blade pitch to a higher angle. This acts as a brake on the crankshaft. If the rpm rate decreases, as in a climb, the blade pitch is lowered and the crankshaft rpm can increase. The constant-speed propeller thus ensures that the pitch is always set at the most efficient angle so that the engine can run at a desired constant rpm regardless of altitude or forward speed.
Constant-speed propellers may have a full-feathering capability. Feathering means to turn the blade approximately parallel with the line of flight, thus equalizing the pressure on the face and back of the blade and stopping the propeller. Feathering is necessary if for some reason the propeller is not being driven by the engine and is wind-milling, a situation that can damage the engine and increase drag on the aircraft.
Some controllable-pitch and constant-speed propellers also are capable of being reversed. This is done by rotating the blades to a negative or reverse pitch. Reversible propellers push air forward, reducing the required landing distance as well as reducing wear on tires and brakes.
A Scimitar Propeller is shaped like a scimitar sword, with increasing sweep along the leading edge.
All propellers lose efficiency at high speed, due to an effect known as wave drag which occurs just below supersonic speeds. This powerful form of drag exhibits sudden onset, and it led to the concept of a sound barrier when it was first encountered in the 1940s. In the case of a propeller, this effect can happen when the prop is spun fast enough that the tips of the prop start traveling near the speed of sound, even if the plane itself is not moving forward. This can be controlled to some degree by adding more blades to the prop, absorbing more power at a lower rotational speed. This is why some WWII fighters started with two-blade props and were using five-blade designs by the end of the war. The only downside to this approach is that adding blades makes the propeller harder to balance and maintain. At some point, though, the forward speed of the plane combined with the rotational speed of the propeller will once again result in wave drag problems. For most aircraft, this will occur at speeds over about 450 mph.
A method of decreasing wave drag was discovered by German researchers in WWII: sweeping the wing backward. Today, almost all aircraft designed to fly much above 450 mph (700 km/h) use a swept wing. In the 1940s, NACA started researching propellers with similar sweep. Since the inside of the prop is moving more slowly than the outside, the blade becomes progressively more swept toward the outside, leading to a curved shape similar to that of a scimitar.
Grumman E2C Hawkeye - Early Propellor Design
Grumman E2C Hawkeye - Upgraded (scimitar-style) Propellor Design
Typically scimitar propellers are constructed of lightweight or composite materials. The combination of light weight and efficient aerodynamics results in more power and reduced noise.
Contra-Rotating Propellers, also referred to as coaxial contra-rotating propellers, apply the maximum power of usually a single piston or turboprop engine to drive two propellers in opposite rotation. Two propellers are arranged one behind the other, and power is transferred from the engine via a planetary gear or spur gear transmission.
When airspeed is low the mass of the air flowing through the propeller disk (thrust) causes a significant amount of tangential or rotational air flow to be created by the spinning blades. The energy of this tangential air flow is wasted in a single-propeller design. To use this wasted effort the placement of a second propeller behind the first takes advantage of the disturbed airflow. The tangential air flow also causes handling problems at low speed as the air strikes the vertical stablizer, causing the aircraft to yaw left or right, depending of the direction of propeller rotation.
If it is well designed, a contra-rotating propeller will have no rotational air flow, pushing a maximum amount of air uniformly through the propeller disk, resulting in high performance and low induced energy loss. Contra-rotating propellers have been found to be between 6% and 16% more efficient than normal propellers. It also serves to counter the asymmetrical torque effect of a conventional propeller. The efficiency and other benefits of a contra-rotating prop is somewhat offset by its mechanical complexity, plus they can be noisy, with increases in noise in the axial (forward and aft) direction of up to 30 dB, and tangentially 10 dB.
Some contra-rotating systems were designed to be used at take off for maximum power and efficiency under such conditions, and allowing one of the propellers to be disabled during cruise to extend flight time.
See also the Rotary-Wing Aircraft (Helicopter) page for Co-Axial Rotor Design Helicopters.
Contra-rotating propellers should not be confused with counter-rotating propellers—airscrews on different engines turning opposite directions.
Aircraft Engines
Aircraft are one of the most demanding applications for an engine, presenting multiple design requirements, many of which conflict with each other. An aircraft engine must be:
- reliable, as losing power in an airplane is a substantially greater problem than in an automobile. Aircraft engines operate at temperature, pressure, and speed extremes, and therefore need to perform reliably and safely under all reasonable conditions.
- light weight, as a heavy engine increases the empty weight of the aircraft and reduces its payload.
- powerful, to overcome the weight and drag of the aircraft.
- small and easily streamlined; large engines with substantial surface area, when installed, create too much drag.
- field repairable, to keep the cost of replacement down. Minor repairs should be relatively inexpensive and possible outside of specialized shops.
- fuel efficient to give the aircraft the range the design requires.
