The Pennyplane
By John Barker
| This article is intended for
those who have never built a duration type indoor plane, are convinced
they can't, but don't realize that with a little help... they CAN.
Where to start? My preference is the Limited Pennyplane (Most of my friends still call it a Novice Pennyplane, and so do I). Get a plan of a successful plane, e.g. Banks Pennyplane or copy a model of a club member. If you don't have a building board try a piece of plain ceiling tile. Buy sharp dressmaker pins. You need some good quality cardboard for templates. I bought a sheet of picture matt material from Michaels, a local general handicraft store that also frames pictures. Any light color is O.K. The Wing: Draw and cut out
the wing outline template. It will look something like this: Draw and cut out the wing rib
template. Mine look like this:
Assembling the wing: Put the usual wax paper etc., on the
board and pin the wing template securely down on top of it. Make
sure that the board is flat. Find some old medium soft 1/8 x 1/16
strip, cut off a zillion little blocks about 3/16 in. long. Use
these with pins to fasten the LE and TE against the template like this:
Stabilizer and Fin: The method is similar to the wing construction. Should be a breeze, except that you may be using thinner wood. Make
a couple of wing stands like this: You should cover your indoor duration plane with one of the modern plastic films. These vary from .000060 in. down to around .000023 in. One supplier quotes .000006 in (6 millionth's) which I find hard to believe. Any of these will be OK, but you may find the thicker stuff easier to handle. I buy mine from Wayne Trivin and Dick Obarski. It comes in 15 ft. rolls. Expect to waste quite a bit with your first attempts to use it. Covering with this type of material is totally different from Jap tissue or condenser paper. It has no inherent stiffness and is full of static. If you let go of a piece it will collapse into a heap like Handi-Wrap only worse. From a strength point of view its a bit like cellophane. Once you have something covered, it is surprisingly resistant to puncture, but if you get a tear started then watch out. Let's cover the Pennyplane wing and tail along with the pair of wing supports. The covering sequence will be: 1.
The wing and tail must be flat with no dihedral. You may read articles which describe different covering sequences, but start with my way and you will be less likely to get into a mess. But do experiment later. Now for details. Film
Frame: First construct a lightweight rectangular frame whose
inside edge is at least 6.5 x 20 in. However you fashion this
frame, it must be flat to start with and have stiff spanwise sides.
With the film mounted on the frame, you must be able to introduce slack by
pulling the sides together. This will let the covering conform to
the rib contour. One way to achieve this is to have bendable end
pieces made from aluminum wire or possibly strips cut from a soda
can. Another rather more complicated method which works great is my
way. This uses a screw adjustable gizmo that looks like this: Mount The Film: Use a slightly damp cloth to wipe off your work board. When dry, lay out the roll of film down at one end and start to unroll it. Don't worry if it starts to cling together, but be very careful NOT to start a tear. With a brand new razor blade cut off a piece at least 1 inch larger all around than your frame. Go slowly because the razor may snag in the film and tear it. Roll up the remaining film and stow it. Now pick up your cut pieces and wad it up (you heard) real tight. Now spread it out again as flat as you can on your board. It will have fine wrinkles and less static. Fasten it down to the board with about ten little bits of tape and if possible tighten it a little at the same time. Prepare Adhesive: Use any type contact adhesive, including rubber cement, that can be thinned to a rather water consistency. I use Elmers SAF-T Contact Cement. This is water soluble. Check building supplies and craft shops. Carefully lay the frame, topside down, on the film. Using a small brush, apply the thinned adhesive so that it wicks between the frame and the film. Let it dry. Press down on the frame to make sure it adheres to the film. With a razor, cut loose the bits of tape and slowly lift the frame and support it right side up. Covering:
Covering is done with all surfaces flat just as they came off the building
board. Do NOT sand cute airfoil type tapers into the LE or TE.
It will not improve the aerodynamics but it will weaken the members
considerably. Lets start with the sing. Lay it upside down on
the film. Introduce slack by bending the frame wire ends, or by
cranking the handle of my frame. You need just enough clack so that
both LE and TE sit down on the film thus: Dihedral:
Fasten the center wing to the board on wax paper with pins angled across
the LE and TE. Slice almost thru the LE and TE at an angle just
outboard of the dihedral rib so that the rib remains attached to the inner
wing. Lift the wing tip to crack the spars and prop up on a wing
support (you did make the supports I hope). Pin the base of the
support to the board and raise the slider to give the correct dihedral
plush washout if the plan calls for it. Add pins if necessary to
hold the dihedral joints together. Repeat for the other tip and
check that the span does not exceed 18.00 in. Glue the joints with
CA and remove the supports. Make
a jig to set up the wing attach tubes to the motor stick like this: Motor Stick Discussion: The prime function of the motor stick is to support the wing, tail boom, prop and rubber motor. Unfortunately it does more than that. Lets consider what it does when you wind up your motor:
For early flying it is easier to trim if the above effects are minimized by keeping the motor stick stiff. Bear in mind that a stick of lightweight wood (up to 7 lb./cu.ft.) and generous proportions will be much stiffer than a thinner stick of heavier stock. Wing
and Tail Mount Tubes: These tubes are made by rolling jap tissue
around a mandrel and impregnating with cement . Start with the 1/16
in. inside diameter tubes which carry the wing pylon sticks. Cut a
strip of jap tissue about 3/4 in. wide and several inches long. Use
the shank end of an undamaged 1/16 drill as the mandrel. Rub the
shank end of a candle stub and remove any residue with your fingers.
