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Author Topic: Smith 1941 Mulvihill Winner  (Read 205 times)
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calgoddard
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« 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.



Attached files Thumbnail(s):
Smith 1941 Mulvihill Winner
Smith 1941 Mulvihill Winner
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skycafe
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« Reply #1 on: June 04, 2022, 07:47:53 AM »

Awesome model!
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Red Buzzard
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« Reply #2 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.

RB

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calgoddard
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« Reply #3 on: Today at 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.

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