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Old Jun 22, 2007, 09:50 AM
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Measuring Thrust

Is there an easy (and inexpensive) way to measure how much thrust my plane produces?
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Old Jun 22, 2007, 10:18 AM
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It's pretty easy to build your own thrust stand. I did mine in just a couple of hours.
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Old Jun 22, 2007, 12:46 PM
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Livermore, CA
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I just took a 4"piece of 2x4 wood and put a post (1/4" x 1/4") in it sticking up.
I tape or zip tie a motor on it and tape the block to the scale lid.
If its a pusher motor, I just zero the scale and hit the gas.
If its a puller/tractor motor, I push down on the scale to add some weight, and zero the scale, the let off the scale, so the scale shows negative, then hit the gas.
Easy and cheap! Butch
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Old Jun 22, 2007, 01:41 PM
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Will either of these two things work for an entire plane rather than just an engine? I'd rather not remove the engine from the plane.

I'm assuming that you have to put the fulcrum exactly in the center or you'll create positive or negative mechanical advantage, true?
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Old Jun 22, 2007, 05:39 PM
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neerim east victoria australia
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Maybe im wrong but all I do is teather the plane to a fish scale tied to a support with a block of wood to support the scale horizontaly give full throttle and take a reading maybe not very accurate but I can try diff props to see what diff they make
Russ
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Old Jun 24, 2007, 04:09 PM
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Quote:
Originally Posted by jpwkeeper
Is there an easy (and inexpensive) way to measure how much thrust my plane produces?
Hi jpw,

Since thrust is a function of prop characteristics and speed, all you really need is an accurate rpm meter and info about your prop to determine thrust.

Contact flieslikeabeagl, a member here on RCGroups, and he'll point you in the right direction.

Here's his web site, along with a great and free prop/motor/battery calculator.

http://flbeagle.rchomepage.com/

Chuck
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Old Jun 25, 2007, 04:32 AM
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Somehow I think it's probably a lot cheaper easier(And probably more accurate) to read a fish scale than it is to try and buy a tach then try and find the right specs for your prop.
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Old Jun 25, 2007, 07:55 AM
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Measuring thrust is a very tricky thing, (well actually it's dead easy), but getting anything useful from it is the tricky part.

Say you want 'X' ounces of thrust. Well you could get it from say a big diameter and fine pitch or a small diameter and coarse pitch, but the plane will fly totally differently with either. Also the coarse pitch prop will 'unwind' better once it's moving, so static thrust is as I said tricky.

I once fell into the trap of wanting more trust, (measured by how much it stretched a rubber band, no ammeter, no tacho). I increased trust considerably with a bigger prop, the only problem was the plane flew worse. The reason?, now not enough rpm to pitch to to get a good flying speed.

Measure thrust by all means, but also think about how you are going to use it and what for. Speed?, vertical climbs?, 3D hovering?, fast take-offs?, each could use a different prop for the same thrust.
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Old Jun 25, 2007, 08:15 AM
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Quote:
Originally Posted by eflightray
Measuring thrust is a very tricky thing, (well actually it's dead easy), but getting anything useful from it is the tricky part.

Say you want 'X' ounces of thrust. Well you could get it from say a big diameter and fine pitch or a small diameter and coarse pitch, but the plane will fly totally differently with either. Also the coarse pitch prop will 'unwind' better once it's moving, so static thrust is as I said tricky.

I once fell into the trap of wanting more trust, (measured by how much it stretched a rubber band, no ammeter, no tacho). I increased trust considerably with a bigger prop, the only problem was the plane flew worse. The reason?, now not enough rpm to pitch to to get a good flying speed.

Measure thrust by all means, but also think about how you are going to use it and what for. Speed?, vertical climbs?, 3D hovering?, fast take-offs?, each could use a different prop for the same thrust.
Actually I don't want to change the thrust, I just want to know what it is. However, that's a really good point, so I'm going to ask you to expand on it and take the thread in a slightly different direction.

So what do you change to achieve each of the things you listed, and how does the type of plane affect each change?

For example, if I have a slow flyer flying and 8x4 prop, and I want more climbing ability, what prop change do I make?
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Old Jun 25, 2007, 01:58 PM
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For a fixed system, (motor and battery), and just changing the prop, the limit will be current driven. So any change to the prop must be done relative to your max safe system current. This also limits what sizes of prop you can use.

For the same current drawn, a finer pitch will give better acceleration and thrust but then runs out of speed, (limited by rpm). The coarse pitch gives speed, but takes time to get there, (it's sort of stalling while the model is stationary), it take time to 'come on the step'.

The pitch number is the theoretical distance the prop moves in one revolution.
The bigger diameter is going to limit the rpm, for a fixed motor and battery.

There is a formula for prop speed, (model speed), relative to pitch and rpm, best is to use something like MotoCalc, (costs) or Ezcalc (free, but not as clever).

Think helicopter and jet. A big dia. fine pitch 'prop' and has very good thrust at low rpm and is relatively slow. And a small dia. relatively coarse 'prop', (turbine), going like hell. Now the jet is fast, but not great at vertical climb from stationary, unless it has an incredible amount of power.

Same with the requirements for 3D and relatively slow model that needs lots of thrust, and a pylon racing model that needs speed. And a reason why the first group use gearboxes and the fast guys go direct drive.

Try Ezcalc, enter you motor, (or one similar if not listed), enter you pack, and your prop. Check the current, then try different props sizes and try to match the current. The results give you pitch speed, (model speed), and thrust.
This will give you a better idea of the limits than all the waffle I've written above.
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