Posts: 64
Joined: 6/3/2002 From: new port richey ,
FL, USA Status: offline
Need a little help. I recently bought a 1/3 B-USA J3 nad I put a larger motor a G-62 and I ordered a set of sea commander floats. To make these changes correctly I need to know the planes CG. Any one who has bult this plane before, please let me know how far the CG is back from the leading edge of the wing in inches.
Posts: 13360
Joined: 3/20/2002 From: San Bernardino County,
CA, USA Status: online
Almost any J-3 on floats can be set at 40% rearward from the leading edge ( a little more than normal ) with the float step to corrispond with the CG.
Don't forget to add a lower verticle stablizing fin. If you don't....you'll find out why.
Dave.
_____________________________
An engineer says.... "That won''t work". A mechanic says..."Oh yeah, watch this". "Old Age, and Treachery will overcome youth and skill". Revver Bro #4.
Posts: 64
Joined: 6/3/2002 From: new port richey ,
FL, USA Status: offline
Hi Dave
Thanks for the come back. Why put the CG 40% back when the rule of thumb is 30%. And what is the benifit of the lower verticle fin. This is new to me and I don`t have all the answers.
Posts: 19353
Joined: 4/22/2002 From: Willmar,
MN, USA Status: offline
As far as the 40% goes, (Dave if there's a specific reason for this please feel free to correct me) But I find that every plane is different. What I do is to set the CG as indicated on the plans and fly the plane (Without floats). Then, adjust the CG to where you like it. Mark the CG point
Attach the floats so that the step is at the CG mark, and the floats have 1 degree of negative incidence. Now, re-balance the plane so that it again balances at the CG mark.
Now, fly it off the water, and decide whether or not you want to re-adjust the CG for float flying.
The lower vertical fin is to provide extra tail surface to counteract the increased area that the floats add to the nose.
Posts: 13360
Joined: 3/20/2002 From: San Bernardino County,
CA, USA Status: online
....I'm not an "Expert", but let me offer my own experience. Minnflyer knows that planes are very different when it comes to floats. First, the drag they cause....think of it as dragging two small parachutes off the main gear. This starts problems....the first of course, is an increased power requirement.
The second, is the increased frontal area induces yaw, especially in cross-wind flight. This is the reason for the "fin"....to increase the area of the verticle stab, to offset this tendancy. See pic below. This is the fin requirement for a Goldburg Anniversary Cub. It is substancial. Look at a purpose-built seaplane, an you will notice an very large verticle stab. When we "add" floats to a general aviation type plane....we must also "add" some more vert. stab.
The CG thing....Most model J-3,s have alot of negative incedence built into them. This keeps them from climbing wildly, as speed increases, an "zooming" up when turned into the wind....like a typical high, flat-bottomed wing trainer. This is a real problem for a Cub conversion.
Remember that the Cub is no longer a tail-dragger, just sitting there with all the positive incedence in the world, just waiting for a little air-speed, and it's up. It's now sitting there...flat....on them great big floats, and it want's to stay that way. Keep in mind that the plane will no longer "rotate" in the normal fashion either. You have increased the weight, and drag of the plane. Not to mention the water cohesion. Speed and horsepower are our friends now, more than ever.
Moving the CG back on the Cub helps alleviate some of these problems....getting the floats "on plane" for take off, and offsetting some of the float induced drag...that wants to pull the nose down. Minnflyer's mention of 1 deg. of neg. incedence in the floats....results in 1 deg more positive incedence in the wing....when the plane is in the water....again, helping in the take-off roll-out.
I hope this helps....Dave.
_____________________________
An engineer says.... "That won''t work". A mechanic says..."Oh yeah, watch this". "Old Age, and Treachery will overcome youth and skill". Revver Bro #4.
40% seems a bit extreme. If it works, great, but I would be hesitant to make that recommendation to others without knowing their skill level and their particular set up. I have flown tail heavy floatplanes (including a cub) and it is not fun......
I keep the cg in the 25-33% range. If it is any help, the BUSA recomended cg for the quarter scale cub is 4.125" on a 16" chord wing ( or 25.7%) which is very conservative..
_____________________________
All I ask is for a chance to prove that money can't make me happy......
Posts: 13360
Joined: 3/20/2002 From: San Bernardino County,
CA, USA Status: online
Wow.....25%....that's to the nose heavy extreme for me. Then add 12-14 oz. fuel. My first seaplane wouldn't lift-off cause it was nose heavy, that's when I adopted the way back approach. Even at 40 %, with a half tank of fuel....in reality your still in the 33-35% range.
Keeping in mind that the plane will be landing under power...I like the light nose....tips slightly high approach.
But on the other hand....the ol' 1/3 back CG will work for most....most of the time.