- capable of operating at sufficient altitude for the aircraft
Unlike automobile engines, aircraft engines are often operated at high power settings for extended periods of time. In general, the engine runs at maximum power for a few minutes during taking off, then power is slightly reduced for climb, and then spends the majority of its time at a cruise setting—typically 65 percent to 75 percent of full power. In contrast, an automobile engine might spend 20 percent of its time at 65 percent power while accelerating, followed by 80 percent of its time at 20 percent power while cruising.
The design of aircraft engines tends to favor reliability over performance. Long engine operation times and high power settings, combined with the requirement for high-reliability means that engines must be constructed to support this type of operation with ease. Aircraft engines tend to use the simplest parts possible and include two sets of anything needed for reliability. Independence of function lessens the likelihood of a single malfunction causing an entire engine to fail. For example, reciprocating engines have two independent magneto ignition systems, and the engine's mechanical engine-driven fuel pump is always backed-up by an electric pump.
Opposed, air-cooled four and six cylinder piston engines are by far the most common engines used in small general aviation aircraft requiring up to 400 horsepower (300 kW) per engine. Aircraft which require more than 400 horsepower (300 kW) per engine tend to be powered by turbine engines.
Flat engines largely replaced the historically more popular radial engines in small aircraft after World War II because the radials, although they had good cooling, added large frontal area which caused too much drag. It was relatively easy to derive flat-6 and flat-8 engines from a flat-4 design by simply adding more cylinder pairs, and the engines could use many of the same components. The only problem is that in an air-cooled engine there can be cooling problems with the middle cylinder pairs.
An opposed-type engine has two banks of cylinders on opposite sides of a centrally located crankcase. The engine is either air cooled or liquid cooled, but air cooled versions predominate. Opposed engines are mounted with the crankshaft horizontal in airplanes, but may be mounted with the crankshaft vertical in helicopters. Due to the cylinder layout, reciprocating forces tend to cancel, resulting in a smooth running engine.
Internal Combustion (IC) Engines
4-Stroke Gasoline Engine
The four stroke engine was first demonstrated by Nikolaus Otto in 1876, hence it is also known as the Otto cycle. The technically correct term is actually four stroke cycle.
The four strokes of the cycle are intake, compression, power, and exhaust. Each corresponds to one full stroke of the piston, therefore the complete cycle requires two revolutions of the crankshaft to complete.
1) Intake/Induction: During the intake stroke, the piston moves downward, drawing a fresh charge of vaporized fuel/air mixture,
2) Compression: As the piston rises the poppet valve is forced shut by the increased cylinder pressure. Flywheel momentum drives the piston upward, compressing the fuel/air mixture,
3) Ignition/Power: At the top of the compression stroke the spark plug fires, igniting the compressed fuel. As the fuel burns it expands, driving the piston downward, and
4) Exhaust: At the bottom of the power stroke, the exhaust valve is opened by the cam/lifter mechanism. The upward stroke of the piston drives the exhausted fuel out of the cylinder.
2) Compression: As the piston rises the poppet valve is forced shut by the increased cylinder pressure. Flywheel momentum drives the piston upward, compressing the fuel/air mixture,
3) Ignition/Power: At the top of the compression stroke the spark plug fires, igniting the compressed fuel. As the fuel burns it expands, driving the piston downward, and
4) Exhaust: At the bottom of the power stroke, the exhaust valve is opened by the cam/lifter mechanism. The upward stroke of the piston drives the exhausted fuel out of the cylinder.
4 Stroke Cycle, 4 Cylinder Engine w/ Overhead Camshafts
4 Stroke Cycle, 4 Cylinder Engine w/ Overhead Camshafts, Valvetrain Animation
4 Stroke Cycle Engine w/ Pushrod Actuated Overhead Valves
2-Stroke Engine
The two-stroke engine consists of only three mobile parts: Piston, connecting rod and crankshaft. The first two-stroke engine was a gas engine invented and built by Etienne Lenoir in 1860. A two-stroke diesel engine was built by Dugald Clark in 1878.
At the point where the sparkplug fires, fuel and air in the cylinder have been compressed, and when the sparkplug fires the mixture ignites. The resulting force drives the piston downward. As the piston moves downward, it is compressing the air/fuel mixture in the crankcase.
As the piston approaches the bottom of its stroke, the exhaust port is uncovered. The pressure in the cylinder drives most of the exhaust gases (but not all) out of cylinder. As the piston finally bottoms out, the intake port is uncovered. The piston's movement has pressurized the mixture in the crankcase, so it rushes into the cylinder, displacing the remaining exhaust gases and filling the cylinder with a fresh charge of fuel.
Now the momentum in the crankshaft starts driving the piston back toward the spark plug for the compression stroke. As the air/fuel mixture in the piston is compressed, a vacuum is created in the crankcase. This vacuum opens the reed valve and sucks air/fuel/oil in from the carburetor. Once the piston makes it to the end of the compression stroke, the sparkplug fires again to repeat the cycle.
