Orcrist | A 2.5m VTPR Glider
Part III: It’s a (vinyl) wrap and maiden flights.
Readers may want to familiarise themselves with the first and second parts of this series before continuing with this follow up article. Also, click any picture in any RCSD story for a higher resolution image. — Ed.
Just the Tip
At the end of the last episode, I had a set of wings that joined the fuselage and that was about it. They still needed work on the tips, servo pockets, control surfaces cutting and hinging before any thought of flying could be considered.
The wing panels were cut oversized from stock slabs of foam, so the first job was to trim the tips down to the required span, as per plan. Additionally, the plan shows a balsa tip giving a bit of tip protection and providing a bit of span beyond the end of the aileron tips.
The wing panels are a veneered foam construction and the trailing edge surfaces are cut from the same panel, then hinged with one of the usual ways — in my case, I’ve bottom hinged with tape.
I chose a bottom hinge for both aileron and flap (as opposed to a more traditional bottom flap hinge / top aileron hinge) to allow for mixing both together to form a single aileron in certain flight modes, whilst still allowing a significant flap travel.
The dimensions of the surfaces were carefully transferred to the panel and then cut with the bandsaw, ensuring symmetry between both wings and allowing for a gap for upward travel, rather than a vertical cut. Once separated, the two edges were planed back to allow for a hardwood facing to be added, giving a little more rigidity and allowing for the gap to be planed to exact dimensions. A similar process was applied to the inner and outer tips of each surface and the panels where they were cut chordwise, these were capped with balsa.
Lastly, a block of balsa was glued to each tip with Gorilla / PU glue and then rough trimmed to shape using the bandsaw.
Once cured, the tips, hardwood false spars and balsa caps were trimmed, planed and sanded to profile, resulting in a nice tidy wing plan, tight panel gaps and straight hinge lines, ready for covering.
Servos
The wing is a four servo ‘full house’ wing with servo operated flaps and ailerons embedded in the wing panel.
You may recall from earlier episodes, there is reinforcement for the servo pockets inside the wing skins, so it was important to cut pockets accurately, where the reinforcement was. The locations were transferred from plan, taking into account the location of the spar (don’t want to cut that!), but also ensuring there was enough depth of the servos too. Originally, I had planned to use some Savox 12g servos I had on the shelf from an earlier project, but in the end I went with new KST HS08Bs (thanks HyperFlight, link in Resources below) as I plan to run on high voltage 2S LiPo for power.
Using the best tool ever (Dremel) and my pocket router jig (see Resources, below), routing out the pockets was a straightforward process with care being needed to make sure I didn’t go too deep.
The pockets were lined with thin ply on the bottom to spread forces across a larger area, rather than direct to the foam and the fore and aft walls were lined with hardwood. More hardwood was used to create a notched frame for the servos to sit in, forming part of the mounting. The careful attention to depth meant these servo frames were perfectly aligned with the upper surface of the servo and rebated just enough to cap with a 1.6mm composite board. The board was screwed to the hardwood ensuring a snug, slop free servo mounting.
Wiring Harness
Servo mounts fettled, the next challenge was to run the servo cables. In hindsight, I could have done this before I skinned the wings, or at least run the channels first. Anyway, a long length of music wire with a slightly roughened tip made its way through the foam slowly but surely by hand from the root to the 1st servo pocket. Easy! Well, yes but the second step from the first pocket to the second was less straightforward.
At the root, there’s plenty of thickness to ‘get it wrong’ without causing a catastrophe (or minor annoyance). But as the span increases, the thickness decreases and the length of the cable run nears two-to-three times the initial length. This causes issues with drag on the music wire (it’s hard to twist and push), makes alignment tricky — you have no way to know where the tip is, either chordwise or vertically.
In the end, I used a drill to rotate the music wire (slowly) and ensured I started with a straight rod, lined up as best I could. Fortunately, I managed to reach the 2nd pocket without hitting the wing skins, twice!
With the cable runs drilled, the next task was to open up the channels to a size big enough to run servo wire through. A carbon rod with sand paper taped to the tip managed this easily. It followed the pilot holes created by the music wire and with the drill used to rotate it. They opened up nicely.
