GP Electricub on floats.. Question on CG..  
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GP Electricub on floats.. Question on CG.. - 5/10/2008 3:05:36 PM   
Geardaddys


 

Posts: 216
Joined: 8/21/2006
From: Woodstock, MD, USA
Status: offline 		I have a Electricub that I've been flying with wheels for the last few years.. Put a brushless/lipo setup in it last year so that if I put floats on it, it would have plenty of power.. Anyway, I got a set of Eflite floats (For the J3 that they also make) and they are about the perfect size for my cub.. Mounted easily and look great.. My question is regarding the CG.. I Flew the plane a few days ago and it was VERY "Pitchy".. It balances level on the stand, where as it used to balance with a slightly nose down attitude. I've added a bit of weight to the nose, but due to the pendulum effect of the floats, I'd need to add a considerable amount to get back to that "nose down" attitude.. I'm up to 4lbs 4oz already which is alot for that wing.. (20.8 oz sq ft. wing loading)

My step is located 1/2 inch behind the cg.. I think that it would help the situation to move the floats up a bit so that the step is on the cg.. All the reading I've read says that you should be behind or on, the cg.. Will it make much difference with the way the airplane handles on the water if I move them up a bit? Thanks for any suggestions! I'll try to post some pictures of it on the water..

John..


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       Post #: 1

RE: GP Electricub on floats.. Question on CG.. - 5/11/2008 12:25:44 AM   
Geardaddys


 

Posts: 216
Joined: 8/21/2006
From: Woodstock, MD, USA
Status: offline 		Ok, moved floats up a bit, and added a few ounces up front.. Flew the plane again today, and it did fly better.. One thing that I realized was that I didn't spend enough time trimming it out the first flight.. It needed A LOT of nose up trim, in fact I think that I used all that the transmitter had to hold level flight at a decent speed.. I guess it just needed more angle of attack to produce the required lift with the floats on it.. The strange thing that I can't get my brain wrapped around is that when I pulled the power back the nose goes up? It was a bit disconcerting on final approach to bring the power back, and see the airplane start pitching up.. Anyway, anyone know what would cause the nose to pitch up when the power is pulled back? Thanks..


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(in reply to Geardaddys)
       Post #: 2

RE: GP Electricub on floats.. Question on CG.. - 5/11/2008 1:07:55 PM   
JimCasey



Posts: 1527
Joined: 3/4/2002
From: Cookeville, TN, USA
Status: offline 		Consider that the floats contribute to the overall aerodynamic characteristics of the plane-they are not just there for landing.

Prop blast over the floats may be pulling the nose down. You may have draggy floats and all that drag is way below the rest of the plane where it pulls the nose down.
I usually find that floats make lift and cause a nose-UP pitch and I have to trim down a little.

If anything your floats appear to be forward of where they ought to be, by an inch or so, but the right place to install floats is where the plane flies best. Scoot them forward a little and give it a try. Since you are using the subfin, you should have adequate lateral stability to handle the floats farther forward. The plane also seems high on the floats. dropping down to where the normal axle would mount to the attachment points( Point of the yellow LG fairing) would still give you plenty of clearance for the prop.

I'm glad you put some power in it. I had the Mark 1 rev zero Electricub close to 20 years ago. Even though I took out all the weight I could when I built it, with the 550 can motor and the 6x1100 NiCd pack, it flew like a full-scale cub, overloaded, on a hot day. Which is to say, just barely.



< Message edited by JimCasey -- 5/11/2008 1:09:39 PM >

_____________________________

Jim Casey/Seaplane Nerd
http://www.smilesandwags.com/Floats.html

(in reply to Geardaddys)
       Post #: 3

RE: GP Electricub on floats.. Question on CG.. - 5/11/2008 8:37:54 PM   
WA Flyer


 

Posts: 21
Joined: 6/24/2004
From: EATONVILLE, WA, USA
Status: offline 		Nice looking cub. I had a plane on floats (PT-19) that exhibited exactly the same characteristic you describe. I concluded that the immediate pitch up with abrupt decrease of thrust was caused by the trim required to offset the thrust line. I made some 2 degree shims for the engine mount to add some UP thrust and the problem went away. After the mod, adding or reducing thrust didn"t make the airplane pitch at all.

I should add that after a thrust reduction, as the airplane slowed down, the nose would gently come down to maintain the trimmed speed.

It seems to me that adding floats to any airplane will necessarily lower the drag center of the airplane and this probably contributes to the original "out of trim" trim situation.

Hope this helps, Paul

(in reply to JimCasey)
       Post #: 4

RE: GP Electricub on floats.. Question on CG.. - 5/11/2008 9:31:21 PM   
Geardaddys


 

Posts: 216
Joined: 8/21/2006
From: Woodstock, MD, USA
Status: offline 		Fellas,

Thanks for the responses.. Some great ideas.. I think that the thrust is the issue.. With the floats it required a good bit of up trim with power on.. I'll try adding some up thrust to see if that helps.. Paul how did you make a 2 degree shim? I may have to suffice for the "add a washer or two" technique.. I have a perfect little "duck pond" just down the street from my house so I'm really hoping to have a decent flying electric floatplane to fly when I don't want to drive to the flying field.. Jim thanks for the input as well.. My step is almost right on the CG now.. The balance is a little better. I really hate to add more wt. to the floats as the plane is pretty heavy now as it is.. As for "Lowering" the plane, I'll look at that.. I thought it looked a bit high. The float kit was designed for the eflite cub, mine is the Great planes kit, and the dimensions are a bit different (mine is a little bit smaller)..

(in reply to WA Flyer)
       Post #: 5

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