2-Strokes vs. 4-Strokes
Advantages of 2 Stroke Engines:
- Do not have valves, simplifying their construction.
- Fire once every revolution (four-stroke engines fire once every other revolution). This gives two-stroke engines a significant power boost.
- Are lighter, and cost less to manufacture.
- Fire once every revolution (four-stroke engines fire once every other revolution). This gives two-stroke engines a significant power boost.
- Are lighter, and cost less to manufacture.
Disadvantages of 2 Stroke Engines:
- Don't live as long as four-stroke engines. The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear-out faster. Two-stroke engines require a mix of oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls.
- Two-stroke oil can be expensive. Mixing ratio is about 4 ounces per gallon of gas: burning about a gallon of oil every 1,000 miles.
- Do not use fuel efficiently, yielding fewer miles per gallon.
- Produce more pollution from:
-- The combustion of the oil in the gas. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit more oily smoke.
-- Each time a new mix of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port
- Two-stroke oil can be expensive. Mixing ratio is about 4 ounces per gallon of gas: burning about a gallon of oil every 1,000 miles.
- Do not use fuel efficiently, yielding fewer miles per gallon.
- Produce more pollution from:
-- The combustion of the oil in the gas. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit more oily smoke.
-- Each time a new mix of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port
Diesel Engine
Both diesel engines and gasoline engines covert fuel into energy through a series of small explosions or combustions. The major difference between diesel and gasoline is the way these explosions happen. In a gasoline engine, fuel is mixed with air, compressed by pistons and ignited by sparks from spark plugs. In a diesel engine, however, the air is compressed first, and then the fuel is injected. Because air heats up when it's compressed, the fuel ignites. Rudolf Diesel theorized that higher compression leads to higher efficiency and more power. This happens because when the piston squeezes air with the cylinder, the air becomes concentrated. Diesel fuel has a high energy content, so the likelihood of diesel reacting with the concentrated air is greater. Another way to think of it is when air molecules are packed so close together, fuel has a better chance of reacting with as many oxygen molecules as possible.
The diesel engine uses a four-stroke combustion cycle just like an Otto cycle gasoline engine. The four strokes are:
1) Intake stroke - The intake valve opens up, letting in air and moving the piston down,
2) Compression stroke - The piston moves back up and compresses the air,
3) Combustion stroke - As the piston reaches the top, fuel is injected at just the right moment and ignited, forcing the piston back down, and
4) Exhaust stroke - The piston moves back to the top, pushing out the exhaust created from the combustion out of the exhaust valve.
The diesel engine has no spark plug, it intakes air and compresses it, and it then injects the fuel directly into the combustion chamber (direct injection). It is the heat of the compressed air that ignites the fuel in a diesel engine.
Piston Engine Configurations
Straight or In-Line 4 Cylinder
Flat or Opposed 4 Cylinder
V-6
Radial Engines
The radial engine is a configuration of internal combustion engine, in which the cylinders are arranged pointing out from a central crankshaft like the spokes on a wheel. The pistons are connected to the crankshaft with a master-and-articulating-rod assembly. One piston has a master rod with a direct attachment to the crankshaft. The remaining pistons' connecting rods have pinned attachments to rings around the edge of the master rod. In the picture below, the top-most piston is the one directly attached to the crankshaft.
Most radial engines have an odd number of cylinders, so that a consistent every-other-piston firing order can be maintained, providing smooth running.
For aircraft use the radial has several advantages over the inline design. With all of the cylinders at the front of the engine (in effect), it is easy to cool them with airflow. In-lines require a cooling fluid to remove heat or complicated baffles to route cooling air, as the rear-most cylinders receive little airflow. Air-cooling saves a considerable amount of complexity, and also reduces weight to some degree. In addition the radial is far more resistant to damage; if the block cracks on an inline that entire cylinder bank will lose power, but the same situation on a radial will often only make that individual cylinder stop working.
These sorts of advantages – light weight and reliability – suggest that the radial layout is a natural fit for aircraft uses.
These sorts of advantages – light weight and reliability – suggest that the radial layout is a natural fit for aircraft uses.
However the radial design also has two important disadvantages. One is that any supply of compressed air from a Forced Induction system (turbocharger or supercharger) has to be piped around the entire engine, whereas in the inline only one or two pipes are needed, each feeding an entire cylinder bank. The other disadvantage is that the frontal area of the radial is always much larger than the same displacement inline, meaning that the radial will often have greater drag. For a low-speed plane this is not very important, but for fighter aircraft and other high-speed needs, this was initially a "killer problem," but was mitigated significantly with the introduction of the NACA cowling in the late 1920s. The large frontal area combined with the durability of radial engines proved advantageous to fighter aircraft at times though, particularly those in the attack role where the engine would act as an additional layer of armor for the pilot.
