Airmaster

Airmaster

If you've ever considered building a tail-first design but balked because canards look odd, the Airmaster may change your mind. With a retractable trigear for appeal, this canard-wing RC model flies well as either twin .40s or a single .60. — AL Masters

Background

• I drew inspiration from flying a full-scale pusher amphibian (LAKE LA-4) in the 1960s; canards draw curiosity from fellow pilots and controllers.
• Canards are often regarded as "weird," so I fitted a retractable trigear and designed the airframe to accept either twin .40 engines or a single .60 to broaden appeal and utility.
• Test flying determines the desirable center-of-gravity (CG). CG behavior can be unpredictable—historically, Dr. Heinrich Focke died experimenting with CG on the Focke-Wulf 19A. On my first prototype test, with twin .40s at full throttle, the model performed a tight loop immediately after liftoff; power was cut and the model landed without damage.

Design and Aerodynamics

• Canard configuration: two lifting wings produces lower overall wing loading.
• Calculated flying weight: 10 lb (160 oz).
• Average wing loading: ~23 oz/ft² (total flying weight ÷ total wing area).
• Canard (front) area ≈ one-fourth of total wing area:
• Canard wing loading ≈ 34 oz/ft².
• Main wing loading ≈ 19 oz/ft².
• Flight characteristics:
• When speed is reduced, the canard loses lift first and the nose drops. Applying up elevator increases canard lift; the model may enter a slightly nose-high descent while the main wing remains above stall, retaining aileron control.
• Recover by applying throttle or dropping the nose (or both). Proper throttle and elevator trim allow nose-high slow flight without "snapping over the top."
• With canard set at 0° incidence, inverted flight and outside loops are good; full-span elevator gives solid control.
• Layout choices:
• Canard mounted on top of the fuselage allows elevator servo and linkages to live inside the canard center section and enables an efficient airfoil rather than a flat-plate canard.
• Canard built and covered separately; attached with nylon bolts for easy removal.

Major Features

• Retractable trigear (retracts for appearance).
• Configurable engine installations:
• Twin .40s (tractor front / pusher rear).
• Single .60 (pusher).
• Firewalls accept .40 or .60 engines; blind nuts provided for both sizes.
• Rugged construction with reinforced fin / rudder supports for pusher installations.
• Optional tip fins (tested then removed without adverse directional stability effects).

Construction Overview

Note: The following condenses the key construction steps and critical dimensions called out in the plans. Refer to full-size plans (page 204) for complete details.

Preparing top sheeting

• Make formed 1/8" sheet balsa for the fuselage top:
• Two sheets, 4" wide x 1/8" thick from 36" stock; cut 12" from each.
• Soak pieces in ammonia water ~5 minutes, drain and wrap around a cylindrical form; hold with masking tape until dry.

Fuselage

• Sides:
• Use two 1/8" Lite Ply sheets, 6" × 48".
• Transfer thrust line and bulkhead locations from the plan; cut and sand one side and use it as pattern for the other.
• Longerons: attach with CYA; upper longeron extends above side to accept top sheeting later.
• Bulkheads and formers: include vertical centerline on each part; ensure accurate fit so sides remain parallel and bottoms are square.
• Slots for fin supports in F-7, F-8, and aft firewall must be precise; test-fit 3/8" balsa sheet for vertical fin.
• Engine selection and mount:
• If twin configuration and engines differ, put heaviest in tractor (front) position.
• Mount and align engine to thrust line; mark and install blind nuts on firewalls.
• Nose wheel:
• Position retract on F-1 and secure with blind nuts. A fixed Fults RF-500 unit can substitute for retracts.
• Assembly:
• Epoxy front firewall in place using F-5 as spacer, epoxy aft firewall in place, clamp until cured.
• Fit remaining bulkheads, verify alignment, lock parts with CYA, then double-glue F-1 and F-5 with epoxy.
• Push fin supports in position, epoxy at bulkhead contact points.

