Prop fans for homebuilts

I would like the benefit of your experience and wisdom concerning propellers. First I'll post a picture of what I want and then I'll explain why I want it, then you can shoot it down.

I want one of these, only smaller

And only one fan, not counter rotating. If I use two engines later then I'll consider counter rotating.

These are my ideas: the reasons that I like this is because I'm designing a pusher, which can always benefit from a smaller diameter, and because I want to see how well I can get away with direct drive.

So, for direct drive you want the prop to be the largest possible diameter for a given RPM, and my target RPM for direct drive is 4000. For this RPM a wood prop can only be like 54" and a standard metal prop can be almost 60" (according to an online calculator).

Swept back blades are required to get the largest diameter for a given RPM.

More considerations - most of the time I think I'll be using less than 200hp but I will be able to get well over 300 and maybe over 400hp for short bursts (later I'm aiming for around 500 - 600hp). With a small diameter this requires more blades but more blades are wasted with the lower power, but ... I'll need more anyway so I live with a little less efficiency at lower power.

I've checked and I don't see anyone making swept back blades that can approach or exceed supersonic, even unaffordable ones. If there are any then let me know. Also, these blades would need to fit into a multi-blade hub, ideally one that can accept more blades later, like go from two to three to four to six blades in increments.

I designed and made a high RPM fan for a V8 once but didn't test it because I realized that where I had it riveted would cause cracks. They were made out of 1/16" stainless steel and curved to make an ideal supersonic airfoil, one that would not give up anything for being supersonic.

So, what do you think.

It can work and NASA has done some pretty good development work in this field back in the eighties. The problem though was that the programs ran out of money so the data never got written up into a good concise report format, accessible to the public. I know two sources of the raw data but i doubt either would be willing to share.

the other difficulty you'll have is that there are very few hubs available that could serve your purpose and to the best of my knowledge, no blades. This means that if you go this route, you'll be the developer, from scratch. Could be pricey.

I think that many blades is just being greedy!

Besides, that many blades probably weigh more than my entire plane...

I came back with some thoughts when back from work

Jan

have you considered a ducted fan?

Originally Posted by radioinred

have you considered a ducted fan?

Ducted fans suffer large losses, require very precise clearances, and are really only applicable to a very limited range of applications and flight regimes.

Originally Posted by orion

It can work and NASA has done some pretty good development work in this field back in the eighties. The problem though was that the programs ran out of money so the data never got written up into a good concise report format, accessible to the public. I know two sources of the raw data but i doubt either would be willing to share.

With the study being done by NASA, doesn't that make the date public domain under the Freedom of Information Act?

Bruce

Yes it does but there is no actual report to release. I'm sure that with some digging you might find copies of the raw data but I'm not sure where. The two sources I know of are private companies that are doing their own development work off of it.

I was working at Gulfstream when we did an unducted fan installation (and testing?) for NASA. I didn't work on it, just watched it being built. I think it was installed on the wing of a G-II with a lead weight on the opposite wing.

There was a huge thick aluminum plate added to the fuselage side in line with the blades - "just in case". It was pretty cool to see.

I think I remember the guys saying it was way too loud and too complicated for the small performance benefit it provided.
Just what I recall.
Wally

I remember a propfan that one of the big companies was testing at Mojave Airport. Since it was proprietary, they had a guard walking the ramp to make sure no one took pictures of it. As soon as he saw us walking up the ramp with a camera (you have to have a camera with you if you visit Mojave), he headed us off and told us we couldn't come close or take photos. When we asked if we could take a picture of him in front of the propfan, his whole demeanor changed and we were able to get lots of good photos. Some of the photos included the guard, but somehow we missed getting him in the frame on some of the photos, but the propfan was there!

Bruce

First, you don't want a propeller doing 4000 rpm due to noise, you will hear it from here to Edmonton, even with smaller diameter it will sound like a hornet (Vespa crabro) And you lose performance also, especially with a pusher working in the wake of fuselage and wings

isn't the wankel's critical RPM in that region too?

if you want good performance go with a PSRU, you need a prop shaft bearing anyway. and you can get a better working propeller.

the nr of blades depends on what you need, to take care of the power, with Power, Speed, RPM, diameter limitations, thickness, and strength as parameters.

Jan

Originally Posted by orion

It can work and NASA has done some pretty good development work in this field back in the eighties. The problem though was that the programs ran out of money so the data never got written up into a good concise report format, accessible to the public. I know two sources of the raw data but i doubt either would be willing to share.

the other difficulty you'll have is that there are very few hubs available that could serve your purpose and to the best of my knowledge, no blades. This means that if you go this route, you'll be the developer, from scratch. Could be pricey.

I just might get into that, maybe my aircraft will be a test bed for propellers.

Until then, what in your opinion is a good prop that is available with a six blade hub and where each blade is good for about 75 - 100hp?

I don't like the stacked 3 + 3 two hub setups.

I've seen there are six blade props available for ultralights but they can't take enough power, and there are six blade props available for airboats, but are they efficient?

Since I want to get max RPM for a given diameter that rules out wood blades.
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When I built that high speed fan for the V8 I had to use stainless steel because aluminum was too weak. It turns out that on a supersonic small diameter fan that the centrifugal forces are so high
at the tips that aluminum isn't strong enough to hold it's own propeller tips on. So I used stainless steel, but even then the planform had to be tapered quite strongly to have good safety margins.

It was an interesting project giving the blades twist when limited to a 2 dimensional rolled piece. Maybe I'll sketch something up and post it here in case there are any enterprising sheet metal workers out there who like to see if they can make things explode, or not explode, as the case may be.
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So the question for the day is, what is best (quality & price) available that's closest to a prop fan.

Originally Posted by rtfm

I think that many blades is just being greedy!

Yes it is, and sexy lookin' too on a pusher.

Originally Posted by Jan Carlsson

First, you don't want a propeller doing 4000 rpm due to noise, you will hear it from here to Edmonton, even with smaller diameter it will sound like a hornet (Vespa crabro) And you lose performance also, especially with a pusher working in the wake of fuselage and wings

isn't the wankel's critical RPM in that region too?

if you want good performance go with a PSRU, you need a prop shaft bearing anyway. and you can get a better working propeller.

the nr of blades depends on what you need, to take care of the power, with Power, Speed, RPM, diameter limitations, thickness, and strength as parameters.

Jan

Thanks Jan, I saw something about this 400RPM problem zone in the Mazda rotaries before. I'll ask on the engine forum.

My thinking on this subject is as follows. I'm looking to see how well I can get away with using direct drive. My plan is to use direct drive at first because it is simple and cheap and I don't care about the efficiency at the beginning. I'll see how well I can make it work with direct drive and if I need to I'll go with a PSRU.

My aircraft project is a very compact pusher so I wouldn't want a prop bigger than 72" no matter what.

With a 72" diameter prop I'll need a PSRU, but what about a 60" prop, that's not tooo much smaller than a 72" and If I can run it at 4000 then I can save a lot of weight.

I'll have to get my propeller book out to calculate the efficiency differences, that will help me decide.

I realize that an engine running at 4000 with four blades will have a higher frequency than one running a 3000 with three blades, but if the tip speeds are the same then the volume should be similar?

I'll ask on the engine forum to see about the 4000 RPM problem area with Mazdas.

Powerfin have 4 and 5 blade hubs.


Jan

Powerfin looks exactly like the kind of thing I've been looking for, and the guy lives in my own state too, so I can go and visit.

I see he is working on wide swept back prop blades, you can see one in the lower left hand of the picture.

And he has GREAT prices!

Thanks!


http://www.powerfin.com/