Giant Ski

While reading through an old issue of Model Airplane News, I came across an article about float-flying off water. It started me thinking about how much fun it would be to fly off snow with skis. First on my agenda was to pick some suitable subjects to modify for ski installation. That was the easy part, because my Stinson L-5 Sentinel and Cessna L-19 Bird Dog were just begging to get out of winter storage and be drafted back into service. They are both 1/4-scale tail-draggers and are very suitable for trudging through snow. After working out a few logistics, I cleared a spot on the drafting table and got started. My intent was to come up with a ski design that was simple, easy to build and would use up some of that “leftover” material we all seem to have lying around the shop.

SKI DESIGN

To get a better feel for the design requirements for skis, I took a quick look through some full-size aviation magazines for possible articles on winter flying. I came across an issue of the EAA magazine Sport Aviation. This particular issue had a short article about winter flying with skis. The article contained some neat color photos of two Piper J-3 Cubs on a snow-covered runway at a grass field. The J-3 is probably the most common aircraft that’s outfitted with different brands of ski; and this supplied me with a few ideas on designing a simple, yet effective, set of skis for my own 1/4-scale models.

After measuring the skis and fuselages of the Cubs in the photos, I calculated their comparative lengths, and used these figures to plan the dimensions of my skis. I then generated a rough draft of the full-size drawings for the 1/4-scale skis following the tried-and-true “That looks-about-right” formula (here’s to good old eyeball engineering!). The length of the skis would be approximately 50 percent of the fuselage length, and the axel pivot point would be at 30 to 40 percent of the ski length aft of the ski nose. For the width, I just picked a number that “felt” right.

MATERIALS

The materials I used for the skis are well-known by all modelers and, depending on the weight of your model, the skis can be made of 1/18, 3/16 or 1/4-inch-thick lite-ply or luane (the plywood material used to skin interior household doors). For models that weigh up to about 15 pounds, use 1/8 inch thick material. For models of 25 pounds or more, I recommend 1/4-inch-thick material (both the L-5 and L-19 are in the lower 20s, so I chose to use 1/4-inch thick lite-ply). I’ve found that metal skis generally mean trouble because snow really likes to stick to cold metal. Wooden skis seem to work better; but just be sure you sand the bottoms silky smooth, seal them well with polyester resin, polyurethane, or epoxy and then apply a good grade of wax. We’ve successfully used beeswax, as well as high grade automotive paste wax. The wax will prevent the snow from sticking and also will allow the model to really slide across the snow.

Lay out the patterns on a flat piece of material and cut the outlines to shape. To get the nose of the ski to bend up and match the curve of the stiffener, a series of cuts is made across the skis top surface. These cuts are only 1/2 the material thickness deep and are only required in the nose area that needs to bend. This process is called “kerbing” and I simply used a utility knife to score these cuts. Just prior to bending this kerbed area, I also fill the cuts with glue so that when all the glue sets, this area will be nice and strong. The center stiffener and the two axle mounts are made of various types of plywood. For 1/4-scale models, the center stiffener is 1/2-inch thick, exterior grade, house construction plywood, and the two axle mounts are 1/4-inch thick, aircraft plywood. I also like to add spacers to each side of the axle supports so that the final thickness is the same as the wheels that I use on that model. This makes the process of switching from wheels to skis and back again, very easy and fast.

The entire assembly is glued together with 20-minute epoxy and clamped in place to cure. After curing, all the areas are sanded and then coated with epoxy and sanded again. Next, they are painted with a couple of coats of paint and topped off with some clear polyurethane or epoxy.

FUSELAGE CONSIDERATIONS

One of the neatest things about this design is the ease with which you can switch from wheels to skis. This is very important when you get that unexpected snowfall and last minute calls from your flying buddies to meet them at the field. It will take only a few minutes to change from wheels to skis.

