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RCME June 1994.

With Chilli Breeze plan/article

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Jonathan M24/06/2019 14:17:14
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Fuel tank - The plans specify 6oz and the SLEC version (red) looks fine as it has a 47mm sq cross-section which will fit the 50mm sq cutout in F2.

Servos - Yes, of course 225s would be better for rudder and elevator. Re wing-mounted or central servo(s), I can see good arguments for both. Torque-rods would be cleaner, but if space is tight (esp for RX battery) then wing-wells would free up a bit of wriggle-room in the fuselage.

Wheels - Mike Delacole specifies smallest possible, 1-1/2" diam but whether these will handle our grass patch will have to be seen.

UC - Built-up wing shows hardwood bearers built into the forward rib-structure - each has a long piece of 1/2"x3/4" section with an additional piece to make it deeper at the inboard end where the torque-stub fits in (i.e. a L with a very long foot). Question is whether a similar arrangement can be let into my foam wings - or would they need an additional 'brace' running fore-and-aft for a more secure anchorage in the foam?

Nigel R24/06/2019 15:09:53
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U/C - foam needs "surface area" in contact with your hard points. I'd go 1/4" ply plate, 1" wide, 5" or 6" long, let in to the underside. On the top, let in a 1-1/2" length of the same stuff. Make sure the torque rod end goes firmly into (but not quite through) the topside piece.

will -024/06/2019 15:23:10
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Yeah red Slec tank rings a bell. Not sure other brands will fit.

U/C - have small wheels but have had to repair the undercrackers as the torque end split and broke free from the beam (built up wing).

Good luck with the foam wing, hope it's not too heavy!

Bob Cotsford24/06/2019 18:27:00
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A tip my mate Dave gave me years ago:-

Take one broom stave and cut off two lengths equal to the depth of the wing. Round of the end of the remaining broom stave and SWMBO will never notice the missing length.

Now bore a hole through the wing of a diameter to match the stave, centre this on Nigel's ply plates at the point where the vertical leg of the torque rod goes through, the plates may need to be 1 1/2" or so wide to accomodate this. Drill the stave centrally for the torque rod, give it a good coat of PVA, Gorilla Glue or whatever and slide it through the wing to span the upper and lower plates.

Sand flush with the wing surface when dry and you will now have a torque rod undercarriage mount that will outlast the rest of the model. If the Missus has hidden the broom any similar dowel 1/2 - 1 1/2" diameter will do depending on the size of model.

Edited By Bob Cotsford on 24/06/2019 18:30:34

Edited By Bob Cotsford on 24/06/2019 18:31:57

kc24/06/2019 19:27:45
5962 forum posts
168 photos

Bob, if I cut the end off SWMBO's broomstick will she still be able to fly with it?

Bob Cotsford24/06/2019 20:20:00
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Posted by kc on 24/06/2019 19:27:45:

Bob, if I cut the end off SWMBO's broomstick will she still be able to fly with it?

She may need to adjust her C of G to compensate kc, Weightwatchers could probably suggest an appropriate spell devil

Jonathan M25/06/2019 07:43:37
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As I'm currently witch-less I've decided to forego shortening my own broom, but have concocted a variation using 1" square softwood. Also going to fit 2" wheels as the 1.5" version will struggle on our patch.

dsc_0114.jpg

I never initially received the plans from Sarik so I called them a week after ordering online. A despatch email duly came through, then a couple of days later two separate envelopes arrived! So if anyone fancies building a CB and wants my second set just PM me your address.

kc25/06/2019 19:44:09
5962 forum posts
168 photos

Nice reply Bob. Fortunately CG is well within limits. I will get a spell cast on me if SWMBO reads this!

Jon there is a theory that it's lighter & stronger if the 2 vertical blocks etc meet in the middle or are common. Possibly needs u/c legs to be longer in the horizontal plane but worth considering. But it means the vertical part is well within the fibrglass bandage and if damage does occur it's hidden in the fuselage so no visible scar.

