Preliminary Test of Double Trapeze Dynamometer

[larry cottrill]

Today I set up the basic structure of the dynamometer for the first time, mostly to see how it would be to set up and get an idea of the sensitivity. I will post pictures here next week.

The bearings are crude - just threaded rod running in steel tubes, but there is no noticeable wobble unless you just push on it from the side. The main carriage is a treated 2x6 plank, weighing about 8 lb. That doesn't seem like much, but for the little engines I do, it makes the sensitivity about right - the first pound of thrust moves the carriage maybe 6 inches. No, I haven't rigged the laser level yet, so I can't tell exactly, but the order of magnitude is right. It's interesting to see how you have to push harder and harder to gain displacement, if you're careful to keep your 'thrust' force horizontal and not do any direct lifting of the mass.

The fuel line extension and high-voltage spark wire extensions are lashed to the front left arm, rigged for an easy arc at the top and bottom so they don't add significant 'springiness' to the top and bottom 'hinges'. There are two rows of pre-drilled holes for my own standard mount spacing [Elektra and FWE models], with 1/4-20 tee nuts hammered into the bottom surface. Thin sheet aluminum will still be used to shield the wooden carriage from the engine heat. A row of vinyl padded hooks down the left side forms a virtual trough for the spark wires and the fuel hose, and offers convenient tie-down points for plastic wire ties.

There is a small screw eye at the front for pulling with a scale for calibration. This will be done via a 0-50 lb dial scale, linked to the screw eye with a piece of SS welding rod on which a 'line level' is hung so you can tell that you're really pulling horizontally - VERY important. If the linearity turns out to be as good as I've predicted, calibration should comprise just making sure it starts at zero and measuring the laser line along the horizontal scale at 20 lb or some such and calculating a single factor - it should then be deadly accurate for the small attainable thrust values I'm after. [The factor will need to be determined for each test setup, because of the varying mass of engines, etc.]

One thing that seems a little awkward is that since the hanger arms are a mere 4 ft long, the engine ends up just a little below eye level! Of course, I am not a six-footer, so that should have been expected. Anyway, no backaches from constantly bending over to make adjustments!

I finally tried getting my FWE going on it for a while, but never could get a start out of it. It did move the stand, though, when a good buzz would kick off for a half second, and even a little nudge on every good bang! So, I think the overall design is wonderfully suitable. I can check linearity once the horizontal scale is finished [1.5-inch square steel tube, selected for straightness] and the laser level is set up in its adjustable mount. Another thing to check is how accurately the whole thing returns to zero at rest after use.

Anyway, I should have photos Tuesday or Wednesday, which will hopefully illustrate the basic setup a lot better than my descriptions. I'll do more after the scale and laser are built on.

L Cottrill

[tufty]

Excellent. Most Excellent.

Simon

[steve]

does this mean that you will be running maggie muggs soon?

It will be interesting to see how your precice thrust measurements with the FWE compare to my crude ones.

[larry cottrill]

Steve -

Maggie is definitely on the short 'to do' list. But, I haven't done anything so far to make mountings for the Black & Decker Leaf Hog, and that will take a while.

To get my FWE running again, I need to re-build the entire intake. My experiments with pinching venturis seem to have given me an engine that refuses to catch hold and run. But, yes, it will be fun to see what I get once I have it running again. Or, of course, I could just build up the second one, a little different from my original, based on things you and Eric have discovered.

L Cottrill

[larry cottrill]

Here are a few pictures of my first try at setting it up, using FWE engine.

All comments / criticisms are welcome here.

L Cottrill

[pezman]

That rules!

Home Despot (sic) sells (or used to sell) a little laser that attaches to levels using a set-screw. It might be a cleaner way to attach the laser to your dyno.

I think that I might just have to build me one of those, although I'll probably use PVC pipe instead (with T fittings at the top and bottom as hinges) -- and maybe Wonderboard as the platform.

[larry cottrill]

That rules!

Home Despot (sic) sells (or used to sell) a little laser that attaches to levels using a set-screw. It might be a cleaner way to attach the laser to your dyno.

You need to be able to accurately set the laser beam in a vertical plane. That's why I chose a laser level rather than just a pointer - it can be set up truly vertical before the test and locked in place. Also, the level lets you set the laser to cast a 'line' if you want rather than just a point. That makes setting up the horizontal scale a lot more convenient, and allows for any slight side-to-side drift of the carriage during its swing.

For accurate linearity, you need:
- The laser line truly vertical at zero thrust
- The linear scale truly horizontal
- The laser line as close as possible to a hinge centerline during travel
All these are required to make sure you are really measuring the tangent of the displacement angle, which is the function that should have a linear relationship to horizontally applied thrust.

L Cottrill

[pezman]

Thanks for the trig lesson ;)

For small excursions the measurement technique makes little difference (sin about equal to tan about equal to theta). The tangent can be extrapolated for large angles only if the arms are initially perpendicular to the floor (need level floor). Sin is linear to less than 2% for angles up to about 20 degrees.

At any rate, neat idea.

[mk]

Very impressive!
Simply, uhm...cool!

[...] It did move the stand, though, when a good buzz would kick off for a half second, and even a little nudge on every good bang! [...]

Just a note:
You probably might want to add a release arrangement that fixes the apperature till the engine runs, don't you?

[larry cottrill]

Just a note:
You probably might want to add a release arrangement that fixes the apperature till the engine runs, don't you?

Ah ... you mean the apparatus. Yes, actually, that might be a good thing to do. Good suggestion.

L Cottrill

[steve]

This kind of reminds me of my primative spring loaded thrust rig wich is hilarious to watch when I get those initial cannon shots during startup. The engine shoots foward and then snaps back violently into position. It is really fun when I get the mixture just right so that the engine dosn't run, but it ignites every time the sparkplug fires (about 5 times every seccond) then it looks like the piston on an old steam engine flying back and forth. I should realy try to get a video of it.

[Chen]

Hi , all

Why don't the thrust tester is designed like the wheelway ? That is a mobile wood platform on the rail and it uses a digital force gauge to pull the platform.

[Chen]

next one picture

[Chen]

next one picture

[larry cottrill]

Hi , all

Why don't the thrust tester is designed like the wheelway ? That is a mobile wood platform on the rail and it uses a digital force gauge to pull the platform.

Chen -

I wanted a design where I didn't need to provide straight and level rails or buy a precision instrument to measure the force. With the double trapeze setup, the thrust force is always horizontal, the linearity should be excellent, and the friction should be negligible. And I have something which can be quickly torn apart and stored up against the wall of the garage. Of course, if I could build it with precision bearings, that would be even better! But I have to work with low cost.

There are a thousand ways such things can be designed and built, and every design has advantages and disadvantages. It just depends on your requirements, and what you are willing to pay for.

L Cottrill