Engine thrust meter

Cribbs74 wrote:How are you able to acheive a true "zero" ? It looks to me that you can only measure thrust between engines.

Ron, theoretically; a  vertically balanced "pendulum" as in my design, should give accurate readings for different engines. Balancing the pendulum to ea. engine is accomplished by counter-weighting the opposite side of the mount, at the top of the pendulum. This can be done with the weight-box... or even simpler; a sm. bottle w/cap, to hold enough water to counter-weight the mount with engine/prop. and possibly a "tach./mount" installed. After mounting your engine, the pointer should read slightly positive on the graph with an empty bottle. Adding water to the bottle will bring it back to "zero".. or "balanced". If using an external fuel tank, it should be mounted on/over the balance point so that; as fuel is consumed... it doesn't unbalance the pendulum.

The pendulum's "length" depends on where the pivot/swing-point is placed. Ideally; a pendulum with a series of graded vertical holes for the swing-pin would help with initial set-up/calibration. You want enough pendulum weight "below" the pivot, to provide a "75% of max." reading on the scale, when testing your most powerful engine/prop. set-up. For testing "Cox" .049 engines; it should not require more than 8 ounces of force, to move the pendulum to it's maximum reading on the scale. This should produce "healthy engine" readings in the 50-70% on the scale ... (4-6 oz. of thrust)

It is important to note; that the movement of a pendulum is NOT "linear" but "exponential". It will take more force to move it, as the angle increases from vertical (zero)...to full extension (100% of the scale). This is where it is more accurate to calibrate the scale in "ounce" increments. The distance between graduations on the scale will be closer together, as the pendulum angle increases.

pkrankow wrote:Simpler is a right angle set of arms on a pivot with one of those fancy digital kitchen scales.  If the lever arms are equal, and the pivot adequately free through use of the "tare" button thrust is directly measured.

Downside is the scale gets oily and nasty, even in a plastic bag. (at least it seemed to, I bought a new scale for the kitchen and use this for shop and hobby things now)

I built one from PVC pipe using round headed carriage bolts as "feet" so it would pivot as free as possible...however the feet never stayed put and my hopped up baby stunt bee pulled the whole contraption off the table!  

The idea is sound.  At some point I plan to rebuilt the apparatus using a thru-bolt and have it all bolted to a weighty plank.  The priority level is very low currently.

Phil

Being on lay-off... "my" priority levels are all out of whack... what I "actually" do vs. what I "should" do.

Here's an idea for the "digital-scale" thrust meter. A balanced "see-saw beam" principal. Either vertical or horizontal arrangement would evenly/accurately load the scales table. If the scale must be "level"... then the vertical pivot would have to be used and the engine mounted facing upward.

[edited]
Vertical arrangement; the beam should be "substantial" (1" sq. alum. tubing) about 3 ft. long with a pivot point "equidistant" from the engine mount centerline, to the opposite end of the beam that contacts the scale. Attach a rubber "foot" to the beam, where it will contact the scale's table. The base/pivot-point would be firmly mounted and have 2 "ears" with bearing holes to support the beams' pivot pin. (see-saw). Mount a "U" bolt over the beams "scale" end, to limit it's travel to about 1/2" above the scale's table. A "cue" lever could be built-in, to lower/raise the beam onto/from the scale's table.

Run the engine "beam raised" with the scale on the other end, centered "under" the beams' rubber foot. "Lower" the beam, to contact the scale for thrust readings. Depending on how much fuel is in the tank, you will get slightly higher thrust readings as fuel is consumed. Having 1/2" of beam-travel permits mounting an external tank independently from the beam, using a long enough fuel line. This would allow accurate measurements throughout the entire engine run.

A shroud/shield should be used for safety... No more "oily scale".

We all know you didn't buy that digital scale for the kitchen Phil....

back to this... I received my order for 2 of these little digital fishing scales in the mail this past week... (for $3/ea. I couldn't resist) and my speed-contest piston/cylinder arrived a day later!




The plan is... to build a sm. test fixture (1' x 2' footprint) engine/motor dyno; capable of measuring thrust via an airscrew. Proposed is; a "tub" to float a buoyant platform with a pylon for mounting a propulsion unit. This mount will have a direct linear pull on the digital scale. Travel limit will be less than 1"... and leveling-cups will balance the float. Safety rails will prevent the float from lifting. An aero-dynamic shield/bag protects the scale from propulsion wastes.

This Dyno is something I've always wanted to build for my Cox engines... and have it be close to accurate. It should at least; give accurate comparisons; along with a tach... between propellers.

The build will be soon.

RknRusty wrote:I'm really interested to see one of these concepts put to the test.
How about this:

You might want to zero it and remove it to crank, then set it on the scale. And for the Bee, fill it, then plug a vent and use an external tank connected to the other vent.

A "pull (hanging) scale" and fixture is what I wanted to go with, in order to have a tach mount... a bracket for an external fuel tank, and "quick-change modules" for either radial or beam engine mounts; to easily remove an engine for plug/cylinder changes etc.  

A buoyant foam block with a central pylon-style engine mount, cut to fit inside a small water tub with minimal "side to side" clearance and about 1" of linear travel, would not have much (if any) linear resistance when floating in/on the water. (think of a motorboat on a trailer... and the same motorboat floating in the water and trying to "pull" either one using a rope) I think this less resistive approach will transfer to a more accurate scale reading, during the running of a prop/engine combo. The digital scale's hook will attach directly to a rigid hook on the back of the pylon/engine mount.

pkrankow wrote:Just hook the scale to the tail of the airplane like a stooge.  Quit wasting yer time getting fancy.

Phil

Sure that would work Phil... I'm sure it doesn't surprise you, that I'm over-thinking this. I do that with everything!

Your idea for "at the field" testing is good too...

I guess I just like the idea a "purpose-built" means of performance bench-testing an engine/prop combo.. before I put it on my airplane.

Roger