Making FF models fly

making-models-deloach

Written by Don DeLoach
Free Flight Sport
As seen in the December 2020 issue of Model Aviation.

excessive spiral stability in a ff model
Excessive spiral stability in a FF model occurs when the vertical stabilizer is too small. This is a sketch from Bill McCombs’ book Making Scale Model Airplanes Fly.

"HOW CAN YOU make a model fly by itself?" That’s the question I most often hear about Free Flight (FF) from non-FF fliers. Indeed, it does seem miraculous that we can get our models to fly at all, let alone fly for long periods.

Remember, though, that rubber-powered FF aircraft preceded the Wright brothers’ manned flight success by more than 30 years. This perspective is especially relevant when discussing the hows and the whys of FF.

Stability Versus Trim

First, to understand how FF models fly, you must separate stability from trim, a common point of confusion. Stability refers to an aircraft’s tendency to return to level flight in case of an upset. Important components of stability include wing dihedral (lateral stability), adequate/effective horizontal stabilizer area (longitudinal stability), and the right-size vertical stabilizer (spiral stability).

Trim is secondary and distinct. It refers to the settings (angles) of the flight surfaces and thrustline.

Lateral Stability and the Importance of Dihedral

Lateral stability is the lateral balance of forces produced by the airplane’s wing. Any imbalance in lift results in a tendency for the airplane to roll about its longitudinal axis. In FF, we achieve lateral stability by adding 5° to 15° of wing dihedral.

If a momentary gust of wind forces one half of the model’s wing to rise and the other to lower, the model will bank or begin to roll. When the model is banked without turning, it tends to sideslip or slide downward toward the lower wing half. Because the wing has dihedral, the air strikes the lower wing half at a much greater angle of attack (AOA) than the higher wing half. This increases the lift on the lower wing half, decreases lift on the higher wing half, and tends to restore the model to its original lateral attitude (wings level); that is, the AOA and lift on the two wing halves are again equal.

The effect of dihedral then is to produce a rolling moment, tending to return the airplane to a laterally balanced flight condition when a sideslip occurs. The restoring force might move the lower wing half up too far so that the opposite wing half now goes down. If so, the process will be repeated, decreasing with each lateral oscillation until a balance for wings-level flight is finally reached.

Although some dihedral is utilized in controlled flight, we use what controlled-flight enthusiasts would term excessive dihedral in FF. We want our models to be so stable laterally that they recover very quickly in case of upsets. Less dihedral is needed for calm-air FF models and Indoor FF, but it is still much more than you’d typically see in controlled-flight aircraft.

Spiral Stability

Spiral stability is the tendency of an aircraft to maintain its trimmed flight path without wandering off or, worse, entering a spiral dive. A FF model is spirally stable if it has the right-size vertical stabilizer (fin).

Too large of a fin and the model won’t climb steeply and might wander off. Too small of a fin and the model will display excessive spiral stability, or "Dutch roll," which is a side-to-side, aft-end oscillation coupled with wing rocking.

Further complicating spiral stability is that wing dihedral affects it. The larger the vertical tail is, the more wing dihedral is needed to balance it, and vice versa.

For most FF models, a fin area of 5% to 8% is ideal. The perfect fin is one that has only slightly (approximately 5%) more area than what would result in a mild Dutch roll.

Longitudinal (Pitch) Stability

This is the ability of a model to recover from longitudinal upsets (nose-up or nosedown). Various factors enter into the calculations here, but we are mainly concerned with horizontal stabilizer (tail) area and the fore/aft center of gravity (CG) location. Horizontal tail effectiveness is affected by the distance from the CG to the tail. A smaller horizontal tail that is slightly farther aft can have the same effectiveness as a larger tail a bit farther forward and vice versa.

William F. "Bill" McCombs, a Princetoneducated Vought senior design engineer, wrote extensively on the subject of FF trim and stability and came up with a number of invaluable formulas. Here’s his formula for finding horizontal tail effectiveness, expressed as tail volume coefficient, or TVo:

TVo = As ÷ Aw x TMA ÷ ACw

Where:

As = horizontal stabilizer area

Aw = wing area

TMA = tail moment arm (distance between the wing leading edge [LE] and tail leading edges [TEs])

ACw = average wing chord (divide wing area by wingspan)

For most FF Duration (nonscale) types, a TVo of 1.00 to 1.80 is common; the higher the TVo, the greater the longitudinal stability. However, ample longitudinal stability is only helpful if the CG is located optimally. To find the CG on FF Sport and Scale models, Bill came up with the following formula:

CG = 16 + (36 x TVo)

Where CG = percentage of mean aerodynamic wing chord behind the LE.

I’ve revised Bill’s formula for the fastest-climbing FF Duration models:

CG = 20 + (40 x TVo)

For most Scale and Sport models that have smallish stabilizers and relatively short tail arms, the CG is between 30% and 45%. For Duration models with larger stabilizers and longer tail arms, the CG often ends up at 60% to 90%.

Flight Trimming

After a model is set up with the proper stability parameters, it is safe to move to the flight-trimming phase. Trimming refers to the angles of the flight surfaces, thrustline settings, and nothing more. It is not possible to "trim out" an unstable condition of a model. When you understand this, trimming becomes much easier to comprehend.

Begin the trimming process by finding a 10- to 20-foot hill from which to perform test glides, selecting a calm morning or evening. If the model pitches over into a dive, raise the TE of the horizontal stabilizer. If the model pitches up excessively and stalls, lower the stabilizer TE. The goal is a floating glide—just on the verge of a stall. If the model yaws abruptly to one side, add the appropriate rudder trim until it glides straight.

Note that rudder trim can be dangerous on a FF model. It can cause an unrecoverable dive on even the most stable model. The best way to make a FF model turn in the glide is not rudder offset anyway—it is horizontal stabilizer tilt. A stabilizer tilted only 3° to 5° is enough to induce an ideal glide circle.

After I have a model gliding well, I move to the power-trimming phase. On a rubber model, this means winding with approximately 100 turns at first and observing carefully because the turns count increases on successive flights. A typical rubber model will need 1° to 3° of downthrust and 1° to 3° of right thrust to achieve an efficient and safe climb pattern.

Use only enough downthrust to cure the symptoms of a power stall; excessive downthrust will limit your climb. The same with side thrust; use only enough to affect a climbing right-hand turn. You don’t want the model turning too tightly—this is inefficient and can result in a crash at higher speeds.

this rear view of a small catapult glider
This rear view of a small Catapult Glider shows the stabilizer tilt that is used to safely induce a glide turn. This model turns left toward the high side of the tilted stabilizer.
the wing dihedral on a typical ff model
The wing dihedral on a typical FF model is between 5° and 15°—much more than on a controlledflight aircraft.

SOURCES:

National Free Flight Society (NFFS)

www.freeflight.org

Making Scale Model Airplanes Fl by William F. McCombs

Available from Susan Creamer

1925 Clark Trl.

Grand Prairie TX 75052

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2 comments

Thank you for the great info in this article! What does each percentage stated in this article refer to? Is the fin area supposed to be 5%-8% of wing area? I don’t know. Please clarify for me.

This is a Great summary. Very nicely done. There should be more in the AMA model aviation media minute and news about free flight. Thanks.

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