Making a Float Plane (2 of 2)

Float Plane – Part 2

In my article Float Flying – Part 1, I wrote an article about all the basic components of a float plane. Now it’s time to put that useful information into practice. In this article, I shall outline the basics and provide tips for installing floats on a wheeled plane.


floatdiagramFig. 1

The most important step is to buy (or make) the correct size of floats. As shown in Fig. 1, to determine the correct size float, measure from the prop tip to the rudder hinge line and multiply the measurement (in inches) by 0.75. For example, the Piper J-3 Cub from Hanger 9 is approximately 46 inches from the prop tip to the rudder hinge line. Multiplying 46 inches by 0.75 will give me 34.5 inches. That means the absolutely smallest set of floats that can be put on this plane is 34-1/2 inches long. With this knowledge, I can look at many different types of floats and pick a set that should complement the Cub adequately. Hanger 9 sells 36 inch floats designed for their Cub that can be installed with only minor airframe modifications. You should always buy or make floats that are a little (but not by much) bigger than what you need. Unless your plane is as light as a glider, the extra buoyancy that larger floats give you will prove to be beneficial.



Once you’ve purchased the floats, the most difficult part will be to properly place them on the fuselage. Usually, pre-made floats will give you general tips on the proper placement of the floats in relation to the center of gravity for your plane. Generally, you want the “step” of the floats to be at or up to zero inches to half an inch aft of the center of gravity. The “step” is the transition on the bottom of the float from the pontoon skids and the aft float section. Most manufacturers of pre-made floats will give you a diagram with their specific requirements.

If your purchased floats don’t come with instructions or you made your own, there is still a fairly easy way to install the floats. The first step is to properly place the floats in relation to the center of gravity for the plane. Take a large sheet of roll paper (assuming you don’t want to write on your building surface) and draw one long line perpendicular to the fuselage on the paper. The line will represent the center of gravity on the plane. Place the floats with the recommended distance from the float step to the plane’s center of gravity. Secure the floats so they don’t move around. Next, remove the existing wheels from your plane and set the fuselage on some bricks or narrow books to raise the fuselage off of your building surface. The fuselage should be placed parallel to the floats and be centered between each of the floats.

Draw marks on the underside of your wings where the center of gravity has been determined for your plane. Use a plumb bob to correctly place the fuselage in relation to the perpendicular line that you drew on your building surface. It can be helpful to have an assistant to make sure the plane doesn’t move while you are setting up the proper placement of the plane. A framing square or triangle can be used if you choose not to use the plumb bob method.

If you’ve done everything correctly, your center of gravity should be exactly 90 degrees over the line on your building surface. The float step should also be properly set back the required distance from this reference line. Now you can mark the location(s) of the landing gear mounts for the floats. Some surgery will probably be required in order to add the necessary wood mounts for the landing gear for the floats. Usually a strip of 1/4″ thick aircraft plywood is sufficient. If the landing gear that came with the floats is the wire type, you can use landing gear straps to secure them to the hardwood mounts. If they are the dural type of landing gears, then be sure to install blind nuts in the hardwood mount. Always glue in the landing gear mounts with epoxy. Since every plane is different, it’ll be up to the builder to determine the best way to install the landing gear mount(s).

By now your floats should be properly placed in relation to the fuselage and you know where to mount the landing gears from the floats. There is one final item to check before you permanently add the landing gear mounts. It is advised that you check the incidence of the floats in relation to the wing. To do this, temporarily shim the floats on your building surface so the top of the floats are level. This represents your 0 degrees. The wing should always have positive incidence to the floats. If your floats are at 0 degrees incidence, the wings should be 1 to 2 degrees positive. If the incidence of the wing is 0 or negative, the plane will “plow” the water instead of properly “stepping up” on the float step when you attempt to take off. You might need to modify how you plan on installing the landing gear mounts in order to achieve the needed positive incidence.

It will take a little work, but if you follow these instructions, your efforts will give you many years of great float flying.


Special Considerations

Another important part of floats is the water rudder(s). It is highly recommended that you have a water rudder to aid in steering when taxiing around the lake. Some floats are already setup for a water rudder and will include instructions on how to install the water rudder and how it will be driven. If you buy or make your own floats that don’t include a water rudder, you can easily add a water rudder to your floats. If you are going to have just one water rudder, it should be on the right float.

I have a set of the Hanger 9 floats which requires one servo to be mounted inside the float. In order to drive this servo, a method of hooking it up to my receiver was needed. I needed to run a servo wire extension from the receiver to the servo. I could have either mixed the water rudder servo with my regular rudder servo or I could have used a Y-Harness off the rudder channel. I chose to use a Y-Harness to keep things simple. I didn’t want to cut a hole on the side of my plane to run the wire extension through. I don’t get to do much float flying, so I needed to find a way of keeping the fuselage clean when I didn’t want to float fly. I also needed to have a semi-water tight method of connecting the water rudder servo to the servo extension coming off of the receiver. To do this, I chose to modify an Ernst charge jack to accept the female end of the servo extension. Now, when I’m doing land based flying, all you can see is a charge jack port. When I want to do some float flying, I just remove the rubber cover from the modified charge jack and plug in the water rudder servo into the jack.

There are many different ways to run wires or hook up a water rudder. There are too many to actually list. It will be up to the builder to determine the best way to install all the necessary linkages and wires. Just remember to keep it water tight. Water and electronics don’t play well together.

I hope you’ve found this information useful and now want to try float flying.