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de Havilland's Mospuito

by Luke Cullen and George Buzuleac

Plan No. 699

Features Construction Articles

Plan Detail:
Plan No. 699
Price: AU$74.00 (3xSheets) 
plus P&H (AU$8.00 within Australia).
Moulded Canopy & Nose Cone: AU$35.00 plus postage.

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Powered with two Rolls Royce Merlin engines the Mosquito's performance far exceeded expectations. Having a top speed of over 600kph it was faster than some single engined fighters of its time.

It first flew in 1941 and a total of 7,781 were built including some which were built at Bankstown in Australia.

Many victorious combat missions were attributed to the Mosquito squadrons making it one of the most famous warbirds of all time. Airborne now has released it 1:7.5 scale model of the Mosquito bomber.

At a sizeable 2.2 metre wingspan it only requires two .46 two stroke engines to give it all the necessary power for sensational scale realism. Being of conventional balsa/ply construction it will appeal to the dedicated, experienced scale builder who thrives on praise and attention at large scale or warbird rally's. This model is a stand out winner!

The prototype I have built for Airborne is modelled on the Mosquito based at No. 1 RAAF Squadron, Laubaun, North Borneo in mid 1945. I have only built one other twin engined model, which was Airborne's de Havilland Comet, and found it a very rewarding plane to fly so I welcomed the opportunity to build one of my all time favourites, the DH98 Mosquito.

building the model

Study the plans carefully and prepare yourself mentally for a major project.
Great care and accuracy must be taken particularly with thrust lines, washout and wing incidences. Any warps or bows in the structure will affect its crucial flight performance.
Make a kit of all the ply and balsa components and group them into their respective areas.

The Wing

The wing incorporates split flaps and ailerons with their respective servos as well as the engine nacelles and retracts. It is quite a heavy load for a sharply tapered wing, therefore careful wood selection is very important. Use strong spruce for the main spar and a good quality ply for the wing brace. Build one side at a time. When one side is finished, build the other around the wing brace with correct dihedral as marked on the the plan.

I covered the plan with cling wrap and then pinned down the spruce spars, aileron and flap spars over the plan.

I then laid all the ribs, W1 - W13 and dihedral brace DB into position (note 2O washout) and placed the top spar over them before gluing them together with cyano. Add shear webbing between W6 - W13.

Glue the false leading edge onto the ribs. Note that the leading edge protrudes into the centre wing section between W1 - W14.

Shape the balsa wedge blocks for the rear centre wing section, do not drill the wing bolt holes just yet. Also add the wedges for ailerons and flaps.

Prepare the 1.5mm balsa skin by gluing the sheets together on the bench using cyano with tape behind the join. I find this a good quick method to use. Sand the sheets flat before gluing them to the top surface of the wing panels. When set lift the wing off the board. Add the square balsa pieces for the servo bays used for the ailerons and flaps.

Now sheet the wing entirely, bottom side first, then double check the 2O washout before locking in the wing structure with the top skin.

Add the leading edge and wing tip blocks then sand to shape. Carefully cut away the ailerons and flaps. I used small Dubro hinges with metal pins to hinge all the control surfaces. Take care with the flap hinges as they are very close to the bottom skin. Angle them into the spar for better security.

The Engine Nacelles

Mark out the nacelle position on the wings. Glue all the formers to the bottom of the wing with the retract mounting blocks epoxied into place. Build up the plywood engine mounting boxes, which also become fuel tank bays rear of the firewall.

Note: Left engine thrust line is 0O, while the right engine has a 2 1/2O right thrust.
The remainder of the engine nacelle and engine cowl is planked with 3mm balsa. the hatch covers and top rear nacelles are shaped from balsa block. Carve and shape the exhaust shrouds from balsa. I made all the doors for the wheel bays but did not use them on the initial flights due to the complexity of the closing mechanism (not shown on plan). They would however give the model a really good scale finishing touch.

After sanding and shaping the nacelles I mounted the engine, throttle servos and the retracts themselves. The engines I used were two OS .46FX two strokes with standard mufflers.

The retracts were Custom Retract units from Peter Gow (see advertisement in this issue) as drawn on the plan. I utilised 180mm Dubro fuel tanks on my model with three fuel lines to the tanks.

Fuel proof the insides of nacelles, the firewalls and fuel tank bays.
The fuselage is constructed in two half shells then glued together. With the engines mounted in the wing, the fuselage is relatively simple to construct.

Frame up the left fuselage side over the plan. With the doubler glued onto F5, add the longerons and wing and tailplane saddles to the formers. Plank the frames. Build up the belly pan (F5a - F10) as part of the fuselage then cut away after completion of smoothing and shaping of the planks.

The tailcone is made up of balsa block, hollowed out for the retractable tail wheel unit.
Trace the plan (fuselage side view) or brush cooking oil onto the plan to make it semi transparent to give you a mirror image. Repeat the construction procedure for the right side of the fuselage.

