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Gentle Curves - Lucas his Skywriter

Build blog - MB2017 - Skywriter

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Lucas Hofman06/12/2016 07:52:31
668 forum posts
375 photos

Thanks for all input. I found another candidate to take out: the tailwheel. Replacing is with a skid will save 5-10 grams. This in effect will save 20-40 gr. in the cowl. The skid under my Ballerina does not seem to hamper manoevring on the ground in any degree.

The order to BalsaCabin is out of the door. With a bit of luck I have the wood in house before Christmas.

John har stirred up something with his comment about inset ailerons. I had already decided to change the airfoil from Lindsay's to a NACA one. I like to know the CL-alfa curves Probably either Clark-Y, with is also 12%, or NACA-3414. The latter is 14% thick, which makes for a smoother stall (this is the airfoil Peter Miller uses in many designs). I have made a J-3 Cub wing with NACA-4415, which behaves very well too. All can easily be build flat on the board. See airfoiltools.com if you want to play around with airfoils.

With all these changes it may be more appropriate to called the modell "inspired by Skywriter" instead of "a Skywriter", but as John says that is the joy of scratch building.

Cheers, Lucas

McG 696909/12/2016 13:37:52
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3305 forum posts
1257 photos

Hello Lucas,

As I wrote before, I find your mods quite appropriate even if they shouldn't really be of help for any 'newcomer' in the MB. indecision

I don't know exactly what airfoil Lindsay used for his Skywriter but it even seems more 'flat bottomed' at the LE than a Clark-Y.

In fact , the Clark Y, N3414 or N4415 should have a very similar behaviour. You could use your Cub wing at 14% instead of 15% as well. Or you could 'pimp' the Clark from 12 up to 14. I think 14% is a good compromise and IMHO, Peter's choice gives a slightly better 'inverted' behaviour with the more curved front bottom part.

If you're interested, I'll try to find a Cl-Alpha graph for them.

... and, you could always call your 'piece of joy' the "LucaSky"... wink

Cheers

Chris

Lucas Hofman09/12/2016 14:31:04
668 forum posts
375 photos

Hi Chris,

For the NACA profiles I have them. Clark-Y has a much sharper stall at a lower angle of attack, especially with low Reynolds numbers. Lindsay wrote me that the airfoil is his own creation.

I was pleasantly surprised with how the Ballerina is behaving, also inverted. And 14% instead of 12% give a 4 mm thicker wing, which decreases the load on the spars and the planking. (also the ribs are a little bit more solid to handle during the build.

John has spoiled the idea of building the wings with strip ailerons. I will use the waiting time til Christmas for drawing new wings with build in ailerons. Current thoughts are to have the same construction as the elevators (core and riblets), but top hinged for good looks and less drag in normal flight.

Lucas

ps. And the name will be "Gentle Curves". And now there are so many changes that "inspired by Skywriter" is more appropriate the "A Skywriter".

Edited By Lucas Hofman on 09/12/2016 14:43:30

Lucas Hofman16/12/2016 19:08:32
668 forum posts
375 photos

Balsacabins Christmas present has arrived:

img_3498 (small).jpg

Enough for one plane and then some:

img_3499 (small).jpg

BalsaCabin knows how to pack there stuff. This has been handled by Royal Mail and British Airways and survived!

Lucas Hofman20/12/2016 07:40:50
668 forum posts
375 photos

First attempt at changes in the upper wing:

upper wing 0.1.jpg

I tried inset ailerons with more cord (like the Ballerina), but it did not look better and made the contruction more complicated. So the changes will be:

  • laminated tip using 4x1.5 mm balsa strip (6mm wide)
  • different way of fittting struts, which makes it possible to cover the wing and glue in the attachment points afterwards (still to be done at the inner points)
  • more open structure
  • slightly smaller ailerons, creating a more "inset" look.

I did a rough calculation about the load on the wing when stalling at high speed (like in a snap roll). That was ca 380N (38 kg). This is high, but given that Peter Miller has measured 24g on one of his models not unrealistic. I do not think the attachment point as drawn on Lindsay's will hold (small glue area's to the ribs, covered by 1.5mm balsa that can be pulled from the ribs). Therefore I want to glue in the attachment points vertically, with ample glue area to the ribs.

The profile will be NACA 3414. Flat from the spar backwards but not forward of the spar. This would make the forward strut attach difficult to fit with the original attachments. Easier with vertical lugs coming out of the wing.

Cheers, Lucas

Lucas Hofman21/12/2016 12:17:11
668 forum posts
375 photos

Experiment with laminating the wingtips: I made a template from 9mm MDF board that is 6mm smaller in radius than the finished shape will be:

img_3513 (small).jpg

The strips (I used 3 times 2mm medium balsa) have been soaking in warm water for an hour or so. After that the strips were layed up together with white PVA glue while flat on a table. The "package" was thereafter wrapped around the template:

img_3515 (small).jpg

Quite a bit of force was still needed. Probably better to use 1.6mm balsa (1/16). It looks good, but tonight we will find out if and how much it will spring back.

Lucas

McG 696921/12/2016 12:59:40
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3305 forum posts
1257 photos

Hi Lucas,

I never tried myself (of course), so this is just a 'projected' thought.

Wouldn't it be easier to bend one strip at the time and pin it at he extremities to the jig?

If you make the first one a bit longer than needed you can pin it down and making the second one a little shorter allows you to pin it at the extremities as well while leaving the first pins in place.

Same procedure for the third one and when done than pin the 'pack' over the total length. I believe it would require less force to bend them individually and the remaining final 'tension' should then be less or even nonexistent.

Just my 'two euro cent'.

