Among the several power-system test devices, like wattmeters and tachometers, another handy tool is the thrust stand. David Fuller has published an article describing a DIY, low-cost design, using simple wood parts and a kitchen scale, here; http://www.rcscratchbuilders.com/col...ruststand.html rcscratchbuilders.com is a new site with the "builders" in mind, both foam and balsa. If you click on the revolving "Beginners" logo you'll find an easy-to-understand description of watts, volts, amps and "C" rating with a formula to calculate (estimated) flight duration.
Dave is also a Wattflyer member (magic612), check out the site!
P.S. I've added this to the "Builders Links" under Tools, Measuring and Testing Equipment) http://www.wattflyer.com/forums/showthread.php?t=14117
Hey Ron - I appreciate you posting a link to my article here. Thank you sir!
Jeff - thanks for the thought on the pusher configuration. The way I have designed this stand, it can be used in either manner. The arm for the 3/8" square stock is centered on the vertical arm, and can therefore be used pusher or tractor style.
I also figure that for most prop sizes that I use (8"-10") the area that is "blocked" by the vertical arm is relatively quite small, and rather insignificant considering that a static thrust stand is never going to be as accurate as an actual flying model anyway. But it's a pretty good way to test out various motor / prop / battery systems in the comfort of one's home, instead of running along side the model while holding a wattmeter! Additionally, for full fuselage style planes, the area of the arm is probably less than the drag created by a fuse, so for all intents and purposes, the difference is really a wash, I think.
And for safety's sake, I prefer the tractor method most times. Keeps the prop in a very obvious "keep clear" area. But that's just me.
When Dr. Kiwi measured the difference, he found that his tractor-style test stand understated thrust between 4.3% and 21.8% compared to the pusher version. In general, the larger the prop, the smaller the difference. Here's the thread.
The Good Doctor's hundreds of motor and prop tests are used as the basis for some thrust calculators, so in that context the improved accuracy is important.
Jeff - my point is that a static thrust stand is never going to give you "real world" performance anyway, so the difference between putting the prop on a thrust stand with a small vertical arm is A) likely to be less than the area of a full-fuselage plane and B) isn't realistic as an measure of actual performance, because it's static (not moving). So I'm not sure how that necessarily "improves accuracy," given the point that if the motor is put on a plane with a large fuselage, the increased "accuracy" just might end up over-stating performance.
As Dr. Kiwi himself acknowledges in that thread, "As you guys mention, the good thing is that the thrust numbers have been understated as opposed to being over-optimistic! I agree with Tom that, in the real world, the nose of any but the most streamlined aircraft will offer some degree of obstruction to airflow."
I'd personally rather understate my performance on the thrust stand, and be pleasantly surprised at the addition thrust I have under actual flying conditions, than have the reverse be true because my "measured" thrust was overstated (since on a marginally thrusted plane, that could prove disastrous!).
I think it's best used as a comparison tool, and for getting some idea of the performance to expect in the "real world." And it's also helpful in choosing an appropriate battery. And I should point out, again, that the thrust stand in the article can be used in either configuration. I actually designed it that way intentionally.