Fuselage cross-grain sheeting

[Geoff S]

I'm building a 60" ws Skyshark Fantasy kit I bought on eBay some years ago. It's designed for a 46/60 size glow engine but I'm building it to be powered electrically using a Pelikan Foxy 4020/10 from Puffin Models.

I'm saving weight where I can and I've discarded the 6mm firewall and fitted a 3mm one further forward to suit the motor. The turtle deck is already film covered stringers but the rear of the fuselage calls for the usual cross grain sheeting (2.5mm in this case) despite the fact there are several 6x6mm struts separating the sheet sides.

I really don't see the need for cross grain sheeting if there is adequate structure to keep the sides rigid and I intend not to sheet at all but to film cover the underside to save more weight.

I have doubts about the need for cross grain sheeting in most models anyway and I feel more or less confident in omitting it but what do more experienced builders think?

Geoff

[Dave Hopkin]

I think I would err of the side of caution and discard the sheeting but substitute corner gussets on the 6mm struts

[PatMc]

The weight saving would probably be minimal.

[onetenor]

Won't the weight saved depend on the specified thickness of the cross graining ? And of course the hardness.

[Piers Bowlan]

I agree with PatMc, why would you want to omit the cross grain sheeting as the weight saving will be minimal and the loss of strength/ torsional rigidity, considerable. The motor you are using can produce 800W on a 4/5S LiPo and suitable prop, much the same as a 46/61 two stroke IC engine I would have thought. I can't see that weight is an issue and you can shift the battery fore and aft too to get it to balance correctly, so no worries about it being tail heavy.

just my 2p worth.

[Former Member]

[This posting has been removed]

[Levanter]

The cross grain sheeting acts in a very similar way to the sheer webs on a wing. It resists a racking movement and therefore keeps the structure straight. You would not be jumping to remove sheer webs on a wing. The corner brackets will have some limited effect but a fraction of the performance of sheeting.

If I was looking to save weight I would omit some cross braces entirely, maybe cut lightening holes if the plane is film covered and on occasions I have reduced the thickness of the sheet and applied two very thin layers at a 45 / 45 orientation or 45 / 0 to keep the outside layer the same grain direction as the rest of the fuselage. Because this effectively you have created a plywood you can be quite generous with lightening holes and get something very light and strong. Use a water based glue very sparingly.

My main thought is like others. Why bother on this type of model. You have plenty of power.

[bert baker]

Hi there,

With regards to the weight of Balsa wood.

I have done a few free flight models where weight is an issue, I was very surprised at the difference in weight from one piece of Balsa to another of the same size.

My local model shop (LMS) has a set of scales making selection better when light weight balsa is required.

Perhaps you LMS does the same well if you have got one,LOL

Bert

[J D 8]

One thing to remember about any electric model is their landing weight is the same as on takeoff,yes motor mount can be lighter due to lower vibration levels but landing stress on the fuz may be greater due to the weight of the battery.

[Engine Doctor]

You could probably reduce the sheeting thickness to 1.6mm/1/16th and still maintain stiffness. If you go for the laminating method ,say 0.8mm at 90 deg, be aware that the glue will have an impact on the weight .PVA would probably be the lightest as most of the weight ie ,the liguid evaporates.

Edited By Engine Doctor on 26/04/2015 10:26:52

[roger graves]

Back in the day when I electrified a Kamco Kadet (Elektradet) with 600 brushed and gearbox I left off all cross grain sheeting , but pinned all cross braces using cocktail sticks no problems.

I think that the cross grain idea stems from early designs where they tried to make them more crash proof.

I was once told that Mr Boeing has never tried to make his designs crash proof, so why do we?

[Geoff S]

Thanks for the interesting and thoughtful replies.

Pat and Piers: True, the weight saving would be minimal but weight saving always is. What is it? Many a mickle makes a muckle

Percy and Levater: I'm not sure the cross grain sheeting is all that important provided, as in this case, the cross braces are strong - 6x6 medium hard balsa let into the 5mm fuselage sides. The sides are 2x 2.5mm laminated together one medium hard the other softer. I had to make them as the originals in the kit were 5mm balsa that was very soft and so brittle I snapped them by catching them as I moved the fuselage in the workshop.

Most of the construction is glued with the Wudcare 5 minute waterproof PVA I used to build the hull of my scale Thames barge. I recommend it as both reasonably fast acting yet still allowing a little wriggle time.

Bert Baker: No local model shop I'm afriad. They've all closed. I find SLEC a quick and reliable supplier and I have a set of digital scales

Engine Doctor: I think lamination in this case is a bit over the top but I've already laminated the fuselage sides and much of the construction is 5 minute PVA. It saves on cyano fumes which can get over powering - in fact I had to build my Multiplex Blizzard foamie in stages because of that and I reserve cyano for when it's needed.

John Davies: It's true that electrons don't weigh any less when they're in the discharged side of the cell but I would have thought landing stresses are best served with longitudinal stiffness particularly as I'm building this as a tail dragger. I've fitted under fuselage sheeting in the past longitudinally rather than across without any issues and I think that's what I'll do here. The fuselage is curved underneath similarly to a Wot4 but I think both 2.5 and 1.5mm will conform to the curve without a problem.

Roger Graves: I agree with you. The cross braces fit tightly into joints on the fuselage sides which is as good, if not better than pins (I use cocktail sticks as dowels and hinge fitting a lot) . As I said, I think I'll fit sheeting but in line, rather than across the fuselage.

