Electric props super sensitive to drilling accuracy

[Percy Attfield]

This week I used the APC 12X6.5 Wide Blade Pusher prop known as the Dennis Adamisin prop here in South Africa and liked it so much I got a second one from Keith Renecle so that I would have a back up if something happens to it.

When I first used it a month ago I had reservations about it because it was noisy, did not really work very well and it is heavy. I also read on Stunt Hanger that there were those that found it noisy so I thought that was just normal for the prop.
 
However Keith Renecle kept on saying it changed his plane from V6 power to V8 power so I started checking the prop carefully and came to the conclusion that I did not drill it accurately when I enlarged the mounting hole to 8 mm – the size of my motor shaft even though I used a bench press drill.

I took it to an engineer friend of mine, he measured it carefully, clamped it and drilled it out so that the new prop shaft hole was centred and bushed it back to 8 mm.  He also shaved the prop boss where it seats against he thrust washer, just to make sure there are no moulding inaccuracies on the seat.

The first time I flew the newly drilled prop I could not believe the result. It was quiet, it sang like a sewing machine and just worked very well and the extra prop weight had no influence on the planes performance.  A real pleasure to fly with it.

I wont drill the second prop, I will take it to the engineer again; I don't want to risk that it may not be straight again.

Those who have reservations regarding the prop or of the noise element should check if they have the same problem as I; a problem with drilling the hole, centred and straight.  I am convinced that this is a contributing factor to the prop noise that is being noticed.

It seems to me that it was good enough to get a prop close to right on an engine; you did not notice the problem unless it was severely out of alignment. For good performance on the electric motor it must be perfect.

Percy Attfield

[William DeMauro]

If you read the notice http://www.apcprop.com/v/html/tech_support_notice.html#holealign on the apc website that will tell you not to go by the holes in the front of the props but to oversize those and use the alignment rings in the back. the back of the prop is also the side to use for balancing.

[Howard Rush]

Percy, there may be a language problem here.  You seem to be writing in English.  By the American definition of engineer, an engineer is not necessarily a person to whom one would take a prop to get the hole drilled straight.  For example, one would not want me or--heaven forbid--my wife drilling his prop.  

[Percy Attfield]

Thanks Howard.

This Engineer is a person that works with lathes, routing machines, that measures with Verniers and other such exact measuring devices. etc.  A hands on person, what would you call that person in American?
Percy

[John Cralley]

Percy,

In America we would call him a machinist or a tool and die maker. 

[Dennis Adamisin]

Percy,

In America we would call him a machinist or a tool and die maker. 

Actually I would call him "Dad".    


Percy, do you know which way the prop was out of alignment - was it along the length of the prop, across the width or some other direction?  was the back of the prop out of square or did your machinist face it off just to be certain that it WAS true?


The 12x6.5 is the heaviest (41g) of the APC pushers we have been using, certainly even a little miss-alignment would make itself heard pretty darned fast.

 

[Jim Fruit]

Thanks Howard.

This Engineer is a person that works with lathes, routing machines, that measures with Verniers and other such exact measuring devices. etc.  A hands on person, what would you call that person in American?
Percy

Percy:

In American, we would call him a South African.

Jim Fruit

[Percy Attfield]

Thanks for all the help with a label for the drilling person.  I will go with machinist.  On reflection I have a son that is a Physics Engineer and I would also not ask him to drill the prop.

