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Author Topic: Frog Talisman build  (Read 3396 times)
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Hovis
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« on: September 13, 2011, 11:30:20 AM »

I'm at the engine-bearer drilling stage with this model but it seems rather front heavy.

The pics show what I have so far - pinned and banded together - not accurately(!).  Front to back balance point is about the middle of the tank.  This is before I've added the front/side reinforcement too, which will make it more front heavy.

If I move the engine back further, I can almost get it balancing on the leading edge but then I'd have to make and install a wing-tank. It still seems to far forward to me.

I think the PAW is probably heavier than other 1.5cc engines but what should I do in ths case?  Will it fly OK or just drop like a V1 when the engine is out?

Also, how much weight should I add to the outer wing tip?
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gossie
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« Reply #1 on: September 13, 2011, 06:06:19 PM »

Where you have the engine and tank now looks to be the correct place.   If you move the engine any further back and the tank into the wing you will probably need to cut the leading edge and that will weaken it.

I think I would tend to leave the engine and tank as shown, trim off the bearers in the front and drill the mounting holes.   Finish the model and add lead to the rear to get it to balance on the LE.  Many CL models have weight in the rear for balance.

Weight in tip?    Just a small lump of lead or whatever should do it to balance out the weight of the lines in the left hand side, or a bit more for safety.  Good luck.
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Daithi
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« Reply #2 on: September 13, 2011, 10:14:59 PM »

When I flew C/L I always balanced on the front wire rather than the leading edge - as for wing ballast, I never used it - a bit of fin offset was usually enough to compensate
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Hovis
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« Reply #3 on: September 14, 2011, 02:34:32 AM »

Thanks guys!  I'll update on progress here.
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Hepcat
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« Reply #4 on: September 14, 2011, 11:56:18 AM »

First things first, does the plan say where the CG should be?  If no position is given then I would guess at 25% of the wing chord but I hope that people who have flown control line more recently than I will give their opinion.

You say you are checking the CG in the condition shewn in the photograph.  It still looks as if there is a little more weight to be added in the rear with the hinging of the elevator and the addition of the fairings (or flaps?) to the trailing edge of the wing.  However there is no propeller shewn, which is quite a heavy item and in the worst position for affecting the CG.  As Gossie has said there is a little to be saved by cutting off the front of the engine bearers and then the wing covering and any finish applied to the fuselage will also help a little to shift the balance point to the rear.

In the end you will almost certainly have to follow Gossie’s advice to add lead in the (tail) end!  Don’t fail to do this because a model with the CG in the wrong place is a dog to fly.

John

 
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Hovis
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« Reply #5 on: September 14, 2011, 12:32:16 PM »

There's no balance point on the plan. I've just drilled the bearers at the recommended location. The PAW sticks out half an inch so there will be a gap between the spinner.  I will probably glue the front end together later and get a better idea of balance.

I have some more questions:

Can I just buy some normal clothing nylon or do I need to get special stuff?

How will I diesel-fuel proof the nylon after it has been doped? I think I may have used household varnish 30 years ago.

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Daithi
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« Reply #6 on: September 14, 2011, 01:18:07 PM »

I used off cuts of nylon from a local drapwery - they usually have one that's light enough (but not too open a weave)

Fuel proofing for the PAW - polyurethane varnish is as good as any
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gossie
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« Reply #7 on: September 14, 2011, 06:13:00 PM »

As Hepcat said you have more weight to go on the rear as you complete the model.  Finish the model and see how things look.

Balance more forward will make it more stable.   Rearward balance and it will be very touchy.   My guess it about the front lead out to the leading edge would be a good starting point.   Use weight though if needed. 

Any old fancy nylon should be fine......I've seen CL models covered in all sorts of patterns.     Dope it's self with a few coats in itself is not a bad fuel proofer against diesel fuel, but a coat of polyurethane as mentioned or even a common varnish will give a nice finish and be proof against the fuel.
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Hovis
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« Reply #8 on: September 16, 2011, 03:46:58 AM »

A update on progress.

This is almost ready to cover but it's not glued together.

It takes 1.4 oz of lead at the tail to balance it at the front of the belcrank.

