District 10 Report – December 1 2020
January 19, 2021

By Trible

The French during WWII had started the war with aircraft that could be described as out of date for the most part. As the battle raged before their capitulation there were a few aircraft being developed that could have help turned tide on the enemy if only they had a little more time to get into production at a great pace. One of those was the Arsenal VG line including the VG 36. One aircraft got completed and was found to be the equal of about anything in the air at that time. This one copy was destroyed on the ground. There were further plans for the design out through VG 40 but never got built.

I was drawn to the design because it looked like a semi scale that could be made into a fully capable competition stunt airplane without having to make a cartoon version out of it, so stretching and changing the outlines that it loses the concept somewhat. I most usually use the RO Jett .76 but had a couple .61’s on the shelf getting stiff for lack of use so I kept the size and hopefully the weight down enough to make that a good choice and get some use from my engines. The construction isn’t too far from norms but you may see a few things that are new to you. I’ll get a running start in the next installment but for now cast your eyes on the subject of our model.

I usually begin construction with the wing.  I really enjoy building wings and it wets my appetite to keeping the project moving forward.  Quite a few use foam wings these days or have wing rib sets made or purchased from a laser cutter.  All good but some of the newer folks perhaps haven’t made up their own rib sets and aren’t familiar with the method best used to do this.  For those this session will describe how to accomplish this task.  To begin we must have a set of plywood or aluminum root and tip rib templates.

In this case I tailored an airfoil planform I believe will best suit this design and power.  When I’m doing a new design I create an airfoil thinking in terms of ‘displacement’-sort of the same concept as hull displacement of a boat or ship.  If you need to float (or fly) a given load the size needs to displace enough fluid to do the job.  Of coarse the more displacement, the more power required to move it.  In our application we not only need to lift the aircraft but also do high G aerobatics with adequate buoyancy.  A small or thin airfoil can’t do this job near as well as an airfoil with more displacement, ie, thicker and larger.  I learned this lesson as a kid building countless 1/2A airplanes and flying many hours in the yard,  both sheet wings and all sorts of built up.  It is to better, to an extent, to go larger and bigger up to the point where you no longer have enough power-and power reserve- to handle to load.  The down side of larger and thicker wings are a bigger surface to confront windy and turbulent conditions.  Certain things about the shape or contour of the airfoil can help some but size matters most.

The first photo shows three sets of templates.  On the left are the ones I use for my .76 size/powered Desperado and Driftwood airplanes.  These are in the 700 sq. in. range and weight anywhere from 68 to 74 ounces.  The middle set are from the original .61 sized Desperados which were about 650 sq. in. and best at around 61-62 ounces.  The set I made for this airplane is roughly between the other two.  The area projected is about down the middle but being a semi-scale could run a few ounces heavier than the .61 powered Desperado which could act a little heavy if I let the finish get too thick.  So the airfoil thickness is more to haul a little more weight.  I also used pretty large flaps on the smaller airplane.  I wanted to make these smaller so traded some flap area for a more buoyant airfoil.  Now I hope to control the weight on this airplane so as to not demand more power out of the engine.  As you can see it’s all trade-offs that I don’t even know how to cipher mathematically so I rely mostly on a fairly long experience.  It has a good batting average for me.

Now that I have two carefully made 1/16” plywood templates with two like holes through-drilled for 4-40 bolts to clamp them together we need the correct number of light 1/16” blanks for each half of the wing.  In the next photo you will see I drew the rough outline of 24 of the largest rib on six sheets of balsa.  These are cut out and 1/8” bolt holes drilled. 12 each are stacked in between the root and tip templates.  Now the balsa is carved and sanded away.  Final shaping needs to be done with a long sanding block across the templates.  Before unbolting I cut the rear 1/8” square spar slots are cut with a zona saw.  I also draw the rib center lines down the front of the rib stack to help line the stack up for the next step.

The stack is unbolted and each rib laid out in order and each rib numbered at the trailing edge.  The second stack is made in a like manner.

We need to trim off the chamfers that we made on the ribs.  Re-stack the ribs in order but this time shove them all forward nose down on the table.  Get the stack straight and precisely position the rib templates back on the stack just exposing the chamfers ( see photo) .  You will need to tape this stack together and perhaps use a few straight pins through the ribs in spots to maintain position.   Use your sanding block to sand off the chamfers.  Before I take these apart I use the zona saw to make one straight cut down the slots for the main wing spars.  This locates the exact starting point to cut the slots on each rib as we progress.

Next I lay out the wing plan on the table and place each rib in the proper place.  I am looking to find the approximate place where the lead out cables will pass through the ribs.  I lay a metal rule from the bellcrank to where you expect it to pass out at the tip.  Mark each rib where the cable will pass.  Do the same for the other control cable.  I carefully locate and draw the fore to aft centerline on each rib.  Not you know where you need to make lightening holes that allow free passage of the cables and large enough to allow for the full range of lead out adjustment at the wing tip with rubbing anywhere on the ribs.  The spar slots are cut at the same time.  I make a laminated, tapering spar with one, two or three thicknesses of 1/8” medium balsa.  Look carefully as to which ribs need which cut out.

The final part for this section is show in the photo where I’ve drawn center lines on the leading and trailing edges.  Rib locations are also marked.  You can also see where I am getting the assembly wing board built up to put the wing together.  Next time we will work on getting the wing framed up.

Dave