The debate about the merits of the radial vs. the inline continued throughout the 1930's, with both types seeing at least some use. The radial tended to be more popular largely due to its simplicity, and most navy air arms had dedicated themselves to the radial because of its improved reliability (very important when flying over water) and lighter weight (for carrier takeoffs).
In the mid-1930s a new generation of highly streamlined high-speed aircraft appeared, along with more powerful V-type engines like the Rolls-Royce Merlin and Daimler-Benz DB 601. This re-opened the debate anew, with the needs of streamlining often winning out. However the Focke-Wulf Fw-190 and Lavochkin La-5 showed that a radial engine fighter could compete with the best of the in-lines, given a proper installation. From that point on many new designs used radials, and after the war the in-lines quickly disappeared from the now-smaller aircraft market.
In the mid-1930s a new generation of highly streamlined high-speed aircraft appeared, along with more powerful V-type engines like the Rolls-Royce Merlin and Daimler-Benz DB 601. This re-opened the debate anew, with the needs of streamlining often winning out. However the Focke-Wulf Fw-190 and Lavochkin La-5 showed that a radial engine fighter could compete with the best of the in-lines, given a proper installation. From that point on many new designs used radials, and after the war the in-lines quickly disappeared from the now-smaller aircraft market.
Pratt & Whitney R-4360 Wasp Major
Pratt & Whitney R-4360 Wasp Major, cut-away
Originally radial engines had but one bank of cylinders, but as engine sizes increased it became necessary to add extra banks. Most did not exceed two banks, but the largest radial engine ever built in quantity, the Pratt & Whitney Wasp Major, was a 28-cylinder 4-bank radial engine used in many large aircraft designs in the post-World War II period.
At least three companies build modern radials today. Vedeneyev produces the M-14P model, 360 HP radial used on Yakovlev’s, and Sukhoi Su-26 and Su-29 aerobatic aircraft. The M-14P has also found great favor among builders of Experimental category aircraft, such as the Pitts S12 "Monster" and the Murphy "Moose". 110 horsepower, 7 cylinder and 150 horsepower, 9 cylinder engines are available from Australia's Rotec Engineering.
Rotary Engines
The rotary engine was a common type of internal combustion aircraft engine in the early years of the 20th century.
In concept, a rotary engine is simple. It is a standard Otto cycle engine, but instead of having a fixed cylinder block with rotating crankshaft, the crankshaft remains stationary and the entire cylinder block rotates around it. In the most common form, the crankshaft was fixed solidly to an aircraft frame, and the propeller simply bolted onto the front of the cylinder block.
In order to generate 100 hp (75 kW) at the low rpm at which the engines of the day ran, the pulsation resulting from each combustion stroke was quite large. To damp out these pulses, regular engines typically needed to mount a large flywheel, which added weight. In the rotary design, the engine itself doubled as its flywheel. Thus, rotaries were lighter than similarly sized engines of regular design.
The cylinders had good airflow over them even when the aircraft was stationary but the engine was running, which was an important concern given the alloys they had to work with at the time. Early rotary engines did not even use cooling-fins, a feature of every other air-cooled design, and one that is complex and expensive to manufacture. Early airplanes had relatively low cruising speeds, and the wind flow at 30 or 40 km/h was often in itself insufficient for cooling engines. Having an engine rotating at a few hundred rpm provided plenty of cooling.
Gnome-Rhone Rotary Engine, cutaway
The Gnome (and its copies) had a number of features that made it unique, even among the rotaries. Notably, the fuel was mixed and sprayed into the center of the engine through a hollow crankshaft, and then into the cylinders through the piston itself, a single valve on the top of the piston let the mixture in when opened.
Gnome-Rhone Rotary Engine, cylinder/valve train detail
The valves were counter balanced so that only a small force was needed to open them, and releasing the force closed the valve without any springs. The center of the engine is normally where the oil would be, and the fuel would wash it away. To fix this, the oil was mixed in liberal quantities with the fuel, and the engine spewed smoke due to burning oil. Finally, the Gnome had no throttle or carburetor. Since the fuel was being sprayed into the spinning engine, the motion alone was enough to mix the fuel fairly well. Of course with no throttle, the engine was either on or off, so something as simple as reducing power for landing required the pilot to cut the ignition. "Blipping" the engine on and off gave the characteristic sputtering sound as though the engine was nearly stalling, though it did not stall as quickly as conventional engines due to its great rotational inertia.
Throughout the early period of the war, the power-to-weight ratio of the rotaries remained ahead of that of their competition. They were used almost universally in fighter aircraft, while traditional water cooled designs were used on larger aircraft. The engines had a number of disadvantages, notably very poor fuel consumption, partially because the engine was always "full throttle", and also because the valve timing was often less than ideal. The rotating mass of the engine made it, in effect, a large gyroscope. This could result in tricky handling. The Sopwith Camel, for example, was known to turn very nimbly to the right, but rather sluggishly to the left. Nevertheless, rotaries maintained their edge through a series of small upgrades, and many newer designs continued to use them.