Cheers

Chris

Lucas Hofman21/12/2016 14:54:30
668 forum posts
375 photos

Hi Chris,

Nice to see somebody is still reading my scribblings!

I started that way, but found that all glue then runs down, since the wood is saturated with water. Keeping the glue surface horizontal kept the glue between the strips. I fixed the bundle in the middle and then slowly bended them over, setting pins every few centimeters.

Somebody wrote about this in the Ballerina build blogs. I do not remember who should have the credit but it looks like a very nice way to get the fiber oriented correctly around the whole curve.

Lucas

McG 696921/12/2016 15:14:27
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3305 forum posts
1257 photos

Hey again, Lucas.

Yes, still reading. In fact, I've subscribed to your "scribblings". wink

Remember, young man. Reading too many build blogs is totally hazardous...

I remember reading it too (Martyn or Chris Barlow?) but not so sure anymore. Any way, I also remember he was squeezing the excess of water out of the wood between his fingers.

Another method seems to bend the strips wet against the jig but without any glue. Leave it to dry completely overnight, then 'unpin', apply some glue between the strips & re-pin in position.

It seems that we both like to 'experiment' a little... yes

Cheers

Chris

 

Edited By McG 6969 on 21/12/2016 15:15:51

john stones 121/12/2016 15:42:49
avatar
11597 forum posts
1517 photos

Still reading also, i bend mine around some thing whilst wet, leave to dry then glue up, did the tips on Dawn Flyer that way and the built up tail on Ballerina i think ?

John

Steven S21/12/2016 17:40:49
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367 forum posts
193 photos
Posted by McG 6969 on 21/12/2016 15:14:27:

Another method seems to bend the strips wet against the jig but without any glue. Leave it to dry completely overnight, then 'unpin', apply some glue between the strips & re-pin in position.

I did this for my Woodys Pusher turtle deck and it worked real well.

Lucas Hofman22/12/2016 07:18:53
668 forum posts
375 photos

More then one way to skin a cat here! the first wingtip came out nice. Next to none "spring back":

img_3516 (small).jpg

The strips for the 2nd had been under water for 24 hours and went on easier. Or maybe it is just the second one that goes easier. This saves (on the Skywriter) nearly a whole sheet of balsa and one ends up with a more solid tip in addition!

Will do the leading edge of the horizontal stabilizing the same way.

Lucas Hofman23/12/2016 20:52:37
668 forum posts
375 photos

Drawing for the upper wing is done:

upper wing.jpg

Send me a message if you want the PDF.

The last wingtip is drying now. I did the leading edge of the horizontal stabilizer the same way. Once one has the routine it goes very quickly.

Merry Christmas and many happy landings in 2017!

Lucas

McG 696923/12/2016 21:44:02
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3305 forum posts
1257 photos

Nice work, Lucas.

It really suits your 'precise' style of progressing.

I won't be building the Skywriter after all, but if I decide to do it some day, I'll be asking for your drawings for sure.

Still following your build anyway.

'Zalig Kerstmis' to you and family.

Cheers

Chris

Lucas Hofman28/12/2016 21:41:56
668 forum posts
375 photos

Lower wing drawing complete now:

lower wing.jpg

Changes compared to Lindsay's design:

  • laminated tip using 3x2 mm balsa strip (6mm wide)
  • different way of fittting struts, which makes it possible to cover the wing and glue in the attachment points afterwards.
  • more open structure
  • slightly smaller ailerons, creating a more "inset" look.
  • moved the bolt to a stronger area.

Construction method is described in the building article for the Ballerina. (pm. me if you need a copy).

The dowel construction is Lindsay's, but drawn here (missing on the RCME plan).

Construction will begin in the new year!

Regards, Lucas

Lucas Hofman02/01/2017 21:28:31
668 forum posts
375 photos

The first 2 formers are cut and one (F3A) is sanded! Always nice to transition from the computer screen to actually cutting wood. I cut up the drawing (actually a copy) and glue the paper templates on the light ply. Black lines on white paper are easier to see when cutting:

img_3566 (small).jpg

I modified the formers slightly (more/bigger lightening holes and no cutouts for the 6x6 strips). But I kept them all in ligth-ply, allowing the cutouts for all the stringers to be made:

img_3567 (small).jpg

The plan is to make a complete kit first (well, maybe the fuselage only) and then assemble it.

Cheers, Lucas

Edited By Lucas Hofman on 03/01/2017 07:54:44

Low pass Pete03/01/2017 09:28:38
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260 forum posts
147 photos

Hi Lucas

You look to be the front runner at the moment. I've still a house of visitors so progress is nil so far.

Ref the cabanes. Have you sourced aluminium for these yet and if so where did you get it.

Happy new year and building.

Peter

Lucas Hofman03/01/2017 09:33:49
668 forum posts
375 photos

I feared I was going to be the only runner....

The aluminium comes from a local building supermarket (Bauhaus). They only had 15x2mm, so I have to adjust the mounting points in the upper wing 5 mm closer to each other. (I choose to leave the formers in the fuselage in place, which limits the spacing of the cabanes).

Happy new year to you too!

Lucas

Lucas Hofman05/01/2017 07:24:45
668 forum posts
375 photos

Last night good progress on cutting out the formers. Probably well knows but I would like to share some of the home made sanding tools that make life easier:

img_3568 (small).jpg

Paint stirrers and solid coffee stirring sticks. The sanding paper is attached with double side tape that is normally used when laying carpet. I cut the sanding paper oversize, glue it and trim with razor blades. That is the last thing you do with the blades! (they start their life cutting covering material, then do wood and end with sandpaper).

Lucas

Lucas Hofman05/01/2017 07:25:56
668 forum posts
375 photos

img_3569 (small).jpg

I love my Proxxon scrollsaw!

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