I have a , strain gage, hanging style scale I bought to weigh planes and the larger parts of planes. I am wondering how well I can measure prop Thrust by looping a line around the tail of a plane on a hard smooth surface and applying power to the motor. The current Alpha 40 ARF test plane takes too little force to measure when dragging it along the floor. I expect static thrust is not the same as flying but would give me some comparible data. Any comments?

I have a friend who uses a digital fish scale in the manner you describe. It does allow you to see variations in thrust with different props. The information is usable to a degree to estimate flight performance once you have some experience with the digital scale and airframe-engine-prop combination.

Thank you oldvet70 for the reply. I too am an old vet of 71 years. This is a Fish type scale. I don't need absolute values just what can be called planning numbers.

Hi!
Why???
Just prop the plane right and you will see what it will do in the air.
For all .40 size high winged trainers flying at sea level, powered by a .40 size glow engine, a 11x6 or 12x4 APC or RAM is the best prop to use. -Simple as that!

Thanks jaka for the comment. I have it as you stated. A 40 size Alpha with an 11/6 prop. I am a retired Engineer and enjoy the numbers on everything I do.

I fly with a club that has about the same system you are thinking about. They went through a testing period with different engines and props and have it posted on there web site for people to look at. Every once in a while it comes out and people start running new tests. It's fun and easy to do. I haven't bothered to use it myself but I think it's using a digital scale so the old mark one eye balls can see it easier.

It's a pretty much useless test except in a limited area such as 3D props where the model is just hanging on the prop. For most types of props where the pitch is about half of the diameter or higher in pitch, much of the prop blade is stalled when it is static.

HighPlains is right on the money. Prop pitch is comparable to the gear ratio in your car. You adjust the gearing in your car to get the desired RPM at a specific speed. When you need plenty of RPM at zero or very low speed (to get the car moving) you use low gear. At slow speed or needing to pull a moderately steep hill, use second gear. At cruising speed, use high gear for best performance on flat ground. Same engine, same car, just adjusting for a different condition. Fixed pitch prop selection is similar except that you make do with a single blade angle and take advantage of the fluid coupling to make up for lack of pitch change capability. You can select an optimum prop pitch for any speed within the operating envelope of your airplane. As an example, a prop that gives lots of static thrust is excellent for takeoff acceleration and for hovering, but won't be worth a darn in high speed flight.

In an airplane that is optimized for cruise, prop pitch and diameter are set in the following fashion:

Step 1. Determine the cruising speed and cruising RPM of your airplane based on the airframe's horsepower required and the engine power available. From the speed and RPM, determine the propellor pitch.

Step 2. Select a propellor diameter which loads the engine to the target RPM at the target Airspeed.


As you can see from this approach, it doesn't make any difference in pitch whether you are flying a 19 sized airplane or a 120 size, providing the airspeed and RPM are the same. You just have a bigger diameter prop on the 120 engine.



Hal Debolt had an interesting rule of thumb for prop pitch which works surprisingly well. This isn't strictly scientific, but it gets you close:

Prop pitch (in inches) equals Airspeed (in MPH) divided by RPM (in thousands).


Dick

Mainly, two conditions can effect your results when comparing static to flight thrust and those are a partially stalled prop(slipping) or it's loaded down(most common).
Here's the best freeware I've ever come across, if you have Excel.


mvvs.nl/prop-power-calculator.xls

I made a test stand with an engine mount on rails. The idea was to see how much weight the engine/prop could lift vertically. I did the tests with different .049 engines and different props.
I used a digital scale to weigh the total weight lifted. It was interesting and fun to see the results.
A good running Cox .049 was able to hold steady at 18 ozs of thrust, which I thought was pretty impressive.