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Outdoor Free Flight Forum => Old Time Rubber => Topic started by: calgoddard on June 03, 2022, 08:50:28 PM

Title: Smith 1941 Mulvihill Winner
Post by: calgoddard on June 03, 2022, 08:50:28 PM
I could not find a build thread for this model on HPA so I decided to start one and post a few questions for those fliers with lots of OTR experience.
I am building this model using a kit from Campbell’s Custom Kits that a friend was kind enough to give me. Attached is a copy of the original plan as published in the April 1942 edition of Model Airplane News. The Smith Mulvihill is basically an over-grown Gollywock. Attached is a picture of famed free flight modeler, Bud Romak, holding his Smith Mulvihill.

The fuselage of the Smith Mulvihill is built from 1/8-inch square longerons and cross-pieces.  The balsa wood sticks in the kit are quite heavy but I am using them anyway.  I don’t want broken longerons and cross-pieces.  The rubber motor I plan to use with this model will be bigger than any I have flown with so far. In Gene Wallock’s list of recommended rubber motors for OTR models for the Smith Mulvihill he recommends a rubber motor that is 24 x 1/8-inches, and 34-inches long.  His list does not give rubber motor weights. The prop is a 16-inch diameter folder with very wide blades. The large amount of energy provided by a motor of this size wound to 85% breaking turns should yield a rapid climb, even with a robust fuselage. Thereafter, thermal hunting will hopefully overcome the negative impact on the glide due to constructing the fuselage out of relatively heavy balsa wood.  

The fin of the Smith Mulvihill has airfoil shaped ribs with the curved side apparently facing left (pilot view).  This fin will pull the model right in the glide assuming that the flat side of the fin is aligned with the center line of the fuselage. I want to fly a right-left pattern, as is traditional for models with folding propellers.  Would this be facilitated by putting the curved side of the ribs on the right side? My Korda C spiraled into the ground to the right until I twisted its airfoil shaped fin to provide left rudder. It has flown very well ever since that adjustment was made. The curved side of the Korda C fin also faces left.

There are two long 1/8-inch square sticks that crisscross on the stab of the Smith Mulvihill. Apparently, they were included to minimize warping. The 1942 article describing the build of this model says these sticks go in the tops of the ribs. This would be tricky to build. I have to include them to make my model SAM legal.  Can I instead put these sticks on the bottom of the stab in grooves sanded into the ribs?

I am thinking of splitting the rudder attached to the rear of the fin where it meets the bottom edge of the stab to facilitate a tilt-up stab DT. I will be flying this model with an on-board LL Electronics RF transmitter to lessen the odds of losing the same.  There was apparently no DT on the first four Smith Mulvihill models that were built. According to the 1942 article all but one of these models were lost.

If you carve the prop blades using the heavy balsa wood blocks included in the kit as marked you will end up with a left-handed prop. This would require counter-clockwise winding and a left-right flight pattern. These would be doable, but awkward. I am not enamored of the prop hardware included in the kit.  I have purchased a Smith stick folding prop and prop shaft hardware from Volare Products.  I know that George Bredehoft’s props and parts are very high quality and should work well with this model.

Title: Re: Smith 1941 Mulvihill Winner
Post by: skycafe on June 04, 2022, 07:47:53 AM
Awesome model!

Title: Re: Smith 1941 Mulvihill Winner
Post by: Red Buzzard on June 04, 2022, 05:23:20 PM
Hi Cal,

You will get a lot of comment on the Smith. I fly mine right/right and if you build in 1/32" of "left" rudder you'll be safe with that pattern. I would also suggest you make that wide blade a good deal narrower (George's prop will come that way) and you will minimize the torque roll to the left with full winds. Mine still went right under power but the whole thing rolled left during the burst. I now fly mine with a 20 x 40" 1/8 inch motor. That will be about 65 grams. Consider using perhaps medium cross pieces as there is quite a bit of wood in that fuselage, use light stuff elsewhere. The wing has a really low A/R and my tubby version needs all the lift I can find. The diagonal braces go on the bottom of the stabilizer as the original plan shows. Notice where the braces cross the spar and you will see the brace is on the bottom. Give lots of thought to anchoring the rudder onto the stabilizer as it is a real weak point if you just glue it to the top of the center rib. Fully wound it is a rocket.


Title: Re: Smith 1941 Mulvihill Winner
Post by: calgoddard on July 01, 2022, 03:31:07 PM
Red Buzzard - Thanks for your comments.

The rubber motor for the Smith 1941 Mulvihill Winner recommended in Gene Wallock's list is 24 x 1/8 - 34 inches long. The folding prop has a 16-inch diameter and blades with a very wide chord. I am thinking I will be launching this model at 40+ inch-ounces of torque. The list assumes you are using TAN II rubber.  I have precious little TAN II rubber left in my inventory of rubber and save it for indoor duration stick models, like LPP.  I will be flying my Mulvihill with motors made of TSS rubber. 

Is a .062-inch (1/16-inch) OD prop shaft adequate, or do I need to use a larger OD prop shaft?

Thanks in advance for you input.

Title: Re: Smith 1941 Mulvihill Winner
Post by: Red Buzzard on July 03, 2022, 01:36:07 PM
Hi Cal,

Your 24 x 34 motor will be about the equivalent of mine (20 x 40), though more torque will be available. If you go with the wide blade prop you will probably see the torque roll to the left during the power burst. Be sure to point it up so you get some altitude while that roll is operating. My experience with both TSS and Tan II seems to show TSS being a bit more forgiving at the upper torque numbers. Where Tan II broke all at once, TSS seems to break in just a couple strands of a 24 strand motor. More often TSS will break a strand or two in the air, so give you warning on your next wind that you are getting close. I've said it before, don't count on getting a bunch of flights from one motor at high torque. If you are lucky you might get two flights. If a motor survives two flights take it out, take it home, and wind it on your torque meter until it breaks. That will tell you how close you were or not. Holds true for both TSS and Tan II.

