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Feb 19, 2020, 09:11 AM
Registered User
Originally Posted by Extreme Sports
Also, how does that model calculate where the CG is? It looks to me as it simply tells you where it needs to be based on the static margin you selected (so 10% ahead of the NP in your example)
Exactly, it's telling you where the CG needs to be. You have to place your gear/battery/motor/etc. so that CG ends up where it recommended.
I do this by taping things in place till I get the required CG, then I mount them properly. With experience, the process becomes intuitive (you kind of know approximately where your motor and battery will need to be).
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Feb 21, 2020, 06:08 AM
Learning to make
Thread OP
Thanks for correcting me about the calc results.

Depending on how heavy the lightweight 1.5m profile fuse ends up, I'm willing to scale up the (heavier) full fuse version to maintain a low enough wing loading.

So, I've drawn up a wing with all the same proportions as the real plane.
The only trouble was that the ailerons were 25% of the wing chord, and started exactly where the 75% kfm step should be.

I chopped 50% off the ailerons and all looks well now. They might be a bit small now (6% of the wing area), but this is what prototyping is for. I think they have a long enough moment to compensate. We'll see what happens on the first flight.....

I've cut out the profile fuse pieces. Now to add some balsa structure to give the motor and wing something to anchor on that won't rip off catastrophically mid-flight. The disadvantage of putting your lift and propulsion out on a stick above the fuse....
Last edited by fossil1999; Feb 21, 2020 at 06:26 AM. Reason: Typo
Feb 21, 2020, 01:40 PM
B for Bruce
BMatthews's Avatar
What sort of roll control response are you after? The smaller ailerons and the large wing tip are likely to cause the aileron response to be fairly weak. In particular at lower airspeeds. Provided you want a response that feels like a trainer you'll be fine. But if you're after any fairly snappy rolling aerobatics you're likely to be disappointed.
Feb 21, 2020, 06:13 PM
Learning to make
Thread OP
As long as the roll response is trainer-like at least, that's sufficient for a (probably flimsy ) prototype. And if it's even less than that... well, wings are cheap.

I'm a bit worried about the motor pod/wing pylon structure. It would be pretty the main design feature of any Skigull model, right?
I've managed to make kfm wings and skis before, but this particular thing is totally new to me...

I've sketched up a possible design that's thin from the front and could be skinned over by foam cowlings .
I'm thinking that the big flat plate is a piece of 3mm foam with 2mm balsa laminated on each side. I understand that's a light way to make something solid, thick and strong?
The middle section of the kfm wing should probably be permanently bonded top and bottom for more structural strength.
The three spars on each side would be made the same way, and I think they'd help the flat plate resist bending loads from acrobatics and wind.

A small concession to practicality can be seen below and to the left of the wing LE. The plate should be a sharp corner there, but I thought that location would be a probable stress concentration.... So I added some additional material.

So, I don't know if this technique is known to be a dead-end.
And I suppose the only way to verify the strength is to make it!
Last edited by fossil1999; Feb 21, 2020 at 06:26 PM.
Feb 22, 2020, 12:02 PM
Registered User
Unless you do rigorous load testing on the ground beforehand (as they do for commercial full scale aircraft), you're not going to know the actual failure loads in advance.
This is a foamy. Take a reasonable guess at build approach and then apply some stresses by hand in the various directions (shear/torsional/compression/tension).
If you get confidence from that, fly it.
If it falls apart, you'll know what to improve/strengthen.
Feb 22, 2020, 01:43 PM
Learning to make
Thread OP
Test as you build... Sounds very spaceX to me.
And reasonable.... Foam and balsa are cheap.
I'll place an order for various thickness balsa sheets and spars, for experimentation.

Might not hear anything until Easter though, as my blissful mid-term school break ends Monday!
Feb 22, 2020, 03:22 PM
B for Bruce
BMatthews's Avatar
A foam core with stiff skins like you suggest is a great way if you want to work with flat stock. You've got the right idea. But I also see a few problem areas.

For the size of model you're considering I'd go with more like 6mm for the foam center core. That'll space the stiff skins out further and let you do more with less.

