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Old May 31, 2012, 09:56 AM
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Quote:
Originally Posted by Mike St View Post
Strakes and canards provide lift at the front end. I have a eurofighter which I believe is about the best pure flyer I know, and I've tested many designs with canards. When set up right, a canard is very sweet. Read up on canards and strakes.
A true canard is very efficent in that all surface areas are lifting to provide stable flight. In a rear tail the horizontal produces down force in stable flight. Strakes increase the effective wing area, decrease the effective aspect ratio, reduce center section stall speed and move the neutral point forward. The larger wing area gives you slower stall speed. The decreased aspect ratio can increase roll response and make pitch a little more sluggish. On some planforms you can induce a tip stall because of the lower stall speed on the wing center section. If you add them to a good flying plane the movement of the neutral point forward without a shift forward in CG can cause instability in pitch.
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Old Jun 04, 2012, 09:59 AM
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Played around a bit this weekend with a seaplane "prototype". Thought I'd start with a plane that flies pretty good, and see what impact modifications that I'd have to make to create a sea plane would have on it.

Took an old delta (~ 24 inch wingspan) pusher with the motor in-line with the wing, and mounted the motor up on a pylon to get the prop above the wing level. Had an 8" prop, so the motor was mounted a bit over 4" above the wing surface.

Put in the same amount of down thrust as in the plans for Otto's Sea Dart.

Sucker just nose-dived into the ground. With the thrust line ABOVE the wing, it just caused the plane to rotate (around the CG?) and nose in. Even with full UP ELEVATOR, could not even get any semblance of flight.

Re-cut the pylon to give even MORE down thrust. Thought this would pull the nose up -- no joy.

Looked at Otto's plans, and found that a line from the motor axis does NOT go through the CG, but rather hits the wing in front of the CG. Not sure of the significance of this.

Tried a VERY severe down thrust at about 45 degrees. STILL could not keep the nose up, although I suspect that loosing a lot of the axial motor thrust was part of the problem here.

I'm missing something, but don't know what. Other designs seem to fly with only moderate amounts of down thrust. Is this pylon just too high for the size of the wing, etc? Or do I need a longer "axis" on the plane to handle a pylon-mounted motor and counter-act the lever arm of the motor above the wing?

I'm attaching a photo of one of the more EXTREME experiments.

Any thoughts or suggestions???
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Old Jun 04, 2012, 01:04 PM
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Quote:
Originally Posted by Flyboy Steve View Post
...Any thoughts or suggestions???
Yeah: Build a Polaris.
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Old Jun 04, 2012, 02:19 PM
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Flyboy, you shouldn't have tried this. Raising the motor above the wing on a pylon causes all sorts of problems. When possible always align the prop with the wing.
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Old Jun 04, 2012, 02:24 PM
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I now understand this much more than I did before I tried it!

However, to make a sea plan (think Polaris) it seems that the motor must be mounted ABOVE the wing surface so that the prop doesn't hit the water.

I was trying to see how an elevated motor + downthrust effected the performace of a "known" plane. It really made it AWFUL!

Now, I'm trying to understand what is DIFFERENT about a Polaris-type plane that makes it possible to fly well with a motor on a pylon.

The alternative is keep the motor in line with the wing and elevate the wing AND motor above the water level (think pontoons on a high support structure). But I'd rather have a Polaris-type plane.

Not trying to re-invent the wheel. Just trying to understand why something like a Polaris works whereas my half-baked experiment did not.
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Old Jun 04, 2012, 03:10 PM
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You are trying something I've thought about but haven't figured out a solution yet. I like my little depron wings and would love to fly them off the water. If you raise the thrust line over or near the CG, like on a wing, you create a huge pitch down rotational moment. To eliminate the rotation you angle the thrust vector down. As you angle down you reduce your forward thrust component and create a large down component. This down component conteracts lift. Think of it as flying a heavier plane with less thrust. A wing relies on reflex for level flight and generating the necessary reflex without a tail is almost impossible.

