How to build a low-cost thrust stand

By David Fuller, Editor - RCScratchbuilders.com

A frequent question that pops up on many of the R/C plane forums is, "What motor should I use for XYZ plane?" As I recall being told when I asked this question a while back (don't we all ask it at some point?), I was rather wisely corrected, "What you should be asking is, 'What plane can I build with ABC motor?'" Well, either way you look at it - plane first or motor first - you'll eventually need to know some important data about the motor / propellor combination you use with a given battery. And the way to do that is with a thrust stand.

"Why bother with a thrust stand?" you might ask. Well, the more performance scratchbuilders wish to eke out of a given aircraft, the more likely they are to eventually start using Lithium Polymer (LiPo) batteries. Compared to Nickel Cadmium (NiCd) or Nickel Metal Hydride (NiMh) batteries, Lipos comparable performance at a 40% weight savings. A battery being one of the heaviest parts of an electric plane, this is a HUGE point! That said, LiPo's are not nearly as "bulletproof" as NiCd's or NiMh's, so care must be taken to ensure that they are not over-extended during use.

A LiPo battery has a "C" rating, indicating how much current can safely be pulled from the battery by a motor without damaging the battery pack. A 1500 mAh LiPo with a "10C" rating can safely supply 15 amps of continuous current to the battery (10C x 1.5 Amp hours = 15 amps). But how to know whether a given motor and propellor combination draws more or less than 15 amps? That's where the thrust stand comes in the picture.

Parts List

Tools required

First, download the plan, then cut each of the wood pieces to the appropriate length. Be sure to measure AND cut each piece very accurately, as the thrust stands accuracy is dependent upon correct measurements.

SIDE NOTE: My scale is a $30 kitchen scale, that is 1 and 1/2 inches high. The screw at the bottom of the horizontal arm accounts for differing scale heights, up to 1.75" high. If your scale is taller than that, you will need to increase the height of the two arm supports on either side of the arm, in which the bearings are placed AS WELL AS the lower portion of the vertical arm by an equal distance. Increase these as necessary for your scale, and for the two support arms, go at least 1/4 (6mm) higher than the top of the scale to be used.

After measuring and cutting each piece of wood, measure the exact locations where the bearings wil go in the arm supports. The OD (outside diameter) of the bearings is not critical, but the ID (inside diameter) must match the diameter of the brass rod you are using as the pivot point. I find that using a T-square with the sliding arm set to the exact measurement I need, placed up against the piece of wood, is the most accurate way to measure a given dimension (especially on wood with rounded corners, as 1x3's often have).

Drill a smaller pilot hole that is one drill size larger than your brass rod. Then drill a partial hole, centered on the pilot hole, with a drill bit the size of the OD of the bearings. ONLY DRILL to the depth needed to make the bearing flush with the surface of the wood! Use a piece of masking tape on the drill bit to avoid drilling too deep. Press the bearings into place, and set the support arms to the side.

Now measure the horizontal arm. Measure the point at which the brass rod will go through this arm. MARK WHICH WAY IS "UP." Now measure EXACTLY 8.75" from this point towards the end of the arm (this is very important for accurate thrust readings). Draw a line down to the bottom of the arm, and across the bottom section. In the midpoint of the piece of wood, mark an "X" at this 8.75" point. This will be where the 1" screw will be inserted that engages the scale later on. Now go ahead and drill the proper size hold for the brass rod.

Moving to the vertical arm, measure the spot on this arm where the brass rod runs through the wood. Note that this spot leaves a fair amount of wood that will extend down towards the base of the stand. This additional wood DOES serve an important purpose - it is designed to be only 1/8" above the base itself, to keep the arm from tipping over backwards accidentally. (If need be, a shim can be glued to the base of the stand to ensure the arm doesn't tip backwards, too.) Measure exactly 8.75" from the brass rod location to the point where the screw will be placed for the 3/8" square stock. THEN, drill the hole for the brass rod in this arm, and a small pilot hole for the square stock screw.