If It Flies - 2011/04
Proper setup of skis and floats
Dean Pappas | [email protected]
Hi, gang. In the December column I dropped an overly simple statement about how rich running helps produce the CL Aerobatics (Stunt) run. Although I fully intended to make that oversimplification to fit the allotted space, it still amounts to a long dangling thread—one I promise we'll return to someday.
The “four-cycling” sound really is the consequence of the engine doing a repetitive strong-fire/weak-fire cycle. That’s why the engine has a slower-sounding note. It’s not the only note you would hear, and a frequency analysis of the sound could easily recover the note that corresponds to the engine RPM. Describing how the strong-fire/weak-fire cycle happens will eventually take up most of a month’s column, but it is amazing how many things happen in the course of one revolution of a two-stroke engine and how they set up the initial conditions for the next revolution.
Enough of that. Let’s move on.
Back at the computer after shoveling snow yet again, I’m reminded of all the enjoyable frozen-finger flying I’ve done from snow, using either skis or floats. Yes, floats—they're handy when water is available too.
As a youngster I flew at the Hackensack Valley Club field in the swamp within sight of the Manhattan skyline. That field flooded each spring. The ankle-to-knee-deep water persisted for only two or three weekends a year, and the decomposing carp left behind as the water receded helped the grass grow lush and green. It also meant many of us in the club had a set of floats handy in the workshop, and maybe even an airplane specially set up for water-flying.
I want to write about the proper setup of skis and floats for flying from both kinds of water: fluid and frozen. I made an observation as a young teenager that surprised me at the time. First I’ll cover the unsurprising part.
When you compare the effect that aligned floats and aligned skis have on the flying characteristics of an airplane, skis have less overall effect. No doubt that is because they are smaller. On the other hand, even a tiny misalignment of skis has a dramatic effect on how the airplane flies, while even substantial misalignment of floats has a comparatively modest effect on the airplane’s trim. How can that be? The answer is in the different shapes: flat plate versus torpedo-like.
Floats: alignment, mounting, sizing
As a general rule, floats should be set up parallel to each other, both when viewed from the side and from above. Their nominally flat top edge should be set parallel to the wing chordline, and the “step” in the bottom of the float should be placed slightly aft of, or directly under, the center of gravity (CG). I’ll tell you why in a moment.
The floats must be mounted firmly, because water is much tougher than you’d think. Even a decent water landing involves substantial impact loads in every direction you can imagine. At speed, water seems to grab things with the intent to rip them apart. If you fly from, or even near, water, you might have seen the results of a water crash. The pieces are smaller and more numerous than after a crash on land. Boat racers (both model and full scale) know this all too well.
Mount those floats like you mean it; any care you take in aligning them will be for naught if the mountings distort and the floats misalign as a result of normal takeoff and landing loads.
How big should the floats be? There are three ways I know to answer this question:
- Manufacturers rate their floats or float-making kits by the range of airplane weights they are suited for. When in doubt, go up in size for water and down in size if you plan to use floats for flying from snow.
- Floats are typically approximately 80% of the overall length of the airplane from the propeller to the rudder’s end. If you look at photos of full-scale floatplanes, you’ll see this four-fifths ratio is typical.
- For those rolling their own: in general, the front end of the floats should be about a half wing chord in front of the propeller(s). The step should be approximately 10% of the average wing chord behind the CG. Since the aft part of the float is normally slightly longer than the front half, that determines the minimum overall length.
With the right-size float and the step located approximately 10% of the average wing chord behind the CG, the airplane should float with the back end of the float in the water but not fully awash. If the aft end of the float is under water, move the floats back and recheck. If the stern is out of the water, move the floats forward. Positioned as described, the floats are set up for a good compromise between nose-over prevention and ease of takeoff.
Water spray in the propeller will literally chew up a wooden propeller in a handful of takeoffs, and it robs an unbelievable amount of power. A nylon or composite prop might be a better choice, but water spray in the prop disc needs to be avoided. The chines, or spray rails, on a float are like skinny downturned gutters on the edges of the bottom of the hull. They deflect water spray to both sides rather than letting it go up into the propeller. A 1/4-inch-wide spray rail on the hulls in front of the propeller might be the only thing needed to solve a bad water-spray problem, but good float designs probably won't need them.
