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F-111 picture

F-111 picture

by Adrian Hoopgood

History

The F-111 is a 2-seat variable geometry, tactical strike and reconnaissance bomber. Built by General Dynamics in the USA, the first F-111s flew in 1964. Plagued with major technical problems, lengthy delays and political antagonism, the F-111 finally emerged as the world's finest strike aircraft.

In 1963, the Australian government ordered 24 F-111s for the RAAF. The first aircraft arrived in 1973 with the rest arriving later. A number of these were lost in accidents but have since been replaced. In recent years, the RAAF's F-111s have been enhanced with improvements and upgraded with modern avionics to take their service life well into the 21st century.

The Model

F-111 picture

Airborne's F-111 is a sport-scale model which, at a glance, closely resembles the real thing. With the success of Airborne's F-14 Tomcat which flies like a rocket but handles like a trainer, the design team ventured into another prop-driven sport jet to add to their plans list, coming up with the F-111.

In the air the F-111 looks sensational! Its long nose and high speed makes you wonder if you're not actually flying a ducted fan model. However history almost repeated itself with the prototype model also experiencing serious in-flight problems which have since been rectified to make it acceptable. Even so, flying this model F-111 should best be left to the experts.

Construction

Building the model is relatively easy. The fuselage is basically a box structure with rounded corners, the wing has only slight sweep back and straps on top of the fuselage, the tail surfaces are simply cut from balsa sheet. Cut out all parts into kit form, then start assembling.

The Wing

F-111 picture

The wing is a traditional built up balsa and ply structure. The plans suggest an option of fully sheeting the wing which I chose mainly because I wanted to paint the model rather than trying to obtain a camouflage colour scheme with heat shrink films. The choice is yours.

Build the wing in two panel, then join together. Pin the lower spar over the plan. Cut the leading edge and aileron spars to length. Glue the wing ribs W1 - W10 onto the lower spar with tabs in place to maintain symmetry of the wing. Glue the leading edge and aileron spars onto the ribs, then glue in the top spar. Glue in the shear webbing against spars and allow to dry.

Lift the left and right wing panels off the building board, then slide through and glue the wing joiners B1 and B2 to make the wing a single piece. Add WB to the rear of the centre section of wing, and WP at the leading. Drill through the LE for the wing locating dowels which can also be temporarily fitted.

At this stage I prepared for sheeting the wing by first joining the 1.5mm balsa sheets sufficient to cover the wing area then gluing to structure top and bottom. Alternatively, you can sheet the D-box and centre sections and use cap strips for the outer wing areas as shown on the plan.

Shape the strip ailerons and temporarily fit onto the wing with nylon hinges. Add wing tips shaped from balsa block and lightly sand entire wing.

Fit the aileron torsion rods and aileron linkages to servo. The aileron servo well is cut out of the underside of the wing. The chine and upper fuselage sheeting over the top of the wing can be made up during the fuselage construction.

Tail Surfaces

Build up the tailplane/elevator and fin/rudder from 9.5mm balsa sheet components cut and joined to shape, I chose not to cut out the lightening holes. Both rudder and elevator use aileron type torsion rods with ball joint linkages to control rods because of their sweep back angles.

The fin has a 12.5mm dowel piece glued onto the top. Trial fit control surfaces with hinges. The Fuselage

The fuselage is made up of two sections, front and rear, both basically box structures integrated together.

Cut out the fuselage sides from 3mm balsa sheets. Add ply doublers to the inside of the front section and wing seat/doubler to the rear.

Glue the longerons and 12.5mm balsa triangle to the sides in preparation for assembly to formers. Glue F3a onto F3 and F4a onto F4. Glue the front fuselage sides onto F2, F3, F4 and F5, then draw the front end together onto F1. Glue balsa triangle gussets into the corners of F1/fuselage sides.

Similarly, assemble the rear fuselage sides onto F6, F7, F8 and F9. Slide the front section through F6 and onto F7 and glue into place flat over building board ensuring all is straight. Add E1a and E2.

Cut and glue the undercarriage blocks into position. Add the underside longeron along the bottom of the rear fuselage section. The wing mount hardwood blocks can then be glued into place. At this point, the tailplane and fin are glued into the rear fuselage, Check the alignment carefully.

Now trial fit the wing onto the fuselage. Check alignment, then drill and tap through for the wing mount bolts. With the wing in place complete the upper fuselage sheeting and shaping of the balsa block rear of the cockpit. Glue W2a over the wing and make up chines from 12.5mm balsa sheet. Carve and sand to blend into the fuselage.

