Control Line: Aerobatics
By Frank McMillan
12106 Gunter Grove San Antonio, TX 78231
Introduction
The past few years have seen more and more CL aerobatics fliers harking back to smaller .35-size aircraft, a trend that may have been fueled by the popularity of Old-Timer and Nostalgia models. Whatever the reasons, I know it's for me — I've really enjoyed the fun of the smaller designs.
One apparent problem has been the lack of suitable engines. The venerable Fox .35 is still available, but many fliers have been looking for more power than the older .35s provide.
Magnum .40 GP
Fortunately, as a result of the Nats issue (November), I had the chance to evaluate a Magnum .40 GP. Derek Barry won the Junior class at the Nats using one of Tom Dixon's Magnums.
Tom Dixon (write for his catalog at Suite 401, 1938 Peachtree Road, Atlanta, GA 30309) provides Bolly props, kits, plans and custom engines for stunt. He has been developing the Magnum .40 GP for about four years as a more docile alternative to the OS .40 FP (of which it is a direct copy). Key changes Tom made:
- Retimed liner timing to tame the engine.
- Adds a small Kustom Kraftsmanship needle assembly with a fine 2-56 thread; the assembly is sealed to the case with five-minute epoxy to prevent air leaks (silicone would work well too).
- A .280-in. bore venturi completes the basic package.
As with any Schnuerle-ported engine, you need to keep the fire going. Tom recommends a Fox 1-1/2‑volt RC short plug, which shields the element from the incoming fuel mixture for a more stable run. Just make sure you have a hot plug.
Judging from several I've seen, the Magnum .40 runs well and really perks up 35-size ships of moderate weight. My test engine weighed just 244 grams.
Propellers and Performance
These engines seem to run well on an 11 x 6 two‑bladed prop or the equivalent. The Bolly props are a good choice:
- Bolly three‑blade: 10-1/2 x 6-3/4
- Bolly four‑blade: 9 x 6
On his 46‑oz., 595‑sq.-in. Thunderbird, Tom likes the four‑bladed Bolly with 65‑ft lines.
Fuel, Lubrication and Bearings
If you're serious about plain‑bearing engines, pay attention to your fuel:
- All‑castor lubrication is an absolute must to preserve the main bearing.
- Cylinder-and-piston combinations in these engines are not overly critical (ABN — aluminum piston, brass sleeve, nickel‑plated crank).
- Run at least 24% castor for lubricating the cylinder and piston; you’ll be pleased with the results.
- Taffinder's PAC fuel mix with 5%–10% nitro is an excellent choice.
Royal .40 (Replacement for the ST-46)
Tom has also been working on a replacement for the ST-46. If you need an engine for your Sig Magnum, the Royal .40 is a good option. Tom flew his model on 70‑ft lines, 10% nitro fuel and an 11-1/2 x 6-3/4 two‑bladed prop. Generally, the same modifications described for the Magnum .40 work well on the Royal .40. Tom reports the Royal .40 and .45 are beginning to prevail in the Southeast and both are readily available.
George Aldrich — Engine Services
Good news: George Aldrich is back in the engine business. George has a reputation for producing superior‑fitting engines. He hones cylinders on his Sunnen, shapes and fits rings and pistons, and has access to superior chroming — you may want a chromed liner on your .60 or .46. He also offers parts restoration on older engines, including regrooving pistons and restoring liner fits.
Contact: George Aldrich 12822 Tarrytown, San Antonio, TX 78233 Tel: 1‑512‑656‑2021
Tech Tip: Talcum Powder and Finishing
Even in Texas the winters make for official building season. While talking with my compadre John Hill about liner problems and balance, we discussed a couple of old but useful finishing tricks involving talcum powder.
Talc as a Dope Filler
Using talcum powder as a filler in dope has been around for a long time.
Recipe and procedure:
- Mix equal quantities (4 oz.) of talc and clear thinner (I prefer a dark color — blue is helpful).
- Dissolve the talc in the thinner, then add the remaining sealer ingredients as your usual base sealer calls for.
- This mix will probably be brushable; thin it further for spraying (spraying yields a nicer, even base).
Cautions and sanding:
- This filler is heavy; you'll need to sand most of it off.
- The dark color helps highlight low spots as you sand. Block sand until you remove the colored filler except on edges or raised areas.
- If you sand down to the surface, recoat with more sealer and resand. Repeat until the base is perfect.
- Thoroughly clean the plane with Prep‑sol (or equivalent), then continue with your finishing procedure. A coat of clear dope to seal the filler is recommended.
Talc for Taxi Lines
A less commonly known use for talc is drawing taxi lines (inked lines) on finished surfaces:
- Dust the surface lightly with talc and wipe off any significant residue.
- Ink your lines; talc helps the ink accept evenly where some areas might otherwise resist.
- After inking, remove the talc with Prep‑sol. You can also spray over the talc; it will disappear.
Closing
Smaller .35‑size designs are fun and, with engines like the Magnum .40 and Royal .40 and the right fuel and finishing techniques, they can perform very well. If you're rebuilding or upgrading, consider the services of Tom Dixon and George Aldrich, and employ the talc finishing tips to improve your models.
Transcribed from original scans by AI. Minor OCR errors may remain.
