Aerodynamics of small glow models

Model Airplane News, Feb 2000 by Lennon, Andy

A guide to choosing planes, engines and props

Small, glow-powered RC models are increasingly popular. They are less expensive to buy and to operate, they're easy to transport, and the necessary field equipment is light. You can fly small models at parks, football fields or any open, grass-covered area, and they can be hand-launched, so a runway isn't needed. Because they're lightweight, they tend to hold up better in a crash and they are quieter than larger models.

Recognizing this trend, the RC hobby industry has a vast variety of products from which to choose. This article offers guidance in small model, engine and prop selection to both novice and experienced pilots.

GUIDELINES

Wing loading (W/L). This is simply the model's weight in ounces divided by its wing area in square feet. It is the first consideration in choosing a model. Its formula is:

W/L (oz. per sq. ft.) = model weight in oz. x 144 / wing area in sq. in.

Advertisements for model kits give wing area in square inches and weight in ounces. The W/L is easy to calculate from this information. The importance of W/L is that it affects the model's level-flight airspeed. The lower the W/L, the slower that speed.

Figure 1 provides flight speeds (left) at various wing loadings (bottom) and at various wing-lift coefficients (curves). Most models fly at lift coefficients (C^sub L^) ranging from .10 to .20 and will stall close to a C^sub L^ of 1.

Using a C^sub L^ of. 10, a model with a W/L of 10 ounces per square foot flies at 50mph and stalls at 17mph. A comfortable landing speed is 22mph. For a W/L of 20 ounces per square foot, the flight speed is 70mph with the stall at 23mph.

For a beginner, the slower flight speed at low W/Ls is more appropriate, as it allows time to correct the model's flight path. Experienced pilots can handle the higher speed of higher W/Ls, particularly landing speeds.

Wing loading, then, strongly influences model choice.

Power loading (P/L). This is similar to wing loading and is the weight-to-power ratio for 2-stroke engines, assuming an engine of 1-cubic-inch displacement (cid).

Its formula is simply:

P/L = model weight in ounces / engine displacement in cubic inches

As an example, a 30-ounce model powered by a .10 engine will have a power loading of 30 / .10, or 300 oz. / cid.

In this author's experience, a P/L of 200 oz. / cid coupled with a good prop selection results in high speeds and out-of-sight vertical climb. Loadings of up to 400 oz. / cid still provide reasonably good performance. The author's Dove, a .15 glow-powered glider, has a P/L of 367 oz. / cid; with an 8x4 prop, it achieves altitude at a surprising rate. Its W/L is 13 ounces per square foot.

Again, the modeler's experience and skill are considerations in choosing an engine size. Larger engine displacements that provide more speed and higher W/L call for more skilled pilots, and vice versa.

ENGINE AND FUEL-TANK SIZE

Small, powerful 2-stroke engines are available with or without throttle control. Without throttle control, the model flies until fuel is exhausted and landing is inevitable. Throttle control permits adjustment of engine rpm at all times, and you can shut it down at your convenience. Landings are at the pilot's discretion, but a throttle servo and appropriate channel on the receiver and transmitter are needed.

A useful rule of thumb for fuel-tank size is 20 fluid ounces of fuel per cubic inch of engine displacement. Tank sizes are:

Tanks that have a rectangular crosssection similar to the Sullivan "slant front" are recommended. The flat bottom permits easy installation on two 3/8-inch-wide strips of 1/6- or 3/32-inch-thick plywood running across the fuselage and braced to its sides with 1/4-inch triangular balsa. CA small, 1/4inch-thick foam rubber pads to the ply, and CA the tank to the pads. This reduces fuel foaming caused by engine vibrations, yet it leaves the tank visible during filling.

RADIO EQUIPMENT

Novices should take the long-term view of a radio purchase. If they anticipate that their interest will prove to be more than a passing fancy, they should obtain 4- or 6channel RC equipment.

Solely to control a model that has only rudder and elevator control, a 4- or 6-channel radio can be used by plugging the rudder servo into the receiver's aileron channel. This provides pitch and yaw control on one stick; the other stick is available for throttle control-or for spoilers, on a glider. As the novice gains confidence and progresses to use the aileron, rudder, elevator, throttle and even flaps, he or she already has the necessary radio equipment.

For models powered by .15 or .20 engines and with wing loadings of less than 15 ounces per square foot, standard servos, a 500mAh battery and a 4- or 6channel receiver may be used.

The author's Dove has four standard servos, a 500mAh battery and a 6-channel receiver. Powered by a. IS engine and with a W/L of 13 ounces per square foot, it performs very well. For smaller models, with servos weighing fractions of an ounce, small 110mAh onboard batteries and 2- or 3-channel receivers are available.