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Even Experienced Modellers Can Make Beginners' Mistakes.

Let's All Learn From This One.

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David Davis20/07/2016 19:24:56
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3600 forum posts
661 photos

I cannot remember how many models I have built since that first Keil Kraft Ajax in 1959 but it must be at least thirty. One day I will make up a list from memory. Suffice it to say that I consider myself to be an experienced builder. I am currently building a Roy Scott BE2e which is a scale biplane with a 2 metre wingspan. Simultaneously I am building a little Veron Cardinal which is a vintage high-wing monoplane of less than half that size.

For Christmas I treated myself to a Chris Foss WOT 4 XL ARTF and fitted an Enya 120 FS.I maidened it last month and found that it flew beautifully like all of Chris Foss's designs. I have assembled ARTFs before and this one went together easily.

About a month ago I ceased to be the only Englishman in my club, here in the middle of France, when chap called Daniel joined. Daniel had a couple of Flair Scouts in his van and an ARTF WOTs WOT. As well as being a good builder, it was obvious that he was a better pilot than I am, so last Saturday I asked him whether he'd like to fly my XL. I took off, flew a circuit, handed him the transmitter and when I was sure that he'd got the hang of the thing, I walked off to my van to fetch my sunglasses.

When I next looked up I could see that something was flapping about at the back of the model and that Daniel was struggling to control it. However he got it down to a safe landing and that's when we discovered that THE ENTIRE FIN HAD BROKEN AWAY!! surprise.

The base of the fin was still attached to the fuselage but it had sheered off neatly where it met the top of the fuselage. I doubt that I could have landed the model in the same situation.

It was obvious that despite all of my experience I must have made a beginner's mistake and cut into the balsa of the fin when removing the covering to provide a wood-to-wood glueing surface. This had obviously weakened the structure to such a degree that the fin worked loose then broke away from the rudder which then flapped about in the slipstream as Daniel brought it in to land.

Perhaps I should have used pinned hinges on this model. At the very least I should have secured the cyano hinges with pins or cocktail sticks. Not that this would have prevented fin from breaking away.

Lesson learned. Now what does a WOT 4 fin look like? blush.

The picture below shows the model ready for repair. The base of the fin which was still glued to the model at the time of the incident, has been removed.

wot 4 xl minus fin.jpg

Kim Taylor20/07/2016 20:19:39
299 forum posts
55 photos

Hi David

I don't think it is, but on the other hand - could this be linked to the well documented hiccup on this kit whereby the fin isn't long enough to touch the tail plane to which it should be glued.

I'm in the process of building (assembling?) one of these converted to electric and I had to glue an additional 2 or 3 mm on the bottom of the fin so that it touched the tail plane when assembled.

Just a thought

Kim

Percy Verance20/07/2016 20:36:59
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8108 forum posts
155 photos

I've seen/experienced this problem too David. Thankfully it wasn't my model, but an Uno Wot trainer belonging to a new flyer. All had been well for several flights until on this particular day it chose to let go while I was flying it. I instantly chopped the engine and sort of glided - semi flopped into the (very) long grass in the adjacent field. There was very little damage, but also very little evidence of glue on the joint either.......

Something of a Wot weak point I think......

Mark a20/07/2016 20:46:42
321 forum posts
3 photos

While I was doing a pre flight check on my XL the other week I noticed some slight movement of the fin even though I had added the extra wood to the base during construction. Just goes to show a pre flight is very important probably saved my XL from a smashing end.

ChrisB20/07/2016 21:01:31
1220 forum posts
34 photos

I had to extend the fin fitting on mine. Glad I did.

Percy Verance20/07/2016 21:02:21
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8108 forum posts
155 photos

I omitted to mention in my earlier post that probably the best way to remove the film covering from the bottom bit of the fin prior to gluing is to (just) burn through the covering with a soldering iron, then peel it off.

David, what's the heavy looking metal bracket thingy which looks like it's bolted to the motor mount arms? surprise

cymaz20/07/2016 22:03:49
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9042 forum posts
1190 photos

I will put some tail bracing top and bottom when I build mine later in the year.

Thanks for the heads up on the problem.

David Davis21/07/2016 04:30:17
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3600 forum posts
661 photos
Posted by Percy Verance on 20/07/2016 21:02:21:

I omitted to mention in my earlier post that probably the best way to remove the film covering from the bottom bit of the fin prior to gluing is to (just) burn through the covering with a soldering iron, then peel it off.