On the Polaris and Sea Dart the motor is located much farther behind the CG so you don't need as much down thrust vector to get everything balanced. On my Mallard Duck, when the head is up, I have a lot of up thrust vector to prevent down pitch. This is a motor above the wing but well in front of the CG. So on the Mallard I get the effect of added lift with a little reduced forward thrust. This might work on a wing but the overall handling qualities might be poor. I've been thinking of a configuration mechanism like on the Duck for a wing so that once your off the water, the motor is moved to centerline.

RC Duck Seaplane flown by Otto Dieffenbach at the Menifee Valley Flyers Float Fly (5 min 19 sec)
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Old Jun 04, 2012, 07:33 PM
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OK, here's my suggestion that I think would work. As I understand your goal, is to make a flying wing fly off water. The way you approached the problem I doubt would work. Try this: Mount the motor on a pylon, directly over the CG, or very close to it. You might need some slight down thrust, to counteract rotation, but it should be so minimal that it won't exceed your lift factor. Definitely use small diameter prop. You'll have to experiment to find the right amount. Don't worry about crashes. Crashing is normal when working out a new design. Your success I think will come by working close to the CG.
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Old Jun 04, 2012, 07:38 PM
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Here's the best solution: Abandon a propeller and use a ducted fan, sitting on top of the wing.
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Old Oct 14, 2012, 10:32 AM
just Some Useless Geek
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Sorry to revive a dormant thread, but I'm slightly interested in getting a seaplane done before the cold weather hits. I think Steve's main problem is one of geometry; the plane has a too-small polar moment of inertia to use an elevated motor mount like that. On all the successful delta seaplane plans the length to span is roughly 3:2. This seems to be the winning ratio for delta park jets in general, so it makes sense that it works for a seaplane as well.

However, for a short-coupled seaplane I like Mike's suggestion of using an EDF for power. With the center/thrust line of the power system pulled way down towards the wing center line you should have greatly reduced pitch rotational effects, even with the small longitudinal polar moment.

Of course, the final solution to the polar moment issue is to lengthen the airframe by hanging a box section out in front, but that kinda defeats the porpoise of the original concept, don't it?
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Old Oct 14, 2012, 10:47 AM
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A couple of things. The myth that epp does not make good seaplanes, is just that , a myth. A flying boat is much moe aerodynamic than a pontoon plane. Here are a couple of EPP seaplanes of mine. Both made from 1.9 pound EPP. Some video of each.
The XPV that Never Was Part 4) Water Trials (7 min 43 sec)

Sharky Amphibian (7 min 44 sec)
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Old Oct 14, 2012, 10:55 AM
just Some Useless Geek
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Okay, how did you manage to prevent the EPP (which is an open cell foam, after all) from wicking water? Other seaplane guys have told me their unsealed EPP planes came out of the water at twice the mass they went in.
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Old Oct 14, 2012, 12:22 PM
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Joel, I am sooooo excited to see your planes! Just what I was looking for !!

I really want to build out of EPP rather than Depron, so it's good to know that this is an OK building material. Did you somehow seal the outer skin?

Do you have any PLANS for either craft? The video has a shot of some plans, but it was kinda fuzzy...

Couldn't tell from the video -- was the Lan Clair a rudder / elevator type control system?

ANY additional info or construction tips would be really useful.

Thanks so much !!
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Last edited by Flyboy Steve; Oct 14, 2012 at 12:29 PM.
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Old Oct 14, 2012, 12:40 PM
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Is this the motor you used? (The link in the video was no longer valid)

http://www.aero-model.com/Hacker-Brushless-A20-20L.aspx

Anyone know of a less-expensive alternative that would work here?
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Old Oct 14, 2012, 03:12 PM
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Pretty sure Don will have a Keda 20-20 that will work . Check out his site and send him an email http://www.strongrcmotors.com/Motors.htm
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Old Oct 14, 2012, 04:43 PM
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steve: back on your wing off water idea, there's a pretty fundamental issue beyond the thrust line, and that's in my experience for water maneuvering, one needs a rudder (either a water rudder or the plane's rudder) and your wing is an elevon setup isn't it? If so, then it'll go either straight (if there's no wind) or wherever the wind takes it.
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