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Sep 28, 2005, 04:49 PM
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Dr Kiwi's Avatar

Thrust Stand - build one yourself.

You'll all have seen photos of my thrust stand, which seems to work pretty well. I've mislaid the original working drawing, but "Chuck" wanted some measurements, so I re-drew it, by tracing from photos, and added measurements and comments as needed.

For what it is worth here are the "pairs" of photos/diagrams. If something is still confusing, please let me know. The actual dimensions of most parts are not critical - mine is the size it is, because I needed something which could take up to a 14" prop. Obviously everything needs to be as square as one can get it, but THE ONE CRITICAL MEASUREMENT is making the distance between (A) "the axle pivot and the thrust line of the motor mount", and (B) "the axle pivot and the push rod which presses on the scale" EXACTLY the same.

Edit: I've added some detail from an earlier thread.

I used decent size (1/4" axle, 3/16" wide, 5/8" diameter), high quality, ball bearings - even shielded to keep the dust out - but only around $2.60 each at Motion Industries - essentially frictionless for this application. The 'axle' is 3/16" brass rod, sheathed in 7/32" brass tube, sheathed in 1/4" brass tube! [The local Ace Hardware had no 1/4" brass rod!]. The 'stop rod' is 3/16" brass rod. The 'bottom of the L' is again 3/16" brass rod sheathed in 7/32" brass tube with thin CA run between to bond the two together for stiffness. The short vertical which presses on the scale, is another piece of 3/16" rod, soldered on to the end of the horizontal rod.

The Ohaus scales I have [CS 2000 and CS 5000 - ~$75] seem to be very good, though they are obviously not in the same league as true Laboratory scales [Sartorius etc] which will set you back $600 or more.

The base is oak ($14 at Lowe's), the rest scrap pine, the streamlining on the front faces of the uprights is balsa 1/2"x1/2" and 3/4"x 3/4" symmetrical LE material.

Cheers, Phil
Last edited by Dr Kiwi; Sep 28, 2005 at 05:30 PM.
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Sep 28, 2005, 04:52 PM
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Dr Kiwi's Avatar

Thrust stand continued

Here is the last "pair" of the photos/diagrams, and a few pictures of the stand in use.

Cheers, Phil
Oct 03, 2005, 03:02 PM
The Big Kahuna of Foam.
bkahuna's Avatar
I don't understand how the second rod (3/16 brass rod) works. The plans say it prevents it from going backwards. Seems like it would prevent it from going forward as well if it is flush with the top of the wooden sides that house the bearings.
Oct 03, 2005, 05:28 PM
Registered User
I can spot one easily corrected problem with this setup, which, BTW, I think is mostly pretty cool. Might build one myself some time.

The plane of the propellor disk should, if extended, go through the pivot at the base of the upright arm. It should appear as if the propellor is aimed down. Another way to state this is that the axis of the propellor should be at a right angle to a line extending from the upright pivot rod to the propeller axis at the propellor.

If you draw a vector diagram of the force exerted by the propellor the way you have it build in the pictures, you'll see that a portion of the thrust from the propellor is exerted perpendicular to the pivot rod such that it does not bear on the scale.

As I said, this is easily fixed by adjusting the angle of the arm that supports the motor/propeller combination.

Oct 03, 2005, 06:33 PM
The Big Kahuna of Foam.
bkahuna's Avatar
wevets... not sure I can visualize what you're saying can you draw a simple diagram?
Oct 03, 2005, 06:39 PM
Will fly for food
I don't think so, the thrust is straight back along the motor shaft axis, so the thrust is perpendicualr to the upright in all cases, therefor the thrust is accurately translated to the leg on the scale.

Doing what youa resaying, would induce a vector not perpendicular to the upright and cause an incorrect reading.
Oct 03, 2005, 07:13 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
More thrust stand designs, ranging from simple to veeery elaborate:

Vriendelijke groeten Ron
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Oct 03, 2005, 07:49 PM
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Dr Kiwi's Avatar
Originally Posted by bkahuna
I don't understand how the second rod (3/16 brass rod) works. The plans say it prevents it from going backwards. Seems like it would prevent it from going forward as well if it is flush with the top of the wooden sides that house the bearings.
No, it is 2-3mm clear of top of the wooden bearing uprights (perhaps I should have said "bored 68mm above baseboard" to allow an extra bit of clearance) - the scale depression is only a matter of a millimeters when the motor is running, so the upright support tilts only a little forward (and of course the 3/16" rod moves even less). Even at, say, 1500g thrust,the rod doesn't move forward and downward enough to touch the wooden bearing upright at all. The rod is not really necessary except to prevent the whole system from tipping backward if I accidentally have the motor leads reversed and the thing pushes instead of pulls!

