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Author Topic: DIY rotary damper style DT timer  (Read 625 times)
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VictorY
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« on: January 04, 2022, 06:34:25 PM »

After having bad luck with the consistency of some silly putty timers that I tried to build, the thought occurred that the same materials could be used to make a lighter timer with a better chance of success in the consistency department. Having raced 1/12th pan cars and remembering the damper tubes on them, I decided to try the concept on a timer. The race car damper tubes work in a linear fashion with lighter oils, but you can get lab grade oils in just about any viscosity quite easily, so I thought that a rotary damper would be achievable. I used two sizes of aluminum tubing with a very close fit, 6mm OD and 5mm OD. The pin is the tip of a T-pin dulled a bit, inserted into a single hole drilled into the inner tube, then glued in place. Plug the other end of the inner tube with something like clay, or epoxy if you think it needs it, so that no lubricant can escape up the inside of the tube. Lube the space in between the tubes and then glue a temporary cap over the end or find a rubber cap that fits.

Mine is very consistent at multiple levels of line tension using rubber to power it. So I ordered some .008 spring wire in hopes of making a spring that will match it perfectly. The 2,000,000cst diff fluid seems to be in the ball park but you can get fluid 10x thicker if need be.

I will probably have spent more in supplies trying to build a decent timer than if I just bought one, but I couldn't stomach paying what the hobby supply places are asking for timers.
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DIY rotary damper style DT timer
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TimWescott
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« Reply #1 on: January 04, 2022, 07:28:19 PM »

I will probably have spent more in supplies trying to build a decent timer than if I just bought one, but I couldn't stomach paying what the hobby supply places are asking for timers.

So if you paid yourself a fair wage for development and labor, how much would you sell these for?
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lincoln
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« Reply #2 on: January 04, 2022, 07:37:42 PM »

Any idea how much your gadget weighs? I've made silly putty timers like yours, but with greater clearances. How much does the speed vary with temperature? I'd have guessed that .008" music wire would be too stiff, at least for a coiled spring. As I recall, that's about the right prop shaft size for a mini stick.

Volare has tiny rotary dampers for $3, and I bet they'd cost less, per piece, if ordered 10 at a time from Digikey. Assuming you knew what specs you wanted.

BTW, silly putty timers have the advantage that materials are on hand the night before the contest. When I was in college, we ordered 5 or 10 lbs of Silly Putty, and I think I still have some of my share. We used to put a little on a chunk of steel and whack it with a hammer. Amazing! If that carpet still exists, I'm sure it still has putty in it. Fortunately, it was mottled and close to the same color. Then there was that time with 5,000 suoerballs in the lobby...   Some years later, I had a job at a company that decided to hand out purple silly putty one day. I think I still have that. And occupational therapy putty from when I busted my hand.
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VictorY
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« Reply #3 on: January 04, 2022, 07:52:49 PM »

I will probably have spent more in supplies trying to build a decent timer than if I just bought one, but I couldn't stomach paying what the hobby supply places are asking for timers.

So if you paid yourself a fair wage for development and labor, how much would you sell these for?

You'd end up paying more for shipping and handling than for the part, IF I were interested in manufacturing them. Wink
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VictorY
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« Reply #4 on: January 04, 2022, 08:00:51 PM »

Any idea how much your gadget weighs? I've made silly putty timers like yours, but with greater clearances. How much does the speed vary with temperature? I'd have guessed that .008" music wire would be too stiff, at least for a coiled spring. As I recall, that's about the right prop shaft size for a mini stick.

Volare has tiny rotary dampers for $3, and I bet they'd cost less, per piece, if ordered 10 at a time from Digikey. Assuming you knew what specs you wanted.

BTW, silly putty timers have the advantage that materials are on hand the night before the contest. When I was in college, we ordered 5 or 10 lbs of Silly Putty, and I think I still have some of my share. We used to put a little on a chunk of steel and whack it with a hammer. Amazing! If that carpet still exists, I'm sure it still has putty in it. Fortunately, it was mottled and close to the same color. Then there was that time with 5,000 suoerballs in the lobby...   Some years later, I had a job at a company that decided to hand out purple silly putty one day. I think I still have that. And occupational therapy putty from when I busted my hand.

1.35g on the weight, plus whatever the mounting plate would weigh. Probably 1.75g in added airframe weight not counting the spring, line, etc. But this unit is built for my big Wakefield model. You could probably replicate it on a smaller scale for models that need less tension holding down the stabilizer.

