prop clipping /amp draw

[DeaninMilwaukee]

I had a strange result and was looking for some feedback from you guys:

I tested amps running through a VOM, but checked rpm with VOM removed from the circuit and got 400 extra rpm from each prop.


On a GWS ips s1 motor, ( 4.14:1) running 10.8 volts on a stock 6x5 SF prop gave 1.9 amps and 7200 rpm.

I then tried a gws 7x6 SF cut down to 6x6 and got 2.65 amps and 6300 rpm.

I'm at a bit of a loss to explain why the extra 20% more pitch added so much more amps. Motocalc only expected about 12% more watts, but I actually saw about 40% more.

Any ideas here guys?



Dean in Milwaukee

[steve lewin]

2 things :

When you cut a prop down you change the pitch. Simplifying a bit, you have removed the end part with the relatively flat angle so your prop on average has a greater blade angle. SO higher pitch.

Also if you have just clipped it you will have drastically changed the shape of the prop. You've increased the blade width and you've cut off the nice sharp tapered tip that makes it efficient. So it will drag a lot more current.

OTOH I'm not a prop expert so these are just guesses based on experiments and a little (uneducated) thought.

Steve

[JMP_blackfoot]

Originally posted by steve lewin
you have removed the end part with the relatively flat angle so your prop on average has a greater blade angle. SO higher pitch.

Uh ??? greater average angle with smaller average radius = same pitch (if prop was helical to start with).
Think of it this way : 2*PI*radius*tan(blade angle)=Pitch

[DeaninMilwaukee]

I did'nt just clip it off, I made a template of the original tip shape and then slid it down 1/2 inch on each tip, traced it and cut it out.

This worked pretty good, the prop looks stock at a galnce, and is still in balance.

I did look at it closer this morning and noticed compared to the 6x5 prop, the blade chord is greater and further, the extra width seems to be all at the trailing edge where it curves into significantly higher pitch.

I have a 10x6 hd prop I'm going to try clipping down to 6x6 and see if I get a better result as the hd props seem to be lower on amps across the board.

Dean in Milwaukee

[BMatthews]

What did the prop SOUND like when you ran it? I've done a fair amount of prop testing myself and have found that the ones that draw way more current than you expect often sound different. This is because the pitch angle has gone past the magic operating point and the blades are stalled while running static tests. In flight with a bit of airspeed the blade's angle of attack will drop and the blade should unstall and operate at a much lower current. This is one of the big problems with static testing and why after a certain point you have to trust predictive programs like MotoCalc and proper in-flight testing.

As for your meter readings inducing a small loss this is normal. To sample the current the meter has a very low internal resistance that picks up a small amount of the voltage. The bigger the scale the lower this shunt resistance. However it IS still a resistance and that is seen as a small voltage drop. If you had two meters, one for current and one for voltage, you could measure the voltage drop across the amp meter. It works both ways. If you were to start using a voltmeter in very high impedance (sort of like resistance) circuits the voltmeter can introduce losses in the circuit. If you take electronics training there is a whole course in any program worth it's salt dedicated to measurement systems and when and how to use them properley and how to guage their effects on the circuits being measured.

For my own work I got around this by using a test supply that uses a voltage regulator with the amp meter before the regulation point. This way I can measure the current but the circuit regulates the voltage at the motor terminals rather than the output of the case. To do this I have the current leads and a second set of sensor wires that only carry the voltage at a few microamps to the regulator circuit. Thus I get around the problem of the amp meter in the circuit.

[DeaninMilwaukee]

Not much sound at all, pretty much like the stock 6x5 prop. I am trying to determine what wire size and length to make a 0 to 10 amp shunt, then I will get some more accurate amp results.

I did get a flight test in on both 6x6's, the clipped 7x6 SF, and the clipped 10x6 hd.

Repeating the previous clipped sf tests, the clipped hd prop drew 0.1 amp less, but got 500 more rpms.

Flight testing: The clipped sf prop did'nt seem any faster than the stock 6x5, speed being the point of this excercise.

The clipped hd prop did seem faster, although it was'nt a huge difference. It did seem better at wind penetration however. Winds were 10-15 for the test.

I need a really calm day and my flying buddy flying his identical plane right next to mine to tell speed differences for sure, but I may be best off sticking with the stock 6x5 sf prop as amps are much lower.
I also need to do a better job of prop balancing the hd prop. Its got a noticable vibration.

We shall see.


Dean in Milwaukee