The Fine Art of Planking
The time-tested method for moulding strips of wood into an organic, monocoque structure which is both light and strong.
This article employs ideas described in an article titled Strip-Plank like a Pro by Henry Holcombe which appeared in the Model Airplane News. See Resources, below, for more information on how this article might be obtained.
The most elegant shapes with compound curves are usually the most difficult to build. Either you have to make moulds and form them out of glass or carbon fibre or you have to plank. Using Henry Holcombe’s techniques planking proved to be much easier than I thought. The pictures show how I planked the fuselages for my 2m electric slope soarer, Rider, and 3.5m electric thermal soarer Sirius.
Henry’s three key suggestions are:
- Modify a stripper to produce angled edges to the planks.
- Start from both edges and work towards the middle.
- To avoid waiting for glue to dry, use small amounts of thin CA glue.
Modify a Stripper and Produce Balsa Strips
For a Master Airscrew Balsa Stripper (see Resources), glue a 6mm (¼”) square strip of hardwood along the edge opposite the blade as shown in the drawing. This tilts the blade. I strip balsa with the sheet hanging off the edge of the bench. You can’t do that with the modified stripper as the hardwood strip has to prop it up. Place the stripper on a flat hardwood surface and lower the blade until it is a minute distance above the surface. This won’t cut right through the sheet but the strips can easily be split off.
Choose light and flexible 2.5 or 3mm (3/32” or 1/8 “) balsa sheet with straight grain. Unless the surface is a major part of the strength quite soft balsa can be used. It is probably best to use 3mm for early efforts as you will probably need to sand out unevennesses. Set the stripping width between 9mm (3/8”) for a large area with a large radius to 6mm (¼”) for small areas of smaller radius.
Cut the first strip. This will have one square edge like section B on the drawing. Reserve it for an edge strip. Turn the sheet 180º — end-over-end, not side-to-side. Cut another strip. This will look like section A. Cut as many strips as you think you will need for the whole job. If you turn the sheet edge-to-edge you get useless strips as in section C.
Plank from the Edges to the Middle
Make sure the formers give a smooth curve by trying a hardwood strip in place. Trim the first square edged strip to length and glue in place. Cut and glue a square edged strip to the other side of the planked area. Pin or clamp the strip. I use balsa cement instead of CA or PVA. It sets quickly, wipes off cleanly and sands well. I found that with balsa cement there was no need to wait for setting.
Cut the next strip, which will have two angled edges, to length. Glue and pin into place. The bigger contact area will help fix the strips together. Glue a square-edged strip to the other edge of the area. Continue to add strips, alternating edges, until the far ends touch. You will need to taper the ends of the next strip. Lay it in place unglued and mark the line of the required cut. Check the fit and glue when happy. The last strip must have both the width and the ends trimmed and tapered.
All that you then need to do is filling and sanding. For filling I used Supalite microballoons made into a paste with Eze-Kote (see Resources) applied with a 0.8mm ply spatula. Holding the piece up to a strong light will show where more sanding is needed.
For maximum strength against both bending and denting, with little weight gain, I used 24g/m² (0.6oz) or 48g/m² (1.5oz) glass cloth applied and coated with Eze-Kote. I use glass because carbon fibre on the fuselage could cause radio signals to be screened out. Eze-Kote takes acrylic paint and varnish well, ideally air-brushed.
Rider: 2m Slope Soarer and Bungee Launch Glider With Motor
Fuselage front ready to plank:
Planked, unsanded:
Filled and sanded with two coats of Eze-Kote, before glassing:
Finished model:
Sirius: 3.5m Thermal Soarer Using Graupner Cirrus Wings and Tailplane
Ready to plank:
Planked, unsanded:
Sanded and glassed using Eze-Kote and 48g/m² cloth.
The model is fitted with a glass fibre moulded canopy:
And here is the finished model:
Good luck with your planking project, and thank you for reading!
©2017, 2022 Peter Scott
Resources
- Bowlus 1-S–2100 Senior Albatross ‘Falcon’ — From the Smithsonian’s outstanding National Air and Space Museum website: “Monoplane glider with strut-braced, gull-type wing mounted high on monocoque fuselage…”
- Strip-Plank like a Pro by Henry Holcombe — A web search did not reveal any source where this article may be downloaded while respecting the original copyright of the author and/or Model Airplane News, where the article appeared (see next item).
- Model Airplane News — From their website: “Published by Air Age Media, the largest multi-media company dedicated to enthusiasts of radio-control planes, helicopters, cars and boats, aviation and diecast collecting, Model Airplane News is distributed around the world…”
- Master Airscrew Balsa Stripper — From their website: “engineered to accurately cut balsa sheets up to 1/4” thick into strips up to 1/2” wide. The design has a lead screw with 32 threads per inch…”
- Eze-Kote — From their website: “Laminating and finishing resin for balsa and foam models. Eze-Kote is a one-part water based foam-safe, low odour, resin alternative to epoxy…”
- The Skyscraper Method for Fuselages — Previous articles by Peter Scott which feature the Sirius aircraft described above. It consisted on two parts, in the July, 2021 and August, 2021 issues of the New RC Soaring Digest.
All images by the author unless otherwise noted. 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.