On your prop shaft question, I use 1/16" shafts almost exclusively. I also keep a couple of pre-bent shafts available in the event you bend one severely. Others will say to use heavier wire so they DON'T bend. Then the load is transferred to something else like a broken blade, hub, or displaced nose bearing. Each to their own.


Title: Re: Smith 1941 Mulvihill Winner
Post by: calgoddard on July 04, 2022, 12:24:51 PM
Red Buzzard -

Thanks for your helpful comments on the rubber motors and the prop shaft.

I measured the OD of the prop shaft supplied in the kit.  It is .062-inches.

In going through my inventory of shafts and bearings for large OTR models, I found a 2mm OD prop shaft already nicely bent to provide the arm for engaging the stop screw.  That 2mm OD is .079-inches.  I already installed the bearing for that shaft and glued it into the nose block with thin CA before reading your response to my question.  This prop shaft came in a kit along with a drive dog that is held to the shaft by a tiny Allen wrench set screw. I need to grind a groove into the prop shaft at the appropriate location for the set screw. The prop shaft and related hardware came with a spring for moving the prop shaft forward, several flat washers, and a ball bearing washer. I believe this prop shaft and related hardware came from Volare Products. It is excellent in terms of quality.

Title: Re: Smith 1941 Mulvihill Winner
Post by: Ployd on July 04, 2022, 09:58:06 PM
Calgoddard wrote;
"If you carve the prop blades using the heavy balsa wood blocks included in the kit as marked you will end up with a left-handed prop".

Seems I am not the only one that found out the prop blanks were wrong when I checked my kit.

Ployd in OZ

Title: Re: Smith 1941 Mulvihill Winner
Post by: Red Buzzard on July 04, 2022, 10:19:18 PM
Cal and Ployd,

Yep, the guide lines on the kit blocks are backwards. If you really want to use them you can transfer them across the blades for proper rotation if you really want to fool with them. I just carved a new one to avoid all the drag of the overly wide blades.

And the Volare prop shaft arrangement will work fine. It's when you try to bend that thicker stuff that it becomes a real pain. I believe you can bush the nose block down to .062" when the time comes...or just replace the original from Volare.

If you PM me your email I can send you a picture of my prop shaft arrangement.


Title: Re: Smith 1941 Mulvihill Winner
Post by: Jack Plane on July 05, 2022, 03:05:02 AM

If you PM me your email I can send you a picture of my prop shaft arrangement.

Or you could just post it here...  :)

Title: Re: Smith 1941 Mulvihill Winner
Post by: calgoddard on July 07, 2022, 01:30:26 PM
Red Buzzard sent me a PM with four pictures of his prop shaft arrangement along with an excellent detailed explanation of the same.

He has had difficulty posting pictures on the HPA website and gave me permission to post his pictures.  Here they are.

Hopefully he will post his explanation of the pictures.

Title: Re: Smith 1941 Mulvihill Winner
Post by: Red Buzzard on July 07, 2022, 07:38:12 PM
Thanks Cal I appreciate the posting.

As suggested, here is the text of my email to Cal relative to the photos. As it turns out, my notes will apply, sequentially from left to right of the photos in the event the file  numbers do not come through:

"Hi Cal,

Company left yesterday and we collapsed. But here I am. I am attaching 4 photos and will add some narrative relative to each.

951 jpg: This is my entire rig. I've used all the stuff shown here with my prop shaft hook-up. My rear "bobbin" is just a piece of aluminum tubing one size larger ID than the OD of my rear peg and as long as my blast tube is wide, plus a little. This makes it way-easier to get a broken motor out without digging around in the fuselage. Just slide the blast tube out, somehow get control of the bobbin and pull the whole writhing mass out of the blast tube. It will make a lively show. All the various Crocketts work fine and can be put on easily through the "D" of the prop shaft.

962 jpg: This is the whole mess disassembled. I had to cut the shaft to get it out, but more on that later. You can see I use a little "drive dog" which is a piece of .055" wire bent tightly around a drill bit to form a closed pig-tail. For lighter motors you can use copper wire or even cotter pins. When you get to bigger motors you will find out when you need tougher stuff. For my Smith motors (20x40" and about 40 inch ounces) I use music wire. That goes into the off-center hole in the prop hub. Engage the 90 degree bend with the drive dog and you're ready to go.

963 jpg: This is the prop shaft I use on all my "old-timer" hubs. The straight shaft is the one I pre-bend and have as spares in my flight box for when something gets bent. Just cut it off in front of the "D" bend, pull the bent piece out, insert a straight one, bend the 90 and you're back in the race...well it's a bit more complex in the doing and in the heat of battle. If doing this at the field be sure to keep all the parts. It is really hard to find the spring and washers or the drive dog in the grass. Also a good reason to have a few little pig tails handy too.

966 jpg: Here it is as you would do it in the field. Note one of the things you have to do is align your fold as you bend that last 90 degrees. If you capture the "D" with a rubber band against your stop, then insert the nose block into your fuselage, you can get your fold almost right the first time. Then it's just a matter of twisting the 90 degree leg while holding the "D" with pliers to do the final adjustment. Some practice is a good thing.

About the only thing left to add is that my current prop, carved on an old-timer (wood) hub is a 16"Dx22"P with the compound hinge bent at 15 degrees. My blades are 1 7/8" wide at 60% of the radius.

Tag, you're it.

Someone else at the HPA website wanted photos, too, but I'm not that good at attaching photos. If you want to post the photos at HPA you're welcome to.

Best Regards,

Bill Swift"

If anyone has other questions just shoot me a PM