Model Airplane Weight
Flying weight affects the performance of all RC models
Perhaps no other factor affects the flight performance of an RC model plane than aircraft weight. This can be easily demonstrated. Take the exact same plane design. Build the aircraft in two variants with one version at a heavier flight weight. The lighter model will fly better. The lighter model will take off in a shorter distance, climb out quicker and handle in a more pleasing fashion.
This effect of light weight applies to all aircraft, be they full scale versions or a radio control model. Minor differences in weight for the same aircraft type will not be noticeable. But if you are not paying attention to aircraft weight as you design a model aircraft or construct one from a kit or set of plans, excess weight can easily creep into your finished project.
Weight is a factor
In full size aircraft, weight is a major factor affecting flight operations. A typical jet airliner can have up to one third of its weight in fuel alone. If you take off in a Boeing 747 for a typical long flight, the jet transport is actually too heavy to safely land right after takeoff.
The assumption is the aircraft will burn off fuel and become lighter during flight, allowing for a safe landing at a proper weight once at its destination.
With most RC model airplanes, and especially electric powered models, there is no real difference between take off and landing weight. We therefore have to be careful to build the model as light as possible. Once the airplane is complete, its flight weight is essentially fixed.
The question comes up as to what is an acceptable weight for an RC model plane to fly well. The answer is “it depends.” You need some method to relate total aircraft weight to the size of the model. This is usually done by making a ratio of the aircraft weight in ounces per square foot of wing area.
Wing loading, by itself, is not enough to properly predict the flight characteristics of a model airplane. Rather, wing loading is an indicator of flight performance that must be considered with other aircraft design factors.
Wing shape, airfoil and aircraft size all play a large role in determining flight performance. A trainer model plane with a Clark Y airfoil will tolerate a higher wing loading as opposed to an aerobatic model with a thin wing cross section.
A larger plane can typically handle a higher wing loading than smaller aircraft. For example, a wing with an area of 2000 square inches could support a loading of possibly 35 ounces per square inch, while a 500 square inch wing could handle perhaps 20 ounces per square inch.
To make the math easy, say each wing half is 12 inches in span and six inches in width (wing plus aileron). This makes for a total wing area of 72 square inches for each wing, or a total of 144 square inches for the complete wing. A square foot equals 144 square inches. The wing loading in this example is two ounces per square foot.
Smaller RC models
Very lightweight micro sized electric powered model airplanes represent a special case for wing loading and weight considerations. For miniature RC planes total weight is the governing statistic. The tiny electric motors and propellers produce only so much thrust and the model is essentially designed around the power plant and electronics.
A great example of this art is the ParkZone Cessna 210 with a flying weight of just 18 grams. There is really no point in figuring out the wing loading on a tiny model of this nature.
Rather, the guiding principal is to keep everything as light as possible. Modelers have added just a piece of tape to a ParkZone Cessna 210 that prevented take off due to this minor increase in weight.
3D foam aerobatic flyers, such as the Extra 300 or the micro 4-Site, use a flat wing section. All lift is produced as a reaction of the wing deflecting the incoming airstream as there is no airfoil section. These specialized models rely on thrust for satisfactory fight performance. They do not “fly on the wing” as with larger and heavier models.
Model weight is critical
We can use these aircraft performance characteristics to our advantage. When building any model airplane, and especially micro flyers, keep the weight to an absolute minimum. If you are designing the model “build in lightness” with the minimum structure required. When building any model pay attention to the grade of balsa or other building material being used to conserve weight wherever possible.
Model aircraft weigh is a critical aspect and predictor of flight performance. Nothing will more quickly and easily improve a model’s airborne characteristics than reducing the airplane’s weight. Keep this insight in mind as you design, construct and evaluate the flying performance of your RC airplane fleet.
Author: Gordon McKay