Vintage Diesel Combat

Adrian as Contest Director.

Model Selection and Construction for Vintage Diesel Models

by Adrian Duncan

In Vintage Diesel Combat, models are limited to designs which were published or in common use prior to December 31, 1970. Under present rules, such aspects as aerofoil section and internal structure may be changed at will, but the design planform must be strictly followed.

The following are some notes based on our experiences to date in British Columbia.

1 . Pick a model that matches your motor. Heavy motors need large wing areas, for example, but large models need more power. A less powerful motor should not be used in a large model regardless of weight - pick a different motor if you want to build that particular model. A small model should be fitted with a light motor to keep wing loading under control. Also, rear induction motors are much easier to mount in an upright-mount design than in a sidewinder design, where they tend to require wing-weakening cut-aways to accomodate them.

2. Be aware of design factors which may limit a model's durability. For instance, wire booms have a tendency to bend and break in certain crashes. They can be replaced by gusseted plywood booms or made replaceble - I use screw-in booms made from thread-ended 4-40 pushrods and screwing into blind nuts. Tail fins are inherently vulnerable to glancing mid-airs and general crash damage. Fuselages tend to break between the trailing edge and the tailplane, and need reinforcing in that area.

3. For a typical 2.5 cc (.15 cu. in.) diesel weighing around 6 ounces, a wingspan of around 32 inches appears close to the optimum. Greater spans impose too much drag for all but the very best .15 to drag around at a good speed, although they do tend to turn tighter. Larger models are a trade-off - speed for manoeverability. Smaller models tend to be faster but less manoeverable. Also, large models are more adversely affected by wind, and vice versa. If you're in a windy area, don't go above 32 inches wingspan

4. Many period designs feature relatively thin aerofoil sections. For tight turns, a minimum 1" thick section is essential. Many late 60's designs use a "flat" 1" section for ease of building and good penetration. In calm air, a "true" aerofoil section gives tighter turns but also increases drag and thus affects airspeed. We use up to 1 3/8" aerofoil sections with good results. In high winds, the "flat" section tends to be just as good, and handles the wind better if anything. Certainly, the "flat" section gives a higher airspeed. It's your choice!

April 26, 1997. Carkeek Park, Seattle WA. L-R Ken Burdick, Jeff Rein, Mel Lyne, Paul Dranfield, Frank Boden, Bruce Matthews and Adrian Duncan

L-R Paul Dranfield (webmaster), Ron Salo, Mel Lyne at Pacific Aeromodellers flying site, Richmond, B.C

5. Many period designs featured spars in the wings. These are not necessary, and add weight and complexity. Omit them. Build a very strong leading edge of hard balsa reinforced in the centre with 1/16" ply and 1/8" x 1/4" spruce backing. Use 1/8" balsa ribs slotted for the trailing edge, and hollow out the leading edge between the ribs to reduce weight, more so on the inboard wing than the outboard to achieve correct balance. Build a strong trailing edge out of 1/4" hard sheet balsa with a 1/8" x 1/4" spruce strip on the front edge to resist shearing. Use 1/8" lite ply for the wingtips, with 1/4" balsa gussets at the inside corners. If the leading edge, the trailing edge and the tips don't break in a crash, the model will survive, so make these components strong! Very light construction can then be used for the balance of the model to keep weight down. Aim for no more than 16 ounces ready to fly, less for a smaller model.

6. Most period designs were intended for the tuned Oliver Tiger or the early PAW's, which weighed up to an ounce less than most modern motors. Therefore, if you mount the motor in the position shown on the plan, it will usually come out nose heavy and unresponsive. We find it necessary when using a modern PAW 15 BR to recess the outboard leading edge in the motor area by at least 1/4" to allow the motor to be moved back by that amount to maintain correct balance. Use this figure as a rule of thumb and see how your first efforts fly.

7. Many period plans show up to 1 1/4 ounces of tip weight in the outboard tip. This was intended to maintain line tension during maneuvering by balancing the line drag on the inboard tip. This weight is totally unnecessary - omit it! Instead, achieve the same effect by such means as using lighter wood for the inboard wing ribs than the outer, hollowing out the leading edge more on the inboard wing, etc. Most importantly, make the inboard wing 3/4" longer than the outboard wing, if this is not already a feature of the plan. More than 3/4" is excessive, so do not go beyond this figure or you will start to "hinge" badly, particularly in calm air. If you follow this advice, you will get a light model that stays out on the lines under all conditions. Don't use more than 15 - 20 degrees of elevator movement in each direction, otherwise the model will over-control and slow down in turns. If you find you need more than this, you're not building 'em right!

8. Cover the model with doped nylon (strong but heavy), Solartex or similar "woven"iron-on material (heavy) Econocoate or Micafilm. Micafilm has been found to be the best of these materials, since it is completely diesel-fuel-proof and requires no doping. Better still, it resists shattering in a crash, and patches are easy to do if a cut must be made to replace a broken rib or similar. Finally, it is extremely light. We use it exclusively. It saves up to 3/4 ounce in weight on a typical model.

9. Make sure your motor pod is well mounted with epoxy and 1/8" hardwood dowels. Use a few degrees of outward side-thrust to really ensure good line tension. Do not use white glue or Zap where diesel fuel could contact them - epoxy is far and away the best. After finishing the model, coat the entire engine mount area with a clear epoxy finish - this will preclude progressive weakening through fuel seepage.

10. Use a securely-mounted two-pipe tank with the pick-up point located at the rear outboard corner and the vent outlet at the front outboard corner. Have the external vent filler nozzle facing forward into the slipstream. Test the tank for leaks and blockages prior to installation (or you'll regret it!). To fill such a tank, you have to hold the model so that the outboard corner is uppermost, and then fill through the vent using the pick-up tube and engine spraybar as the air vent. Such a tank gives a virtually constant fuel head for the entire run. You can easily modify a commercial tank in this way - that's exactly what we do. Use at least a 1 1/2 ounce tank to give an adequate run with a PAW 15 BR.

11. Which model to choose? That's a very subjective question! But to start out, you simply can't beat the good old Vernon Hunt "Warlord". It is easy to build, very strong and a great flyer if built below 16 ounces all-up weight. It's even available in kit form if you're lazy! But it's not very suitable for rear induction motors, since you either have to make a large cut-out in the wing to accomodate such a motor, which greatly weakens the structure, or live with a CG that is way too far forward, thus giving a very sluggish aerobatic performance. An upright mount design is much better for engines of that type, like the MARZ. The "Dongus" is probably the best model of this type,with the "Rogue" a close second. Both of these have the drawback of having those vulnerable tail fins, but they are good performers.

Well, that's what I know at present! My models usually come out around 16 ounce or so with a PAW , and fly beautifully. Diesel combat with a good motor and a well-trimmed model is a real pleasure win or lose, believe me!

Hope you enjoy it as much as I do!
Adrian.