dihedral or straight?

[Bob Chiang]

I am building a flying boat of my own design, and wondering if I should build the wing straight or with dihedral. It will have a rectangular wing of constant cord and thickness, powered by a geared speed 280 (or Johnson) and 6 or 7 700mah NiMH cells. Wing loading should be about 7 ounces per square foot. I am striving for smooth & gentle flight characteristics.

The only straight wing model I've flown is the Sylph (Speed 400 pylon racer). It's very smooth, though of course much faster than this flying boat will be.

So my questions are:
Can a model with straight wing have relaxed flying characteristics?
Is there anything different with a flying boat so that it should not have a straight wing?
If I should build with dihedral, what is appropriate per panel for basically a shoulder wing model with power pod?

Thanks in advance,
Bob Chiang

I'm going to cross post this message to the water forum.

[Sparky Paul]

If you don't have ailerons, you should have some dihedral.
Dihedral on a flying boat makes the tip float struts long.
Your choice. :)

[Dick Huang]

[QUOTE]Originally posted by Bob Chiang
If I should build with dihedral, what is appropriate per panel for basically a shoulder wing model with power pod?

Thanks in advance,
Bob Chiang

Bob,
The aspect ratio will be about 8.6. I recommend 9 degrees per wing panel which will give you a nice turning model with rudder.
Dick Huang :)

[Bob Chiang]

Thanks for the responses, I should have been more explicit: this model will have ailerons, elevator, speed control, and rudder.

I thought about the tip float issue, and considered a straight center section with raised wing tips. The ease of building a straight wing is the main thing tempting me to omit dihedral.

With ailerons, what is an appropriate amount of dihedral?

[Aio_1]

If you have ailerons, dihedral isn't absolutely necessary.
However a flat wing will have less roll stability so more corrections will need to be made by the pilot.
Too much dihedral and the ailerons will not be effective.

At a guess I'd say about 3 degrees sounds right for a plane of 'medium' aspect ratio.
Get some more advice on this though as I'm certainly not an authority on the subject.

Aidan

[Sparky Paul]

I have a Balsa USA Laker. It has a flat wing. Pod mounted motor. Manuverable, nice flier.

[Ollie]

Dihedral results in yaw to roll coupling as well as roll stability. The adverse yaw generated by ailerons alone has to be overcome for coordinated turns. This is usually done by symultaneously giving rudder with aileron control. The degree of adverse yaw is proportional to the coefficient of lift squared. therefore adverse yaw is slight at low angles of attack and high speed but gets worse as angle of attack is increased and speed is reduced. The trade off is between roll stability and the need to use coordinated rudder manually in turns. Yaw to roll coupling also makes axial rolls much more difficult. What you should do about dihedral depends on your flying skill and how you want the model to handle. You can get lots of roll stability with lots of dihedral but at the cost of requiring more piloting skill in turns to make them coordinated and at the expense of axial rolling ability.

[billsmithjr]

about 3 degrees per panel is perfect for this plane - just enough to make it stable not so much as to be un manueverable dont worry about strut length this looks like a pretty standdard set up
just make sure the tip floats are barely out of the water with the plane held level

[strouse]

Bob,

I posted a very similar question a few months ago and also was given very similar advice. Mine was an IPS powered low wing. I went with 3 degrees of dehidral and aileron/elevator. Its as easy to fly as my rudder/elevator high wings. More stable than I imagined. My 2 cents.

WStrouse

[vintage1]

My gut feelo g basd on a few models is that for rudder only operation, high wing or parasol needs 5 degrees, shoulder 7 degrees and low wing maybe even more for relaible rudder only flight. Not sayng less won;t fly, but it can be hard work getting bank on, and worse getting it off.

ith ailerons you are looking to make the model 'neutral' I reckon. Not tightening in the turns as sideslip and fin forcre the nose in, but not self correcting either.

A lot depends on CG HEIGHT and fueslage side area and fin dimensions etc, but 1-3 degrees should be in the ball park. Ive got an almost zero-zero shoulder wing plane here, and in calm air its great - flies true and stays in the turns without tightening. In wind tho, its quite a handful. Once banked by turbulence, it has to be brought back by stick input.

A price I am willing to pay for nice aerobatic performance, but not necessarily the thing for relaxed sport flying.

[Arizona Chuck]

I personally like a little dihedral in a aileron wing. I can't tell the difference in the flying but I think it looks better. If you're building a tapered wing, you can make the top of the wing straight and the bottom will have a little dihedral. A flat wing will sometimes look like the wings are drooping.

[DLC]

For the past several months I have been experimenting with a series of IPS-powered semi-aerobatic planes, hi-midwing, mid-wing and low wing. On the hi-midwing and midwing planes zero dihedral gave essentially neutral stability in roll, i.e., when you gave it a small bank angle, it would circle hands-off indefinitely (on calm days). I used 3-degrees for the low wing and it gave very small positive stability, i.e. when you gave it a small bank angle, it recovered to straight and level in 2 or 3 hands-off circles.

[DLC]

I forgot to mention, all of the planes were aileron ,elevator. throttle control.

[Bob Chiang]

Thanks for all of the comments and suggestions. I am building the wing with about 3 degrees in each panel. Should make for a nice relaxed flyer.

-Bob Chiang
Ithaca, NY USA

[Jasper]

Once built a plane with anhedral (own design). No mathematics used in the design. High wing, YS 120 power, A,R,E F,T, controls. Was vary stable and would fly the pattern. One of my flying friends built one with minor modifications to the underside of the fus', floats on underside of wingtips, same engine and controls. Model flew very well off water. Never measured the anhedral angle. Both models were built from scratch without plans/drawings. My friend liked the idea of the anhedral to get the tip floats closer to the water.

Regards Frank