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Against roll instability, should I add more dihedral/a high wing?

I'm trying to build an RC plane that can handle windy conditions. For that I want extraordinary aerodynamic stability. For longitudinal static stability, I make my planes quite nose-heavy, and compensate for that with a greater tail downforce. That works. What doesn't work is lateral stability/roll stability. In theory, if a gust rolls my airplane, I will have a sideslip. Due to a high-wing configuration, and a dihedral angle, I will have a rolling moment that will counteract the original roll angle. Image from avstop.com, modified

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So I made my plane attempts have great dihedral/polihedral angles, and a really high wing with a solid surface underneath to further help counteract any rolling.

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Any time I went crazy on dihedral and high wings, my planes kept crashing agressively due to very sudden and very powerful side/back flips. At first I didn't understand, as these features should've made my plane almost un-rollable.

However, what I suspect is that these features had the opposite effect. Correct me if I'm wrong, but looking a the first diagram, what I see is that when a plane flying at level flight encounters a sideways gust, it will also make the plane agressively roll over, just as if the sideways gust was from a roll induced sideslip.

Is this a correct assessment? Did these roll stability features make my planes roll around uncontrollably?

Is there a way to have my plane react normally to a sideways gust while having static roll stability? Do I need way bigger vertical stabilizers for these dihedral values?

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Dihedral does not help roll stability to a side force gust. Quite contrary, a gust will roll the plane into a slip, which then will turn the plane downwind (courtesy of your elevator turned rudder).

Firstly, I would not recommend setting a model plane with excessive "static stability" in windy conditions. You want to be staticly stable, but not so much that a gust will pitch you up into a stall. Higher wing loading also helps.

But roll stability from dihedral is good for cruising flight, but no match for a side gust, especially in a model.

What you must do to keep dihedral is to put side area below the CG. This is known as "anhedralling". Sea gulls lower their wingtips. Cessna put the wing (and fuel) on top. Consolidated built their famous PBY 3 Catalina with the engines on top too! All 3 want to roll into the wind, not away. This is the key.

A fourth design, the common paper airplane, does this with gusto. The side "keel" balancing with the upraised wing around the center of gravity is best for cross wind roll stability.

So you may experiment by raising of lowering your CG, adding side area, and glide testing in a cross wind. Your second model seems to have more of a chance. You may try a shorter and more square vertical stabilizer, and a bit of weight (a coin or two) on top of the wing at the expected Center of Pressure.

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    $\begingroup$ Makes sense, but I wonder: Doesn't anhedralling counteract the effect of roll stability? $\endgroup$ – GZoltan Sep 8 at 5:26
  • $\begingroup$ Wing dihedral prevents a plane from rolling on its own. "Anhedralling" prevents a wind shift from rolling the plane. So the best way is not to take away your wing dihedral, but to find another way to balance side gust torque around the center of gravity. Early planes had fixed gear, and shorter (stacked biplane) wings. Longer single wings, mounted low, had many a designer scratching their head (including me). So when you test a design in your "wind tunnel", turn it sideways (and from the bottom too). Know where your CG is on all three axes. $\endgroup$ – Robert DiGiovanni Sep 8 at 10:17
  • $\begingroup$ So on your first model, put the bottle on top of the wing, and the prop directly in front or behind it. (I'm not sure if wearing ruby slippers or chanting "there no place like home" will help). $\endgroup$ – Robert DiGiovanni Sep 8 at 10:34
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I think you’re having a classic dilemma of static directional stability vs dihedral effect. There is always a comprise between two. More dihedral makes airplane more stable in spiral mode. However that makes the dutch roll worse, and in extreme cases unstable. The side flips you are describing sounds like extreme unstable dutch roll mode that eventually end up with uncontrolled flight.

On the other hand, if you don’t want to reduce your dihedral you can make vertical stabilizer larger to avoid unstable dutch rolls. This makes aircraft less stable in spiral, however it’s almost always more preferable to have a spirally less stable aircraft than the aircraft with unstable dutch roll. Because you can easily stop the unstable roll by only applying the opposite aileron, however stopping a unstable dutch roll can get a little bit tricky.

So I recommend either reduce the dihedral or increase the vertical tail volume (tail area x distance to cg).

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