Thermic 70" Adrián Muiño's Build Log

Hello, my name is Adrián Muiño, and I am going to log the building of the “Thermic 70” kit that has just arrived.

I want to thanks Aerosente for the opportunity, and start to work as soon as possible.

 

The Thermic is a vintage free flight model, and what I got was exactly this, even the plans are the original.

 So, there are two modifications that I am going to do and record in this building log:

• it will be electrically powered

• a 3 channel radio control will be added

But none of these changes should spoil the essence of this classic model that will pass from 350g to 550g.

Now it is time to go from planning to execution. Prepare a flat and levered surface, put the plan over it, and cover it with a nylon sheet. The tools and materials needed will be: 30~45 minute epoxy, thin CA adhesive, a hobby knife, a drill, screw drivers, pins, masking tape, a ruler, a squarer, an iron, 1mm and 4mm balsa sheet, 1/8 balsa sticks, and sandpaper.

 

Horizontal stab

 I will start with the horizontal stab. I let the fuse and wing to the end because they are the hardest part.

 Laser cut balsa makes this job enjoyable and fast, but you still need to pay attention to the work before cutting and gluing.

 Properly position the trailing edge (TE) and the spar over the protected plan, place the outer ribs and glue them using CA. Use a square to keep the ribs perpendicular to the work surface and to the TE. To keep the TE and spar placed, use pins. The plain side of the spar goes down, and everything must touch entirely the table before gluing.

 

In the kit I got, the numbers marked in the stab ribs are not the same that in the plan, and the rib #2 needed some work to make it fit, but not big deal.

 Make the leading edge (LE) cutting the 1/8 x 1/8 stick, fix it and sand it. Sand the tips.

 The TE has to be worked preferable with a razor and sandpaper to finish it. Be careful and take your time, because is easy to break the ribs.

In my case the stab will act as elevator, i.e. the whole surface will have movement; I will make it with 3 CF pivot tubes at the spar and a carbon fiber (CF) rod in the front, as old sailplanes use to have, allowing to transport the stab in halves. Two extra ribs are needed, cut them the same size as the inner ones have. The parts will be held together using a silicon tube to fasten the front rod. But I will let this modification to the end.

In my case the stab will act as elevator, i.e. the whole surface will have movement; I will make it with a 6mm carbon fiber (CF) tube tied and glued to the spar plus and inner CF rod attached to the fuse.

A 2mm CF rod in the front completes the modification.

This method was used in old sailplanes, allowing the transportation of the stab in halves. Two extra ribs are needed; cut them the same size as the inner ones have. The parts will be held together using a silicon tube to fasten the front rod, it was placed inside the structure.

Fin and rudder

 The fin has a symmetrical airfoil, so it cannot be build putting the ribs over the table. therefore, first I made the outer frame adding the bottom part (3x3 mm stick). Then I glue the ribs, then the inner “spars” and finally the lower front part of the fin. The result was a very solid structure.

The original rudder was cut in 3mm hard and heavy balsa, I replace it with 2mm light balsa, adding 10mm to increase its area. I used the original rudder as a mold. The rudder was made jointing balsa with the grain oriented in two different directions to make it less flexible.

Use a center marking tool to make a centerline on the TE of the fin and on the LE of the rudder. Before gluing the hinges, I am going to use CA hinges, temporarily join the rudder to the fin, verify if everything is aligned and adjusts whatever is needed. If everything is ok, use a pin to help to center the CA hinges. In the zones where the hinges are placed, I harden the balsa with CA.

Tip: if it is possible, use “hinges glue” instead of CA, it is easy to apply and give some time for corrections.

The wing

 I am going to build the wing as the plane states only reinforcing the central part of the spars. Why? because the model will be notoriously heavier and electrically powered.

The first job is to solve the puzzle, finding and placing every part. Once you are sure that everything is ok you can proceed. I started with the central part. I fixed TE and started to work aligning ribs and spars with a square, and not using pins over the plan that is not very accurate.

I have to make the central main spars again because the ones provided were not straight (they are 1/8) and they were made in very soft balsa. My joint-braces are 1mm higher than the ribs “holes” and the spar, and then I had to modify the #1 and #2 ribs to place them. I also had to make the rib #15 because there was only one in the kit. Check it before gluing anything.

