I would like to do some thrust testing of my engine's. Just to see what props do what. Does anyone have some ideas as to building a test stand.

Just fasten a digital fish scale to the tail of your plane and to an anchor point, start engine advance to full throttle, read scale measurement, its that easy.

LOL, it don't get much simpler than that! If you wanna see a really trick, nice setup, pick up a copy of RC Sportflyer magazine and look at Mike Hoffmeister's engine reviews (you can also find them on Youtube under RC Sportflyer, I believe). He has a ball bearing slide, digital scale, laptop telemetry setup that gives really nice data: rpm, thrust, engine temp, etc.

Good thrust measurements would need to blanket all propwash flow from the sliding unit and engine, so prop thrust as measured is not influenced by prop blast drag.
Lacking this, you will have to compensate for plane drag or measuring setup drag.

I can see where wheel drag will affect simple thrust measurements  I can also see the problems with setup drag.  Is it fair to conclude that any thrust measurements will be on the low side due to these factors?  Do you think it would be more than perhaps 10%?

I would think that changing thrust changes from different props or fuels would still provide a reasonably  accurate difference between thrust measurements.  Something you could make a decision on. 

e.g. If a 12x6 give 6 lbs of thrust using the above method and a 13x6 gives 7 lbs of thrust on the same airplane.  I cannot imagine an error so large that the 12x6 would actually give more thrust.  Am I missing something?

The work has already been done for you. See PE Reiver's prop-power calculator.

AV8TOR

Great spreadsheet.  I I want to do it on my airplane and  at the field.  KISS.

Yes, you indeed are missing something, which is the engine that provides the torque to turn the prop. If the engine cannot handle the larger prop, the smaller prop can, and probably will give you more static thrust. Disregarding the engine: if both props are at the same rpm, the larger prop (diameter OR pitch) will provide more static thrust. Small pitched props (less than 0.5D) tend to loose their static thrust advantage in flight very quickly. This is often explained like driving a car in first gear. This comparison is not valid though, due to very different basic principles.

Guy's I wasn't looking to publish any finding's,lol, just curious as to the minute difference in similar props. Xoar 18x8 versus Bold 18x8. Those two have given me the most noteable difference in flight as of late. The Xoar seem's to do better at lower rpm's and fall off at the upper, the bold seem's to be the opposite. This is just my interpretation of how it feel's in flight. All I am looking to see in testing is the difference in pull of the two prop's thru the rpm range of 4000-7000.

you will find yourself entering a very complicated realm.
Static pull is one of the conditions, where efficiency is zero because plane speed is zero.
In flight, things change a lot.
Many prop manufacturers do not state true pitch, so to get a comparison going, you need to determine the true pitch. You also need to know the zero lift angle of the airfoil used in the propeller, and compensate for that in the pitch number.
Then you will know at what airspeed and prop rpm the thrust has tapered off to zero. This is the second point where efficiency is zero, this time because thrust is zero. In between static and max speed. a lot of things can happen

PE,
I must say that is an amazing spreadsheet. That must have taken a lot of work to create. Great job. Maybe, after years of study I will be smart enough to absorb it all! I know I could never have created anything like that.

It all started in the early nineties. Am still working on it. It's such a great tool I could not advise my customers properly without it.

I unfortunately have lost the link I had to the most recent issue of Pe Reiver's Prop-Power Calculator, so I would appreciate anyone that has it to share it with me. Thanks.

Karol

This one? http://www.fmfk.dk/Prop-power-calculator.xls

I can't open it myself right now, but I saved it for reference. Good luck, hope this helps.

Sincerely, Richard

Thanks Richard, I was able to open your link but it is not the latest revision Pe had made, as one I mislaid the link to had many more brand of props listed and allowed 4 engine comparisons.

Karol

Try this? [h=3]Prop-Power calculator - MVVS - Docstoc.com[/h]
www.docstoc.com/docs/149361600/Prop-Power-calculator---MVVS

Richard, I got it, many thanks and a Happy Thanksgiving to you and yours when it comes.

Karol

Karol, you are very welcome, and thanks for the holiday greetings. I wish you and yours the same hope for a Happy Thanksgiving.

Richard