Throughout the early period of the war, the power-to-weight ratio of the rotaries remained ahead of that of their competition. They were used almost universally in fighter aircraft, while traditional water cooled designs were used on larger aircraft. The engines had a number of disadvantages, notably very poor fuel consumption, partially because the engine was always "full throttle", and also because the valve timing was often less than ideal. The rotating mass of the engine made it, in effect, a large gyroscope. This could result in tricky handling. The Sopwith Camel, for example, was known to turn very nimbly to the right, but rather sluggishly to the left. Nevertheless, rotaries maintained their edge through a series of small upgrades, and many newer designs continued to use them.
As the war progressed, aircraft designers demanded ever-increasing amounts of power. Inline engines were able to meet this demand by improving their RPM, as more "bangs per minute" meant more power delivered. Improvements in valve timing, ignition systems and lighter materials made these higher RPM possible, and by the end of the war the average engine had increased from 1,200 RPM to 2,000. However the rotary was not able to use the same "trick," due to the drag of the cylinders through the air as they spun. For instance, if an early-war model of 1,200 RPM increased to only 1,400 RPM, the drag on the cylinders increased by 36%, since air drag increases with the square of velocity. At lower speeds the drag could simply be ignored, but as speeds increased the rotary was putting more and more power into spinning the engine, and less into spinning the propeller.
Detonation
Detonation (also called knock, spark knock, or pinging) in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The fuel-air charge is meant to be ignited by the spark plug only, and at a precise time in the piston's stroke cycle. The peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive.
Under ideal conditions the common internal combustion engine burns the fuel/air mixture in the cylinder in an orderly and controlled fashion. The combustion is started by the spark plug some 10 to 40 crankshaft degrees prior to top dead center (TDC, or the farthest point away from the crankshaft), depending on many factors including engine speed and load. This ignition advance allows time for the combustion process to develop peak pressure at the ideal time for maximum recovery of work from the expanding gases.
The spark across the spark plug's electrodes forms a small kernel of flame approximately the size of the spark plug gap. As it grows in size its heat output increases allowing it to grow at an accelerating rate, expanding rapidly through the combustion chamber. This growth is due to the travel of the flame front through the combustible fuel air mix itself and due to turbulence rapidly stretching the burning zone into a complex of fingers of burning gas that have a much greater surface area than a simple spherical ball of flame would have. In normal combustion, this flame front moves throughout the fuel/air mixture at a rate characteristic for the fuel/air mixture. Pressure rises smoothly to a peak, as nearly all the available fuel is consumed, then pressure falls as the piston descends. Maximum cylinder pressure is achieved a few crankshaft degrees after the piston passes TDC, so that the increasing pressure can give the piston a hard push when its speed and mechanical advantage on the crank shaft gives the best recovery of force from the expanding gases.
As the Cylinder rises on the Compression Stroke,
A - Pocket of Air-Fuel detonates as spark plug ignites, or
B - Pocket of Air-Fuel ignites prior to spark plug igniting,
either situation results in C - Detonation
When unburned fuel/air mixture beyond the boundary of the flame front is subjected to a combination of heat and pressure for a certain duration (beyond the delay period of the fuel used), detonation may occur. Detonation is characterized by an instantaneous, explosive ignition of at least one pocket of fuel/air mixture outside of the flame front. A local shockwave is created around each pocket and the cylinder pressure may rise sharply beyond its design limits. If detonation is allowed to persist under extreme conditions or over many engine cycles, engine parts can be damaged or destroyed. The simplest deleterious effects are typically particle wear caused by moderate knocking, which may further ensue through the engine's oil system and cause wear on other parts before being trapped by the oil filter. Severe knocking can lead to catastrophic failure in the form of physical holes punched through the piston or head (i.e., rupture of the combustion chamber), either of which depressurizes the affected cylinder and introduces large metal fragments, fuel, and combustion products into the oil system.
Rotary-Piston (Wankel) Engine
The Wankel rotary engine is a type of internal combustion engine, invented by German engineer Felix Wankel, which uses a rotor instead of reciprocating pistons. This design promises smooth high-rpm power from a compact, lightweight engine; however Wankel engines are criticized for poor fuel efficiency and exhaust emissions.
In the basic single rotor Wankel engine, a single oval (technically an epitrochoid) housing surrounds a three-sided rotor which turns and moves within the housing. The sides of the rotor seal against the sides of the housing, and the corners of the rotor seal against the inner periphery of the housing, dividing it into three combustion chambers.
