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Old Feb 10, 2013, 02:06 PM
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richard9999's Avatar
Sydney, Australia
Joined Jan 2007
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In-plane Motor Thrust Testing

This describes an easy method of measuring motor thrust with the motor, prop and ESC in the plane. I am publishing it in the threads of the planes that I own, and in the Power System forum. I have searched for, and not found, a method such as this. If it already exists, I’m sure someone will let me know .

I have looked at various techniques used for motor thrust testing and most of them seem to use some sort of test bench, where the motor, prop and ESC are removed from the plane (e.g. this thread). I have constructed a test bench for myself in the past, where the motor was attached to a pivot arm resting on a digital scale and pushed downwards. This was both fiddly to set-up and felt dangerous, as the prop blades were whizzing horizontally near my eyes as I tried to read the scale, so I have hardly used it (in fact, I forget where it is now!).

While looking at my digital kitchen scale the other day, I realised that there could be a way to measure motor thrust accurately and safely, without removing anything from the plane… and literally on the kitchen table! The test set-up is shown in the photos. Since a digital scale is based on strain gauges, it can operate from any orientation (correct?). I made up a simple “L” bracket so that the scale could hang from it while not touching the table top. A string with two loops at the ends is then taped to the middle of the scale and the loops passed around the horizontal stabiliser. Planes without u/cs must rest on rollers to reduce friction as much as possible - I used 150mm lengths of plastic cable conduit. The “L” bracket is clamped to a table top so that the prop clears its end, and the plane is rested on books if necessary.

Attach the loops, power up the plane (with Wattmeter attached if required) and then switch the scale on and hang it on a the nail on the “L” bracket. Note the scale reading if it is not zero. Spin up the prop, slowly at first to verify everything is Ok and then give it WOT and note the maximum thrust (less the zero thrust if present). Takes about 1 minute, and you can change props and get another reading quickly and easily.

I would suggest that this could be a more accurate measurement of actual motor thrust than a test bench, since it allows for any blockage of airflow by a large fuselage frontal area (e.g. FW-190), it is certainly much faster to set-up. The accuracy of the rollers was checked by comparing the thrust for my FW-190 wheels down vs wheels up with rollers, which varied by only 5gm at 525gm. For testing of more powerful motors, I would use a padded strap to attach to the h. stab, and would add a safety line or strap attached to the clamp.

I use the FC 28-22 1200 kV motor from HK in most of my park flyers – it is light, powerful and easy to swap out. I measured 670gm thrust with a 10x3.8 prop and 525gm with a 10x5. This compares to a published max thrust of 710gm with 10x5 and a user review of 660gm for a 10x4.7. The FlyBrushless website shows max 385gm for a 28-22 1100kV motor with a 10x6 prop. These figures are fairly close, and in any case I am more interested in comparative, rather than absolute, thrust figures, to work out which is the best prop to use. My Art Tech Spit has a stock 4038-15 700kV motor and gives 730gm of thrust with a 10x5 prop. Since it’s AUW is 650gm with a 1500 mAh 3S pack, I have decided to not move it to 4S at the moment.
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Old Feb 10, 2013, 06:51 PM
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Dr Kiwi's Avatar
Chattanooga, Tennessee, United States
Joined May 2003
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Lots of guys hang planes from a digital fish scale on the ceiling - same method.

http://www.rcgroups.com/forums/showp...0&postcount=19
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Old Feb 11, 2013, 02:41 PM
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richard9999's Avatar
Sydney, Australia
Joined Jan 2007
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Thanks for your (again) help Dr Kiwi, and for the good work you have put into the motor database. I wasn't aware of the fish-scale method, which seems very similar to my one, but I don't have a fish scale, while I do have a kitchen scale (and table). My method seems useful to me in determining which prop gives the best thrust vs W used in a particular plane for the limited number of motors I use. In considering alternate motors in the future, I will refer to the database and to user/vendor tests for that particular motor/prop combination.

Richard
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