Making cheap plastic Wot4/Acrowot engine mounts fit for actual purpose!

[Jonathan M]

Recently my ARTF Wot4 gently nosed over when hit by a gust after landing on idle and unbelievably both plastic engine mounts instantly broke!  This is the second time this has happened (last time it was just one mount... but a pain nevertheless).

 

The mounts are ridiculously brittle, but the alternative of retrofitting proper glass/nylon ones to the firewall would involve too much work, as nothing off the shelf would fit the existing mounting-holes etc.  So I ordered another couple of pairs at £5.50 each - not taking any chances, especially as they were out of stock almost everywhere, and one retailer told me they are now in fact discontinued as an item (one would have to order the whole "IC Pack" including tank etc in future)?

 

So this is my work-around to beef up the mounts using 1/16th ply, keying both mating-surfaces before epoxying.  I did also consider drilling a whole series of 1mm holes and binding the ply to the plastic with kevlar thread before smearing more epoxy on - but didn't feel it was worth the risk of setting up extra potential fracture-points...!

 

 

 

Edited August 28, 2021 by Jonathan M

[Nigel R]

As an aside these are available on ebay direct from china for about a third of that cost. 

[Jonathan M]

Thanks Nigel... although of course the cost is marginal compared to the sheer inconvenience of regularly preparing and installing new mounts! ?

 

On the topic of fragile plastics, a club-mate told me yesterday that he'd nearly lost a model recently:  the control surface horns had been made from 3D printed plastic, which are also apparently far too brittle.

[FlyinFlynn]

17 minutes ago, Jonathan M said:

Thanks Nigel... although of course the cost is marginal compared to the sheer inconvenience of regularly preparing and installing new mounts! ?

 

On the topic of fragile plastics, a club-mate told me yesterday that he'd nearly lost a model recently:  the control surface horns had been made from 3D printed plastic, which are also apparently far too brittle.

@Ron Gray Please take note.

[Brian Sweeting 1]

Not something that I am familiar with, only have a glider, but as a replacement for those brittle brackets would a nylon (or plastic) kitchen chopping board be a suitable base for cutting out your own replacements?

[Cuban8]

it does make you wonder if the mounts were already suffering from fatigue cracks and it just took "the gentle nose over at idle" to cause them to fail. Only conjecture of course, but perhaps if cracks were already present and the mount had parted in the air during aerobatics or other violent manouvres, then a wholly uncertain outcome would be worrying. Any signs of a manufacturing defect? I've seen plastic engine mounts break after being smashed into the deck, but never after a simple nose-over. Certainly needs to be brought to the attention of the manufacturers.

I have seen a ARTF model lose a complete engine and firewall assembly - the pilot actually managed to save the model with its CG at its wing trailing edge, by some really good flying and the fact that the model had loads of excess elevator movement!

Edited August 28, 2021 by Cuban8

[Ron Gray]

4 hours ago, FlyinFlynn said:

@Ron Gray Please take note.

Why? I’ve never 3D printed control horns, but obviously the guy used the wrong material to make them from.

[FlyinFlynn]

17 hours ago, Ron Gray said:

Why? I’ve never 3D printed control horns, but obviously the guy used the wrong material to make them from.

Posted May 26

Not sure I would agree with you @FlyinFlynnin respect of servo mounts as it really does depend on their design and what element takes the load. I use them on most of my builds and haven’t had a problem yet (tempting fate, I know!).

 

Because you apparently place a lot of faith in a process that does not deliver the best results in terms of strength and repeatability. It is not just the materials used but a whole raft of other variable parameters, including the very layer by layer construction technique. 3D printing certainly has its place, but the manufacture of high stress components is not it.

[Shaun Walsh]

Sig engine mount

 

How about these?

[Ron Gray]

@FlyinFlynn yup my post refers to servo mounts, not control horns and I still say that it’s the material used in 3D printing (plus of course the associated settings) that is important.

 

Maybe we should just agree to differ and I’ll continue to use the parts as I see fit.

[Jonathan M]

2 hours ago, Shaun Walsh said:

Sig engine mount

 

How about these?

