Planing tail hulls and Icon A5

Martin Seamaster through to the Canadair CL-215 & 415 and the Shinmaiwa US-1.

Re: Planing tail hulls and Icon A5

Postby Gweduck » Sat Mar 05, 2011 4:20 am


A long afterbody (planning tail hull) limits nose up pitch excursions, whereas a hull with a short afterbody allows greater nose up pitch, especially when the secondary step sinks down in the ditch dug by the main step. This difference can have significant effect on the hull’s pitch stability.
.............Ben Ellison
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Joined: Fri Feb 18, 2011 8:21 pm

Re: Planing tail hulls and Icon A5

Postby buzz » Thu Mar 10, 2011 8:08 pm

Hi everybody

Gweduck wrote:
When flying, one quickly learns the trim settings that will give you the desired pitch angle on the step.

Gweduck, I would like to have additional details about the trim setting when your aircraft is planing on the step: do you have to act constantly on the elevator or you set an elevator angle and you maintain it (until the final rotation just before take-off)? I would appreciate if you could reply once again and I thank you for all the informations you gave me, they will be very useful for my project.

For anyone who is interested in knowing better planing-tail hulls, I recommend the following NACA reports (these links don't work well with Google Chrome, use Mozilla Firefox or Internet Explorer):
These documents are the first I found which concern this type of hull. Authors say that the best position for the CG of a planing-tail is after the step (especially for hulls with long afterbodies) and, in order to have a minimum resistance and a larger range of stability during take-off run, the hull has to be in contact with water in two points (around the step and at the end of the afterbody): this is the reason why I asked if the CG in the Gweduck is after the step and I thought it rides on two surfaces. In the reports these concepts are explained in a better way than I can do! :D

Tommaso Lippi
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Joined: Wed Feb 23, 2011 3:36 pm

Re: Planing tail hulls and Icon A5

Postby Gweduck » Fri Mar 11, 2011 12:00 am

Hi Buzz,

In the Gweduck, the minimum drag angle of the hull diminishes as speed increases. Prior to take off the elevator trim is adjusted for the angle that gives minimum hull drag when on the step, about 7 degrees nose pitch angle. When the power is applied for takeoff, the pilot uses substantial back pressure on the elevator control to bring the nose up. This pushes the planning tail down in the water with some force, helping to unload the forward hull, letting the aircraft climb over the hump faster. As the speed builds, the load of the tail on the water diminishes until it finally lifts clear and the hull runs on the step at its minimum drag angle. If the elevator was set correctly prior to the take off, the hull will run on the step hands off and then finally lift into the air when flying speed for that nose angle is reached (still hands off). During this run on the step, whatever pitching moment is necessary to keep the hull at that angle is provided by the horizontal tail. As I said before when running on the step at high gross weight, if the pilot raises the nose so that the tail touches the water, a definite deceleration is felt.

I hope this answers your questions.

………………Ben Ellison
Posts: 10
Joined: Fri Feb 18, 2011 8:21 pm

Re: Planing tail hulls and Icon A5

Postby BenoitLescot » Fri Mar 11, 2011 9:56 am

Interesting what you're saying Ben, so because the step is quite some distance bellow the CG, I would expect that when the plane becomes airborne it will not be trimmed for flight but will tend to increase attitude due to the elevator trim that was necessary to compensate the negative pitching moment of the step dragging in water ? am I right ? Remember I never flew flying-boats :)

In my project, the original hull of the Laté 631 had two steps and it would balance on these two until takeoff, a the cost of extra hydro drag, NACA TN 836 analyses the Laté 521 which has a very similar hull.
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Location: Dubai, UAE

Re: Planing tail hulls and Icon A5

Postby Gweduck » Fri Mar 11, 2011 8:38 pm


You ask some very interesting questions. A hands free take off from dead in the water to lifting into the air, is something I have only done on two occasions. And those were demonstrations to a friend who was skeptical about my claims of hull stability. As soon as it lifted clear of the water, I took the controls and made a normal departure. I don’t remember if the trim setting existing at the time encouraged the nose to pitch up or down. Because of the wing’s proximity to the water, the normal downwash from the wing and its effect on the horizontal tail is greatly distorted, so that one might have difficulty knowing whether a trim change is cause by the loss of water drag, or the aerodynamic effects of the downwash changing as you gain a little altitude. The surface affect can be very noticeable when approaching the water for a landing.

Gweduck is down right now for annual inspection. When its flying again I'll try and get some close up videos of the hull afterbody when planning, as well as an accurate answer to your question about trim change when the hull lifts off of the water.

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Joined: Fri Feb 18, 2011 8:21 pm


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