It’s a Wrap!
I had planned to cover the Orcrist using sign writing vinyl from the outset but now the decision on the design of the covering became more pressing. The glider is a bit ‘non traditional’, so a plane white glider wasn’t really on the cards. A few iterations were played with anchored to a basic two-tone livery. In the end I went for a pink and blue scheme, creating a bit of a talking point, but also offering high contrast for visibility.
I’ve used vinyl for wing coverings previously and like the way it can “mask” some minor surface blemishes where traditional coverings might not. It also offers a huge range of colour options too. My vinyl came from a UK based wholesaler called Metamark (see Resources, below).
There are many YouTube videos on working with vinyl, especially for ‘wrapping’ cars. The basics are:
- Clean the surface your sticking too — no dust!
- Use a new, sharp blade!
- Work from the smallest / tightest curves to the largest areas (you can stretch / expand if needed, but not shrink)
- Use (gentle) heat to help form round curves / to bond the adhesive
- Work away from you / towards the edge (smoothing towards you will cause a ripple)
I use a paint stripping heat gun, but that’s very harsh and care needs to be taken not to either cook the vinyl (it will tear and shrivel) or damage what’s underneath. I suspect a hair drier might work too, although I’ve not tried that.
A section of vinyl was cut with a good excess around the edges to cover one surface. I used the pink as the base colour. Working from the tip, the vinyl backing was removed in small amounts as I worked my way up the wing.
Once the full length was covered, the tips and leading/trailing edges were gently heated and stretched round, ensuring the stretch wasn’t too big, so as to preserve the colour (but eliminating any ripples) and then trimmed.
For the underside, square sections were carefully cut and stuck in to place, creating a high contrast pattern.
Fuselage Covering
Covering the fuselage was a slightly more tricky endeavour. I initially tried to use a single section of a colour on the bottom section, but my vinyl wrangling just wasn’t quite up to it. Undeterred I removed it (gentle heat to weaken the adhesive). I tried again but this time aiming to cover a quarter not half the area. The result was much better, although there’s a very fine join line. You’d struggle to see it! The horizontal edge where the colour changes was achieved by using the factory edge from the vinyl roll. The neat factory edge was lined up and then held in place with masking tape while the remaining areas were stuck down, forming a datum. The next colour was then lined up (again with the factory roll edge), butt-jointed to the previous.
The tail was covered in the same way, but separately to the fuselage; it was just too tricky to deal with such large sections of vinyl and all those contours!
A few design details and flourishes were added and the result is a nice straight joint with no overlap or ‘wobble’.
Hinge the Surfaces
With the wings and control surfaces covered, the surfaces were hinged using Blenderm tape, first the inside of the hinge, then the outer surface. If you do it the other way, you may/will find the tape restricts a little bit of travel.
Another thing I do is to run the tape in sections per surface, not overlapping. That way, if on section fails in flight, the tear shouldn’t (I hope) tear the full span of the hinge.
With both surfaces hinged, the full span of both should travel together smoothly. Mine did!
Control Horns
This is a scratch build, so it seems appropriate that the control horns are scratch-built too (like the rudder and all moving tail). Following similar principles, I used 1.6mm composite board to fashion four matching horns on the bandsaw and hand file:
The control horns were rebated into the surfaces, aligned with the servo arms. Care was taken to insure there was sufficient clearance for full flap travel (near 90 degrees) whilst giving the servo the best leverage versus the hinge.
Once happy with the dry fit, they were glued into place with PU glue. Note the masking tape to protect from any foam creep.
Finally, with the hinges and horns in place, the control arms were made to size using MP Jet (see Resources) machined clevises and M2 steel rod.
Time Flies (And So Does the Orcrist!)
Work and weather combined to stretch the build and maiden flights well into the British summer flying season. I was being quite picky about the wind conditions for its first flight, given the almost symmetrical wing and the slightly over eight build. I picked a day with a forecast wind of 12–18 mph and a hill with a nice big landing area on top, few trees and perhaps most importantly, options to land on the slope too — just in case.