Jonathan M26/06/2019 08:27:09
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Saving the nation's broom-handles for their proper flight-modes...

...and developing kc's extremely sensible theory, how about just using L-shaped ply plates on the underside only, meeting in the middle as before, and with each fat foot mostly under the glassed bandage?

This would spread the twisting forces under the now horizontal feet across a wide enough area of ply over fully intact foam. Simpler to construct, less intrusive (nothing on top of the wing or going through the foam interior), and simpler to repair if needed. This extends the length of the main torque-arm only very slightly so shouldn't make much difference to the springiness of the UC, and total weight surely no more than the through-wing variants.

dsc_0115.jpg

PS Sketch not to scale.

PPS Spare plan now posted to first respondent by PM... for an electric version.

Edited By Jonathan M on 26/06/2019 08:33:16

kc26/06/2019 09:01:05
5962 forum posts
168 photos

Actually what I was trying to describe was just having the vertical supports joining each other in the centre. However what you have now drawn seems very good instead.  But wouldn't it be useful to have the plate in one piece to form a dihedral brace too. then only 1 double groove u/c clamp could be used.  ( allowing for dihedral in some way)   It's debateable whether the ply needs to extend out as far as the wheel but only needs to go just beyond the u/c clamp to be just as effective. Extra width of the ply plate may be better than length.

I also suggest that the clamps are most effective if they are almost at the end of the wire and the others as near as possible to the bend.

 

Edited By kc on 26/06/2019 09:05:57

Jonathan M26/06/2019 09:46:13
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Fun thinking this through together...

A single 'dihedral-brace' piece of ply is doable, but would need to be tapered in thickness towards each end (as well as faired slightly across the width to match the wing camber). It'd be simpler to assume the wing-bandage will give enough binding strength and just make separate ply pieces - although these could then be doweled together at the root for extra security. The aft-running feet can remain separate with their own saddle-clamps (avoids lengthening the main run of torque-spring any further). My sketch wasn't to proper scale, but the feet don't need to be quite that long as shown, and of course the clamps would position right by the bends.

The only reason I left the outboard ends longer (as per the built-up wing plan) was to spread out any shock-force up the main UC leg, but the ply could now of course be wider rather than longer to achieve the same outcome.

I'd radius the corners (say 5mm rad) and scribe these in to avoid the weakness of hard stress points in the veneered foam.

kc26/06/2019 12:10:34
5962 forum posts
168 photos

A flat ply plate could go right across the dihedral by just recessing the middle ribs or foam  a little and building up a balsa filler bit on the outside - with cutouts for the clamps. Much easier than tapering ply in thickness!

Edited By kc on 26/06/2019 12:11:09

Nigel R26/06/2019 12:54:05
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I think you're in danger of reinventing the wheel here...

I wouldn't put the clamps or wire in the area of the bandage. I think it will be too awkward to get the wire clamped correctly withe the glass getting in the way.

I also wouldn't do a single piece underneath, whether L shaped or not. All the torsion loads will then serve only to try and rip it out. This approach will load the glue joint in its weakest direction, and rely mainly on the foam substrate holding together in tension (which it is absolutely terrible at).

If you go with the pieces set into the top surface (or the full depth broom handle), it only has to resist fore (mostly) motion only, the magnitude of which is reduced because it is some distance (i.e. thickness of wing) away from the lower surface. The top wing skin will provides ample strength to resist this movement. If you're at all worried, you could make sure the very inboard end of the leg is just within the glass area, then the top plate is held in place by both wing skin and glass. It's also a quick way to do it, as there is no shaping required for the ply parts, just rout out the 1/4" depth cavities and stick in the ply plates.

Jonathan M27/06/2019 06:36:48
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669 forum posts
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The wheels are actually lower down - we'll get 'round' to re-inventing those shortly...! wink

Talking of lower down (and given that this is all hypothetical at present as I'll only be starting the CB later), how would one deal with retract units in a foam-veneer wing? Same problem?

