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Author Topic: Bad vibes  (Read 665 times)
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RobinB
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« on: February 20, 2012, 10:47:04 AM »

I've been having some timer troubles recently. This is with a Texas 3F III.
The engine is a PAW 19 CT3 diesel.

When first installed, I did some bench checks and the timer ran 12 - 13 seconds from
fully wound point back round to cutoff release.
On the field with the engine running that came down to about 8 seconds!

I know diesels like the PAW are known to vibrate a bit, but I was surprised how much the timer was affected.

I stiffened the timer mounting plate with thin strips of hardwood around the edge,
thinking that the thin ply plate might be flexing / vibrating, and I stuck some thin urethane
strips between the hardwood and the fus for good measure.
After fitting it back together (with longer screws), the timer now seems to be running at its
proper speed with the engine running.
The only snag now is that the whole assembly has come loose due to the holes in the fus having
opened out through all the re-fitting!

Has anyone any hints / tips / sneaky tricks to prevent these problems occurring?


Robin
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Daithi
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« Reply #1 on: February 20, 2012, 10:56:15 AM »

a quick fix for the holes - put a matchstick (or a sliver of hard wood) in the holes with a drop of PVA glue - that will give the screws something to 'bite' into.
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Gnu
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« Reply #2 on: February 20, 2012, 04:03:45 PM »

If you can find a sample of plastic tubing that is a tight fit on the mounting screws ( the tube/ink reservoir from a disposable pen may work), fit a small piece into the hole and glue it in place. It will hold the screws tight. Just don't over-tighten and strip out the plastic.
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RobinB
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« Reply #3 on: February 20, 2012, 04:49:02 PM »

I forgot to add that this model has hardwood bearers, but I've just acquired some alumin(i)um bar stock which I'll use in future.
Hopefully, this will reduce vibration through the fus.

Robin
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DaddyO
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« Reply #4 on: February 20, 2012, 05:13:28 PM »

Hiya Robin

Good fun at the weekend yes? (Cough,cough,cough) Tongue

Anyway, thinking about your fast running timer and a couple of ideas which may be useful (or not) from the days when I was a 'brick swinger' Wink

With the combat jobs I used to mount the engines on either super duper ruskie bearers wot was very expensive and so I only used them in 'posh' models. The cheaper alternative was a dural plate on top of the bearer (I may still have some knocking about)

With the aerobatic jobs a degree of flexibility was needed in the mount itself (I can't remember why this was considered de riguer) There are several types available which might be worth a look at and I was reading only yesterday an old Aerotwiddler article by Henk De J which described what he had done - I'll try and find it for you if you're interested

Toodlepip
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« Reply #5 on: February 20, 2012, 06:09:33 PM »

The screw holes in the bearers have "gotten bigger"?  Drill them out to where a short piece of inner pushrod "snake" (nylon/Teflon) is a tight press fit, rough up the outer surface and press in the "new" holes with a tiny drop of CA.  The screws will not vibrate out, but a tiny bit of silicone caulk on the threads will give you more piece of mind.

Pete
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gossie
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« Reply #6 on: February 20, 2012, 09:11:03 PM »

I usually use blind nuts and screws to hold the timers in my things.

Another way I do it is to use strips of 1/16th aluminium with holes tapped into them, and glued (Araldite) behind where the timer goes.  Timer goes into the slot/hole, screws run through the timer and into the alum. strip to hold it in.

I would only ever use a wood screw type situation on a small rubber model or sailplane.

I have seen people GLUE timers into their models.   Not nice IMHO.
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RobinB
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« Reply #7 on: February 21, 2012, 05:30:58 AM »

Gossie, that threaded alloy strip method is just what I'd decided to do. Thanks.

Pit - no, it's not the bearers. It's the screw-holes i the fus. side that had 'lost their grip'.

The installation kit that comes with the timer has 4 short self-tapping screws - I certainly need something beefier than that on this plane.

I was wondering why I can't remember having any of these problems before. Then I realised - I'd always used air-vane regulated Seelig timers
before on models this size (and I'd got away with screwing them into the fus.)

Robin
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applehoney
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« Reply #8 on: February 21, 2012, 02:11:18 PM »

Quote
I have seen people GLUE timers into their models.   Not nice IMHO.


Hmmm . .the last timers I screwed into place were the bulky old ED clockwork ones, if anyone remembers such

Then came Tatones, later followed by KSB, KBS  and Graupner.      Engine and d/t ....    every last one has been glued into place.

Still are.....

Now I learn it's not nice........      hmmmm
 
 
 
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gossie
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« Reply #9 on: February 21, 2012, 04:43:59 PM »

>I have seen people GLUE timers into their models.   Not nice IMHO.

Hmmm . .the last timers I screwed into place were the bulky old ED clockwork ones, if anyone remembers such

Then came Tatones, later followed by KSB, KBS  and Graupner.      Engine and d/t ....    every last one has been glued into place.

Still are.....

Now I learn it's not nice........      hmmmm

Ah well, they glue modern cars together these days rather than using screws, bolts and nuts, so I guess you have been ahead for years Jim.

Okay for cars.  In fact I have a boot lip spoiler glued with Sikaflex on my car, but timers?   No, sorry, I do prefer to screw them in properly rather than gluing them in.  

By the way, I had an ED Clockwork timer in a couple of Swiss Miss's way back in the '50s........Would be nice be nice to still have it.
« Last Edit: February 21, 2012, 05:47:56 PM by gossie » Logged
Daithi
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« Reply #10 on: February 21, 2012, 05:01:37 PM »

well there's always velcro (or cable ties fed through the fus)  Grin
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RobinB
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« Reply #11 on: March 27, 2012, 03:37:49 PM »

The timer on that model seems to be running normally now and, after a full day's flying on Sunday, it's still in place!

I ended up letting a pair of 1/16 x 1/4 spruce strips in to the fus. side and fitting some neat little 2-56 T-nuts behind the strips.

I got them (and suitable socket-head bolts) from www.modelfixings.co.uk - they are the smallest T-nuts they stock.

Robin
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Sandgroper
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« Reply #12 on: March 28, 2012, 06:44:19 AM »

I use 2mm T/blind nuts to mount small electric motors,a smidge smaller than 2-56,use them on timers too but I have no problems with 2mm self tappers in 3/32 ply with cyno reinforcing of the threads,I do not use Hanks screws I prefer the 1.7mm or 2mm  self tappers they have a little more clearance in the holes

http://www.hobbyking.com/hobbyking/store/__124__58__Hardware_accessories_-Screws_Bolts_Nuts.ht


Phil

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