David, what's the heavy looking metal bracket thingy which looks like it's bolted to the motor mount arms? surprise

When I moved to France last year I planned to rent a house for twelve months to see whether I liked living here. Kitchens in rented houses in France are often very basic and many tenants supply and fit their own kitchen units, taking them with them when they move. The kitchen in my house came with a sink unit with three cupboards underneath. I bought the old cooker from the previous tenant for 50 € but I did not have anywhere to store pots and pans so I bought three galvanised industrial metal shelving units from Bricomarche for about 65€ each. Each shelf is capable of holding 75kgs (165lbs) when reinforced with the L shaped brackets supplied with the shelving units. As my pots and pans do not weigh anything like 75kgs I did not bother using them, but one of them came in handy as a remote glow plug bracket! I was able to tune the engine much more easily than I would have had the cowl been in place! It's my intention to fit the cowl eventually and to bolt the remote to the side.

I've been actively house hunting recently and have seen a beauty but it's right at the top of my budget. ( Isn't that always the case!) It already has a superb fitted kitchen but the shelving units will come in handy in the workshop.

Thanks for the tip about burning off the covering Percy. I have an Acrowot ARTF to assemble and I'll use a soldering iron on that.

Rich too21/07/2016 06:28:15
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3057 forum posts
1070 photos
Posted by Percy Verance on 20/07/2016 21:02:21:

I omitted to mention in my earlier post that probably the best way to remove the film covering from the bottom bit of the fin prior to gluing is to (just) burn through the covering with a soldering iron, then peel it off.

David, what's the heavy looking metal bracket thingy which looks like it's bolted to the motor mount arms? surprise

Cutting the film is something I have never liked doing - thanks for the tip yes

John Olsen 121/07/2016 06:49:52
446 forum posts
23 photos

I've had the fin fall off on a Great Planes 40 size Stik. The root cause is that the bottom edge of the fin that goes into the fuselage has the grain running fore and aft, so that the balsa is being stressed across grain. The bits of vertical grain on the leading and training edge don't engage significantly with the slot in the fuselage, so it is very weak...but not obviously so unless you are looking at one with all the covering stripped off. They really need some reinforcement in this area, which I have done using bits of kebab stick for dowels.

Other than that weakness, they are a good plane and fly well.

John

David Davis21/07/2016 06:52:17
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3600 forum posts
661 photos

So having lost the fin I set about building a replacement.

I measured the height of the rudder, added the depth of the fuselage and cut a suitable length to make up the rear part of the fin out of 4"x1/4" balsa sheet. The notch at the corner will match the extreme rear of the fuselage. The existing covering film will be cut away.

wot 4 xl fin repair (1).jpg

I then cut a diagonal based on the TLAR principle, "That Looks About Right!" and from the off-cut made up the front part of the new fin. I decided to make the top of the fin a little wider than I remember the original to have been in order to give the top hinge some extra support. I could have used the triangle from the off-cut as the front part of the fin but it would have looked a bit rum so I cut a larger triangle using the smaller one as a sort of guide. I then sanded the edges flush and glued them together over the Cardinal plan referred to above. Note the very small triangular filling piece at the bottom.

wot 4 xl fin repair (3).jpg

 

wot 4 xl fin repair.jpg

With a bit of final cutting and sanding it'll probably be alright. Please ignore the specious pencil marks, spatial awareness has never been my strong suit. The fin will be a little larger than the original and will be epoxied to the top of the tailplane and to the top of the fuselage, front and rear as well as to the fin-slot.

The whole exercise has reminded me of the enlarged fins that were fitted to RE8 trainers in the First World War to make the aircraft more stable. These were cobbled together, er.. sorry, these were "engineered"  out of whatever was available and I doubt if many were identical.

Edited By David Davis on 21/07/2016 06:58:54

cymaz21/07/2016 06:54:13
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9042 forum posts
1190 photos

Job doneyes

David Davis21/07/2016 07:00:10
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3600 forum posts
661 photos

I may take up your suggestion of tail bracing too Cymaz.

Shaunie21/07/2016 08:41:50
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944 forum posts
78 photos

What about a triangular fillet between the top deck of the fuselage and the fin? I always feel that there is a stress raiser right where your fin broke and a fillet helps to spread the load a bit.

Shaunie..

