Orcrist | A 2.5m VTPR Glider
Part I: Picking the design, making plans and getting the build underway.
If you have an interest in RC gliders (which I guess you do, if you’re reading the New RC Soaring Digest!) and you’ve been on the internet, you have probably come across people doing aerobatics with RC gliders. If you venture further into that rabbit hole, you’ve likely stumbled across the phrase ‘VTPR’, you might even have Googled it and know it stands for Voltige Très Près du Relief or “aerobatics very close to the ground”. As with many specialties like dynamic soaring and thermal duration, there’s a whole range of purpose-built designs that have evolved to match the task at hand: full aerobatic flight, in close proximity to the slope.
For those with a VTPR curiosity and time to invest, SlopeAerobatics.com is a great starting point. See Resources below for this and all other links referenced in this article.
What Exactly to Build?
You’ve might have seen a few of these designs: Le Fish, Voltij and Ahi, all qualify. For my taste, the Le Fish and Ahi are a little small and the Voltij a little pricey for something designed to do aerobatics close to the ground. Another option is to self-build which leads me to the Excalibur, a 2.5m French design that pops-up on numerous videos but is fickle (based on my English language Googling) to find much info about, let alone plans. There are a few 1:1 sized silhouettes and some wing plans, but they leave much for the builder to interpret. Then which construction technique? Foam? Wood? Composite/moulded?
Enter a post I discovered on RCGroups by Mike Seale (again, see Resources for link) where he faced much the same dilemmas as I. Fortunately, he’s produced a plan for a built-up fuselage and even included a wing plan and foam cutter template.
The Best Laid Plans
Equipped with the plans (post #1 in the RC Groups thread), the next task is to print them 1:1 scale on a normal home printer. I don’t have a plan printer, so I need to tile and print, then stitch back together with tape.
There’s a few ways to do this, but the two I have used with success both involve Adobe Acrobat DC Reader. The first way is ‘simply’ to use the Poster settings in the print dialogue. If you ensure there is some overlap between tiles, you can use these to ensure things are aligned when you stitch the tiles back together later.
The draw back of this method is you get lots of empty space ‘printed’ as well as any supporting text, that is not just the plan.
The second way (and my preferred method) is to use the Take a Snapshot tool in DC Reader to select only the bits you need.
The tool is often buried a few menus down, but once you’ve located it, it should stay in the first level of the Edit menu.
Take a Snapshot toolSimply drag a box around the parts of the plan you need to print (aiming to nearly fill a single page of your printer’s paper size) and press Ctrl-P. In the printer window, check Actual size is selected and print. Repeat selecting / printing the areas you need, aiming to overlap so you can align until you have a set of plans to tape together, without the stuff you don’t need!
Back to The Build
With my freshly printed and taped plans, I cut the plan into a set of formers and the fuselage templates. The fuselage template was used to trace directly on to the side material, twice (two sides) and the former templates were glued to the ply (I used a thin ply which, with hindsight, should have been a little thicker!)
The build method is similar to a Phase 6 (and other Chris Foss builds) and probably many other ‘traditional’ balsa builds: Triangle section is glued to one surface and acts as the jointing surface to receive the adjacent part.
Owing to an oversight on my part, 1/2” triangle was substituted for the 3/8” on the plan (doh!) which meant I needed to adapt the formers to match the larger materials.
Additionally, I added triangle sections vertically to the formers — ensuring a good surface area for the adhesive. (Remember how I said the ply was a bit on the thin side?) the secondary benefit was that this helped ensure the formers were square during bonding too!
Lastly, the tops and bottoms of these vertical sections were trimmed to meet snuggly to the longitudinal sections on the side.
Happy with the trial fit of the formers, showing they fitted well and the adaptions had worked, the next step was to repeat adding the triangle sections to the the other fuselage half.
By the time I had adjusted the formers for the thicker triangle section, there wouldn’t be much wood left at the corners of the cut-outs, as you see here, I left the lower sections whole; a few grams of extra won’t make much difference.
