What's the difference between positive and negative dihedral? I'm guessing one is bent upwards and the other is downwards?

Dihedral is defined as an upward angle from horizontal from root to tip, as viewed from fore or aft. That is to say when viewed normally, right side up and all that, the wing tips are higher than the wing root. Anhedral is the opposite; negative dihedral if you wish to put it those terms. Not bent per se, but two wing halves joined together, the joint being other than 180 degrees.

I'm building an Extra 300s and the manual states to put positive dihedral into the wings. That means the wing tips are higher than the root right?

When viewed from the front, right side up and all of that, wing tips higher than the root, yes.

Whewww, okay, thanks.

You're building an Extra and you don't know what dihedral (positive or negative) is? Are you sure you're ready for that plane? Isn't there a picture of that somewhere in the manual or on the plans, with the amount to be put in called out?

Dr.1

What brand Extra 300S are you building that requires positive dihedral?

The P40 Warhawk has dihedral. You can also call it positive dihedral.

The Cheap $40 trainer has anhedral. You can also call it negative dihedral.

Typically, purely aerobatic designs have very little dihedral. The most modern ones have none.

Here is a picture of a 300 and if you look very closely, you'll see a degree or two of dihedral.

The title of the picture says it's the first 300LP.

you'll see a degree or two of dihedral.

I see a flat top wing with a taper on the bottom. I guess that's technically dihedral.

Dr.1

Some aerobatic model designs include a small (1-2 degrees) of dihedral in the wing to reduce adverse roll coupling with the rudder. A plane with a truly flat wing that has the rudder above the centerline will have a tendency to roll opposite the turn due to the induced moment from the rudder. Adding a small dihedral counteracts this tendency.

Dihedral also increases roll stability by increasing the lift on the "down" wing, which tends to right the plane. Anhedral has exactly the opposite effect, which reduces roll stability. An interesting bit of dihedral trivia is the B-52. If you look closely at the plane, you'll see the wing is anhedral when sitting on the ground. When the plane is loaded, the weight of the plane in flight causes the wing to become level or slightly di-hedral. A bad side effect of this design is that if the plane rolls to far, the wing looses the dihedral, and the ailerons are no longer effective enough to counter the roll. A few of them have been lost due to pilots overbanking at low altitude. If you've ever seen the video of the model B-52 that crashed in England a few years ago, it probably crashed due to the same effect.

Brad

Yes, that is dihedral. The upper surface of the tapered wing is flat, but structurally, the wing has dihedral. The centers of the tips are raised compared to the center of the root. A truly flat tapered wing would have the leading and trailing edges flat, the upper and lower surfaces would not be flat (following the taper), and the tips would be on the same center as the root.

Years ago, I built a Lanier Laser 200, and the wing was joined with the top on a flat surface, using this same principal. The dihedral was thus designed in as a goof proof process, even I couldn't mess that one up!

I'm building a Graupner Extra 300s and it's my 16th airplane. Been flying for 8 years. Never heard the term negative dihedral though. Thought mostly every plane was positve just second guessing myself and wanted to be reassured. Thanks.

Poor pics in the manual and it's rather vague. It says: "The two wing panels are joined with positive dihedral, that is, there should be about 6mm clearance at the center of the joined wing when it is laid down on the workbench inverted."

The 'inverted' part is what threw me for a loop causing the confusion.

Negative dihedral would be something like an Air Force C-5. Right?

Yup, the wings of the C5-A or the (somewhat) horizontal tail of the F-4 Phantom. They both have anhedral or negative dihedral.