Thin some Ambroid (or similar) about 50/50 with acetone. Lay the
tissue flat on the work board and proceed as in the diagrams. At (1)
paint the mandrel with the cement. (2) roll back to pick up the
tissue. (3) roll forward to start the first layer - - use of the
brush will help eliminate any slack - - none allowed here. Continue
rolling and adding cement for several turns. Cut off the spare
tissue and twirl between your fingers to lay the end flat and tighten the
coils. (4) immediately pull the tube off the mandrel with your
finger nails and let it drop on the work board. You will ruin a few
until you find it is easy. The trick is getting step (3) OK.
Make several spares and when dry store them on snug fitting rounded balsa
sticks. With an 1/8 drill and tissue about 1 inch wide make the tail
mount tube, plus a spare or two. Now for the tail
boom. Fasten it down on the building board with pins and
blocks. Set the horizontal stabilizer in place and cement it.
Likewise for the vertical. If you have a droop down tail, block it
up to Wing
Mount Assembly: Sand the lower 1/2 inch or so of the wing pylon
sticks for a stiff fit in the motor stick tubes. Insert them kin the
tubes flush with the bottom. Pin the motor stick to the work board
with the pylon sticks vertical. Support the wing using the props you
used earlier so that the wing sits at the correct height between the pylon
sticks. If all is well the sticks should rest lightly against the LE
and TE. At this time you should include any wing twist called for,
usually some left wing wash in (TE down). I use less than 1/8 inch. Before making the prop and flying hints... let's have some fun. Add some ballast to the nose to make the model balance at about 65% of the wing chord. Set the wing at a slight positive incidence relative to the tail. Set the right tail tip about 1/4 to 1/2 inch down. Test glide in your best clear space indoor (air off). Adjust wing setting until almost stalling. It should turn slowly left and amaze your friends by its lack of speed. Maybe not. It reminds me of a morning when I was giving an indoor flying demo to a bunch of about 80 sixth graders. My Pennyplane was steadily climbing to the gymnasium roof accompanied by ooooo's and ahhhh's from all except one boy who asked "Sir, can you make it fly any faster?" Propeller
Basics: Your plan will give you a good idea of the propeller
construction. It will have thin molded light sheet blades attached
to a single stiff spar. Molding
the Blade Twist: Propeller
Assembly: For the spar cut and sand a piece of medium hard 3/32
in. or 1/8 in. to the shape shown on the plan. Make a tiny hole in
the center and insert the prop shaft. Bend and cement like this: Balancing: Clean off any ballast from the front of your motor stick and insert the propeller. Make certain it revolves freely. If one blade appears much heaver than the other do some careful sanding. Don't worry too much about static balance. Go fly it. If it wobbles it means that the blades are set at or flaring to unequal angles. Check and tweak as necessary. Suffice to say that usually the wobble is affected more by unequal blade angles than by static balance. Rubber Band Flying: Weight the complete model without rubber. It must be at least 3.1 grams. For power TAN 2 is the best. However, it only comes in widths suitable for outdoor flying, 1/4 in. etc. If you model is close to the nominal 3.1 grams, you will need some cut to .075, .080 and .085 in. for starters. If your model is heavier, the sizes will have to be bigger. The way to get rubber of various odd sizes is to call Indoor Model Supply or get someone who has a stripper to cut it from your 1/4 in. strip. Please DO NOT ask a friend to do this on a contest day. Whatever you choose, make a small loop, say 4 in., lube it (I use STP Son of a Gun protectant). Break it in, then stretch wind it until it breaks. Calculate the breaking turns/inch. Make an 18 inch loop of .080 and wind it to about 70% of breaking and then back down to 50%. With these turns trim the model close to the stall with wing post settings and the desired left turn with stabilizer tilt right side down. If it climbs at this 50% (backed off) turns, the motor is probably too thick or too short. Similarly if it sinks, the motor is too thin or long. Either way, just wind up some more until you get a decent flight. Make notes for each flight... trim settings, motor size, weight and flight time. Count the number of turns left at the end of each flight and calculate prop revs... (Turns wound - turns left) x 60 / flight time secs. = RPM. Set a target flight duration for your site (be realistic). Calculate a motor length assuming you use 90% of breaking turns... Length (inches) = RPM x Duration Minutes / 90.9 x break turns per inch) Bear
in mind that a short motor will not run long enough, but an extra long one
will be too heavy. You need to do a lot of flying to get the motor
just right for one flying site. |