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Repairing undercarriage on a foamy

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Nigel Heather17/09/2018 10:49:33
104 forum posts
7 photos

I’ve just returned to the hobby after many years to find the world has gone electric.

To ease myself in I bought an acrowot foam-e. It has been up a few times and I have been largely impressed.

But yesterday it suffered a mishap. I flew it even though my brain was telling me ithe conditions were too gusty but my heart was telling me that there wouldn’t be too many more opportunities this year.

Anyway, I brought it in for what was goung to be a smooth landing but, just as it was approaching the mown strip a gust caught it and pushed it down short into the 4” grass. But it still went in flat, had it been a few yards on it would have been a lovely landing but it ended up on its nose.

No problem, I thought, had a lot worse with my IC planes. But when I reached it I realised that the undercarriage had been ripped off and the prop snapped. Replacement prop already on the way - but how to repair the undercarriage.

The plastic mount has been snapped and ripped out on one side, still in on the other. Looking at the design, there is a thin ply plate on each side which is ebedded in the fuselage and goes through the plastic mount. This plate on one side has snapped.

I can see no way of removing/replacing the ply plate - I can think it can only be positioned at manufacturing when the fuselage is in pieces - might explain why there are no spares for this area either.

So best I can do is as follows

CA, epoxy or wood glue on the two halves of the plate and hope they mate up - you can’t see it once assembled so it is just fingers crossed.

Uhu Por on the foam and plastic.

CA or epoxy to glue the two halve of the plastic mounting plate together.

Any other suggestions. I see that some people drill and tap the plate to using M5 nylon bolts to attach the undercarriage to the mounting plate rather than the steel self tappers supplued in the kit. Having seen the plate I can’t see that working for long. The plastic plate is no more than 1mm thick, but is thickened by circular bosses where the self tappers go in. But these bosses are 5mm in diameter so the guys who have drilled out to M5 will have removed all that material and just tapped into 1mm of plastic.

You could get away with M3, maybe M4 nylon bolts but would they be man enough, or just shear off on good landings.

Cheers,

Nigel

Colin Carpenter17/09/2018 10:58:18
551 forum posts
35 photos

Nigel . Very common .Gorilla glue back in. Take screws and Dremel through at head 50 % so it becomes a weak point ! Once you do this it will never happen again ! I did it on flight 50 and it never occurred till it died over 300 flight later ! 😁😁😁. Colin

Nigel R17/09/2018 11:21:04
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2968 forum posts
471 photos

Foam isn't very strong is it!?

Small bits of ply pull out very easily.

I've stuck fibrous reinforcement (cereal box cardboard, or even just normal paper) on the outside of mine (PVA, but gorilla glue is probably better), to strengthen areas like the U/C mount. This provides a somewhat tougher surface to spread the load of a smaller ply plate onto. Also works on bits like hatch openings, tailplane mounts, etc.

I don't have an Acrowot but would assume the area could be toughened up like this somehow.

Biggles' Elder Brother - Moderator17/09/2018 11:34:54
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Moderator
15748 forum posts
1460 photos

A quick comment on adhesives for this:

1. Gorilla glue is great - but be careful, it expands like mad! Have a couple of practice goes with it in some scrap if you haven't used it before!

2. Epoxy is generally good on foam, but it can go brittle over time. TBH it will probably out live the model if you fly it a lot as a hack in the winter - so no problem really! (BTW that isn't a comment on your flying!!! Its true of all of us!)

3. Personally I don't rate UHU Por for structural repairs like this - not strong enough in my view. Its fine on a really small, indoor, style model or for largely cosmetic repairs, but not under carriages.

4. CA is great - but bear in mind conventional CA attacks foam - the stuff just dissapears before your eyes under a drop of CA! So if you use it be extra careful about any unintentional spread, or use something like Pink ZAP foam safe CA.

It will all go back together and be fine - we've all been there with a foamy!

BEB

Nigel Heather17/09/2018 11:37:32
104 forum posts
7 photos

Thanks for the tips. Yes a bit of an eye opener about foam - but I don’t know why I am surprised.

I’m sure the ply would pull out of the foam, but you just can’t get to it.

Even if you coul get it out you would not be able to replace it because it is much longer than the mounting plate slot that is the only access. Also the assembly sequence must be, push the mount in, then slide the ply though the fuselage and the mount locking the mount in place. But I can see no way to acheive that with an assembled fuselage so can only imagine that you have to do it part way though assembly.

Denis Watkins17/09/2018 12:23:06
3797 forum posts
49 photos

The easier option Nigel, is a ply plate on the fuselage belly, covering as much area as the model offers

Remount your undercarriage to the plate by your chosen method

Nigel Heather17/09/2018 15:30:48
104 forum posts
7 photos

Posted by Denis Watkins on 17/09/2018 12:23:06:

The easier option Nigel, is a ply plate on the fuselage belly, covering as much area as the model offers

Remount your undercarriage to the plate by your chosen method

I suspect that would be best - make a new mounting plate - but how to fix it.