I glued the external ribs first (#9 and #3, letting the two inner ones to the end), and once everything was squared, I followed up with the rest. Glue the ribs using first CA and then with aliphatic resin in the joints, let it dry. Everything has to be touching entirely the table, except the inner ribs that are smaller to allow the 1mm balsa sheeting. The LE was left to the end, and the work over it is the same done in the stab.

The TE has to be worked before joining the 4 wing parts, preferable with a razor and sandpaper to finish it. Again, be careful and take your time, because is easy to break the ribs.

The reinforcement was made between the two spars adding an “X” structure in the inner haft of each wing.

Tip: good-fitting joints are the clue for a light and yet strong frame, do not try to substitute them with glue. A big ball of epoxy is not better than a small quantity smartly applied.

I started the wing extremes by the tip, take your time to find and place the parts, they are not numbered. In my kit I have to add some balsa to one of the wing tips. Sand the LE, the TE and the tip.

And they are almost done...

 

When joining the 4 parts, the most important thing is to have them with the same incidence. To do so I use a ruler over a flat surface keeping the spars parallel to the table in both parts. Use 45 minutes epoxy give you time to maneuver. Let the epoxy dry a night.

The fuselage

The original model had not servos, battery or motor, so the fuse has to be modified to hold all this stuff and deal with the subsequent stress. Consider that the original fuselage weight was around 150g, and now it will weight in the order of 300g, so their strength has to be improved.

I started doing the sides, witch is easy but take times, use the hardest 3x3mm balsa you can get to do it, and make sharp and accurate cuts. 

F1, F1A, F2 and F2A have to be replaced to make room for the motor, so do not glue them. 

Once you have the sides done, join the extremes with marking tape and start gluing the central formers, take your time to align them well. Remove the tape from the nose once you get near it.

Unfinished but still very good looking…

Important note: The flat part of the wing in the intrados, must be levered with the lower part of the stab, it will give 1° of incidence, to do so, you will need to adjust the wing mounting. The motor must be 3° negative compared with the stab.

 

I went with the simplest way to hold the extra stuff, adding a sandwich of 1mm balsa sheet and 6mm foam in the center as holder and as main structure, a method that I use in several of my own designs. 3mm balsa can be used too. Here is a plan of the development, do it 2mm bigger and sand the excess.

There are several commercial and non-commercial ways to mount the motor.

I made it with 3mm plywood glue with epoxy and glass fiber.

The photo shows where the motor will be placed.

This is another option (not used in this case). Mounting the motor in the “modern” way, but you will need to make a cowl and the CG will be even more advanced.

The new structure is embedded in the fuselage.

It will not be seen once it will be covered.

I added an extra support for the wing.

The servos will be placed in the exterior of the rear part of the fuse. Just fix 3x3 balsa sticks to the fuse surrounding the place where the servo will be placed. Use the servo as a guide.

The stab will pivot over an embedded CF tube, attached to the fuse with two balsa planks drilled in their centers. Take your time to align this little CF tube and to glue correctly, it is a precision job.

Getting all together

 Here it is how it is going...

And one more...

 

I also made a window to access the inner stuff. 

This is the wing mounting, remember to check the incidences!

The power plant

 Where and how to install the electric motor?

I was temped to put the motor as pusher in a cabana over the wing, but I am worry about the effect of an elevated CG. I have a 3 channels powered-glider made this way and it is hard to recover from a relative high bank angle. I have seen several models with .0xx Cox engines mounted this way. Just one recommendation if you want to go this way, buy a motor that allows you to use a small prop, i.e. a high Kv motor, for example for 11.1v packs pick one with 1500rpm/V.

But I will install the motor in the nose with a folding prop and a spin, witch is aerodynamically and statically better.

It will move a folder propeller placed in a spinner that has almost the size of the nose, and that allow installing the motor in the conventional way (i.e. with the firewall between the prop and the motor, saving a lot of work because no cowl is required. The fresh air will not hit the motor, the Lipo and the ESC, but as they are over specified and the BEC is switching, I am not worry.

Please refer to the following link to see why I choose the AXI 2212-20, the Jeti Spin 22 ESC and the Polyquest XQ 11.1v 700mAhr 30C.

 http://www.rcgroups.com/forums/showthread.php?t=1103369#post12995341

Just to re-check, I ask a friend, fanatic of Cox, and he told me that a standard .049 Baby Bee can spin a 7x4 at 10,500~11,000rpm, and this was the kind of motor used in 2m sailplanes. My AXI will move effortless an 8x4 will be at 9,900rpm.

