4 blade wood props: Mr. Rabe your thoughts?

LAST EDITED ON Oct-22-03 AT 01:51 PM (CDT)
 
Did a search on this and really couldn't find an answer.
Aaron had some correspondence with someone (??) who has experience in making 4 blade wooden props.

I would like to make some for an Al Rabe Bearcat! ST 51 for power, I have a bunch of 12X5 Rev Ups, and several types of epoxy.
So....... let me hear from y'all.
Thanks!
Bill

Bill,

Watch out. I've made several 3 and 4 blade wood props as has Alan Resinger. While I was pretty happy with the outcome, they probably aren't all that safe. I've never had one fail, but they make me very, very nervous.

>Bill,
>
>Watch out. I've made several 3 and 4 blade wood props as has
>Alan Resinger. While I was pretty happy with the outcome,
>they probably aren't all that safe. I've never had one fail,
>but they make me very, very nervous.
>
>

Randy Powell

I, too, would get a bit concerned about them shedding a blade. Not only could this cause injury or worse to someone in the "line of fire" (whoever is flying is in the line of fire) but it could do a number on your engine and plane before you got it down.

Might I suggest getting a Bolly prop of the appropriate size and pitch. Besides, these are easily re-pitched if you need a little more or a little less.

Bill,
As someone whose made quite a few 3 bladers a few years back I'll agree with Leonard 100%. The possible problems are just too great and the final outcome might be a big disappointment. Just get a Bolly or an Eather and adjust the pitch.
Alan

Bill,

Just to kick you--and others who might have the same ideas--while you're down, I find it important to emphasize what you might not know and that is that Alan Resinger really knows his stuff when it comes to this subject.

I still have a three-blade wooden prop Alan gave me years and years ago. I have looked at it many times, admiring the workmanship, attention to design and detail.

Hey, I have tossed a few props in the garbage simply because the grain direction didn't much suit me. But, until now at least, I would not have flinched at spinning Al's home-brew piece at, say, 15K.

HOWEVER, if Al says "Don't do it!" even after producing a bunch of the things, I would take his advice very seriously.

LAST EDITED ON Oct-23-03 AT 01:01 AM (CDT)
 
In all honesty, I have very little experience with three blade props and none at all with four blades. I do agree with all of the warnings above to the effect that handmade wooden props which are spliced to add blades are dangerous. I wouldn't consider running one of them on my airplane. If you must try a three or four blade prop, at least buy a professionally made epoxy/carbon composite.

Having said that, My opinion is that blade disc area is more important to generating static thrust than multi blade props (however neat they may look, and many stunt flyers are not too objective when it comes to rejecting trendy ideas). I suspect, and am in the process of increasing diameter and reducing blade area to increase disc area and, at the same time, raising RPM a little to get a bit higher on the torque/horsepower curve. My best prop, so far in a 14.5"x4.0 on my BBQB Bearcat. Last year I ran 14"-4.5 props which turned 8800 RPM static. This year, I wanted to increase the disc area. This loads the engine more so I reduced blade area. While reducing engine load by reducing blade area, I continued the reduction until the static RPM was up to 9500. The higher RPM required a flatter pitch to maintain the same 5.5 lap time. Prior to launch the static thrust of this new prop is impressive, particularly when compared to piped 65s turning 10k plus.

You pays your money and takes your choice, but then, you also asked.

Thanks, Al, and everyone else! I did ask, and got the answer.
I can go the CF prop route if I really want a 4 blader. I appreciate all the input. I had seen wood multi blades in the past, and only considered a 4 blade due to the much simplier method to make one compared to a 3 blade.
I don't think it will be worth the effort.

Al, thanks for the input on the blade disc area, and higher rpms/torque/HP curve. I made a discovery of my own, quite accidentally, about this with my current USA-1. Went to a larger diam, less pitch set up for lap time, and found it to fly much better!

Thanks, guys!
Bill

HI Bill. Back in 1965 I made a four bladed prop by cutting notches into the hubs of two wood props. I epoxied them together, bolted it on my OK Cub .049 B and it dragged the engine down to abut 7,000 rpm which is about 2,000 under it's peak on a good day. The prop promptly came apart into three pieces, with two blades staying on a very vibrating .049. End of that experiment!! Ha, ha, missed me. Ruined the Baby Barnstormer.

Everybody has already beat me to the punch line on this. But I think it is important to empahsize what everybody is saying here. Pay particular attention to what all of these people are saying, especially Al, Ty and Leonard.

