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Dihedral

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Richard Clark 204/07/2020 00:22:11
424 forum posts
Posted by alex nicol on 03/07/2020 22:21:30:

Hi Richard,

For what it's worth hi wing aircraft will have an element of self righting due to the pendulum effect caused by the weight of the fuselage hung below the wing. A low wing aircraft will have less or no self righting as the weight is on top of the wing and as soon as you bank there is no self righting effect as the fuselage assists the bank. With regard to the wing looking droopy the model in the image has a span of 82" has no dihedral and no sign of any droop

 

img_20200628_201348.jpg

You are correct and your plane looks fine.. In fact I have often thought of building the similar  Ugly Stick and probably will sooner or later. I shall omit the dihedral shown on the original Phil Kraft plan.

Incidentally don't believe this nonsense about ailerons being near useless on a model with lots of dihedral. The Junior 60 has a huge amount of dihedral which I kept for 'atmosphere' yet mine flies fine without using the rudder at all. I just fitted inset top hinged ailerons (strip ailerons wouldn't look right) and set it up with 50% differential. The ailerons are 'crisp' and there is no visible adverse yaw at all. I gradually reduced the  differential and it makes no visible difference though you might find it visible sat in the cockpit of a real plane. But AFAIK there   no way to actually try that.

'Experts' dreams up these myths, mostly to fill the pages  of a model magazine   and all the other 'experts' just repeat them. .

How does an aircraft turn?

Ailerons only - The aircraft rolls somewhat and slides sideways down the resultant 'gravity slope'. The vertical fin at the back resists this slide and so the plane weathercocks into a turn.

Rudder only - Rudder offset makes the tail slide sideways, slowing one wing and speeding up the other. So more lift on one side and the plane rolls somewhat. From then on it is the same as the ailerons above.

Edited By Richard Clark 2 on 04/07/2020 00:32:11

John Stainforth04/07/2020 00:57:09
367 forum posts
64 photos

Richard,

I beg to differ on how aircraft turn.

With a well-coordinated turn that uses both aileron and rudder, there is no lateral slip of the plane either in or out of the turn. With an aileron only turn, a plane generally tends to slip out of the turn, so there is certainly no weathercocking into the turn. The fundamental reason why a plane that is banked (using ailerons) turns is because the lift vector then has a horizontal centripetal component that pulls the aircraft round in a turn; the vertical component of the lift is reduced, hence the need for coordinated up-elevator to increase the angle of attack and thus lift.

With a rudder only turn, the yaw slightly increases the speed and lift on the outer wing, as you say, but this is pretty ineffective. In the context of this discussion of dihedral, the main cause of a rudder-only turn is the increased angle of attack of the outer wing and reduced angle of attack of the inner wing. Single channel, rudder-only models were given very pronounced dihedral more for this reason than for stability.

Martin Harris04/07/2020 01:12:00
avatar
9411 forum posts
256 photos

Doesn't dihedral provide an increase in angle of attack of the accelerated wing in yawed flight, providing the majority of the unbalanced lift and hence the increase in bank?

My Extra - mid wing and no dihedral - seems to exhibit very little banking effect with rudder only although it may be that there's some unconscious aileron compensation when flying flat turns - I'll have to try it with the stick locked centrally next time I fly it.

Edit: A bit of duplication - John had posted before I refreshed the page...

Edited By Martin Harris on 04/07/2020 01:38:12

Bob Cotsford04/07/2020 10:16:46
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8646 forum posts
483 photos

High wing stability - pendulum effect or resultant side-slip causing partial blanking of the higher wing? Discuss.

Primary cause of turning - weathercocking or inclined lift vector when banked? Discuss.

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