DTs – Dethermalizers – Mysterious? – Complicated! – are you READY?

Dethermalizers – the Next Big Step in your Free Flight Evolution.  Beginners often ask a lot of questions, especially on the field while watching.  One that I often answer in the negative is “do I need a DT?” – or – “what stuff do I need for a DT?”  What I tell beginners is “until you start building, trimming, and flying well enough that your models are repeatedly flying away, you do NOT need to complicate your modeling with DTs.  Come back when you are having models fly away and I’ll help you out.”

Why do I say this?  Because the beginner needs to learn to build, trim, and fly.  There is no sense in putting a DT on a model that isn’t trimmed to fly for more than 30 seconds.  Those three initial skills need to be developed to the point that the model is capable of flying OOS before the modeler needs to worry about how to install and operate a DT.  Until that time, it is an unnecessary complication.  I flew for a couple of decades before I started installing DTs.  And since then, I’ve put on plenty of DTs that don’t get much action (because the plane isn’t up to snuff, yet).

So, bunkie, you lost your plane?  You’ve lost several?  Then you need the Salvation of Free Flight – the DETHERMALIZER!  (No, not the Korda Dethermalizer OT Cabin – Class D 1941 Nats Winner – WITH a DT!)  A properly working (hopefully #1) DT will disturb the balance of your perfectly soaring model and it will no longer be able to soar efficiently and it will be kicked out of the thermal and descend to the ground (hopefully #2) and save your model (hopefully #3).

Hopefully #1 – DT’s, regardless of their type (see below), can malfunction.  During the construction of your model and installation of the DT, you will want to ensure that the operation of the timer and of the release are smooth and glitch-free.

Hopefully #2 – Sometimes, properly functioning DTs are overcome by the strength of the thermal and they sail off Out-Of-Sight, regardless of how smooth and perfect the operation.  A different style of DT action (see below) might have saved the model.  Also, “where” the model comes down may prohibit retrieval – maybe it is stuck high in a tree, or in the middle of a lake, of somewhere else where you cannot find it.

Hopefully #3 – sometimes the model will descend to the ground, but because of the type of DT action or some freak occurrence (tangled lines, etc.), the model will suffer damage when it hits the ground – or whatever it hits.

Dethermalizers have been around since the late ’30s or early ’40s.  As mentioned above, Dick Korda named a model he designed “Dethermalizer”, possibly because he integrated a DT into the design.  This article won’t be so much of a How-To, but more of a discussion the types of Actuators and the types of Actions.  All of the DTs that I know of have an Actuator that control when and how the DT Action takes place.  And the model design incorporates a method of Action.


Actuators are the things that count down the time and trigger the Action.  Count down the time?  Yes – you want the DT to operate after a set time and not before.  This time will be determined by YOU, the Builder and Flyer.  Most of the time, I set my TIME based on the maximum official flight time (most often – 2 minutes).  Sometimes, I will set the time shorter and this decision is based on the field conditions – maybe it is a small field, or a particularly windy day and 2 minutes will carry the model off the field.

Regardless, all types of Actuators are basically switches that, after an assigned time, the “switch” is activated and the Action is initiated.  I will now discuss the various types of mechanisms that we most often see on the flying field (I won’t be discussing the antique Austin timers, although they still work).  Here are the types I will be discussing:  Burning Fuse, Mechanical/Clock, Viscous/Button, Electronic.


This is one of the simplest and possibly most reliable DTs that we have.  It is basically a short piece of cotton cording that has been impregnated with a chemical that burns easily.  This fuse is lit with some sort of lighter (my favorite is an electronic, rechargeable, arcing-plasma lighter).  The fuse fits snugly into an aluminum tubing with a short length extended.  Generally a small rubber band is lightly wrapped around the fuse close to the tubing and as the fuse smoulders and burns down, it will contact the rubber band and melt or burn it and the release of the rubber band triggers the Action.  The snug installation into the aluminum tubing will extinguish the burning fuse.

a Burning Fuse installed on the tail of an Embryo model. The length of fuse from the exposed end to the small rubber band regulates the amount of time before the Action is triggered.