Speed should be fairly consistent in regards to temp change. Silicone based oils are supposed to be stable across a broad range but we noticed a little bit of change with large swings in temp between summer and winter racing. Not much though. We might change the shock oils on our offroad cars 10cst up or down between seasons. No changes in onroad where shaft speeds and range of travel are much more conservative.

I had most of the materials on hand between my model building horde and my rc car racing supply tote. I only purchased the bottle of 2M diff fluid when I realized that my thickest silicone based fluid on hand wasn't going to be in the range that I needed for a 2 min+ timer.
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raggedflyer
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« Reply #5 on: January 05, 2022, 01:41:09 PM »

Just to be certain that I understand the concept, the fluid completely fills the annular gap between the inner and outer tube and doesn’t migrate from the open end in normal use?
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PeeTee
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« Reply #6 on: January 05, 2022, 04:14:29 PM »

Why not use the plastic dampers made by Ace Controls, they weigh 0.6gm each and can easily be adjusted to rotate at 1 rpm!

Peter
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kaintuck
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« Reply #7 on: January 05, 2022, 04:49:49 PM »

peetee, is this what your referencing?
https://us.misumi-ec.com/vona2/detail/221000523005/

marc
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Yak 52
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« Reply #8 on: January 05, 2022, 05:24:03 PM »

Why not use the plastic dampers made by Ace Controls, they weigh 0.6gm each and can easily be adjusted to rotate at 1 rpm!

Peter

I still have some of these available if anyone wants one. Cut the lugs off and they are 0.4g Wink
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Re: DIY rotary damper style DT timer
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VictorY
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« Reply #9 on: January 05, 2022, 06:32:49 PM »

Just to be certain that I understand the concept, the fluid completely fills the annular gap between the inner and outer tube and doesn’t migrate from the open end in normal use?

Correct. The oil is so thick that the surface tension prevents it from migrating.

And this is just the first iteration. I'm sure you could half the length of the tubes and use thicker oil to get the same result.
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VictorY
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« Reply #10 on: January 05, 2022, 06:43:29 PM »

As for those dampers available for less than the $20-25 I've seen on hobby sites, they still require modification to be usable in a model, and that's if you can find or make the right spring to power them. Once I get the final design down and figure out what kind of spring specs are going to match it well, I can make them for less than a dollar's worth of materials.

I'm not saying don't buy timers from hobby suppliers. We need to keep them in business. Just sharing for those who feel like tinkering.
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raggedflyer
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« Reply #11 on: January 06, 2022, 05:01:14 AM »

[quote And this is just the first iteration. I'm sure you could half the length of the tubes and use thicker oil to get the same result.e][/quote]

Sounds ok then so long as dust/ dirt cannot enter the ends to mix with the oil.

I guess it’s down to matching the annular volume (ID and OD and length) with the oil viscosity to determine a reduced length. Presumably the parts could be simplified to a plastic outer tube and internal plastic rod cranked or shaped on the end to engage / disengage with the release line.
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spr
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« Reply #12 on: January 06, 2022, 09:39:54 AM »

Check https://export.rsdelivers.com/browse/mechanical-products-tools/engineering-materials-industrial-hardware/anti-vibration-levelling-components/rotary-dampers

There are several suitable dampers for dt purposes and won't cost a fortune.
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PeeTee
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« Reply #13 on: January 06, 2022, 10:57:44 AM »

The dampers shown in the RS catalog (kindly shown by spr) are all from Ace Controls and you need the FRT-E2 and FRT-G2 with the maximum damping torque. I used to buy these from Ace and gave much of the stock to aeromodelling friends to use as DT dampers. From memory these cost about £1.20 to £1.50 and now cost about £1.70, not worth mucking about with pieces of tube, I could be down the pub instead!  Cheesy Cheesy

I recall that Ralph Sparrow discovered these 20 odd years ago and got Mike Woodhouse (Free Flight Supplies) to take over the ordering.

Peter
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RalphS
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« Reply #14 on: January 06, 2022, 02:19:31 PM »

I recall that Ralph Sparrow discovered these 20 odd years ago and got Mike Woodhouse (Free Flight Supplies) to take over the ordering.

I have been retired for 25 years so it must be 30 years since I got the early ones. Grin  Looks like re-inventing the wheel.  Anyway it is something to do in the middle of winter.  B..... cold here up north! I can't be bothered to do anything.

