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Dihedral

Old 06-11-2020, 02:51 AM
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JotaMo
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Question Dihedral

What it would be the effect/result on the airplane performance/flying characteristics of having less dihedral than the original design? The reason of my question is that I am building a 1/4 scale Sig Spacewalker II and I messed up the building of the 2 wing panels center section, and now I realized that I have much less dihedral, maybe 50% less, than what it supposed to have. To replace the center ribs and position them correctly, at this stage, it is impossible. The easiest way to correct my mistake is to make 2 new plywood dihedral braces with less dihedral, but I am not sure what it can happen when flying the plane.
Old 06-11-2020, 04:10 AM
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speedracerntrixie
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It will have less roll couple with rudder application which in my book is a good thing. Some will feel it will have less stability ( self leveling) however unless you build the wing almost flat I doubt you would actually notice the difference. I have a feeling that the amount of dihedral designed in was mainly for scale appearance.
Old 06-11-2020, 05:03 AM
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Fidd88
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Originally Posted by speedracerntrixie View Post
It will have less roll couple with rudder application which in my book is a good thing. Some will feel it will have less stability ( self leveling) however unless you build the wing almost flat I doubt you would actually notice the difference. I have a feeling that the amount of dihedral designed in was mainly for scale appearance.
Fully agree with the above. I would suggest that the primary effect of less dihedral is most likely to be less dynamic-stability in roll, so it'll make the model a bit of a handful with a cross-wind or gusting conditions. (I used to watch Antonov transports with marked anhedral landing in severe cross-winds at RAF Manston - which was bloody scary!)
Old 06-11-2020, 11:21 AM
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Thank you guys for your responses, I feel more comfortable now in moving forward with my project.
Old 06-12-2020, 08:45 AM
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Hi,
Dihedral improves lateral stability, in other words, it improves the model's tendancy to roll wings level by itself after any sort of disturbance during straight and level flight.

Originally Posted by Fidd88 View Post
so it'll make the model a bit of a handful with a cross-wind or gusting conditions. (I used to watch Antonov transports with marked anhedral landing in severe cross-winds at RAF Manston - which was bloody scary!)

The model should be less affected by crosswind the less deflection it has from the 180 plane. In other words, all other factors staying the same, a wing without di- or anhedral will be the most stable during crosswind take offs or landings.

During crosswind take offs or landings the plane is flown out of balance, also called cross controls. Ailerons into the wind and opposite rudder. This is to align the longitudinal axis with the runway and direction of travel, but to fight drift caused by wind. Any plane with dihedral would be more difficult to fly cross controls as the dihedral wants to roll the plane in the same direction as applied rudder. Also, a wing with dihedral or anhedral tends to be picked up or pushed down by a gust of crosswind more than a wing without any.

Big transport planes with top wings such as Antonovs, C17s and C5s have anhedral. Anhedral decreases lateral stability. This is intentionally designed into the plane to offset the huge stability created by the pendulum effect of hanging a lot of mass below the wings.
Old 06-12-2020, 12:10 PM
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Everything mentioned so far is true. "Pendulum effect" is the key phrase in helping to visualize what dihedral does.

Also, if your ailerons happen to quit working while in flight, it would not make coordinated turns as well on rudder alone, if the dihedral is less. However, it would to flat turns much better. I happen to love doing flat turns, especially in front of full scale pilots that say you can't turn a plane on rudder alone.
Old 06-12-2020, 12:19 PM
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Originally Posted by Outrider6 View Post
you can't turn a plane on rudder alone.
Of course, but it is not the most efficient, comfortable or safest way to turn.
Old 06-12-2020, 12:31 PM
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Fidd88
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Originally Posted by Outrider6 View Post
Everything mentioned so far is true. "Pendulum effect" is the key phrase in helping to visualize what dihedral does.

Also, if your ailerons happen to quit working while in flight, it would not make coordinated turns as well on rudder alone, if the dihedral is less. However, it would to flat turns much better. I happen to love doing flat turns, especially in front of full scale pilots that say you can't turn a plane on rudder alone.
Many years ago, when I was an instructor, we'd fly a circuit in a Cessna 150 using no primary flying controls whatsoever. It'd be perfectly possible to land it likewise, but that was a liitle more than I was personally prepared to venture with someone else's aeroplane! The partial opening of doors, port and starboard gave yaw, and therefore roll without use of ailerons, the throttle and trim gave pitch control. If anyone is silly enough to try this without thousands of hours under your belt, it's your own silly fault if you prang it trying!

Old 06-12-2020, 12:36 PM
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Originally Posted by Fidd88 View Post
Many years ago, when I was an instructor, we'd fly a circuit in a Cessna 150 using no primary flying controls whatsoever. It'd be perfectly possible to land it likewise, but that was a liitle more than I was personally prepared to venture with someone else's aeroplane! The partial opening of doors, port and starboard gave yaw, and therefore roll without use of ailerons, the throttle and trim gave pitch control. If anyone is silly enough to try this without thousands of hours under your belt, it's your own silly fault if you prang it trying!
off topic, but we have done something similar. Shifting weight around the plane in order to make it maneuver. The reaction is very delayed and I would certainly no try to land the plane like that unless it was absolutely necessary.
Old 06-12-2020, 02:38 PM
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speedracerntrixie
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Very few models can flat turn with zero aileron input, this one can as well as doing knife edge loops, vertical eights and knife edge Cuban eights rather easily.


Old 06-12-2020, 05:46 PM
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Originally Posted by JotaMo View Post
What it would be the effect/result on the airplane performance/flying characteristics of having less dihedral than the original design? The reason of my question is that I am building a 1/4 scale Sig Spacewalker II and I messed up the building of the 2 wing panels center section, and now I realized that I have much less dihedral, maybe 50% less, than what it supposed to have. To replace the center ribs and position them correctly, at this stage, it is impossible. The easiest way to correct my mistake is to make 2 new plywood dihedral braces with less dihedral, but I am not sure what it can happen when flying the plane.
If you have not installed the dihedral braces you still have not joined the wings yet correct? In this case place the wings together supported at each end with the proper dihedral and measure the gap. Make up two balsa ribs a bit oversize a little thicker than 1/2 the gap and glue to the root of each wing panel. Sand flush with sheeting then sand new angle to achieve the proper dihedral angle. You will be fiberglassing the center section anyway.
Old 06-13-2020, 11:21 AM
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jester_s1
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A lot of the effect will depend on how the plane normally flies. If the plane normally has positive roll coupling with yaw, it will now have less. If it normally has no roll coupling with yaw, now it will have negative coupling. Dihedral also helps the plane self-correct in a sideslip stall, which you'll lose now. Overall, you might like the effect, or maybe not.
Old 06-16-2020, 02:55 AM
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JotaMo
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Unfortunately I had joined the wings, and I discover the issue with the dihedral when I was glueing the plywood braces, that obviously did not fit. Well well.

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