Proposed Design: 'Thrust Bucket' Dynamometer w/ Multipliers

larry cottrill · Post by **larry cottrill** » Wed Jun 23, 2004 8:31 pm

Here's one that could be built by just about anyone. The only "special" part is the 'bucket', a little weldment comprising a W-folded steel sheet with two end plates welded on.

When Ben Brockert and I were destroying metal samples in the Dynajet, one amusing thing we found is that a flat reflector close to the end of the tailpipe will heavily affect the running of the engine. If the plate is very close, even slight reflection of the pulse wave back into the pipe will keep the jet from continuous operation. The 'bucket' shown here is designed to prevent that, with the idea being absolute capture of the entire exhaust stream. It is, in effect, a 'thrust reverser' that splits the blast wave and mass and redirects them forward around the outside of the tailpipe, thus preventing the pressure wave from starting back forward through the pipe. By doing this, it can be used very close in, as shown here, totally avoiding spillover losses and almost eliminating shear losses in the air behind the pipe. The width of the bucket should be at least twice the tailpipe exit diameter of the largest engine you want to test.

This should be usable swinging in a vertical plane [as shown] or in a horizontal plane with the hinge axis vertical. All the hardware is perfectly ordinary material. You can obtain a scale with a range of up to 50 lb with a dial face over 2 inches diameter at any good sporting goods store; simply check/calibrate it using some known weight. The mutiplier positions shown allow x2, x5 and x10 force multiples, so even very small thrust values could be accurately 'weighed' with the device.

Comments?

L Cottrill

Anthony · Post by **Anthony** » Wed Jun 23, 2004 10:02 pm

I'll tell you you did a good job when I'll understand how it works (shocked!).

As usually, the drawings are amazing hehe.

How do you calibrate it?

Bruno Ogorelec · Post by **Bruno Ogorelec** » Sat Jun 26, 2004 5:42 pm

Interesting. I would be afraid of the 'bucket' interfering with rear-end 'breathing'. Wouldn't thrust be different for a jet that reingests its exhaust gas in the suction part of the cycle, as compared to sucking in fresh air? There's considerable difference in density.

larry cottrill · Post by **larry cottrill** » Sat Jun 26, 2004 5:59 pm

brunoogorelec wrote:Interesting. I would be afraid of the 'bucket' interfering with rear-end 'breathing'. Wouldn't thrust be different for a jet that reingests its exhaust gas in the suction part of the cycle, as compared to sucking in fresh air? There's considerable difference in density.

Bruno -

There could be an effect like that, to some degree, if the bucket is very close. Perhaps I have the tailpipe positioned too close in the drawing. However, I think it's easy to make too much of the potential effect you're talking about. Even a jet that does breathe some of the ejected gas back in is also going to take in a lot from the sides, all around. The inward flow will not be the narrow 'shotgun' pattern of the high-speed outward flow, but will tend to come in from every direction; one reason for the tailpipe flare on the Dynajet, for example. But, it's a problem worth thinking about, and worth testing somehow.

L Cottrill