Piers Bowlan27/06/2019 07:28:30
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If I were considering retracts I would definitely do a built up wing as it can be properly engineered and more likely to survive a hard landing than retracts screwed to wood, bonded to foam. That's just me.

Nigel R27/06/2019 08:28:19
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laugh

On a foam wing? Fly from a concrete runway!

I've seen several methods used.

1/2" square hardwood beams, as long as practical, two beams per retract unit.

1/4" ply plate, which can be reinforced using 1/4" dowel at right angles (i.e. poking up into the foam)

I've done the first, which was ok, providing every landing was greasy smooth. The cons with the second, if things do get torn out, it causes a lot of damage.

A light model helps - less stress on the gear. And the smoother the strip the better. As per Piers comment, with a built up you can engineer in some reinforcements to take the loads back to the spar structure.

Bob Cotsford27/06/2019 09:02:22
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7931 forum posts
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With all due respect to Nigel's comment above, I've had a few models with retracts mounted in veneered foam wings, my favourite method being to embed a U-shaped 1/4" ply plate into the foam then fill in above the ply with soft balsa shaped to blend in with the wing profile. I fly from grass and haven't ripped a leg out - yet!.  The ply plate method gives a lot more gluing area than does the wooden beam way of doing it.  As for keeping the model light, yes it's always a good idea but my 63" span Crescent Tornado was nearly 8lb and was always flown from grass.  The ply plates are glued in with Gorilla Glue (the foaming type) these days.

One example is illustrated in this thread

Wheel wells.jpg

Edited By Bob Cotsford on 27/06/2019 09:11:46

Nigel R27/06/2019 11:05:04
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2985 forum posts
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I probably wasn't very clear about the ply plate damage statement, I meant if the plate with dowels is torn out it will take a fair chunk of foam with it. Although with the dowels, there is even more glue area, and better leverage for the structure to resist the torsional loads from the gear.

Bob you are right, the ply plates can be plenty strong, ensuring lots of gluing area is the thing with foam I believe.

Is the foamed PU glue a little flexible? I used epoxy in the past, I suspect it neither reached into the foam that well, nor flexed much. Both would tend to make a poorer bond with beaded foam. The foaming glues would seem to reach into the foam much better.

Bob Cotsford27/06/2019 11:45:50
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7931 forum posts
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Over the years I've tried various glues such as PVA and epoxy to bond bearer plates for fixed and retractable gear and while they work, they are relatively heavy unless you have a very accurately cut cavity to take the mounting plate or beams. Gorilla Glue does expand into the surrounding foam filling any cracks and cavities giving a 100% bond. Once dry it is absolutely rigid, and heavy arrivals seem to bend the leg before the the plate comes out.

I often use GG as a filler around snake exits and the like, once set for 24 hours it can be cut, sawn and sanded quite easily.

Jonathan M28/06/2019 07:52:54
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669 forum posts
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I can see the issues more clearly now.

Retracts would be slick to factor in, but I'd foresee the wheels and wells getting right covered in fuel residue in my IC version, which a fixed U/C would avoid.

Either type though leads to a possible third solution for securing bearers in a foam wing - which occurred to me having seen Bob's/Piers' half-buried plate for retracts.

This would have slots for ply plates (either two narrow ones as 'cross-beam' foundations for fixed U/C surface-bearers or single wider ones with cutouts for retract casings) first cut into the wing from the L/E of the foam, i.e. before the balsa L/E is later glued on. These slots would be long enough to penetrate down the chord of the wing (the direction in which torsional forces need to be contained) and the ply then buried in with expanding glue. Excavations would then be made through the underside of the wings - either for fixed U/C bearers to contact with and be secured to the buried ply cross-pieces, or for retracts directly down to the cutouts etc.

The main technical challenge would be in accurately boring down the middle of the wing section to 'mill-out' the foam, but if the ply plates were left long enough initially (to be flushed off later before the L/E is glued on) then the milling out could be a bit oversize, the sacrificial overhangs would provide a reference and adjustment-point for accurate alignment down the centreline of the chord (or angled as desired), and the expanding glue would do the rest.

Just an idea.

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