[ken]

This is a spinoff of this thread:
http://www.hoverclub...n...c=1228&st=0

The deviations from the original thread actually started out almost immediately, but I think they started to seriously deviate around post 50 or so. I'm not looking at the thread, so nobody get offended if I picked out your post and it doesn't apply.

The discussion is now rapidly going toward a topic in which I have intense interest and not so much expertise. I need to find my modern propeller and duct design book and see if I can understand some more of it. My math reaches its elastic limit about half way through the book, and the ideas get blurry.

I've built some props, but they're all UH props from their template. My next prop was going to be something I worked up, I know enough from the book to calculate pitch and airfoil shape, but it would look pretty much exactly like a UH prop except for the pitch, which probably won't be on the UH template.

After that, I was thinking about swept blades, but they'd probably be out of wood at first.

The knobby/wavy props you guys are talking about now, they bring to mind the wavelength formula, c=lf (speed = wavelength * frequency). I can't make a lamda here, so you gotta live with an L.

I did some of the math, though, and I figured out that the likely prop noise frequencies have too large a wavelength for the ripples to cause destructive interference.

So the next thing that comes to mind is that the leading edge bumps cause high and low pressure ripples throughout the entire airfoil, but by the time the wave reaches something else (a duct support spar) it no longer hits as one big wave, but is spread out over several tiny events which cancel each other out.

[kach22i]

I think there are several ways to get something out of this. Follow a book or proven design, or go off the deep end and invent something. If you go off the deep end, you can use some kind of software for a steep price I assume. An alternative is to do some trial and error, which may result in you blowing up your oven or losing your head to an exploding blade coming off a hub.

I think you could also just look at a material and say what can I do with wood? What can I do with aluminum? What can I do with carbon fiber? What would happen if you combined more than one material?

Chris's cedar prop looked so delicate, crafted and artfully done (thick to thin) I had a hard time imagining it ripping though the air as actual mechanism. Looks can be quite deceiving though.





Nate's ripple curved blade fan was held up in delivery, I never got to see it. I don't know if it ever showed up.


I did take some pictures of the RevTec, hope they shed some light on the topic.

This post has been edited by kach22i: 29 June 2007 - 09:55 AM

[ken]

Has anyone found a prop whose leading edge is swept forward?

It seems to me that would help reduce FOD on ducts or maybe even the prop itself.

[proteus]

Ok, I just talked to a buddy of mine who was a machinists mate on a "boomer" (nuclear "strategic", not attack, submarine) in the early nineties. Here's what he told me:

Yes, the curved blades (flukes) were done precisely to reduce sound generation and foster a smoother transition of the water off the "surface" of the prop.

You need more blades to compensate for the narrower chord in order to maintain a given "thrust".

Use odd numbers of blades to eliminate any harmonics being generated.

The switch to curved blades dropped the sound generation to unheard of levels compared to the older "straight blade" props.

Now, the Navy is shrouding the props to reduce noise even more (imagine that!)

I wish I had the skill to carve a decent prop - I would love to make a five blade curved thruster. If nothing else, it would look so cool!

[H2O MAN]

Ok, I just talked to a buddy of mine who was a machinists mate on a "boomer" (nuclear "strategic", not attack, submarine) in the early nineties. Here's what he told me:

Yes, the curved blades (flukes) were done precisely to reduce sound generation and foster a smoother transition of the water off the "surface" of the prop.

You need more blades to compensate for the narrower chord in order to maintain a given "thrust".

Use odd numbers of blades to eliminate any harmonics being generated.

The switch to curved blades dropped the sound generation to unheard of levels compared to the older "straight blade" props.

Now, the Navy is shrouding the props to reduce noise even more (imagine that!)

I brought this up to Michael Murphy last week when I was looking over his RevTech.
The NAVY is using the new blades and shrouds on subs and torpedoes - very quiet.

[ken]

Doug,

You do have that skill. It doesn't take rocket science. Start by making a two-blade prop, they're the easiest. Once you start and get interested in it, it can become almost an obsession.

I've wanted to make a three-bladed one for some time now, but haven't ever gotten around to it. And then, my prop holder on the wall would need to be changed. Maybe I could just change the lift fan holder and use that?

I'm amazed at the number of people who think they can't make a prop, and they managed to get a working hovercraft. If you can make a hovercraft, you can make a prop. It might not be a good prop, but it'll work. You need to build a couple in order to get a good one usually, unless you're already a good woodworker and understand what's going on.

[proteus]

Ok, I don't have the time....

I would need to build two comaprable props - one two blade, specific pitch, and the "airscrew" prop in order to make a decent comparison. Both calculated to the same pitch and thrust amount. I think it would be more than just cutting a super wide blade to an airfoil shape and then cutting a curve out of it. The chord would have to follow the curve.....

[ken]

The final product, yes you would be talking about a lot of work. I somehow missed the curved part on the five bladed prop, but if you're really interested I think you could work your way up to it. I'm not "there" yet either, but one of these days I might get enough energy and time to make a curved blade prop.

[proteus]

Here is what you are going for. Less blades, of course.

Attached File(s)

•  propellor_full.jpg (18.9K)
  Number of downloads: 48

[ken]

Wait.

That has two layers on it?!?

[proteus]

It's a mockup. The real ones are classified. That's why whenever a sub is under repair, the first thing that happens is that a cover goes over the propeller - or it's removed completely.

[ken]

Interesting.

This link just showed up on another forum:

http://www.wood-carver.com/gemini.html

It would sure make a blade easier to duplicate for that 5-way arrangement you wanted, Doug.

[ThrustProp]

Interesting.

This link just showed up on another forum:

http://www.wood-carver.com/gemini.html

It would sure make a blade easier to duplicate for that 5-way arrangement you wanted, Doug.

That Gemini is for copying existing physical props or models. Rob Russell watched me carve his prop one day, and went home and used the Gemini to copy it. He said I made the original a LOT faster. I'm sure there's a lot of pro's to it, but Rob said he'd never bother to do that again.

[csorgatz]

Don Bender was telling me a similar story. A duplicator has it's uses, but appearantly prop-making isn't one of them...

That Gemini is for copying existing physical props or models. Rob Russell watched me carve his prop one day, and went home and used the Gemini to copy it. He said I made the original a LOT faster. I'm sure there's a lot of pro's to it, but Rob said he'd never bother to do that again.

[ken]

Interesting.

I was thinking that getting a consistent blade would be neat. I'm pretty fanatical about that on my props, for obvious reasons.

I guess you'd almost have to make a mold and use carbon or similar to make the blades, and put them out one at a time.