Once again, thanks for the replies.

Geoff

[Stevo]

E.D. Youbeat me to it! Laminate two pieces at 90 degrees...

[kc]

You could use diagonal struts instead of crossgrain sheeting or merely use some cross grain sheeting and some gaps.

Whenever you fit cross grain sheeting or diagonal struts just try to see how stiff it is before and after. This will tell you how worthwhile it is.

[bert baker]

Hi there,

I am sure slec would sort wood by weight if required.

[Bob Cotsford]

I'm with the 'keep it' brigade, use very light 3/32"(2.5mm in foreign) sheet, it won't weigh much but it will add a lot of torsional stiffness between wing and tail. If it was a vintage floater then ok, leave them off, but for an aerobatic model, don't spoil the ship for a few grams of soft balsa.

[Piers Bowlan]

I agree, Bob has put it very clearly. Aircraft design, more than any other form of engineering, is about compromise. AKA: the most strength/stiffness for the least amount of added weight. It is not about absolute weight or absolute strength. Longitudinal sheeting weighs the same as cross sheeting but longitudinal sheeting is likely to split in an 'arrival'. This is a hobby, there are no rules, it is up to you, so enjoy! However there is 'best practise' so ask Peter Miller what he thinks ( he has designed more models than we have had hot breakfasts!)

[Peter Miller]

IT is amazing how a relatively minor thump can cause a fuselage split lengthways. Now you can go anyway you like and follow any or all the suggestions made.

MY only comment is this.I cover the bottom of all my models with 1/16" sheet with the grain across the fuselage. Sometimes just stuck to the 3/32" sheet sides. sometimes with an extra strip along the bottom of the sides in the bottom corner.

I do not suffer from lengthways splits even in quite heavy thimps.. But then, that is just the way that I like to do things.

OH, I fitted an Eagletree G meter to one model. PUlled 24.8 G. Nothing failed.

[Steve Hargreaves - Moderator]

+1 for keeping it....reduce the thickness by all means but as the other chaps have said the torsional rigidity the sheeting will add is out of all proportion to the weight penalty.....the days of making electric models extra light to maximise performance are a long way behind us....

[Geoff S]

Peter Miller: I'm a geat admirer of yours and even have a copy of your book 'Designing Model Aircraft' which I use a reference but I've only designed, built and flown a couple myself. So I know you speak from vast experience. However I'm fairly experienced myself in what are euphemistically called early arrivals and I find that model structure behind the wing seems to survive remarkably well and I've never suffered longitudinal splitting.. In fact I could have a collection of relatively undamaged tail structures as memorials to departed flying friends

I find your measured G levels quite astonishing but, having been involved in the design of rotating radio telemetry systems in gas turbines in the distant past. I know how high G can get - 20,000 at quite small radii at around 7000 rpm!

Thanks everyone for your contributions re my dilemma. Bottom sheeting a model is about the last thing I do in construction because I like as much access as possible when installing push rods etc. I'm approaching that point now and I've decided to stick with transverse sheeting but possibly only 1.5 mm.

At the moment I'm struggling with shaping the 2 pieces of balsa that continue the line of the fuselage each side of the fin. I'm using sacrificial tailplane and fin pieces but, as usual, trying to stick everything well enough to allow planing/sanding whilst still being easy to remove for final fitting is a pain

Geoff

[kc]

Depends on the job but spot gluing with cyano on just a tiny part might work, then sanding away any remaining balsa afterwards. Maybe Copydex type of glue and peel off the glue after. Or double sided sellotape can be useful. Maybe pins inserted at an angle.

[Peter Miller]

Posted by Geoff Sleath on 28/04/2015 12:01:55:

At the moment I'm struggling with shaping the 2 pieces of balsa that continue the line of the fuselage each side of the fin. I'm using sacrificial tailplane and fin pieces but, as usual, trying to stick everything well enough to allow planing/sanding whilst still being easy to remove for final fitting is a pain

Geoff

A few spots of CA will do the job.I use that method on all my models.I just spot glue to CA, the thicker stuff is fine Leave it and you can carve and sand away

When done jjst slip a knife blade between the pieces and gently lever apart.

[Geoff S]

Thanks, Peter. In the end I used some double-sided Sellotape which worked quite well. I find using pins never works well because they always seem to to get in the way of my plane/sander or don't hold well enough.

This so-called CNC cut kit has needed a lot of fettling as a lot of the parts aren't really all that accurate. It's a very old kit and I can't help thinking it was Skyshark's first attempt at CNC before they embarked on the warbird kits for which they're better known. The Fantasy was available for a while as an ARTF model but I don't think it is now. It was intended to vbe a quick build but, as so often is the case with me, it's tking rather longer than I intended

Geoff

[Peter Miller]

The older "Pre-cut" kits were ready to cut to shape. That was before the days of computers etc. Still, I remember when the height of absolute luxury was a Kiel Kraft Junior 60 t when you actually got a set of bandsawn wing ribs.

Or you got the FRog kits with die crushed wood parts but normally you just got printwood, usually rock hard. The Kiel Kraft Ladybird kit with masses f printed 1/8" plywood. I learned the art of using a fretsaw on that one!!

ARTFers don't know that they are born.

[J D 8]

KK Ladybird was my first free flight power model,hard work it was and second the fretsaw bit.