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September 24, 2018, 16:06:19 PM

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1
RC Flight Chatter / Re: Wot4 Foam-e Mk2 wing fixing
« Last post by Michael_Rolls on Today at 08:37:45 »
Usually they are spot on..




Sent from my SM-A300FU using Tapatalk
That's normally been my experience as well - haven't felt like following through, but perhaps one day the enthusiasm will return.
I did have one entertaining kit about 20 years or so ago (can't remember for sure which it was so I won't risk maligning anyone in error). A tail dragger, the CG according to the instructions would have been quite a bit on front of the point at which the wheels touched the ground, IIRC 95mm had mistakenly been printed as 65mm
Mike
2
There will be Mayhem slots. Bring your Delta's or Gremlins Folks.
Cheers
Richard
3
TV, DVD & Home Media / Re: Then and Now
« Last post by pooh on Yesterday at 17:01:14 »
PDR, thanks for the detailed background, always good to have an expert explain. I'm just a cynical bas***d and use the curve and my limited understanding to explain why buying an extended guarantee is (probably) a waste of money.

In my own business, I see electrolytic capacitors rated for a mere 1000 hours, yet I have equipment I constructed, using such capacitors, in service 24/7 for 18 years without a failure. I fact the only notable failure was the explosion of a complete unit. My poor excuse is it got struck by lightning...
4
Radio Gear / Re: Flysky fs-i6 spares/
« Last post by flynn on Yesterday at 16:30:17 »
https://www.banggood.com/search/flysky-fs-i6-spares.html?sbc=1

either mode - $7.29
priority direct mail - 12 business days $2.63
expedited shipping - 8 business days $14.14



except they are under re-stocking at the mo....perhaps worth keeping an eye on?
5
TV, DVD & Home Media / Re: Then and Now
« Last post by PDR on Yesterday at 16:27:06 »
Only if they want to go bankrupt!

The "bathtub curve" is the sum of three curves:

1. The probability of latent manufacturing and material defects causing failures over time (an inverse-exponential curve, MTBF decreases with time)
2. The probability of wear-out failure-modes occuring (an exponential curve, MTBF increases over time)
3. The probability of "random" or complex failures which, as predicted by the central limit theorem, approximates to a "constant failure rate" (constant MTBF).

The first of these can be mitigated using process controls and burn-in testing. The second one defines the "product ultimate life" which can be tested for and predicted using Weibull analysis. The third one should ideally represent the in-service period where MTBF is both known and constant. As a result the manufacturer can make a reasonable stab at predicting the likely risk-cost of any given warrant period. So they choose a cost they can tolerate and then calculate the warranty period that should deliver this. Now remembering that the whole concept of MTBF is based on the exponential distribution (so even though it is the MEAN time between failures at that actual time you would expect 63% of the population to have failed*) you clearly don't want to be anywhere NEAR the MTBF time when your warranty period ends. If you were then you could expect your warranty costs to be around 63% of your costs-to-sale, and no one carries that much profit margin.

The usual number used is called the "B10 life" or "B10 component life" - that point at which 10% of the items made would be expected to have failed. Depending on whether (and how much) burn-in or other type-1 (above) mitigation is done this would typically be at anywhere from 3% to 9% of the MTBF. So if the manufacturer offers a 2 year warranty it usually means they expect most of the product sold will last AT LEAST another eight years, and probably longer, unless there is a spoecific wear-out failure mode that would truncate it. There are alos minor variations in these sums depending on whether it's a "Free Replacement Warranty" where they will repair/replace failed items free of charge, or a "Pro-Rata Warranty" where the customer would be expected to contribute to the repair or replacement cost in proportion to the amount of the expected life that had already passed (as for typical 3rd-party mechanical breakdown "warranties" - see also "Betterment Clause").

NALOPKT(&EFGAS)

If you read the above you should see why a manufacturer may be able to give a warranty at no extrat cost, but the "store checkout" warranty will ALWAYS be more expensive. The Manufacturer can mitigate the warranty costs through process control and testing, but the retailer has no control over the probability of failure and is just taking a punt...

[I'll come clean - a few weeks ago I gave a 1 hour seminar to the Divisional Management Committee of our electronic equipment division on subject of warranty risk - how it is calculated and what is needed to manage it as a business risk, so I still have the slides if anyone wants to see them!]

PDR

* if that isn't immediately clear look up the properties of exponential curves and integrate one from 0 - 50%)
6
RC Flight Chatter / Re: Wot4 Foam-e Mk2 wing fixing
« Last post by itsme on Yesterday at 16:16:19 »
What has surprised me is that I had an E-wottie a few years back which did a lot of work until the elevator servo failed in flight (after about 5 minutes and yes, everything worked OK on the ground) so  I bought another one last year. With Veronica's illness and death I didn't get round to flying it until this April - and it is WAY tail heavy - I was quite proud of myself for getting the thing down in one piece (until the breeze caught it and a semi-cartwheel tore the wing loose). Haven't got round to doing anything about it yet - motivation at the moment is very low - haven't actually gone flying since that day. However, can't understand how, when the first one flew perfectly out of the box, number two - with identical radio gear and battery size, weight and location - could be so way out of trim. Checking after the event, I would have thought the CG was reasonably located  - but it didn't seem it whilst in flight. Trouble is, I never made a note of the CG position on number one, so have no idea how they compare.
Mike
Usually they are spot on..