There is only one modification needed for the fuselage; two pairs of eyehooks need to be installed to act as attachment points for the cables. Install two in front of the landing gear, one on each side. Attach the skis’ nose bungee and safety cable (more on these later) to these eyehooks ahead of the landing gear. The other two eyehooks go aft of the landing gear, (again, one on each side of the fuselage), the rear-extension limiting cables will be attached to these. To make these attachments sturdy, I simply epoxy some hardwood blocks inside the fuselage and permanently screw the eyehooks into place (see photos). I leave these in place all year long, so I do not have to make any changes when the weather makes an unexpected turn. I painted these eyehooks to match the fuselage and this way, they just get camouflaged and disappear very nicely.

SKI SETUP

To set up your skis properly, there are two basic, yet very important alignments to maintain.

Toe in: The skis must be parallel to each other, as well as to the fuselage centerline (a function of the landing gear’s axle toe in adjustment).

Angle of attack: The skis’ angle of attack must be approximately 10 degrees positive while the aircraft is in flight (a function of the bungee and aft limiting cable adjustments).

The nose bungee is big rubber bands that lift the tips of the skis. To limit how high the ski noses rise, you have to adjust the lengths of the rear-limiting cables. I like to make these adjustments on the workbench with the skis mounted on the axles (held in place with wheel collars) and the airplane’s tail propped up. To get the required 10 degrees of ski nose-up attitude, I keep the skis flat on the bench and then raise the tail so that the plane’s nose is set at a flight attitude of negative 10 degrees. A stack of paint cans works very well here! If you’ve set everything up properly, when you lift the model off the bench, the bungee cords will lift the noses of the skis and make the aft limiting cable taut. When the model is placed on the ground, the aft cables should slacken and the skis should lie flat. It’s important that they also be able to pivot freely on the axles. As an added safety measure, I suggest you run a “safety cable” alongside the nose bungee. This cable is adjusted when the model is sitting on the ground in the normal “at rest” attitude. The safety cable is attached at the same spots as the bungee, yet at this attitude, this cable should be taut. The idea here is that in the event of a bungee failing, you do not want the ski to turn nose-down on you in flight as it makes for a very messy landing. To make it easy to attach the bungee cords and cables, I install line connectors or some other form of “quick disconnect” device at the fuselage attachment points. Old control-line connectors work well and you might also find similar connectors in a fishing-tackle store.

To make it easier to remove the wheels from my models, I replace the usual wheel collars with cotter pins that go into small holes drilled through the end of the axles.

TIPS ON SNOW FLYING

With all the shop work finished, now it’s time to head to the field. The toughest part is waiting for the snow and then having it arrive at just the right time, like on a Friday night so that Saturday can be a day at the field with nice fresh snow. I live on Long Island, NY, and we don’t usually get much snow, but last winter we had so much snow that it was difficult to get to the field! Regardless of how much snow we get, when we get an opportunity like this, the “Snow Bird Squadron” gets together and makes it to the field for some really great, off-ski flights.

When flying off snow, remember these tips:

• You’ll need to apply slightly more power to taxi. If you have no ski attached to the tail wheel, the rudder will also need a blast of power for turning.
  
• You’ll need more power for takeoff, and the skis will have to “plane” on the snow before you’ll be able to build up air speed. To overcome torque, apply the throttle gradually and smoothly and feed in the rudder as required (just as if you were flying off a green runway). You may need a bit more elevator to prevent the model from attempting to nose over, but once the speed builds up and the skis are “on plane,” you’ll be able to release the elevator. When it’s equipped with skis, your model will not fly as fast because skis increase drag. When flying with wheels, don’t expect to pull up as steeply.
  
• Increase power during landings and use a slightly nose-high, three-point, or wheel-landing approach to keep the tips of the skis up. For short-field operations with my L-5, I particularly like the “I have arrived, three-point, plop” type of landing. The fun part for me is just shooting touch-and-go’s one after another.
  

Using scale-snow skis is a really easy way to extend your flying season. Before heading out, make sure all your radio gear is up to snuff. Cold weather wreaks havoc with batteries, as well as people. Just dress warmly, you don’t want frostbitten ears, toes or flying thumbs and be sure to take along some hot coffee or hot chocolate. Oh yes, and sunglasses are definitely in order—enjoy!