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Apparatus and method for measuring static thrust?

Old 02-11-2007, 09:18 PM
  #1  
UncleBob
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Default Apparatus and method for measuring static thrust?

The thread title is almost the question. How do you measure static thrust? What is the recommended method?

Any good ideas? I have an idea but I want to hear the wisdom of the group before I spout off.
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Old 02-12-2007, 12:32 AM
  #2  
Rabbitcreekok
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UncleBob

Here is one option. For $36 you get a thrust stand and a scale. It looks like it would do the job. I don't have one yet but I plan to purchase one in the future.

http://www.bphobbies.com/view.asp?id...07&pid=V786359

I don't think I could build something for as little as $36.
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Old 02-12-2007, 04:20 AM
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jdetray
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Hi Bob -

Thrust stand designs and information

Making your own motor test stand is pretty simple and inexpensive if you already have some scrap wood and a small digital scale. Otherwise, buying a kit is a good solution.

Dr Kiwi Mk III Thrust Stand, the best of the "simple to build" test stands:
http://www.rcgroups.com/forums/showthread.php?t=505683

It turns out that the pusher configuration of the Dr Kiwi Mk III yields more accurate results that tractor designs.

Other homemade thrust stands:
http://www.rcgroups.com/forums/showthread.php?t=421578
http://www.rcgroups.com/forums/showthread.php?t=272215
http://www.rcgroups.com/forums/showthread.php?t=476666
http://www.rcgroups.com/forums/showthread.php?t=18573
http://www.rcgroups.com/forums/showthread.php?t=328085
http://www.rcgroups.com/forums/showthread.php?t=262089

Thrust stand kit (already suggested by Jim).:
http://www.bphobbies.com/view.asp?id...=V786359&img=l

- Jeff
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Old 02-12-2007, 05:01 AM
  #4  
Dr Kiwi
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Well Jeff - thanks for saving me the trouble of hunting down those links!

Cheers, Phil
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Old 02-12-2007, 07:08 AM
  #5  
jdetray
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Hi Phil -

Certain questions recur frequently enough that I've assembled text files to address some of them. No sense hunting through a bazillion bookmarks every time!

I recently built my version of your Mk III design that works beautifully, though it's not as nicely finished as yours.

- Jeff
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Old 02-12-2007, 12:06 PM
  #6  
ragbag
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Default My Version

As you say, some scrap ply and a postal scale from Office Depot.

Think I have one of these on RCGroups also, a looong time ago. Don't hang around there much anymore.


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Old 02-12-2007, 01:27 PM
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Default Another approach

http://www.badcock.net/cgi-bin/powertrain/propconst.cgi
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Old 02-12-2007, 08:31 PM
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UncleBob
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Some of this seems like a lot of worry about almost nothing, given that static thrust really doesn't apply much to actual flight. Worrying about pusher vs tractor and such seems pretty academic. Interesting, but not really terribly useful in practice is it? Did I miss something?


In any case, my question is well answered! I'll likely get the kit, but Dr Kiwi has the slickest stand setup for sure. Pretty.
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Old 02-12-2007, 11:55 PM
  #9  
ragbag
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Originally Posted by UncleBob View Post
Some of this seems like a lot of worry about almost nothing, given that static thrust really doesn't apply much to actual flight. Worrying about pusher vs tractor and such seems pretty academic. Interesting, but not really terribly useful in practice is it? Did I miss something?


In any case, my question is well answered! I'll likely get the kit, but Dr Kiwi has the slickest stand setup for sure. Pretty.
On a cold winter night and with nothing to do while the aliphatic glue dries you can get readings on prop changes, amp draw and just have a good time entertaining yourself.

Maybe test the new ESC that came in the mail this afternoon while you were at W**K.

Keeps me out of the bars also, can't afford to drink and buy lipo's.

Oh, I live in Florida, did my time in winters.


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Old 02-13-2007, 12:44 AM
  #10  
UncleBob
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Ha - good point. I guess I'm more interested in "will it make plane X fly" and "what's going on with the system in the air", but I can see how it would be fun. I have to admit, I ordered a dozen different props last week, so shame on me.
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Old 02-13-2007, 04:41 AM
  #11  
Dr Kiwi
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Originally Posted by UncleBob View Post
Some of this seems like a lot of worry about almost nothing, given that static thrust really doesn't apply much to actual flight. Worrying about pusher vs tractor and such seems pretty academic. Interesting, but not really terribly useful in practice is it? Did I miss something?


In any case, my question is well answered! I'll likely get the kit, but Dr Kiwi has the slickest stand setup for sure. Pretty.
To some extent it is an exercise in futility, since it may bear little relationship to in-flight performance (unless one does nothing but hover).

But one must have some basis for determining whether "plane x" will fly with "motor y" and "prop z" - it is all very well to give watts/lb and rpm (pitch speed) - obviously at least the second of these is VITAL. But somewhere, sometime, someone, actually had to measure "thrust in relation to prop size and rpm" to provide the basis for all the predictive equations.

