Barnz316

My Feedback: (26)

Hello all,
I have a question about floats and mounting them to the C.G.. I balanced the plane with a empty tank of fuel to find the CG but realized that it would be WAY off with a full tank. Should I just go with a happy medium and balance it with a half tank? Any advice would be greatly appreciated. Thank you

FHHuber

You balance the model with empty tank to set the airplane's CG. Then fill the tank and MARK the forward CG with full fuel. Mount the floats for that CG location.

Its OK for the thing to float a little tail-low with empty tank. It is not OK for it to float nose low.

The CG of the plane should not substantially change wth the floats mounted correctly. The center of boyancy of the floats will be pretty close to thier center of gravity. Once the floats are mounted... you re-check the aircraft CG with empty tank to verify its the same.

JimCasey

My Feedback: (1)

I'll bet the full tank doesn't move the CG very much.
On a 6-pound airplane, an 8-0unce tank 5 inches in front of the CG will move the CG by less than 4/10 inch. If you're being really attentive, you may notice you need to add a click of down trim after you've flown nearly all your fuel out.

All Thumbs

For what it's worth: I'm only on my first float plane, but I've had good success mounting my floats with the step exactly 1/2" back from my empty tank CG. The plane is a 40-size Hobbico Superstar with the empty tank CG at 3-7/8". The idea of worrying about the CG changes with fuel had never occurred to me, but I couldn't be happier with the performance I ended up with (my CG is a fourth of the way back from the front of the recommended range).

Chuck

Barnz316

My Feedback: (26)

I've balanced the plane with an empty tank and with a full tank and the difference between the two is about an half of an inch. The instructions say to mount them slightly forward of the CG but others have told me to mount them behind the CG. Some say to find CG with an empty tank, others say with a full tank. If someone could help me with this, I would be GREATLY appreciative.

All Thumbs

I've not seen one place where it was written to place the step AHEAD of the CG. The best explanation I've heard involving the reason behind step placement is to visualize the step as the main gear in a tricycle gear setup, and those wheels are behind the CG. Your plane will sit on the step and rotate on it as you're taking off. The rear of the floats are for stability when you're sitting still or just slow-speed plough taxiing.

The few sources I've read, and the comments I've gotten in this forum pretty much make the CG and step nearly coincide, with the CG slightly ahead. I've always "assumed" that made the plane stable when it was running at high speed up on the step. The manual that came with my GP 40-size float kit says to mount them this way. They make no reference to a filled tank and merely say "CG". To me that would mean the normal reference CG which is with an empty tank. They say move the step 1/2" BEHIND the CG for the 60-size floats, 3/8" for the 40-size floats, and 9/32" for the 20-size floats. I just made mine 1/2" and I've never looked back.

Now I suppose that what I'm suggesting seems like the opposite of what FHHuber is suggesting, and I have no idea why you'd want your CG on the rear of the floats, but I've only built one floatplane and I'm not the final word. At any rate, I would normally think that if you're close you're in good shape. JimCasey (and/or others), come to our rescue and give us the physics behind that step placement , but I know I'll never change mine...it works perfectly.

Chuck

All Thumbs

For what it's worth, here's a picture of the data in the manual that came with my float kit (obviously for use with all three sizes available). I'm curious what brand of floats you're putting together?

Chuck

Edit: I think what GP has written here is a little confusing as to what order you do things...I forgot about that. Your step location is at 1/2", period (not varied like I wrote above, sorry). You would need to know the planned final location of your CG in order to mount the floats. After they're mounted, you'd set the CG to match what they've listed, but I'm certain they mean the final result is having the step 1/2" back from the final CG (otherwise your step would be even farther back!). As you can see, they suggest a more forward CG than what you'd use without the floats.

Barnz316

My Feedback: (26)

THANK YOU Chuck for your reply and please don't think im arguing with you but the instructions to my floats state "the step of your floats need to be positioned on or slightly forward (1/4) of the CG"(see image below). The only thing I can come up with is the difference between the float designs. By the way, im building u.s. aircore floats. Like I said, please don't think im arguing with you, I just want to make sure I get this right. I really dont wont to be picking up the pieces off of the bottom of the lake. [&o]

Im looking for an educated answer or general rule of thumb to help me with this problem. Or should I just mount them according to the instructions and hope for the best. Oh, Im putting the floats on a .40 size, Extra Easy 2, if it matters.[img]

Barnz316

My Feedback: (26)

Sorry, the image didnt load on the last reply[img][/img]

All Thumbs

Gotcha. Well...myself, I'd go with what the designers are saying. It looks like those floats have a more forward step than the GP design, doesn't it? The majority of my float is the front part. Discussions are good, and like I said, I'm new at this. It appears to me that the float design has a lot to do with it. The other sources I've looked at were referring to more scale-like floats like the GP design, but I think you better go with what Aircore is telling you. I'm tapped out for info now . Maybe someone else can wrap this up. BTW, I can't read any of the lettering...do those instructions give any extra clue as to how they're referring to the CG?