Dennis the prop hole was off set in the length mostly and a little towards the one side. When I say mostly it was less than a millimter out. I think the shaving of the base just confirmed that the base was in fact true. I would say the hole was not straight, it started in the centre at the front and ended very little offset at the back so the arc was not true, that was probably the main problem.
Percy

[RandySmith]

I would always try to ream the prop and not drill one, 8mm reamers should not be hard to find. Drill bits wander off course all the time

Regards
Randy

[Andrew Borgogna]

Hey Percy
Got your email thanks, but I don't think I will be using carbon fiber props in electric.  Regarding your centering problem, my experience with APC props, and this goes back to when APC started making props, is the front hole cannot be trusted for accuracy.  The rear portion of the hole where the insert goes is centered and the inserts are accurately centered.  I don't drill any of the inserts to size I ream them with a tapered ream and so far I have not had any problems with off center vibration.  I remember in the early days of APC props flyer's complaining the props were drilled off center, but they were not using the inserts that come with the props and that was the problem.
Andy

[Larry Renger]

Fit the proper spacer to the prop, ream it if you must to fit the shaft, then put it on a balancer.  If you set the balance perfect, the aerodynamic unbalance is negligible.  The speed dudes and rubber Wakefield gurus both go with 1 blade props! That about covers the range of speeds and thrust we need to worry about.

Balance is the key and the Top Flite magnetic balancer will let you get it right in all 3 dimensions.  Used propely, (soory for the pun) the prop will stop in any position with no tendancy to move.

I regularly run engines in the 20K + region, so balancing IS an issue.  I just wiped out a crankcase by not re-balancing a prop.  (the crankshaft miked out perfect end to end, and fitted a new case beautifully).

 

[Dean Pappas]

Hi Gang,
Isn't the title of this thread amusing, though.
E-props are obviously no more sensitive to balance, there is just no imbalanced engine pounding away to mask any imperfections in prop balance.
Think how much longer the airframes will last ... unless of course we get lazy about prop and spinner balance!
The point is well made, though, that this leaves us open to actually hearing some of the resonances that an out of balance condition can excite.
These are often hidden by "normal" engine vibration, in which case you inexplicably end up with a short-life airframe. We've all seen this.
Oh yes, and serious aeromodelers and drag racers eventually train to be better Engineers! Nyaaah ...

Best Regards to All,
 Dean P.

[Dennis Adamisin]

Hi Gang,
Isn't the title of this thread amusing, though.
E-props are obviously no more sensitive to balance, there is just no imbalanced engine pounding away to mask any imperfections in prop balance.
Think how much longer the airframes will last ... unless of course we get lazy about prop and spinner balance!
The point is well made, though, that this leaves us open to actually hearing some of the resonances that an out of balance condition can excite.
These are often hidden by "normal" engine vibration, in which case you inexplicably end up with a short-life airframe. We've all seen this.
Oh yes, and serious aeromodelers and drag racers eventually train to be better Engineers! Nyaaah ...

Best Regards to All,
 Dean P.

...and note that Percy's (and others) observations were about NOISE, not vibration level - the imbalance manifests itself first in noise before it became noticable as a case of the "shakes".  A nice tight covering job turns the model into a boom box.  Perhaps on an airplane with a sheeted foam wing (audibly inert) it might not have even been noisy? (i.e., problem would have been masked)

The 12x6.5 Wide typically runs about 1000 RPM slower than the 13x5.5 - meaning the tip speed is around 18% lower (quieter)  However, the 12" is also 7g (20%) heavier than the 13" - any imbalance is going to manifest itself earlier & larger.  That leads back to DesignMan's comments about balancing the prop - this is a root cause.  I am a slacker about balancing props, but I see now I'm gonna have to adopt a new philosophy...

Summary of this thread seems to be:
* When you drill out an APC, index off the hole in the back side of the prop.
* Use the APC centering sleeves
* If you think it is running noisy then look first to checking & fixing the prop balance
* Then try Plan B   

[Darkstar1]

Balance is the key and the Top Flite magnetic balancer will let you get it right in all 3 dimensions.  Used propely, (soory for the pun) the prop will stop in any position with no tendancy to move.



 

I also check the pitch to make sure its the same on both sides. I've have seen it at times to cause a resonance
problem if one blade has more or less pitch than the other.

[Larry Renger]

The backs of some APC props have a lot of "flash" making the rear surface concave.  I sand all this off until the back surface is flat.  Then I balance the prop.