Design and build an RC Bird Model

Model Airplane News, Jun 2002 by Hoey, Bob

Modelers have been attempting to build and fly bird models for a very long time, The challenge of doing this successfully, however, Is obvious to any builder-after all, birds have neither rudders nor vertical tails. Over the last 11 years, I have used RC models to try to understand how soaring birds fly without vertical tails. I have developed four bird models (Raven, Seagull, Turkey Vulture and Pelican) that fly reasonable well without vertical fins or rudders. This research has given me a pretty good understanding of the stability and control of gliding birds. I can now design glider models of birds that require surprisingly small design concessions and look quite realistic in flight. The bird's complex control methods cannot be duplicated exactly, and our flying abilty is limited to viewing and controlling from a remote viewpoint, so we may have to cheat a little to achieve stable, controliable flight. Nevertheless, we can use nature's aerodynamic methods for stability and control.

DESIGN METHOD

Start by observing and photographing the bird species you wish to imitate. Try to get bottom, side and front views, but be prepared for a real challenge, since these critters are continuously changing shape. Slides are best because they can be projected onto a wall, and your subject's shape can be traced with a pencil. Varying the projection distance also allows different views to be scaled to the same size. Use a little artistic license and develop a 3-view.

Next, apply the simplified stability and control criteria described herein (center of gravity [CG] location, wing dihedral and sweep). Make design alterations to your 3-view to bring these numbers within reason. The final step is to design flexibility into the structure to allow for trial-and-error development of your design. (Provide for changes in dihedral, CG and tail area, for example). Gliding flight without a vertical tail is certainly possible, but stability will be less than that of a typical RC glider.

PITCH AXIS

The complex planform of a bird's wing requires more attention during the design to properly locate the CG. The wing's mean aerodynamic chord (MAC) and aerodynamic center (AC) can be located using the procedure described in the "Click Trip" URL at the end of this article. First flights should be made with the CG close to the aerodynamic center. Bird models are typically very short-coupled in pitch, and the tail area may need to be enlarged, much like any scale model. A 15- to 20-percent increase will improve trim ability and require less attention to maintain glide speed. Using a slightly reflexed airfoil (typical for flying wings) also avoids large tail deflections.

ROLL-YAW AXIS

The complex shape of a bird's wing, and a bird's lack of a vertical tail, make the design of the wing critical to successful flight. Wing sweep and dihedral are the critical features. Dihedral varies over the span of most bird wings. Land birds (hawks, buzzards, eagles, etc.) usually have small dihedral near the root and increasing dihedral near the tip. Sea birds (gulls, pelicans, albatross, etc.), on the other hand, often have negative dihedral but high sweep in their outer wing panels. A method for defining the total dihedral effect for the wing is also described in the "Click Trip." These calculations are for the wing only and do not account for the destabilizing influence of the body, heads, beaks and other lateral areas forward of the CG. Artificial vertical fins in the form of "feet" (ventral fins) or clear circular discs near the wingtip, toed-in about 20 degrees, can be beneficial for early flights. The required fin area is usually quite small (6 to 8 square inches) and can be reduced or removed as flight experience increases. Note: electric motors at the front have been tried and are quite destabilizing. If you intend to try a propeller, use one of the above techniques to add some vertical fin area.

STRUCTURE

The outer wing panels of a real bird are extremely light compared with the rest of the structure (they're just feathers!). Outer wing panels should be kept as light as possible to keep roll inertia low. Heavy wingtips result in unwanted rolling oscillations. Unusual wing dihedral patterns (gull, for example) can be duplicated by using a full-depth balsa spar and cutting it to the desired dihedral shape. For thin wings, glue carbon fiber to the top and bottom of the spar for added strength. Sheeting the leading edges (top and bottom, back to the spar) will provide the necessary D-tube section for torsion strength. The model's weight will likely be considerably less than that of a real bird, especially for larger species, but this difference is mainly in the fuselage (body weight and shoulder muscles).

CONTROL

Two methods for pitch control have been tried-a standard elevator at the rear of the tail and the single-piece tail hinged at the wing's trailing edge. The elevator at the rear is the most effective.

Because of the large adverse yaw associated with ailerons, they don't work well when there is no vertical tail. A rolling tail (horizontal tail that tilts around a longitudinal axis for controlling turns) works OK depending on the CG location but is not very powerful. If used with a pitch-stable airplane (forward CG), the model flies well, but the control action is opposite to the way birds really fly (the model will turn in the same direction as the tilted tail, but no one will notice!). Drag flaps on the lower wing surface (or spoilers on the upper surface) also work well for initiating turns, but they aren't very efficient or realistic. Since they rely on dihedral effect to control turns, they often require larger wing dihedral than that observed on a real bird.