Glue, Glue and More Glue
Let’s have a quick chat about adhesives. To attach the paper templates/plan cut-outs to the base ply, I used an aerosol spray adhesive similar to Spraymount. For the triangle section and the many other wood-wood joints I alternated between impact adhesive (Evo-Stik Impact) and aliphatic wood glues (Titebond III and Gorilla wood glue). The selection basis tended to be how long I wanted to wait while clamping vs time available for building — use the aliphatic when I could leave things overnight and the impact when I had a few hours to do lots of build activities, taking advantage of the quicker grab time (almost instant) of the impact adhesive.
For gluing the triangle sections, where they need to conform to shape, I found the impact adhesive was preferable: pinning the sections to the outer ply was a non-starter given how thin it was, where-as the impact adhesive (if properly applied to both surfaces and flashed off) gave near instant adhesion with minimal slip.
Around this time, I began to realise that my choice of ply might be a little thin. Not wanting to re-do a bunch of work and waste materials, I tried a few remedies to strengthen the walls, which between formers and triangle sections, were a little flexible to the touch.
The first attempt was by gluing cross members, much like stringers across the expanse. It was OK, but a ‘PITA’ to do for each area, there would be three in need of attention on each side.
Subsequently this was removed it and I just doubled the whole skin on the internal panel, butted tight up to the fore and aft formers.
By this point, I had also glued the formers to one half, ensuring they were aligned, and the control runs for the ELE and RUD were fitted. (hot tip: the RCGroups thread!)
Give Me Some Relief
This is one way to help the triangle sections conform to the shapes of the sides and later, the taper of the fuselage towards the tail is to relief cut into the sections: about every 25mm / 1” I cut into the section, on the side facing the inside of the curve, but not all the way through, to allow it to compress.
Here, I’m thinking ahead to joining the two fuselage sides, showing the added relief cuts to the triangle sections to allow some flex. (It’s a double edge sword: In the left-hand side, I chose the stiffest sections, now I need it to bend!)
Gluing the two halves was a multi-step process. The triangle sections and formers help to keep the length aligned, but the taper of the nose and tail need some thought to ensure an even taper.
First up: bond the mid-section, where the two halves are parallel. An indication of the strength the triangle sections / ply formers: the two jugs are five litres of fluid (~5kg / 12lbs) each and the bars are lead filled stainless box section (400g / 14oz) each.
Once cured and with some Heath Robinson (Google it) weights and shims, glue the nose: bending the nose sections together, evenly while the glue set. Remember the almost instant impact adhesive — it works very well in this situation!
It was a similar story at the tail end: glue the last of the triangle section, while forming the gentle taper (with weights and shims), but leaving the last former and remaining length from the last former back to the tail unglued. The reason being, we need to remove some triangle material to get the correct taper and to allow the tail to be fitted.
The result should be the basics of a fuselage, straight, true and light!
As mentioned, I have left the tail section of the two halves, from the rear former back unglued while I work the tail and the all-moving tail surfaces.
That’s It For Now
I hope that my efforts here might inspire you to create your own VTPR glider — or any design for that matter. If you have any questions, please do not hesitate to leave them in the Responses section below, and I will do my best to answer them.
Until next time!
©2022 Marc Panton
Resources
- SlopeAerobatics.com — “dedicated to the joy and satisfaction inherent to flying RC glider aerobatics on the slope. The website has an international focus and is meant to showcase the different styles of aerobatic R/C slope flying popular around the world…”
- Le Fish — “The Le Fish plan was intended as a first step towards my stated goal above, to create a lightweight EPP aerobatics glider that would allow me to take what I love about flying a Weasel and go even further…”
- Voltij on aeromod.fr — “Avec le Voltij, nous avons réussi à concilier des objectifs antagonistes : une aile parfaitement adaptée à l’acrobatie, mais aussi performante pour prendre de l’altitude…”
- Voltij on air-rc.com — “With the Voltij, we succeeded in reconciling conflicting objectives: a wing perfectly suited to acrobatics, but also efficient to gain altitude…”
- Ahi — “Inspired by years of ‘in your face’ flying with our Weasel and Alula gliders, the Ahi takes your slope expression to the next level of performance…”
- Orcrist 2.5m on RCGroups — The RCGroups thread that proved to be the source of so much valuable information.
- ExCali — the California Excalibur VTPR Glider Project on RCGroups — “We are undertaking our most ambitious project yet, a California reinterpretation of the world famous Excalibur VTPR glider…”
- What does VTPR stand for? — Now you know.
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.
