ROW That Free Flight


from MAN, June, 1960; author unknown

It should be noted that the article was written from the perspective of 1960 AMA gas rules which required 175 ounces per cubic inch of displacement; thus the 1/2A model example is shown as weighing 8.5 ounces. In recent years, we have had no weight required rule for AMA gas, so you can fly that 5 ounce 1/2A [which is no easier to build now than it was then].

The basic formulas should also work well with rubber models.

Raising a contest model off the water is easy with floats of the correct size, design, construction, mounting and trim. Raising your contest model off water does not require a sleight of hand or supernatural aid if you attend to a few details. Not concerned with the use of "hotted up" fuels and motors or some entirely new and radical ROW design, these details relate to floats-their size, design, construction and mounting-along with final flying trim.

Most modelers are content to add floats to an already proven free flight; it's quick and foolproof if the model weighs only the minimum allowed and the floats are not too large. However, over-sized floats are common and wreak havoc with flight performance.

Determining the proper float size isn't difficult.

FIG. 1A
PROCEDUREExample 1/2A Free Flight
Step 1 Weigh model to be used1/2A weighs in at 8.5 oz
Step 2 Multiply model's weight by 111%8.5 X 1.11 = 9.4 oz
Step 3 Divide weight found in Step 2 by .589.4 / .58 = 16.2 cu in
Step 4 Multiply amt. of cu in in Step 3 by 135% to get Corrected Displacement16.2 X 1.35 = 21.9 cu in

Fig. 1-A gives the step by step procedure along with the use of a 1/2A free flight example. Experience has shown that well constructed floats, complete with struts and fittings, will weigh approximately 11% of the model's weight. So in step 2 the model's weight is increased by this percentage. In step 3 the total weight to be supported by the floats is divided by the weight of one cubic inch of water. The resulting quantity (in cubic inches) is the minimum displacement necessary to float the model, and as with icebergs, the greater part of each float would be submerged. Increasing the minimum displacement by 35% (step 4), insures that a "safe" portion of each float will be well above the waterline. The chart in Fig. 1-B gives this "Corrected Displacement" for various model sizes.

.049
FIG. 1B
ENGINEMODEL WT.TOTAL WT. MODEL & FLOATSCORRECTED DISPLACEMENTFLOAT WT.
.0203.5 oz3.9 oz9 cu in.4 oz
8.59.422.9
.07412.614.2331.4
.0917.219.1441.9
.152628.9672.9
.1934.538.3893.8
.2339.944.31034.4
.2951.657.51345.7
.3560.767.41576.7

The next stage of float design involves dividing the Corrected Displacement proportionally between the front and rear floats. Decide where the floats wiIl be mounted; in the front the wire struts are usually sandwiched between the motor's radial mounting plate and the firewall or bolted to the beam mounts. The stabilizer's trailing edge serves as the rear mounting point. Measure the distances from these points to your model's point of balance or CG.

Fig. 2-A illustrates this arrangement. The calculation procedure is given in Fig. 2-B along with the, familiar 1/2A example. Each step is self-explanatory. When finished, round off the displacements to the nearest cubic inch, as in the example, TD would end up as five cubic inches and ND as 17 cubic inches.

PROCEDURE FIG 2B
STEP 1Balance rule states that: NFM X ND = TFM X TD
STEP 2Solving for ND: ND = (TFM X TD) / NFM
STEP 3Since ND + TD = CD, substitute formula in Step 2 for ND
[(TFM X TD)/NFM] + TD = CD
STEP 4Solve for TD in Step 3
STEP 5Using known TD solve for ND: ND = CD minus TD
Example 1/2A FF with NFM=7" TFM=24.25" CD=22 cu in
STEP 17 X ND = 24.25 X TD
STEP 2ND = 24.25TD / 7
STEP 3(24.25TD / 7) + TD = 22
STEP 43.5TD + TD = 22 or 4.5TD = 22 so TD = 4.9 cu in
STEP 5ND = 22 - 4.9 so ND = 17.1 cu in

Each of the two basic float setups uses a three-point support. The one dealt with here utilizes a single front float and two at the rear. You may reverse this arrangement to maintain a more realistic, scalelike appearance, but there will be a couple of disadvantages.

