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Dihedral vs. polyhedral

Old 10-25-2009, 03:51 AM
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FlyWheel
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Question Dihedral vs. polyhedral

What are the differences in the flight characteristics between a dihedral (flat, w/one bend @ center of fuselage) wing and a polyhedral (flat w/multiple bends, usually near/at the tips, or tips curved up) wing? Which would be easier for a beginner to fly Would it depend on the intended use of the plane (sailplane, park flyer, etc.)? Obviously neither would be a very good idea for a stunt flier of any kind.

Are there are any signifigant differences in handling at all? We'll assume both are being used on an otherwise identical aircraft.
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Old 10-25-2009, 05:01 AM
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Sky Sharkster
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Default Dihedral Breaks?

Hello FlyWheel,
Without much technical background, I can only give you an observation based on years of flying gliders and powered sailplanes;
Polyhedral wings are very stable, even when the total amount of dihedralled wing area is less.
"V" dihedral (single break dihedral) wings are slower to respond to rudder input. This may be because polyhedral wings generally have less overall dihedralled area.
Virtually all modern competition R/C gliders with rudder/elevator control use polyhedral or tip-only dihedral.
Most modern Free Flight models use polyhedral and many now use four or five dihedral breaks.
Roll-coupling ("Dutch Roll") is greater with single-break dihedral than polyhedral.
Polyhedral wings are generally slightly heavier than single-break dihedral wings.
Single-break dihedral wings are easier to build.
The vertical lift component of any wing is greatest when it is flat, parallel to the ground. As it is tilted upward (either by banking or from dihedral) the lift of the tilted wing is less. Most polyhedral wings have nearly flat center panels, with greater angle on the tips. This means that during a turn or bank, a greater section (or area) of the wing is still generating near-optimum lift, it is more "flat". Only a small section of the tips are at an extreme (low lift) angle.
This article explains how roll-coupling is greater towards the tips. It seems to indicate that dihedral is most effective on the outboard section of a wing;
http://www.rc-soar.com/tech/spiral_eda.htm
JMO, Hope it helps!
Ron
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Old 10-25-2009, 08:39 AM
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Ron nailed the core of the difference when he said "dihedral is most effective on the outboard section of a wing"

This is for the simple reason that the outer parts of the wing have a greater moment arm than the inner parts.. So dihedral is most efficient if it increases as you move out along the wing.

You can get the same dihedral effect from straight 'V' dihedral, so the same stability and rudder response, but you would have to add more dihedral. Large dihedral angles are inefficient because any dihedral means that the lift produced by the wing as angled inward and the inward lift component does nothing to support the weight of the plane.

So if efficiency is very important, like a thermal soaring glider then polyhedral is definitely worth thinking about. If it's just a sport model and a couple of % of efficiency is not an issue then simple straight dihedral will do the job and is easier to build.
Attached is a Excel spreadsheet that will calculate equivalent dihedral angles of two and three panel polyhedral compared to simple 'V' dihedral angle and give efficiency of each.

Attached Files
File Type: zip
Dihedral.zip (5.7 KB, 590 views)
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Old 11-22-2009, 06:36 AM
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Originally Posted by Sky Sharkster View Post
Hello FlyWheel,

This article explains how roll-coupling is greater towards the tips. It seems to indicate that dihedral is most effective on the outboard section of a wing;
http://www.rc-soar.com/tech/spiral_eda.htm
JMO, Hope it helps!
Ron
Wow, that looks like a cool article.

I made a free flight design that uses winglets. I guess it's extreme polyhedral when you think of it.

The entire wing is flat except the last 2" of the outboard is 90 degrees vertical.

I'm going to check out the formulas and see if it holds for 90 degrees.

My model is at http://www.rubber-power.com. Free plans are available.
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Old 11-22-2009, 09:52 AM
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JetPlaneFlyer
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GeneralSocial

For a flat wing with vertical 'winglets' the formula would 'work' and it would correctly calculate that the EDA was zero.

Here is the calc:
The EDA of the inner panel is: 0(1-0) = 0
The EDA of the outer panel is: 90(0-0) = 0

Total EDA = 0+ 0 = 0

So vertical winglets dont have any equivalent dihedral effect at all.

You will probably now ask..." But how come my model flys fine if it has no dihedral?".. This is due to a secondary effect of the wingles; if they are placed high above the CG of the plane then they will have a stabilising effect as the side force they generate causes a stabilising rolling moment, similar to shown in this diagram: http://www.centennialofflight.gov/es...II/TH27G11.htm

Although they may work to keep the model stable they are a very inefficient way to do it....

Steve
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