Note: in fact the metering made over the PQ + Jeti + AXI + Robbe prop show RPMs well over12,000, woww!

If the wind conditions are good, a smaller power plant can be used, e.g. AXI 2208-26, Jeti Advance Plus 12A and Polyquest 7.4v 1200mA XQ, or the same motor used but with a 7.4v lipo and a 10x6 prop.

Some tips for the electric motor installation:

* Keep the receiver as far as possible from the power plant

* Keep the motor and ESC cables short

* Let the system breath, letting the air pass thought it

* Use friendly plugs; to fight with the plugs inside this fragile model can be catastrophic

 The negative incidence of the motor is critical for a comfortable flight, and it has to be fine-tuned during the first tests. I started at –5° and was ok with the 8x5.

The radio gear

 My servos weight 9g and have 2kgcm of torque. I strongly recommend using good quality servos, particularly for the elevator. Cheap servos will not center perfectly making your thumb work a lot. For safety reasons I pre-tested them before the installation using the cycling feature of the Jeti Box, cool!

 Before installing the servos, as I said I verified the CG, and I realized that this electric Thermic was nose heavy. So, I will install the servos in the tail near the moving surfaces, something that I like because allows short and precise links, and an easy mounting.

 

The 2.4Ghz Assan receiver does not have any particular requirement for its installation, except for keeping it far from the power plant as any receivers. I attached it with Velcro. My version has the short antenna, so no annoying cables will be hanging from the model.

The covering

 I like to add a light hand of dope after everything was glued. To do so, I made it adding alcohol to the universal cement, avoiding this way a trip to the hobby shop and using 100% of the dope made. After this step, a light sanding is advisable.

 Regarding the covering, on one hand there is the traditional coverings, like tissue or silkpan. They are light, they look old-fashioned, but awfully they are weak, and keen to be broken during transportation and in landings.

 On the other hand thermal covering will add strength. There are at least two options, the common covering and the light one. I like the transparent ones, but my problem is that I do not like to work with either of them. Finally, some people use Maylar, or some other no traditional coverings.

 Here there are some figures:

 Monokote (Tan)            5.6gm/sq ft

Coverite Micafilm 3.9 gm/sq ft

Dope over silk              2.0gm/sq ft

Japanese Tissue  0.65gm/sq ft

 I use the alternative available, regular thermal covering. According to my calculations it will add 20g more than the light one. Anyway my advice is to use light thermal covering.

 Before doing anything else, drill 1.5mm holes in all the internal balsa sticks and in the trailing edge of the stab and fin. Do the same with the ribs of the wing. The holes will allow the inner air to escape, making easier the covering.

 First, try to cover some useless balsa to adjust the temperature of the iron. Cover the parts in this order: the wing is the easier part, first cover the bottom of central part and then the top. Later cover the bottom of the tips, and then the top. Make sure that the covering is not making uneven tensions over the wing. I use a heat gun to end the job.

 In tail surfaces, first cover the bottom that will cover the leading edges and end with the top. These parts are small, so the film must be well stretch to achieve a perfect termination.

 I made a hatch in the side of the fuselage to place the lipo. The covering will acts as hinges and rare earth magnets will keep it closed.

I made the canopy with a pop bottle; I sanded it and painted it with acrylic.

 The fuselage is the hardest part to cover; a friend of mine told me to do it side by side, there are 4 parts. If you do it with a single sheet of covering from the top to the bottom, or in hafts, the structure can be damage when the covering got its final size. I started with the upper part, then the lower part, and finally the sides.

I really recommend to use light covering, regular covering is too much for this fragile structure

Final arrangements

The attachment of the wing was made with rubbers; it allows some fine-tuning to the CG if the movement of the lipo is not enough. The rods are fixed to the inner sandwich structure, conforming a strong and yet light cell.

I used 45 minutes epoxy to glue the fin to the fuse. Before the epoxy hardens, use a square to verify if the fin is perpendicular to the stab. Use masking tape to temporally fix the fin until the epoxy dried

  When mounting the control horns, use them as a guide to drill the moving surface. The horn holes must be in the same line that the hinges are to avoid unwanted differential effect.

