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I have a Kitfox III on aerocet amphib floats and was curious about modifying the rigging to move the floats forward. My CG is at the front limit and it is noticeable because floats tend to plow with the water line more than 1/2 way up the nose bumper. I vaguely remember someone talking about moving the floats forward. Is there a known procedure for doing this? Cut the rear supports shorter? Any info is greatly appreciated.
Matt
Hi Matt,
Sorry I missed your question, didn't see it until just now. I have a 912ul Kitfox 4-1200 on Aerocet 1100 amphibs that I have been flying a bunch of years now, so I know exactly what you are going through. In fact I was just pondering how to cure that issue while working on my Kitfox in the hangar last night.
If you move your floats forward, unfortunately you are going to make your forward CG issue worse as you are moving a lot of weight forward. Granted, you will get more floatation forward, which will help the bows sit higher. But by moving the floats forward you also will be moving the step forward, which will most likely result in porpoising that may or may not be controllable. Either way, you will not like the way it will handle in the water. So as far as I'm concerned moving your floats is not an option (unless yours are rigged further back than the original Kitfox/Aerocet rigging specs for some odd reason).
If your Kitfox is like mine, it probably looks perfect when sitting in the water without anyone in it. But load a person in it and it goes deep in the water all the way across the float, and the front bows actually sink deeper until they are just barely touching the bottom of the black rubber bumpers. So idle taxiing, especially downwind or quartering or crosswind will have water going over the top of the float all the while you taxi. Add enough power and the bows will start to raise up, but you are likely to get a lot of water spray through the prop all the way through the transition onto the step, where you are planing out on top the surface of the water.
So why do our floats do that? It's actually really simple...our floats were originally built for a 950 lb gross weight Kitfox Model 2. And if you put our floats on a Kitfox that never goes past 950 pounds all will be well. The floats will in fact support 1100 pounds (I proved that one day by mistake), but anything over 950 pounds and all of the reserve bouyancy is gone, which is why at around 1,000 lbs they start to go deep in the water. Research "Reserve Bouyancy" on the net and what I am saying here will make a lot more sense. Anyhow, using my airplane as an example, when I crawl in it with a full tank of fuel, on the floats it will weigh right around 1150 lbs, so all of the normal buoyancy is used, and actually exceeded. Of course we know the floats still work, but in reality we probably need another 150 - 200 lbs of buoyancy to get them to sit higher in the water. Unfortunately, we don't have that additional buoyancy. However, there are things you can do to improve the situation. Here's my list;
- When idling in the water, either go fast enough to plow, which raises the bows, or go really slow so you are not putting the float into a "full displacement" mode, which essentially means you are pushing the float as deep in the water as it will go. I fly a 912 Rotax, and they tend to idle fast. So I worked on mine to get it to idle slower, and that helped my bow low at taxi issue significantly. The only time it really bothers me now is when taxiing downwind or with a crosswind. But I generally can live with that.
- If you have large fuel tanks like I do, put shut off valves on each tank and then learn to operate without so much excessive fuel. My plane holds nearly 29 gallons of fuel, but my engine on average burns only a bit over 3 gallons an hour. So I will dump 8-10 gallons in my right tank and operate on that while keeping only a few gallons in my left tank as reserve. Doing that saves me almost 100 pounds of fuel weight, which is really beneficial because that fuel is actually contributing to a forward CG. So as little a thing as that sounds like, it makes a big difference in where the bows ride when taxiing.
- I used to worry about taking passengers because the bows were so low with just me in the airplane. But then I realized when I put a passenger in the bows actually ride higher, because the passengers weight is actually behind the CG, which helps move more of the load back across the larger part of the float where it has more buoyancy. So dumb as it sounds, it actually works better with 2 people than it does solo.
Fwiw, the Aerocets on my Kitfox are absolutley marvelous when on the step. They are super maneuverable and handle like a dream. My issues are only when off the step, going slow. And more of that is psychological than anything, as I have never had an issue from being "underfloated". But I have been flying floats for 3 decades, so maybe that helps? However, like you I would love to improve the situation. So here are my thoughts how we can relatively easily cure that issue.
Idea #1 - The easiest fix. Install a sheet metal extended spray rail on the inside of each float from the front bumper down the float about 2 feet. That will provide no additional floatation, but if the spray rail is bent at a 45 degreee angle to the float it will hydrodynamically "Lift" the float bows up whenever you have forward speed. So the faster you taxi the more the bows will rise up. And not only will it also help reduce water spray through the prop, it will reduce the time it takes to get on the step, resulting in an even shorter takeoff.
Idea #2 - Using the same general concept, make an extended spray rail out of foam and fiberglass. And make it bigger than the metal one described above, maybe even extending it down towards the step all just above the normal water line so that you get the benefits of the foam for additional floatation. Might sound odd, but if you have ever floated foam in water it is nothing short of amazing how much weight a small piece of foam will support in the water. I once tested a 4 ft x 6 ft piece of 2 inch insulation foam, and it supported about 200 lbs of weight before starting to go under water. (ever watch the kids at the beach all standing on a 2 inch thick semi-rigid 4' x 12' floating panel in the water? It will handle a lot of kids...)
Idea #3 - If you apply the foam floatation concept to this next idea, you will probably gain at least 200 lbs of floatation to your Aerocet 1100 Kitfox. Make a template of the sides of your floats and add a piece of 1 inch foam to each side, shaped into the existing tops and bottoms. Add in those built in spray rails in the process, and you will essentially be adding the equivalent of 2 of those 4'x6' 2 inch foam sheets to your floats. Granted, the only bouyancy gain will be what is under water, so my math is only approximate. Obviously this could be a big job forming the foam to shape so it looks good, and then glassing and painting and attaching. But I know this would really help bouyancy issues as I measured a lot of small floats over the years and typically a float of the same length but that is 2 inches wider will have 200-300 lbs more floatation. So I know this would work.
I love the handling of my Aerocets, the storage lockers, and how well they allow my Kitfox to fly. So I will continue to put up with them until one day when I do these modifications I've described. But by all means, if you have any other ideas, throw them my direction. I'm sure there are more ways to solve this issue, and maybe even easier?
Paul Seehafer
Note attachments;
1st pics is my Kitfox floating at Osh SPB - aircraft sits very nice, almost perfectly rigged. When not loaded it looks like any other float plane.
2nd pic shows the airplane taxiing out with just me and approx 20 gallons of fuel. Notice how low the bows are. Bringing the bows up with either aluminum spray rails or a foam and glass extended spray rail would probably eliminate most of the issue in my opinion.
Great post Paul. Very generous.
I always learn from and enjoy your posts on float flying. :)