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May 11, 2019, 10:58 PM
Registered User
Thread OP
Thanks Colonel.

Last catch-up. The trailing edge of the wing will be secured with magnets. It's usually best to have a separate hatch for battery access but in this case access through the wing opening is the most practical and the magnets will make this quick and easy.

There was recently some discussion on this forum on the topic. The consensus seemed to be, don't! Many seem to have experienced/witnessed/heard of the plane that did a sudden loop when the wing momentarily lifted from its seat. With rubber bands this is quite humorous; not so with magnets.

However the trailing edge fastening of a low winger is surely the perfect case for magnets. In normal flight the wing lift will hold it in place and a cambered wing is trying to pitch down as well. I'm well on the way to convincing myself. In the unlikely event that I attempt an outside loop then only 1/3 of the load will act on the trailing edge.

So, it's done. The magnets are 3 X 3mm.
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May 12, 2019, 07:26 AM
'Douglas' to his friends.
Quote:
Originally Posted by Footrrot
...The trailing edge of the wing will be secured with magnets. It's usually best to have a separate hatch for battery access but in this case access through the wing opening is the most practical and the magnets will make...
I've thought of doing this myself a couple of times, and have used magnets in their traditional 'hatch' role on a few 'planes. The trials I did did not inspire me, though, for wing retention. I found that it's quite difficult to get a reliable, long-term bond between epoxy and the magnets, and so usually employ a film over the face of the magnet, itself securely glued to the woodwork. However, the attraction between the magnets is greatly reduced if there's any gap, even as little as thin film. It would work with bigger magnets, but I think that, for small, light 'planes where the technique would be best suited, the very weight of suitable magnets plays against them.
I'll be interested to see how they fare for you, hoping that your faith in the technique is not betrayed.

May 12, 2019, 08:46 AM
I like real wooden aeroplanes!
Sundancer's Avatar
Fear not Footrot, the magnet fixed TEs is perfectly practicable for model like the Estrellita. My electric R/C version of George Woolls' subsequent design, the La Paloma biplane has the bottom wing fixed just like this - dowel at the LE and two pairs of 5 mm dia rare earth magnets at the TE. Furthermore the interplane struts also use magnets for fixing. It has done a LOT off flying in the last four years with no problems whatsoever. I also have several other models - scaled up KK Sportster, electrified Lulu soarer, Size 9 single float flying boat, Smeed Flipper biplane etc that use magnets teamed with either slide-in screw head locking bolts or dowels at the leading edges, again, no problems at all. The little Flipper relies on magnets only and it is very impressive the way it flies apart into the various components in a rough landing without damage whilst remaining perfectly OK in flight.
May 12, 2019, 11:34 PM
Registered User
Thread OP
D: - Your comments raise an issue I haven't seen mentioned before. There is no film over my magnets - the faces are in perfect contact and they make a satisfying click when the wing is installed. But I did use kitchen film between the magnets when epoxying them in place. This means there should be a tiny gap between them, but the structure springs so the magnet faces touch . Only a tiny amount of spring but this produces a force opposing the magnets.

Shortly I'm going to cover the wing - this will further increase the amount the structure must spring for the magnets to touch. Again, only a tiny amount, but will this significantly weaken the net attraction? Taken far enough, the amount of spring could be just slightly less than the pull of the magnets. In that condition a small disturbance would overcome the magnet; spring in the structure would eject the wing like a jack-in-the-box.

Thanks D for raising that issue. I will make sure that the wing seat has enough clearance to avoid interference with the magnets.

SD: - I didn't know (or more likely I forgot) that you had substantial experience of magnet wing fixing, so thanks a lot for the reassurance though I'm still just a little apprehensive. The absolute finality of the way magnets come apart is somehow disturbing.

