Rotor Pitcheron Scratch Build UK - Page 3 - RC Groups
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Jan 21, 2018, 11:09 AM
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The Solution

I solved this problem by putting 3mm (1/8”) balsa between the core and the templates.
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Jan 21, 2018, 11:14 AM
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My Perseverance got Rewarded

After much profanity I have finally produced the required number of usable cores. Note that I say usable, not perfect. All my cores need to have their leading edges addressed. In particular, my wing cores all have “thick LE syndrome”. I think this is because:
  • My bow could do with more tension.
  • 73cm (28.5”) is starting to get rather long and encouraging sag/lag.
  • Lifting the wire over the template LE is a delicate matter with a high failure rate.
  • Wire sag/lag exists in all traditional foam core cutting but it is accentuated by the rapid change of direction at the LE.

I have just checked Phil Barnes’ DVD to see why he doesn’t have a problem with LEs like I did (I should have looked at the DVD again before cutting!). Here is what I missed:
  • His cores are all shorter than mine and I expect his bow is properly tensioned reducing the effect of wire lag.
  • When cutting the top surface Phil Barnes places the wire at 1/16” (1.5mm) above the airfoil centreline to start the cut. Therefore the wire hits the templates some distance into the core where the templates are already at a backward slope. This is better than having to lift the wire vertically up to get it started over the template LEs like I did. Note taken.
  • His LEs are not right straight after cutting either (phew! I was starting to get a complex). He has the experience and therefore just knocks the LE into shape with a worn 100 grit sanding block (there is hope for me yet).

My wing cores are all OK except for the fat LEs which I will make right with a long sanding bar. I wasted a lot of blanks trying to get workable cores. I will chalk this down as the cost of education.
Last edited by Kenneth Paine; Jan 21, 2018 at 11:20 AM.
Jan 21, 2018, 06:36 PM
It's time for me to fly
JimZinVT's Avatar
Ken I had exactly the same tearing of the foam when I wrapped the edges of my cardboard templates with copper foil tape. That's a clever solution shimming the templates out away from the foam.

For the leading edges, I make the templates with a small curve (maybe 5mm radius) rather than a hard corner where the lead-in meets the leading edge. It leaves a small ridge along the LE that's easy to sand off. You still need to help the wire a little through the transition but easier than with a hard corner there.
Jan 22, 2018, 09:07 AM
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I agree that a sharp corner to start the LE from is asking for trouble. After this experience I am considering using little ramps as illustrated below and then just rounding the LEs with a sanding bar. I am also considering cutting future wings upside down. I have noticed quite a few modellers do this and it makes sense to me for two reasons:
  1. When starting at the LE It is easier to push the wire down against a template than to lift it over the template.
  2. The wing's top surface should be more accurate.

Regarding the second point, here is my reasoning: A bagged wing panel can also be viewed as a collection of errors in that, starting from bottom the foam shuck, you have your bag layer, your breather, your mylar, your bottom laminate, your core and then your top laminate. Even if your core is pretty close to what you intended to cut, do you think the top surface is anywhere near what it is intended to be after all this? I don't.

If you cut the core upside down then the bottom shuck should be pretty close to the airfoil top surface shape is intended to be. The only additional distortion to the wing's top surface comes from the bag, breather and mylar. The remaining cumulative distortions are sent to the bottom surface of the wing where they are less of a problem.

Jan 22, 2018, 11:50 AM
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I wish I had seen this first...


I wish I had seen this post of Joe Cornier's before I had made my templates and cut my wings.

Note the use of scrap foam shims (photos 6 & 8) at the ends of the core's LE. While the wire ends cut through these shims on theirway to the LE, the centre of the wire reaches temperature and this minimises sag.

Sheer brilliance!
Jan 25, 2018, 11:42 AM
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Making the tail feathers good

Today I gave my tail feather cores some TLC with a sanding bar and their shucks a quick pass with a sanding rod. I am very pleased with the results.

I sanded cores until most of their sheen had turned dull. I also shaped their leading edges which were not fit for purpose, specially the tips of the stabs which had a 1.5mm (1/16”) flat spots for leading edges due to errors and limitations of the templates. I am very pleased with the results; note that the tips are very small, only 45mm in chord (9/5”) and 3mm (1/8”) thick. I am sure that none of the cores approach the intended NACA 0007 section but they are far better than sheet balsa with a rounded LE and TE.