I have some 14g (0.5oz) fishing weights - how many should I put in the wingtip box?

I bought some nylon yesterday, now this is where I do need help.  I have covered with nylon before but I don't remember how I did it (~30 years ago).  I don't think I made a very good job of it then. Smiley

Should I glue the whole lot together first or cover it whilst it's in parts?

What shapes do I cut out e.g. to cover the fuselage?

How do I handle wing tips - should I cover the bottom of a wing then the top?  How do I handle the complicated areas e.g. wing roots, engine bearer area etc?

When I cover the tailplane, how do I handle the area around the hinges?

I've not found any suitable step-by-step text/pics on the web for this yet - are there any?

I have nylon, shrinking dope (thinned 50/50) and polyurethane varnish at the ready...
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Daithi
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« Reply #9 on: September 16, 2011, 08:37:30 AM »

What I did with nylon was to apply it wet - use dope to attach it (it will blush but that will disappear when you give it a couple of coats after it's dried).

As I said before, I never used weights in the wingtip - the centrifugal force of the whole thing scooting round at a high rate of knots is normally enough in itself - a bit of fin offset should be all that's needed to keep the nose pointing outwards from the circle - wingtip weights tend to make it a bit crabbed in flight with the outboard wing lower than the inboard.

Covering - it's usually easier to do the wing first and then assemble (but with nylon I normally used full strength dope - the structure is normally beefy enough to take that and nylon doesn't shrink as much as tissue)/I always cheated - I did a 'wraparound covering' (took thee nylon from the trailing edge, round the leading edge and then to the other t/e in one piece). That way you cover both sides at once and have a very neat and smooth l/e.

Wood areas - just sanding sealer, dope, sandpaper and a lot of elbow grease followe by polyurethane (brush several coats well into the engine and tank area to make a good 'skin'). You could brush dope the nylon onto the wood but I never did

Normally hinges are applied after covering

Fuselage - experiment with paper to get the rough shape - when you apply wet nylon it will it will pull into place easily.
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TimWescott
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« Reply #10 on: September 16, 2011, 02:36:27 PM »

It's looking good.  If you want to mess with it, make the cover to the tip weight box removable, and fine-tune things after you get the model flying.  I'd make a tail weight box, too, also adjustable.

Starting CG should be somewhere between the leading edge and 25% of the chord back.  Right on the leading edge it'll tend to drop like a rock when the engine goes out, but it'll be very stable in the air.  At 25% it'll be much more controllable, but it'll start getting twitchy.

You can also cover with plastic film, or you can splurge and get silk. 

For film, either cover the whole thing with film, or paint the fuselage with rattle-can and cover the wings with plastic.  There's a whole process -- in the US the brands to use are Polycrylic for primer and fill, then Rustoleum Gloss Protective Enamel for the top coat.  Diesel fuel is easier on paint than nitromethane, so you have less to worry about.

For silk, in the US folks go to Dharma Trading (which may sell to Commonwealth countries, too) and get 5M or 8M china silk.  That's the straight-weave stuff that's good for models, and it's a lot less expensive than Esaki silk.  Then just paint with dope in the old traditional way.
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Hovis
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« Reply #11 on: September 17, 2011, 06:39:58 AM »

Here are some pics of the nice sanded airframe prior to my covering experience.
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Hovis
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« Reply #12 on: September 17, 2011, 06:48:34 AM »

Here are some pics of my covering disaster!

I covered the rudder. Not too bad but a nasty 'spike' mess at the sharp corner. Then I covered the front of the tail plane underneath.  Looked alright but I couldn't get the nylon to stick along the hing line.  Yes, I should have covered it before hinging.  I peeled the tailplane nylon off.

The fuse and tail will be painted  Smiley

Then to one wing.  Oh dear. What a nightmare. Perhaps I'm using the wrong nylon but it's very hard to make it stick after it's turned a corner.  The wing tip is just a lose bag!  I used full strength dope too.  After another coat, the wrinkles are just as bad.

I'm going to peel it off and try again now.  This time I'll use two pieces rather than one.

