Thoughts on mounting full lotus sponsons inboard on a Catalina?

15 posts in this topic

Posted (edited)

I like the looks and results people have reported after mounting their sponsons inboard, at one of the jury strut attachments.   I didn't want to weld on the struts to add tabs, so I mocked up some brackets out of plywood that use P-clamps to attach to the 3/8" jury struts right at the cluster welds.   The plan is to do some slow speed taxi tests and make sure the sponsons still have enough leverage at this closer distance to allow me to re-board the boat over the side.  I think that's the most extreme static stability situation. 

If it seems workable, I'll fabricate these out of fiberglass.  I weighed it in my head, and decided I'd rather have a sponson shear off rather than deform the jury strut.   I plan to make a little destructive test rig and make the fiberglass just thin enough so that this is the likely outcome.

Thoughts?  Advice?  Cautionary tales? 

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Edited by cowlove

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Posted

as long as your not dipping a wing tip on a good cross wind that seems like a great plan.  Why do you not want to have them further outboard where the original fiberglass floats were mounted?

I am trying to learn all I can from you folks that are flying these before I complete mine. 

:BC:

 

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Posted

I believe you might be able to make sharper turns when going full blast on the step during water maneuvering using ailerons into the turn ....and on a hard surface you might be able to land with more crosswind before striking the upwind side sponson ....that's the good side of having them inboard.

The downside:

I would be concerned about digging in your wing tip (maximum leverage) when doing sharp turns using aileron on the step,  there is a real danger to make the Cat swing around wildly, maybe even turn over...

Last but not least, ...You will be pretty slanted on one side when floating around.....for slow taxiing in water it needs more rudder when trying to go straight.

I have mine outboard at the "normal" location and I find I can slide on the water doing tight enough turns with wings pretty level. I also prefer having the Cat floating more level/horizontal when having the sponsons more outboard. 

My 2 cents :)

 

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Posted

Just seems like they'd be up too high?

I have a lot of lake amphibian time, and while taxiing or sittting still you are always resting on one sponson.  If the other wing is too high then the wind can get under it too much and bury the downwind sponson.

Id say when resting on one sponson, you would want the other one about 16"-20" out of the water, and maybe less if you are inboard like that.

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Posted

Maybe you have the FL instructions?

Here's a few pages, let me know if you want more.

 

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Posted

I had not seen the FL instructions, thanks for that.    I'm not sure different the floating angle will be.  The new location is only about 1" higher than stock, but of course it is much more inboard and will result in a greater angle to the center of buoyancy and larger list in the water.  How much larger will depend on how low she sits in the water, I guess.   I can adjust the height a little bit as needed. 

Agreed, catching a wingtip would be all around bad. 

-Jim

 

 

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Posted

One last thought I have is that moving the sponsons inboard you should actually increase their buoyancy as you reduce the leverage force. I find the sponsons when fitted outboard just enough to keep not submerged when in a heavier sea. With the sponsons more inboard you might have trouble not dipping the wing tip when slow taxiing in a swell making it harder to keep your track....Hope that makes sense.

I find this interesting brainfood when trying to do do things different, there are always some things to loose and other things to gain.

Cheeers and long life to all the Cats and other flying devices

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Posted (edited)

Oh and for your destructive testing,  in the lake amphibian the sponsons were under the most stress in side load (while skidding a high speed turn where they could catch a side edge).   They only "sank" when going really slow or while sitting still when wind grabbed the underside of the opposite wing.  Once you get the speed up above 20 mph you had enough aileron to keep em up and also they were skipping along fast enough where it wasn't likely that you could dip one. 

Bracing for side load will be something I'd look into carefully if I were mounting them up.

Edited by MN Kitfox 2

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Posted

I made the change.  I'm not that great a welder, so I bought a 1" tube (whatever the width of the float is), and split it where the float flange is, bending it down for a hole, and a bolt mounting point on the float.  I also drilled through the pipe with a hole saw for the vertical mounts- and since I pinned them with bolts, they are adjustable.  It looked to me as though the nose needed to be down a bit on the lotus floats, and flying found that to be the case- Not a great picture- but it's on this site: 

IMG_1663.JPG.72437c8e78a255222410bc1e425

If you zoom in on it, you might be able to see what I did.  The little red dot at the tip is an aerodynamic touch- a superball ground down to fit.  Note the nose of the float is still a bit high.  I brought is down a bit more after this flight- as per the tufting.

Jack

Austin, TX

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Posted

Oops- missed it.  You're looking for an inboard mount. You might try and contact Don- he had a thread under catalinas (don's catalina), and had moved his sponsons iniboard as well.  I only have the one jury strut- he had two, and moved it in to the outer of the two (yeah, it reads confusing to me as well, but there are pictures in the thread..) 

 

Jack

Austin, TX

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Posted (edited)

I threw on a wing to visualize the geometry of the test sponson location.  It's actually a fair bit lower than I thought, and the angles are quite different.  I drew lines on the picture below to help visualize any problems.   

