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Jet Ray plans are available from: Airborne Plans Service for AU$38.50 plus P&H2 (AU$3.00 within Australia).  
Moulded Canopy: AU$27.50 plus P&H (AH$6.00 within Australia).
Plan No. 565
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Jetray Picture

The Jet Ray is a sport model delta which is loosely based on the Douglass Skyray, an American interceptor fighter of the 1950's.

The style is categorised as "stand off scale" as the similarity to the Skyray is remote. Nevertheless the Jet Ray is a strikingly different model in its own right. This tail-less aircraft with swept back wings and and extremely low aspect ratio will certainly draw attention from all modellers from Old Timers to the ducted fan fraternity.

Despite being a prop driven model the Jet Ray represents a good introduction to "Jet flying".

CONSTRUCTION

Wing

Jetray Picture

The wing is built directly over the plan - covered in glad wrap.

I chose to use two servos for the ailerons thus being able to use the elevon facility of the FUTABA 8UAP computer radio.

Pin the right and left trailing edge sheets and the rear 3mm x 6.5mm spruce spars to the plan. To obtain a few degrees of dihedral and positive washout pin the trailing edge sheets flat to the building board and build the upper wing surface facing down, i.e. flush with the plans.

Fit and glue the ribs W1 - W7 to the rear spars and trailing sheet. Complete the framing of the rear section by adding the bottom rear spruce spars and trailing edge sheet, angulate the spar slots individually before gluing each rib: and add the trailing edge spar. We now have a fairly rigid, warp free trailing edge to the wing.

At this stage, fix the right and left balsa leading edges to align the front of the wing ribs. Dress the remaining rib slots with a file and add the front 3mm x 6.5mm spars and the balsa 3mm x 3mm sub spars.

Glue in the undercarriage blocks. I suggest these be extended from W2 to W4 for extra strength. Add the various strengthening fillets and sheet the wing.

The ailerons are shaped from 6.5mm balsa and sanded to shape.

Fuselage

Jetray Picture

I chose to sheet the top and bottom decks with 3mm balsa to reduce the fuselage weight - the plans suggest balsa triangles and 6.5mm block. This modification simply requires a rounding of the top and bottom of formers F2 - F8 and addition of 6.5mm sq. stringers.

Laminate the fuselage doublers to 3mm balsa sides - I chose 2mm 3 ply doublers to reduce weight.

Mark the position of the formers on the inside and the wing cut out on the outside of each fuselage side.

Drill the engine mount holes into F2 and glue this to the sides. Now add formers F3 to F8. Sheet the top deck from F2 through to F8 and sand to shape.

Fix the nose undercarriage system to formers F3, 3a and 3b - for a steerable nose wheel the 3mm piano wire will need to extend to the top of the fuselage and a nylon bracket fixed to accept a pushrod for the rudder servo.

Glue in a 3mm balsa fuel tank floor and sides and add the captive nut to F2 for the engine mount. Fuel proof the forward section between F2 & F3.

Sheet or block (with 6.5mm balsa) the whole of the fuselage belly - having carefully marked the wings cut out position on F3 between F3 and F3b. Add the dorsal fin.

Now for the exciting time - the cut away of the wing belly between F3, the angulated cut between F7 - 8. Take your time and do this neatly. I suggest a trial cut-out of the wing shape on a piece of 3mm balsa and see how well it fits the wing - modify the shape of this trial till you are satisfied and translate the end result to the fuselage for an accurate wing profile cut out.

Jetray Picture

Add ply ends to both sides of the rear fuselage cut away. Trial fit the wing to the fuselage and also the cut-out wing belly to the wing - rework the fuselage cut-out sides and formers for a good fit with the wing axis running along the fuselage centreline to get a zero degree angle of attack - accurate building leads to good aerodynamics.

Fit the front wing locating dowels and rear mounting blocks at F7 to the wing underbelly PRIOR to gluing this section to the wing. After trial fitting the wing and underbelly glue this to the wing.