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Daithi
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« Reply #13 on: September 17, 2011, 10:03:41 AM »

The trick is to soak it well and pull taut as you apply it (use bulldog clips on the overhang on the wraparaound to hold it tight). Then full strength dope (thinned dope isn't great on fabric)

Nylon doesn't shrink the same way as tissue (with water)
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Hovis
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« Reply #14 on: September 17, 2011, 10:20:56 AM »

Thanks - I've just been hanging pegs around the edge. Bulldog clips would be much better.

The second try looks a lot better although there are white patches on it.  A coat of dope didn't appear to remove it. Perhaps that's the thinners.

I've nearly done 3 out of 4 wing panels. I'm rapidly running out of dope.  I thought Solarfilm etc finishes were expensive, now I'm changing my mind. (£3.95 for 125ml of dope!)

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Hovis
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« Reply #15 on: September 17, 2011, 12:18:36 PM »

It's taken me all day and a whole pot of dope!

This time I made a much beter job. Where I put on another full coat of dope, the nylon sank a bit and then reset itself a bit lower - you can see it around the ribs on the left of the picture.  The wing on the right of the pic hasn't had a second full coat and looks much better and less patchy. The white stain-like marks by the leading edge are from the dope - perhaps where it mixed with the water?

I'm wondering if this nylon is pre-shrunk because I stretched it over the frame, sprayed it with water and it didn't loosen/sag at all.

Now I need to paint and fuelproof the rest...

Will the polyurethane varnish hide the white marks from the dope?
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Daithi
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« Reply #16 on: September 17, 2011, 12:25:18 PM »

You don't spray it - you put it on soaking wet and stretch it over the frame and let it dry. The 'blyushing' will come out after a couple of coats of full strength dope (or a drop of brasso will cure it - I found that by accident one time when I spilt some on a wing  Cheesy )
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Hovis
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« Reply #17 on: September 17, 2011, 12:47:08 PM »

On the first try, I tried soaking it and then stretching it but it just introduced loads of creases which were hard to pull out. Spraying it was much neater.

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Hovis
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« Reply #18 on: September 17, 2011, 12:49:06 PM »

after my first covering disaster, I found this was useful:

http://cbhmodels.co.uk/nylon_covering.htm
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Daithi
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« Reply #19 on: September 17, 2011, 01:12:32 PM »

Spraying won't work - it doesn't shrink but it WILL stretch when wet
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« Reply #20 on: October 01, 2011, 09:05:24 AM »

Finished model ready for flying.
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Saint Alfonzo
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« Reply #21 on: December 07, 2011, 03:17:29 PM »

Thats looking really nice.. How did the flying go??   Smiley
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Hovis
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« Reply #22 on: December 07, 2011, 03:29:41 PM »

Thanks! It hasn't flown yet.  I had trouble getting diesel fuel but eventually managed to get the stuff together to make my own.  I've only done some bench running and not been able to get any flying done. Now that the days are so short and the weather is bad, it looks like I'll have to wait until spring.  Sad

I'll post a flight/crash report here when I do get the chance to try it.
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Saint Alfonzo
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« Reply #23 on: December 07, 2011, 03:47:46 PM »

I'll be interested to hear how it goes then.. I just love the old kit model;s from that ( my) era!!
 Surprised you couldn't get hold of diesel fuel easily, I get mine from the local model shop ( well actually its a local sports and model shop.. the modeling stuff occupies one small corner!!).. Where abouts ( roughly ) do you live, if your in the south there's quite a few c/l sport/ competition events to get along to..Smiley
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Hovis
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« Reply #24 on: December 07, 2011, 04:23:17 PM »

I'm in Cambridgeshire. I can order fuel and collect it from Bury St Edmunds but it costs a lot in (car) fuel to get there.

I found a friendly pharmacist who was able to get me some ether and I was able to mix fuel for the first time.  It was very satisfying, seeing an engine running on my homemade fuel.  I ran another PAW 149 and some of my larger diesel engines on a 1/3 mix but couldn't get my smaller DC engines running properly on it.  Adding some octyl nitrate (can't remember the actual name) that I bought off Ebay fixed that though.

I'm quite worried about getting dizzy and falling over since my last CL flight was in the early eighties! I don't think I'd be any use in a competition but I'd love to go and see some. 

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