Dick Sarpolus’ Thermix ’13

Build your own thermal hunter Article, photos, and design by Dick Sarpolus Read the full article in the August 2014 issue of Model Aviation.

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RC sailplanes have changed during the past 45 years. Now there is carbon fiber, vacuum-bagged fiberglass wings, ailerons, spoilers, flaps, crow settings, and highly efficient airframe designs. My first RC sailplane was built with the wing and stabilizer from a large-class FF model, with a fuselage made to hold the radio gear. Launching was done by hand towing—like FF towline gliders. Although running across a field towing an airplane was good exercise, I sure didn’t like it. Next I tried hi-starts, with shock cord or surgical tubing for the slingshot power. This again was a lot of work and not much fun. Our first winches were gas-lawn-mower-engine powered, with a chain drive and clutch setup turning a large drum to wind in the line. Turnaround pulleys were another item to figure out. The gas-engine winches were noisy, messy, and we had plenty of broken lines and broken wings. Electric winches were next, and somebody finally figured out that the old Ford starter motors with the long shafts would work for our needs. Today the winches use small, efficient, powerful modern electric motors. I consider the best advancement in technology from those early days is the use of a powerful electric motor and folding propeller in the nose of today’s RC sailplanes. I’m not talking any sort of contest flying—just a sunny summer day, soaring for fun, looking for the thermals, and enjoying flying and easy launching without a winch or hi-start. I wanted to do some sailplane flying and I wanted to make wood chips and sawdust to do it—not go out and buy an ARF. I wanted to do this retro style, for nostalgia’s sake. When looking back at the early model sailplane activity, nobody comes to mind faster than Frank Zaic and his Thermic series of glider and sailplane designs. Frank Zaic and his designs are a part of our aeromodeling history and well known at least to those of us who appreciate and have a nostalgic interest in the background of our activity. Frank designed sailplanes before World War II, and many were produced in kit form by JASCO and later Jetco Models. This isn’t the place for a review of Frank’s interesting life and model aviation activities, but it’s a topic worth researching if you enjoy that kind of reading. He’s in the AMA, National Free Flight Society (NFFS), and Society of Antique Modelers (SAM) Halls of Fame and his series of Model Aeronautic Year Books from the 1930s into the 1960s make for worthwhile reading. Most of his publications can still be found today. I built Frank’s Thermic 18, Thermic 20, and Thermic 36 hand-launched gliders in some quantity as a kid in the 1950s, and later on used his classic Thermic 100 wing as the basis for a few RC sailplanes. The instantly recognizable, widened and tapered wingtip shape on many of his larger sailplanes came from a full-scale German sailplane of the 1930s, the Minimoa, designed by Wolf Hirth. That wing shape is occasionally still seen today, likely for its nostalgic value rather than for any performance benefit. Some enthusiasts have built replica Thermic 100 sailplanes. Plans and reproduction kits are available for true nostalgia flying. I decided to use Frank’s general Thermic 100 wing shape and pod-and-boom styling, coupled with some building techniques for an old-style sailplane, but have an electric motor in the nose. This project is intended to be an easy-flying model for some fun on a nice summer day, floating around and looking for that thermal lift, knowing the motor in the nose is there to help out if needed. It’s built with balsa, basswood or spruce, and plywood. When I found I could buy the basswood wing spar material in 24-inch lengths, I laid out the wing with four 24-inch panels and 3-inch wingtips, for an overall wingspan of 102 inches and roughly 900 square inches of wing area. I built the wing in one piece, and I knew it would fit in my minivan. If the wing size is a concern, I’d make it with a one-piece, 48-inch center section and have plug-in tip panels, attaching music-wire joiners to brass or composite tubes. I like the looks of a pod-and-boom fuselage and you can get a fiberglass tube to use as a boom. I used 1/4-inch-thick basswood top and bottom pieces with 1/16 plywood sides for an easily built, all-wood boom. Using balsa sides and plywood doublers for the fuselage section to hold the radio gear, the electric motor, and the battery pack, provides an overall structure is more rugged than light weight. The motor I used—a BP Hobbies’ A2826-4 with a 14 x 8 folding propeller and a 4-cell LiPo battery pack—is more than is needed for relaxed flying. A smaller power setup could be used, but I enjoy the straight-up climb capability at times. A 2,500 mAh battery is plenty for climbing high, cutting the power, and looking for thermal lift. A pack of up to 4,000 mAh can be used if you want to count on longer flights without worrying about finding the elusive thermals.



