Prop pitch, rpm, and performance

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TiPi

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Keep it, you might appreciate a little bit of extra material for a refurb (or trim a chewed up tip).
 

meglin1

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I'm wondering why hardly anyone makes their own prop? Not even people who build their own airplane?
This is a very difficult unit. It is possible to achieve good performance with several approximations, but it is not easy to provide a high resource, resistance to temperature and humidity changes, and resistance to damage. Masters accumulate this experience for decades. In our aviation University in Kharkiv, a few units have retained this experience and technology.
 

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blane.c

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I'm wondering why hardly anyone makes their own prop? Not even people who build their own airplane?
I am guessing because you are likely not to build one prop more likely to build three or maybe four to get one that works correctly and you can buy one that is already figured out for your plane or from a plane that is fairly close.
 

Vigilant1

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I'm wondering why hardly anyone makes their own prop? Not even people who build their own airplane?
Some possibilities:
- It's an entirely different skill set than building any type of airplane. It's very much an art, and one mistake can result in the need to start again from scratch.
- It requires different tools
- While it's possible to spend a lot on a prop, it's also possible to get a very satisfactory wooden prop for less than $500.
- The folks who know how to do it already have the materials on hand, the tools, and the experience to do the job efficiently.
- (Most important to me) The results of a failure are very dramatic and serious. Loss of a prop blade in flight is a real attention-getter, rivaled only by a wing spar failure, loss of a primary flight control, or flutter.

It's not rocket science, and many homebuilders have carved their own props successfully. Just like anything else in this hobby, if a person thinks they'd actually >>enjoy<< doing it, then they can give it a shot.

I do most of my own home repairs and improvement--plumbing (I'm cursing at a Moen shower valve right now), electrical, structural, painting, etc. But, I don't enjoy taping and mudding drywall. I >can< do it, but I'm not fast at it and I don't enjoy it. So, I hire someone else to do it. And if I thought getting it wrong could kill me . . .
 

blane.c

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Some possibilities:
- It's an entirely different skill set than building any type of airplane. It's very much an art, and one mistake can result in the need to start again from scratch.
- It requires different tools
- While it's possible to spend a lot on a prop, it's also possible to get a very satisfactory wooden prop for less than $500.
- The folks who know how to do it already have the materials on hand, the tools, and the experience to do the job efficiently.
- (Most important to me) The results of a failure are very dramatic and serious. Loss of a prop blade in flight is a real attention-getter, rivaled only by a wing spar failure, loss of a primary flight control, or flutter.

It's not rocket science, and many homebuilders have carved their own props successfully. Just like anything else in this hobby, if a person thinks they'd actually >>enjoy<< doing it, then they can give it a shot.

I do most of my own home repairs and improvement--plumbing (I'm cursing at a Moen shower valve right now), electrical, structural, painting, etc. But, I don't enjoy taping and mudding drywall. I >can< do it, but I'm not fast at it and I don't enjoy it. So, I hire someone else to do it. And if I thought getting it wrong could kill me . . .
Yes I can do mudding and taping too, don't enjoy it, I like to hire it out. My father said "it is the last 1/4" that everyone see's" and he felt it was better left to professionals whereby many homeowners feel it is something they can do themselves it generally looks like they did it too although of course there are exceptions they ain't the rule.
 

lr27

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I've carved some little props for models, but even there most modelers don't even want to try. I carved an oar which would have come out nicely if I'd laminated the blank so it wouldn't warp. Would much rather carve a prop than do drywall. I suppose if you're using one of those duplicator gadgets, that would be less fun.
 

Dana

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...The results of a failure are very dramatic and serious. Loss of a prop blade in flight is a real attention-getter, rivaled only by a wing spar failure, loss of a primary flight control, or flutter.

It's not rocket science, and many homebuilders have carved their own props successfully. Just like anything else in this hobby, if a person thinks they'd actually >>enjoy<< doing it, then they can give it a shot.

