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Old Aug 16, 2006, 12:02 PM
IAD
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New England, USA
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Help!
Flying Boat Advice...

I'm working on an 'almost-looks-like-it-could-be-scale' flying boat, a somewhat bizarre mix of Catalina, Dornier, and seagull.

Specs. (Projected)
66" span
500+"^2 wing area (undercambered)
17 oz. AUW
4x IPS S2 drives @ 4-5 oz. of thrust per.
5 channels: Rudder/diff. throttle, elevator, aileron, throttle, front motor disengage. (Disconnects the front motors from the ESC.)

I'm confident of my abilities in terms of making it behave well in flight, but I'm a bit sketchy on the rise-off-water part of the flight envelope. Lacking any previous experience, I'm putting a good deal of faith in a very generous thrust/weight ratio, and a large wing.

Mainly, I have concerns about the hydrodynamic drag resulting from the stub wings/floats. Right now, most of the rear half of the stub wings would be sitting on the water, with the leading edges a good ways above the surface. (They are at positive AOA, relative to the wing.) To me, it seems like a lot of surface area in contact with the water?

The step is located about 0.75" aft of the CG, which I gather is pretty standard.

Any input would be appreciated.

~Luke
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Old Aug 16, 2006, 12:22 PM
Registered User
Dresden, Germany, europe, planet earth
Joined Aug 2003
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Build it! This one is just too cool to be not built.
The underwater area seems okay, as long as you get enough air under the sponsons. Sharp edges at the rear end and out at the tips may help breaking water suck.
But I would recommend to build her bigger. 80" span would be fine, and prevents tip stalling from those sleek wings...

Regards,
Kuni
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Old Aug 16, 2006, 12:31 PM
IAD
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Ok... I just ran some displacement calculations, and the waterline shown is for 25 oz., AUW. (Far about the design weight, so it wouldn't sit that deep, after all.)

I'm not overly worried about tipstalling, but I see what you mean... However, with a wingloading of only 4.8 oz./ft^2, it should be quite a floater. (And not just on the water.) Maybe some wing fences would be useful? (80" would be gigantic, twice as large as I've ever designed/built before!)

~Luke
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Old Aug 16, 2006, 11:04 PM
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That design is SUPERCOOL!!!!!!!
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Old Aug 17, 2006, 08:27 AM
Balsa Flies Better!
Stamford, CT
Joined Oct 2000
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Hi Luke

I think you've got a different problem- the sponsons won't have the flotation to torque roll the airplane level if you dip a tip- and believe me- that happens a lot. Also- flying on water is pretty rugged on an airframe- I haven't seen many relatively large lightweight craft out there. I have no idea of how hard it would be to unstick that airplane- but I can tell you that surface tension really works against you the smaller and lighter you go. Slow speed is brutal- getting off the water is where momentum helps.

Also- Peter Wenk who died a few years back drew up lots of waterplane plans including the Do 26 and the 314. (I have both- started the 26, but I think it'd need a major rethink at this point.

Sam
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Old Aug 17, 2006, 09:14 AM
IAD
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I see. So, for starts, I might want to think about enlarging the sponson tip floats, to help keep things level..?

The fuselage structure will be carbon reinforced, and fully sheeted... 1/16" on the bottom, 1/32" on the top. I think it will hold up alright. (Also, I can always add more carbon ribbon if necessary.) I'm not too worried on that point.

I have noticed float/flying boats tend towards heavier/faster... You guys are probably right, this should be a scaled up to a quad Speed 400 job.....

Of course, the 'Flying Hydro' 3D ships are pretty light, and they seem to make out alright. (Admittedly, they more or less fly on brute strength.... )

I suppose I could always build it, and if it can't get off the water, just add a set of wheels in the nose and sponsons. No matter what, I think it would be a pretty nifty plane to see flying around.

~Luke
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Old Aug 17, 2006, 09:19 AM
IAD
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Oh, just a thought...

What about a dynamic stabilization system... A set of small low-aspect ratio 'ailerons' mounted below the water line, linked to a gyro, to keep the hull level on takeoff and landing?

~Luke
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Old Aug 17, 2006, 09:35 AM
IAD
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Re: lightweight flying boats... Aside from the Hydrofoam designs, I came across this: http://www.art-tech.cn/english/Artic...?ArticleID=149

Lots of surface in the water (the wing roots appear to sit in the water), not much to keep it level (the wingtip floats don't touch unless it's already going over) and the takeoff speed wasn't particularly high. Total weight is only 15 oz, with ~240 in^2 wing area.

The biggest difference I spot (aside from one motor, vs. four) is the wing area... But I can't see how doubling the wing area would be a bad thing.

~Luke
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Old Aug 17, 2006, 09:54 AM
Balsa Flies Better!
Stamford, CT
Joined Oct 2000
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Luke

Problem with the dynamic system is that it relies on forward motion for effectiveness- something you haven't always got on the water. The Coot works because the entire wing is the float- as it tips over further, the displacement on that side increases, which provides a positive righting moment. The problem with your design is that when the tip hits- it will be stable-forms a tripod. I'm a sailor- when the mast hits the drink, it takes a lot of righting moment to get it out.

In terms of the hydrofoams- basically they're very well designed to plane quickly- lots of flat surfaces. I suspect that they are not very pleasant in any kind of chop but I'm guessing here.

Currently I'm messing around with my own flying boat- it's not that easy to get the hull to do what you want.

Sam
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Old Aug 17, 2006, 10:47 AM
IAD
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Gotcha. I was operating under the assumption that tipping is most likely to be an issue when moving, so...

