• This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn more.

CampRobber Scratch Design/Build

#1
Hello, Flite Test Family. This is my scratch design/build. It's my first, and my first RC at all in a long time, so I'm keeping it simple. It's roughly a "Balsa FTTT": 24" length, 40" span, 4 channel control.

IMG_20191215_130354.jpg

The fuse is 1/16 balsa sides, 1/8 lite ply bulkheads, and 1/8^2 stringers. It's 2" square at the wing and tapers a bit towards the ends. Forward of the wing there is a single lite-ply deck designed so the speed control and radio would attach on top and the battery below, from the opposite side.

The wings are straight with a Clark-Y airfoil and barn-door ailerons. They're constructed with 11 3/32 ribs, top and bottom 3/8x3/16 balsa spars, leading edge sanded from 1/4^2, and trailing edge sanded down from 1/8x1. No sheeting or webs. They are joined at the middle by aluminum tubes through the first two ribs such as to produce 5 degrees of dihedral.

The tailplanes are made of 1/4 stick. There are guide holes in all the aft bulkheads for elevator/rudder pushrods.

The first motor I bought was just the cheapest bldc on amazon, the dubious orange 2212. I ran it to max thrust, one prop blade fell off, and the vibration ripped out the firewall. Is that why they're cheap? I've glued that back in and replaced the motor with an 1806.

IMG_20191215_232304.jpg IMG_20191215_232125.jpg IMG_20191215_232154.jpg IMG_20191215_231906.jpg IMG_20191215_232108.jpg IMG_20191215_231748.jpg

These are some things that I wish I'd done differently so far:
  • firewall better attached
  • sheet wings at least where the rubber bands go
  • either half ribs or front stringers in the wing
  • wing ribs 3/32->1/16
  • wing spars balsa->basswood and smaller
  • at least a few sheer webs
  • wings joined by spar
  • tailplanes spar/ribs construction instead of flat sticks
  • tailplane leading edges meet inside fuse
  • wingtip device to reinforce last rib against monokote
  • fuse sides without exposed engrain (?)
  • smaller alignment tabs all around
  • included flaps
So that's something for next time. It's snow/mud season here so I've ordered a flight sim to play with until it gets a bit warmer out.
 
#6
Yes, it's laser cut. I wrote a computer program that draws the foil and inserts the spars, (that was going to be a lot of work because I was originally planning a tapered wing) then I edited it in librecad for the ailerons and such. The fuse is just drawn in librecad. I have a 40 watt Epilog and it cuts 1/16 balsa at 100/100 speed/power.

Apparently there's commercial software (profili, devcad, etc?) that does this stuff but I'm a programmer so DIYing that part isn't a huge deal.
 
#7
I rebuilt the ailerons. They're 8g instead of 4g each but are much more solid.

The first version used a plywood rib-type thing that extended as a control horn, and balsa tripling that was supposed to make it possible to cover. But I would have had to sand through that ply to bevel the front edge.

The new ones are laminated out of 3 1/8" balsa sheet layers, with grain oriented chordwise in the middle and spanwise outside. I'm using nylon control horns. Unfortunately the control horns have .063 holes and my 0.039 (?) music wire has some play.
 

Attachments

#8
Covered the tail and dry fitting the control surfaces. I had some problems covering the tail, with the monokote pulling away from the inner corners. It needs a 3S2200 and 44g more ballast to get the CG right, but there are a few more things to add. I've been playing with RF9 and it's becoming clear I need a *lot* of practice.
 

Attachments

mach1 rc

Well-known member
#10
Covered the tail and dry fitting the control surfaces. I had some problems covering the tail, with the monokote pulling away from the inner corners. It needs a 3S2200 and 44g more ballast to get the CG right, but there are a few more things to add. I've been playing with RF9 and it's becoming clear I need a *lot* of practice.
Really starting to look nice
 
#13
This is a prototype all-flying v-tail actuator I've been working on. For this first build I'm using a conventional tail but longer term my goal is aerodynamic performance and drag reduction -- and I don't like building hinges and control horns anyway so I thought I'd experiment with all-flying control surfaces.

IMG_20191223_002607.jpg IMG_20191223_005431.jpg IMG_20191223_011836.jpg IMG_20191223_011726.jpg

One of the laser cutter tricks I've learned is that plastic and wood have low friction against each other so you can use them as a bearing surface. This uses two pieces of acrylic trapped inside a plywood sandwich -- a rotating one that forms the stabilator pivot, and a sliding one that acts as a wedge to drive a crank attached to the stabilator. The actual crank arm is 1.7" but the wedge angle provides further mechanical advantage so it's equivalent to a 2.83" horn.

The fins shown use a NACA0015 foil. The dimensions of the mechanism can be tweaked but this one goes to +/- 8 degrees. The polars at RN=100k suggest the foil won't stall until 13ish, but I'm not sure how downwash figures in. One of the things I like about this is that it is super easy to change the tailplane aerodynamics later.
 
#17
Almost done. The electronics bay has a balsa cover secured with nylon screws and the battery bay has a plastic cover attached with velcro strap. I still have to tweak the control linkages a bit and I think I should probably monokote strip over the hinges because the gaps are kinda bad. Also the wing could use some kind of saddle-type thing to fit more snugly. AUW with a 3S2200 installed is 567 grams or 20 oz, resulting in a wing loading of 10.8 oz/sqft.
 

Attachments

#18
CR7_3750k1.JPG


Practicing the dark arts a bit on nye. The 28ga nichrome wire was only $6.49 on amazon and a 4x8' slab of foam at the box store is $20. The tube is 3/4" fiberglass from mcmaster. The fittings are lasercut from cheap plywood, woodglued together and epoxied to the tube.

The balsa/monokote wing weighs 150 grams. Minus ~20 grams each wing for servo and linkage, 110 grams.

The 12" section of foam with the same profile weighs 20 grams. 40" of it would weigh 67 grams.

The monokote weighs about 30 grams though, and for a fair comparison, the foam ought to be finished somehow.

The foam would also need some kind of spar to compete with the balsa's strength.