Anyone running Zenair (or other) Amphib's? Thoughts?

[JoeRuscito]

I haven't posted anywhere, mostly because it would be a long novel. Anyone who is in need of, or wants to hear the whole dissertation is welcome to PM/get my number and I will do so verbally!

That said here is the summary, this is all highly dependent on initial CG and empty weight.

Three variables to deal with: Height above the floats, angle of attack, and fwd aft position. The first two while somewhat related are easier to determine, but do have some personal preference mixed in. I went with 5 degrees nose up relative to the bottom of the fuselage to the float top (on ground not floating). When the floats are floating this gives me a little more like 7-8 degrees AOA on the wing. As for height, I wanted to be a little higher to protect the prop and fuselage from spray. Roughly 23.5 inches measured at the rear attach point to the top of the float (perpendicular to the float).

Now the more complex variable took two attempts. The gear was weighed after being removed and the empty CG of the airplane was calculated to be 9.84 in. Note: In level flight. This is different then the CG when floating, and that is dependent on the variables above as well as your vertical CG position, which very few of us calculate. There is a balance between flight performance/cg and float performance/floating CG.

The first position the fuselage was too far back on the floats. And while it floated fine and wasn't unstable it took significantly longer to get off the water, close to 60 sec at gross! In this case the CG was only about 2 in FWD of the step (when on level ground).

I ended up moving the fuselage 5 in forward relative to the floats, which put my in flight empty CG (without wheels) almost directly over the CG or center of balance on the floats. Said another way, the CG is unchanged when installing the floats.

Happy to discuss more, even after doing it, it's easy to get all the variables jumbled. All in all you don't want it too far forward or too far aft, but somewhere right in the middle. Ha!

[av8rps]

Good explanation of what can be sometimes a very difficult thing to get accomplished.

A good friend of mine has built about a dozen airplanes, most of them different makes and models, and including some biplanes. And all of them were put on floats. And every one of them had great off the water performance. So one day I asked him how much time he spends trying to get the rigging right on all those different planes? He explained that his method is quite unusual compared to what others do, but it works well and is not very difficult or time consuming. So here is how he does it.

He said that since he typically sells airplanes on land gear and not floats, that he always has the CG of the aircraft on wheels. So when he puts it on floats his obvious goal is to have a similar CG as when on wheels. Following his experience and general basics like the distance of the floats to the fuselage, the throat (incidence) angle, spreader width, etc., he installs the floats. Once that is done he literally puts a pipe under the step of the floats and rolls the airplane around on a flat floor until the aircraft sits level. Where the pipe is in relationship to the step tells him if he needs to move the floats fore or aft. So then it is a simple thing to shift the floats to the optimum position, and he's done!

I thought that was a bit crazy until one day when I was talking to JJ Frey (who used to run EDO float corporation and also wrote the book "How to fly floats") and he said that sometimes they did the same thing at EDO, as rigging so many different types of aircraft was difficult without a lot of trial and error costing a lot of time and strut material. So I guess my friend's method wasn't so unusual after all.

I guess I'm just lazy. I try to copy rigging of other people (like you) that have already figured it all out! So thanks again for sharing.

Paul