Meet the Snowflake
My new plane with flight surfaces made from 9″ foam plates. If it looks like a Sierra Nevada Special, that is because that is what it is, but with three wing panels instead of two. Best way to get the span and dihedral without a complicated V bevel joint at the wing saddle.
The limitation was making parts from the 6″ diameter flat disk in the center of the 9″ foam plate. The foam wing panels are 3 1/16″ x 5 3/16″. Dihedral gore is 1/32″ deep at 1 1/4″ from leading edge. This produces 1 1/2″ dihedral at the tip. Maximum camber is 1/4″, also at 1 1/4″, no incidence. Camber is formed by creasing along six lines spaced 1/4″ apart. Tailplane is 5 3/4″ span, 1 3/4″ tip chord, 2 1/4″ root chord. Fin is 2 3/8″ span, 1 1/2″ tip chord and 2″ root chord, with creased 1/4″ wide rudder panel. The 47.4 square inch wing area is asking for a 7″ to 10″ diameter prop. I used a 7″ Peck-Polymers plastic prop, Midwest bearing and shaft. Standard right handed pigtail rear hook made from a straight pin. Stick is 1/8″ x 1/4″ x 16″ with 1/8″ square nose block. Tailplane taper is 3/16″ in 2 1/4″. It weighs 10.3 grams empty. Motor shown is a 14″ loop of 1/8″, which is good for test flights. This will get it up in the air and run out of turns high enough up that turning circle and descent rate can be checked. Some right thrust will be required under power, right rudder for right descent. Test flights will tell me what motor cross section to use when I go to a double length, thinner motor for maximum turns and longest flight.
This would be a good plane for the forthcoming Foam Plate Airplane Contest.
Snowflake got 2 minutes on its first outing. Like its namesake, it comes down slowly. Here is a 2 minute flight video:
There was frost on the ground. At first my fingers were burning from the cold, but they soon went numb. It was difficult to get the O-ring at the end of the tightly wound, lubricated rubber motor onto the wire hook without being able to feel anything. I lost it once, had to make up lost turns with finger turns. Two minutes is about the best I could get on this field. I might get a few more turns into the motor. It had few left when I picked it up. I put in 2,400 out of an estimated 2,690 maximum turns, 89%. I used a 22 1/4″ loop of 0.045″ x 0.086″. If you don’t want to strip rubber, stock 3/32″ is the closest. That will get a good climb without having to wind to 90% of capacity. A longer motor might get more time.
The seagulls were on the ground. When they saw the plane going up, they made a run for the expected thermal. A bunch of them came right toward the plane when it turned into the group. They scattered to both sides. They continued searching for lift, flying around the plane, but eventually settled back to the ground. I’ve had the same thing happen with hawks in another location.
There is no plan. It isn’t necessary to have a plan, as everything is rectangular or trapezoidal. All the necessary dimensions are given above. Anyone who is in a hurry can build one from that information. The build is essentially similar to the Sierra Nevada Special.
Just got back from some photo flying. Got a good one of it flying overhead.
I made a second one. Couldn’t get it to fly right in the descent. More right rudder, twist wings, nothing. Got it to go straight, but not turn. Not something you want when flying from a small park. Finally I tried twisting the stick to tilt the tailplane. Made it worse. So I tilted it the other way. Now it flys OK. The tail must have negative lift on it. A tilted tailplane produces a side force that turns the plane.
Build Your Own Snowflake
This complete, illustrated tutorial will guide you step by step through the process of building your own Snowflake. This may be used by individuals or groups like scouts, school classes, senior centers or recreation programs. There are a lot of steps, but none of them are especially difficult. Be patient and take your time.
This picture shows the parts you will make. There are only 18 parts in this airplane and 6 of them are preformed. The plastic prop, prop bearing/hanger and steel prop shaft will be bought. The rubber motor is cut from a length of strip and tied into a loop. The motor is looped around a hook on the back of the prop shaft and a wire hook attached to the back end of the fuselage stick. The wing mounts to a saddle made of balsa parts and held to the stick by two dental rubber bands. The flight surfaces are cut from foam plates. The wing is creased to form a cambered airfoil shape. This curve improves both the aerodynamics and the strength of the wing compared with a flat plate.
