Bad Experiment in Viscous DT Timer

Often when things do not go as planned; studying the reasons why can lead to a great learning experience. Example, one of the cheapest and lightest weight devices to bring down a free flight model airplane (DT or dethermalize) in a specified time before flying away is with a “viscous timer”. Basically a “rotary damper”, a device that slows down rotary motion is pulled by a “tension spring”. When a line with a loop that goes around and an arm comes off; either the stabilizer or the wing of the model plane moves upward causing the airplane to come down. There will be free flight modelers that think this method works pretty well and others that will avoid using this method completely.

Free Flight Discus Launch in DT Mode

Ikara Viscous DT TImer on P30 Model

Living in a cold climate I try to keep busy with my model airplanes even when I cannot fly outside, this winter I decided to review how all of my DT devices operated and time the length of a cycle until it dt’s. Other DT devices I have are electronic, mechanical with wound spring, and a burning fuse, the first two types are accurate but can be more expensive and heavier. What I was doing was like a science experiment and I realized in actual operation outdoors conditions would be different; mainly the temperature in my house is constant.

Badge Timer on Test Board

First I started with a P30 rubber model that had the Ikara brand viscous installed, running cycles of the timer continuously while watching TV; I recorded the DT times on paper. In looking at the data of the times over a couple of days there was one pattern I noticed of the first run normally took the longest and then the times were a little more consistent. This prompted questions to online model forums and I learned that this can be a characteristic of the fluid in the rotary damper, a couple of suggestions I received were to not move the rotary damper too fast and to work it a couple times before using the timer for a flight. 

Volare Spring Winder

In my experiments I tried two more viscous timers and also constructed my own springs with a winding device I purchased from Volare. I asked so many questions on model forums I am sure people are tired of me asking about viscous timers. In that in some ways this is like a mousetrap car in the use of a spring and the concept of leverage I consulted with Doc Fizzix the mousetrap car expert. I also searched Internet for any article related to this type of timer.

My plan is to write two type of articles with what I am learning, one for free flight modelers that I will submit to Free Flight Digest and another that is more about the science of how this seemly simple device functions. My hope is also to show students how to install in a fairly simple rubber powered airplane such as a Peck Sky Bunny.

About the bad experiment, what I found was that my operating the timer constantly with no break in between was causing my data of recorded times to be really inconsistent. After realizing that I waited several minutes before running the timer again. This is consistent with actual use in a model plane as it will normally take a few minutes to retrieve the model before flying it again. Now I was recording times that were consistent, I will write about why this is in future articles.

Bill Kuhl
http://www.ideas-inspire.com

Related Links

http://www.oaklandclouddusters.org/resources/Documents/Articles/In%20the%20Workshop/Rotary_Damper_Timer_Build.pdf  - Manuel Cisneros article on Viscous DT

https://www.docfizzix.com/  Doc Fizzix Mousetrap Cars

http://volareproducts.com/  - spring winding device

https://freeflight.org/  - National Free Flight Society