How hard is it to design your own plane?

[David Lewis]

If you build from plans, it's plans building. But if you design it first and draw your own plans, it's scratch building.
In increasing level of difficulty:
1. RTF
2. ARF
3. kit
4. plans
5. modified plans (e.g. enlarged)
6. scratch (one-off)
7. scratch (production)

As someone else suggested it's a good idea to start with non-scale (sport) scratch-built gliders. I like paper gliders but balsa wood is also good. The next logical step after that could be non-scale rubber free flight.

When designing a glider, there are 3 performance parameters you can optimize:
1. Minimum sink rate (i.e. maximum endurance)
2. Maximum glide ratio
3. Minimum stall speed

You will balance all three, or emphasize one over the other depending on how you want the glider to fly, or the contest events in which you're going to compete.

[David Lewis]

The design and development method I use, I first sketch a rough preliminary drawing and then, based on the wingspan, fuselage length or wing area I'd like to end up with, calculate the scale factor. In this example, the scale factor of my drawing turned out to be 6.35.

1.jpg

Next I multiply all the dimensions on the sketch by 6.35, and lay out the parts on cardstock, balsa, construction paper, or similar material. After I build the prototype, I test fly it and figure out what revisions are needed. In this case, I found it necessary to make it easier to form the airfoil and dihedral joint, and to attach the wing to the fuselage. Also I had to reduce fuselage girth in order to save weight.

3.jpg

[David Lewis]

Based on the experience of building the prototype and the flight test program, I make a second sketch. This sketch, however, is not to scale. It's a schematic for recording dimensions taken directly from the prototype. When you convert from 2D drawing to 3D reality something usually gets lost in translation. The 3D model is better for tweaking and visualizing the final configuration.

2.jpg

I then lay out the schematic dimensions on my construction material and build the second prototype.

4.jpg

On the second prototype, I switched from simple dihedral to semi-hedral, added a wing mounting flange to the fuselage sides, and added two wing ribs.

5.jpg

The wingspan is 290 mm, about as big as I can go using construction paper. The second prototype (cardstock) weighs 13.7 grams. The third prototype is made from construction paper and weighs 7.0 grams. It is one delightful-flying glider. With scissors and some glue, you can try a large number of ideas in a short time. For example, as aspect ratio goes up, induced drag should go down. However, Reynolds number of the airfoil also goes down. It would be interesting via experiments to determine the optimum trade off.

[BillParker]

http://www.dumasproducts.com/product_info.php?cPath=137_51&products_id=230

[davidchoate]

Bill, Does That 50" 190 Fly Good? I really like Guillows Planes, but find they are a bit small for RC. I have been thinking very much about enlarging the Plans on 400 kit. I want to use retracts,flaps.If I could sqeeze that into a 1/16 scale It would weigh to much.

[BillParker]

I fly U-Control, and as such yes, the 190's I have do fly well. Easy built makes 50" P-40, Spitfire, Hurricane, and some civilian planes, but no big 190. I blew up a 500 series 190 to just above 40" and it works well, and I have a 36" House of balsa and it rocks u-control... I own some RTF planes, but need a flight instructor, so I've grounded myself, rather than destroy em...

[p40qmilj]

i do it all the time! i've written 4 books with plans as well as one designs. i also love to moify gullows plans to get the spams the way i like them. the secret is to be as scale as possible and build on what you know

jim

[David Lewis]

In designing a scale model, my first step is collecting documentation. The 3-view is the centerpiece of my portfolio. If the 3-view is inaccurate it will negatively affect subsequent steps. It's rarely a good idea to slavishly duplicate your subject but any deviations from scale should serve some purpose (e.g. simplifying construction or improving performance) and use accurate data as the jumping off point.

I always compare several 3-views against each other and cross check against photos as the final arbiter in case of discrepancies.

[David Lewis]

The final paper version flew so well, I decided make it out of balsa. Test flights with the paper prototype indicated that the wing center section and the horizontal stabilizer needed to be made out of 1/16" contest balsa. Everything else is 1/32". If you look at the top view, the centerline of the fuselage is aligned with a grid line on my magnetic building board (red arrows).

When designing a model airplane wing, there are 5 engineering issues to address:
1. Airfoil
2. Dihedral
3. Torsional rigidity
4. Strength-to-weight ratio
5. Quick and easy to build