Rx for R.O. W.
October 1967 American Modeler
Seaplanes have always been a popular topic with modeler and full-size pilots alike. There's something about watching an airplane take off from or land on the water that is awe-inspiring. Flying without the constraints of a narrow runway certainly has its advantages, but there is an added element of risk with seaplanes because of potential damage that can be caused by water entering the airframe or even damaging it. The possibility of drowning, even after making an otherwise perfect landing, exists for the full-scale pilot, and the modeler can lose equipment that otherwise might be salvagable. This article is pretty extensive and give a lot of food for thought concerning taking on rise-off-water (ROW) operation. I looked up the tail number (N3763C) of the Cessna 150 shown, but either it has been retired or it hasn't changed owners in lo these many years.|
Rx for R.O. W.Having covered the floats and hulls in the last issues, Part II discusses the important aspects of good model airplane design for water operation. How to waterproof, converting land planes, taxiing, landing and takeoff of seaplanes.
By George Wilson
There are two approaches to seaplanes: 1) Convert a landplane to a seaplane and, 2) build an airplane which is initially designed to be a seaplane. Obviously, the latter approach should give the most satisfactory results. On the other hand, it has been solidly demonstrated that a landplane with floats added can be an excellent water performer. Seaplane Configurations: The first consideration is whether to use one or two main floats. A single main float is easier to build and causes less drag both on water and in the air. A single main float - on a single-engined airplane - also has the inherent advantage of minimizing the amount of spray that passes through the propeller. Figure F shows configurations most frequently used. Notice the spray paths that are possible during the early part of takeoff run. At this time it is important that the propeller devote all its effort to pulling (or pushing) the airplane through the water. When spray goes through the propeller, power output is seriously reduced. A seaplane riding on the step will often lose this position and sink almost back to the waterline because of loss of speed resulting from a splash of spray. An incidental factor, is propeller erosion. If wooden props are used they will quickly loose their tips as a result of water (spray) impinging upon them. Nylon props appear to withstand this abuse.
The superiority of the single main float design is attested to by its choice for full scale aircraft. Only on rare occasions such as the Italian Savoia-Marchetti S-55 has a two pontoon seaplane design been evolved except for conversion of a landplane to a seaplane.
As an aside for those interested in the engineering and mathematical side of our hobby, the scale effects of hydraulics are not similar to those of aerodynamics. In the latter case we have to deal with a very non-linear situation (Reynold's number) which makes the application of full-scale data to models completely impractical. The hydraulic case is essentially linear - the equivalent to Reynold's number is the Froude number - and, therefore, full-scaled data can be scaled easily for model use. Another way of saying this is that water will flow around model floats in the same manner that it flow around full-scale floats. Airflow around models is quite different than full-scale, requiring changes in area, dihedral angles, decalage, etc.
Single main floats require support on each side to prevent the airplane from falling to one side or the other. This is normally accomplished using "tip floats."
In many respects full-scale experience fits typical model floatplane requirements
- the 1967 Cessna 150 (for example) on floats.
"Sponsons" attached to the main float have also been used - like small wings mounted at the waterline, which provide the floatation necessary to prevent capsizing while floating. Properly designed sponsons provide some added lift. Sponsons never provide the positive protection against capsizing that tip floats do. On the other hand, it could be theorized that they would be less apt to cause "water looping." As a practical matter, the tendency for seaplanes with tip floats to water loop is very small. The drag caused by these floats appear to be small with respect to that of the main float. Additionally, the inertia of the fuselage and main float helps maintain a near straight path even when a tip float touches first on landing or digs in a bit on takeoff.
Sponsons can be rigidly attached. Tip floats should be attached flexibly and must have break-away capability. All things considered - and perhaps for the basic reasons that they are more like scale - the author prefers tip floats. It should be noted here that the same rules apply to tip floats and sponsons as apply to main floats. In either case their size should be minimized to reduce drag both in the water and in the air.
Most flying boats make fine landplanes by the addition of wheels. For temporary usage, these may be attached to the bottom of the float using rubber bands. Care should be taken to prevent the gear from marring the under surfaces of the float - a layer of plastic foam will usually provide the necessary isolation.