Ralph
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VictorY
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« Reply #15 on: January 06, 2022, 06:16:32 PM »

Can those light duty dampers do 4+ minutes with a pull heavy enough to secure a large twin rudder empennage? Where can a person find a chart that shows what damper and spring combos result in any given duration/rpm, and for what class of model they are suitable?
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VictorY
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« Reply #16 on: January 06, 2022, 06:31:08 PM »

[quote And this is just the first iteration. I'm sure you could half the length of the tubes and use thicker oil to get the same result.e]

Sounds ok then so long as dust/ dirt cannot enter the ends to mix with the oil.

I guess it’s down to matching the annular volume (ID and OD and length) with the oil viscosity to determine a reduced length. Presumably the parts could be simplified to a plastic outer tube and internal plastic rod cranked or shaped on the end to engage / disengage with the release line.
[/quote]

Contamination didn't seem to be a problem for the open ended linear damper tubes on our race cars, and even onroad racing is an extremely dusty environment with loads of rubber particles on top of the dust and sand. A rotary style damper would have much less chance of ingesting contaminants than a linear damper pumping in and out constantly. Here are some damper tube parts from one of the latest 1/12th pan cars. Very simple but also easily tunable and serviceable. Notice the dimples in the inner tube to hold extra oil.
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Re: DIY rotary damper style DT timer
Re: DIY rotary damper style DT timer
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flydean1
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« Reply #17 on: January 06, 2022, 06:32:01 PM »

For such a slow pull, you may have to use some sort of lever arm with the weak spring moving the damper, and attached to the long arm of the lever.  The short arm of the lever engages the DT line to the stab, Usually a metal split ring us used to keep the DT line from climbing around the bend and locking the lever.
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calgoddard
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« Reply #18 on: January 06, 2022, 07:11:42 PM »

In DT systems that utilize rotary dampers it is best to de-couple the force that moves the damper from the force that holds down the stab.

See:

https://freeflight.org/Library/TechLibrary/DTButtonMounting.pdf

In the first drawing in the article, the rotary damper is labeled as DT.

I have used this de-coupled DT system extensively and it works well.  It avoids "stab creep" which is the gradual increase in negative stab incidence in DT systems where the same spring or rubber band that holds down the TE of the stab pulls on the damper.  As the tension on the spring or rubber band lessens during the latter stages of the flight, the TE of the stab gradually lifts up a tiny amount which results in a stall.

In my de-coupled DT system I use a significant pulling force to hold down the TE of the stab.

I hope this helps.

PS - It is my understanding that the original Silly Putty timers were developed and perfected by Stan Buddenbohm decades ago.
« Last Edit: January 06, 2022, 07:26:12 PM by calgoddard » Logged
flydean1
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« Reply #19 on: January 06, 2022, 10:55:02 PM »

That was what I was attempting to say.  You did it better.  I have used the same arrangement on several 1/2A models and they work very well.

I have also used lever systems on larger up to C gas to lessen the pull on the DT scroll on my Texas MAX timers.
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VictorY
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« Reply #20 on: January 07, 2022, 07:02:18 PM »

I had a mouse trap style mechanism on my Wakefield for the RDT system but removed it when I decided to go old school and tinker with a homebrew DT. I haven't ever wound my own springs so that was part of it too. It will be fun to experiment with coil diameters and number of winds needed to get the tension and length of pull that I need.

If necessary, I will either add the mouse trap lever back or put a turnaround on the spring line so there will be twice as much tension on the DT line going to the stab as there is on either the spring or the damper.

The more time spent working on models means less time wasted staring at a computer screen.  Grin I don't even get many chances to fly my FF models. Not a single one of my contest worthy planes has been sent up with a max number of winds as there are no fields near me large enough with the central Texas winds almost never being calm. Hopefully I will get to make a trip to Hamilton, TX this year and see what my fleet can do.
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VictorY
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« Reply #21 on: January 11, 2022, 08:01:12 PM »

The spring wire was a little late arriving but I finally got to wind a few springs and get some numbers from my timer. The spring I settled on for longer run times, 180 to 240 second range, is 37mm long and has a 2mm O.D., made from .008 beryllium copper spring wire. Three revolutions of the timer clocks in at just over 4 minutes with about 15 seconds of error, so three and a quarter revolutions will guarantee a full 240 sec max flight before DT'ization. These times were observed in an 80 degree room. I'll have to try some tests outside soon to see if the cold will change anything.
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