The first flight was from Thurnham Castle in Kent on a gloriously sunny day with a nice southwest breeze. After a few minutes confirming I had control while it flew in my hand, a firm chuck resulted in the glider soaring away, climbing a little and only needing a couple of clicks of trim. The ‘bale out’ space ahead was not required and gradually a ‘feel’ for the basic flight controls was gained. I didn’t push the envelope, but there was plenty of authority in pitch and role. A few minutes of floating around confirmed the wing was not particularly good at lift generation, but it was stable in turns and as an airframe, it could penetrate too. All promising signs for its intended purpose: aerobatics in proximity to the edge of the ridge. Alas, Thurnham is not the ideal slope for that type of flight — the ridge has a nice rounded crest, no sharp incline or lip to create a strong up draft, but ideal for a slope session with a ‘normal’ glider.
The next maiden was down by the seaside at Beachy Head. As a slope, it’s a bit more pronounced in incline, but again, misses the boat slightly due to the bushes near the top. My Alpina soared away perfectly — the Orcrist struggled to make it beyond the bushes! Next slope!
Lessons Learned And Thoughts
I’ve enjoyed the build — so no issues there. I’ve not built much before (just the normal kits where you fit servos or cover a few things) and certainly not from scratch. I’ve therefore learned a few things along the way:
- Build lighter. I have a tendency to overengineer things. The tail could be much lighter and I may strip off the tail skin and lighten the balsa underneath.
- Build lighter still! The wings are overengineered: the spar could be half the thickness. The epoxy soaked into the veneer, I will use a different glue next time and I’m not sure I needed to glass them either!
- Cut / route the servo wire channels before skinning the wings — it will save time and reduce risk!
Talking of wings, the resulting panels were a little thinner than the plan versions. talking with ‘those who know’ they have suggested a cooler cutting wire; the suspicion was I had the wire a bit hot and the resulting kerf was a bit more than expected. This, coupled with using veneer instead of balsa sheeting resulted in a pair of quite skinny (thin) wings. I will almost certainly make another set and aim for a bit more thickness. (and weight saving).
Thanks for reading this series, and if you have any questions please leave them in the Responses section below and I’ll do my best to answer them.
©2022 Marc Panton
Resources
- Orcrist | A 2.5m VTPR Glider — Part I: Picking the design, making plans and getting the build underway.
- Orcrist | A 2.5m VTPR Glider — Part II: Building, cutting and skinning the tail and wings.
- Metamark — Sign materials & vinyl supplier
- HyperFlight — A well known supplier of RC gliding equipment. (ships internationally)
- MP Jet — Supplier of excellent, precision engineered RC components. (ships internationally)
- What a Tool! Servo Templates for Dremel Rotary Tools — My previous RCSD article where I explain the finer points of routing with Dremel tools.
- Dark Grey Styrofoam — The foam is listed as ‘styrofoam’ with a density of 33kg/m³ (~2 lbs/ft³). They do other colours, but not in the ‘large size’ sheets — however, it’s all the same density.
- Koto Wood Veneer — This veneer is very similar to Obechi.
- 25g/m² Ultra Lightweight Close Weave Glass Cloth, 950mm Wide — I’ve had both glass and epoxy from this supplier — friendly and quick service. Their epoxy hardener has a slight blue tint which is helpful when mixing which isn’t noticeable once cured.
- EL2 Epoxy Laminating Resin — I’m using this laminating resin with the ‘fast’ hardener, pot life 15–20 minutes in my shed at 15C. Cured in ~10 hours (ie. overnight). The cloth samples are handy for odd jobs and repairs where you don’t need a meter of material, such as the servo pocket reinforcements.
- My LHS: Addlestone Model Centre — A proper model shop with materials, kits, and parts. Don’t forget to support your LHS (local hobby shop)!
- Orcrist 2.5m on RCGroups — The RCGroups thread that proved to be the source of so much valuable information.
All images, videos and drawings by the author. Read the next article in this issue, return to the previous article in this issue or go to the table of contents. A PDF version of this article, or the entire issue, is available upon request.
