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Old 10-19-2008, 11:43 AM   #11
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Re: How to measure the static thrust HELP!!

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I maybe should have also explained what I usually do with my thrust measurements. I have a specific rpm that I go for on each engine after I do a couple of tests. When racing especially as some would know different engines will not give the high rpm but are real workhorse's lower down on the scale. I take several props of different pitch's and measure the static thrust and write it down in my little black book. I then test fly several of the prop's to see what there performance is. If the engine is bogging then I reduce the pitch or if screaming I raise the pitch. Now since I am not racing will experiment with a few props to get what I feel is comfortable with out having it over rev. Now the question is how do you know what your engine is doing while up there. You can go by sound and what it looks like or like me when I take my time is an old Tele Tach that I had mounted in a test bed plane. My daughter read the readout while I was flying as it was not very safe trying to read and fly at the same time. It was all part of the game to go faster. Now I do it for sport and just want it to run smooth with good performance.
John Davidson
Keep the shiny side up and the wheels down
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Old 10-19-2008, 02:15 PM   #12
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Re: How to measure the static thrust HELP!!

This doesnt have to be too confusing...

Yes, in general, a larger diameter, lower pitched prop will produce more thrust VS a smaller diameter, higher pitched prop - and likewise, once airborne the same props would see the smaller, coarser pitched prop haveing a higher top speed. However, this is also VERY much dependant upon the drag of the airframe. Hence, why I stated that static thrust is more usefull for selecting a prop for 3D and prop-hanging, where the flight characteristics of the plane are not really part of the picture.

Say you wanted to hover your 5 lb fun fly plane with a .46 engine - you will see a higher thrust with something like an 11x4 or 12.25x3.75 prop than with a 9x8. In that situation, its very usefull.

But in general, once you get a prop that can fly the plane, you can start fooling with various pitches and diameters untill you get the best of what you want (take offs, acceleration and deacceleration, verticle down line braking, ability to pull through maneuvers such as snaps and over the tops of humpty's and stall turns, etc.)

The highest static thrust is not always the correct choice, but it may be depending upon what you want from your airplane.

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Prop max RPM Idle max CHT max EGT max static thrust

APC 13x8 11,400 2,200 374 F 494 F 3.6 Kg / 7.92 lbs
APC 14x4 W 11,500 2,200 394 F 474 F 4.1 Kg / 9.02 lbs
APC 14x5 N 12,400 2,200 370 F 521 F 3.9 Kg / 8.58 lbs
APC 14x6 10,600 2,150 387 F 581 F 4.5 Kg / 9.90 lbs
APC 14x8 10,000 2,000 349 F 444 F 4.2 Kg / 9.24 lbs
APC 15x4 W 10,700 2,000 368 F 521 F 4.7 Kg / 10.34 lbs
APC 15x6 10,000 2,000 321 F 564 F 4.5 Kg / 9.90 lbs
APC 16x6 9,300 2,000 432 F 426 F 4.9 Kg / 10.78 lbs

This is a table of values I just took for a 4 stroke engine - in this case, choosing the prop based on both the engine rpm (what max rpm that the engine is supposed to get on the ground) and the measured thrust can be helpfull in selecting a prop for a particular airframe. Now, again, this is just one thing to help you... but its both interesting, and can also tell you things like the 14x6 is giving the same thrust as the 15x6, but with the 14x6 prop you may exceed the recommended running rpm in the air when the prop unloads. So, I would start with the 15x6 and go from there, rather than the 14x6. Its just a tool to help you select props.

I think we may have gone further than the OP had asked to go, but hey, its all interesting stuff

Andrew Coholic -MAAC #26287L

1/2A to giant scale, IMAC, SAM, R/C sport, turbine jets, Heli's...
if its got a wing or two and an engine - I like it!
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