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Author Topic: Prop Development  (Read 363 times)

Online Motorman

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Prop Development
« on: September 11, 2017, 12:39:27 PM »
I need an E prop for a full size stunt ship I'll be building this winter so here it goes. I found the blade area distribution I wanted on the Top Flight power point series. These props are very inconsistent in air foil and pitch but sometimes you can find two props you can work with as I did on an 11x5 pattern I made.

In this case the hobby shop didn't have many 12x6 props to choose from so I decided to work one blade and copy it then make the pattern with composite blades. Probably can't see it in the pic but I added a piece to the tip to make it 13" dia.

From this mold I can make a prop with any number of blades from 11" to 13" dia. and re-pitched for any application. I like building versatility into my prop designs and molding process. I originally wanted to make it just thick enough to be used on glow engines as well as electric but it came out on the thin side because, I sanded the Phillips entry out of it to make a slight under camber so, we'll see.

Next step will be to notch the hubs and glue them together.

Motorman
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Offline Paul Smith

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Re: Prop Development
« Reply #1 on: September 11, 2017, 12:53:43 PM »
It seems like, in the world of stunt anyway, electric motors are producing the same power as piston engines.  So why would they need different props?

A few years ago, electric motors were weak, so they could use light, thin props.  But times have changed.  Can they still get by with weaker props?
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Offline Tim Wescott

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Re: Prop Development
« Reply #2 on: September 11, 2017, 01:27:44 PM »
It seems like, in the world of stunt anyway, electric motors are producing the same power as piston engines.  So why would they need different props?

A few years ago, electric motors were weak, so they could use light, thin props.  But times have changed.  Can they still get by with weaker props?

Less vibration.  A 2-stroke engine generates one honking' big torque pulse per revolution -- a 4-stroke (and probably a 2-stroke running in 4-stroke) produces a honkin' bigger torque pulse once every two revolutions.  Depending on the construction of an electric motor it'll generate torque somewhere between utterly smooth and a whole lot of pulses (probably several dozen, but I can't work it out in my head) per revolution, each pulse being proportionally smaller than from a 2-stroke.  And, a single-piston engine will always and ever be imbalanced.  I'm not sure how that reflects to strain on the prop, though.

Igor Burger makes CF props that have moderately thick blades and some sort of core -- I'm not sure if it's balsa or foam, though.  He dis-recommends them for IC.

Someone, whose name I can't remember, was using big quadcopter rotors for props on his electric stunter -- they were fine in the air, but much less robust than regular APC electric props when they suffered a ground strike.  He lost a blade in practice for the 2014 WC (2012? someone help me out with names and dates please).  That created a honkin' big imbalance that ripped the nose of his airplane off.  He got a loaner plane, but finished well into the middle of the pack.
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Online Motorman

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Re: Prop Development
« Reply #3 on: September 11, 2017, 05:06:43 PM »
Also, piston engines turn more rpm which makes for flutter at the tips if the prop is too thin.
There will be a sunny day and we will fly our airplanes.

Offline Steve Helmick

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Re: Prop Development
« Reply #4 on: September 13, 2017, 08:56:56 PM »
The main problem/thing with electronic stunters is battery capacity vs. weight. If you can use a prop that turns easier (i.e., thinner, less undercamber, low drag tips, etc), you might be able to downsize your battery and save some weight. I reckon this would allow a fella to order a new set of batteries of a different configuration. It must be interesting, because some folks really dig it.  S?P Steve
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Offline Traian Dorin Morosanu

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Re: Prop Development
« Reply #5 on: September 14, 2017, 06:29:36 AM »
First of all is good to have some baseline to start from.
Yes most props have a wild pitch control in their finished products.
If you want to make a good prop then a good mold is in order. Now if you want a good accurate mold there in only one way to do it and that is CAD CAM. Moreover if you want a good prop then you get in the CAD CFD analysis to fine tune airfoil prop shape etc. Only then you will produce a stellar prop and even then that will be good for only one diameter (small range).
If you have good molds then you can play with things like layout etc to get max out of the prop.
For Electric you want to keep the blade center of mass as close to the hub as possible (that means hollow molded or solid core) for IC that is counter productive and weak so fill all the prop with CF 100% and you are good.

Another way to get the design in the ball park is to make a bunch of props out of hard wood install them and test them for pull. For this an electric setup with feedback for RPM and power used is important. You can set an RPM say 10000 (typical what we use give or take) and once it comes up to the correct RPM record the pull and amperage used. Obviously the goal is to get most pull for least Amps for a specific pitch.

Look around and see generally what diameter what pitch and what RPM are used usually.

There is a lot of work ahead let me know if I can help with something.

By the way I am also working on a prop design for electric in the 13in range.

And by the way I used to make the Gator Props for IC motors. I still have the mold for a 2 blade prop. I might use it to make some E-Props from it.

Good luck
Each goal you meet is a moment of happiness
Happiness is the harmony between what you think and what you do. Mahatma Gandhi


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