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Dassault Mystere Falcon 20

by Mat Downes

Plan No.626

Features Construction Articles

Plan Detail:
Plan No. 626
Price: AU$55.00 (2xSheets) 
plus P&H (AU$4.00 within Australia).

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This model provides challenges for the builder due to its’ cylindrical and tapering fuselage, swept back wings, nose cone and elevated tailplane. There are several incidences to consider. Building experience is preferable, but the finished model is so pleasing to the eye that you may like to give it a go purely as a model builder. It is definitely not a model for learner pilots.

You can start construction where you like, I started on the wing.
As with any model built over the plan, use cling wrap to protect it from glue. Use good quality material and a flat building board.
Trace the wing ribs onto balsa sheet and trim to accurate size, remembering there are two of each. The template and router system is a bit of an overkill for this model.
Pin the bottom main spar to the plan. Position the ribs as per the plan and use the dihedral template to get the correct angle for W1. Add the top spar, leading and trailing edges, gussets and shear webbing. Note the 2 degrees washout achieved by the larger packing pieces on each rib as they move towards the tip. Before sheeting this half, prepare and install dihedral braces B1and B2. When glue has set, cut out part of W1 to allow for servo mounting. Sheet top of wing where required.
Remove from the plan and complete the bottom sheeting. Be careful not to pull the washout out at this stage.
Build the other half but don’t put the shear webbing between W2 and W3. Sheet the top as above, but only sheet the leading and trailing edges underneath at this stage.
Test fit and join the two halves. Use 35mm blocks under the tips to achieve the required dihedral.
Prepare and install the balsa block (trailing edge) between W1s and W2s and the undercarriage mounts UM. Don’t fit B3, the balsa block or the wing locating dowels until later. The shear webbing and remaining sheeting can now be installed, note the sheeting at the tip as this is not common in wing construction. Wing tips, aileron tips and the rib caps complete the construction. Shape to shown profiles and sand lightly.

The fuselage is built in two halves. The left (port) or near side if you’re a horse rider is built over the plan with the ink side up. You will need to place the plan face down to build the right (starboard) side, unless you want to build two fuselages! To see the lines dampen the back of the plan with paraffin oil, or kerosene, or vegetable oil.
Formers with the exception of F9 are cut from 3mm ply. The plan shows half formers, so tack two pieces of ply together, overlay your template (I photo copy the plan and rub turps on the back of the paper which is face down to have a transfer effect). You will now cut two symmetrical halves from the one template. This method guarantees accuracy. Note the bottom is cut off F6, but retain the off-cut.
F1 as you see is 2 off. One is cut in half (use a fine blade saw) to use in fuselage construction and the uncut one is glued on when the fuselage is eventually joined.
Spruce longerons are pinned down; note the top one only goes to the tail and the bottom one to the wing cut out.
Place the fuselage crutch over the plan and accurately mark the position of formers F2 to F5. Mark the starboard crutch so that you have a mirror image for later. Include the position of the wing mounting brackets.
Slip formers over the crutch to marked positions and place the assembly onto the longerons. F1A is at right angles to the plan as the engine thrust line is achieved by wedging the engine mount. When satisfied all is square and plumb, glue together. Don’t glue to the bottom longeron. This makes joining the halves and installing F3 easier. The middle longeron can be fitted and glued to each former, back to F6.
When dry or set, pin the 6.5 x 3mm balsa spine to the plan. Slide the dummy jet engine mount into F7 and F8. Place this assembly onto F6. Pin and glue to the spine over the plan. Now pull down the protruding middle longeron taking care not to lift the assembly from the plan. Use weights to hold in position while the glue sets.
I sheeted from the top of the fuselage round, (grain running lengthways), using 2, 100 x 3mm sheets from F1 to F6. Leave an overhang at F1 to cover the second F1 when it is installed. You may opt to have a second balsa former at F6 to take a join. If you’re a top balsa worker continue all the way to F8, but my experience says plank from F6 back. Sheeting covers the wing cut-out area at this stage. By using only two sheets there is now sufficient room for access to mount the nose wheel hardware, fit F3A and clamp when joining the fuselage halves.
Repeat the building process for the other half, don’t forget to turn the plan over!

Joining the Halves
First thing to do is cut out the excess sheeting in the wing seat area. By working from the inside you can push pins through the sheeting to give an outline of the fuselage crutch. Cut the sheeting carefully and leave a good amount of access for trimming when the wing seating is fitted.
A note here worth remembering is that the wing is tapered and swept back. The profile of the wing seat cut out shown on the plan is actually at W2 not W1 so the leading edge area of the seat will need a fillet if you are too severe with the knife. Don’t install the seat planking at this stage.
Test fit the fuselage halves and sand if necessary for a neat fit. Glue and clamp. Use plenty of rubber bands, clamps etc. Fit F1, F3A, nose wheel hardware, engine mount blind nuts and fuel tank tray. Fit the bottom longerons and complete the sheeting.

