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Feb 16, 2020, 12:53 PM
Learning to make
Thread OP
Idea

Full fuse kfm Rutan Skigull attempt (WIP)


I had been trying to design my own foamie..... But the idea of making all the outlines from scratch daunted me.
I'd rather learn more about the aerodynamics responsible for keeping our foamies in the sky and how to cut foam! Maybe practicing making plans from an existing design is better.

I've seen the Rutan SkiGull. Inspiring design. I think it would work great as a belly lander/take-off motorglider.
First up is the pics of the real deal! These images from http://www.burtrutan.com/skigull/ , and everything belongs to Mr. Rutan.
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Feb 16, 2020, 01:07 PM
Learning to make
Thread OP
First order of business was making templates suitable for cutting foam with.
Funnily enough, I found three-views in the testimonial from Rutan at the bottom of this purchase page for CAD software: https://www.darcorp.com/sharkcad-pro...utic-software/
i guess we know now what Mr. Rutan used to design this plane.
Feb 16, 2020, 03:52 PM
Learning to make
Thread OP
Made outlines from the CAD views. A bit too simplified, maybe, but will suffice for the "chuck glider" stage.
Feb 16, 2020, 04:13 PM
Retired CAD guy
birdofplay's Avatar
Yeah,
When they won't even state a price without BONDING with you
THEN it's too expensive at any price.

Sound like you might be a Student.
If so Rhino3d is ~$200 and it's a terrific 3D CAD!
Feb 16, 2020, 05:18 PM
Learning to make
Thread OP
Words of wisdom!
I'm using paint.net to produce the templates, but I might make a model in Solidworks if I need to.

The main problem I'm facing at the moment is making the longeron and former templates for the actual model. Maybe time to loft a fuse?
Or take the K.I.S.S. route! And make simple longerons and formers that only approximate the fuse shape. Decisions decisions...
Feb 16, 2020, 08:44 PM
Registered User
rajazo's Avatar
Nice project Fossil!!
If it is your first scratch plane go for a simplified version (profile), test it and fly it as soon as you can. Enjoy your plane and find all the problems in a easy to rebuild or repair plane. In parallel, model a more advance fuse and when your are happy with the first version replace the fuse and so on.
Is really important find the correct point of balance to get a gentle or sporty plane. This will help you find a correct position to all of hardware included in a more advance fusse.
I used this method in my first projects and keeps me happy and Interested in the project and far from frustration , even with unhappy maiden flights.

BTW, this plane could be a great rc sea boat (beautiful, sporty and stable) if you can solve all hydrodynamic problems of the scaled counterpart. A great third stage for your proyect

Good luck
Feb 17, 2020, 02:09 AM
Registered User
This is the kind of plane that definitely can be done, but at the same time will almost certainly have multiple failures before success.
Some planes are just more difficult to implement at typical medium model scale.
Making this one really big (80 inch span or more) will make things a lot easier in the final analysis.
Feb 17, 2020, 04:19 AM
Learning to make
Thread OP
Quote:
Originally Posted by rajazo
Nice project Fossil!!
If it is your first scratch plane go for a simplified version (profile), test it and fly it as soon as you can. Enjoy your plane and find all the problems in a easy to rebuild or repair plane. In parallel, model a more advance fuse and when your are happy with the first version replace the fuse and so on.
Is really important find the correct point of balance to get a gentle or sporty plane. This will help you find a correct position to all of hardware included in a more advance fusse.
I used this method in my first projects and keeps me happy and Interested in the project and far from frustration , even with unhappy maiden flights.

BTW, this plane could be a great rc sea boat (beautiful, sporty and stable) if you can solve all hydrodynamic problems of the scaled counterpart. A great third stage for your proyect

Good luck
Good idea! A proper flying prototype and the fancy looking product... I like the sound of that approach, as foam is cheap!

I'm making this out of water resistant XPS foam board, so hydrodynamic research is definitely on the table for later on. This plane deserves to have the "all terrain vehicle" characteristics of the real thing.
Feb 17, 2020, 04:28 AM
Learning to make
Thread OP
Quote:
Originally Posted by nuteman
This is the kind of plane that definitely can be done, but at the same time will almost certainly have multiple failures before success.
Some planes are just more difficult to implement at typical medium model scale.
Making this one really big (80 inch span or more) will make things a lot easier in the final analysis.
Doubly agreed! The plan is for a 1.5m wingspan. Maybe bigger with a collapsible wing.

I think a major focus point will be the structure that holds the motor above the fuse... It has to be strong, light and simple! And if it can hold the motor, it can hold the wing too.
Feb 17, 2020, 06:13 AM
Learning to make
Thread OP
Now, the construction work that I've done....
Two "chuck gliders" of 75cm wingspan. They each weigh about 15grams.

The first one was made 100% from 5mm green XPS underlay I found at B&Q. This foam has good lightness and stiffness, but it is brittle.
The nose, and the fuse just forward of the tail, have both broke clean through twice on hard encounters with the ground.
So the design will need to pay attention there. Perhaps additional reinforcing at the tail to spread the load, and an EPP nose to absorb the impact.

The glider also revealed a stalll problem.... Give it enough airspeed when you toss it, it pitches up until it stalls, and then drops nose-down like a rock and lands upside down.
The glider also made me realise this plane has similar proportions to the U-2 spyplane. And that's not a paragon of stability like a Cessna is.
There must be a way! The real SkiGull has a gentle stall, if you believe Mr. Rutan.


Then I found another kind of foam at B&Q. This stuff is white and a lot more floppy, but flexible, not brittle. I bought a bunch of 3mm.

