dihedral or straight wing?

[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 science forum.

[pda4you]

Can a model with straight wing have relaxed flying characteristics?

Sure I had a 1/4 scale clipped cub with 0 dihedral and clark y airfoil. It was very relaxing. If you want to have self correcting caracteristics I would put some dihedral in. IMHO it also looks more natural to have some dihedral.

Mike

[Ironsides]

You have not indicated whether you have ailerons or rudder only.

Dihedral assists turning with rudder only. I will defer to others on the amount of dihedral.


David

[pda4you]

Yep - David is right - with 0 dihedral you will need ailerons.

[Bob Chiang]

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

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?

[pda4you]

What is the span and airfoil (flat bottomed, semi-symetrical etc)?

On homebuilts I usually use the TLAR method (That Looks About Right).

Mike

[Bob Chiang]

36" span, 8" chord not including ailerons (maybe 1 1/4" wide). Airfoil is flat bottomed with some Phillips entry.

[pda4you]

I would do about 2 inches total.

[lrsudog]

A couple of things to consider (especially with a flying boat) is that a bit of dihedral will make the plane much more forgiving during landing, and will allow a bit more fudge factor regarding roll attitude when you land. Lot less likely to dig in a wingtip or tip float if they are each an inch higher.

The other thing is that takeoffs in breezy conditions will be much easier, since the dihedral will induce a certain amount of self leveling, and help get both tip floats out of the water at the same time.

[Climate]

The other thing is that takeoffs in breezy conditions will be much easier, since the dihedral will induce a certain amount of self leveling, and help get both tip floats out of the water at the same time.

This is true if the plane is facing in to the wind, but in a crosswind, any dihedral will make the plane want to dig in the leeward tip float which will in turn cause a groundloop, er waterloop.
A better option might be to build a flat wing and then add angled tips to give the wing more stability.
Of course the type of airfoil used will determine the overall flight characteristics, more than dihedral.

Peter

www.climatemodels.com

[lrsudog]

I would agree with regards to the angled floats, but I fugured that was a given.

I also would try to avoid cross wind takeoffs & landings in breezy conditions on water.

Dihedral, angled floats, what have you.

[Bob Chiang]

Thanks to all for the comments and suggestions. I am building the wing with about 3 degrees per panel (1" per side on a 36" span wing). Despite any Pattern flyer wannabe tendencies in me, I'm intending (and powering) this model for relaxed flying.

Please see my next question posted separately.
Thanks again,
Bob Chiang
Ithaca, NY USA

[Neo02]

Hi Bob,

despite common belief, even pattern plane do have dihedral in their wings. It's almost always cut into the wing's profile: assuming that the upper side stays at 0°, the under side has a slight dihedral because the root rib is much bigger than the end rib. This varies from plane to plane, but dihedral surely helps when performing rolls and other aerobatic maneuvers.

As for your question, I totally agree with what has already been said: dihedral helps even if you have ailerons, makes the turns look much nicer and "round".

Regards,
-Fabrizio