PropCart: STEM for Aviation and Windpower

Abstract

Abstract PropCart is a three wheeled model vehicle with a variable pitch pusher propeller and rubber band motor. This paper presents the construction and testing of this vehicle as a platform for teaching concepts of linear and rotational mechanics, and elementary aerodynamics of airfoils applied in aircraft and wind turbines. Building and testing the vehicle is proposed as a 12th Grade STEM project: SCIENCE component: Physics; Linear and Rotational Mechanics with practical determination of basic and derived quantities in particular uniformly accelerated motion. TECHNOLOGY component: Design and construction of the vehicle. ENGINEERING component (overlapping technology): Measurement of thrust of the propeller at different setting angles using video data of propeller angular velocity and cart linear velocity; enabling determination of relative wind and angle of attack, calculation of Reynolds number and identifying stall angle without the use of a wind tunnel. MATHEMATICS component: Data analysis using Excel. A teaching course based mainly on the Khan academy physics programme is provided to support concepts used in the propeller cart project. PropCart is based on a four wheeled vehicle with fixed pitch propeller, devised by David Newton (1999). A redesigned three wheel vehicle with a variable pitch propeller was used in a practical project in a 12 week introductory engineering course at the University of Canterbury (2002) for 15 Petronas students. 12 students completed the project: Test videos for each student were taken in PAL format at 25fps for time intervals of 1–2 seconds for propeller setting angles of 15, 30 and 45 degrees; with student choice of the number of rubber bands used and the number of windup turns. Readings of linear and angular motion were taken with software which could select frame by frame display. Linear and angular displacement against time were plotted against time and time squared. Separate experiments were conducted on the performance of the propeller with the cart fixed with motor axis normal to an annular scale graduated in degrees. The propeller using 2 rubber bands was wound up by 30 turns for each test. Setting angles from 0 to 90 degrees were used. Graphs of angular displacement against time were compared over one second and plotted on a common time axis for setting angles 0 to 90 degrees. The graphs show that stalling occurs at angles greater than 15 degrees. Cart linear motion and propeller angular velocity within a one second time interval were related. Angular velocity of the propeller against time was a piecewise function: At low setting angles near 15 degrees tending to an acceleration sub-function followed by a constant velocity sub-function the latter giving constant thrust and constant linear acceleration of the cart. The angular displacement against time squared graph showed non uniform decreasing acceleration, with the rate of decrease increasing with time squared due to drag; further supported by the test results of the linear motion of the students’ vehicles. A large number of activities are detailed for the teaching of linear and rotational mechanics. For example, the rotational inertia of the propeller can be determined by mounting it as a compound pendulum; this activity importantly uses the parallel axis theorem. Taking the propeller blade as a flat plate airfoil: The relative air flow at mid span can be determined as falling along the resultant of air relative to the cart and air relative to the reference point on the blade: Enabling the determination of flow angle and angle of attack for a given setting angle. A parallel linkage with adjustable link lengths was developed for measuring setting angle by reflecting a laser beam off mirror foil on the blades, this is briefly discussed.