As the rotor turns, its motion and the shape of the housing cause each side of the rotor to get closer and farther from the wall of the housing, compressing and expanding the combustion chamber similarly to the "strokes" in a reciprocating engine. However, whereas a normal four stroke cycle engine produces one combustion stroke per cylinder for every two revolutions (that is, one half power stroke per revolution per cylinder) each combustion chamber of each rotor in the Wankel generates one combustion 'stroke' per revolution (that is, three power strokes per rotor revolution). Since the Wankel output shaft is geared to spin at three times the rotor speed, this becomes one combustion 'stroke' per output shaft revolution per rotor, twice as many as the four-stroke piston engine, and similar to the output of a two stroke cycle engine. Thus, power output of a Wankel engine is generally higher than that of a four-stroke piston engine of similar engine displacement in a similar state of tune, and higher than that of a four-stroke piston engine of similar physical dimensions and weight. This design also allows the Wankel engine to have a much higher redline as there is less friction working against the internals of the engine.
As the rotor turns, its motion and the shape of the housing cause each side of the rotor to get closer and farther from the wall of the housing, compressing and expanding the combustion chamber similarly to the "strokes" in a reciprocating engine. However, whereas a normal four stroke cycle engine produces one combustion stroke per cylinder for every two revolutions (that is, one half power stroke per revolution per cylinder) each combustion chamber of each rotor in the Wankel generates one combustion 'stroke' per revolution (that is, three power strokes per rotor revolution). Since the Wankel output shaft is geared to spin at three times the rotor speed, this becomes one combustion 'stroke' per output shaft revolution per rotor, twice as many as the four-stroke piston engine, and similar to the output of a two stroke cycle engine. Thus, power output of a Wankel engine is generally higher than that of a four-stroke piston engine of similar engine displacement in a similar state of tune, and higher than that of a four-stroke piston engine of similar physical dimensions and weight. This design also allows the Wankel engine to have a much higher redline as there is less friction working against the internals of the engine.
Advantages
Wankel engines have several major advantages over reciprocating piston designs, in addition to having higher output for similar displacement and physical size. Wankel engines are considerably simpler and contain far fewer moving parts. For instance, because valving is accomplished by simple ports cut into the walls of the rotor housing, they have no valves or complex valve trains; in addition, since the rotor is geared directly to the output shaft, there is no need for connecting rods, a conventional crankshaft, crankshaft balance weights, etc. The elimination of these parts not only makes a Wankel engine much lighter (typically half that of a conventional engine with equivalent power), but it also completely eliminates the reciprocating mass of a piston engine with its internal strain and inherent vibration due to repetitious acceleration and deceleration, producing not only a smoother flow of power but also the ability to produce more power by running at higher rpm.
In addition to the enhanced reliability due to the elimination of this reciprocating strain on internal parts, the construction of the engine, with an iron rotor within a housing made of aluminum which has greater thermal expansion, ensures that even if severely overheated the Wankel engine can not seize, as an overheated piston engine is likely to do; this is a substantial safety benefit in aircraft use. The simplicity of design and smaller size of the Wankel engine also allow for a savings in construction costs, compared to piston engines of comparable power output. Additionally, the shape of the Wankel combustion chamber and the turbulence induced by the moving rotor prevent localized hot spots from forming, thereby allowing the use of fuel of very low octane number without pre-ignition or detonation.
A further advantage of the Wankel engine for use in aircraft is the fact a Wankel engine can have a smaller frontal area than a piston engine of equivalent power.
In addition to the enhanced reliability due to the elimination of this reciprocating strain on internal parts, the construction of the engine, with an iron rotor within a housing made of aluminum which has greater thermal expansion, ensures that even if severely overheated the Wankel engine can not seize, as an overheated piston engine is likely to do; this is a substantial safety benefit in aircraft use. The simplicity of design and smaller size of the Wankel engine also allow for a savings in construction costs, compared to piston engines of comparable power output. Additionally, the shape of the Wankel combustion chamber and the turbulence induced by the moving rotor prevent localized hot spots from forming, thereby allowing the use of fuel of very low octane number without pre-ignition or detonation.
A further advantage of the Wankel engine for use in aircraft is the fact a Wankel engine can have a smaller frontal area than a piston engine of equivalent power.
Disadvantages
The design of the Wankel engine requires numerous sliding seals and a housing that is typically built as a sandwich of cast iron and aluminum pieces that expand and contract by different degrees when exposed to heating and cooling cycles in use. This can lead to a very high incidence of loss of sealing (compression loss), both between the rotor and the housing and also between the various pieces making up the housing.
Just as the shape of the Wankel combustion chamber prevents pre-ignition, it also leads to incomplete combustion of the air-fuel charge, with the remaining unburned hydrocarbons released into the exhaust.
