Friday, February 23, 2018

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

Related Links  - Manuel Cisneros article on Viscous DT  Doc Fizzix Mousetrap Cars  - spring winding device  - National Free Flight Society 

Monday, February 5, 2018

Catching Up in February

I have not posted anything in a couple of weeks, so time for an update. The best news came in an email I received from a student I had helped online build model airplanes for the Wright Capacitor event in Science Olympiad several years ago. I have to give her much of the credit for asking good questions and being so responsive by email. She was thrilled to receive 6th place out of 61 teams at the national competition and decided to pursue aerospace engineering in college and has an internship lined up at Boeing this summer.

Witch Hawk 500

Wright Capacitor Model

This winter I have been busy building another fleet of free flight model airplanes, the building went pretty fast but finishing up all the ending details has not gone as fast. Of course I had to try some new things; one was the tissue over mylar covering technique on the New Gollywock rubber model I am building. I had covered with mylar once before and that went pretty well but I must have not applied the dope as consistent as needed to attach the tissue. Some places look real good, other spot have some sagging tissue, from a few feet away it looks fine and the finished structure is rigid and light.

New Gollywock

After building and flying my first glow powered model in several years last year the Basic Yeller powered by a Cox PeeWee .020 I decided to try more glow models again. I started building my largest free flight yet, the Witch Hawk 500 to be powered by a K&B .19. This is my first model with a mechanical timer for motor cutoff and DT, the timer is a Texas Timers MaxIIIA. I wanted to do everything by the book, so used the mounting kit and the plastic dust cover. Covering is Polyspan on surfaces and silkspan on the fuselage.

Basic Yeller
1/2A Streak Free Flight

Two smaller glow models are under construction also; these are the ½ A 2018 One Design models at the Nats, Streak and Smarty. BMJR had a sale on the two kits so I purchased both. The Streak is covered now but still needs to have clear dope and some trim, Smarty is in final construction stage.  I have purchased airbrush equipment but still waiting for the pressure regulator to arrive. There are plenty of Youtube videos on spray painting that I have been watching before experimenting with the equipment.

Flying Aces Moth

Another rubber power model kit that I am finally building is the Peck kit of the Flying Aces Moth. I also have the Volare short-kit that I might build with any improvements I see are needed. Instead of the included plastic propeller I purchased a prop blank from Volare, the wood propeller is almost half the weight of the plastic propeller. This model will have a viscous DT, I used Manuel Cisneros article on Rotary Damper Timers as a guide for setting up a DT system for this plane.

Icon A5 Electric RC

Flash X18 on Frozen Lake Winona
When the temperature and wind are not too bad I have been doing some winter flying both RC electric and some simpler free flight models. The frozen lake close to where I live gives a huge unobstructed flying site but so far there has been little snow for cushion when the model lands. Crashing on ice is about like crashing on concrete. When there was a little snow I had fun doing touch and go’s with the Icon A5 park flyer electric RC. I have been test flying the Flash X18 rubber powered free flight that Dick Bertrand uses in his Free Flight Rescue program. With some minor tweaks I am getting more consistent flights.

Bill Kuhl

Related Links