The SWITCH:  burning through a rubber band that is preventing the ACTION

Benefits of the Fuse Type:
– cheap
– installation is generally simple
– it is relatively light weight
– it is generally very reliable

Downsides of the Fuse Type:
– they are dependent on an igniter – a lighter of some sort – and electronic ones need to be charged.
– setting exact time is a guessing game as the rate of burn can vary with conditions
– if the fuse is damp or the day is rainy, the fuse may not light or stay lit
– a poor start of the burn can extinguish and result in no DT
– FIRE – these use live fire and they can start fires
– some modelers have burnt their models while lighting the fuse
– if the model lands before the desired time, the fuse/fire could light the grass on fire
– as a fire hazard, this type is PROHIBITED in some locations

Overall, these work well.  If the fuse is lit properly, it WILL burn down at a regular rate and it will self-extinguish.  Just make sure your rubber band is tight against the fuse and your Action is positive and you will have a light-weight, reliable Actuator.


These are called “Clock” types because of the way they work:  you wind a spring and it unwinds; just like an old-fashioned clock or watch.  There are commercial versions or you can make your own.  Being “clocks”, they have a very reliable countdown.  The spring is regulated somehow to slow the unwinding of the spring.  There are some very complicated multi-function Clock Timers.  These are used to manage several aspects of the model’s performance:  engine shut-off, variable incidence tails, DT action.  The ones shown here are simple, single-function timers.

Mechanical/Clock DTs. In the back are examples of a Commercially-produced single-function Mechanical DT. In the front is a home-made one. The arms with weights are added to regulate the rate at which the spring unwinds.

The SWITCH: usually the unwinding and release of a line that is preventing the ACTION

Benefits of the Mechanical/Clock Type
– these are very reliable – they count down at a constant rate.

Downsides of the Mechanical/Clock Type
– they are often too heavy for small models
– commercial versions are becoming very hard to find
– homemade versions take a little bit of experimentation to get them to work right
– homemade version require a wind-up toy – not all wind-up toys have the right clock
– they can get dirt in them
– I have had them “not work” but that is probably lack of skill in construction or setting


Sometimes these re referred to as “Button” timers – Bob Munson perfected the viscous timer and had a Button (small) and a Badge (larger) version.  Homemade versions usually are made from rotary dampers that are found in various automotive applications:  slow opening compartment doors, etc are regulated by viscous rotary dampers.  All of these work in the same manner:  they have a chamber with an oil or compound.  In that compound is a “paddle” that is commented to a rotating shaft.  The fluid or goo resists the rotation of the shaft and this is usually at a fairly constant rate.

Munson Button and Badge Viscous Timers.

a homemade Viscous DT Kit that used a commercial rotary damper


There is another homemade viscous type that is made with aluminum tubing and Silly Putty.  These can be made with a little work.  You can find instructions HERE (PDF in the FAC Library).  There is an early version where a “trench” in the glider fuselage was filled with Silly Putty and a wire dragged through it.  I cannot find a link to that right now.  There are several how-to’s online regarding the tube/Putty versions.

The SWITCH: usually the unwinding and release of a line that is preventing the ACTION

Benefits to the Viscous/Button Type
– they are lightweight
– the original Munson timers were very reliable (current version, not so much)
– homemade versions are cheap to make and material is available

Downsides to the Viscous/Button Type
– some are temperature-sensitive
– all are dependent on a constant rate-of-pull (rubber band, spring, etc.)
– pull sources do not always have a constant rate of pull.


Some electronic gurus have been able to produce electronic countdown timers that are light weight and programmable.  The results are a relatively light appliance that is very reliable and very accurate – to 10 seconds; some are to the single second.  There are now various products, some with multi-function, some are band burners, some trigger micro servos, and some with battery chargers built in.

one type of commercially available Electronic DT

The SWITCH: burning a rubber band or activating a servo that is preventing the ACTION

Benefits to the Electronic Type
– very reliable and very accurate
– relatively light weight
– installed properly, they can be swappable between models.

