Spearhead Jr (oz16014)

About this Plan

Spearhead Jr. Radio control sport model, for Mills .75 or similar engine.

Quote: "For the past few years I've been flying the SAM/Texaco RC events. I'm enjoying new friendships at new flying sites with old design models which still give me the thrill of flight. As what I've built with my own hands circles higher and higher in a lofty thermal, I have a transmitter ready to safely terminate the fun at will - super nice RC fun!

I was looking through a bunch of old black and white snapshots recently and, from about 1943, I found my favourite model - a Spearhead Junior (oz220) by Jerry Stollof. Mine had an Ohlsson .23 and it flew and flew through the war years. It was finally lost in a booming thermal about 1945, happy memories. Also, I'd recently broken-in on my test stand a brand new engine - the totally modern redesign of the famous Mills .75cc diesel now made by Irvine. Suddenly '2 plus 2' made a perfect '10'. The light flashed bright - I'd build a new Spearhead Jr, the neat new Mills .75 would power it, I'd use two of the teeny-tiny new HS-50 Hitec feather servos and the matching 555 miniature receiver would drive from my X-347 With 300 square inches of wing area and displacement not over .049 and with a 17oz flying weight, I could SAM / Texaco contest fly the model in the Open 1/2A Texaco event with a very competitive 8oz wing loading - and have bunches of fun too!

My new Spearhead Jr runs 2 to 3 minutes with an 8 x 4 turning near 8000 rpm. Climbs are up to 500 feet to maybe 900 feet of altitude. The model wobbles visually in / near lift and full up elevator trim with a bit of left rudder trim allows it to nicely circle in Florida thermals. Without lift, flight times are near 8 - 10 minutes with plenty of relaxed stress-free air-time; nice. With thermal lift, you can fly to your heart's desire. Airborne battery drain from the 270mAh pack is practically nil - when my X-347 beeps, it's time to land. Real pleasure!

Let's clear the bench, grab the box of mostly scrap wood, spread the plans for our Spearhead Jr - and start building.

Fuselage: building begins with a flat horizontal crutch, built directly over the top view of the fuselage. The 3/16 x 3/8 side pieces of the crutch should be hard / strong / firm as they become the bulk of the fuselage strength. Cross pieces can be soft / pulpy and I used rock hard maple at the forward end of the crutch to hold the engine mounts.

Bulkhead F1 is from 1/8 ply and the landing gear wire is 3/32 bent to shape and sandwiched / glued to the front of Fl with more plywood. The formers above and below the crutch are built directly over the plan, then removed and glued to their respective positions. Two F10 pieces from 3/16 form the wing saddle and F11 and F12 form the fuselage backbone.

I suggest you build as per plans - don't add any more to the structure as the original from the 1940s was considerably heavier than this modern 17 oz version - but do select good, firm, straight balsa. If it's not in your scrap box, a trip to the hobby shop is worthwhile.

A sheet of 1/64 ply is cut to pattern shape and glued forward of F2 to form the rounded cowl. Add 3/16 sheet below the cowl to complete the front of the fuselage, be sure to leave a bottom slot to allow for fuel drainage.

I added short lengths of strengthening carbon fibre yarn to the rear side of F2 - you might want to add a small strengthening gusset between the forward internal end of the 1/8 wing hold down dowel and the forward side of F2. I decided that a sheet of 1/32 ply would be fitted between F2 and F4 (from the crutch up to the wing saddle) and 4 small plastic screws would hold it in place. The on/off switch mounts to this ply as well as the charging receptacle. Removal of the ply would easily give access to the servos / receiver / NiCad pack and the pull-pull (closed loop) cables too. Small pieces of 1/8 ply are tapped and glued in place to hold the 4 plastic screws..."

Spearhead Junior from RCM&E May 1999.

Direct submission to Outerzone.

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