To build this model, you’ll have to round up the materials you’ll need and cut up a copy of the plans for paper templates so you can trace and cut out the parts. I’ve been getting my balsa, basswood, and plywood from National Balsa. I took my time, cut out all the needed parts, and made a complete kit before I started construction. Then I made a number of changes while building the airplane, throwing some parts away. Hey it was the prototype! You won’t have that problem, because the plans now reflect the final version and I’m happy with it. Because you’ll be building your own model, if you want to make changes and have some different ideas to try, go ahead and make it the way you want. This is another advantage of scratch-building instead of buying an ARF. And if you ever need to make some repairs on the airframe, you’ll be able to make the spare parts you need yourself. I cut the plans sheet into pieces, and built the wing panels on a flat building surface. I put waxed paper over the plans to protect them. I laid the lower spar and the lower center section sheeting pieces down on the plans, and positioned the ribs over their locations on the plans. The opposite-side wing panels were built over the same plans to get the correct rib spacing, but with the ribs heading in the opposite direction. I used vertical-grain balsa spar webbing in the center panels only, and with the top spars, LEs and TEs glued in place, I removed the wing panels from the building surface. The panels were put together with the plywood dihedral and polyhedral joiners. I wrapped the center section joint with a strip of 6-inch fiberglass cloth and epoxy.


This model uses a simple, basic structure of balsa, basswood, and plywood. No exotic materials or techniques are required. It has a polyhedral wing and control is via rudder and elevator.



This view of the nose section shows the motor with folding propeller bolted in place. Cooling air-inlet holes are used in the removable hatch, which uses a spring-loaded latch.



There is plenty of room inside the fuselage for the battery and radio gear.

The tail surfaces were built up over the plans, and I added a little dihedral in the horizontal stabilizer so its tips wouldn’t catch on the ground during a landing. That was probably unnecessary, and the stabilizer would have been easier to build flat. I drilled and tapped the hardwood pieces in the stabilizer 1/4-20 for two nylon bolts to hold the tail assembly to the fuselage boom. The fuselage pod-and-boom sections are easily built up, and the boom glued into the holes in the pod formers. I put several cooling air inlet holes in the removable plywood hatch, which is held in place with a spring loaded latch mechanism. The strong magnets available today for this purpose could also be used. Nylon tubing-style pushrods to the elevators and rudder are inside the boom. The wing is held to the fuselage with the plywood tab at the LE and one 1/4-20 nylon bolt at the TE. The motor is bolted to the plywood firewall, with plenty of room for the ESC and any reasonably sized battery pack. The servos are installed under the wing position. You don’t need a towhook on the bottom of the fuselage unless you want a “pure” sailplane. Nylon pinned hinges are used on the elevators and rudder, and your favorite type of iron-on covering will do. I used a 60-amp ESC with built-in BEC circuitry to power the receiver and servos. I didn’t even think about any hand-launched test glides before the first flight. I simply powered it up and hand launched it. The airplane jumped out of my hand, and headed steeply up. I put some downthrust in the motor mounting before the next flight. My first flights were made in the late fall in cool and even cold weather, so I haven’t done much thermal soaring yet. I like the easy way the airplane flies, and will probably do my piloting from a comfortable lean-back chair in the summer when enjoying that sunny-day thermal activity. —Dick Sarpolus

The Thermix is designed to be easy to build and fly while paying respect to the sailplanes that influenced its design.




Type: Electric sailplane Wingspan: 103 inches Length: 55 inches Wing area: 970 square inches Weight: 64 ounces Wing loading: 10 ounces per square foot Power system: 700-plus-watt brushless motor; 60-amp ESC Propeller: 14 x 8 folding propeller Battery: Four-cell 2,500 to 4,000 mAh LiPo Radio: Four-channel; two servos Construction: Built-up balsa, plywood, and basswood



Frank Zaic autobiography www.modelaircraft.org/files/ZaicFrank.pdf BP Hobbies www.bphobbies.com


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The thermix looks like a great project. Being the lazy person I am with not a lot of building time available, I wonder if anybody else is thinking what a quick build it would be if there was a lazer cut wood kit available.

George, are you still interested in laser cut parts? If so I'm trying to see how many of us are. It looks like there may be 4 so far...you, one other original, plus Eliott Scott, and me. Once I know how many there are, and who they are, I'll send an email to everyone to see what everyone wants. At that point I plan to see what company is interested in doing the cutting for the lowest price. It'll take a little time, but should save each of us $$. Denis

I'm getting a pricing from National Balsa for this but am interested if anyone has been successful in getting laser cut estimates. I'm finding I need to do the legwork in getting a .DWG file for an estimate and have had no luck in finding if AMA Plans can provide this format. I've cut up the ribs and formers and am trying to get them sized correctly. Once that's done I'll have an estimate from that company for wing ribs, fuselage formers, and dihedral braces.

