Unified Engineering Spring Term 2005

Problem P8. (Propulsion) L.O. G

Use the nominal Dragonfly propeller for the conditions (a-c) shown below. First draw velocity triangles in the blade-relative and absolute reference frames at inlet and exit to the blade row, then estimate the power imparted to the flow per unit area.

For the purposes of this assignment, assume that the flow exits the blade parallel to the zero-lift line. [Although this is in general a poor assumption for propellers. For the "sparse" blading of a propeller (compared to a compressor), assuming the exit flow lines up with the blade will over predict the turning. At typical conditions for your props, Professor Drela estimates that the turning is over predicted by about 70%.]

a) Radial station r = 0.7 R_tip, V = 5 m/s, 5000 rpm

b) Radial station r = 0.7 R_tip, V = 8 m/s, 5000 rpm

c) Radial station r = 0.7 R_tip, V = 5 m/s, 8000 rpm

d) A typical propeller efficiency curve is shown below. Why is the efficiency reduced at low values of the advance ratio (l = V/wR) and again at high values of the advance ratio?