You can use the bearing and shaft from a 6″ propeller assembly, as I did, or you can order the Prop Hanger and Shaft separately. You can make your own shaft from 0.039″ steel wire, which matches the diameter of the wire on the 6″ propeller assembly.
The stock rubber motor is 52 1/2″ of 3/32″ rubber strip. This will make a 26″ loop, with an estimated maximum turn count of around 3,000 turns.
While you are there, get a winder.
You can put more turns into a motor with less chance of breaking if you use motor lube. You can make your own lube by mixing together equal volumes of green soap and glycerin. The green soap is slippery and the glycerin is a moisturizer.
It is easier to get the tightly wound motor onto the hook if it has an O-Ring at the back end.
Heavy 3/16″ dental bands are used to hold the wing on.
You will need several sizes of balsa wood, which you can find at local hobby, hardware, art and craft stores, or you can order from Sig or Midwest. Experienced builders can use lighter wood. Inexperienced builders may be safer with harder wood, especially for the fuselage.
The fuselage stick is a 16″ length of 1/8″ x 1/4″. (If you make it 15″, you can get two fuselages and two rib pieces from the standard 36″ stick. This would work well for a group build.)
The nose block is 1/2″ of 1/8″ square. This should be medium to hard wood.
The rib block is a 3″ length of 1/8″ x 1/4″. It can be light wood, or it can come from the same stock as the fuselage stick.
The hold down stick is 3 5/8″ of 1/16″ x 1/8″. It should be medium to hard wood.
The wing saddle face plates are made from 1/16″ x 3″ sheet balsa. Each plane will need about a 1 1/4″ length. The grain goes vertical.
Each plane needs a rear motor hook. These are easy to make from a straight sewing pin, 0.020″ steel wire or you can buy them.
Each plane will require four foam plates. We have been calling these 9″ plates, although the package says they are 8 7/8″ diameter. Make sure there is at least a 6″ diameter flat circle in the center. Some 8 7/8″ plates have only 5 1/2″ of flat area. You can use 1 mm Depron foam if you prefer, but planes made for the Foam Plate Airplane Contest must be made from plates.
Workplace and tools
You will need a well lighted, flat surface to work on. You may want to protect the surface with a work board or several layers of newspaper. Cutting is done on a piece of corrugated box cardboard, to prevent cutting into the surface of the table.
I keep my tools in a box, but I get tired of pawing through a jumble of tools looking for just the one I want. For a given project, I will need only certain ones. It is convenient to lay them out in plain sight across the back of my work table in roughly the order in which they will be needed.
The foam parts are made by cutting around cardboard patterns or templates. We must make the templates first.
Put marks at 1 1/4″ and 3 1/16″ from the edge on each perpendicular. (Note that I made a mistake on the one along the ruler, I marked at 1 3/4″. That had to be corrected. Both mark locations must be measured from the same edge.)
One edge must be cut on a curve so the cambered wings will fit together with a dihedral angle between them. (This will become clear later when you see how the wing panels fit together.) At the 1 1/4″ mark along one edge, put a mark 1/32″ inside the edge. This will be the high point of the curve.
Draw a smooth curve through the three points; the leading edge, the high point and the trailing edge. The tangent to the curve at the high point should be parallel with the base line. You may need to move the curve and do it in two arcs.
Use the curve to guide the pointed knife in cutting the curved edge. Make several shallow cuts, rather than trying to cut through in one pass. It is easier to make precise cuts with gentle pressure. The blade is kept tangent to the curve, so it will not cut into the curve. You will need to move the curve and make the cut in two arcs.
Along the straight edge, make 6 marks at 1/4″ intervals, starting from the leading edge. These mark the positions of the camber creases. (I now suggest making a seventh crease, 1/2″ beyond the sixth.)
Mark a point halfway along the leading and trailing edges, 2 19/32″ from the ends. These mark the center line of the center wing panel. This template will be used to make all three wing panels. Note that I have written a label on the template so it can be identified later.
Use the knife and the curve to cut along the airfoil. Make several shallow cuts, rather than trying to cut through in one pass. It is easier to make precise cuts with gentle pressure. The blade is kept tangent to the curve, so it will not cut into the curve. You will need to move the curve and make the cut in two arcs.
