Measuring static thrust

[Air-Gar]

What is the accepted method for measuring static thrust in a model airplane? Or is there an accepted method? What information is available on this subject?

[Andy W]

Lots. Search the Power Systems and Electric Talk forums.
Unless you're building a 3d model, or something else designed to hover, in my opinion, it's a useless parameter to worry about measuring..
..a

[natoquick]

What I've done is take an old model and attach a spring scale (like for fishing or something) to the tail end of the aircraft. Run up the engine, hold onto the scale, and read what it says. A very simple way to measure the thrust.

[peterangus]

Air-Gar

What is your interest? Hovering or climb-cruise?

If the latter, then as Andy W says, static thrust is of little value. Pitch speed is much more important.

If hovering is your thing, then natoquick's suggestion is fine. Be aware that this measures not pure thrust, but thrust minus drag, which is what you require. A bluff bulky fuselage will affect the reading more than a slim streamlined one would. [I'm assuming engine-on-the-nose]

Information? For every ounce of information there's a ton of dis-information.

[Air-Gar]

Thanks peterangus. I am interested in thrust for hovering. I'd like to test some different types of props to see which one(s) produce more thrust at a given RPM in an electric power system. I believe I have a pretty good method - hanging my plane by the tail to a digital fish scale, then taring the scale and running up the throttle. This seems to give consistent, repeatable results.

[BMatthews]

...I believe I have a pretty good method - hanging my plane by the tail to a digital fish scale, then taring the scale and running up the throttle. This seems to give consistent, repeatable results.


It's a fine method as long as the torque does not jam the scale somehow.

[Andy W]

The calculation results from any of the multitude of programs out there are close enough - certainly as accurate as any home built thrust measurement system. You need to know two things - will it fly safely, and will it overload any of the components. If the calc programs say you're OK, go fly, and if needed, tune the setup from there, keeping an eye on static currents and component temeratures as you go.
..a

[Air-Gar]

Andy,
Can you list the best two or three calculation programs that you are aware of. I'd like to check them out against each other and against my measurement method.
Air-Gar

[Andy W]

Electricalc, Motocalc, PCalc. For identical parameters, they all put out different results, and none will match reality! :)
..a

[Tim Green]

Air-Gar, your method's sound for measuring thrust (minus force made by propwash hitting fuse and that part of the main wing nearest the fuse).

I've heard that highly pitched props can just thrash air during static testing, not showing the thrust they'd show when the plane was moving. So watch out for that effect.

[adam_one]

I guess that the prop pitch speed refers to propeller's rpm times the pitch.
However, the static rpm that we easily can measure with a tachometer is lower than when the model is flying…
Is there any reliable method to measure the actual pitch speed?
Thanx.

[peterangus]

adam one

If the model is hovering, then the conditions are identical to those on static test.

For wingborne flying, calculation of actual pitch speed would require an in-flight rpm measurement.

[adam_one]

If the model is hovering, then the conditions are identical to those on static test.
Yes, I understand that, so the pitch speed is more relevant when the plane is not hovering.

I've recently read an article in the last issue of Fly RC Magazine where they show a nomogram to predict the model's speed in level flight.
It is based on the following formula:

static rpm x nominal pitch x 0.000947 x 1.20 = speed (mph)

They use a so-called "corrective factor" of 20% to adjust the static rpm and nominal pitch to level flight values.
They also state that the nomogram is not 100% accurate but close enough for the intended purposes.
I just wonder how close it is... any opinions? :)