Thermal brake: ViscoDT vs. glow cord

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On my P30 models, the thermal brake has been triggered using a glow line up until now. A tried-and-true and fairly safe method.

I've often read reports and watched videos about ViscoDTs. In these systems, a viscous mass slows down the drum as it rotates. The drive is provided by a rubber band or a spring in the release cord, which is attached to the ViscoDT on one end and to the tailplane on the other.

I joined a group order from gummimotor.ch and ordered five ViscoDTs. One of them was installed and tested in the Modell Contester last summer after I received it.

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The DT was glued to the side of the fuselage. Small brass and copper tubes serve as guides for the release cords made of fishing line. A small plastic bead was slid onto each cord and threaded through the opening twice. Loops were tied at the ends of these release cords. One of these loops is hooked onto the rear wing, and the rubber band is tied to the second. The plastic bead can now be used to adjust the angle (35 degrees) at which the rear wing folds up.
The rubber band also has a loop, which is hooked onto the wire of the DT. The length of the rubber band is such that the rear wing rests on the support even with minimal tension. Now it is ready to be launched.

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I wasn't satisfied with the results (release time/flight time, smooth deployment, etc.), so I looked for solutions. First, I shortened the wire pin so that the loop of the rubber band was just barely hooked on and released cleanly after deployment. Further tests were conducted last fall using this setup.

A second model was equipped with such a ViscoDT: Secundus. The setup was the same, except that the rubber band is not attached to one end of the release string but in the middle of it. To obtain meaningful data, the rubber band is the same length as on the Contester model.

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This allows for more revolutions on the drum compared to the rubber band (max. 5 revolutions), which will hopefully result in a shorter release time/flight time. In addition, the fishing line unwinds more smoothly and evenly, which is not guaranteed with the rubber band (despite lubrication).

The trigger times (measured manually with a stopwatch) were then determined under dry conditions. The results are shown in the two tables.

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Contester (Rubber Band)
Revolutions DT    Time to release / Approx. flight time
       1                     36 sec. 35
       2                   1 min. 03 sec. 76
       3                   1 min. 19 sec. 92
       4                   1 min. 25 sec. 94
       5                   1 min. 37 sec. 54
       6                 Rubber slips off the drum

Secundus (fishing line)
Revolutions DT    Time to release / approx. flight time
       1                     49 sec. 85
       2                     50 sec. 14
       3                   1 min. 10 sec. 68
       4                   1 min. 25 sec. 18
       5                   1 min. 41 sec. 85
       6                   1 min. 59 sec. 16

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Week of March 9, 2026: Beautiful, calm weather with pleasant temperatures already in the morning. Thermal activity began to form as early as mid-morning! Flight altitudes! We have a flight altitude restriction at the model airfield due to its proximity to the regional airport.
Ideal test conditions. Power unit: 10-gram FAI Rubber TAN Super Sport 1/8" with 1,000 RPM. The release times achieved in flight matched quite well with those measured on dry ground. The thermal brake released without any problems. The models drifted to the ground like a withered leaf.
After that, test flights using the glow line to trigger the thermal brake. With the glow plug used, 1 cm results in a burn time of approximately 55 seconds. Triggering worked without any problems.
Yes—the flight altitude of 150 meters was exceeded a few times. That's just unavoidable in free flight, since once the model is released, it's no longer possible to control it.

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Initial conclusion: The ViscoDTs are perfectly adequate for casual everyday flying. For competition use, I'll continue to rely on the glimmer line, as it ensures that the specified flight times can be reliably achieved. For this reason, the spring steel hooks on the fuselage and tail wing will remain in place.

Peter

[calgoddard]

Peter -

You need to make two critical changes to your viscous damper de-thermalizer (DT) system.

1) decouple the pulling force on the viscous damper from the stab as shown here:

   https://www.youtube.com/watch?v=Fdr_uRYuYCE

2) use a coil spring made of .009-inch music wire instead of a rubber band to pull on the viscous damper arm. The pull force of a coil spring is far more consistent than the pull force of a rubber band.

I have not been able to convert a commercial viscous damper sold as a conventional part for adequate use in a DT system because the arm has insufficient resistance. The damper shown in my video is a purpose-built damper called a Badge Classic which used to be sold by FAI Model Supply. It is no longer being manufactured.

I suggest you use a radio dethermalizer (RDT) from BMK of the UK if you want true reliability and accuracy. It only adds 3 grams with its LiPo battery. You push a button on a hand-held transmitter when you want to terminate a flight. Alternatively, you can use an electronic band burner timer from BMK into which you program a precise DT time.

We are not allowed to use fuse timers in the western United States due to wild fire danger. If you are satisfied with your fuse DT, keep using it in green Switzerland but be aware that I understand manufacture of DT fuse has been discontinued.

[calgoddard]

Peter -

See my prior post.

Two pictures are attached.

The first picture shows a viscous damper DT with a mousetrap on one of my Three Nite P-30 models. Its structure and operation are explained in the video I referenced in my prior post. There is a YouTube link to the video in my prior post. The viscous damper is a Badge Classic damper designed specifically for use in free flight model airplane DT systems.