Before gluing the two shells together install the control rods to the rudder, elevator and tail wheel retract unit.
Glue the two fuselage halves together checking for true alignment.
Decide on whether you will use the clear pre-formed plastic nose cone which is available from Airborne or make a solid balsa front end, depending on which Mosquito variant you wish to build.

Epoxy the wing mounting plate into the fuselage. Position the wing accurately on to the fuselage and drill and tap for the nylon wing bolts and blind nuts.
The belly pan under the wing is detachable and held on with spring latches. This allows for access to wing mounting bolts and radio gear which must be kept as far forward as possible to aid in balancing the model.

Tail Surfaces

The tail plane and fin are of built up construction of balsa then covered with 1.5mm sheet. This is a simple and lightweight method but take care to keep the tailplane straight. The tailplane can now be fitted to the fuselage, carefully checking the incidence with the plan. The elevators and rudder have a central sheet with ribs added to the outer sides. Sand to shape and hinge onto the tailplane and fin respectively.

Covering & Finishing

After the entire airframe was finished I covered it with 21g fibreglass cloth (personal choice only) with the exception of the rudder and elevators which were covered with Sig Coverall.

I then hinged and pinned all the flying surfaces. I applied the fibreglass cloth with dope and then, to fill the weave, I thinned the dope and sprayed 8 to 10 coats before priming the airframe. I then pained the underside of the airframe with the light blue. I then proceeded to mask and paint the remainder of the airframe with the green base coat. I then sprayed the tan camouflage markings before applying the roundels and lettering. All the roundels and lettering are cut in vinyl. The airframe was then sprayed with a two pack clear with a matting agent for the scale effect.

Radio gear used was all JR 577 servos with the exception of 371’s on the throttle and a 591 on the elevator. I used one servo for each aileron, flap and rudder and separate throttle servos. I used Dubro servo savers to adjust the throttle servos to make synchronising the transition from idle to full throttle easier. I put the aileron and flap servos as close as possible to the surface and only needed short pushrods. the rudder and elevator servos were as far forward as possible and I used carbon tube as their pushrods. I used a Dubro adjustable valve for the retracts and a medium tank.

The propellers I used were APC 11x7’s. The engines had the baffle removed from the muffler for better performance. This was a good combination even with the 100mm spinners. The engines run at 12,000rpm and sound fantastic as they produce the harmonies of a twin.
The all up weight of the model is 6.3kg with the balance point at 25% of the wing root chord.

Flying

The day arrived for the test flight, somewhat windy but warm and fine. I did a test run of the engines and tuned them to run at similar revs at full throttle. I then cycled the retracts a few times to check how many operations I would get from a full tank. After refilling the retracts tank I then taxied the Mosquito and went to full throttle to see the tendencies it had.

de Havilland's DH98 Mosquito

 

running up engines ready for take-off

 

complete set of balsa and ply components
All conponenets cut out prior to commencement of building

 

partially built wing
Underside of wing prior to installation of retracts and finishing off the sheeting

 

complete wing

 

partially built engine nacelle

 

front end of nacelle
The work in the cowl is a credit to the builder, Luke Cullen

 

wing with retracts fitted
Retracts supplied by Custom Retracts suit the Mozzie perfectly

 

complete fuselage and wing

 

complete fuselage and wing 2

 

painted fuselage and vac formed canopy and nose cone
The fuselage is conventional build up construction with ply formers and balsa sheeting

Choose whether to have the clear nose cone or paint over it suit your particular scheme.

Clear nose observation cone and full canopy are available from Airborne as well

Mozzie in flight

Like most war birds it wanted to turn to the left until the rudder had enough authority. With all the checks finished I taxied out and lined the Mosquito in the middle of the strip. I then advanced the throttle slowly and gained some airspeed and then went to full throttle. It took a long scale like takeoff rollout but when airborne flew with a good power margin and only a small amount of down trim. This model is quite large and unlike many scale models does fly a scale speed, i.e. not too fast. At full power the Mosquito handles as if it is on rails... straight as a die until you begin to manoeuvre it.

I did a succession of fast and low fly buys and it looked great. I did a few slow barrel rolls and loops, this model does it all easily. At height I attempted to stall the Mosquito to see what it does. It gently noses over and drops a wingtip but recovers instantly to corrective action.

Spinning was also attempted with recovery after three turns, no problems encountered here provided you have reasonable height.

The Mosquito sounds really great with the harmonics of the twin engines on song. So far I have not experienced any engine cutouts which gives credit to the reliability of OS engines.

Landing the model presents no problem, with about 25 - 30 degree of flap it lands very slowly and you can just steer the model to a really smooth landing. Any more flap (45 - 50 degree) dramatically increases its rate of descent. The flaps are not just for looks and are very effective.

The Mosquito gains a great deal of attention wherever I have flown it. In the air, rolls, split esses, loops and low-level full throttle passes are where this model excels.

On the ground it stand majestically, attracting the attention of a lot of people. In particular it seems to draw out the more mature people amongst us who are only too glad to relate a nostalgic story or two about this much loved hero of WWII.


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