Conversion of Landplanes: The easiest way to convert a landplane is to add floats. The most popular system is to add a pair of main floats - as is done in most full-scale conversions. When a low-wing airplane (or biplane) is converted. it appears highly desirable to consider the use of a single main float with tip floats under the wings. In either case, three things must be considered: 1) additional fin area. 2) increased power, and 3) waterproofing. The latter problem will be covered later, but it must be mentioned here since it is critical to the decision of whether or not a conversion should be attempted. If you are starting from scratch and not just adding floats to an existing model, you can handle the waterproofing properly. On the otherhand, if you are adding floats to an existing airplane, have a look at the part of the article on waterproofing before you proceed. It takes more than clipping your receiver in a plastic bag to do a minimum waterproofing job. There is nothing quite like a few soaking-wet balsa parts for changing the balance of a model; or, worse yet, one section of the rudder or elevator filled with water and not noticed until the model starts stalling all over the sky shortly after takeoff.
Because of the added weight it will be well to use a little added power. You shouldn't have to go more than one motor size higher if you use the proper floats. Your model will tend to fly and take off slower with floats. For this reason it may be well to try a propeller with less pitch than you would normally use. Matching airspeed with propeller pitch is something most of us do to maximize the motor's performance. However, the choice of propeller should be reviewed as part of the conversion.
Increased fin area is necessary to compensate for the profile area added by the floats. The fin, and rudder of any airplane assures that it will fly in a straight line if none of the control surfaces are actuated. The basic rule states that the side area of the airplane behind the center-of-gravity must be greater than the side area in front of the CG. This is a fancy way of saying that the airplane is like a weather vane; the area behind the CG makes the airplane "weathercock" with its nose forward. A "sub-fin" located under the normal fin and rudder, twin auxiliary fins near the elevator tips, or auxiliary fins at the end of the float(s) are generally used. An area about one-third that of the total fin plus rudder area is generally satisfactory.
Waterproof hatch construction requires flat surfaces. Seal it with silastic.
Waterproof toggle switches mounted on the hatch. Switch on before starting engine.
Floats must be well braced, securely mounted and very durable. Lots of dope and wax.
If the model tends to wobble from side to side in the glide and is over-sensitive to rudder commands, additional fin area should be added. Many people agree that a larger than necessary fin in this type of model is desirable to promote added directional stability. In "thermal hunters" and airplanes requiring extreme maneuverability it is not the case. In a seaplane, inherent stability would appear to be an asset in any case.
Although a number of stunts are possible with a seaplane, it is not an ideal stunt configuration. Most seaplanes have a low CG which results in what is known as a "pendulum" stability. The fuselage and floats hang like a pendulum from the wing, It is interesting to watch some seaplanes when they right themselves after a maneuver; their lower sections actually swing back and forth like a pendulum. At high speeds in turns this same pendulum effect can cause excess banking. A docile airplane like the Seacat can lock itself into a turn with its wings vertical if a tight, high speed turn is called for. Once understood, the situation can be guarded against or, if it happens accidentally, it can be "unwound." One wrong control can cause a spectacular spiral dive, followed by a large splash.
In the first section of this article the necessity of rigid float mounts was pointed out. We mention it again here to re-emphasize importance. It would be a good starting point in your conversion planning - along with the waterproofing investigation - to assure yourself that rigid float struts can be added to your particular model. In most cases no serious problems will be encountered, but a little far-sighted planning at this stage can avoid disappointment later.
Taxiing the Seaplane: Perhaps the greatest joy in seaplane flying is the takeoff and landing surface that water provides. Most important are facts that takeoffs and landings can always be into the wind and that the water is much softer than the hard ground. With a water rudder and with the motor at a medium speed your model seaplane becomes an "air-boat." It can be taxied from the shore to a good spot for takeoff and, after landing, it can be taxied back to the place on the shore where you are standing. "Proto" ground handling in seaplanes is much easier than in landplanes and can save a great deal of awkward handling in chase boats or wading in muddy ponds. This can add greatly to your pleasure.
Water rudders can, and should be, small. A six-foot model will handle well with a rudder 1/2" high and 1 1/2" long. Unlike air, water is dense and, therefore, a control surface is much more effective than air rudders of similar area. The rudder should be small to minimize the water drag it causes. A balanced rudder with about 1/3 of its area in front of the rudder post is desirable, especially if you are using low-power servos. If you are using rubber-driven escapements, it is virtually impossible to make them powerful enough to drive an underwater rudder. Incidentally, don't let this fact stop you from trying escapement controlled seaplanes. Once up to speeds where the air rudder becomes effective, a well designed escapement controlled seaplane will handle well on the water and in the air. Here again, the Sea-cat is an excellent example. Many of these models have flown very well as originally designed with escapements.