I would be interested in an estimate as well. Due to time, etc., the closer I can get to a kit - the better.

I also would be interested in a Kit.

Jim, are you still interested in laser cut parts? If so I'm trying to see how many of us are. It looks like there may be 4 so far...you, one other original (George), plus Eliott Scott, and me. Once I know how many there are, and who they are, I'll send an email to everyone to see what everyone wants. At that point I plan to see what company is interested in doing the cutting for the lowest price. It'll take a little time, but should save each of us $$.

Tried ordering two sets of plans, but the webpage is stuck at the "Review and Continue" step, impossible to tell whether my credit card information was received. Will just fill out the form in the issue and mail with a check, but I order plans frequently from MA and would like to see this problem addressed.

Just ordered the plans. If Dick says "it easy to build" it will be. I know everyone has a busy life and wants "instant gratification" Building, especially from scratch is very satisfying. Thanks Dick! Mike Rubin

Anyone ever put ailerons and or flaps on one of these??

I made a Thermic 36 in 4th grade in the'70s, my introduction to models more significant than sheet balsa tossers. I've been in love with the lines ever since. I later kept trying to build pod and boom fuselages but had very limited access to supplies and never managed to make a rigid yet light enough boom. I was a pre-teen so I don't judge the failure harshly and I had a lot of fun. That said, this design is like a Cupid's arrow right to my modeler's heart. I may change a few things (and more than likely find they don't work and use your exact approach in the end) because that's fun, but this is the first model since I've returned to the hobby that I just have to build out of traditional materials instead of dtf etc. And I am so looking forward to balsa dust instead of bits of foam! Thank you.

...I, too, would like to know how many are interested in a laser cut kit, and, once we know how many there are, what it would cost each of us. Let's say we give it a week...let's check in next Saturday, the 23rd. Anyone know Dick Sarpolus to see if he has had any laser cut kit inquires?

I cannot understand the drawing of the horizontal stabilizer. It calls for 3/8" balsa which establishes the thickness and then calls for 1/2" hardwood for the attaching stabilizer. Also, how are the hardwood pieces attached? I am also pondering the linkage and wondering how on attaches to the stabilizer through the boom as indicated in the drawing?

What i have planned to do is: Cut the 1/2 x 1/2 down to 3/8 x 1/2. I then glued the two pieces on the center section and glued the two front and back pieces to the hardwood. I made a cardboard pattern and marked the centers of the holes for the 1/4 thread. I have not really figured the Vertical Stab linkage as of today. I think i can get all three golden rods in the rear of the tube, just like they are in the front.

Here is what Dick had to say about the stab dimensions: . What I didn’t make clear enough on the plans, other than the note “optional stab dihedral 1 ¼” under each tip”, was that the ½”x ½” hardwood was needed because of the dihedral built into the stab. The thicker wood could be trimmed to the shallow vee shape of the dihedralled stab.

What did Dick do about the Elevator control hookups at the horizional stab? I can plainly see three control rods inside the fuselage. I have been studing on how to make the wires hook up on the stab. I'm just about to bend a connector wire to hook up buth sides of the stab together. Any info is greatly Appreciated.

Looks like only 2 of us are interested in combining for a possible 'package deal'. Anyone else interested??? Denis at [email protected].

I finally fogured out the control hookups. The two elevator control rods are on eack side of the fuse. The rudder comes out od the top of the fuse. The Horizional stab and elevator have diheadreal . the plans do not show this detail. I used some hardwood and cut it to the diheadreal and glued to the three sections of the stab. Does anyone know what the reccomended throws are for the rudder and elevator? I to would be interested in laser cut parts, either a short or full kit.

What type/size control rods did you use?

So far it looks like there may be 4 people interested in laser cut parts. Once I know how many there are, and who they are, I'll send an email to everyone to see what everyone wants...wing ribs, fuselage parts, etc. At that point I plan to see what company is interested in doing the cutting for the lowest price. It'll take a little time, but should save each of us $$. So, this is the last call...is there anyone else who would join us in an order? Denis

I am interested in a full or short kit. Is it too late to join in on this?

The last thing i'm having trouble with is the wing plate at the LE. The plan is not clear about this part. Does anyone have any ideas?

I am interested. Am I too late?