I do most of my own home repairs and improvement--plumbing (I'm cursing at a Moen shower valve right now), electrical, structural, painting, etc. But, I don't enjoy taping and mudding drywall. I >can< do it, but I'm not fast at it and I don't enjoy it. So, I hire someone else to do it. And if I thought getting it wrong could kill me . . .
I could see making my own prop(s). I would enjoy it if I didn't have so many other demands on my time. I don't see it as a major death risk, as long as you're not departing from conventional design. It does require attention during the laminating/gluing process, as well as balancing, but neither of which are particularly difficult. But it is a lot of work. If I was doing it, I'd want to do it with a CNC router, with only finishing work done by hand... anything else would be too much work for the time I have available. But maybe as a retirement gig?

I am guessing because you are likely not to build one prop more likely to build three or maybe four to get one that works correctly and you can buy one that is already figured out for your plane or from a plane that is fairly close.
If you have a common airplane/engine combination. I got recommendations of 44 to 54 inch pitch for my plane, which is a huge range. One of the reasons I went with Sterba is because he will rework it at no additional cost... but the downside to a totally hand carved prop may be accuracy. I was looking to reduce the 47" pitch to 44, but as near as I can measure it it's at 46" after rework. I think there's still room for improvement, but it's quite possible that he/I might overshoot and lose instead of gaining.
 

blane.c

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I don't know how Sterba does it, I've watched some video that the tips are extra long and pinned into the jig, the tips are trimmed towards the end of the finishing process. But there are likely a lot of ways to do it.
 

blane.c

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Are you supposed to measure the chord line or as best you can off the bottom, and is the station to measure at a standard?
 

lr27

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If you want to be consistent, you need to measure the chord line and then add a little, since airfoils usually aren't at 0 lift at 0 angle of attack. For instance, I seem to recall the Clark Y has to be 2 or 3 degrees negative before it gets to 0 lift. So, I think, everyone is waving their hands and giving approximate figures. If the other guy was measuring from the flat part, then for any meaningful comparison, you'd better do that too. But it still won't be very close unless the airfoil is accurate. Then there's the issue of pitch distribution along the blade. I think this issue is part of why props seem like a black art.

I gotta admit I've forgotten most of what I used to know about props. I even knew what an advance ratio was. Still, I didn't know all that much in the first place.
 

Vigilant1

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If I was doing it, I'd want to do it with a CNC router, with only finishing work done by hand... anything else would be too much work for the time I have available.
I agree, CNC or a prop duplicator jig would be the only way I'd attempt it.
 

blane.c

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You can 3d print a blade now. So CNC shouldn't be a problem. The nice thing about resorcinol is you can add heat and reduce the drying time dramatically, so if you can tent your layup and raise the temp you can be cured in a few hours after.
 

Dana

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Everybody measures it differently, so it's hard to compare props from different manufacturers. Both Sensenich and Sterba told they measure off the back face. That's fine for Sensenich, the blades are thin and the back face flat, and my number agreed with how it was marked, but the Sterba's back face isn't flat. I measured it at points 1/8" from the leading and trailing edges, a later attempt with V-blocks at the leading edge didn't agree so well.

As to where, if the prop is true pitched, i.e. constant twist from root to tip, it shouldn't matter where you measure it, the pitch will be the same. But some prop makers vary the twist for various reasons. The standard place to measure it is at 75% radius.

The blade thickness and camber also affect things. I'm getting similar performance from the wood prop with significantly less pitch than the metal prop, presumably because, as lr27 pointed out, the thicker section and camber of the wood prop means the zero lift line is at a higher angle than the back face.
 

vhhjr

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I have made several props and it is not a trivial task. The first few were made with a draw knife and templates then I was given a router duplicator. With the duplicator and a master blade you can turn out a decent prop more quickly. Still lots of sanding to get the final shape right. For non flight props I used soft, knot free wood such as Popular. For flight props I laminated walnut and Maple. It's a lot of work. For flight props I bought a couple adjustable prop hubs and that makes it easier because you can try different pitches without having to make several fixed pitch props. For ground test props I made the hub shown below.
 

