Control Line: Aerobatics
Frank McMillan 12108 Gunter Grove San Antonio, TX 78231
Wind tunnel and motivation
Frank Williams, an aeronautical engineer currently working on NASA's Space Shuttle program, acquired a portable wind tunnel about a year ago and has performed some experiments of interest to control-line modelers. The tunnel has roughly a 12" diameter test section capable of producing velocities up to about 50 mph. It is eight feet long and powered by a 1/3 hp motor with straightening vanes, screen mesh and a contraction section — the kind of setup you might find in a high‑school physics lab. The balance measures two components (lift and drag) to the nearest 0.01 lb. The flow may not be perfectly laminar and the model support is not ideal, but the tunnel is far better than sitting in the shop guessing at the flow around a section.
A lot of experimental data on wing sections and control surfaces exists in textbooks, but much of it was derived at Reynolds numbers, velocities and chord lengths quite different from those encountered in model flying. More applicable low‑Reynolds data is starting to appear due to interest in RPVs (recoverable pilotless aircraft), wind-turbine research, and RC soaring. Herb Stokely's "Soar Tech" and the annual National Free Flight Society symposium reports are useful sources for low‑speed aerodynamic material that can be transferred to control-line applications.
Stab/elevator wind‑tunnel tests (Mike Denton and Frank McMillan)
One set of winter evenings, Mike Denton and I spent time looking at stabilizer/elevator combinations in the tunnel. We tested several combinations of shapes and hinge‑line geometries and observed some interesting phenomena. For this report I’ll describe the effect of a simple hinge‑line seal on a relatively common sheet‑stab/elevator shape.
Test setup:
- Parallel plates installed in the test section about 3 in. apart to approximate two‑dimensional flow.
- Test specimens spanned the 3 in. width; stabs were 3 in. long and elevators 3 in., giving an overall chord of 6 in.
- Sections were cut from 3/16 in. sheet balsa. The stabilizer leading edge was slightly tapered and rounded to about a 3/16 in. diameter contour. The elevator had a triangular leading edge and straight taper to the trailing edge.
- The original open hinge‑line model had a "concours" (show‑quality) gap of less than 1/32 in., almost invisible unless held to the light. I did not expect much gain when I sealed that gap with tape on the bottom side, but the results were surprising.
Test procedure:
- Measured lift and drag for elevator settings every 5°.
- The stabilizer attitude was fixed and aligned parallel to the tunnel axis for all tests.
Results
Open hinge line:
- The lift‑coefficient vs. elevator‑deflection curve for the open hinge model showed a fairly normal variation and agreed well with available data from other sources, which validated the tunnel and balance.
Sealed hinge line:
- Sealing the hinge produced a significant improvement in lift. At 20° elevator deflection, the sealed section produced as much lift as the unsealed section produced at its maximum (about 45° deflection). Past about 25° deflection the sealed section’s lift falls off and approaches the unsealed section’s magnitude at 45°.
- At 25° deflection, for this particular section, the sealed configuration produced roughly 25% more lift than the unsealed section at the same deflection.
- Implication: small control‑surface gaps that seem insignificant (e.g., a 3/32 in. gap on beginner planes) may be disadvantaging flyers more than expected. Even tight gaps on pro stunt ships may be costing a few percent of performance — possibly more.
Flow phenomena observed
At low Reynolds numbers some unique behaviors occur. For the sheet‑stab section tested, at moderate and higher lift coefficients we observed a laminar separation bubble at the stabilizer leading edge. By inserting a fine wire with a streamer thread into the test section we could visualize the flow: just downstream of the leading edge the flow separated into a "bubble" about an inch long, then reattached to the surface. The reattachment produced a turbulent boundary layer downstream, which in turn delayed further flow separation farther aft on the surface — a natural reorganization of the flow.
A nicely contoured NACA 0010 section actually showed slightly poorer performance than the slab sheet section in these tests. There is still much to learn; if building a new stab/elevator today I would focus on making it light.
Builder of Model (BOM) rule and appearance points
The Builder of Model (BOM) rule has been one of the few parts of our event that has remained essentially unchanged in intent since the beginning. That does not mean it cannot or should not be changed, but opinions have been expressed, both privately and in print, about its relevance given modern life and competing demands on people’s time.
Current practice at many local contests permits fliers to enter with planes that do not comply with BOM provided they waive any appearance points. My informal opinion is that this is good for the event because it encourages participation; more people flying is better for the event now and in the future. My endorsement, however, stops at local contests.
The National Event Directors have not knowingly permitted violations of BOM at national events, nor should they while the rule remains in effect. When we enter we sign that we will comply with the rules as written, and that commitment should be honored. If the Precision Aerobatics community decides BOM should be dropped, then work through the official rules process to change it — and accept that appearance points would have to go with it, since there is no effective way to enforce appearance if BOM is removed.
I’ve been flying stunt for thirty‑nine years; BOM has been a basic topic that has stood the test of time. I believe the event has endured partly because BOM required competitors to persevere to achieve high standards. Like an old friend, I’ll hate to see it go.
Northwest Regionals
The Northwest Regionals is known as one of the largest and finest control‑line meets. Held at a permanent CL site at the Eugene, Oregon airport (three asphalt and four grass circles), the meet includes virtually all competitive events.
- Date: May 25–26 (this year)
- Contact: John Thompson
1145 Birch Ave. Cottage Grove, OR 97424 Tel. 503/942-7324
Frank McMillan 12108 Gunter Grove, San Antonio, TX 78231
Transcribed from original scans by AI. Minor OCR errors may remain.