Making 3 or 4 blade props from two blade props is at best a difficult, time consuming effort and unles it is done right, can be very dangerous. I made a 4 blade wood prop once by notching two 2-blade props at their hubs. On the test stand, in the first minute of running, one blade sheared along the grain from the notched hub. Luckily, the blade did not hit anyone.

There is a method where a flat plate disk with a machined ring near its circumference fits in a machined grove on the front face of the hub where the blades meet. Then, bolts go through the disc and each blade (two bolts through each blade on larger props - like 12" diameter and up) and tap into a rear plate (which can be the spinner backplate) with lots of epoxy around the hub and between the front and rear plates. This can make a fairly secure assembly. I have used this arrangement before and have seen others use it. (This is not my original idea, but was necessary in the era when there were not any suitable commercially available 3-blade props for the ST .46s that I was flying.) I have had some ground impacts (as in crashes) where the blades were sheared off, but the hub assembly remained intact.

I would now only be comfortable with a home made multi-blade prop if it was laid up in a form with carbon fiber.

With the multitude of props that are on the market, particularly the carbon props and other composite materials, I see little advantage to making your own multi-blade propeller unless you enjoy making your own props. There are easier ways to extract some additional bit of performance from your engine prop combination than making your own multi-blade props.

This is not to say that there might be an optimal 4-blade or 3-blade) prop for your particular model. But it makes sense to explore what is already commercially available before going to the trouble and risk of making your own.

Here's a thought.

Use two two bladed props, DON'T remove any material from the hubs, somehow bond the front of one hub to the back of another, and mount them on, if necessary, an extra-long propshaft (possible on some engines, notably Fox Combat Specials). So two blades rotate in a plane in front of the other two and you'd need a strangely cut spinner. So what? Yeah, there's the wood-crush issue, though.

Anyhow... Anyone try a single blade yet?

Isk, here's a question

Aerodynamic issues aside.

Given that there are some very, very clever timber adhesives around these days, is there any reason to expect that your twin props, 4-blade would not be just as effective if the boss of both props was spot milled so the two were assembled so the blades rotated in one plane.

When dealing with wood products, I find myself constantly reminded of a demo that an old woodwork teacher ran for my class when I was in junior high school--about 40 years ago--where he demonstrated that the wood adjacent to a joint, properly made, and glued with PVA (white woodworkers glue) was stronger than the timber adjacent to the joint.

I have to wonder whether such an approach would work for our models.

Cheers

I guess I'd be more worried about slippage (especially after some wood compression) than about blade strength, in this case.. you might end up with an X-shaped prop, which would look dorky.. not to mention that when the front prop slips with respect to the back prop, so does the prop nut.

Perhaps tiny wooden pins or dowels might work? The glue should, but even if it holds, the wood next to the glue might not.

>
>.. the wood adjacent to a
>joint, properly made, and glued with PVA (white woodworkers
>glue) was stronger than the timber adjacent to the joint.
>
>I have to wonder whether such an approach would work for our
>models.
>
>Cheers
>
>Geoff

Suggest that you go back and read Post #7 above and the second paragraph of Post #8 above. Without some form of reinforsement to enhance the strength of the joint, I would not want anyone around a wood prop that has been glued together at the notched hub no matter how good the joint or how good the glue.

The problem with replying to these posts and offering information is that nobody ever seems to be swayed from their preconceived notions. It is an exercise in frustration.

>The problem with replying to these posts and offering
>information is that nobody ever seems to be swayed from
>their preconceived notions. It is an exercise in
>frustration.
>
>Al

Years gone, Walt Musciano made 4-blade props for some of his Scale jobs by notching the hubs and gluing a pair of two bladers together. I once saw a prop Charlie Bruce made this way for a P-51, but he said: "Its for display"- you could barely see the joints where it was made and fitted together.
In today's lawsuit happy climate I would be extremely cautious about using anything but the best and safest equipment I could get. Unless you are on your own property and your nearest neighbor is 5 miles over the next mountain. Even then I'd be cautious.

Hi Al,
No, it is good that you replied. I am now sure that the effort would not be worth the potential hazards, especially since the availability of CF props. Your insights, along with Keiths, Randy's, Dan, etc., clearly made a point to me.
Also, the mention of blade disc area/rpms was a very nice addendum.
Thanks again!
Bill

>The problem with replying to these posts and offering
>information is that nobody ever seems to be swayed from
>their preconceived notions. It is an exercise in
>frustration.
>
>Al

A kingdom of self made men...

