I have a 40 size trainer that has a set of GP FG floats. I have to run this thing to 40mph on the water to get it to lift off. Could it be the wing is not big enough for the extra weight. Running a 40 FX OS on it and a 10x6 prop. Boy is she fast as a hydofoil ! When I get it to lift off ie: full up elevator....well it almost does a back flip !!!! I have increase the throws on elevator to max and the floats don't have water in them. The floats are 38" long, the rudder hinge line to back of spinner plate is 46 and the step is about 2.5 inches behind leading edge and the floats extend about 6 inches past prop arc. I bought this on RCU, so the plane is a unknown so not sure where the CG should be. I don't have any trouble landing, and it will just fly real slow. The only in flight tendencies it has seems to be squirrly pitch up and pitch down. Any ideas folks ?

Floats are a bit too long. 70-75% of fuse length of 46 should be 32-34in.
However your'e stuck with them for now. Check the angle of incidence of the wing to the floats. Put top of floats level and if you have an incidence meter on the wing make sure it is 1-2deg. pos. Also make sure the floats are mounted securely. If the can act like the are independently controlled both up and down and side to side[very wobbly] this can also lead to weird take-offs. It could also be too heavy. You state step is 2.5 in. behind leading edge. not familiar with the a/plane. Step needs to be on or very close to balance point of the plane. Squirrelly in pitch[up and down]? sounds tail heavy to me. Make sure you check the balance.Just a few thoughts,
Cheers, Dave.

yep, stuck with the floats....on this plane at least. I will check the other points you mentioned Dave, especially the tail heavy. I purchased thinking it had been flown before, though it looks all new, could be tail heavy as you mentioned, this too would keep it planted in the water as well. Thanks...Butch

Sounds like a float wing incidence problem..As Mogman said you need 1-2 deg of pos between the wing and float... You can get this by adding spacers to your front float mount, either to the fuse or the top of the float.. As far as your float step goes, you should be close to the C/G... But if it is hard to shift the floats forward, you can still try it at 2.5 behind. I have a Norseman on floats and my step is about 2.5 in behind the C/G and its fine on take-off.. Like my drawing looks like a Caveman did it O well you get the point...

Thanks CF, drawing is good....appreciate the extra effort in doing so. Time to get the old bubble level and incidence meter out and get to work.

Butch

What conditions were you flying under? A little head wind and some ripples on the water will generally decrease the take off distance by a great deal. Smooth water will make the floats stick like glue. Vee bottom floats will stick more than flat bottom.
Seaplane

I believe the step to be way too far behind the CG. The step should be a maximum of 0.5" behind the CG. If you consider this for a moment, the plane is always trying to rotate around the CG, but is having a hard time doing so due to the position of the step being so far back as to act as a lever arm that, due to the force of the water at the far end of the lever, is keeping the plane from rotating. Therefore, when you are finally able to yank the plane off the water by exerting a lot of rotational force (pitch) by having a lot of up elevator, the pitch when the plane is released from the water is such that it does the backflip you describe due to the amount of elevator employed.

CF - What is your method of getting the drawing attached? I've tried to do this previously but the website won't allow uploading a Word document (.doc). What is the extension (.XXX) of your drawing?

Planes with tricycle gear have the mains a little farther back than that, and they are able to rotate. I use the guideline of putting the step under the point corresponding to 40% of Mean Aerodynamic Chord.

Right click on his drawing, then left click on the "Properties" choice of the dropdown menu that appears. You will see that his drawing was a ".jpg" file.

Thanks Seaplane and Newc. The big ole lake was so smooth you could see the trees on the opposite shoreline reflecting. This trainer is call a SMART 40. I cannot find anything about it on the web. Since I did not build nor put the floats on, I have no way of knowing the CG . I tell you this thing is running like a bat out of #$%% with full up elevator and then all of a sudden straight up and almost over. Good my reactions are fast enough to release before it acutally does a loop. My delimma is still where is the CG on the plane. Without this, how do I ever get the step moved forward correctly. BTW, the only way to move the GP floats forward is to change the attach point on the fuse bottom since the wire gear fits in a groove on the top of the floats. Do-able though. I guess since the plane flys like crap I can trash it and build my own ( don't enjoy building) or cut it up and move the floats forward and see what happens. There is a mathmatical equation for determining the CG based on wing root, lenght or something like that...I will have to try to find it.

