estradajae

Hi everyone....

When I was 15 or something I made a wood prop for an old 25 os engine.... ended up very symetrical and balanced...but no idea of the pitch... The plane didn't flight..that was the time when all the planes I made didn't flight at all and were destroyed at the first "flight" because they were very very heavy...

I think I can work with wood good, and I was wondering if I could make some big props....I recently bought a 25cc engine that needs something between 16x8 to 16x6.. I ordered this two menz props, which I'm planning to "reproduce".... but it would be interesting having a little bit more of knowledge about how can I calculate/estimate the pitch...and the considerations.... does anyone knows of a cad program?

Any info, experiences??

I've googled some, but it comes mainly to real scale planes...and they start to work with CNC and so...so there it is pointless to me.

Is there a forum here in this matter?? I haven't found it.

Saludos,

Jorge

pe reivers

you need to saw the proper angles in the wood block at the proper diameters, then plane the blade thrust side to the bottom of the sawn grooves.

The proper angle formula is:
 tan(alpha)*Pi*D=pitch (use all same units, and you will be fine)

You can see that with a fixed pitch, alpha (the angle) will decrease if D (diameter) is farther out to the prop tip

estradajae

Thanks Pe

I did a little more research and found the same thing....My question goes also on the fact that most of the props I've seen looks like the pitch don't change linearly from hub to tip... the trailing edges don't follow a straight line.... But I guess I'm over slippery ice there...that may be some of the secrets of the propeller constructors, and are product of intensive aerodynamic simulations??

Saludos,

Jorge

pe reivers

shape to your liking, once you have the angles planed in and smoothed. You will se the trailing edge get it's shape along the way.
Take care to use at least 15% foil thickness with wood. More as you near the hub, and have the thickest point of the airfoil not forward of 1/3rd the blade width. (bad pull, noisy).
You also can experiment with different pitches toward the tip.
Example, near root have 6" pitch, increase to 8" toward 2/3rd diameter, then flatten out to 7" at the tips. Good for 3D

estradajae

Thanks Pe,

That is nice piece of info...

Saludos,

Jorge

blw

My Feedback: (3)

Pitch may still be determined at 75% of the span, from the hub.

Lnewqban

I would say it is a risky businnes.

A propeller is basically a twisted wing, but it is buit to work under big stresses, mainly centrifugal and bending forces.

The section closer to the hub carry the maximum efforts.

That is one of the reasons why big wood propellers are laminated, or plastic ones are reinforced with some fiber.

estradajae

I don't think is that risky as you think....

Laminated props are really big props.... and there are also big props made of a single piece of wood...
I'm talking about props in the 10-18" diameter range.... If wood wasn't a suitable material for making props, there wont be any there...

Off course wood choice is a determining factor here...

TFF

Make gage blocks or templates of every 2 -3cm of the blades pitch and shape, so you can compare each side. Make sure the grain of the wood is straight through the hub area. I love hand made props.

pe reivers

I have made balsa props as my first ones for rubber motors to practice.
Then of hardwood. The tropical woods are too brittle and splinter because most have quite open pores.
Excellent woods are:
Red beech (brown) . It must be dense. avoid the low density stuff
Rock maple (brownish), straight grained. Avoid the twisted grains (bird's eye etc.)
Lemonwood (yellow)if you can get it. Check for cross-grain. That is not good for props.
Birch (whitish)
Robinia (AKA Locust) (light brown)
The brown and white/yellow makes a fine combination for laminated props.

Best is to split the wood with an axe before planing. That will show what you have got.

Jim Thomerson

There have been a number of articles on how to carve props in the old model airplane literature. Apparently it is a lost bit of craftsmanship. When I was in highschool @ 1953, I carved a 14 x 6 prop for an Ohlsson 60. I had a piece of white, straight grained wood off a packing crate. Don't know what happened to that prop.

estradajae

As soon as I finish my Thesis here for the University, I'm going to carve some props....in the mean time who wants to carve some wood and show some results??? after all I think it is lots of fun... who knows...maybe one of us come with super props that outperform some out there , especially on the price...

Saludos,

Jorge

Kmot

My Feedback: (24)

Check this out:

http://books.google.com/books?id=Jnd...age&q=&f=false

pe reivers

Also check out http://www.mh-aerotools.de/airfoils/javaprop.htm if you want to experiment with airfoils and different AoA and Re-numbers.
Martin Hepperle made it all too easy for us with this Java applet.

em14

KNOT - That was a great piece of info and from a long time ago ... thanks for posting it!
Leo

estradajae

For some reason I can't see the content of the book on google books...I think there are copyright issues here with this book

PĂ© looks interesting the page you suggested.
I just would like to be able to understand how to get interpret the polar curves....I studied electronics engineering...and no Idea how to get the useful infos from the polars.... I have the same problem when choosing the airfoil for my planes =(.

Has anyone picked up already a piece of wood and made a prop after this thread??

saludos,

Jorge

pe reivers

He who perseveres will succeed.