Forced Induction
Forced induction describes the process of compressing (forcing) air into an internal combustion engine. In the process of forced induction, a gas compressor is added to the air intake, thereby increasing the quantity of air, and ultimately oxygen, available for combustion. An internal combustion engine without forced induction is considered naturally aspirated. A naturally aspirated IC engine uses only the downward stroke of a piston to create an area of low pressure in order to draw air into the cylinder through the intake valves. The pressure in the atmosphere is no more than 1 atm (approximately 14.7 psi), so there ultimately will be a limit to the pressure difference across the intake valves and thus the amount of airflow entering the combustion chamber. Since the forced induction system increases the pressure at the point where air is entering the cylinder, a greater mass of air (oxygen) will be forced in as the inlet manifold pressure increases. The additional air flow makes it possible to maintain the combustion chamber pressure and fuel/air load even at high engine revolution speeds, increasing the power and torque output of the engine.
Forced induction is used to improve engine power, efficiency, and emissions without much extra weight and minimal modifications to the engine architecture. Two commonly used forced induction technologies are turbochargers and superchargers, turbochargers being the most commonly used.
Supercharging
Turbocharging
Intercoolers are also found on turbocharger systems in order to cool the compressor discharge air flow (and so increase its' density). Since the turbocharger operation is dependent on exhaust gas pressure, the extra engine power produced by the turbocharger system is primarily available at higher engine RPM's. The term "turbo lag" means that the extra power is not available as soon as the throttle is increased at lower engine RPM's, since exhaust pressure must increase and the turbine wheel rotational speed must increase ("spool up") accordingly in order for the system to be effective.
Nitrous Oxide
Nitrous Oxide is a chemical compound with the formula N2O. At room temperature, it is a colorless non-flammable gas, with a slightly sweet odor and taste. At elevated temperatures, nitrous oxide is a powerful oxidizer similar to molecular oxygen.
A property of nitrous oxide is that at about 565 degrees F., it breaks down into nitrogen and oxygen. When it is introduced into the intake tract of an internal combustion engine, it is sucked into the combustion chamber and, on the compression stroke, when the charge air temperature reachs 565 deg., a very oxygen-rich mixture results. If we add extra fuel during nitrous oxide injection, the effect is like a super charger or increasing the compression ratio of the engine.
Nitrous oxide has this effect because it has a higher percentage of oxygen content than does the air in the atmosphere. Nitrous has 36% oxygen by weight and the atmosphere has 23%. Additionally, nitrous oxide is 50% more dense than air at the same pressure. Thus, a cubic foot of nitrous oxide contains 2.3 times as much oxygen as a cubic foot of air.
Nitrous oxide is stored as a compressed liquid; the evaporation and expansion of liquid nitrous oxide in the intake manifold causes a large drop in intake charge temperature, resulting in a denser charge, further allowing more air/fuel mixture to enter the cylinder. Nitrous oxide is sometimes injected into (or prior to) the intake manifold, whereas other systems directly inject right before the cylinder (direct port injection) to increase power.
One of the major problems of using nitrous oxide in a reciprocating engine is that it can produce enough power to damage or destroy the engine. Very large power increases are possible, and if the mechanical structure of the engine is not properly reinforced, the engine may be severely damaged or destroyed during nitrous oxide application.
Turbine Engines
Turbojets consist of an air inlet, an air compressor, a combustion chamber, a gas turbine (that drives the air compressor) and a nozzle. The air is compressed into the chamber, heated and expanded by the fuel combustion and then allowed to expand out through the turbine into the nozzle where it is accelerated to high speed to provide propulsion.
Turbojets are quite inefficient (if flown below about Mach 2) and very noisy. Most modern aircraft use turbofans instead for economic reasons.
A - Low Pressure Shaft, B - High Pressure Shaft, C - Other Structures:
1. Nacelle, 2. Fan, 3. Low Pressure Compressor, 4. High Pressure Compressor, 5. Combustion Chamber, 6. High Pressure Turbine, 7. Low pressure Turbine, 8. Core Nozzle, and 9. Fan Nozzle
A Turbofan is a type of aircraft gas turbine engine that provides thrust using a combination of a ducted fan and a jet exhaust nozzle. Part of the airstream from the ducted fan passes through the core, providing oxygen to burn fuel to create power. However, the rest of the air flow bypasses the engine core and mixes with the faster stream from the core, significantly reducing exhaust noise. The rather slower bypass airflow produces thrust more efficiently than the high-speed air from the core, and this reduces the specific fuel consumption.
Designs known as "Propfans" work slightly differently and have the fan blades as a radial extension of an aft-mounted low-pressure turbine unit.
Turbofans have a net exhaust speed that is much lower than a turbojet. This makes them much more efficient at subsonic speeds than turbojets, and somewhat more efficient at supersonic speeds up to roughly Mach 1.6, but have also been found to be efficient when used with continuous afterburner at Mach 3 and above. However, the lower speed also reduces thrust at high speeds.
