Beginner's Guide to Free Flight

Written by Don DeLoach Learn more about the original form of heavier-than-air flight How-to As seen in the July 2012 issue of Model Aviation.

What is Free Flight?

It’s a thrill, a challenge, a puzzle. It’s other guys like you, the world around, striving for the same graceful beauty of flight. It’s comradeship across all human barriers. It’s bull sessions through the wee hours. It’s fierce competition, with the highest of sportsmanship. It’s a battle against nature … her perversity … her law of gravity. “Free as a bird,” describes God’s most unchained creation. Man’s is a model airplane soaring birdlike in a thermal. You created it. Vicariously you soar with it, with its freedom. Free Flight is the mist of the dawning’s calm as you test. It’s the noonday sun as your model thrusts for the heavens. It’s the cool drink after a dusty chase. It’s the piercing scream of a peaking engine … the silence of the glide. It’s sunburn and poison ivy and weariness to the marrow … made worthwhile. —Bob Hatschek Model Aviation and Free Flight Hall of FameFree Flight is the original form of heavier-than-air aviation, dating back to Alfonse Penaud’s 1871 rubber-powered Planophore. Much has changed since that first 11-second flight in Paris, but the essence of FF remains the same. It is about the purity of flight, and confidence to make an aircraft fly stably and efficiently, with no piloting after the launch. Probably the easiest way to get involved in FF—or model aviation, for that matter—is with a simple, handheld catapult glider. Plenty of kits exist from various sources, and RTF models are legal for AMA competition.

A young competitor launches his P-30 at a contest in Marion KS, in 2010.

One of the best is Stan Buddenbohm’s Scout. It is a simple, 16-inch wingspan design that is mostly balsa, easy to build, and flies superbly. Others include the Cata-Piglet from Campbell’s Custom Kits and the Sting series by A2Z. The idea behind the Catapult Glider event is straightforward: the models are adjusted to launch vertically from a 9-inch, handheld, rubber band-powered catapult. In less than two seconds, they reach speeds in excess of 100 mph and heights of more than 100 feet. That’s exciting, but then the magic happens as the gliders slow down at the top of the launch, their noses drop, and they transition into slow, circling, floating glides of roughly 5 mph. From a good launch, a well-trimmed catapult glider can remain aloft for approximately 90 seconds without thermal help. Catapult gliders aren’t difficult to adjust for flights provided one understands the dynamics involved. Rudder offset controls the roll/transition and is effective mainly at launch speeds. Stabilizer tilt and center of gravity (CG) are generally only effective during the glide. And incidence changes affect both launch and glide. Begin your initial flight trimming by setting the CG at the plans location and hand gliding the model in calm conditions at a local park. Look for a gradual left glide turn with no tendency to spin or dive.

An F1B model is on the winding stooge at the 2010 Southwest Regionals.

If the model dives, add incidence (stabilizer trailing edge [TE] up) until the model is at the edge of a stall. If the model spins or banks drastically, you probably have a crooked fin or wing. A proper launch should be pitched up roughly 45° to 60° and banked right at approximately 45°. Reverse this scenario for a left-handed flier; bank left at launch and the trim should be reversed for transition to a right glide circle. For more than 30 years, the best starting point for powered FF has been the P-30 model. True to its name, this is a simple-to-build-and-fly competition class that provides loads of fun at a low cost. General specifications are a 30-inch wingspan and length, 40-gram minimum weight, and a commercially available 91/2-inch diameter plastic propeller.

A competitor at the 2010 Southwest Regionals winds his P-30. The P-30 is an ideal entry-level event for FF competition flying.

An excellent and competitive P-30 kit is the PAL Model Products Square Eagle, thousands of which have been built in the past three decades. The Square Eagle can be built in a week of evenings by even the most inexperienced builder. Basic familiarity with stick-and-tissue construction techniques is helpful but not required. Probably the most important thing about building FF models is recognizing the importance of precision. Sloppiness, at even the earliest stages of construction, will show up later with warped flight surfaces, and a model that is difficult to adjust for flight. Work on a completely flat tabletop surface. A hollow door from a home store makes a good flat surface. Some builders go a step further and work on 3/8-inch thick (or thicker) glass tabletops. For most traditionally constructed FF models (balsa wood, open structure), you’ll need a surface you can stick pins into as you frame up structures over full-size plans covered with plastic kitchen wrap. A good pin board is a 2 x 4-foot acoustic ceiling tile. Even better is the 1/2-inch thick sound-proofing fiberboard available at home supply stores in 4 x 8-foot sheets. Both options are inexpensive; the sound proofing is my favorite because it’s slightly denser and holds pins more firmly. Small rubber-powered models, such as the P-30, are almost always open-structured balsa frames covered with an ancient but superb material: Japanese tissue. This fine tissue is still made by the Esaki company in Japan, as it has been for generations.

Several young modelers participated in Catapult Glider at the 2009 Nats in Muncie IN.