Downsides to the Viscous/Button Type
– some are very expensive
– all require programming that can be complicated
– all require on-board micro-batteries that need charging
– the batteries are so small (capacity) special chargers should be used
– charging on the field requires a power supply
– band burners may break the thin ni-chrome wire
– band burners require light rubber bands that WILL burn with the short hot pulse

All of the types listed above do one thing:  they provide a predictable delay from the start to the time the “switch” initiates the ACTION.

Types of ACTIONS

The ACTION is what takes place once the “switch” is thrown.  These Actions can take place on the Tail, or on the Wing, or Other locations.


Probably the most common is the Tail DT where the horizontal stab is hinged.  Typically a fine line is run from the tail, along the side of the fuselage, to be connected to the DT.  When the DT activates, the line is released and the tail, hinged at the front (usually), pops up (due to spring loading or a rubber band pulling) to about 45 degrees – like up elevator to an extreme.  This will cause a nose-up condition in flight that balances out and the model sinks slowly and flatly to the ground.

The Tail-type is probably the easiest to install and operate, although there has to be enough tension in the line to keep the tail in flying position and not allow it to creep into even a degree of “up” where none was intended.  Also, Tail-types “can” still be carried away by a strong thermal, even after the DT Action takes place.  It is not common, but can happen.

Some models, because of the design where the tail intersects the fuselage, will require a two-piece split horizontal tail pivoting on a rod/tube combination.  These are more complicated installations, but Scale models often require this if the Tail Type is chosen.

the previously-shown Fuse-type DT and the Horizontal Stab popped up.

a Scale model with at Split Horizontal in DT’d position.


Some models – and modelers – prefer a Wing DT.  There are two types here, too:  a Pop-Off Wing and a Pivoting Wing.

A Pop-Off Wing DT does just what it says:  once the Action takes place, the wing becomes detached from the model and all aerodynamic function is lost.  The plane falls to the ground – sometimes very rapidly (possibly damaging the model).  The wing is typically attached to the fuselage by a long string and, if done right, can act like a maple leaf and slow the descent.

Another type is the Pivoting Wing DT.  Similar to the Tail DT, the wing is hinged at the Trailing Edge.  Again, the desired angle is about 45 degrees.  This causes the same disruption as the Tail DT, but instead of a flat descent, the descent is nose-down and generally faster.  Again, this can result in damage to the model.

The legendary Jack McGillvray had a Fairey Barracuda that pivoted the wings at the high point.  You can watch the video HERE thanks to Tom Hallman.

hard to see, but there is a Burning Fuse DT in the middle of the wing. The Fuse burns the rubber band and the wing pops off. The tip of the wing is attached to the tail of the fuselage by a fishing line.

hard to see, but this is a Wing DT that pivots at the Trailing Edge.


There are other less-common Actions.  In Hand Launch Gliders, some would use a section of pop can on the fuselage.  It laid flat while under normal flight, but the Action would release it and the natural curl of the aluminum can would disrupt the airflow around the fuselage and spin the glider down.

Other gliders use a more modern DT where the fuselage pivots just behind the wing. This is a very positive Action.

Returning to the ’40s, Chet Lanzo created a model where the wing went through the cabin and the upper part of the cabin was hinged at the rear and popped up, causing a disruption in the glide.


This is not meant to be THE definitive paper on DTs.  I am just sharing common types of Actuators and Actions for the person just starting into DTs.  It will give them things to consider.  I am sure that my lists of Actuators, Actions, Benefits, and Downsides are incomplete – you may have pages to add, but this is what I came up with this afternoon.  And lastly, how you route the lines and adjust for travel and such is clearly not included in this writing.

Of course, one should always research the subject.  There are tons of articles on DTs in the FAC Library.


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