Peter Miller21/07/2016 09:53:03
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10756 forum posts
1259 photos
10 articles

The fin on my Marauder (Sept 16 RCM&E) is virtually butt glued to the tailplane with the smallest of fairing blocks.

I have two 1/4" dowels set into the fin and these go through the tailplane and into a 1/2" sheet tailplane platform.

For film, I have two ways of doing it. I mark where the film must end and cover up to that line. I allow it to go 1/32" intothe joint.

Method 2. I pul masking tape down on the balsa. Apply the film with a slight overlap onto the masking tape.Cut the film over the taped area and then pull the tape off before finally ironing the edge down

Manish Chandrayan21/07/2016 10:18:59
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622 forum posts
73 photos

Most kits from yesteryears did have those fillets at tailplane and fin junction, but slowly the practice seems to have been discontinued. Fillets (nicely faired in) do the job very well.

An old soldering iron for film removal from the ARF's is always better than doing it with a blade. The iron can also help in opening up servo cutouts and other tasks. Smaller holes are best dealt with a heated piano wire piece. This not only cuts the film without damaging the balsa underneath but also seals the film edges.

Wire bracing is also a good trick and adds tremendeous rigidity to the entire unit. While using screws to secure the anchors for bracing, use of a metal tube of sufficient length let in the wood surface ensures that the screws when tightened do not crush the wood

Bottom line is most of us do commit errors that we should have not. Good to see the model survive. Testimony to the design and flying skills of the pilot on the sticks at that time

Peter Miller21/07/2016 10:27:56
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10756 forum posts
1259 photos
10 articles
Posted by Manish Chandrayan on 21/07/2016 10:18:59:

Wire bracing is also a good trick and adds tremendeous rigidity to the entire unit. While using screws to secure the anchors for bracing, use of a metal tube of sufficient length let in the wood surface ensures that the screws when tightened do not crush the wood

I insert a short stub of 1/4" dowel into the T/P or fin for the bolts. Also use 8BA solder tags to attach the wires.

Struts made from K&S 1/4" sreamlined brass tube work very weell for tailplane bracinf.

PatMc21/07/2016 10:48:59
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4326 forum posts
524 photos

I just cut the film using a sharp scalpel, peel it away from the area to be glued & use pva as the adhesive. If I don't think the design uses enough support between tailplane, fin &/or fuselage I modify the joint.

Where possible I fix tail surfaces using some form of screw or screw & peg system rather than gluing them permanently in place.

Peter Christy21/07/2016 11:13:43
1729 forum posts

I must put my hand up and confess to a stupid error I made only a couple of weeks back, involving a WOT4E-Foamie. I'd bought it as a test hack for checking out new or repaired radio gear, assembled it as per instructions. On checking the CofG, I found it to be quite a way aft of where the instructions said. I added about 50gm of lead to the nose to get it right, and then headed to the field.

To my surprise, the model was way out of trim, constantly trying to dive sharply into the deck. And whilst frantically fumbling for the elevator trim - yes - I managed to turn the TX off!!!

On most transmitters, this wouldn't have been a problem. I immediately realised what I had done, and switched it back on again! Unfortunately, this was a computer Tx, and it took some seconds to re-boot! Worse, I had programmed a "throttle cut" switch, and the Tx would only boot if the switch was in the "cut" position - which of course it wasn't because I was flying at the time!

The poor old Wot4 spun into the crops, sustaining quite severe damage to the fuselage, though everything else survived OK.

I've now replaced the fuselage, but in the process discovered that there is apparently an error in the kit instructions. The CofG should be 80mm back from the LE, not 70! I discovered this warning on the website where I got the new fuz.

The repaired model balanced spot on the correct position without the need for any lead up front, and flew straight off the board.

So anyone building a Wot4E-Foamie, watch out as the printed instructions indicate the wrong CofG!

But that still doesn't excuse my "dumb thumb" moment.

And another word of advice: Whilst a throttle cut switch is a good safety idea on an electric model, it should NOT be set so that the Tx will not boot unless it is activated!

My check list now goes: Tx ON, throttle cut SET, and only then power up model.

Lesson learned - and after 50 years of RC flying!

--

Pete

Manish Chandrayan21/07/2016 11:38:01
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622 forum posts
73 photos

K&S tube ends squashed, flattened and drilled appropriately at the ends and suitable size carbon rods epoxied in to other ends is another variation for rigid and light bracing smiley

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