Sent from my SM-A300FU using Tapatalk

7
TV, DVD & Home Media / Re: Then and Now
« Last post by pooh on Yesterday at 15:50:00 »
does the supplier offer an extended guarantee? if so, how long ?

There is the "bathtub graph" of failures over time, so called because it is shaped much as a section cut across a bath. A fair number of failures occur early in life, then there is a plateau with very few, then a rapid rise after a certain time. You can bet that the insurers have worked out the likely failure-free life, and set the extended guarantee to end just before the rapid end-of-life failure rate increase.

Of course that is a statistical calculation, spread over many units, so you may still get occasional failures during the bottom of the bath period.
8
RC Flight Chatter / Re: Wot4 Foam-e Mk2 wing fixing
« Last post by Michael_Rolls on Yesterday at 12:12:08 »
What has surprised me is that I had an E-wottie a few years back which did a lot of work until the elevator servo failed in flight (after about 5 minutes and yes, everything worked OK on the ground) so  I bought another one last year. With Veronica's illness and death I didn't get round to flying it until this April - and it is WAY tail heavy - I was quite proud of myself for getting the thing down in one piece (until the breeze caught it and a semi-cartwheel tore the wing loose). Haven't got round to doing anything about it yet - motivation at the moment is very low - haven't actually gone flying since that day. However, can't understand how, when the first one flew perfectly out of the box, number two - with identical radio gear and battery size, weight and location - could be so way out of trim. Checking after the event, I would have thought the CG was reasonably located  - but it didn't seem it whilst in flight. Trouble is, I never made a note of the CG position on number one, so have no idea how they compare.
Mike
9
RC Flight Chatter / Re: Wot4 Foam-e Mk2 wing fixing
« Last post by itsme on Yesterday at 11:04:59 »
Although excellent at what their job is- shake the box and go flying- these wot thingies are not long lasting. If you spend time on a few mods, however, they can give good service for years. Battery cover- I line the edges with mirror ply. Extra glue to the motor fixings. better wing bolt fixings (as above) and coat all delicate edges in cyano to harden them. I have just spray painted an Acrowot foam-e as I detest the stick on decals but its not easy as masking tape of any kind picks off the paint. I bought some De Luxe Materials Foam Armour at the Nats. Advice- dont bother. May be epoxy will harden the foam, but this stuff wont even stay on it.
10
RC Flight Chatter / Wot4 Foam-e Mk2 wing fixing
« Last post by PDR on Yesterday at 10:44:39 »
I have one of these which I tipped out of the box (I can't say I "built" it) a year or so back. I haven't flown it, of anything else, at all this year due to other commitments - mainly the fact that I spend 4 days a week north of the Wall helping the Wildlings develop Lancashire's anti-white-dragon defence systems*.  So it has been sitting ready for use on the bench in the workshop since before Xmas.

Anyway, yesterday I cleared the bench because I've got the bug to start some building again (more anon), so I picked up the Wot-e and noticed that the wing seemed to "rock" as if the bolt was loose. I gave the TE a gentle one-finger tug to test this theory and the wing just came off - the wing bolt pulled right through the wing, which I thought showed a distinct lack of gratitude and a general refusal to be a team player...

For those not familiar with the model - the wing is made up of a number of EPO  mouldings bonded together, with some moulded plastic (look like ABS or HIPS - certainly not nylon) fittings for the LE location dowels and the rear bolt location. The rear bolt location moulding is essentially a flat plate to spread the load, with a tube leading down to the bolt seating face so that the (metal) bolt head is well below the surface of the wing. The bottom of this tube with the bolt-seating face has completely detached, so nothing is holding the wing on at all. My theory (FWIW) is that having been in the workshop with the wing bolted on it since it last flew on a frosty day last October it has been through temperature cycles from about -6degC to +35degC, and the expansion of the foam of the wing has overstressed the part. Fortunately this isn't hard for me to fix - I can turn-up a flanged collar in either dural or stainless that will just drop in. I'm just glad I spotted it before I took it flying again!

But it has made me think more about the storage conditions for my models - especially the consequences of temperature cycling. I've fallen into the habit of leaving wings bolted on until I actually need the space, but I think from now on I'll make a point of loosening metal wing bolts in foamies. It's no great hassle to back the bolts off a couple of turns and then re-tighten them on arrival at the field.

Anyone else suffered a similar failure?

PDR

* my Better Third suggested my job title should be changed to "Head of Marriott Points Acquisition" as I've spend 3 nights a week in the place for over 15 months (~246,000 points this week and counting). I just wish they gave air miles on the company jet...
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