While I've been catch-up reporting the last couple of weeks I've also been slowly plodding onward. I've decided where everything will fit, made up push-rods and control horns, figured out where the control rods will exit the fuselage and cut holes, weighed each item and calculated the CG. On a light model covering and finishing moves the CG rearward quite a lot due to the large area and moment of the tail surfaces. I guessed an allowance for that. It looks as if the CG will be OK.
May 13, 2019, 02:27 AM
So I'M meant to be in control?
Colonel Blink's Avatar
As he has mentioned, Sundancer has lots of experience with flying models with wings retained by magnets; I have none. However, it has always been something I have shied away from, on the grounds of the basic physics that if a banded wing lifts off it's seat under aerodynamic forces, the force applied by the bands increases with the separation distance; with magnets it very rapidly reduces. So all the time the aerodynamic forces are lower than the magnetic force it's deep joy. But the second it exceeds the magnetic force and the wing lifts off it's seat, unless by chance you reduce the force the wing will simply carry on and it's goodnight campers and hello terra firma.

However, on a slow flying lightweight rubber conversion as long as you make sure that the magnets are a bit stronger than you think they need to be you should be fine!
May 13, 2019, 03:54 AM
I like real wooden aeroplanes!
Sundancer's Avatar
Just to be clear; I wouldn't want to retain the wing SOLELY by magnets except in a very small light model such as the Flipper. What I do is to have a mechanical retainer of some sort either at the leading edge - dowel in tube - or under the main spar position - bolt in wing with head locating under ply plate - and then magnets at the trailing edge which lock the wing in position. Now I have NEVER seen a wing lift off against the tension of rubber bands STARTING AT THE TRAILING EDGE. A moments thought will indicate that it MUST be the leading edge that lifts first - hence the mechanical fixing there. There isn't going to be any serious force trying to separate the magnets.

In the case of the Estrallita, being a low wing model, unless negative G aerobatics (whether intentional or not!) occur there will not be a problem anyway.
May 13, 2019, 06:07 AM
So I'M meant to be in control?
Colonel Blink's Avatar
That's a good point - the magnets will have the mechanical advantage being further away from the LE fulcrum of the retainer than the centre of lift/pressure or whatever it is

(This is a great 'clubhouse discussion', isn't it? )
May 14, 2019, 11:02 PM
Registered User
Thread OP
Sure is Colonel, and thanks everyone for your thoughts. I feel more confident now - I think.

This next episode really belongs on the "stupid" thread. I usually put a wire joiner between the two elevator halves. So I fastened them down over the drawing, bent the wire, and cut the little groove for the wire to sit in. Then, in a moment of sheer daftness I filled the little groove with balsa cement and set the wire in it.

I realised nearly straight away what I had done. Somewhere else on this forum I can remember solemnly warning the ignorati not to use balsa cement around a wing centre section because it can cause dreadful warps. I knew exactly what I had done and what was going to happen.

Nevertheless I thought, if I leave this fastened down flat for a really long time I might get away with it. Well I didn't and you can see in the photo what happened.

It took some time to accept that this was absolutely unfixable. Once that sunk in it didn't take long to make two more elevator halves. This time I knew where the horn was going to go, cut neater grooves, used a lighter gauge wire, and used slow setting epoxy to set the wire in.

Saved .4gm as well.
May 16, 2019, 09:22 PM
Registered User
Thread OP
Today's story also features balsa cement, but not as the villain this time. The plan shows a fairing on the undercarriage legs supported by a dog-leg bend in the wire. I didn't bother with the dog-leg but simply glued the 1.5mm balsa fairing on the back of the wire. I know this works because I've done it before on a model about three times the weight of the Estrellita. This u/c survived not only some dodgy landings but also the very violent demise of the model. As evidence I submit the yellow exhibit in the photo.

I cheated a little by making the fairing narrower than drawn. This reduces the stresses on the fairing while also increasing its flexibility slightly. The wire was sanded and coated with balsa cement which was allowed to set. A groove was made up the front of the fairing. To make the groove I used a broken off piece of a 1.5mm diameter file like thing that was sold as a fret saw blade that cut in any direction. The cut off end of a steel wire would work. Stick the fairing in place with balsa cement.

