Got tried of guessing which prop worked best and what RPM so I built this test stand and really was surprised how a prop turning 800 RPM less than another could put out 3 lbs more thrust

This is not built for NSSA but does what I want it to do.

Pro Zinger 20-10 @ 7100 RPM put out 17 lbs thrust
Troy built model 22-8 @ 5700 rpm put out 20 lbs thrust.
Pro Zinger 22-8 (no rpm reading) 18.5 lbs thrust

This on a 50cc motor with zero break in time

Milton

Milton,

A 22X8 Pt models Cf prop (TBM sells them) wil give you around 6800 rpm and about 25lbs of thrust depending on the brand of engine.

Nice stand!
As long as you have a "standard" prop -you can try different mods to the engine and you will then see what power % increases are occuring
changing props proves ZERO about the engine - unless you know what the hp is .
for example :
on testing electric motors -we look at thrust (as you do) but also the watts required for that reading .
So if I change props and see more thrust -I will always see more watts being consumed.
Irrespective of the prop used
If you change props and see more thrust -at the same rpm - you will obviously be getting more power from the engine by simply changing the prop.
How can that be?
Prop efficiency at zero airspeed is the only answer .
Where the thrust stand will really be of value -is in comparing the engine mods -with standard loads you use (props you have that have a known static thrust at a certain rpm)
Or - simply use it to tune a given setup -
different props which provide different thrust at different rpms will tend to confuse which does what except if you do all your flying in a fixed hover.
anyhow - nice setup -it can provide lots of useful info.

Been tinkering with gas engines for about ten years and still don’t know anything for sure, You are right about different props giving more thrust with same rpm,

Trouble is the props are not cheap, Spent over $200.00 last week alone them.

Also been working with electrics for about three years and there is were you can get confused, Amp draw, Voltage under a load, Watt in watts out. And on and on. No wonder people get confused when they have never set one up.


Milton

kewl i know what u mean about props i do one of two ways buy a bunch and try them or ask RCU whats working best and run that one

yep -just plain ol cut n try works best .
on static tests-you can get lead down the primrose path if all you look at is static thrust - or rpm
More rpm means --less load
More thrust means -more static thrust.
In flight the whole picture can change drastically .
Bottom line -you have to use props which allow best engine output and by simply changing mufflers (sorry bout that ) you can make a huge change in results
If the same prop turns faster -- you have increased torque and that is all good stuff. That will show up on the static test as more thrust.
Change the prop - you change the test - apples and oranges

Is the distance from the eyebolt to the pivot the exact same as the distance from the pivot to the block that pushes on the sensor/scale?

AJC

Is the distance from the eyebolt to the pivot the exact same as the distance from the pivot to the block that pushes on the sensor/scale?


AJC
[/quote]

Yes
Milton