It’s wintertime in the Midwest, and we have recently had snow on the ground. Here are some of the things that I have learned about an ultralight in the snow, first with wheels, then with skis:
Using wheels in five inches of medium heavy snow makes for very high drag on the ground. On takeoff, full throttle moves the plane forward but the high drag of the snow makes it difficult to attain takeoff speed. Eventually, as the plane attains enough speed, the wheels break free, and the plane zooms ahead and upward, at last free of the snow drag. This takeoff experience is likely similar to a floatplane trying to overcome the water drag of the floats and break free into the air. Takeoff can feel like it may never happen.
Upon landing, the high drag makes for a short landing roll and feels like catching the arrestor cable on a carrier landing.
Drum brakes pick up snow and form ice on the internal parts. This makes for woefully ineffective braking once you roll back onto a dry plowed surface. It is conceivable that the wheel could freeze after takeoff and choose not to roll upon landing. One would be advised to anticipate this.
The main wheels tend to slide sideways in snow. Taxiing straight can be a challenge. Watch this during crosswind taxiing.
Once you lift off of the snow, flying over snow is like flying over dry ground. This seems obvious now, but it was a welcome relief after the clumsy, draggy feel of the plane in snow.
I have a friend in the “cool stuff” industry, and he gave me a sheet of half-inch thick “ultra high molecular weight polyethylene,” or UHMWPE for short. This slippery stuff looked like airplane skis, even in the flat. A table saw easily cut the UHMWPE. Holding one end over a glowing burner on the wife’s stove for five minutes softened the UHMWPE and made it formable by hand, if your hands are inside a pair of welding gloves. This was an easy way to turn up the tips of the skis. I made an aluminum angle iron structure to attach the skis to the axles of the plane. A router was used to radius the edges of the skis. Cables and bungee cords were used to limit ski rotation and to bias the ski to a horizontal orientation to minimize in-flight drag and prevent ski tip dig-ins upon landing.
The skis were sized for a snow pressure loading of 1.25 pounds per square inch. This was calculated as 490 pounds gross aircraft weight divided by (2 skis X 7 inch width X 28 inch length). This size appears to be a good compromise, but would likely be a bit small for use in deep powder snow. I haven’t had a chance to test in deep powder yet. For comparison, cross-country skis are about 1.67 psi, snowshoes are typically around 0.89 psi, and snowmobiles are in the vicinity of 1.33 psi.
Moving the plane from the hanger to the snow is a bit of a challenge. Some pilots swap wheels for skis after rolling the plane to the edge of the snow. I chose to add “hanger wheels” that penetrate about an inch through the bottom of the ski. These are 4” diameter hard rubber wheels from ("save big money at…") Menards. They look primitive but make the plane roll easily on concrete. However, they do NOT roll so easily on frozen gravel, and become a real quagmire on muddy gravel. The best method to use to move your plane depends on the present condition of the path from your hanger to the snow.
Below is some information gathered from direct observation, various pilot’s handbooks, and from local graybeards (thanks, guys):
Skis significantly reduced the drag of the five inches of heavy snow.
There is high degree of variability in the resistance of various types of snow.
There is a high degree of variability in the resistance of the snow from foot to foot, especially in old snow. Crossing previous tracks may make for an abrupt change in drag. This made my nose-wheel plane feel reminiscent of a tail-dragger, abruptly yawing in response to variability in the snow surface. Watch out for patches of raw mud!
Watch for drifts on the lee of obstructions. Picture Prairie Lakes after a night of wind from the west.
Packing a path through the snow by taxiing several times creates an easier takeoff roll, both with skis and with wheels.
Use ailerons to rock upon one ski to break free of high drag snow condition (a la float plane).
Skis may freeze to the snow if parked for a period of time. It may require extreme effort to break the skis free and get sliding. Rock the plane free before climbing in.
Skis with snow frozen to their bottom surfaces slide like Velcro. When parking, place wood boards under skis to prevent this Velcro syndrome. Spraying WD-40 or Pam on the bottom surface may help.
Slushy snow squishes out (forward) from under the nose of the ski at high taxiing speeds, and under some conditions this can be heard passing through the prop. Wood props would likely prefer to avoid this occurrence. Slush may squirt far enough to hit a tractor prop, so get the nose ski up as soon as possible.
I observed no aerodynamic differences between the skis and wheels once airborne.
The main gear skis tend to slide sideways. I considered routering lengthwise grooves in the bottom of the skis, but the experts said no good. I added a snowmobile skag to the bottom of the front ski to get steer authority, which makes a huge improvement. The main gear skis still slide sideways easily. This makes for slewing around turns, and making ‘donuts’ of about one wingspan diameter is possible with the right combination of power and rudder.
Crosswind taxiing is quite interesting. The crosswind pushes the main skis sideways, while the nose ski (with the skag) tracks and holds a line. The sensation is very much equivalent to crabbing in the air in a good crosswind. Lively but manageable.
It’s hard to screw up a snow landing. Snow drag quickly kills your airspeed once your skis touch. Icy snow will be different.
“Touch and gos" in fresh snow are fun! It’s neat to see the tracks from your previous landing and takeoff, and try to beat the distance or match the landing mark.
The snowplow loaf at the edge of the hangar road may be difficult to traverse. Preplan your path to the strip.
Taxiing – once you get it moving, keep it moving. Do your takeoff checklist before you start your taxi.
Flying in light snow flurries is a good skid/slip indicator, allowing you to “see” the air that you’re flying through. This is a good way to recalibrate your yaw string.
Skis have no brakes. There is typically quite a bit of drag from snow, but watch for patches of ice. Run-up is something you might want to do while taxiing. Or, you can let your skis freeze to the snow.
On overcast, “flat light” days, the snow surface texture and distance may be hard to judge, especially unbroken snow, making the landing flare interesting to problematic. Those “blue blocker” sunglasses may help. Land near someone else’s tracks.
It is difficult to see where those old tires that mark tiedown locations are buried. This makes for a rude surprise when you taxi over one.
Snow can gather on top of skis, adding weight. A better design would have no voids to catch/hold snow on the top surface of the ski.
Landing at a distant airstrip or at a later time may be interesting, especially on warm, sunny days. The snow surface may be different than the one you left, and it is difficult to read the snow from altitude.
Frozen lakes look really attractive for landings. How do you judge ice thickness? Watch for signs of other users on the ice, including ice fishing.
Do not land in cornfields. The stalks and stubble partly hidden by soft snow may tangle skis. Ugh!
Flying with skis has been an adventure, creating a chance to fly in the white months! Thanks for the advice and assistance of flying friends! Dress warmly!
About the author
Roger Kellogg works as an engineer at Argonne National Laboratory. He began flying ultralights in July of 2003. Roger has recently played a major role in the financial turn-arounds of Leading Edge Air Foils, California Power Systems, and Aircraft Spruce. He looks forward to flying his Aerolite 103 to Oshkosh for AirVenture 2004, probably without the skis.
Where to now?:
For Sale: One Airport, Low Miles!