I spent a lot of time shooting Highpower rifle when I lived in Ohio. I used to shoot with various friends. One of my friends used to bring his sons to shoot with us on occasion. One afternoon we were teaching the young lad to shoot prone. He insisted on putting the butt of the gun under his arm and placing his eye on the rear sight. We told him that was incorrect and commenced to show him the proper position. He started to argue with his father and me, insisting that "he knew what he was doing".

Since we had him pointed downrange and were single loading the rifle, I said, "fine, pull the trigger".

He did.

The resulting gash above his eye (from the rear sight of course) was bleeding only mildly, he was more upset that we were laughing at him. Of course his mother nearly killed my friend and did not speak to me for several days. She simply did not understand. This kid had to learn all his lessons hard, it was his nature.

This story is certainly not directed at anyone, in fact I share the child's nature, that is why I recognize it so easily.

>>The problem with replying to these posts and offering
>>information is that nobody ever seems to be swayed from
>>their preconceived notions. It is an exercise in
>>frustration.
>>
>>Al
>
>A kingdom of self made men...
>
>I spent a lot of time shooting Highpower rifle when I lived
>in Ohio. I used to shoot with various friends. One of my
>friends used to bring his sons to shoot with us on occasion.
> One afternoon we were teaching the young lad to shoot
>prone. He insisted on putting the butt of the gun under his
>arm and placing his eye on the rear sight. We told him that
>was incorrect and commenced to show him the proper position.
> He started to argue with his father and me, insisting that
>"he knew what he was doing".
>
>Since we had him pointed downrange and were single loading
>the rifle, I said, "fine, pull the trigger".
>
>He did.
>
>The resulting gash above his eye (from the rear sight of
>course) was bleeding only mildly, he was more upset that we
>were laughing at him. Of course his mother nearly killed my
>friend and did not speak to me for several days. She simply
>did not understand. This kid had to learn all his lessons
>hard, it was his nature.
>
>This story is certainly not directed at anyone, in fact I
>share the child's nature, that is why I recognize it so
>easily.

So do I.
Don't even get me started on this one- I could type for two hours here.

>So do I.
>Don't even get me started on this one- I could type for two
>hours here.
>
>Jake

I should say I USED to follow this path more often than not.

Eventually the blood gets in your eyes from the constant beating of the head against the proverbial brick wall.

Gets old seeing red all the time...

>>So do I.
>>Don't even get me started on this one- I could type for two
>>hours here.
>>
>>Jake
>
>I should say I USED to follow this path more often than not.
>
>Eventually the blood gets in your eyes from the constant
>beating of the head against the proverbial brick wall.
>
>Gets old seeing red all the time...

Yeah, but suppose the wall finally crumbles and you are left standing there- bloody and disheveled perhaps, yet finally free?

LAST EDITED ON Oct-23-03 AT 05:13 PM (CDT)
 
With a 15 hour time difference, it was very late. I missed the detail of the earlier posts.

Mea Culpa, Geoff

An interesting application of Isky's stacked prop idea -

Peter Smart from the U.K. used stacked props to simulate the counter-rotating food processors on the Tupolev TU-95 Bear when he built this huge FF model. They look particularly realistic in flight.

Could be just the thing for that semi-scale Miss Ashley...

Hmmm...

God bless!

Was the TU rubber or electric or CO2 powered?? Big difference at very low rpms. As to one bladed props, these are used or have been used in speed events with a heavy counter weight (I still have three) and at one time in rubber powered free flight, mainly Unlimited rubber or now Mulvihill. Very slow rpms and lots of inital torque, but nothing approaching even a .15.
I would be very leery of any home made wood prop assembly at the rpms we use, namely 8,500 plus and at the torque range of our engines. Simply put, unless you truly know just what you are doing and the end results, DON"T DO IT. Caps for emphasis. Get a carbon fiber prop made by a reputable source.

Keith,

>>There is a method where a flat plate disk with a machined ring near its circumference fits in a machined grove on the front face of the hub where the blades meet. Then, bolts go through the disc and each blade (two bolts through each blade on larger props <<

This is essentially the method both Alan and I used. It does make for a solid hub. This was demonstrated last summer when I was using one of the 3 blades I made (or it could have been Alan's) and had to make an inverted landing. One blade sheared off, but the hub was intact.

I still wouldn't use one again. My experience last summer just made me nervous. Every time I started the thing, I was paranoid. It's just not worth it.

Actually, I don't see much wrong with Mike's suggestion of stacking the props. In order for an engine to turn them they would have to be small and, in my opinion, unsightly. This would also reduce disc area but no more in theory, than other 4 balde props.