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Gentle Curves - Lucas his Skywriter


Lucas Hofman
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Last out with the Tucano, middle of the pack with the Ballerina but first out with the SkyWriter.

Mine will have a slightly different setup: 4 cells instead of 3. Mostly because I have both 2200mAh and 3300mAh packs, but also because that lowers the amperage, which implies a smaller ESC. A 500W output can be generated with a 800kv motor (on 4S) driving either a 12x6 or 11x7 propeller. A Turnigy Aerodrive 3542 800kv and a Turnigy AE45 ESC play well together and generate about 500W with an APC-E 11x7 propeller.

Looking at the drawing there is ample space for both types of battery, but not that much possibility to move the pack back or forth. Better make the battery tray so it can accept both sizes:

img_3468 (mobile).jpg img_3469 (mobile).jpg

New techniques to be tried out with this build:

  • making panels and rivets. I have never glassed a model.
  • laminating curves. Shown on some Ballerina build blogs. I will try to to both tail surfaces and wing tips. I think that will save some weigth while increasing strenght.

But first I have to create a shoppinglist for the Balsa Cabin.

Lucas

Edited By Lucas Hofman on 28/11/2016 15:01:25

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Posted by kc on 28/11/2016 15:03:23:

Is there much scope for rivets or much that can be glass clothed in this model?

Not too much kc. If you read the article about the Skywriter you will see that Lindsay glassed the forward part of the fuselage, and with primer build some panel lines to simulate aluminium panels. He "made" som rivets with a soldering iron too.

Lucas

Edited By Lucas Hofman on 28/11/2016 18:56:37

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You are right Lucas - I hadn't read the article so didn't realise he glass clothed it! Good luck with that - it seems lots of extra & messy work! If you see Tim Hooper's Blackburn article you might be tempted to use the same method of aluminium sticky tape instead of glass cloth. Very realistic alloy effect on that model.

 

If you are going to the Balsa Cabin also check out their range of fittings etc.   A friend who visited them recently said they have a good range of all the bits needed.

Edited By kc on 28/11/2016 19:32:23

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It looks a nice easy area to cover with glass cloth Lucas. Put a little thought into whether you want to use a water based resin or an epoxy finishing resin though. Water based is very easy to use but not quite as tough as epoxy. Easier to sand too!

Definitely have a look at Danny Fentons videos on you tube too. He's very good and explaining each step!

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Chris and kc, thanks for the advice. It will be some time before I have to read up on how to make panels.

Currently measuring all parts to make a material list. Since one cannot get decent balsa in Norway I will order (as last year) from the Balsa Cabin. Shipping to Norway pre-christmas may take some time.

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While studying the plan, and comparing it with other plan I become more and more convinced Lindsay overdimensioned the Skywriter. I am contemplating the following changes:

- all formers behind the cockpit in 3mm balsa instead of light ply

- bottem in 1/16 cross grain (with lightening holes) instead of 2mm without holes

- sides in 3/32 instead of 1/8

- balsa lower main spar instead of pine

- leave out wing sheeting aft of the main spar (except in the center section), but probably add 1/16 webbing between upper and lower spar

- mini servo's in the fuselage instead of standard

- pull pull to the rudder (mostly because I like it, and have some wire around.

- leave out the longerons in the fuselage

- lightening holes in the cores of the wing tips/ elevator/rudder

- HK film instead of solartex

- less (or) lead in the nose due to a lighter tail.

I think it is possilble to trim off about 400 grams from the 2400 Lindsay ended up with. And a fair bit of the price of the wood too....

Any comments?

Lucas

Edited By Lucas Hofman on 03/12/2016 15:27:54

Edited By Lucas Hofman on 03/12/2016 15:53:44

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I think he may have gone with lite ply because of the notches for stringers (strength) you could put another hole in and save a bit of weight i suppose ?

Yep film for me as well and i'd have no problem using decent mini servos in it, i'm pretty happy with it as is, but i've got an inkling for inset ailerons and a f/glass cowl.

