The last few tests have shown that under full power Goliath was unable to produce enough thrust to lift itself. Analysis shows that Goliath should have sufficient power to lift itself, but it's difficult to say why Goliath is not hovering without additional data on the actual thrust and engine power being produced. The electronics for measuring the thrust produced is almost complete. Measuring the engine power requires measuring the engine RPM and a hall effect sensor is being installed that will be triggered by the magnet on the flywheel.

Meanwhile the design analysis has been revisited again to make sure there wasn't something's that been missed. The power required calculations were shown to be sound, so a closer look was at the propeller design was next.

Originally when designing the shape of the propellers, analysis was performed using simplified blade element theory. This consisted of some code borrowed from a Matlab code that I got from a University of Cambridge website. An attempt to make a spreadsheet out of this was unsatisfactory and Prop Designer was tried instead. Aircraft propellers and helicopter rotors work on the same principle, but operate in different regimes. Since the hover condition (static thrust) was the design condition for Goliath, Prop Designer gave an answer, but the software warned it may not be a good answer. With the deadline for the 2014 Hackaday Prize looming, it was decided to push forward and design the propeller with an angle of attack at the maximum L/D of the airfoil to get an efficient design.

Taking a closer look at the propeller design required having a more accurate analysis. This time a python code was written to analyze the design and the issues encountered previously were resolved. While the new python code is giving the best results for the propeller design so far, it still needs to be validated with data. However if it is right, then the new analysis shows a reason why Goliath isn't flying and more importantly how I can change the design to get Goliath flying.

Below is a plot showing the predicted power curves for different propeller designs using the new code.

The current design has a angle varying from 20 degrees at the root to 12 degrees at the tip. The problem with the design could be that it produces too much thrust at lower RPMs. The predicted power is too much for the engine to get above 2400 RPM, limiting the power to 21 Hp. The predicted thrust is at this condition is 220 lbs. Goliath weighed 238 lbs when last tested, so this could be why Goliath isn't flying.

If this is indeed the case, then the fix is to lower the tip angle. The plot above shows 3 additional designs where the tip angle is decreased in increments of 2 degrees. The last design in the list has a tip angle of six degrees and looks to be the most promising. This design could allow the engine to run up to 3300 RPM generating 27 Hp and the predicted total thrust is 290 lbs which should provide more than sufficient thrust.

The next test will provide thrust data and possibly engine RPM data. This will help validate the software and provide a good path forward to get Goliath flying.