airhead

good evening (EST anyhow) ...

I finished building the Great Planes .40 float kit. I am currently trying to attach it to my Northeast Aerodynamics Train-Air .40. The center of the Center of Gravity range is about 3 and 3/16 of an inch behind the wing's leading edge. If I was to attach the floats without making any changes, the step of the float is between 3 and 4 inches off ... yikes!!! The Great Planes instructions say that it should be 1/2 inch behind the COG.

Is the difference between my step and COG ok?

If not ... what to do?

I was thinking of moving the gear wire hold-downs (lack of better term) on the floats (forward landing gear ... the gear that is part of my plane) back. This would decrease the amount of space between the two wires. Is that okay?

Is there a better way of resolving this issue?

Please help, I am dying to fly off of the snow ...

Thanks,

Dan

FHHuber

For flying off of SNOW, the step location is nowhere near as critical as off water. it would probably work as-is. (if you haven't thrown the aircraft CG off, but sounds like you checked that.)

For flying off water... with the step of those floats is that far behind the CG.. you'd set the plane down in water and watch it dip the prop in the water. (maybe even turn turtle, and use the wing as a floatation device...)

The front of the floats should be noteably forward of the prop hub to fly off water. The profile area of the floats being that much forward is why many light planes need added fin area when equipped with floats.

JohnBuckner

My Feedback: (1)

Dan with the step that far off to the rear you will never be able to rotate for takeoff.

For that airplane the the step should be either at or no more than 1/4 inch aft of the CG. The CG with the floats should be in the same position as when flown on wheels. The floats should be roughly around 75% in length of the distance between the tail and prop. You need to move the floats forward and make struts as neccessary. Using existing wheel main gear is not always a great idea.

Also the floats should be installed so that the airplanes wing has a few degrees positive incidence to the floats top deck. In other words the tail sits slightly low.

After the floats are properly installed and the CG may have changed a little you can move the battery a little or if weight is neccessary it should be added to the floats themselves and in that way you do not have to fool with the CG every time you switch back and forth with wheels.

John

4*60

My Feedback: (41)

Have you got the tip of the floats at least 2.5 inches in front of the prop?

airhead

In reply to all that have kindly answered me ...

FHHuber - I was hoping to fly it off of water when things thaw out here in Massachusetts

JohnBuckner - thanks for the info ... would you recommend move the connection points (for the landing wire) on the floats? or would you suggest moving the landing wires themselves (where they attach to the plane)?

4*60 - I have yet to check the amount of float in front of the prop. What is the relationship of the amount of float in front of the prop to the CG (I love learning this stuff)??

Thanks -

Dan

JohnBuckner

My Feedback: (1)

Dan if you already have wire truss strut setup (I thought you were attaching the floats to the existing main gear and adding a second to the rear.

If the trainer has a flat bottom you mearly position the floats fore and aft and that will be where you use the landing gear straps to anchor the two struts accross the bottom. Inside the fuselage at these two points glue two plywood plates accross the bottom and that will give an anchor point for the langings gear straps.

The purpose of the float bows in front of the propellor is to help prevent noseovers Landing, Takeoff and when taxiing downwind. This usually works out pretty good if you size your float length to about 3/4 of the fuselage length but not always as there are some airplanes that just don,t fit the formula. Its not locked in stone and I have flown some that the bows did not quite make it out to the propellor but of course required carefull operation on the water.

4*60

My Feedback: (41)

And you can move the floats further in front you are then getting your step in position at or near the CG. If the tips were at the prop you could move them 3 inches forward and voila!

The plane will dip less at the front and the prop will pick up less or no water, when taxiing, also.

You should have mounts on the floats that allow for fore and aft mount flexibility. You move the float attach points allong the float to change the setup.

Just my .02.
.

Ed_Moorman

My Feedback: (1)

I am not a pro at float flying, but I do fly off water and I have read everything I can find from the past several years about floats and from what I can tell, the idea that the step must be 1/4 inch behind the CG is bunk.