I don't mean to be argumentative, but I do have one question: I'm looking at the design, and I can't really see it achieving a statically stable configuration, expect either right side up, or completely upside down. If I did drag a tip, the takeoff run would be terminated due to drag and the resulting ground-loop, but because of large span gullwing, it shouldn't be get more than a few degrees over before the (reasonably buoyant) wingtip returns the aircraft to level. (See attached.) The wingtip's displacement at 19 deg. as shown is causing about 12 oz. of righting force, acting near the end of a 33" lever arm. And most of the weight of the aircraft continues to act on the high side, at angles of less than about 37 deg.. (At which angle, the outer panel of the wing is providing more than 30 oz. of righting force, almost twice the projected AUW.) I'm not saying I'd manage to get off the water, but I just don't see how the aircraft could enter a stable tripod configuration by partially capsizing.

(Granted, the aircraft could tip forward, about an axis defined by the bow and the wingtip, but any flying boat can do that, regardless of float configuration.)

Regarding planing: Assuming this is a still-weather design (anything with a 4.8 oz/ft^2 wingloading is, generally speaking) changing the hull shape a bit to get it to plane quickly might not be a bad idea? (Why design the hull to cope with chop, if the rest of the aircraft can't cope with the winds that would cause it..?)

~Luke
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Old Aug 17, 2006, 11:11 AM
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Sao Paulo, Brasil
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Nice design, dragon style

Suggestions:

1) Install motors on a nacele over wing, the thrust line will be lower, making it to fly better, and will have less trouble with reinforcements needed due to the pod.

2) Surface is not a big deal if your design let it to "fly" on water instead of swimming on it. Keep corners, step and trailing edge of hull sharp, to avoid the Coanda effect. Your overall design resembles a "tunnel hull", will probably work well.

To try it without spending too much time, you can make only the part of hull that will touch the water on cheap foam, add some weight on CG, install a cheap DC motor with propeller with nicd/nimh battery and a on/off switch and try it on a swimming pool.
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Old Aug 17, 2006, 11:47 AM
IAD
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Ok, I'll sharpen up the fuselage aft of the step, and maybe try and run some tests.

Yes, the motors are rather high-set. Reinforcements I don't mind, but the high thrust-line was a little disconcerting. However, I've talked to others about this, and I was told it probably wouldn't be that big a deal. I don't mind trim changes, and with some upthrust, it shouldn't be too bad anyhow.

Anyhow, I can always drop a pair of pylon-less nacelles on, without much difficulty, if the elevated setup proves unworkable.

(I will admit that here, I'm just being stupid... I really like those pylons. )

~Luke
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Old Aug 17, 2006, 12:06 PM
Balsa Flies Better!
Stamford, CT
Joined Oct 2000
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Hi Luke

My theory isn't good enough to explain what my practical experience has shown. I understand where you're coming from- you're asking good questions. I see the righting moment you've got. All I can say is that I know it takes a lot of force to get a sail out of the water- and that on several of my floatplanes- even with tip floats- when the wing hits the water- it can stay stuck.

Example- my current airplane- tips are 18" off the centerline- airplane weighs 2.2 lbs- tip float is 4 cu. in. Wing tip float. If I did the airplane over again- I'd increase the tip float volume. Theory says that should provide sufficient righting force- it's a flat bottom hull-but it doesn't- unless you can get the airplane moving.

Suggestion- do the experiment- what you'll find is that wingtips don't float well- especially long skinny ones.

Regards,

Sam
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Old Aug 17, 2006, 01:15 PM
IAD
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4"^3 is only about 2.4 oz. (0.15 lb.) of buoyancy... Given that it's acting with on the end of a 18" lever arm, the 2.2 lb. net weight only has to act on a 1.35" lever arm for the system to become statically stable. Assuming the mass centroid is a little distance above the water line, this is pretty realistic. Add in the effects of surface tension, and I see how the wing could get stuck. 2.2 lb. is a lot, compared to 0.15 lb...

I'll definitely run some tests, since in my case, I think surface tension would be the determining factor in whether or not the wing 'sticks', and I won't even try to calculate values for that.

I will say this, though: at the angle at which the wingtip is 1" above the water, the sponson shown in the renders is providing 13 oz. of buoyancy, acting at about 3" from the centerline. This gives a righting moment of 39 oz.*in.. As it continues over further, the righting moment increases, to a maximum of 75 oz.*in., when the sponson is completely under water.

Your tip float has a maximum righting moment of 43 oz.*in., plus the AUW of the airframe is twice as much, which depending on mass centroid, could be a good or bad thing. (Since you say the tip would get stuck, I'm guessing it was bad, in this case.)


As far as I know, the geometry of the float should only change how it behaves dynamically (planing/nonplaning)... In a static situation, a high aspect float of the same displacement as a wider one should provide the same amount of buoyancy.

~Luke
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Old Aug 17, 2006, 08:25 PM
Balsa Flies Better!
Stamford, CT
Joined Oct 2000
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Hi Luke

Thanks for doing the calcs- it would have been a lot of looking at old physics texts for me. I must admit- I'm about ready to give up on this thing for taking off the water- let me start a new thread- don't want to hijack this one.

My comment wasn't about my tip float which is 7" by 3/4" by a 0-1.5" taper- not really what I'd call high aspect ratio- it was about the skinny tips on your design which I have a terrible feeling are going to be very, very sticky once they're down.

Good luck with it-

Sam
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