These are the four templates you will need to cut out the sheet parts. Make the fin and tailplane according to the dimensions given. Mark the centerline on the tailplane and punch pin holes 3/32″ in from each end on the centerline. Mark a rudder hinge line 1/4″ forward of the fin trailing edge. It starts 3/8″ up from the base. Poke pin holes 3/32″ down from the top of the hinge line and at the bottom corner of the rudder.
Cut Out the Foam parts
Place a plate over the cutting board, locate the tailplane template in the flat area of the plate and cut around the template. The foam is soft, little pressure is required to cut through it. Keep the face of the blade flat on the edge of the template and draw the knife slowly, just like drawing a line with a pencil.
Place the fin template on the remaining flat area of the plate with its trailing edge aligned along the cut of the tailplane trailing edge. Cut around the fin template. You get two parts from one plate.
Place the fin on a block with the holes marking the rudder hinge line aligned over the edge. Press the rudder down, forming a crease on the rudder hinge line. The rudder should be bent toward the right side of the fin.
Now place the wing panel template in the flat part of another plate and cut around it, as before. This will be a wing tip, with one straight edge and one curved edge. The straight edge will be the outer tip, the curved edge will be where this tip panel joins the center wing panel. This curve must be cut very precisely. We must be careful to keep track of the orientation of these panels; leading edge/trailing edge, upper surface/lower surface. They must be properly oriented when they go together to form the wing.
The crease marks are on the under surface of the wing. That and the position of the curved edge tells us that this is going to be the right wing tip. Put an ‘R’ in the corner near the leading edge and the curved end.
Flip the template over, place it in the flat part of another plate and repeat the process of cutting out a wing panel. Pay attention to which edge is the leading edge. I marked it on the plate before cutting it out.
Flip the pattern over, aligning the leading and trailing edges of the template with the leading and trailing edges of the wing panel, and aligning the centerline holes in the template with the centerline marks on the plate, and cut along the curved end. Be sure the curves are oriented in the same way, so the leading edges of both ends correspond.
That completes the cutting out of the foam parts.
forming the wing camber and joining the panels
Another way to do this is to place the wing panel on a piece of paper, align the edge of a block or ruler with the corresponding crease marks, and pull the paper up. You must press down firmly on the block to keep the paper from slipping out.
If you are a hero, you can spread glue on the edge of one panel, bring the leading and trailing edges of adjacent panels together, bring the high points together and hold the panels in place as the glue dries. Watching glue dry is not one of the more exciting things to do in life.
Carefully align all the intermediate panels so their edges are firmly butted against each other. Put a block behind the other panel, push the tape across the joint and press it firmly in place on the other panel.
Make the fuselage
I had already cut all the sticks to length. Measure, mark and cut in the miter box with the razor saw. If you are planing to do a group build, you might want to cut parts in bulk.
Mark the heavy end. This will be the front end where the propeller goes. The tail goes on the back end. The stick has a front end and a back end. It has a top and a bottom. The wing and fin go on top, the propeller bearing, tailplane and rear motor hook go on the bottom.
Cut the 3/16″ x 2 1/4″ triangle off. This is waste. Cut into the waste, rather than into the stick. This cut will be finished with sandpaper, so leaving a little waste is better than cutting into the final part.
Place the stick on top of a block with the surface to be sanded extended a little over the edge. Back it up with a piece of hard 1/8″ x 1/2″ or similar. Place the edge of the sanding block flat on the table and gently sand down to the line. The sides of the stick are parallel with the table top, the sandpaper is perpendicular to the table top, so the sanded surface will be perpendicular to the side surfaces of the stick.
GLUE TAILPLANE TO STICK
make the wing saddle
With a block of wood, align the flat edge of the rib template with the end of the 3″ wide 1/16″ balsa sheet. Cut along the curved top edge of the rib. Make several shallow cuts, rather than trying to cut through in one pass. It is easier to make precise cuts with gentle pressure. The blade is kept tangent to the template curve, so it will not cut into the template.
Press the bottom edge of the side plate against a block of wood, place a 1/4″ stick and a 1/16″ stick on top of it and press them against the block. Draw a pencil line on the side plate along the 1/16″ stick.