Dave.
_____________________________
An engineer says.... "That won''t work". A mechanic says..."Oh yeah, watch this". "Old Age, and Treachery will overcome youth and skill". Revver Bro #4.
Posts: 64
Joined: 6/3/2002 From: new port richey ,
FL, USA Status: offline
Hello Gentlemen I am taking all this info in. I have mounted the soreader bars on the sea commander floats for my 30% J3. Before I go any further, I think I will fly the plane using the wheels first and establish the proper cg and come back and finish mounting the floats. As previously mentioned, I bought the plane off the second owner and it already had the B-USA floats on it. When I first flew the plane it wouldn`t lift off and it was just getting up on a plane when I ran out of room at the lake after about 500ft and I ran into the opposite bank and broke the right float. I felt kind of dumb but now I feel that the plane may not have been set up properly. That won`t happen again with these floats.
Posts: 13360
Joined: 3/20/2002 From: San Bernardino County,
CA, USA Status: online
Here's a couple more things I will offer....
1- At the rear of the floats, either on the spreader bar....or the float mount.....make it "slotted ", so you can easily adjust the angle if necessary. 2- On my next seaplane, I'm going to employ a trick the "Pattern boy's" use. Set the fuel tank centered right over the C/G. Just say 1/3 for sake of example. Now the C/G will be the same, from the begining of the flight....till the last.
Keep in mind that 16 oz. of fuel weighs almost a pound. Say your plane is empty, and you set it in the water. See how it sits ? Tips down in the front, huh ? Now drape a one pound bean-bag over the cowling. Yikes, even worse. See the advantage of the fuel tank loc. ?
3- also I would say if you have down thrust in the engine....remove it. A little right thrust is of course usually a good thing.
Dave.
_____________________________
An engineer says.... "That won''t work". A mechanic says..."Oh yeah, watch this". "Old Age, and Treachery will overcome youth and skill". Revver Bro #4.
Posts: 1541
Joined: 3/4/2002 From: Cookeville,
TN, USA Status: offline
Tank on the CG: Good idea. That's why full-size planes have tanks in the wings. But with a r/c plane, [COLOR=crimson][SIZE=3]you'll need a pump.[/SIZE] [/COLOR] There's one advantage of seaplanes with pod-mounted engines: the tank is on the balance point and still close-enough to the engine so it does not need a pump.
Adjusting downthrust: Maybe a good idea, but only if the plane will never be flown from wheels again. My experience is that floats cause some up-trim, which eliminates the need for removing downthrust. The floats are a lot more streamlined than wheels: even though they are bigger, they don't really cause a lot more drag. (Remember the Schneider Cup? The fastest planes in the world had floats, not wheels.
_____________________________
Jim Casey/Seaplane Nerd http://www.smilesandwags.com/Floats.html
On a 40+ lb model, the location of the fuel tank is not going to make a noticeable difference to the cg whether it is empty or full. The good news is that nearly every gasser has a pump, and there is plenty of interior room to work with.
On a 40 -60 size sport plane - yes, there is a definite change in trim as the tank empties, so putting the tank on the cg would be an idea, provided (as mentioned above) that there is a pump, and that there is room to accomodate it, and all the radio gear.
Reg Sal`s dad...if you can get your hands on a copy of HANSEN SCALE VIDEO # 38, there are several minutes focused on a one 1/3 scale cub on floats, including an interview with the pilot and lots of nice flying ,landing and taking off shots..
_____________________________
All I ask is for a chance to prove that money can't make me happy......
Posts: 456
Joined: 9/25/2002 From: HOUSTON,
TX, USA Status: offline
hELLO guy,
Since I was your FIRST response it is appropriate to come back and at least show you that I DO have the SeaCommander floats, albeit in the smaller version of the CUB. This is H9 1/5 Scale. Going for the 1/3 scla floats right after this one is well established. Maybe going for them with the 1/4 scaler as well. Good luck RICK
Posts: 64
Joined: 6/3/2002 From: new port richey ,
FL, USA Status: offline
Thanks Gentlemen
I thiink I have enough info to confuse me...No, only kidding. The response was great from everyone. I am going to fly the 1/3 j3 with the wheels first, then set up my floats. The G-62 Wil pull the plane just fine and the floats speak for themselves. I will set the floats according to the best CG that I can determine. I think I will start using the 33% rule and take it from there.
Printable Version
All Forums >> RC Airplanes >> Seaplanes >> Where is CG on 1/3 B-Usa J3 
Where is CG on 1/3 B-Usa J3 - 
Reg Sal`s dad..... - 
Reg Sal`s Dad....... - 
Where is CG on 1/3 B-Usa J3 - 
RETURNING -