FREE FLIGHT INDOOR
Bud Tenny, Box 830545, Richardson TX 75083
Correction
In the February 2003 column, a description defined propeller pitch as pitch = 3.1416 x tan(y/x). Actually, diameter is part of the equation, as explained in detail below.
Propeller Formula
A reader asked for an explanation of the basic propeller pitch formula presented in a previous column:
pitch = 3.1416 x tan(angle) x diameter
This formula expresses the pitch of any individual station along the blade where:
- Pitch is expressed in inches.
- "tan" is the tangent function.
- Angle is the angle of the station under discussion, relative to the propeller shaft.
- Diameter is the diameter of the circle traced by that blade station as the propeller rotates.
A typical propeller assembly fixture uses a 45° triangle to support a blade while it is being attached to the spar. If that triangle is placed at three inches from the shaft, the diameter is twice that radius, or six inches. The 45° angle makes pitch calculation easy since tan(45°) = 1.0. So the pitch of the propeller blade at that station equals 3.1416 x 1 x 6 = 18.8 inches.
If a flat-blade propeller is required (no blade twist or camber), each station along the propeller has a different pitch. Assume the blade angle is 45°. Example stations:
- Station 1 (inches): Diameter 2 — Pitch 6.28
- Station 2 (inches): Diameter 4 — Pitch 12.57
- Station 3 (inches): Diameter 6 — Pitch 18.85
- Station 4 (inches): Diameter 8 — Pitch 25.13
- Station 5 (inches): Diameter 10 — Pitch 31.42
Much of the thrust (and rotational drag) is generated by the tip. Since the area near the hub generates very little thrust, most blade outlines have the greatest width at midblade and taper to a narrower tip. The usual indoor propeller blade is twisted so that the pitch remains the same at every station. Solving the formula for angle gives:
tan(angle) = pitch / (3.1416 x diameter)
Example for a constant pitch of 22 inches at different diameters:
- Diameter 2: tan(angle) = 3.50 — Angle = 74.1°
- Diameter 3: tan(angle) = 2.33 — Angle = 66.8°
- Diameter 4: tan(angle) = 1.75 — Angle = 60.3°
- Diameter 5: tan(angle) = 1.40 — Angle = 54.5°
- Diameter 6: tan(angle) = 1.16 — Angle = 49.4°
- Diameter 7: tan(angle) = 1.00 — Angle = 45.0°
When you consider blade outlines used by fliers today, they typically taper to a point near the three-inch-diameter mark, with maximum width at midblade, then taper toward a tip with a rounded outline. Wingtips and propeller tips develop a tiny vortex caused by spanwise flow that spills off at the tip. Also, the propeller tip travels farther each revolution than the center of each blade, where the maximum width is located. So the blade outline has that shape for reasons besides appearance.
Tip vortices have severe effects in full-scale aircraft, varying in intensity with the airplane’s weight. For years I was convinced that indoor models, especially indoor rubber-duration models, were too light to generate the effects. I have been disabused of that notion by two trusted friends with considerable experience in aerodynamics.
Wing wash (another name for wing vortices) can affect the stabilizer in some flight attitudes. Some designs set the horizontal stabilizer higher than the expected vortex or set the wing just high enough above the motorstick to allow it to be removed from tissue sockets, then tilt the tailboom down roughly 15° so the stabilizer is below the expected downwash path.
Indoor News and Views (INAV)
For those who do not subscribe to INAV, Carl Bakay has created an exceptional publication with the aid of Steve Gardner and Dave Haught in the US and Nick Aikman in the United Kingdom. Unless you know all there is to know about indoor models, you need INAV. They expect to publish six issues a year for $15, payable to Tim Goldstein, 13096 W. Cross Dr., Littleton CO 80127.
Indoor Supplies
Tim Goldstein has a steadily increasing number of indoor items, and he features precision-cut select balsa. Contact Tim at [email protected] for more information.
Slanic Notes
Junior Doug Schaeffer and Jim Richmond made the two longest flights regardless of class at the 2002 Indoor World Championships in Slanic, Romania. Doug and Jim led their teammates by a good margin, and these flights led the Junior team and Open team to victory by a large margin and have been homologated as US records.
Science Olympiad (SO)
I have noted in previous columns that young participants in Science Olympiad (SO), with guidance from model builders in the Wright Stuff event, have become excellent indoor fliers. All US Junior team members at the Indoor Champs in Slanic—with their towering win over the other teams (50 minutes between first and second place)—came from the SO program.
There are many SO model kits available. One that I have been privileged to review came from Ray Harlan; the Bambino is carefully crafted to produce a model capable of making good flights.
Two things are outstanding in this kit:
- A detailed selection of wood sizes and lengths gives the builder a head start in constructing the model (each strip of wood is selected by weight and color-coded to match the intended use).
- The full-size plans are supplemented by 11 pages of detailed, step-by-step instructions that can lead an inexperienced builder to success with little outside mentoring.
Contact Ray at 15 Happy Hollow Rd., Wayland MA 01778-3521, (508) 358-4013 (voice), (508) 358-4013 (fax), or [email protected] for pricing and other helpful information.
Testing
Having good rubber is critical to making long flights, and Jim Richmond ([email protected]) is king of the long flight. He tests all of the rubber he uses. I asked him how he does the testing.
"My rubber test formula is the same as the one I cooked up in the 1960s: 'Torque at 1/2 turns (unwinding) times total turns divided by the loop weight.'"
"This has proven to be accurate enough for most evaluations even though it only looks at the middle torque. When doing rubber tests, I record the unwinding torque in 20-turn increments."
These readings can then be compared with similar ones from other tests for further evaluation. Additionally, I sometimes use the torque measurements to plot charts for really close comparisons.
"For each test, I record the date and ambient temperature as well as the rubber batch and the number of wind-ups represented. My torque readings are in oz.-inches and the rubber weight is in grams."
Northwest Activity
Albany, Oregon, is an active indoor area, with five contests each year in the South Albany Gym. Bob Stalick and Andrew Tagliafico are the leaders. The contests draw participants from as far as Yreka, California (280 miles). Photos by Bob Stalick from a recent contest show:
- Jim Longstreth with his MiniStick.
- Jerry Powell of Yreka, California, with his 1.2-gram AMA Easy B.
- MiniStick winner Andrew Tagliafico, who posted 8.58 (minutes) in the 36-foot ceiling.
- SO models of all ages: Caitlyn Gilbert's first SO model (built from a Midwest kit), Marty Thompson using SO as a learning tool to break into indoor, and Jessica Buffard in her third year of competition as a seasoned SO regular.
- Tony Mula's Pfake Pfokker scalelike fun model.
MA
Transcribed from original scans by AI. Minor OCR errors may remain.
