How To Scratch Build RC Planes
Over the next few weeks I’m going to diversify from my tutorial style of posts to take you through the build of a new model that I’m currently working on.
Although the majority of my visitors are newcomers to flying rc model aircraft, there will hopefully be some of you who will be sufficiently interested to want to know how to scratch build rc planes.
For me a major part of the pleasure in our hobby is actually deciding on a subject and then obtaining or drawing up a set of plans for it followed by the building process to completion.
The end result is a completely unique model that no-one else will have when you turn up at your field. I find it a little disappointing when I turn up at our club field to find several identical ARTF models lined up. I can honestly say that none of my scratch built planes are duplicated at our club.
Now I don’t expect you all to start drawing plans but there are a good selection of model subjects available from plan publishers. The range of subjects covers simple designs right through to very advanced scale models that require particular skills and expertise to complete.
I have been designing and building my own projects for some years now however, my current build is one designed by a friend of mine back in 1975. Although the original was for 40 size glow engines, I have adapted the design to take electric power. I have a particular affection for models with a retro feel to them and this one is just such a subject. As you can see, the plan has suffered the effects of time and wear.
Another modification to the original, besides the electric conversion is the inclusion of mechanical retracts. The only reason for this is the fact that I happen to have a set of suitable trike retracts lying idle. There is no good reason why they should not be replaced by a set of the latest all electric retracts so readily available now or left as a fixed undercart as the original drawings.
Once the model is completed and flown I will be re-drawing the plans to show the changes required to accommodate the electric components and retract installation.
You have seen the condition of the original plans and because I didn’t want to cause any more damage to them I decided to re-draw the Wings, Stabilizer, Fin & Rudder. The original design called for foam cored wings covered in Balsa or Obeche veneer .
Where I live in Spain there are no foam cutting facilities and I don’t have a ‘hot wire’ foam cutter so I decided to draw up a set of wing ribs. Now this requires a certain amount of geometry knowledge for which I have to thank the perseverance of my school maths teacher all those years ago.
Having re-drawn the wings, projected the rib set and cut them out, I decided to build the fuselage first. Don’t ask me why! When you are scratch building you are free to do whatever you prefer unless there is a good reason to build in a particular sequence. This will usually be indicated by the designer either on the plan or in any additional instructions included.
There are four basic components that make up the main structure:
- Fin & Rudder Assembly
- Stabilizer & Elevators
The simplest constructions are the Fin, Rudder & Stabilizer so these are the parts I decided to build first.
In each case the structure is a frame from 6mm (1/4″) Balsa covered on either side by a skin of 0.8mm (1/32″) balsa. Prior to covering the Rudder it was sanded to a tapered section at its trailing edge. the Stabilizer and Fin are flat section with rounded leading edges.
The Elevators are solid medium density 7.5mm (5/16″) balsa tapered to 1.5mm (1/16″) at their trailing edges. The method of moving the Elevators is quite unusual in that, because they are swept backward, there is no connection between the two surfaces.
A Fuselage mounted 90 degree crank right at the rear drives a split connector to ball links mounted on the inner edge of the elevators as shown in the diagram below. This means that they can be driven from a single fuselage mounted servo via a pushrod.
Here is a photo of the end of the crank protruding from the rear fuselage.
The crank is cut from a piece of 2mm PCB resin board. The connection to the servo forward in the fuselage is by way of a length of 4.5mm (3/16″) Ramin with wire pushrods at each end terminating in clevises to both servo output and crank.
The ball links on the inner ends of the Elevators are mounted on 1.5mm (1/16″) marine ply extensions. I will show photos of the completed installation in due course.
Building The Fuselage
The sides are cut from 1200mm (48″) x 100mm (4″) Balsa sheet, 3mm (1/8″) thick. It can be difficult to source this length of sheet so I took a standard 1M (39″) sheet and spliced an extra short length on to one end.
To do this I cut the main sheet at an angle of 45 degrees and from another sheet cut another piece again at 45 degrees and glued the angled joint using aliphatic resin glue.
My technique for doing this is to lay the main sheet on the bench and apply a length of masking tape along the join line. I then place the mating piece hard up against the angle line of the main sheet, pressing it down firmly on to the masking tape.
I then fold the mating joint edges back and run a bead of glue along this joint line. I then bring the additional piece up into line with the main sheet and add strips of tape across the joint line to hold it together. I then place weights on the sheet to keep it flat on the bench until dry.
The beauty of this method is that once dry the masking tape can be peeled off cleanly as the glue will not adhere to the masking tape adhesive.
1.5mm (1/16″) ply doublers run from the nose to just behind the rear edge of the wing mounting area on each side. These are stuck to the Balsa sheet sides using contact adhesive.
Whilst these side panels and doublers were drying out fully under weights, the fuselage formers were cut out from the appropriate thicknesses of plywood where required and the rear formers made up from strips of 4.5mm (3/16″) Balsa as directed on the plan.
Once the sides & doublers were fully dried the front formers were glued to one side laid flat on the plan (covered in clear polythene sheet to protect it from glue) ensuring each was truly vertical. These were allowed to dry before the other side was glued to the other sides of the formers, again checking for true square setting, and clamped in place using weights.
The turtle deck formers were then added and stringers glued in place ready to take the 1.5mm (1/16″) covering. The photo here shows this basic structure with the stabilizer, fin & rudder placed in position.
At this stage, to enable me to locate the exit points for the rudder closed loop control wires I decided to install the elevator and rudder servos in the space above the wing seating.
Whilst doing this I also decided to install the front nose leg and its retract servo. Having done this I cut and installed the pushrod to the steering arm from the rudder servo.
Then I made up the elevator pushrod and installed it inside the rear fuselage connected to the crank and to the servo output arm.
This picture will give you some idea of the internal servo layout as viewed from the underside. The front of the fuselage is to the right and the rear to the left.
The servo to the right is for the nose leg retract operation whilst the top left servo is for rudder and steering. The nearest one drives the elevator.
Over the next 7 days I will be making progress on the rest of the fuselage and assembling the rear flying surfaces to it. I will go into detail on how I complete the removable cockpit assembly and fabricate the moulded glazing.
The cockpit has to be totally removable for the purpose of changing the power Lipo after each flight.
In the meantime please feel free to visit my website www.rookiercflyer.com where you will find all the basic information you will need to get started in RC model plane flying.
I hope you have enjoyed this initial insight into how to scratch build rc planes. So join me again next time and we’ll progress with the build together.