Stage Five – How I Scratch Build RC Planes
Last time I said that I hoped to be able to show you the Hawklett partly covered. Well, later in this post I will have a photo showing just that.
Build progress doesn’t always go exactly to plan due to unavoidable delays and every day life interferences. This last week has been one of those weeks so unfortunately progress has not been as good as I’d wished.
Never-the-less, this demonstration of how I scratch build rc planes has made some progress so let me talk you through it.
Finishing The Cockpit
As you can see from the first photo, having received the new bottle of ‘canopy glue’ I have been able to finish off the glazing.
It’s quite amazing what you can do with a nice clean soft drink bottle, a plug made from scrap balsa wood, a hot air gun, bits of scrap balsa for the interior furnishing and Humbrol matt paint! Eventually I will be outlining the canopy frame using lining tape.
Initially I had intended to rely on the strength of the magnets to hold down the rear of the canopy/battery compartment cover. On reflection I decided that it would be prudent to latch down the rear of this component to avoid loosing it in high ‘G’ manoeuvers. Consequently, I have fitted a spring loaded catch at the top of the fuselage immediately behind the cockpit. This latch engages with a tube let into the rear former of the cockpit assembly.
Covering The Wings
The picture on the right shows the top of the wings fully covered in 1.5mm (1/16″) balsa sheet. Two sheets of 100mm wide balsa were joined edge to edge. My method of doing this is to apply a run of masking tape along one edge of the first sheet. I then place the second sheet hard up against the first one pressing down on the masking tape.
I then fold the masking tape in half with the two sheets back to back and run a bead of white glue along both mating edges. When they are laid flat on the bench the excess glue is squeezed out of the join line and wiped away using a damp cloth. I then pull the edges together tightly using pieces of masking tape across the joint. This is left to dry weighted down with various heavy flat objects.
Once dry the sheet was placed over one half of the the wing surface and lightly pinned in place whilst the outline was marked for cutting to size. Once cut it was attached to the curved surface of the wing using white PVA glue applied to the complete frame and rib edges. Whilst drying it was held in place using copious pins and masking tape.
The underside outer sections were done in the same way. Full length sheets were not practical due to the need to cut out the wheel wells, leg recesses and undercarriage mounting plates, to say nothing of the aileron servo mounting/access plates.
These areas were covered using cut pieces of 1.5mm (1/16″) balsa sheet joining edge to edge and glued with thin CA adhesive. You can see from this photo that this area is fairly busy and needs care to get the covering correct.
Having completed the covering, small cracks and gaps were filled with lightweight filler which was sanded smooth when dry.
Checking Incidence of Wings
As this model is an aerobatic type with fully symmetrical wing cross section, the wing is set a ‘0’ degrees of incidence. This means that the centre line through the leading edge to the centre of the trailing edge is parallel to the model’s datum line.
I know, you’re going to ask me ‘what is the datum line?’ Well here’s an official dictionary definition: “The horizontal or base line from which the heights of items are reckoned or measured as in the plan of an aircraft, etc.”
Often this reference line will be drawn through the centre of the thrust line unless the engine or motor has down thrust built in. In this case a datum line is usually drawn below or above the main drawing and everything is measured and positioned above or below this line.
OK, enough of this technical stuff, let’s get back to the incidence check. The stabilizer of this model is set parallel to the centre line so in order to check the wing incidence I had to set the fuselage inverted on blocks so that a spirit level laid across the stabilizer showed ‘0’ degrees. I placed the wing on its seating and checked that it was also at ‘0’ degrees.
I did this using an incidence meter I made myself. Here is a photo of it in position. The level indicator is a smart phone on which I have an app. that makes the phone into a spirit level. It has a simulated bubble and digital numeric indication of the actual angle (most useful).
The two Grey/Blue slides have a ‘V’ cut out, one for the leading edge and the other for the trailing edge. The deepest part of the ‘V’ each side coincides with the centre line of the wing chord. The small table on which the smart phone sits is parallel to the slide bar. This ensures that the phone reads an accurate angle of incidence for the wing.
Fixing Wings To Fuselage
Prior to finishing the wings I had marked and drilled the plywood former either side of the retracted nose wheel to take the two short wing locating pegs.
The matching location for the pegs were marked on the wing face plate through the holes in the former and drilled out. The pegs were then glued into place and the final fit checked. So far so good!
Once the underside fuselage fairing was glued in place and sanded to a smooth profile a 1.5mm (1/16″) ply plate was glued to the rear of this fairing to take the wing bolts. The hole positions were marked and 3mm pilot holes drilled through.
The wing was placed in position on the inverted fuselage and, using the pilot holes, the appropriate positions for the blind nuts were marked on the 6mm (1/4″) plywood plate epoxied into the fuselage. These pilot holes were then enlarged to take 6mm nylon wing bolts as shown here.
The locating holes were drilled to take the centre boss of the blind nuts. These were placed into position with a thin smear of slow setting epoxy resin applied to the face that is pulled up into the plywood. Steel bolts and washers were used to pull the nuts and their barbs into the topside of the plywood plate. these were left to allow the epoxy sufficient time to set hard.
When the epoxy had cured the steel bolts were removed and I was able to fit the wings on place and secure them using the nylon bolts.
She’s really starting to look like a flying machine now – quite satisfying I think! I promised you some covering at the beginning of this post. Well I’m a man of my word so as a token gesture I have covered the motor access hatch, the anti-glare panel on the cockpit/battery hatch assembly and also used black lining tape to finish off the cockpit! Here’s a photo of the full assembly so far.
We’re progressing quite nicely and next time should see some serious covering done. I do hope you’re enjoying this build blog, I certainly am.
I think there is a great deal of satisfaction to be gained and I strongly recommend it to you. To scratch build rc planes does not have to be expensive or onerous and you will learn techniques that will make you a better modeller in the long term.
If you have dropped across this post accidentally and enjoyed it, please take a look at my main website www.rookiercflyer.com. Also you can find the earlier posts in this series to see the full build log. Number one is here, take a look and enjoy.
Catch you next week.