The second picture shows the installation of a BMK RDT receiver on my Three Nite P-20 model. The RDT circuit board has a hole formed in its left end through which a small CF post extends. The CF post is glued in the left side of the fuselage with CA. A dental rubber band on the forward end of the DT line is stretched over a Nichrome wire on the RDT circuit board and secured around a pin on the circuit board. When you push the button on the BMK transmitter, the signal is transmitted to the RDT circuit board through the tiny whisker antenna. After rapid processing, the Nichrome wire is heated up and severs the dental band. This releases the DT line and allows the rubber band at the tail end of the fuselage (not visible in the picture) to pivot the trailing edge of the stab upwardly The RDT circuit board and the LiPo battery are each secured to the fuselage with a No. 10 rubber band that wraps around the fuselage. This arrangement allows easy transfer of the RDT and LiPo battery to a different model.

The advantage of RDT is that you can terminate a flight once a max has been achieved. You can also terminate a flight if the model begins to head for trouble, like a grove of trees. You can also terminate a flight if the model starts behaving badly due to some improper adjustment or installation, such as inserting the nose block upside down resulting in up-thrust. You can also terminate a flight early if you are just making trim adjustments to improve the climb phase under full launch torque  and you don't want to wait for a full flight to end before retrieving the model and making another adjustment.

Sometimes I just rely on the RDT and don't also install an RF locator beacon or a GPS locator beacon (and their additional battery or batteries) on the model to save weight. A P-30 is big enough to carry both the RDT and locator and batteries without too much degradation in potential flight duration. This is not true with smaller models such as a P-20.

[calgoddard]

Peter -

You might have more consistent results with the viscous damper sold by Ikara compared to the viscous damper you are using. Here is a link to this part:

http://www.ikara.eu/index.php?nid=6259&lid=en&oid=953456

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Mike
Thank you very much for your answers and comments.

The "green Switzerland" you mentioned isn't actually that green in the summer anymore. Due to the heat waves and droughts in recent years, regional fire bans have been issued time and again, and the trend is on the rise.

Regarding the launch spring: Do you make these yourself, and what kind of flight times do your models achieve?

Thanks also for the link. I'll ask my Czech friend Stan to bring one of those DTs back this summer.

Best regards,
Peter

[calgoddard]

Peter -

I buy the DT coil springs that I use. This video shows how you can make them yourself:

https://www.google.com/search?q=coil+spring+winding+mandrel+tool+.009+music+wire+volare&sca_esv=9a3fab58edaf9218&biw=1280&bih=585&sxsrf=ANbL-n4e90kgMNBP1M2rZNcFGFVmNxrVNw%3A1773678277555&ei=xS64abnHIaT0kPIP8LbN6A8&ved=0ahUKEwj5vbe96qSTAxUkOkQIHXBbE_04ChDh1QMIEQ&uact=5&oq=coil+spring+winding+mandrel+tool+.009+music+wire+volare&gs_lp=Egxnd3Mtd2l6LXNlcnAiN2NvaWwgc3ByaW5nIHdpbmRpbmcgbWFuZHJlbCB0b29sIC4wMDkgbXVzaWMgd2lyZSB2b2xhcmUyBRAhGKABMgUQIRigATIFECEYoAEyBRAhGKABSMg5UPILWKcycAF4AZABAJgBiQGgAdISqgEEOS4xNLgBA8gBAPgBAZgCGKACwBPCAgoQABiwAxjWBBhHwgIFECEYqwKYAwCIBgGQBgiSBwQ4LjE2oAfzjgGyBwQ3LjE2uAe8E8IHBjAuMTcuN8gHPYAIAA&sclient=gws-wiz-serp#fpstate=ive&vld=cid:f62a663e,vid:X5TiqdEd7mM,st:0

Wow - that's a crazy long link. If it does not work, search for this video by Maxfliart on YouTube:

Mini Coil Spring Winder Tool - For DT Springs

I have the Volare Products spring winder tool shown at the end of the video, but  it is just easier for me to purchase 10-20 coil springs at a time from glider expert Stan Buddenbohm.

I have over ten flyable P-30 models from low tech to high tech, all but one of which I built myself. I tend to fly one of my Three Nite P-30 models in contests since I designed it myself. Usually it will max, i.e., achieve a flight over 120 seconds unless it gets caught in down air (outside of a thermal). I have achieved flights over 3 minutes in a flyoff, the longest being around 5 minutes. 

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Mike
I have another question about springs. How long are the springs you use? I found a manufacturer here where I can order springs made from 0.3 mm spring steel wire. I think this is a good alternative to the rubber bands I'm currently using.
I'd have to order the electronic parts from abroad again, since there are no suppliers among the model shops in Switzerland.

Peter

[calgoddard]

Peter -

See the attached pictures.

The coil spring I use in my DT systems has a loop at each end. It is just short of 3 cm long. The outer diameter of this coil spring is 2.92 mm.

This coil spring is wound from .009-inch music wire that converts to about 0.23 mm.  I am not sure but 0.3 mm steel wire may be too large for a DT spring.

The easiest way to purchase .009-inch wire is to purchase the high E string for an electric guitar.

I am not sure why, but in the US small gauge steel wire is typically referred to as music wire. Maybe this is because it is used for the strings of an electric guitar.

Don't use a rubber band or a segment of elastic cord to provide the pulling force on the damper. It will fatigue with each stretch and will not give you predictable DT trigger times. Also, a rubber band will degrade from exposure to UV from the sunlight. The same is true for elastic cord.  If not over-stretched, a steel coil spring will exhibit the same pulling force each time it is stretched the same amount, even if the coil spring is stretched thousands of times.

[Stunthenk]

In Europe it is called pianowire. Definitely musical origen!