Taxiing should be done at medium speeds - just short of getting up in the step. At low speeds steering becomes sloppy and weathervaning becomes a controlling factor. Therefore, at least three-position motor control is desirable for effective water handling. Note that with a well chosen medium speed many seaplanes without water rudders will do a reasonable job of taxiing.
In addition to the water rudder and motor control, several other factors will influence a seaplane's water handling characteristics. Since airplanes with high wings tend to catch the wind easily and will weathercock or actually capsize when broadside to the wind, they tend to have the poorest water handling characteristics. With them it is often impossible to turn it into the wind for takeoff, or to taxi back to the starting point. Any seaplane pilot who has flown high wing lightplanes on floats will tell of similar problems. One of the best light seaplanes is the Colonial Skimmer. This plane has a low profile when on the water; its propeller is well shielded from spray; and it has won the praise of many pilots. This airplane should make an excellent scale model seaplane; the plans from a Berkley kit would be a good starting point.
In any case, seaplane flying is good fun but to get the most from it, water handling characteristics should be carefully considered. The potentials in this area alone make land flying relatively unattractive.
Waterproofing: This is one of the most important factors. The subject can be divided into two major parts: first, waterproofing the seaplane itself and, second, protecting the radio equipment. In. a well-designed model seaplane, equipment protection may not be a problem if the airplane provides water tight compartments for the equipment. If there is any question about water reaching the equipment, it is best to wrap it as much as possible in plastic bags closed with rubber bands looped around the wires going to each unit. Receivers and batteries lend themselves to this type of waterproofing. On the other hand, servos do not since they must be mounted to the airframe and must have both electrical and mechanical connections. It is therefore, most desirable to design for "watertight" equipment compartments - if possible.
A major consideration in waterproofing is the kind of water you will be operating from. Fresh water presents little difficulty. Radio equipment has been thoroughly immersed in fresh water, then dried carefully without any apparent deterioration. Salt water is different and immersion should be avoided. If it does occur, immediate flushing with good clean fresh water is in order, followed by careful drying, using forced air if possible. Most vacuum cleaners can be connected to blow rather than suck air. This is a handy source of warm air for drying. Salt water will corrode all common metals, including motors, fasteners, control linkages, landing gear struts, switches, etc. If you fly from salt water, oil or grease all exposed metal before and after flying and make sure you wipe the salt water off metal parts as soon as possible after a flight is made.
Motors appear to cause little difficulty - if you remember to run them immediately after they have been dunked! The liberal amount of oil in the fuel provides considerable protection. Motors start quite easily after being submerged and having ingested water. A few flips of the propeller and a little shaking will remove most of the water. Running out a prime or two (inserted through the exhaust port) will cause the remaining water to be thrown out. A wet motor sometimes starts better than a hot dry motor since the wet motor is cool and is less liable to quickly boil-off or evaporate the methanol in the fuel. Most wet motors will restart quickly, quickly spitting out water, sputtering a bit and then operating normally, all within a few seconds.
The most fundamental problem in waterproofing is the plane itself. The problem is to prevent the airplane from soaking up water. The immediate effect of absorbing water is to change the balance - particularly serious when the covering is punctured and water fills some of the Volume within the structure, such as the Volume between two wing ribs. And this Volume is quite significant.
Long nylon pushrod guide keeps water out of fuselage.
Because of this probability it is well to recheck a seaplane's balance several times during an extended flying session. A secondary problem is that of drying out the structure (the balsa wood) once it has soaked up water. Air circulation within the structure is poor. The water and vapor is trapped, causing the drying process to take a long time. To lessen the problem seal the inside of the structure, as well as the outside. The framework should receive at least two coats of dope, diluted no less than two parts dope to one part thinner. Spraying will save time, but be sure to apply enough dope to thoroughly seal the wood. It is desirable to use extra bulkheads in certain cases to create compartments which are essentially watertight. Typically this may be done to isolate the rear of a fuselage just in front of the opening for the elevator. If practical, elevator and wing openings should be permanently covered for obvious reasons.