Cut through blocks to detach wing from the fuselage. Add the dorsal fin.Build up the cockpit/front fuselage hatch with balsa block. Install the optional strut brace to F2 if desired and sheet the underside fuselage between F1 and F2.

Install the engine mount with engine in place. Carefully check thrust lines as these are very critical on this model.

Trial fit the fuel tank, fuel lines and throttle cable.

Build up the nose cone from 9.5mm balsa blocks with balsa triangle in corners and ply nose ring. Carve and sand to shape to blend in with fuselage.

Now install the servos under the tailplane and set up linkages to control surfaces. Access to servos is through a hatch underneath. Fit the nosewheel strut assembly unit to F3b. Cut and bend the main undercarriage wires and fit them onto the mounting blocks.

The underside of the fuselage can now be sheeted. Complete the dummy engine intake and shock cones by carving out the balsa blocks to shape. Round off all corners along the fuselage. Similarly complete the tail pipe area with balsa blocks carved to shape. I chose to hollow out the tail pipe blocks for a better scale effect. Lightly sand over the entire fuselage in preparation for covering and painting.

Covering and Painting

F-111 picture

Being a fully sheeted model the surface was first smoothed with dope and talc, then covered with tissue and dope. Further coats of dope and talc were brushed on and when dry sand smooth until a silky smooth finish was obtained.

The F-111 was then painted with enamels in camouflage colours on top and cream on the underside. To remove the gloss off the finish the model was coated in clear matt varnish. The inside of the tail pipes were painted in orange day-glo.

Roundels and insignias were painted on to complete the model. Several options for painting the F-111 are available but I preferred a contrasting colour scheme to differentiate the top from the bottom. Install the engine, fuel tank, radio control gear and fit the wheels to the model, then balance it to the Centre of Gravity shown on the plan.

Flying the Model

F-111 picture

Initial flights were frightful to say the least. The F-111 was difficult to keep straight on the take off run and once it did leap into the air, climbed erratically with nose pitching up very steeply. At one point it stalled and tumbled out of the sky, the only saving grace was the contrasting colour scheme which helped with orientation during corrective action. The model was quickly landed while still in one piece.

After debriefing it was decided that the Centre of Gravity, which was first set at about 25 per cent may be too far forward. It was subsequently moved back by adding weight to the tail.

The next flying session was very short lived with the F-111 taking off, climbing vertically then again dropping out of the sky, this time nosing into the ground - crash!

Apart from a demolished nose, the rest of the structure got away with it lightly. The model was repaired, this time with added downthrust, deleted right thrust, increased rudder area and balanced to a C of G well forward at 10%, far more forward than what would be considered normal.

Third time lucky.... The F-111 now flew respectably. It now took on a real jet like performance.... fast with a very wide aerobatic envelope. Rolls are slow and positive and loops required plenty of sky and when diving out of the manoeuvre it looked magical! Rocketing past on its fly-by you'd swear you're at a real airshow watching the F-111 about to do a fuel dump and burn.

However flying this model is far from comfortable. You will soon discover its speed range for safe handling is within a very narrow band. Too slow or too fast it will tend to have a mind of its own and will be difficult to control. If you over speed the F-111 in straight and level flight, it will have a tendency to nose pitch upwards, and unless quickly corrected by throttling back it will perform a high speed stall and possibly a snap roll,

Throttling back will bring the model back into full control if in trouble. Conversely flying too slowly may also result in stalling and snap rolling. Although all control surface areas are relatively large, control response is not so great so care needs to be exercised at all times. The model will certainly spin in spectacular fashion but keep the power off or you won't recover.

The rudder is not very effective for steering on the take off run so apply full power and get into the air quickly. Should the engine cut out the F-111 will glide OK provided the air speed is maintained by pushing the nose down with elevator.

Approach speed to land must be kept up to avoid stalling. Electronically mixed flaperons would be an advantage on this model. Trimming this model requires a lot of fine tuning. Summary

Airborne's sport-scale F-111 is a spectacular model to see flying in the air and it looks great on the ground. It is relatively easy to build but flying requires a skilled pilot at the controls. Successful flying is only possible within a narrow band but once mastered will put on a performance similar to any ducted fan model.


SPECIFICATIONS F-111

Type: Sport Scale Prop Driven Jet Wingspan: 1.4 metre Length: 1.3 metre Weight: 3.1 kg Engine: .46 two stroke 4 channel radio control required

F-111 plans are available from:
Airborne Plans Service

AU$50.60 (2 sheets) plus AU$3.00 P&H2
Moulded Canopy: AU$27.50 plus AU$6.00 P&H

Plan No. 643

Click here to order this plan

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This page was last modified on the 21-May-02