Place the wing saddle on its side on a block, with the scrap spacer still in the gap, and use the vertical sanding block to sand the upper surface of the rib smooth along the entire length and side to side.
glue the fin to the stick
Glue the wing to the saddle
Place the saddle on the tabletop. Align the pinholes on the wing centerline with the center of the rib and press the wing down on the rib. Make sure the leading edge of the wing corresponds to the leading edge of the rib.
MAKE THE REAR MOTOR HOOK and glue it to the fuselage
The diagonal cutter leaves a sharp chisel end on the wire. This can cut into your rubber motor, causing it to break when it is wound up. File the sharp end round. Start by filing the end down flat, then twirl the wire between your fingers as you file across the end at different angles.
Motors can twist off a hook. The motor will have a clockwise twist in it. With a right handed pig tail in the hook, the motor will twist itself onto the hook. Put the end of the pin in the round nose pliers at an angle where it has about a 3/32″ diameter.
Twist the wire around the pliers to form a right handed helix. Roll up the fingers of your right hand and point your thumb out. A right handed helix goes in the direction the thumb points when going along it in the direction the fingers point.
Half of the remaining straight piece is bent down at a right angle. These last three straight pieces lie in the same plane. Since this is short and hard to grip, and has a sharp end, use a block of wood to bend it.
Make the propeller assembly
We want a 7″ prop with shaft and hanger. These can not be bought as a complete assembly, so we must make it up. I replaced a 6″ prop from an assembled unit with the 7″ prop.
Many people scrape the back of the heavy prop blade with a razor blade. This doesn’t work very well, because the straight edge of the razor does not fit the compound curvature of the prop blade. Also, scraping from side to side across the blade risks going over the edge and cutting your finger. I use a kitchen paring knife with a curved edge near the tip.
If we grab the end of the shaft with the pliers and bend it over, we will likely put a curve in the shaft. That can jam the prop and prevent it from freewheeling. To avoid that, we grip the shaft tightly and bend the end over by pressing it against a hard object.
Insert the front end of the stick into the open box end of the prop hanger. Put the bearing and shaft on the underside of the stick, so the prop hook aligns with the rear motor hook. This should be a snug fit.
Slide the stick into the slot under the wing saddle. Make sure the front, more curved, part of the wing goes forward. A good place to start is with the leading edge about 3 1/2″ from the front end of the stick. The wing slides along the stick to adjust balance.
Make the motor
We will use two motors, one for flight testing and trimming and another for flying. (If the flight motor is more than twice the distance between the hooks, you can double it up for flight testing.) The flight test motor is shorter and thicker than the motor used for duration. We need a motor that will quickly get the plane up just high enough to check climb and turn, then run out of turns so we can check descent. We want a steep, steady climb, circling right, a right cruise and a gradual descent, turning right, with the prop freewheeling. The test motor must be long enough to go slack when it is unwound, so the prop can freewheel. It will not be fully wound, it is wound just enough to get high enough for the test glide. The hooks are 13″ apart, so we start with a 14″ loop of 1/8″ rubber.
Experience with similar planes suggested that a 22 1/4″ loop of 0.045″ x 0.086″ rubber would work well for this plane and that is what I used to get the two minute flights. Making this motor required using a rubber stripper. If you don’t have a rubber stripper, the nearest standard size is 3/32″ (0.09375″ wide) strip. Make the loop at least 26″ long. The dimensions of the motor determine how many turns you can put into the motor and thus how long the motor will run.
Hold the motor about 2″ from the ends and dip both ends into a solution of knot lube. Knot lube is two volumes of motor lube mixed with one volume of water. If you tie knots with straight lube, you will be able to see the knots untie themselves. Knot lube is not so slippery, just enough to allow us to tie tight knots in the rubber without tearing it.
Motors must be protected from dirt and light. If you have several motors, they must be identified. Some people store their motors in paper envelopes with the data written on the envelope. You can put your motors in plastic baggies with the data written on a paper tag. These should be kept in a light tight box.
fly the snowflake
We start in the shop by balancing the plane with a motor on it.
Put the knot end of the motor on the rear motor hook. With a short motor that is going to be finger wound, there is no need for an O-ring. We put the knot on the rear hook because if we put it on the prop hook, the tag ends would rub on the stick at every revolution, slowing the prop.
