View Single Post
Old 11-19-2014, 03:22 PM   #21
Senior Member
Join Date: Oct 2009
Location: Junction City, WI
Posts: 517
Thumbs up Re: I still need float-rigging info


Originally Posted by Lynn Matteson View Post
Sorry if this becomes redundant, but I haven't heard a word of response to a month-ago request for float-rigging info for my Zenair floats on a model IV. And I can't seem to get off a private message either. I know you float-fliers are out there, and I'm anxious to join the party. : )

Lynn Matteson
Ok Lynn, I have some info for you. I did some calculations using a variety of float install information I've aquired over the years, and then called a few people that I consider to be float experts to see what they thought about how to best float a Jab 2200 Speedster? The good news is that we all came up with approximately the same numbers. The bad news is that I can't tell you exactly how we came up with that, as we all used different methods to arrive at our numbers . But after talking about what would be best for your situation we all agreed that the best thing to do is to use experience rather than formulas. Many seem to think that float installation is a science, but the reality is that it's more like an art. So in the end the numbers we decided to use are based on our collective experience more than what you might find in some textbook. But we're confident that it's more likely to help you than if you were left to do it by the book.

But I do have to make this disclaimer; None of us are formally trained on any of this. We're just guys that have been flying these kind of floatplanes for a lot of years, and have learned mostly through our own trial and error. So don't be surprised if you find that you may have to tweak things a bit after you've installed the floats on your airplane. With homebuilts since no two are alike you almost always have to do some trial and error before you get it right. And while I'm pretty confident we can get you 85-90% accurate to where your floats need to be, it's likely that you may need to tweak the throat angle and/or step placement if you want to extract the top performance from your new floatplane. And if by some stroke of dumb luck our numbers work well without any tweaking, then you can just consider that a bonus you get to enjoy from all of our years of fiddling

So let's cut to the chase;

- First, set your spreader bars so that the floats are no more than 8 ft wide at the widest part. Typically that means that if you drew a centerline down each float lengthwise, from the center of each float (with Zenairs that would be the channel that you walk on), that measurement should be 6 ft. Our experience has shown that a lot of people put the floats too far apart, reducing their off-the-water performance of the float. Most people don't know this, but the bow wave coming off the front of each float helps to hydrodynamically lift the other float next to it in the initial part of the takeoff run.

- Second, set the floats up for about a 22 inch height from the top deck of the float to the bottom longeron on your Kitfox. That should give you enough prop clearance, while still benefitting from the use of ground effect to help make the wing lift sooner. You will notice when looking at most of Zenairs float install pictures that they tend to mount their floats further away from the airplane than what I'm telling you. That's not to say what they're doing doesn't work. But if the aircraft is kept closer to the float it typically results in having the wing closer to the water, which makes better use of ground effect (or should I say water effect?). The theory is simple, as the aircraft moves forward the air becomes slightly compressed under the wing, which helps to create lift sooner. Or at least that's what some of us believe (plus, sitting lower on the floats also helps with stability on the water)

- Third, level the Kitfox using the bottom door frame tube (this is very important). Then locate the most aft (rearward) center of gravity for your Kitfox, and run a plumb bob down from that area of wing to the floor.

- Fourth, slide the floats under the aircraft until you have located the step 3 to 5 inches aft of the rearward Center of Gravity range for your Kitfox (3-5 inches from where the plumb line is hanging). On my Kitfox equipped with Aerocet floats (which handles and performs marvelously), that positions the step directly below the middle of the 3rd (from the front) landing gear attach bracket (see the photo I attached of the float rigging dimensions for my Kitfox to see what I am describing. But note that the 23 inch measurement I am showing in the picture was done only so you could have an easy way to see EXACTLY where my step is located). Your step location should be somewhere near where mine is in the picture. And you probably will find it even further aft as the location of the step on my Kitfox is as far forward as you would want a set of Zenairs. If you put it further forward you are likely to have water handling issues (e.g. porpoising), and too much forward CG.

The Zenair amphib is about 2 ft longer than my Aerocet amphib, so when the Zenair is installed the bows with all the nosegear weight is much farther out in front of the airplane than mine, and likely will cause the aircraft CG to move quite a bit forward. So it is possible that you might need to move the float even further back than the 3-5 inches. The good thing is that having to move the step back because the float is long doesn't do much to affect water handling. The worse it does is (maybe) extend your water run for takeoff.

But have the step too far forward and you can cause all kinds of weird, and even scary water handling issues.

- Fifth, set the float angle. On an aircraft with a high power to weight ratio you can get away with just a 2 to 3 degree (tail low) angle on floats. But for most normal aircraft that number is more like 4 to 6 degrees. In the case of your Kitfox I would recomment going with a 4 to 5 degree angle (using the top of the float and the bottom of the door frame) as the Kitfox wing does generate a lot of lift for its size, and your overall wing loading is still relatively low.

The last two photos attached are just there to show you how the float should look when the airplane is at high speed on the step. Commonly a floatplane will ride on only about 4 inches of the float, the area just ahead of the step (also known as "The sweet spot"). It may look as though the back of the float is skimming too, but it is essentially that small 4 inch area ahead of the step doing all the work. That's why it is ultra critical to have the step in the right location.

One last suggestion is to study the photos of my Kitfox closely, as even though I have Aerocet amphibs and you have Zenairs, the concept is the same. So if you can just make a few minor changes to your rigging as explained above, it's likely you will get great enjoyment from your new Kitfox floatplane.

I will follow up this post with some more pics that I also feel might help you.


(please forgive my late editing - I had a computer crash earlier)

Last edited by av8rps; 11-19-2014 at 06:54 PM. Reason: Better clarity
av8rps is offline   Reply With Quote