RC Skis (oz11781)

About this Plan

R/C Skis. Plan showing how to build snow ski undercarriage for RC models.

Quote: "The best article I have seen regarding the design and application of Snow Skis for RC planes, written for Flying Models Magazine, April 1999, by a gent from Timmins Ontario Canada - where they know their snow!
The included plans can be scaled up or down to suit your size of model. I used 1/16 in (1.5mm) thick birch plywood 3 in wide x 12 in long (12mm X 305mm) for the main skis and 1/32 (.8mm) birch ply 1-1/4 in wide x 4 in long (32 x 102 mm) for the tail ski on .40 - .60 size models weighing up to 7-1/2 lbs. (3.4 Kg). They also scaled down very well for my 1/2 size Sporty 40. Regards, Mike Foster

Quote: "Fly in the Snow using R/C Skis. By Andy Woitowicz.

Use these skis to fly from the fluffiest powder snow. They can be sized to adapt to any size model.

Those of us fortunate enough to enjoy all four seasons have available to them a unique flying experience. It allows the opportunity to get out of the shop, limber up the batteries and get some fresh air and exercise all at the same time. Yes, I'm referring to winter flying off of snow. This, of course, calls for skis. There have been a number of ski designs and various commercial products available. Some worked reasonably well and some not so well.

A lot can be learned by observing full size aviation. Aircraft with tricycle gear on skis are a rarity. These work well on hard packed snow, but on soft virgin snow - which is what is found on frozen lakes and the like - they are a disaster. The problem is that the force of a high thrustline acting on the drag of the main skis causes the nose ski to dig in. Also, the weight and vibration of the engine in a model pounds the nose ski into the snow compounding the problem. You would need a good blast of throttle with full up elevator, to keep the nose ski light until you gain some forward speed. Otherwise, you'll be whipping up your own custom made snow storm.

Then comes the problem of landing. On soft snow, perfect two pointers are a must every time. Possible certainly in full size, but not guaranteed every time in models.

No problem you say, the snow is soft. Yes, but when the front end digs in, the nose ski, being in such close proximity to the prop, generally gets whacked. The result is a nicked prop that should be replaced and a ski that needs to be repaired.

The solution to these problems short term is to hold the tail of the airplane down, keeping the nose ski light and rapidly advancing the throttle just as you let go. This works but is a pain. Not to mention that after landing, you have to taxi back at a somewhat uncomfortable speed. Otherwise, the nose digs in and - puff! - you have another snow storm. Taxiing back to your spot is difficult at best and is aggravated by irregularities in the snow surface such as drifts, footprints or changes in snow density.

None of these problems exist on a properly designed, ski equipped, tail dragger. The main skis support almost the entire weight of the aircraft and the tail ski follows obediently behind. Set up correctly, a tail dragger on skis allows you to taxi over virtually any kind of snow, hard or soft, smooth or bumpy. Those modelers who have never flown a tail dragger and are afraid of it or have always had difficulty, will find that a tail dragger, on skis, is a pussycat compared to the wheeled version.

Flying off of snow opens up many new fly-ing sites such as lakes as well as fields that are not normally suitable for lack of a run-way. This lets you set up the pit area to your advantage, allowing for instance, take-offs directly into the wind. The runway is limited only by the size of the lake or field, and any difficulties with the aircraft veering off to the side due to torque are not serious. This allows you to practice with your tail dragger without the fear or intimidation that a narrow runway may cause. You can practice high speed take-off runs, and learn to use the rudder to compensate for torque.

If you've already got a tail dragger, you're half way there. Depending on the design, most trikes can be converted to dragging their tails. The nose gear, of course, comes off. On a trike the main gear is placed so that the axles are behind the balance point while a dragger needs them ahead. This can usually be accomplished quite readily with a new cut-out in the fuse bottom and some plywood. What is important is that the axles be directly under the leading edge of the wing. Too far ahead and handling becomes squirrely while too far back results in frequent nose-overs, For the tail there are many commercial tail brackets available that can be readily retro-fitted and it is im-portant that the tail ski be made steerable.

Many model ski designs follow full size practice in the use of elastic cord at the front of the ski and a stop cord at the rear. If your aircraft is scale, this is appropriate. If you're flying sport this is unnecessary and cum-bersome. On a nose over, the stop cord can be strained to the breaking point and during general handling the cords always seem to get in the way.