DISPLACEMENT MODE: The sponsons will actually be closer to the water.   I think it will be OK, as long as both sponsons aren't forced into the water at super-max gross.  Based on where I remember the water line being then the plane was overloaded with sacks of concrete, there is a chance this might happen and it would require changing something.  I'll check this on slow speed taxi test day.   There also needs to be enough stability to board over the side, which is about the most extreme test of static stability that I've seen the plane subjected to in practice.   The full lotus sponsons are smaller, and the close-in location will cut the lever arm in half, but I am predicting it will be fine.  

ON STEP: The sponsons are a little further from the water than stock, but not as much as I'd feared.   It appears that the sponson will still amply prevent the wing tip from hitting the water, assuming flat seas.  I'll gradually work up to testing this gently in later step taxi tests.  Maybe :)    

LAND: For wheels, it's a win win win.  It looks like I could almost drag a wingtip if I wanted.  A guy killed himself in a Catalina by scraping holes in his full lotus floats on land, then using them in water, so it's good to see these little fellers a little further away from the concrete.   

 

 

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Edited by cowlove
typo

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Posted

Thanks for the full lotus instructions, and thanks Jack for your aerodynamic research.  I don't have the same specific reference plane that the instructions talk about, so I think I'll start off with the same AOA as the stock sponsons.  If the hydrodynamics work out, maybe later I'll orient them for aerodynamics instead. 

Here's a picture of the AOA and fore/aft placement.    

IMG_20171030_181101.jpg

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Posted

Actually, the guy (Jim) that went in on Inks Lake had a different problem- his Catalina had been altered, and had storage areas hollowed out into the *inboard* sponsons (which cover the wheels) accessible from the cockpit.  I thought it was a neat idea when I saw it - a good place to stick charts and whatever, but he had a bit of separation between the hull and the sponson (caused by learning to land a tailwheel aircraft in a plane without suspension), and water started jetting in.through those openings.  To my knowledge, he did not have a seaplane rating. Long story short, if you even *think* you have a lot of water in the fuselage, do NOT try and yank it off the water.  I have an automatic bilge pump in my cat that has a switch for automatic, and manual.  If the pump is running in either mode, I have an indicator light that comes on. Checklist on landing- bilge pump on automatic.  Checklist on takeoff, pump on manual, and a gentle climb off the water for the first 20 seconds, switch to auto, check for operation light, and only then...  Jim took off heavy, had water, pulled back hard, and the water ran to the tail, shifting CG, and stalling him. Knew Jim, knew his plane, and have since landed on Inks Lake, thinking of him.

If you watched my youtube video, you'll note a caption in there after my first landing "checking for water". I do not have a floorboard in the rear of my cat (only a midget/child could sit back there- I tried once), so I can readily check for water.  There is *nothing* in the cat design to stop a sudden rush of water(weight) to the tail, other than a small step.

$,02

Jack

 

 

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Posted

Well said Jack and I will add my procedure to this vital item....besides having the bilge pump on, when I am ready to take off, I throttle up (about 75%) and rapidly chop the power, then look under the floorboard/seat space if there is any water (cutting the power makes the water rush back forward under your feet if there is any). This simple test is a confirmation there is no water when taking off and also is a safeguard if I were to forget the pump. There has also been a crash of a Seamax M-22 because it had taken on water when beached, on take off pilot forgot to switch on bilge pump, the weight of water in the tail did not allow the pilot to recover. This is deadly..

 

 

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Posted

That version of the event makes a lot more sense than what I'd read somewhere: that it was the wing-mounted sponsons that got scraped up in crosswind landings.  Sorry that you knew him, Jack, I hope my mention of it doesn't seem irreverent in this context. 

Yes, I'm also pretty paranoid about water in the bilge too.  Here's some of my handful of tricks for worrying about it:

1) These battery-powered water-detector alarms are light and reliable, and I never had false alarms.     https://www.amazon.com/Zircon-Alert-Electronic-Detector-Sensor/dp/B01J39MJ9I/ref=sr_1_1_sspa?ie=UTF8&qid=1509547198&sr=8-1-spons&keywords=moisture+detector&psc=1 

I kept one in the tail, zip tied to the frame just inside the landing gear spring bolt inspection cover, and I also kept one just behind the step of the hull, where it would detect any water sloshing aft into the tail.    Later, after I dealt with a pretty bad gash in the aft fabric from taxiing into ice, I realized how steep the angle is from the tail to the bilge is.  Even in plow taxi, water drains very effectively from the tail area into the bilge, and I'm confident any leak in the aft area will be apparent as water in the bilge.  

2) I have a little self-adhesive mounted mirror (non-fisheye worked best) that lets me glance backwards and down into the bilge without moving my head.   I give the bilge a glance once I'm on step. 

3) In my plane, there's a small void space between the sponson and the hull.  I drilled holes from the interior of the hull into that void space, and through that into the foam-filled sponson itself, so any appreciable water in the sponsons will drain into the bilge, and the leak will be evident. 

 

 

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