Greater surface area will increase drag, and the float spread necessary for stability will demand a more extensive strut system. Two things must be considered before the cubic inches are molded into an efficient float shape. First, the float should be capable of leaving the water's surface quickly. Second, it must be "clean" or drag free. Because of these requirements, we've chosen the popular sled type. The upswept nose prevents digging in, and water does not cling to the tapered rear. The thin- silhouette offers little frontal area, yet a large planing surface. Here the rear floats differ slightly in that they're narrow and deep to keep the fuselage well off the water.

Fig. 3 is a typical float plan. Although designed specifically for Half Alpha (a 1/2A F.F. that appeared in the July '59 issue of M.A.N.) these floats will fit any other .049 powered model that weighs the minimum 8.5 ounces and possesses, nearly the same TFM/NFM relationship as shown in Figs. 2-A and 2-B.

Marks are spaced at quarter-inch intervals along the plan's sides. Connect them. They'll aid you in enlarging the plan to a desired float size and in plotting the upper and lower curved surfaces of each float. More important, the grid is used to compute the displacement of any float, large or small. Simply ocount the squares within the float outline. From this determine the side area in square inches and multiply that amount by the float's width (also in inches) to get the cubic inches displaced. The rear floats are drawn side by side only to simplify the displacement finding and construction.

Choose medium hard balsa for the float's bulkheads, sides and all crossbraces except those to which the wire struts are attached. Use only spruce for these if you want to get more than a short season's use from a set of floats. Medium soft to soft balsa will do for the planking. Rear floats are planked with thinner balsa and have fewer braces. Thinner planking may be used on top of the front float too. Naturally, as float sizes increase, thicker wood is used, for example the bulkheads and sides of floats suitable for a .23 powered model will be 1/8" sheet; 1/16 to 3/32" planking will also be needed.

The wire struts vary in size too. One-sixteenth music wire may be fine for the 1/2A models, but only 1/8" wire is satisfactory for the .23 floats. Wire can run a size smaller on the rear floats. All mountings must be rigid. When in doubt, use next larger wire size.

Construction proceeds in this fashion. Bind and cement the brass tubes and wire struts to the spruce braces. Erect the bulkheads and sides and cement in the crosspieces. Build the rear floats inverted on their only flat surface; later remove them from plan and glue in the wire strut and its brace. Apply bottom planking first so joints may be double cemented. Then add top planking. A good sanding and rounding of all corners prepares the way for two coats of sealer. Cover smaller floats with tissue, medium-sized floats with silk or nylon bottom and tissue top surfaces and the giant varieties entirely with fabric. Doping may be tiresome but don't stop until the floats have a glasslike finish.

Enough water can be absorbed by poorly finished floats during the float test to change the flying trim completely. Finally add the fuel-proofer and a coat of wax. Bolt the floats to the stab's trailing edge. Small plywood squares reinforce these areas. Mount the front float and bind the angle braces to the main strut with small rubber bands.

Set the front float so it has three to five degrees positive incidence when checked against the fuselage center line or line of thrust. Test glides and, later, power flights ought to be made over a grassy-soft field. Usually it's necessary to remove incidence from the stab. You might notice that the turn tends to straighten out and that climb is slightly shallow. Correct by resetting the fin's tab and remove any downthrust. When satisfied with the climb, pull-out and glide, head for the pond.

Here's where the model won't forgive the faulty launch. Don't attempt to push the model downward until the water meets the waterline. If you do, the plane's initial forward surge wilI cause a wave to wash over the floats' top surfaces and there's no hope for a successful ROW. Instead, rest the model lightly on the water's surface release hands-off style and it'll skip off in three or four feet.

Continue testing, playing with the front float angle and glide trim. The object: to obtain consistent ROW's with the float at a minimum angle of incidence where best glide occurs.

aalmps 10/99; REV 2/09