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blane.c

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Everybody measures it differently, so it's hard to compare props from different manufacturers. Both Sensenich and Sterba told they measure off the back face. That's fine for Sensenich, the blades are thin and the back face flat, and my number agreed with how it was marked, but the Sterba's back face isn't flat. I measured it at points 1/8" from the leading and trailing edges, a later attempt with V-blocks at the leading edge didn't agree so well.

As to where, if the prop is true pitched, i.e. constant twist from root to tip, it shouldn't matter where you measure it, the pitch will be the same. But some prop makers vary the twist for various reasons. The standard place to measure it is at 75% radius.

The blade thickness and camber also affect things. I'm getting similar performance from the wood prop with significantly less pitch than the metal prop, presumably because, as lr27 pointed out, the thicker section and camber of the wood prop means the zero lift line is at a higher angle than the back face.
I wonder if Sterba uses a profiled block? So the profile would fit the bottom curvature and the bottom of the block would be flat. If I was doing a lot of propellers I would have some kind of simple jig(s) to make life simpler.
 

TFF

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It would be plenty easy to build a jig to compare stations. Center hole in hub being the anchor point for measuring. Roughing out the blades is pretty easy. Most people doing this will be pretty consistent because they are trying to be.

Most home brew props will be a Clark Y RAF6 mix of an airfoil, carved normal. Dealing with under 150 mph and 150 hp a home prop will be pretty forgiving, but with that changes will also be hard to notice without big moves.

I think the bigger issue is making good blanks. Finding consistent wood along with good glueing will eat up a lot of money. Cheaper than a Sturba prop but would still take $150 in wood and another $150 in glue. How many clamps do you own?

The man who use to demonstrate prop carving at Oshkosh could probably rough out a prop in under two hours, probably closer to one. He used epoxy and was using red oak the day I watched making a prop for a O-200. He said his biggest issue was going too thin and blades fluttering. He made the point that he was not trying to be cutting edge. If you made the blanks, a local cabinet shop might have a CNC machine capable to cut you one out.
 

BJC

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I met a VP 1 builder, circa 1967, who designed and built his own propeller, and wrote a “how to” pamphlet that he sold through Sport Aviation. He also built a removable cockpit enclosure for winter flying.

Lots of props have joints at the hub to make use of shorter boards. Three bladed props have all short boards.

Glue should cost less than $50. https://www.thepaintstore.com/DAP-Weldwood-Plastic-Resin-Glue-p/00203.htm


BJC
 

Dana

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Ed Sterba told me he uses a rasp followed by a drum sander to shape the props.

The Clark Y is the traditional airfoil for wood props (as well as a lot of other things). Metal props are usually a lot thinner, with a sharper leading edge, than you can do with wood. A traditional wood pro with a wrapped metal leading edge has to have a fairly large leading edge radius. But props (like Sterba) with a polyurethane leading edge can be a lot sharper, almost like a metal prop. But the choice of airfoil isn't that critical for a prop. A sharp leading edge on a wing would have a nasty stall, but on a prop it doesn't matter.
 

blane.c

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I met a VP 1 builder, circa 1967, who designed and built his own propeller, and wrote a “how to” pamphlet that he sold through Sport Aviation. He also built a removable cockpit enclosure for winter flying.

Lots of props have joints at the hub to make use of shorter boards. Three bladed props have all short boards.

Glue should cost less than $50. https://www.thepaintstore.com/DAP-Weldwood-Plastic-Resin-Glue-p/00203.htm


BJC
How do I know it is resorcinol? I looked in dap literature but it doesn't specifically say resorcinol anywhere I see. Is it just common knowledge that it is the right stuff?
 
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