The design presented here is neat, streamlined and requires no modification to your existing landing gear. As illustrated, the long spring wire restrains the ski in both directions. Attachment of the spring is di-rectly to the axle and is infinitely adjustable for angle of attack. Commercially available, single-split collars are used for the larger sizes and solid axles. Nuts and star lock-washers are used for threaded axles.

The single-split collars are available at hardware and industrial supply stores. Collars with 1/8-inch ID are used for .20 to .40 size aircraft, They can be drilled out to as much as 3/16 inch and this will accommodate most applications. Bigger than 1/8 inch are available ready to go but they are much larger, heavier and more expensive than is necessary. As pictured, a washer must be installed in the split to prevent drilling off center. For threaded axles, a nut of the correct size is drilled out for a slip fit. Star lock-washers are needed and they are used to se-cure the spring (nut assembly. The split collars are simpler to use and adjust and with care, they can also be used on thread-ed axles.

The collars and nuts are drilled to accept the spring wire which is soldered in place. The wire is retained at the rear of the ski in a piece of plastic tube, secured by a chunk of ply. Do not secure this end rigidly, The wire must be allowed to slide in the tube for proper operation.

The tail ski is generally too small on the smaller aircraft to incorporate the single wire spring loaded feature so a conventional elastic/stop cord set-up is used. This works well because nose-overs are of no consequence and the tail assembly is not so large as to be cumbersome. Larger aircraft can incorporate the same set-up for the tail as the main gear. Using an ordinary non-split set collar works well at the tail because the loads imposed are not as great.

Regardless of the restraining method, placement of the axle is very important. You will notice that the axle is not placed at the mid-point of the ski, On people-carrying skis the boot harness (axle) is placed farther back from the center. This is done so that the rear of the ski is loaded more than the front and this forces the ski to assume a positive angle of attack at all times. This can be illustrated by taking a long piece of wood and making a mark at the mid-point, Now place it on a piece of soft foam and press down, with your finger, on the mark you made. This will compress the foam down equally. Now move your finger back a bit and you will find the rear pressing down more than the front. This produces a positive angle of attack and this is the effect you want on snow.

Some ski designs fail to take into account that they are replacing wheels of a certain diameter. Most of the commercial units I have seen have this flaw. Axle placement should be such that the stance of the air-plane is unchanged, This is especially important if you insist on a trike geared set-up because the prop is dangerously close to the front ski to begin with. Even on a tail dragger, if the axles are put at or near the bed of the ski, this brings the prop unnecessarily close to the snow, No harm done perhaps but instead of a nice aggressive positive stance to your airplane, it looks as though it's in danger of sinking out of sight.

The plans presented are for a .25 sized pat-tern ship. Don't forget that you need to make a left ski and a right ski by way of the place-ment of the ply block at the rear of the ski. So far this design has been used on an .049 Ugly Stick (oz5175) and on a .45 sized fun flyer. The dimensions shown can be reduced to 80% size for 1/2A, and for a .45 sized aircraft increased to 150% size. Access to a reducing/enlarging copy machine makes this an easy task. If you want to experiment with a more stylish outline, feel free but the dimensions shown in the box have worked well.

Finishing is done with your favorite method but should be as fuel proof as you can make it, Also, the bottom surface must be very smooth and must be waxed with an appropriate ski wax. If you have snow to fly from, there should be a ski shop nearby that can advise you as to which wax to use, I use glider wax intended for waxless skis. Not necessary but very convenient is the addition of Teflon to the bottom of the ski. This is available from industrial suppliers in 1/64-inch thickness and is etched on one side to accept epoxy. Wax is not needed. This allows you to keep on flying even on wet sticky snow. (Ed, Note: This Teflon or a similar material is used by woodworkers as a facing on table saw fences. It can be purchased from Woodcraft Supply, 210 Wood County Indus-trial Park, PO Box 1686, Parkersburg, WV 26102-1686; www.woodcraft.com).

For those who haven't tried or have been disappointed with snow flying, give these units a try. I can guarantee that they will handle any snow conditions, smooth or rough, soft or hard Touch and goes are a blast and landing mistakes on soft snow are much more forgiving.

Questions or comments? Contact me at Andy Woitowicz, 170 Cedar St, N, Timmins, Ontario P4N 6J3 Canada."

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