About this Plan
Cherokee. Free flight rubber model.
Quote: "Vintage Lightweights, Part 2. Andrew Longhurst concludes his discourse on this popular class with a close look at one of his favourite designs.
The Cherokee is a very special plane in that it always flies! It is the easiest to fly rubber model ever designed. When I first got this plan, it was built almost as an afterthought amongst a myriad other projects. At any rate it was a low priority; mainly because of its bizarre (now I call it pleasantly quirky) appearance. However, come the clay to go and play on the flying field, its desire to fly - and indeed fly away - proved stunning. It just wouldn't land without a D/T, so be warned.
Another thing is that its ratio of build time to fun, is quite unequalled. So, in its funny way, it is an ultimate, a peak of aeromodelling design. How can you resist twelve hours and the cost of a few pence to build one? When you have done that, it will produce a performance of two and a half minutes or so, which is enough to get you into the fly-off of most Mini Vintage competitions. Once in the fly-off, it's anybody's thermal!
BUILDING: THE FUSELAGE: There is no need to use heavy wood but don't use pith either. Nice straight-grained wood, with each 36 inch length of 3/32 square weighing one gram. It is much better to strip it off a sheet, rather than use shop bought strip. Firstly, it will be lighter and secondly it will result in a straight fuselage, rather than one which is banana-shaped. Instead of building the two sides first, this model is better built using a top and a bottom, owing to the 'Flounder' fuselage styling. The wing mountings are the only fiddly bit. Bend the four 18 swg poles and glue them into place with spots of balsa cement. Get the right angle tops aligned nicely before you bind them in with cotton thread, and cover with more cement. When you fit the noseblock, try to arrange for 1/16 each of down-thrust and right side thrust.
WING: Because this wing is of a sparless design, the lead-ing edge becomes aerodynamically critical. If you buy a pre-formed 1/4 in LE from your model shop, the wing will work. Home carved ones sometimes do not, usually because they are too pointed. Again, a medium weight for all wing wood will do - enormous strength is not required, as the biggest stress it will suffer is from tissue shrinkage, rather than flight loads. Dihedral breaks can be butt jointed with Cyano or Epoxy. With these glues, there is no need to reinforce the joint in any way at all.
TAIL: Lighter wood can be used for the fin and tailplane, as we are not going to subject it to the full tissue shrinkage loads. For transportation, the fin can be provided with two cocktail sticks at the bottom, which poke into two rolled paper tubes glued to the tail leading and trailing edges. If these project down by about 3-4 mm, they can be made to locate the tail in the fuselage to provide positive location (see sketch). This is extremely important, as all light-weight models are very sensitive to fin adjustment under power and if your fin is on the move, wreckage is guaranteed.
THE PROPELLER: Providing the model is of reasonable weight and has the right warps, the performance will depend on the prop. Recent experiments with a 12 inch Keil Kraft plastic prop have shown that a hand carved balsa prop will outperform the plastic one by a factor of 50% (1.5). Therefore, it pays to get it right and the main thing to remember is that the correct twist in the blade, and therefore the correct pitch, is much more important than ultra thin blades, or undercam-bered blades, or surface finish. In fact, thin blades warp, as do blades which have tissue covering. Pronounced undercamber is useless, as it just absorbs power. Therefore, go for constant helical (twist) pitch, checking with a 45 degree segment from a child's protractor. The pitch you are looking for is 40 degrees at half radius and 23 degrees at the tip, give or take no more than 5 degrees. Keep the undercamber low (flat bottom is perfectly all right) and the blade thickness to between 2-3 mm..."
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