videorov

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Which seems to fly the thermals better. The straight I guess will fly faster?
Im talking wings up around 4ft each side.
The straight wings are easier to build with hot wire cutting process.

skylark-flier

I'm certainly no expert on this - however, from my experience, if you turn the wingtip "up" you're going to get a somewhat "squirrelly" bird any time you maneuver because of "tip-stall". This can be deadly on landings - totally destructive if there's enough stall. A straight wing or one with washout (turning the wingtip down) is actually the more preferential set-up for most wings, whether powered or gliders.

Dave

TIA

OP are you talking about turning the wingtip up or down by way of front to back as in leading edge down trailing edge up "Twist" as it were, or are you talking about sweeping the wingtips upward as in polydihedral as the wingshape of a hobie Hawk? (The yellow plane in the picture)

videorov

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talking about sweeping the wingtips upward as in polydihedral as the wingshape of a hobie Hawk? (The yellow plane in the picture)

But not all the way across the wing. Some wings turn up towards the wingtips and some have a joint where the start to turn up.

I think the straight wing all the way out to the tip is easy to make with the hot wire process and will be a faster wing.
The turned up wing tips slow the plane down to make it easier to fly for people.

Aurora_60

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From what I understand, The dihedral affects the stability of the aircraft by positioning the CG low relative to the wing. The wieght of the fuse at the bottom of the "V" shaped wing will act as an uprighting ballast. This will make the plane want to self-correct itself in a banked attitude back to level flight. This wont affect the speed of the plane, just the roll attitude. The speed is affected by drag from the profile of the airfoil.

Weather or not to build a straight wing or a "bent" one depends on what flying characteristics you want. But in a sense of aerobatics vs. stability, not speed.

DM

da Rock

There's a reason you see so many model sailplanes that look alike. They perform the task better. The task is twofold. You have to find the lift, then you have to utilize it.

What you see in so many sailplanes is what does both jobs. By having a couple of degrees of dihedral at the fuselage with "turned up tips" will give you good hunting performance. The center 'flatness' helps the glider cruise while you're running off looking for lift over on the other side of the field. The upturned outboard sections will give you better response to lift one side of the wing feels while your hunting, so you'll get better indications you've found it than you'd get from a wing that was flat except for the center dihedral.

Look at any sailplane catalog and do some of that.

BTW, you'll notice that polyhedral wings usually don't have ailerons. You'll also notice that flat wings almost always have ailerons.

scaflock

There's actually a lot of things that go into a wing design. You have to take into consideration the main type of conditions it's going to be flown in as well as just how it's going to be flown. (Thermal or aerobatic etc) Very few full scale sailplanes use a poly wing yet are able to find lift just fine. When you scale the size down to your typical model span, you need to increase the stability hence the poly layout on the majority of model sailplane wings. Flat wing models tend to be more squirrely and better suited to aerobatics than a plan form with a strong dihedral or polyhedral. Airfoil choice is also going to have an effect in how a plane flies. Thicker airfoils while they provide more lift will fly slower than a thinner airfoil. (The Paragon comes to mind. Wing is pure lift but it's a slow flier unless you ballast it up quite a bit.On the other hand a Windfree is a thinner airfoil and will thermal just about as well.)

For my money, the Drela line of airfoils offer the best compromise between lift and speed. He also proven that you don't have to keep the same airfoil the entire length of the wing. By using a slightly different airfoil at the tips you can cut way down on tip stalls etc. Take a good look at some of the models that use his airfoils and you'll find that they change from root to tip and may actually use 3-4 slight changes over the span to optimize the flight characteristics over a wide span of flight conditions. My current design project uses the Drela AG-40 thru AG-46 foils along the length of a 16 ft 4 in wing and is intended for use in XC flying. As such it needs to be able to find lift but still be able to move out when needed.

wyowindworks

Are you talking about polyhedral?






or an upturned tip like this?

videorov

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upturned tips.

Aurora_60

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The polyhedral and upturned tips will have affect on the aircraft the same way, that being the roll axis. The upturned tips will transition smoother through the roll process than that of the polyhedral. Doing away with both will provide an aircraft more rsponsive to the roll axis and will hold a banked, or out of level flight, longer.
If you are cutting your own wings, I would say to make both a Flatish wing, with just a bit of dihedral, and a stock version. Experiment, thats what the hobby is all about!


DM

wyowindworks

The upturned tips will not increase the drag/slow down the craft. The theory with the upturned tip is that it moves the tip vortex farther out and thus increase the effective span. Whether this actually happens is quite debateble. Even in full scale gliders many tip designs get carried on because of mythical claims rather than hard proof. There a many highly compentative TD ships that do not have them.

BMatthews

Flat wings are easier to cut. That's pretty obvious. But as mentioned they are not automatically faster.

Wings with serious polyhedral are intended to be steered around with rudder instead of using ailerons. Using ailerons for rolling takes away the need for any dihedral at all. But the reason you find that a lot of thermal gliders have three piece wings with a small amount of dihedral in the outer panels is because it tends to make the model groove better in thermal turns. The dihedral takes away much of the need for small corrections to prevent the turn tightening up. This makes the model easier to fly when out at farther distances where it's tough or impossible to see the wings clearly to know if the turn angle is degrading or not.

If you decide that you want some dihedral but do not want to go with a 3 part wing then you can consider putting the dihedral in at the wing roots for a basic simple V angle. Two or three degrees isn't much but it will help to stabilize the model in turns and in straight line flight. For three degrees of dihedral you would angle the flat panel up to 2.5 inches at the tip for a 48 inch panel. For 2 degrees the tips would be jacked up by 1.75 inches. That much will do a lot for making the model easier to fly with less corrections while at the same time help hold the tips up to avoid snagging during landings.