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Old Feb 21, 2013, 07:30 AM
Two left thumbs
Muncie, IN
Joined Sep 2006
4,230 Posts
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Blowing off?

In this book http://www.amazon.com/The-American-F.../dp/087021070X the author discusses "blowing off" an overladen or underpowered plane by following in the wake/slipstream of a lighter or more powerful one. Have any of you heard of this and/or applied it to models whose hulls or floats just don't like to unstick in smooth water?
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Old Feb 21, 2013, 01:22 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
12,897 Posts
I occasionally have a problem with 'blowing off', but I put that down to mainly an age thing.

But I have heard of fliers cruising round in a circle to create some ripples to aid lifting off calm water.

Wow!, calm water, it feels like years since I have last seen calm conditions whether air or water.
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Old Feb 21, 2013, 05:11 PM
Parkcityflier
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Park City, UT, USA
Joined Aug 2001
1,463 Posts
I've never seen or heard of this technique and I've been a full scale pilot for over 40 years, although not with a whole lot of water experience except with flying RC. I suppose that if your airplane just couldn't quite make it onto the step, if you crossed behind a larger aircraft blowing a strong slipstream behind it the momentary bump might be enough to break the float suction and get you onto the step. You get a similar result when you cross a wake, which sometimes works. Doing this with two RC airplanes would be pretty difficult, if not impossible. The thing to do is to pick a day when there is a small "chop" to the water, rather than off of glass smooth water, if you have an airplane that is hard to get onto the step and airborne.

Jim
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Last edited by parkcityskier; Feb 23, 2013 at 09:19 AM.
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Old Feb 21, 2013, 05:26 PM
Airplane crasher
CapMike's Avatar
USA, CT, Shelton
Joined Jan 2008
2,732 Posts
When running a smaller boat that was weighted down to capacity I used to have to turn a circle to create a wake. The idea there was to give it something to hop up onto. So the theory does work, and can work for a plane to get unglued from the water. Just doing slight rudder turns can also do the same I believe. By rocking from side to side it allows some air under the hull breaking the suction. Something else one can do is add a reverse chine to the hull. By doing this it brings in air as well as causes a diversion of the water forcing it to rise.
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Old Feb 21, 2013, 05:45 PM
Art Schmitz
United States, TN, Crossville
Joined Jan 2012
392 Posts
Many years back I worked with John Elber. He worked summers as a bush pilot in the Hudson Bay area. John did relate that they would line a rather stock Cessna 172 on floats behind a Cessna 180 to get the weaker plane off the water.
art ( private pilot )
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Old Feb 23, 2013, 01:10 AM
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mosquitowings's Avatar
USA, OK, Warr Acres
Joined Feb 2004
507 Posts
I too have heard of this problem, but here in Oklahoma there is no such thing as still water except in the bible!
Jim
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Old Feb 23, 2013, 09:01 AM
Two left thumbs
Muncie, IN
Joined Sep 2006
4,230 Posts
Quote:
Originally Posted by mosquitowings View Post
I too have heard of this problem, but here in Oklahoma there is no such thing as still water
Jim
Oh??? http://stillwater.org/ But maybe the town's named for the water they use when brewing moonshine?

Like most of us, I used to fly full scale, but have NO seaplane experience.
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Old Feb 23, 2013, 09:41 PM
59 years of RC flying
Daedalus66's Avatar
Canada, ON, Ottawa
Joined Feb 2006
16,042 Posts
I've had models that couldn't get off the water without the aid of waves. The problem is generally not insufficient power (assuming that the model has enough to fly decently) but float design or setup that prevents the model from rotating enough to give the wing about a 10 degree angle of attack. If the floats are relatively straight on the bottom (i.e., they don't angle up much behind the step), the wing cannot achieve enough angle relative to the airflow to generate the necessary lift (up elevator simply pushes the tail down until the back of the floats is in contact with the water, it can't in any other way lift the nose). With such a model, your only hope is to find or generate a wave that will pitch the nose up enough to get the needed lift. Following another aircraft might do this.

As well, perhaps following another aircraft can generate some upward air flow as the downwash from the wing bounces off the water. This would be equivalent to increasing the angle of attack.

Keep in mind that symmetrical airfoils typically need a couple of degrees more angle to generate a given amount of lift than flat bottom types. Also, increasing camber by lowering the flaps somewhat can generate more lift at a given angle.

Finally, power obviously does help!
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Old Feb 24, 2013, 11:56 AM
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Dennis C's Avatar
Grass Valley California
Joined Jun 2004
2,460 Posts
If you do not have enough power the technique would be an invitation to tip stall.

If you have enough power then it is a set up issue, You need to insure you have the step in the correct position relative to the CG and that you have the correct incidence of the float and the wing.

If you want an enjoyable experiance then be sure and take the time to fix it.


Dennnis
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