Thanks again for the input...I know nothing of float flying other than I have fallen in love with it since I sorta have my own private flying field, especially in the winter months when the lake traffic is pretty no existent. I have tons of nitro and dirt flying experience and full scale, but water flying sure has been fun.

Butch

flyifr, a rough way of finding the C/G on the wing is it should be at or near the front main spar in the wing itself.. It should be close to this, but trainers are fairly forgiving so if you are close to this, it should still fly OK.. As far as that step goes, balance the plane, change or check the incidence and try it again. If it still does it then think about moving the floats forward.. In the picture you can see the black tape is the C/G of the Norseman, and my step is 2.5-3" behand it.. I installed the floats to the scale dimension and after this I realized where the step was. I thought O well I'll just try it anyways, well it took-off and flew great... I haven't changed anything since




Yes Jim is right, my picture is a Jpeg. I just took a photo of it with my digital camera, and then edited it to size...

JIM CASEY..that is it, I did not read all the posts...MAC. Do you have the formula for MAC?

Thanks again CF for the info....one thing for sure something is not right and I have a lot of great advice to try though it will be a few weeks before I check the fruits of my efforts. With the holidays here, will be end of the month before I get to fly at the lake house again. Thanks

Butch

On a trainer with a hershey-bar wing, the wing chord and the MAC are the same. The wing chord is how far it is from the leading edge to the trailing edge. So on a .40-size trainer that's usually about 10". The balance point is usually at 30%. pick up the plane with your fingertips under the spar. That's usually where the Balance point should be. <Scientific content alert> The plane balances on the spar because the spar is the strong part of the wing, and that is where the lift forces act through. If they put the spar somewhere else in the wing, when the wing lifted the plane the lift forces would twist the wing and it...would...be....bad. </Scientific content alert>

So with a 10" wing, expect it to balance at 2.5 to 3 inches, and put the step of the float at 4 inches.

If you have swept or tapered wings, search out "Sparky Paul" for a graphical method that works pretty well.

I think your problem is that you need more incidence. OR something weird, like you sanded the step on your floats round. If you have flaperons, droop them a little for takeoff.

Hi

I agree with jims last post, move the step should be about 4" back from the le. and the way it may have been sanded may have placed the step further forward than it appears.

The easy solution if the floats arn't very moveable is to build the bottom of the float up to move the step back.

On my first water plane that had been sanded rounded at the step, when i finally worked out what was up i added 2 balsa triangular wedges similar to aileron stock as a sort of extra step and the problem was solved.

Simon

But also, don't forget to check that you have about 2º positive incidence in the wing!

Since your question is "where is the CG", not "where should the CG be?" there's no need to get all worked up about mean effective (or aerodynamic) chords, etc. The CG is simply where chordwise the plane balances.

Hold the plane with a finger under each wing on each side of the fuselage such that the plane is level. That's where the CG is. Then use this point to establish that the step should be 0.5" back.

As mentioned, also make sure that the wing is 1 - 2 degrees positive incidence relative to the floats.

just to split a really fine hair, the center of gravity is a point in space. If you freely suspend the airplane from any point, (Wingtip, left wheel, propeller.....) the Center of gravity (or more correctly the center of MASS) will be directly below the point where the plane was suspended. The center of MASS will most likely be somewhere inside the fuselage.

You have just told him how to find the Balance point, which is a point along the wing chord where the plane will balance fuselage-level if you suspend it from that point. If the center of mass is above the balance point the plane will fall over. That's why you have to balance a low wing plane upside down.

I know....picky picky.

