Airboat & propellor selection.

[Roger]

Hi, i've been working away on a project for my kids to play with (really...!) I've covered most of the mods and changes here http://rcsoaringcommunities.co.nz/viewtopic.php?f=13&t=872 but in the process of working on the model it prompted me to ask Bruce if he knew of any prop performance data available. This (as I have discovered) is a big area just in itself, I am wondering about setting up a test-rig to compare the performance of certain props against on another.

For instance, ignoring the motor side of the input, which prop will give the best performance for your model from the least power input. Measuring static thrust is one thing, but it does not tell you everything about how that prop will perform on your model - it will tell you if you could "prop hang" your 3D airplane etc, but will the prop generate enough thrust to keep your model in the air at normal flying speed. This is not so much of an issue these days with the advent of cheap high performance brushless motors and lipo power, however I like to know that my model is set up in the best possible way.
Factors to note is that the thrust a propellor generates drops away as the model's flying speed increases, so while a prop may generate a lot of static thrust, the dynamics of that prop may be that it generates very little thrust at the high flying speed required for a hot liner. Conversely, some props may generate only a very small amount of static thrust, and not start to really perform until they get some flying speed, this is because the propellor airfoil is actually stalling when the propellor is in static air and requires some airflow over it to start to "fly" properly. Air density and temperature are also going to play a factor, more dense air will provide more thrust...

At the moment with my airboat I have actually had better performance out of my old Johnson 540 brushed car motor than the cheap hobbyking bushless combo I bought. I ran the 540 with an 8x3 prop on a 7.2V battery, and compared the brushless with a 9x4.7 prop which turned out to be much slower. Changing the prop to a higher pitch smaller diameter 7x6 produced similar results to the 540 with the brushless, but it was still slower - it was drawing much less current though and ran for a considerably longer time. Yesterday I ran the boat with an 8x4 prop on the 540 motor, and got a very slight improvement in overall speed. However the model did not accellerate as fast, and the motor was labouring a lot harder to get to the model to the same speed. My guess is that I was achieving the best performance with the 8x3 prop on that voltage - so I added two more cells to the NimH battery last night so today the battery is charged and delivering 11.6Volts fully charged, down to a discharged voltage of 9.6Volts - theoretically this should give better performance all the way down - and judging by the test run up in my lounge, it should be quite impressive! I'm going to compare both props again today.
I realise this is a very non-scientific approach, however with this sort of model, it is quite easy to compare the performance with different propellors and settings with some similar constants.

Ideally, I would like to run a test rig in a wind tunnel, with a movable motor mount sitting on an air cushion (like an air hockey table). The motor mount would be linked to a load cell, the motor would be brushless with a lot more power than is required to drive the prop. Instruments required would a tachometer, amp meter and volt meter, thermometer and barometer - as well as an air speed indicator (or two) for the air speed in the wind tunnel ahead of the prop, and behind the prop. Then each prop would be set up in turn and run through its paces at a range of wind tunnel air speeds, taking readings of current and voltage and thrust etc at a range of pre-determined prop rpm's. These results could then be charted on a graph for comparison with other props. The motor power supply would need to be regulated and not from a battery etc to eliminate another variable from the equation. The reason that the air cushion would be used is to eliminate friction out of the pulling load applied by the motor. Of course it could also be possible (with some correct callibration) to mount the motor directly to a load cell. With the advent of this forum, perhaps there is a chance of someone out there setting up a test rig like this whereby we might be able to start sharing results with similar test rigs.

Roger

[Roger]

Uploaded video of my latest runs with the airboat, additional commentary provided by my 5 year old...
http://www.youtube.com/watch?v=3TJuUQSILu0

[Roger]

I trialled the airboat at the weekend running a 3S lipo, good thing I had an amp meter handy too...! the thing went ballistic on the water, poor little motor screamed its head off with an 8x3 prop, very fast and rather uncontrollable - and LOTS OF FUN!
However I noticed a slight change in pitch a second or so after full throttle applied, brought it in and checked the current draw (which was very low when only running the 8 cell nimh) and it was up at 34 amps... Not huge, but was more than the speed controllers rating, and the battery's 20C is 32amp, with a burst to a little higher at 30C. So, while it was good fun, I shut it down and walked away with everything still in one piece...!