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Variation in thrust and propeller pitch

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Gareth Jones18/09/2010 20:14:24
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I have been experimenting with Snogg, the PT boat featured in the September edition of model boats.   I decided to try fitting a Speed 700 motor to replace one of the Speed 600's and this showed a significant improvement in speed and ran much cooler so I purchased a second Speed 700.  I retained the original 30 mm propellers but increased the battery voltage from 7.2 to 9.6 volts.  I measured the speed controller input volts, amps and watts using a Wattmeter, the prop speed using a laser tacho and the propeller thrust using a set of digital fishermans scales. (all very nerdy but I like to be methodical!)
 
The interesting point that came from these tests was that I had much more thrust from the left propeller than from the right.  Both were doing near enough the same speed, LH 8800 rpm, RH 9000 rpm  with similar input power, LH153 watts, RH 142 watts at full throttle.  However the LH propeller gave 900 grammes of thrust, the RH only 550 grammes.  I tried an alternative propeller on the RH side and it gave about the same thrust as before, around 580 grammes.  I then put the propeller from the LH side on the RHS, reversed the motor direction of rotation and got around 850 grammes of thrust.
 
The only logical explanation is that the pitch of the propellers is different so I decided to try and measure the pitch.  The photo below shows the original LH and RH propellers.

 I fitted a penny washer over each one and used that to try and measure the height at the tip of each blade

The photo below shows the best of my propellers, the one that was originally on the LH side.  While it was slightly more evenly pitched than the others, it still varied quite a lot and on average seemed to have less pitch than the RH blades.  I plan to try and set one of the RH propellers to a similar but even setting to see the effect.
 

 
 
Has anyone noticed anything similar or got any ideas, I assume all three blades should be symmetrical or is the design more complicated than that?
 
 
 
 
 

 
Colin Bishop18/09/2010 23:16:41
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They should be the same and the blades symmetrical. Looks like a bit of a quality control issue to me. The blades would appear not to have been soldered in evenly.
 
Correct alignment of the blades is something you would normally take for granted, I've never thought to check it before myself - some food for thought there. I assumed that props would normally be assembled on a jig which would keep everything in the correct place.
 
Congratulations on the Snogg article Gareth, I found it very interesting.
 
Colin
Gareth Jones19/09/2010 10:52:02
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I thought about this a bit more last night and I dont think that the pitch of the blades is that much different.    I think that when the propellers have been fitted or removed the blades have been bent along the line of their roots which is the weakest point when you are holding the propeller and tightening up the lock nut.  It would be very difficult to twist the blades and affect the pitch.  Consequently its perhaps the efficiency of the propeller thats been affected somehow.  I will stick to my original plan and try and set one of the RH side propellers (Left hand pitch I think, always get confused over this), so that all the blades are set at the same tip height.
I am thinking of writing 'Snogg the sequel' as the test results from various different motors (and propellers) has been quite interesting and enlightening. 
HS 9319/09/2010 20:37:22
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I have stoped using compasit props and only use prop shop ones , they are a bit more expensive but we did some tests with props not long ago and found they where the most consistant and gave the best performance/runtime of all we tried.
some of the props tested

 
 
Peter
 
 

Edited By HS 93 on 19/09/2010 20:46:14

Gareth Jones25/09/2010 17:33:47
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I have now done some more work on Snogg's propellers and here are the results.  Firstly I measured each blade tip height as shown below using the 'penny washer'.  For my best propeller, which was originally the LH side one, I took an average of all three blades, which came out as 10.5 mm.
 
Then I carefully bent the blades on one of my RH propellers to match, setting all three blades as symmetrically as possible.  I also made a minor adjustment to the LH propeller to match the blades to the 10.5 mm setting.  Note, in bending the blades I dont think I am changing the pitch very much, just evening up the circle which is swept by each blade.
To avoid bending the blades again when I refit or remove them I made a tool from a couple of scrap ply disks, three small bolts and some heat shrink as below.  The tool can be held in my fingers with the bolts up against the edge of the blades, reducing the chance of bending them.


I then repeated the performance measurements as before.  The figures below show the way in which a fishermans digital scale is used to measure thrust and the Wattmeter measures the power into the speed controller.


I ran the tests twice, firstly with the propellers in their original position.  Secondly, with the props swapped over side to side and the motor wiring swapped to provide the correct rotation.
 
Just to remind you originally before matching the blade heights I had:-
 
LHS motor 8800 rpm, thrust 900 grm   RHS motor 9000 rpm, thrust 550 grm

After resetting the blades

LHS motor 8700 rpm, thrust 930 grm   RHS motor 9400 rpm, thrust 740 grm
 
After swapping props side to side
 
LHS motor 9350 rpm, thrust 870 grm   RHS motor 9300 rpm, thrust 1065 grm
 
My conclusions are that matching up the blade heights has improved the performance of the propeller.  The penny washer method seems quite effective as a way of measuring the differences between blades.
I think I probably bent the blades myself originally when fitting or removing them and the tool should help prevent this happening again in future.
There is still a significant difference between the two propellers, even after they have been matched and swapping them from side to side shows the effect persists so its not the motors or prop shafts.
Overall changing from a 7.2 volt Speed 600 motor to a 9.6 volt speed 700 has roughly doubled the thrust and the motors run much cooler and more efficiently.  The current at max power has increased from about 17 amps to about 20 amps so the original speed controllers are OK.
The propellers are now fitted so that the RH side rotates anticlockwise, when looking from the back, LHS rotates clockwise.  I think this is the opposite of what is normally recommended but since it seems to give the maximum total thrust they can stay where they are. 
Its been an interesting few weeks.  Maybe if I can find some good looking 30 mm propellers at the Blackpool Show I might have a go at repeating the tests to see if they are better matched or more efficient.
 

 

 

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