Made chuck glider No.2. This one has a kfm2 wing and the tail/stab are scaled to 120% percent. I also gave the horizontal stab negative incidence to the main wing, in hopes of fixing the stall problem. No joy.... the stall MIGHT be gentler, but it's still there.
Also trialed taping plastic straws to the wing LE to make it more aerodynamic.


To do:
Fix the stall problem, develop Glider No.2 farther.
More drawing.
Motor mount structure.
Powered flying 1.5m prototype.
Feb 17, 2020, 09:08 AM
Registered User
rajazo's Avatar
points to Check
Don’t discard CG problem... tail heavy could be the excessive climb origin.

Don’t put excessive force when you throw the glider.. Excessive force in a wrong position could produce a unwanted climb or pitch. It is just a glide test

Check the horizontal stabilizer size too, it should have an area between 15 to 20% of total wing area
Feb 17, 2020, 12:21 PM
Pro Hoarder
turbonut's Avatar
Cool project..I was wondering when someone would build the skigull
Latest blog entry: In flight
Feb 17, 2020, 12:26 PM
B for Bruce
BMatthews's Avatar
You can learn a lot from simple chuck gliders. But a key thing is to avoid THROWING it. A gentle push is all you need and all you want. 15 gms is very light, it will take no more than a very light push given a 75cm to leave your hand at flying speeds.

Quote:
The glider also revealed a stalll problem.... Give it enough airspeed when you toss it, it pitches up until it stalls, and then drops nose-down like a rock and lands upside down.
That description is a pretty good description of what happens when you have the CG too far back. In particular the nose down flip and landing upside down after a short half out side loop. You need to move the balance point forward with some nose weight and then adjust the stabilizer's rest angle to give you some up trim to counter the extra nose weight.

Keep in mind that a key trimming factor is that the CG/balance point location works hand in hand with the wing to stabilizer trim angle. You cannot change one without changing the other.

But the CG location is still the boss in this. You set the CG at some point to make the airplane stable in pitch. Then you adjust the stabilizer to set the flight speed.

It looks a lot like your wing is way bigger in relation to the rest of the design when I look at your pictures and at the drawings of the real plane. If this is the case because you think you need more wing area just keep in mind that the bigger wing even with what looks like a bigger stabilizer still means that the balance point will need to be fairly strongly forward to be stable. Like potentially as far forward as 25 to 27%. There are a few online CG calculators you can use to determine where the CG should be placed.

On small and light gliders the generously rounded nose of the folded over KFM airfoil isn't doing you any favors. If it'll hold up and not curl badly try a single flat plate without the fold over. Or if needed make the test model smaller so you can use a single layer of 3mm foam. You can still learn all you need from test gliders that are only around 30 to 45cm span. And then the light foam can be strong enough.

And if I'm right and you did make the wing and stab larger I'd suggest you get things back into scale so the wing to stabilizer gap is the correct proportion to the wing and stab as shown in the top view of the real one. As you say you've ended up with a rather U2'ish look where the real one in the top view is not that radical. So get things back into the proper proportions first.

Plus you can learn all
Feb 17, 2020, 12:26 PM
B for Bruce
BMatthews's Avatar
You can learn a lot from simple chuck gliders. But a key thing is to avoid THROWING it. A gentle push is all you need and all you want. 15 gms is very light, it will take no more than a very light push given a 75cm to leave your hand at flying speeds.

Quote:
The glider also revealed a stalll problem.... Give it enough airspeed when you toss it, it pitches up until it stalls, and then drops nose-down like a rock and lands upside down.
That description is a pretty good description of what happens when you have the CG too far back. In particular the nose down flip and landing upside down after a short half out side loop. You need to move the balance point forward with some nose weight and then adjust the stabilizer's rest angle to give you some up trim to counter the extra nose weight.

Keep in mind that a key trimming factor is that the CG/balance point location works hand in hand with the wing to stabilizer trim angle. You cannot change one without changing the other.

But the CG location is still the boss in this. You set the CG at some point to make the airplane stable in pitch. Then you adjust the stabilizer to set the flight speed.

It looks a lot like your wing is way bigger in relation to the rest of the design when I look at your pictures and at the drawings of the real plane. If this is the case because you think you need more wing area just keep in mind that the bigger wing even with what looks like a bigger stabilizer still means that the balance point will need to be fairly strongly forward to be stable. Like potentially as far forward as 25 to 27%. There are a few online CG calculators you can use to determine where the CG should be placed.

On small and light gliders the generously rounded nose of the folded over KFM airfoil isn't doing you any favors. If it'll hold up and not curl badly try a single flat plate without the fold over. Or if needed make the test model smaller so you can use a single layer of 3mm foam. You can still learn all you need from test gliders that are only around 30 to 45cm span. And then the light foam can be strong enough.

And if I'm right and you did make the wing and stab larger I'd suggest you get things back into scale so the wing to stabilizer gap is the correct proportion to the wing and stab as shown in the top view of the real one. As you say you've ended up with a rather U2'ish look where the real one in the top view is not that radical. So get things back into the proper proportions first.
Feb 17, 2020, 01:53 PM
Learning to make
Thread OP
Thank you everyone so much for this information. Thanks to it I have found "problems" with my approach.
For starters, I never made a chuck glider with the original skinny high aspect ratio wings of the real thing.
I increased the chord right off the bat in order to satisfy an arbitrary wing loading required for an arbitrary AUW I selected.
So I will go back, re-proportion the templates as precisely as I can to match Rutan's CAD views, modify the chuck gliders to match.

Also, I'm using Rutan's CAD views to make a CAD model of the skigull of my own. Could be needed for determining fuselage cross section shapes.


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