Propeller for my 5 cylinder radial

[gbritnell]

If you have followed my thread on the 5 cylinder radial engine you will know that I had trouble with the plastic 3 blade propeller, not enough mass. My plan was to build an aluminum prop after I got the engine running and the adjustments sorted out. I mounted a brass flywheel to the engine and got it running which was posted as a video clip.
Late last week I drew up an aluminum prop with hub and started construction. The hub was first. I made it from 2 pieces that would clamp onto the blades when finished. I drilled and reamed the center hole and drilled and tapped the the clamping screw holes for 1-72 socket head screws. The machining was fairly straightforward the only tricky part was putting the counterbored recess into the bottom of each pocket. I thought about making some kind of setup for the lathe but that would have involved using an angle plate mounted to the face plate with a lot of indicating and adjusting so instead I made up a small counterboring tool much like a miniature woodruf key cutter. The part could then be held in my dividing head as it was for most of the operations. I indicated the first hole plumb and square and then double checked the other 2. Between the 3 holes they were only out .002.
The blades were roughed from a piece of 1.00 bar stock. The diameter was brought down to .865 and the tang to fit into the hub was machined. The other end of the stock got a center drilled hole.
The blades were then put into the vise on the mill and the stock was cut to the hub boss diameter of .281
I didn't take pictures of the turning and flat milling as it was just normal machine work. In this picture you can see how the prop ends fit into the hub.
gbritnell

 

[gbritnell]

The next operation took some thinking. I needed to cut one side of the blades with an angle of 1.568 degrees. I had thought about making a piece of steel with the proper angle and clamping the pieces into the vise but that would have only got the flats done. After that I needed to machine the airfoil shape and the blades wouldn't have stuck up high enough to get that done. The solution was to use the dividing head. When the blades were initially turned I had put a center drilled hole in the outboard end. My plan was to use the tailstock for the dividing head to support the outer end. The tailstock was set at the proper angle and a blade was mounted in the chuck. The tailstock is made so there is some room to angle it up and down from the horizontal position. The only problem is the tailstock needed to be blocked up to get to the center hole. I put some blocks under the corners to get close and then adjusted the tailstock center out until it located properly in the center hole. I verified it's position by indicating both the top and sides of the blade blank.
gbritnell

 

[gbritnell]

The top edge of the blade was then machined to the proper thickness. I did get a little vibration on the finish cut so I pinched my fingers on the stock to dampen the vibration. The blades are going to be thicker than would be proper for a true shape but this is to add more mass. If it works as I hope I can always cut the blades down at a later date.
Next up was to put the airfoil shape onto the blades. I had made a step-off chart up to use a .375 ball mill. This time I started from the heavy side figuring it would be easier to climb cut against the stock and hopefully prevent any deflection while cutting. The outer end of the blade is only .125 thick.
gbritnell

 

[gbritnell]

With the machining completed I then cut some shape to the trailing edge of the blades. I made up a template so that each blade would be as close as possible to the other two. I then started filing to smooth out the airfoil and putting the radius on the leading edge. The corners around the root of the blade were also radiused and smoothed.
gbritnell

 

[gbritnell]

When I machined the blade blanks I had left extra stock on the hub end for clamping in the dividing head. With the filing and sanding complete I sawed the extra material off and filed it to the proper width of .09. Each blade was then fit into a pocket in the hub. This is shown in the picture in the first posting.
I then set up my buffer and polished the blades. It might be a little premature if more material needs to be removed but I thought they would look better with a nice shine on them.
In the next couple of days I'll fire the engine up and see how well this works out.
gbritnell

 

[ozzie46]

Beautiful job George. Did it take a lot to balance them? Wouldn't want to see that engine shake apart.

Also are you going to paint the tips in say yellow and red for visual effect? The blades will disappear when running and do quite nasty things to hands and fingers if accidentally put thru the prop arc.

Ron

 

[gbritnell]

Hi Ron,
Yes, I thought red would show up the best.
gbritnell

 

[petertha]

That is so cool, nice work. What maintains the blade's pitch setting in the hub? The 4 clamp bolts & friction on the stem part? Maybe you could even dial different static thrust (load) settings in that way, kind of how a variable picth works?

 

[cfellows]

Beautiful prop, George. Great work

Chuck

 

[mu38&Bg#]

What size propeller did you try? I see a Master Airscrew in the background. Even single cylinder RC engines run and idle well with light wooden props. A mutli should not be as critical of flywheel. At ~15cc I'd use a 14-6 to 15x8 plastic prop and three blades would have plenty of mass. What sort of idle RPM are you trying to achieve? I'd guess 1500, maybe 1200 RPM is as good as it will get.

I'm concerned of the safety of that prop. What is the weight of the blade and the radius to mid span? What is the retention groove diameter? By guessing with what you've posted I think you'll exceed the yield strength of the retention groove at around 4200 RPM. Is the hub made of steel?

White tip paint is the most visible on a dark or neutral blade. A white prop can have red tips. Contrast is what makes it visible.

Greg

 

[metalmad]

Love your work George !
Pete

 

[gbritnell]

Hi Greg,
I had an 11 x 6, 3 blade master airscrew prop on it when I first started the engine up. I couldn't get the rpm down below about 2500 rpm, I'm guessing from working on these engines so long. As can be seen in the video of the engine running with the brass flywheel I can now throttle it down to about 13-1,500 rpm. I'm not trying to achieve anything below 1000 rpm but I would like to get close to that number. Even though the blades are made from aluminum the mass isn't so great as to pull the bosses from the ends. As a test I am going to make up a bar with the same boss on the end and put it into a test block, then using my friends hydraulic press see how much pressure it takes to push the boss off of the bar. I'm guessing it should be 3-400 lbs.
This prop is 13.5 inches in diameter and the pitch is variable.
gbritnell

 

[mu38&Bg#]

I mixed some units last night. You should be OK, but I wouldn't exceed 6500RPM.

Greg