I sanded the shuck or beds because there was noticeable more kerf between the root and tip that at the root and tip because of the templates acting like heat sinks for the hot wire. If the cores and beds had not been sanded, then the core would only come in contact with the beds at the root and tip and there would be a gap between the cores and their beds mid span.
Jan 25, 2018, 12:02 PM
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Originally Posted by agarchitect
Nice work. Smart to make three at once. Less work than starting from scratch three times.

Well, now that I have messed up so much, building three models doesn't seem such a good idea. All this rework is getting rather tedious.

Jan 25, 2018, 01:01 PM
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Which means you are learning a lot in this multiple build and hopefully guys watching will learn also is why we go through this extra effort to post the good and the swing and a miss. The main thing is you keep going forward Kenny and this should be one lesson in itself. Build on your getting to the food section of the build and if your not sure about something ask and it could save you some time and effort!
Jan 25, 2018, 01:14 PM
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Thanks Joe.

You wouldn't believe how much help and inspiration your posts and those of others who also take the time have provided to me.

Jan 26, 2018, 10:45 AM
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Solving the thick leading edge problem.

My big worry was whether or not I would be able to fix the thick leading edges on all my cores. I am pleased to say that my method works.

The LEs get thick towards the middle due to wire lag I assume. Because the LEs are fine at the root and tip for the first inch or so of the span, I decided to use these as templates. To limit how much my sanding bar will cut into the thicker portion of the LEs, I protected the ends of the LE of my test core with masking tape and also applied masking tape to the ends of my sanding bar so that I can sand away until there is no resistance and the pieces of tape on the bar and LE are rubbing together.

I am VERY pleased with the results. I look forward to fixing the remaining 5 cores.
Jan 27, 2018, 12:39 PM
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Isn’t this weird?

Whilst sanding my wing cores I noticed that not all parts were coming in contact with the 30" sanding bar. Think of the core having 5 segments along the span with 1 being the tip, 3 being the middle and 5 being the root. Regarding the top surface, the sanding bar was cutting hard into the tip (1) and middle (3), sliding relatively lightly over the root section (5) and not touching the sections in between (2 & 4). For the bottom surface the opposite was true: sections 2 and 4 were getting cut but not 1, 3 and 5. See photo 1 below. The shucks suffer similar irregularities.

The reason is because the core is distorted as shown in photo 2. Why would it have distorted? My bench is straight and the wire should be straight. I can only think of:
  • Heat induced distortion (only the root third of the semi-span had angel hair and it was short)
  • Internal stresses (but all blanks were skinned)

Answers on a postcard

All my cores are bowed. As a consequence I decided to downsize to my 10" sanding bar with 200 grit to ride over the crests and into the troughs along the span (photo 3). Sanding with a cross-hatch motion takes care of the high spots without introducing flat spots.

The results of my decision can be seen in photo 4 which is another core approaching completion.
Last edited by Kenneth Paine; Jan 27, 2018 at 12:44 PM.
Jan 27, 2018, 01:28 PM
It's time for me to fly
JimZinVT's Avatar
Were the core blanks weighed down evenly across their whole surface while hot wiring? If not maybe there was some lifting during the cutting process?
Or maybe due to low bow tension? Although I'd expect that to cause more more symmetrical distortion.

I have seen some warping of my cores even if the blanks were skinned first. I guess skinning doesn't always relieve all the stress. But the wings turn out flat when I vac. bag them while weighed down in the beds.
Jan 27, 2018, 02:14 PM
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I used more weights than that shown in the photo below. I ended up using three 12" steel bars at each end and two 6" steel bars in the middle. I did check for the need for shimming but since the blanks were only 20mm (4/5") thick, the weights were enough on their own.

We'll see how they turn out after bagging.

I had this problem with the ready-veneered wing panels of my Simprop Solution XL kit (thread here: ).

I should have remembered from that build that aluminium extrusions are straighter than my humble productions and even high quality German kits, therefore using long bars is futile. You can see how much grey and tan primer stayed on the wings of my Solution as filler to avoid cutting into the veneer too much.

Last edited by Kenneth Paine; Jan 27, 2018 at 02:21 PM.
Jan 30, 2018, 02:26 PM
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Wing Twist Mechanism

Have you any plans for how you are doing the wing twist mechanism/linkage arrangement?

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