Canard

• Build over plan on waxed paper.
• Pin 1/4 × 1/2" rear spar and 1/2 × 1/8" sub-leading edge over the plan.
• Install 3/32" balsa sheet ribs; fit (do not glue) 1/8" plywood braces into the slotted center ribs.
• Top and bottom 1/16" plywood center sections: select grain as shown on plan to allow forming.
• Drill two 1/16" pilot holes in bottom center section for canard bolts; accurate center-to-center dimension matters for servo clearance.
• Add vertical-grain hard balsa fill blocks, top plywood, drill through with No. 21 and then No. 21 to No. 21? — (plans call for final holes with No. 21 pilot then No. 21?); final bolt hole is .159 (No.21) and later enlarge to No.21 front/back? — follow plan drill sizes shown.
• Complete canard: top/bottom sheeting, rib caps, 1/8" triangle leading edge. Tapered elevator stock used for canard tips.

Elevator

• Cut from 3/32" × 1/2" elevator stock; cut lightening holes.
• Drill 1/16" holes for Robart hinge points; drill matching holes through canard rear spar.
• Remove top material to accept 3/32" plywood insert for top-mounted control horn.
• Add 1/16" balsa sheeting to both sides; hinge points installed after covering.

Wing

• Ribs: trace and cut one set, use as template for the other; sand in pairs.
• Spars: 1/4 × 1/2" balsa spars, cut to length.
• Dihedral braces and rib doublers from plywood; make left and right doublers.
• Glue rib doublers, drill for retract arm lines.
• If no retracts, use fixed-wire gear mounted to plywood doublers or formed aluminum gear.
• Build panels:
• Pin lower spar assembly over plan; position ribs 1–12; glue spar, sub-leading edge, and trailing edge.
• Add 3/32" balsa TE sheet, hinge supports, and vertical-grain web between ribs 1 and 12.
• Build second panel similarly.
• Join panels using 1/8" plywood dihedral braces and appropriate dihedral spacing (first panel elevated 2-1/4" as shown on plan). Glue all joints with CyA.
• Wing mount:
• Add plywood wing mount flush with top of ribs 1 and 1C, bolster with 1/2" triangle stock.
• Use soft balsa blocks drilled for bolt access (1/2" blocks, 3/16" holes).
• Make/fit main gear retract mounts (Rhom-Air or Spring-Air are acceptable).
• Main gear struts: 5/32" wire.
• Wheel wells: template and cut bottom sheeting for clearance, reinforce with 1/8" scrap balsa as indicated.
• Finish: tips, leading edge, rib caps, and center section sheeting. Open wing bolt access holes and true up.

Ailerons

• Cores from 3/8" × 1-1/2" aileron stock; add 1/16" light balsa sheet.
• Drill hinge pin holes with 7/64" bit; drill 3/32" hole for torque rod.
• Transfer hinge locations to wing; epoxy brass tubes on wing trailing edge for hinge pins.
• Remove ailerons for final sanding and covering.

Final Assembly and Fuselage Completion

• Sand wing saddle and fit wing to fuselage; ensure both sides meet wing at same angle.
• Epoxy wing hold-down blocks and clamp until cured.
• Drill pilot hole at leading-edge centerline, ream to 3/16" for wing dowel; assemble F-5A over dowel for adjustable mount.
• Drill and tap hardwood blocks for 1/4-20 wing bolts; redrill to 1/4" as required.
• Carve middle longeron in battery/receiver bay per plan; add fuel tank supports, servos, and radio tray.
• Epoxy 1/4" plywood canard mount; use canard to drill holes and tap for 10-32 nylon wing bolts; ream canard bolt holes to 3/16" for the dowel.
• Apply vaseline to dowel hole, insert 5/16" dowel for epoxy assembly of F-5A, then remove dowel from wing after epoxy cures (dowel to be epoxied into wing after covering).
• Add nose top sheeting, trim and shape longerons for smooth curve; fit aft sheeting—compound curves may require soaking or steaming sheet balsa.
• Fit fuselage bottom sheeting, tailcone fillets, tail skid plate with 1/32" plywood doublers.
• Install Spring-Air nose wheel or chosen unit; fit radio gear, Ni-Cads in foam-lined compartment forward.