Some pictures

Bracket #2

The part of the bracket that has snapped off. As you can see it has a cheek either side (the other one is still embedded in the fuselage). These cheeks sit in wells in the fuselage and a piece of ply pushes from the fuselage into the cheek. You can see the edge of the ply that has snapped off.

Bracket #1

The view from the plate - as you would see it on the bottom of the fuselage. As you can see the undercarriage mounting holes are both on this broken piece. The hole to the right is compromised and the break has gone part way through it.

Underside

The bottom of the fuselage where the mounting plate slots in. On the left you can see the well where the broken piece would fit and on the right you can see the part of the mounting plate that is still embedded.

Side view

This picture shows a close up of the size of the fuselage where the plate cheek has been ripped out. To the right you can see the broken edge of the ply plate.

Full side view

This final picture attempts to show how the ply plate works. Most of it is embedded in the fuselage. The hashed bit slots into mounting plate cheek. The plate is too long to be slid out to the back of the plane (to the right in this picture) - the only way it could be slid in and out is from the front. That would have to be accessed through the tiny battery bay and there is foam mouldings stopping it from moving forward. I can't see how you could get it out. I can only think that the ply plates are positioned most of the way when the fuselage is in half. Then the fuselage is joined, the mounting plate added and the ply plates pushed backwards where they lock in place.

I cannot see any way of replacing the plates - and that would make sense because those parts are not available as spares.

So what do you reckon - best I can think is to create a ply plate with wooden cheeks and to glue that into the wells in the fuselage. Have some captive nuts in the ply and use nylon bolts to mount the undercarriage.

Does that sound reasonable?

Cheers,

Nigel

John Rudd17/09/2018 16:03:11
96 forum posts
2 photos

Nigel,

All the advice given thus far is sound...However.....the stronger you make the repair will shunt failure to the next weakest point....

My advice would be to effect a repair that is more resilient, ie have the undercarriage attached by something with a bit more 'give' in it....example rubber bands....if they are strong enough for a wing then man enough for undercart and provide enough give for the less than perfect of landings...

Mike T17/09/2018 18:11:43
410 forum posts
28 photos

It looks to me as if all your problems stem from inadequate gluing of the plate that actually carries the u/c. If it had been properly glued at the factory, it'd probably just have flipped over on its back...

The easiest solution would be to ignore* the broken forward-projecting plate embedded in the fus. Cut out the remnant of the plastic mount on the other side then make up a new u/c plate from 1/4 liteply, with 1/8 hard ply tongues projecting down from it - one into the vacant pocket with the broken plate, the other going down alongside the remaining plate You'll have to excavate a slot for this). Both tongues to be packet out with balsa or lite ply shaped to fill the pockets.

I'd use epoxy to put it all together, though you can use ordinary cyano on this foam. Recover the foam from the plastic u/c plate to reinstate the fus. side. Paint white, screw in your u/c (reinforce the holes with cyano) and your done.

(*If you do all the above, then cut away the fus side with a sharp blade to access the 'old' and new plates where they abut each other, you could glue in a ply plate to bridge them. Cyano the fuse side back in (after the inner surface has been relieved to allow for the ply plate)

A picture would paint a thousand words; unfortunately, I haven't got one...

Nigel Heather17/09/2018 18:49:41
104 forum posts
7 photos

@Mike T

Thanks for the suggestion - pretty much what I had in mind.

Although not sure I will go as far as bridging the ply plates - but an interesting idea.

Cheers,

Nigel

Mike T17/09/2018 18:58:17
410 forum posts
28 photos

Nigel - the good thing about working with foam is that with a new blade, you can cut chunks out then put them back in virtually invisibly and with no loss of strength. An alternative would be to cut a slot tight alongside the plates and to push some 1/16 ply in to this, then hit it with the thin cyano...

Nigel Heather18/09/2018 00:23:36
104 forum posts
7 photos

Would 4mm nylon bolts be okay for fixing the undercarriage or should I go up to 5mm.

I want to go with something that will shear rather than pull the mount out but not to break on perfectly good if a little hard landings.

Chris Walby18/09/2018 07:12:40
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952 forum posts
228 photos

Nigel, IMHO just take a step back, did you remove the U/C due to a hard landing? You stopped using massive drag and basically something had to break to dissipate that amount of energy!

Glue it back together, add a little reinforcement and try not to add too much weight, 4mm nylon screws should be okay. A trainer foamie I had used 2.5 mm steel screws for that reason, easy to sheer off in the event of a accident, but easily replaced.

