Pushover (oz11172)

About this Plan

Pushover. Single channel RC sport pusher model, for up to .75 diesels or .049 glow engines.

Quote: "Interesting little single-channel pusher, for happy hours of small-field flying. Pushover, by Dave Greenfield.

The layout of this model is a little unorthodox, but it has been developed in this way to meet certain desirable design requirements. Basically I wanted a fairly attractive 'park flier', that would serve as a test model for a flying-boat development programme. Well, we don’t all have ponds or lakes handy, but I have found that this version has been good fun, and it has fulfilled its requirements.

With this layout one can legitimately dispense with the under carriage and, of course, over grass this is a good thing. Undercarts are for tarmac - skids are for grass! A brief word on undercarts - this is the first time I have not used one as, in the past, I have relied on the drag caused by the u/c to reduce the thrust drag couple. With this layout you have the thrust drag force arrangement of the low winger, with the stability of the high-winger.

Of course, an u/c could be fitted but, as it is not needed, you have the bonus of reduced overall weight. Thus, the wing loading is lower, the airspeed is less and consequently the model is quite docile. Talking of wing loading, it should be between 10 oz/sq ft and 12 oz/sq ft. Aim at 11 oz/sq ft, which is 16-1/2 oz all-up weight, for 1-1/2 sq ft of wing.

Prop and engine are well out of the way, safe from damage and dirt and, of course, most of the sludge from the engine exhaust is swept clear of the model. With the engine where it is, the slipstream is clear of the wing, which means that the wing is moving through clean air. Make sure that whatever way you choose to rig your model, the slipstream does not play directly on to the tailplane. On the other hand, the slipstream playing directly onto the rudder has its advantages, one being that the model will twitch its tail either way, on right or left signal, whilst stationary on the ground. (This, I hope, will eliminate the need for a water rudder in the flying boat version).

Take any engine up to .049. Give preference to an engine that will turn a prop in either direction. If your engine turns in one direction only, get hold of a prop that will push. (A good range of small diameter Tornado nylon pusher props is available through Ripmax dealers. -Eds.) In the model in the photo, I have used a Beech four-bladed prop, which I made from two double bladed props halved at the centre and metal bushed. Ply discs have been added, one each side. Balsa cement is a satisfactory adhesive if the finished prop is doped well.

CONSTRUCTION: Just as someone giving you directions concludes by saying: you can’t miss it, someone describing construction techniques says: It’s quite straight forward. Well, it is.

Fuselage: Because this is simple enough to build without detailed instructions some space can be given to techniques.

Use 1/16 or 3/32 sheet balsa for the sides. Cut to shape and make sure that 1/16 in slot is allowed to accommodate top decking under wing mount pylon. Add 1/16, 1/8 and 3/16 uprights and 3/16 doublers at the nose (grain vertical). Use contact adhesive for the doublers, as this remains flexible. Make up a former to take your escapement bearing plate. Hold the tail end of fuselage together, and glue the escapament former in position. Make temporary cardboard spacers to hold the fuselage sides to their correct curvature rearward of the escapement former. These spacers are made by cutting parallel slots 1/16 and 3/32 in wide, the correct distance apart, and then sliding them over fuselage sides.

Cover the fuselage top and bottom rearward of the escapement former with 1/16 balsa. The tailplane platform is 1/32 ply, drilled to take 1/8 in locating dowels from the tailplane, and scored underneath. The angle of this platform can be slightly greater than the dihedral angle of the tailplane. Fit the 1/32 ply fuselage end plate, drilled to take the 18 swg control rod extension, and with rectangle cut out to take the tailplug rubber hook.

Soak the nose of the fuselage in hot water, and bend it inwards towards the nose. Hold until dry, with rubber bands, possibly fitting a temporary spacer at this stage.

Meanwhile make the wing pylon by inserting the formers, which become more trapezium shaped towards the rear. It is best to glue these in place on the fuselage top decking, constructing the escapement inspection well at the same time, which entails cutting away some of the top decking. Allow this assembly to dry before bending the pylon sides in to the 'streamlined' shape.

Tie the fuselage sides together at the nose with the 1/8 balsa former. It may be necessary to insert a temporary spacer between the sides in order to maintain the curvature. Do not remove it until the 1/32 ply fuselage bottom has been fixed. Fit the noseblock and shape it roughly. (The skid, which is made from 1/8 spruce, is fitted after the fuselage is paper covered.)

The top of the cabin, which is really a hatch to cover the radio compartment, is built up of balsa with provision made for the switch. One tip here is to cut the two curved backbone pieces (i.e. do not bend them from straight) oversize to begin with. Use contact adhesive for fixing in place as, if properly used, no pins are needed while the glue dries.

Wing: Normal techniques are used for the general construction. It is made in two halves and held together by aluminium dowels through the paper tubes. No. 8 knitting needles (about 5/32 in) are fine. The engine mount is sandwiched between the two wing halves and is also 'threaded' onto the metal dowels.

Tailplane, fin and rudder: Make fin from 1/8 sheet balsa with tip from three laminations of 1/32 ply, arranged in such a way as to allow the 22 swg rudder bearing pivot to plug in. The rudder is in 1/8 balsa, capped top and bottom with 1/32 ply which has a hole for the pivot (hinge).

This rudder hinge system is very simple, clean and efficient. Make sure that the bottom of the rudder is kept clear of the tailplane retaining bands by slipping a short length of plastic sleeving on to the lower pivot first. The advantage of this system is that different rudders can be used (ie rudders of different size and throw) which are easily interchangeable on the flying field.

The tip plates on the tailplane are purely for structural reasons (as far as I know). They keep the sheet balsa from curling. Make the tailplane in two pieces. Fit centre fillets from 1/4 in TE section. Form the dihedral angle and then glue to either side of the fin. The gap between the tailplane halves, rearward of the fin, takes a piece of balsa in which the lower first wire is embedded. Either sandwich this wire swg 20-22 in between two pieces of 1/16, or slot a piece of 1/8 with a fine saw.

Engine mount: This is made from 1/8 ply. I prefer to make my own 'ply' from two laminates of 1/16, or three of 1/32 (this is 'thick' 1/32 and the extra glue brings it up to a 'thin' eighth!). The mount sandwiches between the wing halves, and threads on the metal dowels. A practical point here is to tack the mount to one wing half until you are sure that the thrust angles suit you (or, rather the model ), and finally glue it permanently when satisfied. I found that I made several different engine mounts for the prototype, using different thrust lines. That was when I found out that the slipstream should never play on the tailplane! The side thrust for beam-type mountings is cut into the mount itself. If radial mounted engines are to be used, I would suggest bolting (and perhaps eventually riveting) 20 g aluminium angle plates, one each side, to the engine mount, and drilling them to take the engine bolts. I even suggest sidewinding a radially mounted engine.

The engine could be cowled with a thin aluminium bonnet type cowl, bent oval in end elevation and bolted to the mount. A locating pin can be embedded in the top of the mount to engage in a hole in the top of the cowl. Cut your pattern from card and fit it by the trial-and-error method.

Conclusion: No one can claim this to be a contest model, although it can 'perform' in its own way, but it is a sweet little flier, that can be used to teach the neighbours to fly (if you have that kind of neighbour) and can give lots of trouble-free fun."

Pushover, Radio Modeller, April 1971.

Direct submission to Outerzone.

Supplementary file notes

Article pages, thanks to RFJ.

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