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Old Sep 06, 2004, 10:11 AM
Dr Kiwi is offline
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The Dr Kiwi Test Stand revisited


A forum member asked me for measurements of my crude, but well-used and oft-illustrated test stand, so I drew up a diagram for him. I thought others might be interested, so here it is for general consumption (along with a few other photos to illustrate details not well explained by my poor quality rendition).

I've attached the drawing with measurements [not great quality, but I hope it will serve the purpose]. Also, some photos showing you how I made my base plate - depending on the type of scale you have, yours may have to be quite different. The structural upright part of the stand was all made from ~1"x1/2" pine - actually sold by Ace Hardware for ~$2 as a 3' long garden stake or sign post!

If you modified the design a bit you could accommodate larger props - I can just squeeze a 12" in there if I mount the motor exactly right! As you know from the other photos I've posted in the forum, all the gear (battery, Whattmeter, flashlight for tach, Rx, ESC, and obviously the motor) is loaded on top of the base plate, so that one only has to tare the scale to get a direct reading for thrust. The electrical gear is held by velcro, the flashlight sits in a crude foam "holder", and the leads are held in place by rubber bands looped over the various dowels. I have made up numerous motor mounts to accommodate everything from LensRC and Feigaos up to Phasors, and even ducted fans. These are simply screwed on to the flat/wide face of the vertical post.

If you have any questions just ask.

Cheers, Phil
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Old Oct 31, 2004, 07:34 AM
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Thanks for posting this. I finally got tired of guessing and built it yesterday. I used ¾” x ½” pine (since I couldn’t find 1” x ½”) and sized it to handle a 14 in. prop. I also used ¼” hardwood dowels throughout, including 2 to connect it to the base. The only screws used were those for the motor mount. Works great!

John
Old Oct 31, 2004, 08:01 AM
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I went a little different way when I hacked mine out. It's 12" square, so a 14" prop is no problem. It's made up of a scrap particle board base, some 1/8" dowels and some 3/8" square stock. Lashed and CA'd together. So far no problem at 24oz of thrust...
Old Oct 31, 2004, 01:08 PM
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John - Beautiful job - now mine looks really amateurish! How have you attached the base to the scale (?velcro or little rails beneath it on 3 sides)? If you get a biggish motor/prop on there pushing 20oz thrust it's going to want to wander around.

Cheers, Phil
Old Oct 31, 2004, 03:40 PM
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Phil, I covered the bottom with the 1/4" latex foam used to pad slimmer gas tanks and radio equipment. It's extremely non-skid. The Mamba in the picture developed 24 oz of thrust and didn't move a bit.

John
Old Nov 06, 2004, 05:13 PM
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Variations on a theme...


Under Construction.

Thanks Doc!

mike
Old Nov 06, 2004, 05:35 PM
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These devices don't measure thrust, they measure thrust minus drag.

Obstructions to airflow in the slipstream should be minimised and streamlined.
Old Nov 06, 2004, 06:16 PM
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I agree that the design is not ideal [I tossed it together in an afternoon], and certainly the "bits" above the prop could be better streamlined, but I wonder exactly what percentage error we are inducing by having relatively small obstructions several inches behind the prop [and its much the same for every motor/prop we test, so our comparisons might still be reasonably valid - even if our absolute numbers are not spot on]. I imagine the most significant absolute error is in having an essentially stalled prop - but again, that's how it is for every prop we test, so the results are, to a considerable degree, comparable.

Cheers, Phil
Old Nov 06, 2004, 07:16 PM
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And theres the fact that most planes do have some drag in the prop blast so it may be closer than you think.

Also I made mine so the motor/prop can be mounted on top/ pusher fashon (notice top unused hole in pic) if I wanted to, to minimize that if need be.

peterangus, show us your test stand, we've shown you ours.

mike
(of course you'd also need to match temp., humidity, altitude, airspeed... if you want to get picky )
Last edited by umrk; Nov 06, 2004 at 07:23 PM.
Old Nov 07, 2004, 06:22 AM
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Quote:
Originally Posted by Dr Kiwi
I agree that the design is not ideal [I tossed it together in an afternoon], and certainly the "bits" above the prop could be better streamlined, but I wonder exactly what percentage error we are inducing by having relatively small obstructions several inches behind the prop [and its much the same for every motor/prop we test, so our comparisons might still be reasonably valid - even if our absolute numbers are not spot on]. I imagine the most significant absolute error is in having an essentially stalled prop - but again, that's how it is for every prop we test, so the results are, to a considerable degree, comparable.

Cheers, Phil
So now you need to build a variable speed wind tunnel, so you can get in-flight numbers.
Old Nov 07, 2004, 10:11 AM
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Yup. The back bell is a quickly thrown together version. I have a much better one in the works for a more powerful fan I got for it.

Gordon
Old Nov 07, 2004, 10:25 AM
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I for one think it is extremely nice of everyone to share both their collected data and means used to collect it. If you feel their data is inaccurate then collect your own or use someone elses.
Most of us here are hobbiest and some seem to think we are all wanabe rocket scientist.
My wind tunnel is provided by my revolving propellor and mother nathure.
My extremely hitech thrust stand can be viewed at post # 291 this thread
https://www.rcgroups.com/forums/show...ghlight=lensrc

Charles

Charles
Old Nov 07, 2004, 11:36 AM
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Quote:
Originally Posted by Dr Kiwi
I agree that the design is not ideal [I tossed it together in an afternoon], and certainly the "bits" above the prop could be better streamlined, but I wonder exactly what percentage error we are inducing by having relatively small obstructions several inches behind the prop [and its much the same for every motor/prop we test, so our comparisons might still be reasonably valid - even if our absolute numbers are not spot on]. I imagine the most significant absolute error is in having an essentially stalled prop - but again, that's how it is for every prop we test, so the results are, to a considerable degree, comparable.

Cheers, Phil
Your published thrust data (given prop and rpm) is allways lower then my measurements. The difference is about 10...15%. I think it is because of the drag of your stand (the parts above the propeller). My stand is a pusher, nothing in the path of the propeller wind (of course, this layout has its own shortcomings too).

However, as in most cases the aeroplane itself is in the path of the propeller wind your measurement maybe closer to the practical applications.
Old Nov 07, 2004, 11:44 AM
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Nice, Gordon!

Are you implementing a means of straightening and equalizing the air flow / ressure?
(I'm probably expecting too much on a hobby setup.)

mike
Old Nov 07, 2004, 12:30 PM
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It has a honeycomb flow straightener (that's the white stuff in the intake bell) and three layers of screens behind that. Dave Robelen, retired NASA wind tunnel area, is my input for wind tunnel design points, and also the builder of a few hobby wind tunnels of his own.

Gordon


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