khodges

My Feedback: (1)

Setting up my first floatplane--where exactly does the step go--AT the c/g, or just behind? The plane is a 80 inch H-9 Cub, the floats are GP's .40 size. I fiberglassed them, put a micro servo in the right one for water rudder. Instead of Y'ing the water rudder to the rudder, I used a separate channel and mixed it to the rudder. This way I can set the end points so I get more servo travel on the water rudder. This plane will probably stay set up for floats, already have a Cub on wheels.

Wayne22

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I had one of those! Great fun! What engine do you have in it...?

I prefer the step on my planes to be right where the wheels would be on a land plane, however, generally accepted practice is to place it slightly behind the cg.

MJD

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Interesting - why do you want more travel on the water rudder than the air rudder? Sounds like a recipe for water-looping. I would have thought you'd go with less travel due to the authority of the water rudder.

Mike D.

JohnBuckner

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For what its worth. On a float plane running at step speed i,e, planning (either landing or T/O), if the step is at or behind the CG it will have the same directional qualitys as a tricycle geared airplane and for the same exact reason (CG ahead of surface contract point).

An airplane running at step speed that is has the step ahead of the CG will demonstrate the same directional difficultys as a taildragger for the same reasons (CG behind the point of surface contact with the CG wanting to swap ends with that point).

For a powerful sport type plane CG a little ahead of the step can work OK simply because they are seldom on the water at planning speeds to make much differance. But if you want nice scale like water operations that many appriciate with airplanes like Cubs without water looping tendencies. Then my best suggestion is to set the step anywhere from at or just behind the CG perhaps up to no more than a half inch on that size airplane.

John

jrf

My Feedback: (551)

John:

Without disputing your conclusion, I suggest that you consider the fact that a float plane at high speed on the water does not have a contact "point" with the water. It has a contact "patch" extending three to six inches in front of the step. The contact "point", for purposes of your analysis might be the point where the water first strikes the float keel, of perhaps the center of the "patch", but it is certainly not the step.

Since the effective center of pressure on the floats (your contact point) is always several inches ahead of the step, even at high speeds, it is not likely that it will ever get behind the CG.

The behavior you describe may be true, but you might want to rethink your explanation of why it is happening.

Jim

JohnBuckner

My Feedback: (1)

jrf I can only respectfully dissagree. Wording semantics will not change the fact that the center of pressure, contact point or first structure on the aircraft to make contact (in the case of a landing) with the water will in fact be a tiny portion of the step. If the step is forward of the aircraft CG this is when the directional instability will manifest itself, just as in a conventional (taildragger) geared landplane.

Now consider the takeoff, agine the wording of your choice: waterline, footprint, center of pressure or planning surface will progressively get smaller as the aircraft rises out of the water and without a doubt that "point" will be just at the step as the liftoff is reach. Agine its at that point and a period of time before that the instability will manifest itself, simply put when all physical contact with the water is just at the step and if the step is forward of the CG, instability will result just as with a wheeled taildragger for the same reasons.

Just for the record I am not an engineer hydrodynamic or aerodynamic of any kind but I am a survivor of many installations done for myself and a lot of other folks and I,ve never had on installation perform poorly as is a common sight at many events due to poor installations. I never said a forward step and aft CG would not work with some types of airplanes. What I said for those beautiful water departures with the aircraft rising gently up onto the step and leaving the water at a nice shallow scale angle with Cub or cub like airplanes a step either at or just behind the CG is in my opinion is a far better way to go.

My recomendation continues to have the step at or just behind the CG.

John

khodges

My Feedback: (1)

Not one to take sides, but my own train of logic agrees with yours, John, that the point of ground resistance, be it wheels or the footprint of a float "on the step" should either be at or behind the c/g. This would tend to stabilize and reduce the tendency of the plane to yaw.

As for more travel on the water rudder, MJD, I should elaborate more. If I were to have the same travel with both rudders, as in a Y'd connection, it limits the authority at low taxi speed. I plan to use negative exponential to increase the travel at low taxi speed, when I would use a greater control input, and as I increase speed for my takeoff run, the decreased stick excursion of my rudder input would hardly move the water rudder at all. Desensitizing the water rudder servo around center, it shouldn't affect directional control at all, once I have enough wind across the tail for the air rudder to be effective. At any rate, I can flip a switch and disable the water rudder completely, if I want.

nony-- I have a .72 Saito in it. Probably overkill, but I had the engine available. I mounted it inverted, have an on-board glow, just for absolute reliability over the water, but I expect that, as in my 1/5 L-4 with an inverted .65, it will be fine without the glow. At least, with the glow plug underneath, it will make for easier starting.


I've still got to paint the floats, mount them, do a final engine install, and put the Rx and glow driver in place, and mount the ventral fin. As I don't expect to ever fly this from land, I'm not even mounting the tail wheel. I'll get some pics before I maiden it (while it's new and in one piece)

Wayne22

My Feedback: (2)

good choice! I ended up with an OS 70 after trying several engines - biggger and smaller. This gave the best overall performance.