Ohka (oz15847)

About this Plan

Ohka. Control line scale model.

Quote: "Ohka. A Kamikazee control liner. A 1.5cc powered flying bomb by Jym and Dave Leddy.

THE YOKOSUKA MXY-7 'Ohka' (Cherry Blossom) was designed by the Japanese during World War II as a piloted missile intended for suicide attack. The mark II on which our model is based made its first powered flight in November 1944 and its first operational use was on 21st March 1945.

The first section contained a warhead of 2,645 lbs of high explosive. The second section contained the cockpit which was equipped with only very basic controls and instrumentation and the third section contained three solid propellant rockets. The 'Ohka' was carried to its area of operation by a specially adapted GM42 'Betty' twin engined medium bomber. Once released the Ohka would glide to the target and when in a dive it would accelerate to over 500 mph by firing the rockets.

Our model is not quite so fast but still has a handy turn of speed with a good 1.5 cc motor on 35 ft lines.

Wings: Cut out all the parts and glue the 1/16 ply reinforcing centre piece to the main spar. Pin the main spar to the plan on one side (with the other half of the spar up in the air!) and add the ribs, leading and trailing edges. When this has dried unpin from the building board and build the other side. The wing is now covered in 1/32 balsa sheet and the wing tips are added after this sheeting is completely dry. The wing can now be covered in lightweight tissue and then put to one side.

Fuselage: The fuselage is built on a 1/8 balsa spine. Cut to shape, up to and including F8, and pin over the plan. The two 3/8 x 3/8 bearers are glued to the spine but are packed up 1/16 with scrap balsa. This will ensure that the bearer face is an the centre line. Now add former halves F3B- F8B except for F4 lower. When all is dry remove from the plan and add all 'A' formers, the tank and F-2. Glue in the block between F7 and F8. This can be slotted and grooved to allow for the elevator joiner and control horn. At this stage the bellcrank can be bolted to the upper bearer.

Tail Unit: The tailplane and fins are all made from 3/16 balsa. Although the sheet between the two elevators could be strengthened it is about the only part of the tail that has not been repaired on the original model, in spite of all the flying with many different pilots. Don't worry about the small size of the elevators, they are quite effective.

Fit the tail unit to the 1/8 spine on top of the supporting blocks and ensure that there is sufficient room in the slot to allow free movement of the elevator. If you are going to cover the tailplane in lightweight tissue it is best to do it at this stage. When you are satisfied with the movement glue the tail in place and when dry connect the elevator to the bellcrank with the 16 swg piano wire push rod.

Back to the Fuselage: Now is the time to fit in the wing. Take great care to ensure that it is square. We made a model for control line of the Bachem 'Natter' and failed to notice the wing had dropped 2° before the glue set. The model was finished and test flown by our mate Bryan Shelley and he swears there could never be another model like that (he has since moved out of the area!). Formers F4L can now be added and when the wing fixing is dry, the fuselage can be covered. This can either be planked with 1/4 x 1/16 balsa strips or can be covered in moulded panels of 1/16 sheet.

For this process, panels of 1/16 medium balsa are cut oversize, and after soaking in hot water are taped around suitable formers. They are then left to dry overnight and when dry can be trimmed, fitted and glued in place. Soft block is glued between formers Fl and F2 and sanded to shape. The lead outs are made from control line wire and must be fitted before the panelling is complete.

When all the covering is complete and dry, make the diagonal cut shown on the plan which removes F48. Fill in the hole with 1/8 sheet and add three dummy rocket nozzles made from scrap balsa.

Finishing The original models were covered in lightweight tissue and given several coats of sanding sealer. Grey enamel paint was applied as the main finish with black under the canopy and at the rear end. The final finish was matt polyurathene varnish. The canopy was cut from a larger commercial moulding and I think would have looked better with a pilot under it.

Flying: Balance on the front lead out and fly on 30-35 ft lines. Take off can be hand or dolly launched, we have tried both. The Ohka is fast but stable and creates quite a stir, although people don't seem to want us near carrier competitions with it..."

Note this article is an extract from post on HPA by ilgk48 of the complete Aeromodeller magazine, see https://www.hippocketaeronautics.com/hpa_plans/details.php?image_id=13350

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