Cheers, Phil
Last edited by Dr Kiwi; Oct 03, 2005 at 08:08 PM.
Oct 03, 2005, 08:03 PM
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Dr Kiwi's Avatar
Sorry, wevets,

You are incorrect. The horizontal force from the prop thrust is exerted at a distance of 232mm from the axle. The rod depressing the scale is perpendicular to the upright and also at 232mm from the axle - the horizontal thrust (x distance) exactly equals the downward force (x distance) on the scale.

I know it works that way because I calibrated the system by hanging, vertically, various known masses from a nylon line attached to the end of the motor mount and then running around a ball-bearing pulley, such that the force they exerted was horizontal, just as a motor's thrust is. I hung a 100g mass from the line - scale read 100g; I hung a 750g hammer from the line, scale read 750g.

Cheers, Phil
Oct 03, 2005, 10:05 PM
Registered User
Hay Doc,
I see your keeping bizzy!! I was thinking about a thrust stand to put the planes on!
Last night I put a just built pusher, with a Customcdr, nose down on my scale. I held the tail lightly, zeroed the scale, then applied power. I got about 12ozs. I then tried my Potensky PAF and got only about 5ozs. It seems to work ok with pushers, but not with tractors. So! What about a N-gauge model train track, with a flat car to tie the plane to and then a lever like yours to push against?? It may not be real accurate, but close enough for simple folk. Any suggestions?? Thanks, Butch
Oct 04, 2005, 08:47 PM
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Still looking for Ideas!! Butch
Oct 04, 2005, 08:56 PM
Registered User
ryanl2006's Avatar
Butch- What about getting a fish scale and just tying a line around the tail of your plane. Fish scales are not the most accurate out there, but they are cheap and would get the job done.
Oct 04, 2005, 09:00 PM
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Dr Kiwi's Avatar
Originally Posted by Butch777
Still looking for Ideas!! Butch
If you can make it frictionless it may work. The other problem may be interference to the airflow to and from the prop. That's, I think, why you got such disparate results for your "hold by the tail tests". That's one reason why I canned my vertical stand and went to the horizontal one - with high quality ball races on the axle it is essentally frictionless and the airflow into, and past the prop, is not seriously impeded by any bench or walls.

Cheers, Phil
Oct 05, 2005, 03:24 PM
Registered User
I'm sorry Doc,
The 12 ozs I got on the scale was from a Customcdr, 2cell w/ a 7.5 wattage prop and the 5ozs was from a Potensky 025 motor geared, with a 8X6 prop on my PAF.
when I maidened my scratch built, Customcdr powered pusher and it went stright up on launch, but my PAF is real tame and won't even complete a loop. I was just thinking of ways to test a whole plane, rather than just the motor. So like if the plane seems to not be running so good, one can just put in on a thrust stand of sorts, to give it a check-up!!
What about suspending the plane from 3 threads, infront of a 90 degree lever, that pushes the scale?? Butch
Apr 11, 2006, 09:34 PM
I think your stand has and error induced by the air drag of every thing behind the prop.
Visualize your hanging weight test, but now put a 40-100 MPH wind blowing the vertical arm back. The stronger the wind the more load it would take off the scale and the lower the thrust reading. You could actually measure it by using a motor not connected to the stand to generate the wind and air drag. I suspect it could go into double digit grams.

To minimise the error you can stream line the vertical support and all behind the motor, or better yet test motors in pusher mode. That would require moving the motor to the other side of the vertical. You would still have some effect from the airing toward the prop, but my guess is it would be an order of magnitude smaller, as the velocity a few inches in front of the prop is much lower than the velocity behind. Of course you could stream line and minimize frontal area of the vertical etc. and reduce the error even more.

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