No, you are not too late. Tomorrow I am sending letters to Dick, as well as the AMA office to see who may have contacted them versus this blog. Joseph please send me an email directly to: [email protected] (Note there is but one 'n' in my name, not the usual 2). Once I get email addresses from each interested person, we'll be able to work out the details. Thanks for your interest and patience.

Communicating via this blog takes a lot longer than I ever expected. Sorry all. One way or another, the following people have expressed interest in a laser kit: George ____?____ (Expressed interest early. No word since) Eliott Scott (Sent me a direct email. Need confirmation...see below) Scott Hamm (Sent me a direct email. Need confirmation...see below) Jim Griffis Joseph J. Dziedzic Denis Kutch So, it looks like there could be 6 of us. Will each of you please send me an email directly to confirm interest AND provide a valid email address? I am sending emails directly to Dick and to AMA to see if they know of anyone else. Once we have a 'real' list,I'll send emails to everyone so we can work out the details. Thanks, Denis [email protected] PS It's not a mistake, there's only one 'n' in my name

I did finish my Scratch-build. It turned out looking good. I have not had time to test fly it . The wind here had been way too strong. It's supposed to be warmer this weekend and they are calling for less wind. I'll report back if i get to fly it. Scott

This blog may have lost its following, but if anyone out there wants to join the 4 of us already on board in getting a laser cut kit, this is your very last chance. If interested, send a direct email to me at [email protected] single 'n' in my name is correct.

Hello to all, Well the wind was perfact and partly clear skies today. My test pilot Earnie checked the controls and Aw-ed at the bird. I grabbed her up and headed to a clear part of the field. A full throttle takeoff and she was airborne and flying stable after trimming the elevelator slightly and a little harder trim on the rudder. She flew very well , but it took all the rudder could od to make a right turn. Earnie landed her after a long run about 1" off the ground for about 30ft. We looked and the tail section and noticed it was tilted, I loosened the screws and was able to find a scrap of balse and wedge it in. Back to another flawless takeoff with no adjust ments on the rudder at all and a slight trim on the elevelator. Now for the fun part. Straight up at full throttle to just about out of sight, Just as Dick described in his article. I am very well pleased with dicks contrubution to the hobby. Thanks Dick and the AMA for another fine job. No for the laser cut kit. If anyone is interesed you need to contact Denis ASAP. It's in the works. Scott Hamm

Hello to the Group, The thermix 13' kit is in the final stages. Currently we have 5 members who want a total of 6 kit's, me taking 2. There have been some changes made to the drawings that will make the kit more builder friendly and they will have more build options, such as 3 part wings. I'l like to personalyy like to thank, Roger Marshall ( Camden Custom Cutters in Camden SC), Denis Kutch and the others who have taken the time to make this possible and last but not least Dick Salous for his fine designed Plane and helping us in the approval of the drawing changes. While theres still time, If you'd like a kit contact Denis Kutch at: [email protected] Thanks to all who replied and hope you will order your kit as well. Scott Hamm

As Scott said in his May 26 posting, We've made quite a number of "builder friendly" modifications/improvements to Dick's fine basic design. The plans have been redrawn on CAD so rib spacing is consistent, PLUS both wings are shown! For a PDF of the revised plan and tex info about the changes, send an 'e' to [email protected]. Roger Marshall (Camden Custom Cutters in Camden SC) ordered enough material for 8 kits, but 6 are claimed, sold, committed...gone. So 2 are available, and more can be provided.

Are there any kits left, if so how much is it? is the main wing a single 103" or does it come apart - we don't all have mini-vans :) ?

Hi Richard, Yes, there are kits available. Hopefully, you were able to open the plan image I posted. We made a number of corrections and modifications to Dick's initial CAD drawing...actually re-drew them. The wing can be broken down to as few as 4 sections. I'm going to build mine this fall/winter as a 4' center section plus 2 outer sections--I drive a VW New Bug. The reason for that is I plan to add spoilers in the center section. The kit is available from Roger Marshall of Camden Custom Cutters for $115 + shipping. My CAD drafter redrew the plans, and I ask each builder to send him $25 for a share of his costs. Also, if you want to know more, send me an email, and I can reply with a copy of the info on everything we modified...Denis

I would also be interested in a kit...!

Jude, Did you ever contact Roger Marshall? Are you still wanting one of the kits? If you are interested, I can send you information about the differences between 13.1 and 13.2 before you commit to buying. Denis

As a former model builder and flyer ( Playboy with Ohl I was very interested

Frank, It's been months since I checked this blog. Did you ever contact Roger Marshall? Are you still wanting one of the kits? If you are interested, I can send you information about the differences between 13.1 and 13.2 before you commit to buying. Denis

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