There was a recent article in RCM, as I recall, about converting fun fly planes to floats. Now they have a lot more elevator authority than a trainer, but it's worth a read. He recommends 3 inches back. I have also seen it written several times to locate the step at 40% of the MAC. 40% would give you about an inch and a half on a 12 inch chord-more if the wing is broader.

It was explained that the step is similar to the main gear on a tri-gear plane. If the main gear is right on the CG, then the plane rotates easily, but can also tend to skip of landing. With the step further back, the plane takes more elevator to rotate, but tends to skip less. On a plane with the main gear further back, when you touch, the nose drops and you get less bounces.

One of the problems with "store bought" floats is if you get the step too far back, the amount of float that extends out in front of the prop is short. The shorter the floats in front of the nose, the more critical it is that you make a good landing or you can dig the nose in.

I am flying an old Sig Fazer with a 5-year old Tower .46. My step is 2-2 1/2 inches back from the CG and I have had no trouble at all taking off and my landings have been without skips.

The bottom line, in my opinion, is there is no universal truth when it comes to step location, only opinions. The authors all say they like this location and use it on all their planes. They never tell you what else they tried and what they didn't like. And no one has ever run a test, to my knowledge, with the step at various locations. I may have to do this one of these days.

You are probably too far back, but I wouldn't panic. Move it forward to about half of what you've got, 1 1/2 inches, and try it. You also might try running a long strip of 1/8 ply, maybe 3/16, about an inch wide, set vertically between your attachment points with several holes drilled so you can adjust the mounting fore and aft.

I also think much of the landing problems are from poor landings and would result in about the same thing on water, grass or pavement. If you are going to get your plane into a semi-stalled condition with the nose up, set up a high sink rate and plop it down, you can look for a bent nose gear on the ground or a nose over in the water.

JohnBuckner

My Feedback: (1)

Ed, The gentleman is using a conventional forty size trainer, not a funfly type with huge elevator authority. That airplanes setup as he described will not get off the water and if it does get bounced off by a wave it will most likely be in an uncontrolled state. The info I provided will work well and I have no need to test excessive rearward step/CG relationships for my airplanes simply because I have seen the results of just that type setup countless times over the years by others and it is seldom successful particularly on trainer or cub like airplanes.

If you care to beleive that is Bunk then fine but that is of course only just my opinion based on considerable personal experiance.

John

airhead

once again ... thanks for all of your replies!!

sooo ... it looks as though I am going to simply undo the straps (my term for what holds down the) on the floats and move them back. this will push the floats forward and beyond the prop (3 or so inches). this will also rectify my CG/step issue.

last question before i perform the work ...

will it matter if the back gear is sitting a couple (2 or 3) inches from the back of the floats? if so, then it appears as though i will have to abandon the gear and where it sits on my airplane (aka - the back landing gear of the tri-gear plane).

Thanks in advance!

Dan

JohnBuckner

My Feedback: (1)

Dan in my opinion if enough rigidity can be acheived the actual positioning of the struts is really not important. If the float position itself i.e. incidence, distance of the floats apart, step/CG relationship and both parallel are all approriate then the airplane will be a delight. Even a low powered and heavy airplane can be a joy on the water with float positioning being the key.

The only possible disadvantage I can see with front struts a bit far behind the middle of the float would be a bit flexible or in the case of a very hard landing possibly float damage.

John

JimCasey

My Feedback: (1)

I'm having trouble imagining how if you move the floats forward so that the step is in the conventionally accepted location, the floats stick 3 1/2 inches in front of the prop, but the main gear is almost at the transom, and the front gear is behind the middle of the float.

The step is almost always at 50% of the length of the float. The Main gears are close to the balance point so they go close to the step. If you have a taildragger, then the mains attach a little in front of the step and the rear struts attach about 5" behind the step.
If you have a trike gear trainer, the mains attach almost exactly where the step is, and you can make a t-shaped strut that attaches where the nose gear normally attaches. But the float should only stick forward of the prop by 20-30% of the prop diameter.

So having both struts attached behind the step and a big chunk of the float in front of the prop just sounds like there is a fundamental misunderstanding going on.

Maybe I'm just missing something.