All of the jet engines used in currently manufactured commercial jet aircraft are turbofans. They are used commercially mainly because they are highly efficient and relatively quiet in operation. Turbofans are also used in many military jet aircraft, such as the F-15 Eagle and in unmanned aerial vehicles such as the RQ-4 Global Hawk.
Turboprop engines are a type of aircraft powerplant that use a gas turbine to drive a propeller. The gas turbine is designed specifically for this application, with almost all of its output being used to drive the propeller. The engine's exhaust gases contain little energy compared to a jet engine and play a minor role in the propulsion of the aircraft.
The propeller is coupled to the turbine through a reduction gear that converts the high RPM, low torque output to low RPM, high torque. The propeller itself is normally a constant speed (variable pitch) type similar to that used with larger reciprocating aircraft engines.
Turboshaft Engines are used primarily for helicopters and auxiliary power units. A turboshaft engine is very similar to a turboprop, with a key difference: In a turboprop the propeller is supported by the engine, and the engine is bolted to the airframe. In a turboshaft, the engine does not provide any direct physical support to the helicopter's rotors. The rotor is connected to a transmission, which itself is bolted to the airframe, and the turboshaft engine simply feeds the transmission via a rotating shaft.
Water Injection
Water Injection is a method for cooling the combustion chambers of engines by adding water to the cylinder or incoming fuel-air mixture, allowing for greater compression ratios and largely eliminating the problem of engine knocking (detonation). This effectively increases the octane rating of the fuel, meaning that performance gains can be obtained when used in conjunction with a supercharger or turbocharger, altered spark ignition timing, and other modifications.
Many water injection systems use a mixture of water and alcohol (approximately 50/50), with trace amounts of water-soluble oil. The water provides the primary cooling effect due to its great density and high heat absorption properties. The alcohol is combustible, and also serves as an antifreeze for the water. The purpose of the oil is to prevent corrosion of water injection and fuel system components. Because the alcohol mixed into the injection solution is often methanol (CH3OH), the system is known as methanol-water injection. In the United States, the system is sometimes referred to as anti-detonant injection, or ADI.
Due to the cooling effect of the water, aircraft engines can run at much higher manifold pressures without detonating, creating more power. This is the primary advantage of a water injection system when used on an aircraft engine. The extra weight and complexity added by a water injection system was considered worthwhile for military purposes, while it is usually not considered worthwhile for civil use. The one exception is racing aircraft, which are focused on making a tremendous amount of power for a short time; in this case the disadvantages of a water injection system are less important.
In an IC piston engine, the initial injection of water cools the fuel-air mixture significantly, which increases its density and hence the amount of mixture that enters the cylinder. An additional effect comes later during combustion when the water absorbs large amounts of heat as it vaporizes, reducing peak temperature and resultant NOx formation, and reducing the amount of heat energy absorbed into the cylinder walls. This also converts part of combustion energy from the form of heat to the form of pressure. As the water droplets vaporize by absorbing heat, it turns to high pressure steam (water vapor or steam mainly resulted from combustion chemical reaction), that would add engine output. The alcohol in the mixture burns, but is also much more resistant to detonation than gasoline. The net result is a higher octane charge that will support very high compression ratios or significant forced induction pressures before onset of detonation.
Piston engined petrol military aircraft utilized water injection technology prior to World War II in order to increase takeoff power. This was used so that heavily-laden fighters could take off from shorter runways, climb faster, and quickly reach high altitudes to intercept enemy bomber formations. Some fighter aircraft also used water injection to allow higher boost in short bursts during dogfights.
As a general rule, the fuel mixture is set at fuel rich on an aircraft engine when running it at a high power settings (such as during takeoff). The extra fuel does not burn; its only purpose is to evaporate to absorb heat. This uses up more fuel, and it also decreases the efficiency of the combustion process. By using water injection, the cooling effect of the water allows the fuel mixture to be run leaner at its best-power setting.
When used in a turbine engine, the effects are similar, except that preventing detonation is not the primary goal. Water is normally injected either at the compressor inlet or in the diffuser just before the combustion chambers. Adding water increases the mass being accelerated out of the engine, increasing thrust, but it also serves to cool the turbines. Since temperature is normally the limiting factor in turbine engine performance at low altitudes, the cooling effect allows the engines to be run at a higher RPM with more fuel injected and more thrust created without overheating. The drawback of the system is that injecting water quenches the flame in the combustion chambers somewhat, as there is no way to cool the engine parts without cooling the flame accidentally. This leads to unburned fuel out the exhaust and a characteristic trail of black smoke.
B-52 Strategic Bomber - Take-Off
For early B-52s, water injection was seen as a vital part of take-off procedures. For later versions of the B-52 as well as later turbine-powered bombers, the problem of taking off heavily loaded from short runways was solved by the availability of more powerful engines that had not been available previously.
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