Balsa will work for the stiffener skins if that is what you have. But keep in mind that balsa is strongest along the grain. So I'd angle the wood grain so it runs along the line of the diagonal you show in the sketch that runs through that delicate corner you are worried about.

And if the three strips you show are reinforcing strips that will be in the center core then that's a nice bit of structural design right there. You've nailed the directions that the major loads will occur. do up that set of braces, fill in with foam then skin over it all. Lovely....

There is a further issue with the wing. Keep in mind that to withstand structural loads that foam needs to have large glue areas. Small lines of glue as you'd have here would just rip away. Also the pylon to wing joint itself would be one helluva load riser and result in any mild loading breaking the wing right at the pylon joint.

I'd say you might want to add short load spreading spars to the lower side of your KFM airfoil made from hard balsa or house construction softwood like pine or spruce. These stub spars should glue against the internal braces of the pylon to aid with getting a more solid joint.

Another way to do it that most folks would go with is to make the pylon part of the fuselage and the motor nacelle part of the wing. Then skin the pylon as you are thinking and add a flat wing mount platform. Then rubber band the wing and motor nacelle in place or use a couple of nylon screws instead of the rubber bands. This obviously means you'd need a power connector for the motor Or just rig up a way to remove the wing and cradle it against the fuselage with the motor leads between the two.

Some ideas to think about or perhaps something that will inspire you towards an even better option as you grind on towards the Easter break.
Feb 28, 2020, 04:08 PM
Learning to make
Thread OP
Great to know that I'm on the right track.

Increasing the foam core thickness is no problem. Might be worth investigating going all the way up to 10mm for the core.

Great point regarding angling the balsa grain with the corner, I didn't think of that.
Neither did I think of putting the reinforcing strips inside the foam core. That's a great idea to save space!

The wing will certainly need dedicated spars to transfer the load evenly to the pylon. Maybe even a narrow balsa sheet.

As for the alternative methods of making the plyon/motor mount/wing structure...
My favourite is taking part of the centre wing panel, making it a permanent part of the pylon, and using a detatchable rod-and-tube system for the outboard portions of the wings.

Turns out that Rutan made the wings removable like I want to do. See the still I took from a video.
Trevor Budge has a lot of enlighting Skigull video on his channel:
Mar 01, 2020, 12:39 PM
Learning to make
Thread OP
Just about finished the important parts of the full fuse CAD model. I can justify this as practicing with the CAD software we have to learn in one of my courses.
Mar 01, 2020, 05:28 PM
Registered User
Dickeroo's Avatar
Originally Posted by fossil1999
Just about finished the important parts of the full fuse CAD model. I can justify this as practicing with the CAD software we have to learn in one of my courses.
You might find this document of interest.... looking forward to your maiden flight.
Mar 05, 2020, 09:00 AM
Learning to make
Thread OP
Originally Posted by Dickeroo
You might find this document of interest.... looking forward to your maiden flight.
Thank you sir, that was an interesting read.
I'd have to learn some more aerodynamics to understand it better, but it interesting all the same.
Kfm3 for the win!

I'm trying to get all my ducks in a row for the Easter holidays. Placing an order for #11 blades, balsa sheets and dowels, tools, etc. And looking into a hot wire cutter.
There are a few unknowns though....

Such as the coverings that modellers put on their planes to make the surfaces look all shiny and slick. I have no idea what to look for.

I'd only attempt to put something like this on the airplane once everything else has been tested to work, but I'm trying to do some research in case some of the required preparations have a long lead-time to order.

And so far on my searches, I understand that I need to buy "WBPU" (and skip the fiberglass) to coat the foam surface with if I want a light, non-structural hard glossy coating? And that there's lots of surface preparation involved.

There was something about a coffee filter technique too.

Could anyone kindly provide a link to some thread where I can learn more about these coatings? I'm sure that I'm missing something.
Mar 06, 2020, 08:27 AM
I Look, Listen, and Learn
Timbo383's Avatar
This one covers more than just newspaper...some good hints and tips when applying various types of skins.
Mar 06, 2020, 08:43 AM
Learning to make
Thread OP
Originally Posted by Timbo383
This one covers more than just newspaper...some good hints and tips when applying various types of skins.
Thank you! That is exactly what I was looking for.

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