By the way, that step on the bottom of the float is there to break the Coandă-effect attachment of the water to the hull, making takeoff much easier than it would be otherwise. Aviation pioneer Glenn Curtiss invented it, along with the aileron. The diagram shows the preferred alignment of floats to the airframe.
The only thing I haven’t covered is water rudders. I am merely a part-time water flier, but I’ve never used them. Most pilots I know who do use them create some sort of linkage or spring-loaded mechanism to pull them up out of the water for takeoff, because it makes a big difference in takeoff performance. Full-scale fliers normally lift them out of the water as soon as they have enough air over the rudder for acceptable control authority. If I regularly flew from water, I’d put steerable water rudders on at least the starboard float.
Skis: setup for snow and takeoff
Skis are another matter altogether. For one thing, they can be much smaller than floats, especially on hard-packed snow. Large tires work on hard-pack too. If you plan on flying from soft or freshly fallen powdery snow, skis need to be longer and wider—practically as large as floats in deep powder—and the attachment method becomes more critical.
Skis must be free to allow the model to rotate nose-up for takeoff as well as maintain perfect alignment in flight. The spring or cable attachment point must be directly above the ski pivot, or binding will result. The best method I have seen for setting up skis for snow is to allow them to rotate nose-up for takeoff but be spring-loaded back to parallel with the wing for flight. By doing this, the airplane can sit flat on the snow, rotate nose-up for takeoff, and then the skis return to parallel with the wing chordline for flight. It's amazing how much aileron trim a little tweak can require.
The setup: a strong but flexible cable attaches the tail of the ski to an upright that is attached to the main landing gear leg. Adjust the cable length so that when taut the ski is parallel to the wing. Keep the cable taut with a rubber band or spring (rubber bands aren't all that good in cold weather) stretched between the upright and the front or tip of the ski. If the model has tricycle landing gear, the nose ski can be treated similarly or simply locked at the correct angle. The mains are the ones that need to rotate for takeoff and the landing flare.
There is one other difference between skis and floats that falls in favor of skis for some airplanes: skis offer little or no side area, while floats have plenty of it low on the airframe. Not only that, but floats have more side area in front of the CG than aft of it, which reduces yaw stability.
In the case of my trusty sport model, the Carl Goldberg Tiger 60, this is no problem—the long tail provides plenty of yaw stability. Shorter-tailed airplanes might develop a tendency to sashay slowly from side to side or even drop the tail into turns. This can turn an otherwise enjoyable aircraft into a complete dog. The cure is added vertical fin. Full-scale pilots typically add a pair of semicircular fins to the underside of the horizontal stabilizer—one on either side. I’ve seen a modeler add a fin to the top of the stern of each float to fix the yaw stability deficit so the airplane reverts to its usual self when wheels are replaced. You could even use thick clear plastic if you don’t like how it looks.
So why do skis create more trim problems than floats when misaligned? The lift created by a flat-plate flying surface, even a fairly narrow one such as a ski, is somewhere between three and five times that created by a float-shaped or fuselage-shaped object with the same projected area and the same angle to the oncoming air. The difference also partly depends on the size of the object and the airspeed, or the Reynolds number.
Do you remember when I wrote about computing the neutral point for a blended-body/wing-type airplane? I claimed that the area of the nose and tail of the fuselage (as viewed from the top) figures into the calculation. A refinement: because the nose or tail is a skinny tube-like object, that area counts only about one-third as much as the same area would if it were part of the wing or stabilizer. Aerodynamic texts include tables and graphs of this area-effectiveness multiplier for a variety of fuselage-like shapes and for objects such as drop tanks. They’re useful for approximation, although professionals now often use computational fluid dynamics modeling—a virtual wind tunnel.
I'm out of room, but I enjoy the daylights out of water-flying and hope you might too. I’ll be back next time. Until then, have fun and do take care of yourself.
—MA
Transcribed from original scans by AI. Minor OCR errors may remain.