With the longer motor, we can double it. Put the O-ring on the rear hook, pull the two strands through the prop hook and loop the other end onto the rear hook.
We can bend the prop hanger a bit to the right. Motor torque will roll the plane to its left, causing a left turn. At high torque, this will become a left power dive. To avoid that, we make the plane fly right, against torque.
Gather the things you will need on the field. This could include a winder, motors, stopwatch, video camera and extra batteries, foot stooge, flight log and pen. For quick finger wound tests, the motor might be all you need. You could include a baggie of lube. These can be conveniently carried in a belt pack. Don’t forget the retrieval pole.
If you wind with the motor hanging down slack, it will wrap around the hook and the shaft, forming a knot and jamming against the stick. To avoid this, pull the motor back in line with the shaft and put a few turns into it. Use your index finger to turn the prop slowly in a clockwise direction. The ramp on the front of the prop will engage the shaft and turn it. Notice how the rubber twists.
Now you can shift your grip closer to the prop and begin winding faster. Count turns in groups of ten, like this: One, two, three, four, five, six, seven, eight, nine, ONE, one, two, three, four, five, six, seven, eight, nine, TWO, one, two, three, four, five, six, seven, eight, nine, THREE, one, two, three, four, five, six, seven, eight, nine, FOUR, one, two, three, four, five, six, seven, eight, nine, FIVE, one, two, three, four, five, six, seven, eight, nine, SIX, one, two, three, four, five, six, seven, eight, nine, SEVEN, one, two, three, four, five, six, seven, eight, nine, EIGHT, one, two, three, four, five, six, seven, eight, nine, NINE, one, two, three, four, five, six, seven, eight, nine, TEN. Ten tens is a hundred. When you get to TWENTY, you will have two hundred turns in the motor. This is a good place for the first test flight.
Hold the plane under the wing with one hand and hold the tip of the prop with the other. Hold it at a comfortable height, shoulder high or over your head. Have the wings level and the nose pointed slightly up. Rear back, swing forward and step forward, pushing the plane forward and releasing the plane and the prop at the same time. Do not throw it hard. Push it forward at close to flying speed. The propeller will immediately take over and pull the plane through the air. After a couple launches, you will see the speed and attitude the plane wants to go right after release. Try to match that condition when you launch it.
Test flights are made with a 14″ loop of 1/8″ rubber to check climb, turn and descent. Short flights are made to allow fine tuning before attempting a longer flight that might get into trouble. The goal of the trimming process is to get the plane to make a steep, steady climb, long cruise and slow descent, turning uniformly to the right. This involves adjusting the wing position on the stick to get proper longitudinal balance, right rudder for turn in the descent and right thrust to get proper right circle in the climb, in that order.
Your first flight might look something like this.
Observe carefully and describe what the plane is doing. It makes a steep, slow climb, a brief stall, turning left, levels off and flys steadily, straightening out, then begins a gradual descent, turning right. This tells us that the wing is in the right place, because the cruise and the descent are good, and the rudder is turning the plane right in descent, but it is turning left in the powered climb. The correction is to bend the prop hanger to the right so the prop will pull the plane into a right turn under power.
With the plane properly trimmed, we can try a longer motor for more turns and a longer flight. I used a 22 1/4″ loop of 0.045″ x 0.086″ rubber. Winding a long motor requires a way to stretch it out. If you tried to finger wind it, the motor would roll itself into a ball around the prop shaft. If you have a friend to fly with you, your friend can hold the prop as you stretch the motor and wind it. If you fly by yourself, you will need a stooge to hold the prop as you wind the motor.
The foot stooge is placed on the ground and held securely in place with a foot. The propeller is engaged in the hooks, the motor is looped into the prop hook, the O-ring is placed on the winder hook and the motor is stretched as much as possible. Turns are put in until the motor begins to tighten, then the winder is lowered toward the tail until the desired number of turns are in the motor. Then the O-ring is taken off the winder hook, the winder put in a pocket, the stick is held near the rear motor hook and the O-ring placed on the hook. The prop is disengaged from the stooge, the stooge is put in another pocket and we are ready to launch.
This is the first two minute flight of the Snowflake.