And it's not really true that we honor Carl Goldberg by writing his initials next to the center of Gravity.

Sure as heck is, and to show how similar we are, I almost went back in and edited my comments to change from center of gravity to balance point, but decided to not do it since all the previous comments had been about CG.

Glad that you posted as you did!

Now let's tackle the question of "How in the heck can a Bumblebee fly?"

Perhaps I am missing something. If your balance point or CG, whatever you want to call it, is off, aligning the floats to this incorrect point will not fix the problem. I think one needs to know the location of the PROPER CG/balance point before you can properly align the step of the floats. I agree with the post above that you can find the CURRENT balance point/CG of anything by trial and error. Whether or not it is the correct location is another thing. I would start over with this setup. Find the formula for MAC and LEMAC online. The other option would be to take a rough guess. Once you think you have the proper CG, you can mark it and align the step of the floats to it. Then, when the floats are mounted, you have to re-balance the plane/floats to get the CG back to the location you found without the floats. Usually floats require "nose" weight because so much weight is added behind the CG. Just my opinion...

When I built floats for a PT-20, the kit included parts for a ventral fin, are you using a ventral fin ? Just a thought, may not be necessary on this plane. Good luck in your problem solving.

Okay if you want to find the Mean Aerodynamic chord, here is a LOT more than you wanted to know:
http://users.adelphia.net/~pjburke1011/cg.htm

Once you know the MAC, the CG should be at 30% if your plane has a horizontal stab. 20-25% for a flying wing.


"How in the heck can a Bumblebee fly?"
He can fly Real good!

Thank Lamolliever, I think my issue is going to turn out to be tail heavy. Like I said in earlier posts, can't try a thing until weekend after next. Since these are GP FG floats, I know it is not an issue with rounded over step as mentioned. To me, it looks as if the step is about 3 inches back from leading edge, but the wing is at my lake house, so this is from looking at the wing saddle. As the plane sits without the wing, it sit on the aft of the floats, so unless I find that the wing moves this much, I am thinking I have a tail heavy situation. Easy enough to fix at least temporarialy with stick on weight. Will post result around the 29th or 30th. I appreciate the input and the knowledge of those that have "been there, done that".

Butch

Butch,

I think your issue is that there's not enough relative incidence between the wing and the floats. If it's a flat-bottom wing you want at least 2 degrees. If it's symmetrical or semi you want more maybe 3 to 5. I've seen many float planes behave as your does and increasing the relative incidence cured 'em all.

BTW, I looked at the GP floats on their web site. Shouldn't be too hard to move the floats fore an aft but I'll bet they're close enough.

CR

Well folks, you all were on the money with your help. When I got to my lake home today, I set the wing in the saddle and bolted her down and check the cg by use of front spar...cg was pretty close, just a tad tail heavy, but this was with empty tank. I leveled my bench and leveled the plane/floats and WAH LA !!! the leading edge of the wing was visibly pointing down !!!!! I put my incedence meter on and sure nuff..2.5 NEGATIVE. I as CF-DRG said and added a 3/4 shim up front under the gear and now I have right a 2 degrees positive....and it look better to the eye as well. I took it down to the lake, placed her in and fired it up and after about 40 yards of run, off the water she came.....BEAUTIFUL !!!! Did you guys see the big GRIN !! Wow, what can I say but thanks for all your assistance. I have never met someone involved in aviation that I did not like and was not willing to offer a helping hand. Also, the plane handled much better in the air as well...I was able to trim it out to level flight and not fight it like I was before. I guess the difference between the wing incidence and the float incedence was making something go aloof, as it handles fine now. Thanks again JimCasey, CF-DRG, Lamorillrever, Newc, Mogman, Charley...I sure glad I asked !!!! Happy New Year folks....good luck to you in 2007.
Butch

Butch.... glad we could help. We've all been there before and others helped us. This is a great hobby, and the people in it are the reason. Good luck and don't hesitate to ask questions.have a good New Year.
Cheers. Dave.