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Author Topic: Static thrust tester  (Read 603 times)
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flydean1
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« on: March 18, 2018, 10:47:21 PM »

Looking for ideas for testing static thrust on small gas engines and electric. 

Not remotely interested in complex electronics, strain gauges, etc.  Just something simple and workable easily made with basic modeling tooling and skills.
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wmazz
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« Reply #1 on: April 19, 2018, 01:17:52 AM »

Small gasoline or small nitro engines?


Bill M.
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flydean1
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« Reply #2 on: April 19, 2018, 11:40:00 AM »

2-Stroke model airplane engines.  .020-.049
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wmazz
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« Reply #3 on: April 19, 2018, 04:24:07 PM »

My bench and dyno testing experience is with 23cc to 160cc gasoline engines
converted to glow style ignition.

But I have seen a few types for your application.

In the 1998 Symposium Bob Stalick wrote a Paper "Thrust testing small internal combustion engines."

He used a sliding engine fixture that used this spring scale https://us.ohaus.com/en-US/SpringScales

2- short length, ball bearing drawer slides from Home Depot may be an improvement over Bob's
fixture.

The more common style I have seen is using a 5lbs postal scale. A simple 90' arm with equal lengths
(1 foot x 1 foot) would work. The vertical arm has a motor or engine mounted to it. Then you need
a pivot at the 90' corner that raises the 90' arm up to the level of the postal scale. The horizontal
arm is positioned in the forward direction and applies pressure to the scale.

It may be helpful to use a rubber bumper to filter out some vibration.

I will draw a pic later.


Bill M.
 
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Re: Static thrust tester
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flydean1
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« Reply #4 on: April 19, 2018, 05:43:33 PM »

Your latter idea is exactly what I envisioned.  Did remember a rolling static tester like the one you describe.  Seemed really complicated, and the scale and all associated stuff would be right in the prop blast unless a 90 deg pulley arrangement is used.

Really overcomplicates the issue.  Simple cabinet hinge and equal length arms should suffice, and doubt I will need a 5lb scale.  I expect an 02 to have around 4 oz and an 049 no more than 10 but I could be wrong.

Now for my Jett 67.......?
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wmazz
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« Reply #5 on: April 19, 2018, 07:35:26 PM »

Using a postal scale you could also build a reactionary torque style dyno. I have one
in my basement that was made from just plywood.

It would work well with electric motors and carburated nitro engines.

The main disadvantage with testing thrust or torque is changes in air density. On days
that have clear blue skies, your testing rpm will increase. When the air temp gets hot,
that will really mess with your readings!

In these conditions all engines loose HP, but the air density changes the load on the prop
and so most people don't notice the difference. But it does change the engine tuning.

The biggest advantage on hot days with clear blue skies is less drag on the air frame. If
you are flying pylon models, these conditions will increase top speed.


Bill M.


High pressure in the upper atmosphere creates low atmospheric pressure at lower
altitudes. In Riverside the elevation is ~1000 ft, but during the summer the density
altitude can easily reach 4500' to 5500'.
BM
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flydean1
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« Reply #6 on: April 21, 2018, 09:25:32 PM »

My testing will involve only relative differences done pretty well on the same day and shielded from winds. 

Test runs with different props on same engine.  Again, relative values are all that is required.  My torque meters I use for rubber flying have units, but not absolutely accurate.  My rubber is calibrated to the meter, not the other way around.

Well aware of Density Altitude.   I'm a Flight Instructor at all levels of aviation, and hold an Airline Transport Rating.
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