Reinforcements and Critical Details

• Upper vertical fin and rudder:
• Pushers cause extra stress. Sandwich the rudder/fin post between two plywood supports and use epoxy to attach to fuselage—do not rely only on CYA.
• Use medium-hard balsa for rudder to resist prop-tip vortex-induced bending.
• Knife-edge or high-load maneuvers can break the fin or rudder if not reinforced.
• Hinges: treat hinge centers with petroleum jelly before epoxying hinge points to prevent loss of hinge action after epoxy cures.
• Fuelproof and seal engine compartments.

Engine, Propeller, Muffler and Related Notes

• Engine options:
• Twin .40s (tractor/pusher combination).
• Single .60 in pusher position.
• Ballast required for CG:
• Twin .40s: about 9 oz of ballast in nose area (with gear down).
• Single .60 pusher: 1–2 lb lead up front to rebalance (relocating Ni-Cads, receiver, and gear servo rearward can reduce ballast required).
• Mufflers:
• Prefer routing aft engine muffler exhaust rearward; some muffler outlets can be sawn at 45° to direct exhaust.
• O.S. .40 muffler outlet can be rotated for exhaust direction; muffler styles exist for O.S. .40 and HB .40 in pusher installation.
• Propeller guidance:
• Pusher prop must not exceed 10" diameter due to rudder / lower fin location.
• Avoid highly flexible pusher props with a .60 in the aft spot.
• Recommended: three-bladed Grish Magnum series for larger-power absorption without damage.

Ground Testing and Field Notes

• Hard dirt runway testing revealed pebble damage to pusher prop tips: nose wheel occasionally passed through pusher prop wake and struck tips.
• Solution: raise aft engine ~1/8" (one-eighth inch) on the second prototype and redesign upper vertical fin/rudder for clearance.
• Problem did not occur on grass strips.
• Full-size reference: Dornier 335 uses a fendered nose wheel to prevent prop-tip damage.
• Test and iterate early with ground runs to identify necessary changes before first flight.

Fuel Tank and Ballast Placement

• Aft fuel tank: after testing, place the tank clunk in the forward position with the clunk line as short as practical but flexible enough to reach the tank bottom.
• Avoid placing an aft fuel tank too low.
• Add ballast to achieve desired CG with gear down and before filling the fuel tank.

Radio, Control Linkages and Wiring

• Rudder control:
• Use 1/16" music wire through Nyrod sheath; connect to rudder horn with Du-Bro 2-56 swivel ball linkage to prevent flutter-inducing play.
• Servo leads and removability:
• Use servo extensions for easy removal of canard and main wings.
• Extend leads by cutting and soldering stranded extension wire; insulate solder joints with heat-shrink tubing.
• Choked extensions were not necessary with FM or PCM systems on prototypes.
• Muffler and exhaust routing, plus fuelproofing, are important prior to flight.

Flying Characteristics

• The Airmaster is predictable and stable when trimmed properly.
• Retract gear for appearance—looks great.
• Both twin .40 and single .60 installations produce an assertive, well-mannered flier with good roll rate and penetration.
• Aerobatics:
• Performs loops of all sizes, good outside loops, and maintains control if one engine stops.
• At reduced power with elevator full up the model can appear to hover with near-zero forward speed; no sharp stall experienced in testing.
• Landing:
• Nose-high approaches are acceptable; aileron control remains effective to touchdown.
• Feed a little throttle on final to reduce descent rate before running out of elevator.
• CG changes:
• Move CG gradually when experimenting—rearward shifts can change the model from stable to twitchy to uncontrollable.

Final Remarks

• The canard airframe is straightforward to build and offers engine configuration choices.
• Retractable landing gear, clean lines, and the two-wing layout will please biplane lovers—without rigging.
• Full-size plans available (page 204).
• Always fuelproof engine compartments, reinforce critical fin/hinge areas with epoxy, and conduct thorough ground testing before first flight.

Transcribed from original scans by AI. Minor OCR errors may remain.