Prop Block Dimensions

[Ed Hardin]

I just received plans for a Bob White twin fin mulvihill, the only information on the plans for the prop is 22.50 dia x 27 pitch, carve blades from 12 x 1.50 x 1.75 block. Does anyone know where I can find info on how to layout the block for carving.
Thanks,
Ed Hardin

[awringlien]

Hey Ed – Are you still looking for a prop solution for the Twin Fin? Bob’s plans always had a side view and a top view of the prop block. Make templates of those and draw them on the rectangular block of dimensions you listed. Carve block to those outlines (both directions)… then carve the bottom of the prop balde by “connecting” the diagonal between LE and TE. Sand/carve the requisite undercamber and then do the top surface. All of the Bob White designs that I know (beau coupe old and new, Twin Fin and wakefields), had a tip angle of 15-17 degrees. But, to get the pitch desired (27 you said) you would strive to make the blade have a 45 degree angle at a radius of [27 divided by (2 times PI)] or at 4.3 inch radius.

If you don’t have block side & top views, I can send you a copy

[Bill Shailor]

Ed,
If you don’t have the outlines, email me your mailing address and I’ll trace the templates I have and mail them to you. His prop outline was unique.
Bill

[Ed Hardin]

Bill and Andy,
thanks for the reply. I have the outlines for the top and side from the plans, the info on the pitch angles was my next question, so I guess you read my mind on that one. I have two Burdov balsa/glass blades I got from Larry Bagilini that I may put on it, but I also think I will carve the prop as well. Just finished rolling the fuse, I used the Jim O’Riley method from the April 2005 Free Flight Digest and it worked well. I am pleased with my first attempt at rolling one. Hope to see both of you at the NATS, if not before. Thanks again.
Ed
PS: Bill, congrats again on your Moffit NATS win, I was wondering how far off the field did your flights go.

[Bill Shailor]

Ed,
Moffett was a blast! Thanks! The only flight that went off was the 7 minute flight. If it D/T’d on time, I think it would have stayed on. It went off at the far northeast corner of the field, the part that has recently been opened up.
If you are going to carve the prop blades, try to keep the blades relatively thin. The way I do it is to carve/sand the bottoms first, then when the tops are still fairly thick, glass the bottoms. This will keep the blades from breaking when you are sanding and shaping the tops. When the tops are done, and the blades match in airfoil shape and dimensions, glass the tops.
This way you can keep the blades thinner without them deforming at launch.
See you at the NATS!
Bill

[Ed Hardin]

Bill, thanks for the tips on the prop, I think all the props I have carved in the past are too thick so I will try the method you described on this one. I finished the Twin Fin and have the Burdov blades on it. It weights in at 117 grams, much lighter than my old mulvihill. All I need now is a field large enough to fly it. My wife also says I need to buy a Walston radio for it, maybe I’ll give myself one for Christmas.
Ed

[awringlien]

Just for debate – though I don’t want to cause you building paralysis by over-analysis:

I thought Bob designed all that prop blade area in front of the hanger/pivot so that the blade could flex open a little during the power burst – hence providing a little natural Variable pitch functionality. Making them too stiff precludes the blade flexing (of course too hard or too soft of wood selection changes the equation as well!) However, with stiff blades the “skinny” 1/16th MW hub may open up under high torque itself – if you use his Z-hub M/W sizes.

I’m not really disagreeing with Bill’s recommendation – after all I made a super stiff version (laminated balsa core, vacuum bagged with kevlar fabric at 45/45 degrees and .007″ CF hub to tip) and it performed well enough to win Dawn Unlimited (7+ min) at the NATS a few years back. Trust me they were STIFF and not flexing a bit!

Good luck

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