John

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That is a good question Percy. I have no specific reason other than that light models generally fly better (lower stall speed, better climb and acceleration etc.) One gets in a nice spiral where less material means less weight in the cowl, smaller motor, smaller battery etc. The only reason I see to make a plane heavier than it needs to be is when adding ballast to a sailplane to penetrate better against wind.

I am not sure if a lighter model is less robust either. Material that has no function adds loads to a structure on impact. Last winter I crashed an ARF that was completely made of laser cuttet light ply. It went down, not too fast in 20 cm of snow but looked like it has suffered an explosion inside. I am sure a decent balsa structure would have had less damage.

But how about turning the question around: Why make a plane heavier than it needs to be?

Lucas

ps. please realise I fly F3A (aerobatics with 2x2 m. planes that have to be under 5 kg) and F3P (indoor aerobatics where a 60 gr. model is "heavy". One get into a certain mindset....

 

Edited By Lucas Hofman on 03/12/2016 16:06:04

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If you use thinner balsa then you might choose firmer balsa to do the job and end up with the same weight!

It's all a matter of judgement which grade of balsa to use, I suggest waiting to see what grades arrive and then decide on the final size ( order a bit extra! )

However some of your changes seem valid - the plan seems to show 1/16 balsa underside anyway -I don't think Balsa Cabin sell 2mm balsa.    Lightening holes are not desirable in this area.

Fuselage outside longerons seem unnecessary to me so leave them off but perhaps keep 1/8 sides.

Some fuselage formers could be balsa instead of ply. I reckon it would be much easier to make only F5 & F9 notched and leave the F6,7,8 unnotched but slightly smaller to compensate. Notches so close together are more difficut in balsa.

Film instead of Solartex will save weight and money. Closed loop ( push pull) to rudder seems a good idea.

I mentioned before that slightly increasing the nose length might be better than using lead ballast. Adding 1/2 inch in front of F3 might work.

Of course if you change the design you are on your own!

Edited By kc on 03/12/2016 16:42:31

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Great approach, Lucas.

Your methodical improvements are really worth reading but in fact you are redesigning the bird. And that's what makes me still a bit reticent about jumping in.

Your modifications list is really well done but regarding the aft fuse formers in balsa, I think Rosco might be right regarding the fragility of all those notches then. Why not using the Deply trick there: a lamination of 2x 0,8mm ply with some 2mm Depron in between (free too as cut from foam pizza underplates)? The thin ply makes the notches quite strong & the foam keeps the weight under 50% of the 3mm ply.

Following your Excel, you are substituting the spruce spar with hard balsa. I think your webbing idea would also be a good thing then.

I only found one Skywriter build by a member of the Canadian TECAM forum. He adapted different things as well, including lengthening the nose by 1 3/4 inch to avoid some lead but still ended at 5,4 pound (including 5oz lead) with a 3S 5000mAh.

@ Percy > don't you think a 1200mm ws plane @ 2400gr has a tiny bit of obesity?

Cheers

Chris

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To Percy and kc: look at the picture I took today of a 2 m span sport-scale model:img_3481 (small).jpg

and a detail of the fuselage behind the cockpit:

img_3480 (small).jpg

A nice example of a fuse build up of longerons and formers, which just enough material to ensure stiffness. To cover such a structure with balsa would give you more weight but not much else I would think. If we, on the other hand, construct the fuse with load bearing sheet then the longerons can go.

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Thats almost a vintage style of construction although it seems to use a lot of fretted out ply rather than old traditional balsa.

Light models crash lighter and less often! However a model that can be easily repaired is highly desirable. Having ply doublers along the fuselage and spruce spars in the wings usually keeps the model straight enough to repair so I think they are worth their weight.

However bear in mind that personal flying style and conditions affect the matter - some people like a model that goes where it's pointed and not one that is thrown off course by wind. Some people have exposed windy flying sites and want to fly whatever the weather. Therefore a bit of extra weight doesn't worry them. But to me having a lightweight model that will probably get away with a rash manouvre without stalling is worth putting up with getting blown around in a wind. It's all a matter of compromise and opinion.

So Lucas's proposed modifications all seem reasonable except the change of spar material ( in my opinion)

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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

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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

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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

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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

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