"will it make plane X fly" - what do you need to determine that? - enough pitch speed (yes), but what else - watts/lb is such a moveable feast depending on the type of plane, the efficiency of the motor, and most importantly how well those watts are translated into thrust to move the plane.

Cheers, Phil
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Old 02-13-2007, 04:42 AM
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colecaz
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I set the plane on the floor on it's wheels and attach a digital fish scale to the tail. Attach the other side of the scale to something heavy. Open the throttle and read the pull/thrust.
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Old 02-13-2007, 05:52 PM
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UncleBob
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Originally Posted by Dr Kiwi View Post
To some extent it is an exercise in futility, since it may bear little relationship to in-flight performance (unless one does nothing but hover).

But one must have some basis for determining whether "plane x" will fly with "motor y" and "prop z"
Couldn't agree more - I just meant that obsessing over the contribution the drag of the stand might make to the measurement is interesting but sort of stepping over dollars to scoop up pennies as far as practical use goes, it seems.
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Old 02-14-2007, 02:05 AM
  #14  
Dr Kiwi
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Originally Posted by UncleBob View Post
Couldn't agree more - I just meant that obsessing over the contribution the drag of the stand might make to the measurement is interesting but sort of stepping over dollars to scoop up pennies as far as practical use goes, it seems.
I'm a Research Scientist by training, and though my test stand is somewhat primitive, I do like to get things as accurate as I can.

I began with a vertical stand, which aside from being very dangerous for the operator (me), was fraught with problems of ground effect and so was not much good as far as accuracy went. Then I moved to the tractor version, but after some thought, it occurred to me to work out how much of an error the tractor configuration was inducing (efflux air pushing on the vertical arm reducing the downward pressure on the scale) - to my chagrin it was quite significant, particularly with small props - 10% or even more - but less (~5%) for large props.

So I went to the pusher version, which certainly produces consistent figures, and, as far as I can tell, seems to be accurate. Unless the vertical and horizontal moment arms are unequal (and I know they are not) my thrust numbers cannot be HIGHER than they should be - knowing that my stand delivers at least a minimum thrust figure is, I think, useful.

In-flight, with the prop not stalled, useable thrust might be higher - in that case my figures comprise the worst case scenario. It has to get better in the air!
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Old 02-14-2007, 05:24 PM
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I went ahead and ordered the BP kit - once it gets here I'll look it over and see if I can convert it to pusher without too much trouble. Thanks for the detailed explanation.
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Old 02-27-2007, 06:54 PM
  #16  
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So I've assembled the thrust stand kit, and there's no really obvious way to cleanly install and remove multiple motors over the long term. Anyone have a good solution to motor mounting?
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Old 02-28-2007, 12:53 AM
  #17  
Dr Kiwi
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I'm away from home so I can't post specific photos of motor mounts, but you''ll see at least two in amongst this thread: http://www.rcgroups.com/forums/showthread.php?t=645168

For stick mounts, [as shown in the above thread] I use a piece of ply bolted to the mounting arm, with a 10x10 (Photo #1) or 8x5 (Photo #2) stick attached. This allows me to test all sorts of stick mounted motors (with or without gearbox) by employing any of the various aluminum mounts available (from Hyperion , Cobri, and my own custom Jerry Blake tube mounts etc - Photo #1 again) or motors in plastic IPS or GWS 300/400 gearboxes. Even X-mounts can be attached directly to the proximal end of the 10x10 stick for radial mount motors.

I've even used "Aeronaut mounts for Speed 400/600" on a similar ply plate.
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Old 02-28-2007, 12:55 AM
  #18  
ragbag
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Originally Posted by UncleBob View Post
So I've assembled the thrust stand kit, and there's no really obvious way to cleanly install and remove multiple motors over the long term. Anyone have a good solution to motor mounting?
I have a previous post, #6, I attach a 10mm spar with two screws and mount the GWS type of motors to it.
The Axi type I have a plate that the motor is mounted to and attached to the same upright.
Measure from the prop shaft to the horizontal shaft and use that measurement to set the distance from the pivot shaft at the bottom pivot to the center of the scale.

This makes it all equal and square. About as good a guesstamate as you can get.

All of the attachments use the same screw and bolt holes, so there is a good average to help make final decisions.


DR Kiwi can type faster than me!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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Old 03-01-2007, 12:03 AM
  #19  
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The fastest and most accurate way to determine if the plane will really fly. Is to go to full power and see if it wants to almost pull vertically out of your hand. Also, is the RPM scream high enough to do the job.



Very field proven method.
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Old 03-01-2007, 12:14 AM
  #20  
Dr Kiwi
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And as the motor fries, the ESC shrink wrap melts and the battery puffs - you realize that you have over-done it!:p:p
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Old 03-01-2007, 04:11 AM
  #21  
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Default No No No !

You are on the CUTTING EDGE !!!


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