In America, wing loadings for model aircraft are typically given in ounces of model weight per square foot of wing area. Measure the total wing area and convert it to square feet. Weigh the model and convert that to ounces. Divide the weight (ounces) by the wing area (square feet). The lighter the wing loading, the slower the model can fly and the more the model will be impacted by wind and turbulence. Gliders and sail planes usually have wing loadings substantially less than 8 ounces per square foot, have very low stall speeds and are quite susceptible to turbulence and wind gusts. A good RC trainer aircraft for a novice student (such as the Aeroscout 1.1 meter trainer by HobbyZone) will have a wing loading of 13 to 14 ounces per square foot, will have a stall speed around 10 miles per hour, and be able to fly in 10 mph winds. Sport trainer aircraft and smaller sport RC aircraft usually perform well at 16-20 ounces per square foot and have stall speeds of 12-15 mph. RC pattern aircraft need to be able to handle wind and turbulence and will have heavier wing loadings of 22 to 28 ounces per square foot and subsequently have higher stall speeds. Some specialty airplanes such as the E-Flite V1200 racer were designed for very high speeds (140 mph) and have much higher wing loadings of 45 to 50 ounces per square foot. These also have much higher stall speeds and need flaps to allow landings at reasonable speeds. Even with flaps, the heavy wing loading of the V1200 requires the landing approach to be made at 30+ mph to provide a safety buffer against stalling. It should be noted that the size of the aircraft has a lot to do with acceptable wing loadings. The Reynolds number has a lot to do with how much lift a wing can produce. The bigger the plane, the bigger the Reynolds number and the more lift a wing can produce at the same speed. For example, a small plane design that might not perform well with over 18 ounces per square foot wing loading may fly fine at 30 ounces per square foot if the model is scaled up to triple the size of the original plane.

The next parameter that is of significance is called "power loading". It is the power produced by the motor divided by the weight of the model. With electric airplanes being the norm today, most power ratings are given in watts. If you need to convert horse power to watts, just multiply the horse power by 746 watts/HP to obtain the value in watts. The power to weight ratio (or power loadings) are usually given in watts per pound. Slow flying models with light wing loadings may fly OK at 50 watts/pound. Most sport models with wing loadings over 10 ounces per square foot will need 80 to 100 watts per pound just to fly around comfortably and if moderate aerobatic capability is desired, then increase the power loading to about 120 to 140 watts per pound. If 3-D (hovering) aerobatic capability is desired, then 150 to 200 watts per pound will be needed.

To determine what size engine is needed for your model, first determine the total model weight and the desired performance level. For example, given a basic trainer model that weighs 5 pounds and has a wing loading of 20 ounces per square foot. Limited aerobatic capability is desired. The power loading would need to be about 120 watts per pound. Therefore, at 5 pounds, the total power needed would be 5 pounds x 120 watts/pound = 600 watts or 0.8 horsepower. If a glow engine is desired, then most entry level plain-bearing .35 cubic inch sport engines will produce that much power at lower elevations. Above 5000 ft. elevations, it may take a .40 size engine to make that much power.