Chuck

Barnz316

My Feedback: (26)

Sorry about the last image, hopefully this one is better. No, the instructions say nothing about how they're interpreting the CG. It just states that the step should be at or infront of CG. The way Im thinking is that if I mount the step to an empty tank CG, then it will be nose heavy with a full tank and want to "plow" through the water.[img][/img]

All Thumbs

I just don't know if it's that sensitive, but we'll wait for more experienced folks to chime in. To me, the biggest deal is to be able to get up on step. You'll start off with full back elevator until you do. My understanding was that as long as your wing had enough incidence to the top of the floats, you'd get a positive lifting force that will take you off the water. Most of the problems I heard of with ploughing permanently through the water were related to wing incidence...but I'm going to stop giving advice here because I'm not as well read as others .

Your floats look like flat bottoms to me (as best as I can tell). Mine have a bit of a keel to them which probably accounts for the difference in design. Maybe your floats will get up and plane on the water faster. I'm certainly learning something from this thread anyhow...that there's bigger differences than I realized. Perhaps you should shoot off an email to the manufacturer and ask them to clarify the CG they refer to?

Chuck

Irish R.Seer

My Feedback: (1)

If the step of your floats is at 50% of the float length then the step should be positioned right on the CG as shown on the plans. I've been flying off water for 5 years, and this is how Iset it up and it works great.

JimCasey

My Feedback: (1)

The most authoritative source I have found says to put the step at 40% of the mean aerodynamic chord.

What does this mean?
It's pretty easy if you have un-tapered wings: Most trainers have a chord of about 10 inches, and the step would then be under a point 4 inches back from the leading edge.

Aha! But the floats are rigged with the wing at a positive incidence. So is the float under the 40% mac with the wings level, or with the floats level?

I don't know. Once you are this close, nothing is cast in stone.
at this point, it's splitting hairs anyway.

Suspend the plane so it is balanced with the wing at zero angle of attack. drop a line straight down from the 40%MAC and locate the step of the floats there. Rig the floats so with the float level the wings have 3-5 degrees positive incidence. It's a proven starting point

When it sits in the water, engine off, it should sit nose-up, but without the tails of the floats sunk.

It will fly like this, and have manageable taxiing characteristics. You can try variations of different incidence, and cheat the fore-aft location a bit if you want. If they porpoise on takeoff, the nose of the floats is probably a little low. If they're directionally unstable on takeoff, you can probably slide them a bit aft. just make sure they are parallel in all axes, and rigidly mounted.

jrf

My Feedback: (551)

Jim;

Is that information for fullscale or model floatplanes?

Jim

JimCasey

My Feedback: (1)

Well, that was for models, but float rigging is not subject to any Reynolds number effects that I have ever been exposed to, so it should work for full-scale also. I was once taken to task by Mr. Ben Lanterman, who has impressive aerodynamic credentials, for showing a photo of a full-size Maule floatplane as a how-to. To this day I have never understood his criticism.

jrf

My Feedback: (551)

Jim:

I don't have impressive aerodynamic credentials, but I have 25 years of model float flying experience and close to 100 model float planes to draw on and that experience tells me that model float planes set up like full scale have very limited application. Large scale models, flown in a scale manner can work that way, but most sport models require a different approach for best on-the-water performance.

The best model setup also varies considerably depending on things like downthrust, airfoil and power available.

Our area of greatest disagreement seems to be the high positive wing incidence you usually recommend. I have found that 0 degrees works very well for anything with a symetrical airfoil and that semi-symetrical and flat bottom airfoils are easiest to take off if set up with a small amount of negative wing incidence.

The theory is that, as the model builds speed on the water, if left alone, it will seek it's flying attitude. Good, safe water handling requires that the floats be level with the water during that phase of the take off run. Since a model with a highly cambered airfol (like a flat bottom) at cruise speed or above flies at a slightly negative incidence, the floats are best set up to be parallel to the water with the wing at that slightly negetive incidence. Actually, the horizontal stab usually comes closest to flying at 0 incidence, so I (and many of the model float plane manufacturers) set the top of the floats parallel to the stab. The result is that the airplane will run across the water, hands off, at full throttle until you decide to take off, then a slight pull on the elevator will break the model free from the water.

To get back to the subject of this thread, models with a lot of downthrust (like most trainers) under take-off power will try to bury the front of the floats, which usually causes the model to porpoise. I have found that moving the float forward until the step is as much as 1" in front of the CG helps to overcome this tendency. Normally, landing with the step that far forward would encourage the model to bounce on landings, but coincidentally, most trainers land slowly enough that this isn't a problem.

Try it, you'll be surprised.

Jim

Barnz316

My Feedback: (26)

THANK YOU ALL

I am VERY appreciative for all of your help and input. I mounted the floats last night according to JRF's reply and the instructions. The step is 1/4' in front of the CG. All of the water around my area is froze over right now but as soon as I get it on the water, I'll reply and let you all know how she worked. Thank you all again.

All Thumbs

If it was simple and easy, and all theories were set in stone, it wouldn't be fun would it? Personally I think you're in no danger of having a wildly unstable craft with the care you've taken. You're going to have a blast!

Chuck

JimCasey

My Feedback: (1)

I think all the "rules" are just guidelines...good safe starting points. I have set up planes with zero incidence, and with pretty substantial (>5 degrees) incidence, and I have put the step under the CG and a little aft. All are flyable, altho' my results have been happier with more incidence, even tho I can see where that might get me into trouble. Like somebody said earlier: The rules ain't set in stone. If it works, it was close enough to being right. It's all about having fun.