Free Flight
OLD-TIMER Clarence Haught
GRAVITY
As a general rule, center of gravity (CG) locations are omitted from most Old-Timer plans. Many modern-day Free Flighters trying their hand at Old-Timers are confused by this. Establishing CG locations similar to today's AMA gas ships can be disastrous.
A good starting point is:
- Balance cabin models at 1/3 wing chord aft of the leading edge.
- Balance pylon models at 1/2 chord.
The basic difference between the types is the stabilizer airfoil. Most cabin ships employ streamline or symmetrical stabilizer airfoils, while pylon ships tend to use flat-bottom lifting airfoils. The more lift generated by the stabilizer, the further aft the CG can be placed.
Modern Free Flight designs often have relatively large stabilizers—40 percent of the wing area is not uncommon. Most Old‑Timers are around 25 percent. AMA gas ships sometimes have the CG located very far aft; in the case of the Dixielander (a well-known English design) the CG is aft of the wing trailing edge. Use these as starting points, but establish the final CG location during flight testing.
Glide trim is best adjusted by small CG changes:
- Add tail weight to slow the glide.
- Remove tail weight (or add nose weight) to steepen the glide.
Reserve stabilizer-incidence adjustments for regulating climb angle.
Another general rule: pylon models should climb right and cabin models should climb left. Engine torque provides a natural left-turn tendency (depending on engine rotation). That tendency is opposed by the corkscrew-like propeller slipstream and works fine so long as power is not excessive. As power was increased to attain more altitude, models became more difficult to control.
Carl Goldberg's pylon development helped control the more powerful engines used in contest models. The pylon adds directional stability and provides an area for the slipstream to act upon, neutralizing torque effects and giving the pylon model its natural right-turn tendency.
A turn during the climb (power portion) helps avoid looping tendencies and assists a smooth transition to glide. A model trimmed for a straight-out climb will often stall when the engine cuts and lose a great deal of altitude before settling into a glide—if it settles at all. I prefer to control climb turn with a rudder tab. Some fliers employ side thrust on the engine, but this is effective only at initial or slow speeds; as the model accelerates, airflow negates thrust adjustments.
Climb angle is best controlled by stabilizer incidence. Establish an initial setting by hand‑gliding, then increase or decrease incidence to fine-tune the climb. Raising the trailing edge increases incidence and increases climb.
Modern Free Flight designs often induce glide circles by tilting the stabilizer: the model turns toward the high side of the stab as the stabilizer seeks a level plane, tilting the wing into a turn. This method is often ineffective on most Old‑Timers because their stabilizers are relatively small; a lifting stabilizer is required to make stab tilt effective.
Wash-in and Glide Turn
I use wash-in to induce glide turn, because I prefer the glide to turn in the same direction as the climb. This is particularly helpful at launch: a thermal model will usually stay in the thermal rather than turning out. If glide-turn relation is opposite to climb-turn, practice wing-panel wash-in to get the desired relation.
Wash-in is trailing edge down on the outboard panel. During the higher-speed climb, wash-in helps hold the inboard turn while the wing-up maintains climb. Remember that the outboard wing (outside the turn) moves through the air faster than the inboard wing, creating more lift on the outboard wing and tending to roll the model toward the inboard wing. When the engine stops and airspeed decreases, the wash-in creates more drag than lift, thereby inducing the turn while still generating enough lift to nearly equal the faster-moving outboard wing.
Wing warps and field adjustments
Wing warps are easily set by holding the surface over a pan of boiling water (if covered with silk, silkspan, or tissue), twisting to the desired warp, then cooling by rubbing across your leg. This is fine for initial settings, but in the field you can adjust warps by heating the surface in an automobile exhaust—take care to avoid breathing the fumes.
An alternative to warping is to secure a length of shaped trailing-edge stock to the bottom of the wing trailing edge with the sharp edge forward. This method is not as efficient as wash-in because it results in a thicker trailing edge; it adds drag but not much lift. It is easy to add in the field.
Trimming Old-Timers — Basic Rules
- Establish basic center of gravity.
- Establish basic stabilizer incidence by hand-gliding.
- Regulate climb angle with stabilizer incidence.
- Regulate climb turn with a rudder tab.
- Regulate glide speed with minor CG shifts.
- Regulate glide turn with wash-in.
New kit
When SAM approved the 70 percent Buzzard Bombshell for official SAM competition, 4K's Models quickly produced a kit. The model has a 50.4 in. wingspan and is recommended for .19 to .29 engines. The kit features sanded wing ribs, formers, and fuselage sides; formed landing gear; and a steel bellcrank by Sal Taibi of Superior Aircraft Balsa. At today's prices, $39.95 is reasonable.
Four K's Models (4K's Models) 4202 W. 172 St., Torrance, CA 90504
Four K's also produces other Old-Timer kits:
- 72-in. Buzzard Bombshell
- 36-in. Baby Bombshell
- 73-in. Denny Junior for Power Fliers
- Korda Wakefield (rubber)
- Lamb Climber (rubber)
Try your dealer first.
Clarence Haught 3226 Honeysuckle Dr., Coeur d'Alene, ID 83814
Transcribed from original scans by AI. Minor OCR errors may remain.