Canopy, Spine & Tail

At this stage the fuselage really looks like a box! Don't be disheartened - the addition of the canopy section, spine and tail sees this model blossom from an ugly duckling to the unique piece of flying history the Jet Ray represents.

The rear section of the canopy was formed from balsa block hollowed out. The spine and tail section are easily built form 6.5mm balsa.

Add the rear triangular fairings either side of the tail pipe.

COWL

Jetray Picture

The cowl section is difficult - choose your own favourite method. My system is to make a rear cowl backing plate that locates to the F2 formers with 3 pins. Spruce spars join this to the nose ring F1 and 3mm balsa is moulded to form the body of the cowl. The cowl is held in place by 4/40 bolts from F1 to the legs of the engine mount.

FINISHING

There are many options available and considering all external surfaces are balsa, I chose the dope and tissue method.

The whole model is sanded to shape with 150 grade paper and then doped. This is followed up with tissue paper being applied to all surfaces with dope being used for adhesion.

A mixture of dope and talcum powder is brushed on and then sanded with 240 grade paper to a fine finish.

Automotive acrylic was then sprayed without the use of an undercoat to save weight. The first coat is sanded right back before the second coat is applied.

All the decals and decoration are fixed, followed by a light scuff with 500 grade paper and the finish coat of the two pack clear is sprayed on.

HINTS

  • This model is inherently nose heavy- build all areas in front of the balance point as lightly as possible - hollow out formers and ribs, use light balsa and avoid block balsa.
  • Place servos and battery as far rearward as possible.
  • Balance Point - The plans have a suggested range shown - balance the model 10mm rearward of the front suggested point - required 400gm.
  • Use two aileron servos and a computer radio with elevon function.
  • Canopy was purchased mail order from extensive range in the BG/Castle Hill catalogue.
  • Arrange the nose wheel for a slightly nose high altitude on the ground.

ENGINE

Jetray Picture

I chose a reliable ASP 46 2 stroke engine. The Jet Ray flies very well with scale speed with this unit but could be a more brisk performer with a 60 or 65 engine.

FLYING THE MODEL

Having had no experience with jet flying I must admit to being quite anxious about the maiden flight.

As luck would have it well known ducted fan pilot Peter Agnew happened to be flying at the field on the day in question so I asked him to do the honours.

The Jet Ray certainly grabbed everyone's attention, it really is a "boutique" model.

After a thorough pre flight check, Peter taxied the model at various speeds to check for vices - none were obvious. Full throttle was applied and the Jet Ray tracked true and accelerated surprisingly rapidly.

At 30m the plane lifted off without any elevator impact and climbed gently into the sky. It looked magnificent when banked with the large delta-like wing silhouetted against the background.

After 5 clicks of down and 2 clicks of left turn the Jet Ray was flying hands free.

The model loops and rolls beautifully but is a little underpowered with the ASP 46.

Disaster nearly struck several flights later when severe rudder flutter during a high speed run nearly ripped the fin off. After advice from the jet pilots the rudder was permanently fixed to the fin and is now no longer operational.

The plan now shows a fixed fin only as rudder authority was virtually non-existant on this delta.

Landing is a breeze, with the Jet Ray just settling to a slightly nose high attitude when the power is reduced.

CONCLUSION

The Jet Ray is a rare and interesting model that looks particularly attractive both on the ground and in the air.

This model is not for the beginner but is very stable and easy to fly - it represents an excellent step up to ducted fan flying. Have a go, I'm sure you will not be disappointed.



SPECIFICATIONS Jet Ray

Type: Semi Scale Interceptor Fighter - Swept back wing - Tail-less. Wingspan: 1.3 metre Length: 1.25 metre Weight: 3.6kg Engine: 46 ASP 2 Stroke Prop driven Radio: Futaba 8UAP computer unit with elevon function. 3-4 function R/C required



Jet Ray plans are available from:
Airborne Plans Service
AU$38.50 plus AU$3.00 P&H2 (within Australia)

Plan No. 565

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