Fitting the Wing
One point five mm balsa is fitted between the fuselage crutch and the skin. You may find it easier to sheet over the edge of the skin, either way works well. The success of this operation will depend entirely on the trimming job as just described. When complete, fit B3, wing dowels and the under belly block. The wing can be bolted in position. Take care to measure from a common point at the tail to the wing tips to ensure accurate positioning.

The Back End
The fin is very easy to build over the plan. However accuracy is required when the balsa frame above and below the tailplane slot is installed (this is not the triangle support), note the 1.50 negative incidence. Sheet this side of the fin, remove from the plan and cut the tail plane slot gaining access from the unsheeted side. Cut the nyrod exit point. Sheet the other side, include the other nyrod slot and cut out for the tailplane. The sheeting goes into the fuselage so don’t skimp.
The tailplane is built as per the plan. Use the ply dihedral braces, ensureing each tip is 11mm above the building board when joining and that there isn’t a twist in the tailplane when it is glued together. Fit the tailplane into the fin and glue in place, be sure the angles in relation to the fin are equal.
Mounting the Completed Tailplane
The unit can now be mounted to the fuselage, again fitting and trimming. This may take time but the incidence needs to be correct. Use plenty of glue (I use a good epoxy) as access to the fuselage spine is only through the top slot. Rudder, elevator and bracing fillets can now be completed.

Dummy Jet Engines
Find a cardboard tube of appropriate diameter and glue on formers E 1s (there are 2 per set), 2 and 3. Plank with 3mm strips. This is a challenge as the circumference of E2 and 3 is bigger than E1. Three mm balsa will stand a fair bit of sanding. You may cover the pods and install later or install and cover.

Building the Nose Cone
This is a labour of love but the results are pleasing. I laminated scrap together with a rough hollow inside. I then shaped outside to the required profile and hollowed out the inside with a modeller’s grinder. The worst part is that after all the work you need to cut large holes in it to fit the motor. Again please yourself whether you glue it on or use mounting blocks and screws.

Bring it all Together
Fit out the Mystere with control surfaces and hardware. Be conscious of weight and positioning of gear as the model will easily become nose heavy. Remember the full size Mystere has two engines that are mounted behind the C of G and we only have cardboard and balsa!

Speaking of C of G
The plan I built off, being a review plan had no centre of gravity marked so it had to be calculated. Fellow club member Andrew Barber loaned me a great book called “Scale Aircraft Models for Every Day Flying” by Gordon Whitehead. It was originally published in England by RM Books. It explains in layperson terms how to calculate the point. It was an interesting exercise, taking into account wing areas, root and tip cord lengths of wing and tail and the tail moment. All calculated, the position was determined as now shown on the plan. To gain a good balance the hardware was mounted aft as describe earlier.
A good quality covering material and some pinstripe set this model off with a touch of class.

Sunday afternoon at the Hamilton club. Many of the members were aware of the construction project as Wednesday evenings is usually building night at my workshop where we gather to do a little building and talk a lot. So there were plenty present to witness the maiden flight.
The usual checks were done, radio range check, correct control surface movement, and most important of all, engine reliability test. The Mystere is powered by an Enya SS 40. Control surface movement was a hot topic of debate, but being an untried model it was decided to use a good amount of travel because you obviously can’t increase it in flight. However my radio has dual rates and I therefore had a good range of control.

Roll out to the runway, final wiggle of the sticks, a deep breath, and power up. The Mystere lifted off after a long build up of speed and climbed away with minimal trim required. The long run was a precautionary measure as there were no known flight characteristics to go by. Just a couple of up trim clicks were needed. Control surface throws used are, elevator 16mm total, aileron 16mm total, rudder 30 mm total, but governed by the position of the elevators.
Fly it like a passenger plane. It is a fast flying plane, great for circuits and low passes, the odd loop is okay but I don’t think the passengers like them much!

Height was gained and a stall was tried, more screams from the passengers! It drops a wing at low speed, but power up and fly away no problems.
After several minutes in the air there is no choice but to land. On my first attempt I overshot the strip not allowing for the high approach speed. Throttle on, climb away and try again. Next approach was longer and flatter, trying not to wash off too much speed. Yahoo, a successful landing. This is a sleek machine which has an approach speed higher than that of a 40-size sports model.

This project has been well worthwhile. I’ve learnt a lot, enjoyed the challenges and love the look of a great model. The old man reckons he wants to hang it in his office when I’ve finished with it! He’ll be waiting a while.

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The Dassault Mystere Falcon 20

Fuselage Construction

sheeting fuselage 2

wing construction

attaching leading edge

sheeting of wing

wing halfs joined

fuselage with radio gear fitted.


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