 For the first flight, I aligned the stab with the wing levering both flat bottom surfaces (just turn the plane and put a level over the stab and another level over the wing, then fix the servo link in the desired position).

Programming

I programmed my loved Futaba 7UAF that was revamped with the ASSAN 2.4Ghz module and the X8R7L receiver. Here are the throws:

Elevator: +/- 12mm; dual rate at 70%; exponential –40%

Rudder: +/- 20mm; dual rate at 85%; exponential –20%

Throttle: set the lowest and highest point in the ESC

If you radio do not have dual rate, use the 85% of the throw shown

I mixed 4% throttle with stab, lowering it of course

A very welcomed characteristic of the ASSAN 2.4Ghz conversion is that I can keep the same radio and memories, and use it as if it were using 72Mhz module. Furthermore, the airborne time is not limited by the use of the frequency, and the transmitter battery consumption is at least 50% of the original 72Mhz. Cool!

For the Jeti Spin 22 ESC I choose either “Glider + Outrunner” mode or set:

# of Lipo cells: 3

Timing: 22°

Brake: on / soft

Cut of voltage: 3V per cell

Cut off mode: reduce power

Acceleration: 1.5 sec

Throttle curve: logarithmical

Frequency: 8Khz

ESC thermal protection: 80°C

We are almost done,

The lateral balance of the model, can be set lifting the model by the prop shaft and the bottom of the fuselage. Move the lipo pack or add lead in the tip of the light wing.

The C.G. must be located at near the 35% of the wing chord at the center. The measurement must be done with everything on board (ready-to-fly condition). The final adjustment can be easily done moving the Lipo pack, that should be near the motor.

Tip: make a mark to point out where the lipo must be placed; a little change in its position can have a great impact in this light model.

Check each control direction, throw, and if they are centered. The eventual changes must be done over the links and not over the radio trims. Check the programming of the ESC and charge the Lipo.

In the field

Butterflies in the stomach and a lot of expectation surround the maiden day.

But one test is still needed, without the prop check the radio’s operational range with the motor running.

Want to know how it flight?...

It did it deliciously from the very beginning!!!

To know more about the flight performance you can go to this thread:

http://www.rcgroups.com/forums/showthread.php?t=1120820#post13261897

For further adjustments, here is a guide to fine-tune the CG:

http://www.rcgroups.com/forums/showthread.php?t=1070608

I hope this log helps and encourages new modelers to enjoy the building work and feel the peaceful flights of this classic recreated by Aerosente. I am looking forward new challenges as beta builder and test pilot.

Good thermals!
Adrián Muiño

Notes about the author

Adrián Muiño has almost 30 year of experience in the hobby, and he has written tenths of articles for magazines such as Model Airplane News (USA), RC Model (Spain), and Pegaso (Argentine). Today he is specialized in electric flight.
He also is Electronic Engineer, has an MBA, is certified Project Manager Professional and teaches at one of the most prestigious universities in Argentine.

Comments

[Tom Martin]

Great workmanship Adrian. You've taken great images and your descriptions of the steps you've taken are excellent. Very helpful!

I can't wait to see the fully framed fuselage. It's going to be beautiful.
Tom

[german]

Congratulations! for your work!! beautiful images.
Germán,MM y L

[Gregg Kirkwood]

Looks great Adrian. My kit has arrived and will also be electric powered. I have finished and flown a scratch-built Schweizer SGS 1-34 glider - one of Tom's plans and I look forward to building this project.

Gregg
Dubbo, Australia

[Gregg Kirkwood]

My Thermic 70 flew its maiden today and what a brilliant glider! It flew straight from the hand and only required 2 clicks of up elevator to hold on trim. Total weight was 800grams and what was different with mine is that I fully sheeted the main wing and vertical fin, The tailplane is all moving and I fitted a retract main wheel.
Power came from a Turnigy C2826-1350 motor with 18amp ESC and a Turnigy Nano-tech 1800mAh battery. I fitted a 10x6 folding propellor and at no stage during the flight did I need any more than half throttle.
The kit, as expected was great and all material supplied. It was a real pleasure to build!

[Marco A Q Teixeira]

Can I add a conventional landing gear in a glider?

[Adrián Muiño]

Yes, you can add a light one.

But I don't see the need to do so.