ASCI 309 Group Assignment - Luke Morgan

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Embry-Riddle Aeronautical University *

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309

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Mechanical Engineering

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Dec 6, 2023

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docx

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1 Reciprocating engines utilize one or more pistons to convert pressure into rotational motion. Each piston is sealed in a chamber which has an inlet, outlet, and a sparking mechanism. The inlet allows the fuel/air mixture in as the piston moves down, the inlet is sealed and the piston moves upwards and pressurizes the fuel/air mixture until it cannot be pressurized any further. Once the piston reaches the top of the chamber, the spark plug creates a spark and ignites the mixture which forces the piston down. When the piston begins moving back up, the outlet valve is opened to release exhaust. Once the piston reaches the top again, the outlet is closed and then the process repeats (Reciprocating engine). This entire process is done hundreds or even thousands of times every minute based on the power settings. The process of the piston moving up and down is used to turn a crankshaft that is attached to the propeller of the airplane. It converts up and down motion to horizontal rotation to turn the propeller. Some limitations of reciprocating engines would include the power to weight ratio, speed, and RPM. Reciprocating engines utilize the force of controlled explosions in each cylinder, which get extremely hot. To enable this, the engines are made of high density metal to allow high temperatures and to increase the lifespan of the engine.As reciprocating engines become more powerful, they become heavier, thus requiring more power from the engine to gain speeds/lift to overcome the weight of the engine. Speed is also a limit on reciprocating engines, because with these engines, there will also be a propeller to create thrust. Propellers are limited to rotations per minute (RPM), because if the tips of the propeller rotate close to the speed of sound, the propeller will be extremely loud, and lose thrust (Konz et al.). Therefore, propeller driven airplanes can only travel so fast. Propellers operate by using torque from the shaft to turn the blades. The blades are mini airfoils that generate a pressure difference from the front of the propeller to the back of the

2 propeller, a high pressure behind and low pressure in the front (Konz et al.). This pressure difference pushes the airplane forward. Propellers are twisted to create equal lift along the entire blade. This twist can sometimes be up to 40 degrees, but the amount will depend on the purpose of the blade (Konz et al.). There are many different ways to improve propeller efficiency. One way would be by adding more propellers, counter rotating propellers, or variable pitch propellers. This would allow for more chunks of air to be pulled through the propeller, creating a greater pressure difference and therefore more thrust. The counter rotating propellers have an advantage, because the second set of blades are able to recover part of the slipstream rotational energy that would be lost on conventional single screw systems (Carlton). Having the ability to adjust the angle of the propeller has huge effects on efficiency of the propeller. It allows that propeller to be at the most efficient angle for each phase of flight (Konz et al.). For example, as the blade angle is decreased, the engine RPM is higher, resulting in more thrust (Wood, 2022). A low blade angle is most efficient during takeoff. However, during climb and cruise, the blade angle will be increased because the propellers do not need to produce as much thrust during these phases (Wood, 2022). The angle of the blades are increased, causing more resistance on the blades as they spin, thus causing a decrease in RPM. This larger angle allows the blade to take much larger chunks out of the air, causing the airplane to move much further with each rotation. Brake horsepower is the available power from the engine (Mlblevins, 2009). However, as that power is converted from the engine to the crankshaft, it loses some power. The force that is delivered to the crankshaft is the shaft horsepower (Hendricks, 2023). This rotational force is

3 what spins the propeller of the airplane. Thrust horsepower is the actual force of the propeller. This is dependent on the efficiency of the airfoil that is the propeller. If the engine, crankshaft, and propeller were 100% efficient, then the amount of brake horsepower delivered from the engine would be the same as the thrust horsepower ( Thrust horsepower definition & meaning ). As altitude increases, air density decreases. This is because there is less atmosphere weighing down on the air at higher altitudes, and there is less gravity as one increases altitude. Lower air density relates to how densely air molecules are grouped together in a given area. For example, on a standard day at sea level the air density is 0.0765 lb/ft^3, and on a standard day at 30,000 feet the air density is 0.02272 lb/ft^3 (Admin, 2020). Because of this decrease in air density, the propeller will have less performance. The propeller is a rotating airfoil that generates lift along the longitudinal axis of the airplane. To do this the airfoil rotates and creates a low pressure on the front of the propeller and a high pressure behind it, resulting in thrust that pushes the plane forward. When the air density is low at high altitudes, the propeller cannot generate as much thrust as before because there are less air molecules readily available.

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4 Resources Admin. (2020, March 30). How do you find the density of air at altitude? . MassInitiative. https://massinitiative.org/how-do-you-find-the-density-of-air-at-altitude/#:~:text=The %20tropospheric%20tabulation%20continues%20to%2011%2C000%20meters %20%2836%2C089,Standard%20Atmosphere%20Air%20Properties%20–%20Imperial %20%28BG%29%20Units Carlton. (n.d.). Contra-rotating propeller . Contra-Rotating Propeller - an overview | ScienceDirect Topics. https://www.sciencedirect.com/topics/engineering/contra-rotating- propeller Hendriks, A. (2023, May 12). Difference between shaft horsepower and horsepower . Compare Factory. https://comparefactory.com/difference-between-shaft-horsepower-and- horsepower/ Konz, C., Happel, C. C., Turano, D., Daniel, G., Bigger, M., & Leishman, G. J. (n.d.). Reciprocating engine/propeller . Introduction to Aerospace Flight Vehicles. https://oer.pressbooks.pub/introductiontoaerospaceflightvehicles/chapter/reciprocating- engine-propeller/ Merriam-Webster. (n.d.). Thrust horsepower definition & meaning . Merriam-Webster. https://www.merriam-webster.com/dictionary/thrust%20horsepower Mlblevins. (2009, June 1). What is Brake Horsepower (bhp)? . Science Struck. https://sciencestruck.com/what-is-bhp-brake-horsepower

5 Reciprocating engine . Reciprocating engine - Energy Education. (n.d.). https://energyeducation.ca/encyclopedia/Reciprocating_engine Wood, A. (2022, September 28). Aircraft Propeller theory . AeroToolbox. https://aerotoolbox.com/propeller/