The next bit is key I think. Cut a strip of regular weight Polyspan about 15mm wide, grain cross-wise, and wrap it around the front of the fairing and back onto the balsa, with guess what (BC!) rubbed well in. Polyspan is very strong indeed along the grain and useful for all sorts of bracing jobs. Finally, I wrapped the whole thing with red Airspan set in dope.

This all sounds a bit dodgy - you may think the fairing would split off the wire. It doesn't happen though because the glue line is long and the Polyspan stops the balsa cracking along the grain. A long explanation but a totally simple job.
May 18, 2019, 11:19 PM
Registered User
Thread OP
The fuselage, fin, and rudder are covered with late lamented Airspan left over from long ago. It is supposed to be attached with Balsaloc, a heat activated white goo, Not having any of that I experimented. Setting the airspan in wet dope worked well but it has to be rubbed down until the dope sets and that didn't seem practical. Laying it on dry dope and soaking thinners through it didn't work at all. I had read somewhere that ordinary PVA could be used just like balsaloc, and to my surprise that did work - well it seemed to.

Covering the rudder, fin, bottom and sides of the fuselage went very well, no problems at all. However when I came to the upper fuselage quarter panels nothing would go right. The problem appears to be that these panels, although they appear to be plane surfaces, actually incorporate some double curvature. Most covering materials have enough stretch when applying and shrinkage to accommodate this. You don't notice any problem. Not so with airspan.

I needed several attempts at these panels, during which it became apparent that the PVA didn't grip as well as it seemed to in my tests. In fact, while using a very low tack masking tape, in an attempt to get neat overlaps, I managed to peel the covering off one side. The secret of Post-it pads rediscovered. Negative progress.

Eventually I gave up and pronounced it finished. I like the Airspan's appearance of a good quality tissue but I'd sooner people didn't look too closely. It will get two light coats of WBPU to waterproof - if the PVA got wet the whole lot may come off.

I would say that PVA/Airspan is OK if you are sure you have plane surfaces and can get good overlaps. A better adhesive would help but Airspan is always going to be one of the more challenging covering materials.
Last edited by Footrrot; May 19, 2019 at 06:39 AM. Reason: sp
May 19, 2019, 06:16 AM
I like real wooden aeroplanes!
Sundancer's Avatar
Most PVAs are not really heat sensitive in the same way that Balsaloc is - they may seem to grip, but the bond is very weak - but obviously you have found this out!!
May 19, 2019, 11:56 PM
Registered User
Thread OP
In my last post I made a rather silly spelling mistake, since fixed. A covering material that can "peal" would be on a par with the amazing stuff that can be "taught".

Funny how the brain works. I woke in the middle of the night, somehow only then becoming aware of the error. I knew I wouldn't get back to sleep unless it was fixed so I got up, turned on the computer, and went for the edit function hoping I'd get there before anyone else read it - no such luck. SD got there first and now probably thinks I'm an idiot though he was too polite to say. He's probably right but not because I can't spell.

If you looked closely at the photo in my last post (even though I asked you not to), you may have seen that the Estrellita now has a crew. Usually my planes are OK with fairly whimsical folk at the controls but the Estrellita deserves a steadier sort of chap. These pillars of propriety are chosen from Paul Plecan's Profile Pilots. I must have found them somewhere on the net but goodness knows where. I will post them again for anyone needing sober, clean living, wholesome personnel for their model.
Last edited by Footrrot; May 19, 2019 at 11:59 PM. Reason: sp
May 20, 2019, 03:57 AM
Registered User
Footrrot;
Thanks for the pilots. The last one I did I drew myself purely from imagination. It took quite a long time to do ...
And thanks Paul Pecan too.
May 20, 2019, 04:02 AM
'Douglas' to his friends.
Thanks for risking the threatened prosecution by posting those pilots; they'll come in very useful (although I'll probably 'blow them up', as I don't see myself building much 'peanut' scale ever...).
Whilst I'm here, congratulations on a fine build, with delicate covering and an excellent Build Log. Looking forward to First Flight.
May 20, 2019, 11:41 AM
Registered User
@footrrot,
Looks just fine.
Far far better than my Doculam refab of my Solostar...


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