Reciprocating engines, Superchargers,Propellers Lecture 9 Chapter 4

Reciprocating Engines Internal combustion recip. (piston) engine Four stroke cycle engine credited German Nickolaus Otto in Steam was not successful. Why? Figure 4-13 p. 94 Wright Brother’s engine –How did it work?

Reciprocating Engines 1 st reciprocating engines were liquid cooled. 1 st airplanes were so slow that air cooled was not practical The rotary engine was cooled by air. The crankshaft of a rotary is mounted to the airframe & cylinders revolve around it. The cylinders are air cooled even at low speeds.

Air Cooled Engines Pro: Cylinders placed radially around the crankshaft for equal cooling for many cylinders Con: This arrangement allows for high drag with the large frontal area. Horizontally opposed configuration worked well with 2/4 cylinders –Horizontally opposed recip. engines with as many as 8 cylinders producing up to 450 horsepower.

Reciprocating Engine Performance Horsepower- English unit One Horsepower = 550 foot-pounds –It would take one horsepower to move something requiring a force of 550 lbs over a distance of one foot every second Mean Effective Pressure- average pressure throughout a stroke {force on the piston}

Power Power is the time rate at which work is done The number of power strokes per minute is ½ times the rpm because there is a power stroke every other revolution The power in one cylinder is proportional to average cylinder pressure times the length of the stroke times piston area times rpm.

Terms Brake horsepower- horsepower delivered at the shaft Pony Brake- classical method of measuring power output. Shaft horsepower- power delivered to the propeller. Thrust horsepower- amount of power that actually gets converted into thrust.

Terms Rated brake horsepower- the power output at a rpm at standard sea level density. Figure 4-15 p. 98 typical variation of brake horsepower with altitude Economy cruise- 55% power Performance cruise- 75% power Good compromise- 65% power

Superchargers Superchargers utilize a small compressor in the intake manifold that compresses the air received from the atmosphere to a higher pressure. Turbosuperchargers are more efficient because they use exhaust gas pressure.

Turbosuperchargers The exhaust drives the turbine, which is connected to the compressor in the intake (like the compressor & turbine function in a turbojet) The effect of supercharging/turbocharging is that sea level, or rated, power can be maintained up to a certain altitude.

Figure 4-16 p. 100 This shows the effect of supercharging The dotted line represents the power available at a constant rpm with turbocharger. The solid line represents the original unsupercharged engine.

Propellers Propeller (airscrew) is essentially a small wing rotated in a plane perpendicular to the path of flight & developing thrust in the same way that a wing develops lift. Figures 4-18 – 4-25

Propeller Efficiency The efficiency of the propeller is really how much brake horsepower it converts into thrust power-dependent on the ratio to forward speed to rotational speed. Figures What are the different types of props? Pros/Cons? Figures 4-29 & 4-30

Turboprop & Turbojet Performance Figure 4-31

Quiz on Lecture 9 Chapter 4 Please take out a sheet of paper Include today’s date & your name

Quiz on Lecture 9 Chapter 4 Compare and contrast superchargers and turbosuperchargers.