Model Aerodynamics and RC Airplane Design
Aerodynamic considerations for sport and scale RC aircraft
The application of innovative model aerodynamics will vary depending on the type of RC model airplane you are designing. For example, the majority of trainer radio control models look about the same. A good trainer will have a high wing, a flat bottomed airfoil, reasonable nose and tail moments, generous tail surface areas and noticeable wing dihedral.
These design parameters have matured through time and experience. There is not much that the RC airplane designer needs to apply from model aerodynamics to improve the trainer’s flight characteristics.
To gain experience with understanding and applying model aerodynamics the beginning designer can start by taking an existing trainer airplane layout. Redraw the plan with the same moments and areas, but incorporate subtle changes in the wing, fuselage and tail outline shape.
The aerodynamics and flight characteristics of this evolved design remain the same. You can load a design in a computer flight simulator to get an initial feel for handling characteristics. The main learning point will be demonstrating your ability to draft a full size RC construction plan without any worrying about your new design’s flight characteristics or structural issues.
With a scale model design the shape of the aircraft outline is already determined. There are still numerous aerodynamic decisions that the designer must make to produce a well behaved RC aircraft.
Model aerodynamics considerations will include selecting a suitable airfoil section along with preparing proper incidences for the tail surfaces, wing and motor.
Airfoil selection is an important part of your airplane’s aerodynamics and flight performance.
Airfoils vary greatly in appearance and effect on an airplane’s flight and fall into five general shapes: under cambered, flat bottom, semi-symmetrical, fully symmetrical and flat.
The size of your final model has a large bearing on the importance of a proper airfoil selection. Lightweight indoor micro flyers are less dependent on the airfoil shape than larger and heavier models.
Under cambered airfoils were common on full scale aircraft in the first decade of flight and produce a lot of lift (as well as drag) at low airspeeds. This was a useful feature with the underpowered aircraft of that day.
The under cambered airfoil section is shallow as the wing’s strength came from the external cable bracing common in early aircraft. The Finch, Fokker Spin and Chickadee use an under cambered airfoil shape. The aerodynamics of this design ensures positive slow flight performance that is perfect for low and slow maneuvers.
A flat bottomed airfoil, such as the Clark Y, is ideal for general purpose flying. A Clark Y airfoil has good depth for adequate aircraft structure combined with gentle stall and model flight aerodynamic characteristics.
The Clark Y airfoil was used on numerous full scale aircraft from light aircraft to fighter biplanes of the pre-World War II era, thus a great choice for a wide variety of scale RC model aircraft. The Stevens AeroModel line of laser cut balsa kits use a variant of the Clark Y.
Fully symmetrical airfoils are best reserved for large size aerobatic RC aircraft and are rarely used for a lightweight indoor model airplane. On the opposite end of the airfoil spectrum, model aerodynamics show us that a completely flat bottomed airfoil can work well with lightweight indoor 3D aerobatic model airplanes that use powerful motors (where thrust exceeds aircraft weight).
Model aerodynamics comes into play with determining the wing and tail surface incidence settings. Incidence is the angle between the wing’s chord line and the fuselage datum line. Incidence angles are typically built into the aircraft structure.
As a general rule for smaller RC airplanes I set the wing incidence at 2 to 3 degrees positive incidence and the tail surfaces at a zero incidence setting.
An exception to this approach involves slow flying antique models like the Demoiselle or Blackburn. I used a Clark Y shape for the top of the airfoil section, but did not include the bottom area of the airfoil. In effect, I produced an under cambered airfoil shape.
To retain the desired slow flight characteristics, I used 5 degrees of positive wing incidence for the Blackburn. The models fly fine, but the lift produced by the high wing incidence will cause the models to climb if too much power is applied. This model aerodynamic flight characteristic is fine, as I want very stable slow flight handling from these high aerodynamic drag pioneers of flight.
For the smaller model aircraft I use two to three degrees of down thrust on the motor mount. On larger models start with no engine down thrust and adjust if necessary during test flights.
Model aerodynamics considerations indicate that for a model without ailerons, at least one inch of wing dihedral per wing panel is adequate for wingspans up to 50 inches. More dihedral can be added if desired, but is usually not required.
The main concern for the scale RC plane designer is longitudinal stability, are stability with pitch. Longitudinal stability is best ensured with adequate tail length and tail surface area. For most RC models, a horizontal tail surface area of 25 to 30 percent of the wing area will ensure stable flight.
The center of gravity is the last and perhaps most important application of model aerodynamics for enjoyable radio control flight. The CG should be located at 30 percent of the aerodynamic average chord of the wing.
If in doubt with a unique wing shape or unusual model, moving the center of gravity forward provides more stable fight. An aft center of gravity increases longitudinal instability (a “twitchy” elevator control), and can result in loss of control if the CG is too far aft.
Author: Gordon McKay