Hi. Let me show you how I built one of my scratchbuilds. I'll tell you about platform decision, design, buiding process, powertrain & electronic guts selection.
What to build?
First thing to do, is to choose what to build. It mainly depends on what you expect from finished airplane. When you scratchbuild, apart from joy of building it, you build it for - fun (funflier) or special purpose. I needed an airplane to carry short range FPV. So all things to consider in design (in no particular order):
- carrying capacity
- flight time
- overall costs
- ease of build
- available materials (incl. space for gluing some beast together :) )
- time you are willing to spend building (incl. shipping of parts)
It all counts. If you are unclear in any of these, you may run into problems later. It helps if you order them from most to least important so you know, what you can sacrifice in favor of something else - a trade off. My dreamed specs were:
- fly for at least 30 minutes
- carry basic fpv (under 200 grams)
- stable flight, but slightly aerobatic handling
- cheap as beans, must fit in very tight budget
- use materials and guts I already had home
- quick build
After some research I decided to go with flying wing. It was pretty easy to build (hey, only a set of wings!) and stable but not self-leveling like high-wingers. After some wing loading calculations I came up with 1200mm wing span and 350mm root chord. This should be plenty to carry fpv and hefty battery for quite a bit. I got a lost a little while searching for airfoil, so I picked MH-45. It should be nice even for flying wing, with high lift and moderate drag.
In my parts box I found
Turnigy 2830 1000kV motor, 25 Amp ESC, 10 gram oragne digital hobbyking
servos, 3s2200 and 3s1600 baterries. With 9x4.7 motor draws 15 Amps
static at full throttle and should pull about 1kg. That should be plenty
for intended purpose, and hefty battery will last for quite a while.
One could adjust weight/thrust ratio by choosing another motor/prop and
set flight time by battery size. But I like my planes to have
weight/thrust 1:1. I rarely fly full throttle, so this setup gives long
flight times and great punch if I run into trouble and need to throttle
So, guts were OK, no need to order anything. Then I
needed material. Cheap, available - I had some polystyrene laying
around. That white crumply fragile wall thermal insulation thing. I was
aware that it is fragile, and will break in crash. But I was building it
for FPV and in case of crash, it would be the cheapest part broken. I
was building it for flying not for crashing, anyway :).
I applied some candle wax to the edge of template to help wire slide around, and I cut out set of wings with hotwire. I wasted about 2 winghalves until I dialed in cutting speed and wire temperature. Next two sets for me and my friend came out perfectly. Afted cutting, quick sanding with fine sandpaper and cleanup with vacuum cleaner on low power, with brush attachment.
I needed center part of airplane spar-free so I can shift battery and
FPV freely to get CG right, I had to come with some replacement for
spars. Glassfibre! I had some light and heavy GF fabric from HK with
laminating epoxy. I epoxied wing halves together. After hardened, I
added 5x5mm spruce spars starting so that center part of one layer of
heavy GF just covered their ends. They run about to the servos. Just for
safety. Then I covered leading edges with light GF to have some
protection for landings. I cut elevons from balsa trailing edge stock.
Hinged on omnipor (blenderm?) tape. Some masking and water based spray
Red center is all GF, leading edge GF can be seen on the pic, glossy.
Then I embedded servos and cables with extensions, fitted control horns and linkages. I knew that polystyrene definitely would ding, break and crumble in contact with... about anything? Protection layer needed. I bought A3 laminating foils some time ago, just for this purpose. So I ironed it all over the wing, except for elevons and center, heavy GF part. It came out really nice. Laminating foil doesn't shrink (much), but it stiffened wing significantly. Motor mount made from 1.5mm ply, embedded about 10 - 15cm deep in foam.
I was not sure about CG, and flying wings are not forgiving in this area. For maiden I only taped guts to the surface to get CG as per some online calculator.
First throw - nose dive. Broken GF and crumpled core at the very nose :D. Taped the nose, shifted battery back, and it flew. Some more battery shifting and trimming, and it was flying very well. I flied it couple of times like that, just then I taped FPV gear up there.
After some more CG tuning I embedded FPV gear into foam, securing everything with velcro strips. Velcro is glued under light ply plate to the bottom of every tray, so it won't come out at high-G stunts or very bad landing.
I tape all wires and filter to the airframe before every flight to reduce the drag.
Weight came to 395 grams + 105 grams FPV + 170 grams 3s2200. Flight times on 3s1600 are about 30 minutes, on 3s2200 about 45 minutes of calm, gliding flight. More than enough :). Rough calculation from battery size and runtime says that it eats 3,5A cruising. Not too bad. A while ago I bought camera mount from HK and put servo on tilt, so I can look down for flying or horizontal for landings.
I'm really happy how this one came out. Flights are long and steady, handling is nice and power margin makes it agile flier.