Nigel Heather18/09/2018 08:45:01
104 forum posts
7 photos

Posted by Chris Walby on 18/09/2018 07:12:40:

Nigel, IMHO just take a step back, did you remove the U/C due to a hard landing? You stopped using massive drag and basically something had to break to dissipate that amount of energy!

Glue it back together, add a little reinforcement and try not to add too much weight, 4mm nylon screws should be okay. A trainer foamie I had used 2.5 mm steel screws for that reason, easy to sheer off in the event of a accident, but easily replaced.

Yes it was a short landing. It didn’t go in that hard or fast, had it covered another few yards it would have been a perfectly good landing.

But a gust caught it and put it down short into unmown grass. I was expecting to find out t just up on its nose and surprised to find the mount broken and ripped out - but that is probably my lack of experience with foams.

I need to check the size but the undercarriage is held on with small steel self tappers - they didn’t break, they didn’t even bend. This is why I am considering nylon fixings when I repair, leaning towards 4mm as I don’t want a repeat performance, but unsure whether that would be too small for nylon.

Cheers,

Nigel

Mike T18/09/2018 10:17:20
410 forum posts
28 photos

Speaking from my experience doing similar repairs to Wot4 foam-e's, I'd just use the original steel screws, self-tapped into the u/c plate (cyano re-inforced threads). However, tapping the plate 4 or 5mm for nylon bolts would be just as good.

What I find now is that heavier landings tend to result in bent (or broken) wheel axles, rather than damage to the mounting in the fus.

Lima Hotel Foxtrot19/09/2018 15:44:08
avatar
345 forum posts
Posted by Biggles' Elder Brother - Moderator on 17/09/2018 11:34:54:

A quick comment on adhesives for this:

1. Gorilla glue is great - but be careful, it expands like mad! Have a couple of practice goes with it in some scrap if you haven't used it before!

2. Epoxy is generally good on foam, but it can go brittle over time. TBH it will probably out live the model if you fly it a lot as a hack in the winter - so no problem really! (BTW that isn't a comment on your flying!!! Its true of all of us!)

3. Personally I don't rate UHU Por for structural repairs like this - not strong enough in my view. Its fine on a really small, indoor, style model or for largely cosmetic repairs, but not under carriages.

4. CA is great - but bear in mind conventional CA attacks foam - the stuff just dissapears before your eyes under a drop of CA! So if you use it be extra careful about any unintentional spread, or use something like Pink ZAP foam safe CA.

It will all go back together and be fine - we've all been there with a foamy!

BEB

CA depends on the foam. EPO and EPP are fine with every CA and kicker I have used on them. UHU por is fine in areas that require a bit of flex and can be better than CA or epoxy.

John Roberts 920/09/2018 12:28:48
avatar
185 forum posts
167 photos

Nigel,

I have found that 4mm breakable nylon bolts were just a bit too fragile but 5mm ones were the ideal compromise - strong enough to stand a bit of a rough landing but weak enough to snap if you drop the model down heavily.

Just be aware that whilst the breakable bolts are a good way to stop the whole u/c mounting block from getting ripped out of the fuselage you may well find that the battery cover catch starts to pick up damage instead. I suggest that you try to 'fabricobble' a bit of a foam skid to protect the catch because it will soon get torn out otherwise.

Nigel Heather20/09/2018 12:59:28
104 forum posts
7 photos
Posted by John Roberts 9 on 20/09/2018 12:28:48:

Nigel,

I have found that 4mm breakable nylon bolts were just a bit too fragile but 5mm ones were the ideal compromise - strong enough to stand a bit of a rough landing but weak enough to snap if you drop the model down heavily.

Just be aware that whilst the breakable bolts are a good way to stop the whole u/c mounting block from getting ripped out of the fuselage you may well find that the battery cover catch starts to pick up damage instead. I suggest that you try to 'fabricobble' a bit of a foam skid to protect the catch because it will soon get torn out otherwise.

Just the info I needed - will go with 5mm.

Cheers,

Nigel

Neil6720/09/2018 15:38:19
avatar
186 forum posts
3 photos

Nigel,

I cut out some foam and glued in a ply plate with 4mm captive nuts. Added a 'breakaway plate' as recommended on the forum to allow access to any sheared bolts. 4mm bolts are really cheap and seem to last quite well, and believe me my landings can be dodgy at the best of times, as our site can be a tad windy. I use 5mm bolts on a traditional balsa acrowot and they also work well with the same breakaway plate setup. The foame is great fun and so easy to throw in the car with wing on for a quick visit to the site. I agree with the comments about the hatch and they really should redesign the battery space - they must have paid for the moulds by now.....

Enjoy.

Neil

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