How to build a Peck Polymers Prairie "Gyro"

Model Airplane News, Oct 1998 by Friedlander, Bill

HERE'S A SIMPLE, fairly easy approach for those who want to try an autogyro. You start with a 3-channel Peck Polymers* Prairie Bird 50, which is a wonderful fun fly airplane that can be built quickly and will do many maneuvers, including flying inverted. On a hot summer's day, the Prairie Bird will thermal like a soaring glider. When you replace its wing with simple, side-by-side dual rotors, you create a respectable, easy-to-fly introductory autogyro-what I call a "Prairie Gyro." The best part is that you can fly the model either way, depending on which lifting surfaces are attached.

Plans and building instructions for the Prairie Bird are available with the kit, so they will not be discussed in detail here. Modifications during construction to facilitate conversion to the Prairie Gyro are outlined below, and drawings for the autogyro lifting surfaces are included.

MODIFICATIONS

Adjust the length of the nose section. Since autogyros need quite a bit more power than conventional aircraft, a .15- or .20-size engine is used instead of an .049 or .051. A Thunder Tiger* .IS with an APC 8x4 prop seems to work well. Although a .20-size engine would ensure good autogyro performance, a .15 provides plenty of power for the fixed-wing configuration! Because larger engines are heavier, the nose section of the model should be shortened. Moving the firewall (F3) to where the landing gear emerges from the bottom of the fuselage is a good start.

Position components to achieve CG location at F7. The parts placement is as follows: rudder and elevator servos on a lite-ply platform between F9 and F10; receiver packed in Styrofoam and foam rubber between F7 and F8; 4-ounce fuel tank between F6 and F7; battery (four M cells) under the fuel tank; throttle servo on a lite-ply platform between F5 and F6.

It's a good idea to have already constructed the stabilizer, elevator, fin and rudder so they can be positioned first. The engine should be next, then the fuel tank (which should be positioned at the CG so fuel will not affect the CG location) and so forth. The last item to be placed should be the battery because its weight can be used to offset the required positions of the other components.

Landing gear. To minimize ground looping during taxiing and takeoffs, bend the landing gear back.

Cabin treatment. The plans show that part F19 has cutouts to represent the cabin windows. I suggest that you don't remove these but instead CA them in to provide a more robust support area for the wing and autogyro attachments. Windows can be simulated with black paint.

Removable panels are also installed as the front windshield and the top of the rear sloping deck between F9 and Ft 1 to allow access to the servos.

Add extra 1/32-inch ply doublers. The plans call for F18 as a doubler where the forward and aft sections of the fuselage sides are joined. F18 should be replaced with 1/32-inch ply doublers on the inside from F9 to F12.

Install triangular blocks to support the vertical fin. Triangular blocks (3/8 to 1/2 inch on a side) should be installed along the length of the base of the vertical fin to provide a stiffer, more rugged attachment point for the fin.

Install a steerable tailskid. In place of the fixed skid shown on the plans, install a 1/16-inch-i.d. brass or aluminum tube running vertically from where the fixed skid emerges from the bottom of the fuselage to the top of the fuselage. The top of the tube should be aligned with the hinge line of the rudder. A /hs-inch piano wire with a 1/2inch-long, right-angle bend should be trapped between the screws that hold the rudder servo horn. Form the lower portion of the wire into a smooth rearward curve to allow the model to easily slide over grass but to still provide positive directional control while tail-dragging.

AUTOGYRO ROTORS

Model autogyros seem to perform satisfactorily when their disk loading is less than 6 ounces per square foot. Disk loading is the area of the rotor disk in square feet divided by the weight of the aircraft in ounces. In the case of the Prairie Gyro, each rotor has a diameter of 25 inches, and together, they provide a disk area of 6.8 square feet. The completed weight of the aircraft is about 37 ounces without fuel, giving a disk loading of 5.4 ounces per square foot. Adding only 1/2 inch to each rotor blade lowers the disk loading to about 5 ounces per square foot. The diameter of the I-beam rotor supports should also be increased to prevent them from clashing at the centerline of the aircraft.

BLADE CONSTRUCTION

The rotor blades have Clark-Y airfoils. Each blade is 12 inches long, 3/16 inch thick and has a 13/4-inch chord and can be made with two pieces of 3x36-inch hard balsa. Sig Mfg.* offers partially preformed balsa airfoils that can be used as well. The rotors used in my Prairie Gyro were made by gluing 3/16 x 1/2-inch pieces of spruce as the leading edges (LEs) to the balsa aft section of each blade. The blades were then shaped using a Clark-Y airfoil template (CompuFoil*) and vacuum-bagged with 1.5-ounce fiberglass laid on the bias. This provides a glass-smooth surface for the blades; this is important to attain autorotation. The blades can also be covered with heat-shrink tubing or MonoKote*.