 

Those look very similar but hopefully less brittle.  Hard to tell from the photo whether the vertical parts are pre-drilled, would be ideal if they're not as would then be easier to retrofit on ARTF models where the firewall holes/blind-nuts are manufactured in.

[Paul De Tourtoulon]

On 28/08/2021 at 14:20, Brian Sweeting 1 said:

Not something that I am familiar with, only have a glider, but as a replacement for those brittle brackets would a nylon (or plastic) kitchen chopping board be a suitable base for cutting out your own replacements?

I also use the 'white' professional stuff, it is strong enough for a vibrating Rcgf 20cc copy / clone,,,

[ken anderson.]

hello J M.....why not buy/use an ordinary engine mount...as used for years..wont they fit or some other reason?

 

ken anderson...ne..1...IC Engine mount dept.

[Jonathan M]

27 minutes ago, ken anderson. said:

hello J M.....why not buy/use an ordinary engine mount...as used for years..wont they fit or some other reason?

 

ken anderson...ne..1...IC Engine mount dept.

 

Hi Ken

 

 

It's simply because retrofitting any given 'ordinary engine mount' will have different spacing holes, which would mean a significant rebuild of the firewall and blind-nuts, possibly moving the tank back a bit to clear different screw protrusions, also clearance issues with the built-in throttle linkage , etc... all entirely possible but defeats the object of an ARTF.

 

In the meantime, I'm pleased to report a super session with the Wot 4 on Saturday - including surviving a dead-stick on takeoff with a complete somersault when it came down in the dry stubble!

[Robin Colbourne]

Maybe boiling the engine mounts before drilling and fitting would help?

 

Nylon propeller manufacturers, before the days of glass reinforced nylon, used to recommend boiling their propellers to increase the moisture content and render them less brittle.  A quick Google found an RC Universe thread in which someone stated that r/c model car racers boil nylon suspension parts to reduce the likliehood of them breaking too.

When working as a manufacturing engineer, we had an injection moulded plastic bracket which went round a 1" tube on an airliner fuel gauging probe.  Some would snap when being fitted to the tubes whilst others wouldn't.  As a result,  I spent a day at the moulding company to find out what was going on.  It turned out our purchasing dept had asked them to go to 'just in time' production, just making what was required for work in progress at our place.  The injection moulder explained to me that it took 20 minutes for the injection moulding machine to get up to temperature and a post-moulding annealing process was also required.  With the move to 'Just In Time', the time taken to mould the entire order was less than the time it took to get the machine up to temperature.  In the end we reached some sort of compromise, whereby the moulder would mould a larger quantity, they and our goods inwards inspection would test them, then we would have them delivered according to our needs.   This way a failed test would not hamper our delivery of finished units.

 

Why the annealing process is crucial for plastics

This article below explains how the shock cooling occurs when a part is removed from the injection mould, causing stresses.  A long (30 minute) annealing process will increase strength.  I guess an advantage of boiling is that it holds the part at a constant temperature of 100°C and allowing the water to cool naturally prevents shock cooling.

Annealing tips for amorphous polymers 

Edited September 6, 2021 by Robin Colbourne

[J D 8]

                Although a very good flyer I think the artf WOT 4 has very poor fuselage structure that lends itself to style above strength.  Weak where strength is needed and strong where it is not.  

  Firewall [ engine bulkhead ]  is pretty well ok but then F1 is very poor, strong where the wing tab goes but lower down very thin and a great big hole in the middle where the tank comes though. A solid thicker F1 [ tad longer nose for tank to fit in front ] would be much better.

   A real bugbear is the ply UC plate that starts at the bottom of weak F1 and only goes from there to the dip in the fuselage.  A longer plate from firewall to the dip and fixed to F1 would be much better.

 From F2 to F3 is the ply servo tray and rear wing mount making this a strong area, more than is needed. The servo tray would be better fixed in between F1 and 2 for strength and cg reasons.

   The tail plane is mounted in the centre of the side sheets and on the model in the pics has come loose.

    Pics are of a wot 4 that did not fly very well I was checking out for a club member. The multiple repairs to the nose/UC area resulting in the twist you can see in the pic's and the tail plane waggling about in its slot did not help.

  It was decided to scrap it and move on.