Float mounting question
 

I have been having some float mounting issues and am hoping someone can confirm I am on the right track or alternatively, set me straight.
The plane is a Goldberg Eagle II and the floats are a pair of fibreglass 31.5” ones that I was given – no mfr information. I mounted them according to the conventional wisdom: float tops 2 degrees negative to wing and step 1/2” behind CG. The mounting was a wire N strut arrangement. I fabricated two kick-up water rudders actuated by a flex pushrod from a stub shaft in the nose gear bracket.
At first, the floats threw a lot of spray into the prop, but then I learned the trick of full up elevator until the plane gets on the hump, then neutral to take-off speed and then up again to lift off. Everything fine at this point except that the up elevator it needed to get off the water put it into a pretty steep climb as soon as it lifted off.
At one point, it started going into high speed turns as soon as it got on step. Needless to say take off was aborted. After a bit, I discovered that one of the strut X braces had come loose and concluded that the resulting flex in the mounting was causing the floats to go a bit cockeyed, causing the uncontrolled turn. I fixed the strut and at the same time, being a natural born tinkerer, added spray strips to the insides and shortened the rudders, since they were deep enough to stay in the water while on step. This did not cure the high speed turns. If anything they got worse. I got it in the air a couple of times and it flies fine, but most takeoffs were aborted. You can see the S shaped wake in one of the photos.
For unrelated reasons, mostly due to being off the sticks for a few years, I managed to demolish the float struts, so back to basics. The recommendation for incidence is that the top of the floats should be 2-3 degrees negative relative to the wing chord. Looking at float plans (Cunningham and others) I see the bottom just ahead of the step at something like 3 degrees positive relative to the top of the floats. I measured mine up and found that the bottom is actually about 1.5 degrees negative relative to the top. Mounting the top 2 degrees negative results in the bottom being 3.5 degrees negative, whereas the Cunningham design (for instance) results in the bottom being 1 degree positive to the wing.
So now I am thinking that the negative incidence on the float bottoms puts the centre of water drag too far forward when the plane is on step, but before rotation so that any side to side variation causes the uncontrolled turns and that the reason it only started recently is that the drag from the long rudders while on step compensated for the forward drag from the float bottoms.
Thoughts?
Incidentally, the plane is powered by a Webra ball bearing .40 w/ 10x6 prop and has enough power for multiple inside loops and inverted flight (barely) even carrying floats. My lake is at about 3000 ft elevation.

RE: Float mounting question
 

I personally do not have the answer, but here are two websites I used when putting floats on my plane.

[link]http://www.smilesandwags.com/Floatsite/FLOAT%20SELECTION%20AND%20FINE%20TUNING.html[/link]

[link]http://www.seminolerc.com/float_flying_basics.html[/link]

RE: Float mounting question
 

Questions like this are helped enormously if you will include a good perpendicular side view with the top of the float decks level.

Hard to tell from the picture but its possible that the wing to float top decaledge is insufficient to allow the wing to reach takeoff angle of attack before the heels touch the water when fully up on the step. Depth of the step is also a consideration here.

Also in my opinion the step a half inch behind the CG is a bit much on that size airplane which will require and excessive amount of elevator to sufficiently rotate for the takeoff which always cause problems after leaving the water.

On that airplane I would use a range from the step even with the Cg to the step no more than a quarter inch aft of the CG.

John

RE: Float mounting question
 

This attachment is a copy of Chuck Cunningham's article in RCModeler of many years ago.
A couple of things I've added after float flying for approx 25years

I put about 1/2" toe in to the front, it helps to track.
I make the spread about 1/4 to 1/3 of the of the wingspan.
The tip of the float is 2 1/2" in front of the prop.

Good luck

HughO

RE: Float mounting question
 

I believe you diagnosed your problem correctly. When you get up on step, the negative incidence in the bottoms of the floats is letting the plane rock over onto the noses of the floats. It's like trying to push a wheelbarrow really fast.

It is conventional to set the plan up relative to the tops of the floats, but the real parameter of interest is the planing angle of the bottoms and yours is off. Actually your "free" floats have been pretty expensive........

Also, just additional information: the incidence of an Eagle wing is about 1 1/2 degrees positive from the bottom....the chord line is measured from the most forward edge of the dowel to the center of the trailing edge.

Somebody already threw you a link to my website: I don't sell anything but I threw several links on there where to get good, foam-cored floats. It has been reported that RNZ is no longer operational but the website is still up. Plane Fun Floats and Seaplane Supply are both good choices.

Oh, and don't blame the motor for inadequate power to perform inverted maneuvers: it's the Eagle airfoil. The almost sharp leading edge is the culprit. I had an Eaglet50 (same, just smaller) and tried everything to get an outside loop, including terminal-speed dives for airspeed. All I could get was level inverted flight regardless of the position of the stick. With a good set of floats I generally notice IMPROVED inverted performance.

RE: Float mounting question
 

That is a good plane for flying off water. Try backing off on the power to say 1/2 or 3/4 as soon as it gets up on the step. tease the elevator back a small amount. Sounds like your yanking it off before its ready to lift off.
Falcon

RE: Float mounting question
 

Remember: That "2 degree positive incidence" thing is only a "Rule of Thumb"

The best thing to do is to experiment a little and see what the results are.

RE: Float mounting question
 

Scubanero,
I think you are on the right track to solving your problems. I've found some of the trainer type planes have loads of down thrust in the engine which pulls the front of the floats down when excellerating. I think the down thrust was put in to help prevent the plane from nosing up during excellerating due to the drag of the wing being high above the thrust line. When you add floats to a high wing plane you also add drag below the fuselage, so in theory you shouldn't need the down thrust in the engine any more. This is just my opinion and I have no proof to back it up.
Seaplane

RE: Float mounting question
 

Thanks everyone, especially Jim and Seaplane. I hadn't thought about downthrust, which is considerable for the reaons stated, but it would help push the centre of drag forward as well. It was actually a mention on a thread in this forum of the step being too far forward causing the same problem that got me thinking about the water/float contact point while on step. I think the heel angle is OK. I never stalled on lift off, it just didn't look great. The relative length fore/aft of these floats is such that the tip is just even with the prop, but I never had a problem with nose-overs either. Not much expense involved, just lost air time.

I am contemplating building a new set of floats per the Cunningham guidelines this winter, not because I don't think mine can be made to work, but to make improvements to the tip position and the spray control (concave prow). Also, I found a little bit of water in one of mine and like the thought of a foam core.

RE: Float mounting question
 

Although Ralph (Seaplane) is very high on the list of people I want to be like if I grow up, I disagree with him on the downforce thing.
Floats are a lot bigger than wheels but frontal area is not the only factor in drag. I doubt if floats make much difference in drag. Floats are a lot more streamlined than wheels.
Before WWII, the fastest airplane in the world was a seaplane, and it had BIIG floats.

I have never changed downforce angle when adding floats and never found it to be indicated.
In fairness: One of my flying buddies in FL set up a Kadet Senior with zero downthrust. that worked too.

An IggleII is a .40 size trainer. You should need floats 32-35" long. May I humbly recommend that you consult further with Ralph, since he has taken an interest in your predicament. He makes some really spiffy foam-cored floats and is developing a cult-like following here. Save the Cunningham article for later-you can always make your own custom floats when you make a scale Thrashwell-Snailby Belchfire MKIV or aqua-space shuttle.

Ralph's website is http://www.geocities.com/planefunfloats/