What makes Esaki tissue so desirable for FF is its low density (roughly 3.5 grams per 100 square inches) combined with amazing skin strength when it is water-shrunk. This skin strength translates to finished flying surfaces that are much stiffer than the uncovered structures. The downside of tissue covering is it is time-consuming and more difficult than iron-on films. It requires the builder to brush on some kind of adhesive. White glue (thinned 50% with water) or unthinned nitrate dope work well. The latter is mildly toxic, so open a window or wear a respirator. Tissue-covered structures are then dampened with a light mist of water and brushed with two or three light coats of non-tauntening nitrate dope (thinned 50/50 with dope thinner) roughly 5 minutes apart. This seals the pores of the tissue, makes it reasonably glossy and considerably stronger. In lieu of thinned dope, some modelers use Krylon Crystal Clear #1303 out of a spray can; it works well and is actually slightly lighter. As previously stated, the importance of precisely aligned, warp-free structures cannot be overstated. Most critical is the vertical stabilizer; glue it on absolutely straight unless the plans say otherwise. The horizontal stabilizer is also critical. It must be adjustable longitudinally, preferably via a small 2-56 nylon screw on the TE. Small 1/64-inch plywood shims are a passable substitute, although a screw is much better. No warps should be present in the horizontal stabilizer. Remove any you see with a heat gun or hair dryer. Be careful not to get the structure too hot; balsa and doped tissue are excellent fire starters! The wing is a different matter. It should have roughly 1/16 to 1/8-inch washout (TE higher than the leading edge [LE]) in the tips. Unless your plans say otherwise, the washout should be equal in both tips. Again, use your heat gun and get the warps right before attempting that first flight.

Several young modelers participated in Catapult Glider at the 2009 Nats in Muncie IN.

Your first flight with a P-30 should be an unpowered glide with the 10-gram rubber motor installed and the CG located as shown on the plans. Find a grassy spot and gently toss the model forward with the nose slightly down. Shim or screw up the stabilizer’s TE until you see a slight stall. This means you’ve slightly exceeded the upper incidence limit for that CG position. Lower the stabilizer slightly and toss again; the stall should be gone. You’re now ready for powered flights. Your first powered flight should only be attempted in a fairly large field and in light breeze. Wind roughly 50 turns into the motor and release the aircraft, carefully observing it. Chances are that the model will pitch up slightly and power stall or “mush” forward slowly; this indicates a need for downthrust. Most FF models need approximately 2° to 4° of downthrust for optimum flying. You only need enough downthrust to prevent a power stall at full power; any more than this will limit your climb height. Keep increasing turns in increments of 50 until you see the model turn in the climb. The desired climb is a right spiral (left is the direction of torque and is unsafe under high power) using slight right thrust. Most rubber models use roughly 1° to 3° of right thrust to affect a right-spiraling climb. Keep tweaking the thrustline and increasing turns until you’ve reached maximum power, which is roughly 1,100 to 1,200 turns on a typical six-strand x 1/8-inch P-30 motor. For this you’ll need a mechanical winder and a larger field—200 acres minimum—more if you live in a windy area. Set the DT on every flight; I’ve seen models fly away in thermals from modest heights.

Andrew Ringlein displays a proper Catapult Glider launch at the 2010 AMA Nats.

What is a dethermalizer?

Dethermalizers (DTs) are essential equipment on nearly any outdoor FF model. They come in various forms such as a tilt-up stabilizer or pop-up wing, but the concept is the same: normal lift-producing airflow is disturbed greatly causing the model to descend at a much faster rate, thus saving it from thermal updrafts. Most FF plans and kits have provisions for DTs. Be sure to follow these instructions carefully!

The Basics of Free Flight Stability

In order to fly autonomously, Free Flight models must be sufficiently stable in all three dimensional axes: pitch, roll, and yaw. This is opposite of most forms of active-control flight where maneuverability is desirable. For conventional (wing in front, horizontal stabilizer in back) FF aircraft, there is a narrow longitudinal (fore/aft) CG range. The CG position is the bedrock of any FF model; it determines the critical angular settings of the wing and horizontal stabilizer, which enable efficient flight. Because FF models are optimized for maximum lift and minimum drag, airfoils are much different from most RC and CL airfoils. We almost always use undercambered or flat-bottomed airfoils in the range of 6% to 9% wing chord thickness. Thinner is generally better, but it is usually only attainable with strong, high-tech construction materials. Stabilizer airfoils aren’t nearly as critical. They are usually flat bottomed with 5% to 8% maximum thickness, but can also be simple flat plates on smaller models such as gliders. The wings of most FF models are set at 0° to +3° positive incidence. The horizontal stabilizers are set at a range of 0° to -3°. This difference of angles—usually 2° to 3° total—in concert with a safe CG location is what yields adequate longitudinal (or pitch) stability. The tendency of a FF model to diverge laterally (“fall off” on a wing) is largely controlled by the amount of wing dihedral used. Roughly 10° (or the equivalent) on each wing half is needed for optimum performance. This is more than a typical RC model. The idea is to have a model that resists upsets and returns to level, stable flight without dangerous, spiraling dives. FF models must also have horizontal stabilizers that are adequately effective in order to resist longitudinal instability (unrecoverable dives). Most horizontal stabilizers for FF are in the range of 20% to 40% of the wing area. Large stabilizers were common in the older, slower designs, until approximately 1970. Since then, the trend has been toward smaller ones. Generally, the faster the model is, the smaller the stabilizer can be. Long tail moments make the stabilizer more effective, so these models can have small stabilizer areas—even less than 20% of the wing area. The vertical stabilizer area is a final issue of importance on a FF model. It should only be large enough to prevent the Dutch roll or tail wagging. RC models tolerate much larger vertical stabilizers because they are under the pilot’s control. A too-large vertical stabilizer on a FF model can cause spiral instability. This is manifested when the model is resistant to recovering from a spiral dive; extreme cases of spiral instability can cause a crash. Spiral instability is also evident in a FF model’s inability to climb steeply, which can be a major detriment to performance. Welcome to Free Flight! —Don DeLoach [email protected]

Sources:

FAI Model Supply (570) 882-9873 www.faimodelsupply.com National Free Flight Society www.freeflight.org PAL Model Products [email protected] www.palmodelproducts.com Stan Buddenbohm Box 1677 Boulevard CA 91905 www.discuskid.com