As in landplanes the wings and occasionally the elevator of a seaplane get knocked off. If this happens, the entrance or water must be prevented. Gasketed covers can be mounted inside the fuselage at these places, if equipment access through these openings is necessary. In any case, a gasket should be installed around wing and elevator openings. Liquid silastic rubber (General Electric "Clear Seal" for example ) or a closed pore plastic or rubber foam make good gasket materials.
Silastic rubber gaskets are formed by applying a generous bead of the material around the opening, covering it with a layer of Saran Wrap and then mounting the wing or elevator with elastics, applying enough pressure to flatten the bead of rubber and making it conform to the surface of the wing or elevator. The excess rubber that oozes out should be scraped away immediately; it can be trimmed after curing. but it takes a real sharp razor edge to do the trick.
Most plastic and rubber foam seems to be the "open pore" type. This simply means that it acts like a sponge and soaks up water. "Closed pore" sponges or foams will not soak up liquids. Keep watch for the latter type of material if you are planning to go the seaplane route. Oil-resistant, closed-pore, foam-rubber sheet about 1/8" thick is available - if you can find a place to buy it. This makes fine gaskets when cut into 1/4" or 3/8" strips. Corners may be mitered and joined with "Goo" or "Plio-bond." The latter adhesives also may be used to attach the gasket to one side of the opening.
Flat gaskets with square outlines are desirable, since they can be made easily; and they seal tightly with a minimum of pressure applied. A fairly simple method of gasketing hatches in curved surfaces is shown in picture. Double bulkheads are installed at the front and rear of the section which will eventually be the hatch. After planking (or covering), the hatch is cut free and a square gasket seat is installed in the fuselage and on the bottom of the hatch. There will be slots between the front and rear of the hatch and the fuselage; these may collect a little water but this can be quickly removed by pulling the edge of a rag through the slots. Hatches made in this manner can be held down using rubber bands and dowels or wire hooks.
Openings required to pass control rods, switch-handle extensions, and similar devices can best be waterproofed by passing the rod through a fairly snug piece of metal or plastic tubing for several inches, as it passes through the skin of the model. Grease may be used in the tube to assure essentially perfect water seal for all eventualities short of prolonged submersion. The grease can be omitted in most cases if corrosion of the rod itself (within the tubing) is not a problem.
If your seaplane has its motor mounted on the wing, consideration must be given to the installation of the motor control servo and its leads. It must be possible for the wing to break loose without breaking leads or leaving an entrance for water. The recently available nylon tubing/flexible inner cable will allow new systems to be devised for mounting the servo in the fuselage and transmitting the control motion via the tube and cable. A break-loose connection must be included in the tube and cable if this is done. Another method is to use a slip-clutch-type servo which has an external amplifier. The servo in this case requires only two leads that may be connected using snap connections from a couple of old batteries for a plug and connector that separates with little pull required. In one case, connections were successfully built into the bottom of the wing and top of the fuselage. When the wine was mounted, contact was established via spring loaded fingers in the fuselage and brass plates on the bottom of the wing.
The waterproofing problem is not half what it seems to be at first glance. The author's venerable Seacat has now flown for eight years, and only on one occasion has water entered the hatch in the fuselage. We learned to fly radio control using this airplane and, needless to say, it has had many rough landings, including cartwheels, full-speed vertical dive-ins, nose-overs and others too complicated to describe. In the one case of leakage, the hatch had not been "dogged" down with enough rubber bands.
CONCLUSIONS: With small extra effort a well-designed seaplane will fly well. Water takeoff should occur with ease if the foregoing suggestions are followed. Briefly, the five points below should be remembered:
- Proper Design of the floats is essential for good takeoffs. A landplane that flies well can be converted to a seaplane that flies well. Seaplanes designed for the job will perform best.
- Proper trim of the floats and the airplane overall is essential. Good design and construction of the float(s) will minimize float trim problems.
- Rigid float mounts are essential for good takeoffs. Seaplane floats must be rigidly attached; let the water absorb the landing and takeoff shocks.
- Sharp chine edges are essential to reducing float drag and shirring the water cleanly away from the floats.
- Polished float bottoms are essential to reduce float drag and to let the model slip easily into the air.
- Waterproofing is essential to protect the radio equipment, to preserve the airplane, and to maintain flight trim.
- A water rudder and motor control is essential to good taxiing performance which is a major factor in the enjoyment of sea-plane flying.
Posted May 12, 2013