Monday, April 5, 2010

Initial Thrust Testing

This past weekend we built a test platform in order to do some basic thrust tests with our motors and the two different props we have. This gave us a general understanding of how much current we need per motor in order to keep our aircraft hovering. We were able to more accurately determine this because we have our first body finished! There will be more on the body in following posts. The thrust test platform consisted of a lever made from ABS and a metric scale, so that prop thrust generated a proportional weight on the scale. By scaling this thrust by the ratio of the lever lengths we were able to get an approximate measurement of lift (in grams) versus current for our two props.

Thrust test bed:


Video of tri-blade ramping from idle to 8 Amps (12000 RPM):
video

Our findings indicate that the two prop configurations are similar, but at lower currents (as in hovering) the 2-blade prop is more efficient than the tri-blade, and at higher currents (above 5 amps) the opposite is true.

Thrust vs. Current plot:


The difference in efficiency at high and low current output is particularly evident when plotting RPM versus current as in the figure below. Note that at higher currents the dual-blade prop has a lower RPM gain per Amp than the tri-blade, and the tri-blade has a higher RPM gain per Amp than the dual-blade at lower amps. The general difference in RPM between the tri-blade and dual-blade is due mostly to the diameter of the props.

RPM vs. Current



To determine the efficiency of our system, we measure the entire weight of our body (adding on some weight to for wires to be conservative) with a particular battery, and determine the hover time based on battery capacity and the current required to hover.

For example, using Blue Lipo lithium polymer batteries with 2200 mAh (at about 185g) we have a total body weight of 615g. In order to hover we need each motor to generate 615/4 or about 154g of "thrust". With the tri-blade this corresponds to about 2.2 Amps per motor (8.8 Amps total). With the dual-blade this corresponds to about 1.8 Amps per motor (7.2 Amps total). With our 2200 mAh battery, this gives a flight time of:

Tri-blade -- 2.2 Amps*Hour * (60 min / Hour) * (1 / 8.8 Amps) = 15 minutes
Dual-blade -- 2.2 Amps*Hour * (60 min / Hour) * (1 / 7.2 Amps) = 18.3 minutes

This indicates that at hovering, the dual blades are (18.3-15) / 15 * 100% = 22% increase with respect to the tri-blade efficiency. This, however, was not enough to convince us to use the two blade props over the three blade props...the tri-blades are much quieter and, let's face it, look way too badass.

With a 1500 mAh Mystery Lipo we found the following flight times using the same metric:

Tri-blade = 12.5 minutes
Dual-blade = 15 minutes

Here's the mess we made in the undergraduate electrical engineering lab:

2 comments:

  1. Awesome work! I did a similar less scientific test.

    http://www.youtube.com/watch?v=PHZr8VfvyjY

    I intend to spend more time and money on the thrust research and body structure because all the other areas are well covered by open source projects.

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  2. Wonderful! I'm working on a similar project with an organization at MSOE in Milwaukee, and we've been looking for experimental setups to determine what our equations for flight stability need to look like.

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