Sportster 2 (oz12230)

About this Plan

Sportster 2. Radio control sport model.

Quote: "Geoff Northmore takes a vintage free flight design, halves the size and adds micro R/C for calm weather flying fun

This model is a half-sized radio controlled version of a free flight design the Sportster (oz412) by Julius Unrath, the original being powered by a Loutrel Aero - whatever engine that was! I discovered Sportster in the February 1992 edition of Sam 35 Speaks where it was shown as a small three-view, and I assume it must be true vintage to grace those august pages.

There was only one useful accurate dimension - an enlarged wing rib drawn on a grid of stated half inch squares. This gave me the wing chord and so I was able to calculate a constant to apply to the smaller scale three-view. This in turn produced sensible measurements, with some judicious rounding off for span, length and other major dimensions. As I enlarge or reduce plans by hand (all part of the fun!) I arrived at a result which closely follows the construction of the original, even including some non-essential detail which enhances Sportster's vintage looks - I hope!

Some changes have been made for R/C; the tailplane spar was moved forward to enable a concealed elevator horn to be used, some sheeting aft of the firewall, laminated balsa for bamboo and a flat main wing centre section to replace the original central dihedral join. This latter modification helps prevent oil ingress and makes a good wing seat. The fuselage top directly aft of the cylinder head was a bit ambiguous in shape so I used common sense to get a practical result.

My own creation is powered by a Mills .75 diesel fitted with a Tarno carb. This works quite well. Any engine of similar capacity, throttled or not, should be okay. My radio consists of a Fleet Micro receiver, three Fleet micro servos which gives me closed loop for the rudder, a balsa push rod for the elevator and a wire push rod for the throttle. I originally used a 130 mAh battery, since changed to 200 mAh. Lightweight radio gear is required if you are to get a worthwhile performance from the suggested power. This means build in lightness, and do not beef it up!

Construction: This is a simple model using conventional materials. The following notes may assist those less familiar with the building technique required. Remember that careful and accurate construction is repaid by good looks and performance.

Fuselage: Commence by cutting out the ply formers (I used el cheapo salvaged from discarded fruit boxes in my local vegetable market). Cut and drill the engine bearers. Bend up the undercarriage legs from good quality wire and stitch them to the ply, leaving them dangling for the time being. Glue bearers to ply formers, recheck all are square and set aside.

Using hard balsa for longerons, pin them down and build the first side as per the plan. Do not put engine cowl pieces on as yet. When this is dry, cover with cling film and build your second side directly over the first. When dry, separate the sides and lightly sand prior to joining them up to the ply formers already set aside. You did remember to make a hole for the rear undercarriage leg to pass through, I'm sure. Do not bend and glue the upper longeron yet. Leave to set then complete the parallel portion of the fuselage followed by joining up the rear and filling in the remaining spacers. Now you can bend and glue the top longerons in place. Blame Mr Unrath's design if a joint goes ping, not me!

After this, bind the undercarriage legs together with soft wire and solder. I used a gas blow lamp on low hear. Lastly epoxy over your stitching to secure positively. I now installed my fuel tank (a converted 35mm film canister) and drilled holes in the firewall to suit fuel feed and throttle cable. I next decided on my receiver and servo positions and glued soft 1/8 balsa sheet rails inside the fuselage to take their respective bearers for the servos and 1/16 sheet balsa for the Rx. Make sure your control runs are direct and free of interference at the rear end. In addition I fitted a drinking straw tube, exiting the lower fuselage aft of the tail skid position for the Rx aerial.

Make up the engine cowling leaving a drain hole. My upper cheek pieces of thin ply are flexible enough to let me use a nut driver to secure the engine. The bolts are locked together by a short piece of wire soldered to their heads then epoxied to the underside of the bearers.

Make up your hatches. I used a ply tongue at one end and two small woodscrews at the other for the battery hatch whilst the upper hatch has a tongue forward and is then held firm by the wing when it's banded on. I left installing the tailskid and building the cabin until now, the former has a positive gift for impaling things whilst the latter is a bit prone to accidental damage in the early stages of building.

Incidentally, for glazing I used Evo-stick on the cabin frame only and pressed the glazing material into place whilst the adhesive was still very tacky, it seems to work well. I covered the joins with thin strips of masking tape which I later painted. Make the dowel holes and then sand ready for covering.

Wings: Cut out all the ribs but do not try to cut slots for diagonal braces. Make up the laminated tips (the method has been described too often to repeat again). Cut the spars, leading edge and trailing edge overlong, and don't forget to notch trailing edge for ribs. I used a thin, flat file edge on. Commence by building one of the outer wing panels. Pin down the two spars and trailing edge, flush at the root end long at the tip. Glue all those ribs not crossed by a diagonal brace in position. Next come the main diagonals. Now the remaining ribs are offered up, marked and slotted individually before being glued home. Now add the leading edge, dry fit the wing tip, cut the LE, spars and TE for a snug fit and then glue. It's easy to cut bits off, much harder to add a bit on! Finish by placing the small diagonals in place. Now build the opposite panel.

The centre section is best made by gluing one ply brace to each outer section. Position the root ribs accurately over the plan then glue and clamp the braces to the mainspar and sub-spar. You must also prop the wing tips up Sins each side for the correct dihedral. Add the LE, TE, the gussets and fill the gap in the spars. Cover the whole centre section chord with the sheet balsa. When ready, lift wing, complete the lower sheeting from front to rear. You can now sand smooth and blend in. Lastly, add a strip of thin ply over the TE to reinforce the area the wing bands pass over.

Tailplane, fin and rudder: These are made in the same manner as the wing. I used mylar hinges and made their slots before building. Use 1/16 balsa to
separate the fixed from moving surfaces whilst building. The elevator halves are joined by 18 swg wire to which was silver soldered a central brass horn. This joiner runs in plastic tubing which I fixed to the tailplane spar with small pieces of Solartex ironed over. Sand all to symmetrical section, add a horn for the rudder closed loop and the end is nigh!

Covering: I used heavyweight red tissue on the wing and forward fuselage and lightweight yellow for everything else. Dope, then fuel proof the vulnerable areas. Take care, I had a bit of a disaster doing the latter - they didn't mix and I had to either re-cover or cover with paint. I now have a black forward fuselage - have a guess as to what I settled for!

Flying: Ready for the fray, my Sportster weighs in at 19 oz or 500 grms. Check controls, wind the engine up to full bore, a firm hand launch straight ahead and she's away, no problem. The elevator is not particularly sensitive, the rudder is, so limit its movement or use low rates for starters. You might find you need some down thrust or permanent down trim. I opted for a heavier battery plus a bit of down, but do watch the weight. The model will perform the basic simple aerobatics. It drops a wing at the stall, but as yet, I haven't got it to spin. Landings are no problem but take offs, due to the sensitive rudder, need care.

Sportster looks well in the air and, built lighter with better wood selection and lower power, would make a good free flighter - you could easily correct my changes for 'wireless' as the SAM 35 humorists will have it. Have fun!"

Direct submission to Outerzone.

Quote: "Hi Mary and Steve, Thought you might like this update of an oldie, 'Sportster 2'. Though there are numerous 'Sportster' named models in OZ, this one wasn't amongst them. It was revived by Geoff Northmore for R/C and in reduced size having been derived from a S.A.M. 35 Speaks publication. It was published in Radio Modeller, May 1995 so its 25th anniversary is this month.
Hope you're well and staying safe, and thanks for the wonderful OZ.
Best wishes,"

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