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Dihedral Self Correction and Vertical Balance Point.

Old 01-23-2016, 08:29 PM
  #1  
birdDog
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Default Dihedral Self Correction and Vertical Balance Point.

I'm wondering about the capabilities of dihedral in self leveling/correcting with lack of the bulk of weight being below the wing.

Here is a Salvage scrap flyer I cobbled together after getting tired of looking at this wing in the rack.

3ch throttle and tail controls. The problem I can foresee is that the vertical balance point is roughly at the lower surface of the wing.

Does Dihedral have some inherent tendency to correct in this situation or are High wing 3ch planes designed on the principal of the weight causing it to right itself?

I know, I know, "Throw it in the air and find out ya bum." but I'm trying to keep the wight down. Any insight would be appreciated. Thanks!



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Old 01-23-2016, 09:06 PM
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fhhuber
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Dihedral works even if CG is above the wingtips.

simple explanation is:
As the plane banks, the low wing becomes more level and the high wing will be at more of an angle. This makes the low wing more efficient which will tend to bring it back up and makes the high wing less efficient so it will want to go down.
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Old 01-23-2016, 09:52 PM
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I trust you on such matters. Looks like shes getting flung as is. Thank you!
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Old 01-23-2016, 10:12 PM
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Generally you use a couple more degrees dihedral for the low wing vs high wing if rudder-elevator-throttle (no ailerons)

It may tend to turn a bit "flat" That is actually a plus for a climb and glide sailplane...

The swept back LE at the tips ALSO acts a bit like dihedral for stabilizing the model.

Just be ready for turns to not be as responsive as you might want.
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Old 01-23-2016, 10:30 PM
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If anything I can saw a few ailerons in. Still have a basement full of components. Lazy circuits are a nice change. There are more than enough other aircraft to get the blood pumping when I need that. Thanks again.
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Old 01-23-2016, 11:19 PM
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Dihedral (as in the 'V' angle of the wings) has nothing to do with vertical CG and also nothing to do with the 'efficiency' of the wing being increased because it becomes closer to horizontal.
It's actually related to side slip. When a plane banks it tends to slip toward the low wing creating a sideslip flight path. The slip coupled with dihedral causes the AoA of the low wing to be increased thus causing a stabilising roll opposite to the bank.

It's hard to explain but simple once you get your head around it.

Here's a diagram from a NASA's Introduction to Aerodynamics:


The vertical CG position does have an effect like dihedral but dihedral still works without having a low CG.
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Old 01-23-2016, 11:21 PM
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That sums it up very nicely.
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Old 01-24-2016, 03:45 AM
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I built a foamboard wing with zero dihedral and threw it on my 3 channel slow stick. I was able to slowly correct rolls but if it got too far sideways, it seemed to just fall out of the air.
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Old 01-24-2016, 04:06 AM
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Note in the graphic...

L1 > L2
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Old 01-24-2016, 08:31 AM
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Originally Posted by JetPlaneFlyer View Post
Dihedral (as in the 'V' angle of the wings) has nothing to do with vertical CG and also nothing to do with the 'efficiency' of the wing being increased because it becomes closer to horizontal.
It's actually related to side slip. When a plane banks it tends to slip toward the low wing creating a sideslip flight path. The slip coupled with dihedral causes the AoA of the low wing to be increased thus causing a stabilising roll opposite to the bank.

It's hard to explain but simple once you get your head around it.

Here's a diagram from a NASA's Introduction to Aerodynamics:


The vertical CG position does have an effect like dihedral but dihedral still works without having a low CG.
Its a combination of all factors ... not just one.

Nigel
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Old 01-24-2016, 08:41 AM
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Originally Posted by fhhuber View Post
Note in the graphic...

L1 > L2
Correct, but the question is 'why'

The reason that L1>L2 is due to the velocity component from side slip changing the AoA of the wings.. i.e. the sideslip component increases AoA of the low wing (thus increasing L1) and decreases the AoA of the high wing (thus decreasing L2)
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Old 01-24-2016, 08:47 AM
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Also... the wing that is nearer level has more of a vertical lift component while the other has more of a sideways lift component.

There are several things going on...

that's why I said "simplisticly" rather than going into 30 pages of aeronautical engineering.
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Old 01-24-2016, 09:09 AM
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Originally Posted by solentlife View Post
Its a combination of all factors ... not just one.

Nigel
Well, that depends on what you mean by 'all' and what you mean by 'it'.

Dihedral (as in the angling up of the wings) works purely due to one reason, as already described, the sideslip components increases the AoA of the low wing and decreases the AoA of the high wing. That's all there is too it.

Here's a picture of a good real world example of how the airflow 'sees' the wings of a plane with dihedral when it's in a side slip.. Note that relative to the airflow the leading wing has much higher AoA than the trailing wing:


But there are a range of other things that have a 'dihedral like effect' that work independent of actual dihedral.. Some of those are sweepback, high wing position, vertical CG position/keel effect, possibly others....
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Old 01-24-2016, 09:17 AM
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That's a picture of how yaw makes the wind "see" the airplane...
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Old 01-24-2016, 09:20 AM
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Originally Posted by fhhuber View Post
Also... the wing that is nearer level has more of a vertical lift component while the other has more of a sideways lift component.
No, that's not a factor at all, though it is the common (but incorrect) explanation that gets bandied about.
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Old 01-24-2016, 09:21 AM
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Originally Posted by fhhuber View Post
That's a picture of how yaw makes the wind "see" the airplane...
Correct, yaw and side-slip are one in the same aerodynamically.
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Old 01-24-2016, 09:34 AM
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Originally Posted by JetPlaneFlyer View Post
No, that's not a factor at all, though it is the common (but incorrect) explanation that gets bandied about.

I'm sorry... but that is the way that its explained in physics class...
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Old 01-24-2016, 10:03 AM
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Originally Posted by JetPlaneFlyer View Post
Correct, yaw and side-slip are one in the same aerodynamically.
Sorry I have to ask how can YAW be same as SIDE SLIP ... they are two completely different actions.

YAW is the angular change of a unit in horizontal plane. Unit can still be travelling along intended line of track.

SIDE SLIP is the bodily displaced movement at angle to the direction of intended movement that takes it OFF the intended track..

nigel
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Old 01-24-2016, 10:13 AM
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Originally Posted by fhhuber View Post
I'm sorry... but that is the way that its explained in physics class...
Sack the teacher!

Here's some pages out of the very well respected book 'Model Aircraft aerodynamics' by Martin Simons (which is contributed to by world famous professors of aeronautics such as Eppler, Wortmann and Selig among others.)

These are a couple of pages covering how dihedral works. Note the diagram Figure 12.17 which covers the explanation you described.. It's titled "inadequate explanation of dihedral effect'

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Old 01-24-2016, 10:16 AM
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Originally Posted by solentlife View Post
Sorry I have to ask how can YAW be same as SIDE SLIP ... they are two completely different actions.
I don't want to get dragged off in a tangent trying to define terms.. It wasn't me that described the photo in the previous post as 'yaw' and I never used the term yaw in my explanations. Lets say that a plane that is yawed relative to it's direction of travel is sideslipping... happy enough with that?

I'll just stick to using the term side slip.
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Old 01-24-2016, 10:25 AM
  #21  
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I'm sorry that you don't want to get off on a tangent defining terms when you were the one who took the discussion on a wild and unnecessary tangent.
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Old 01-24-2016, 05:19 PM
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I used to get all hung-up on term's for this and that, be it Boating or flying, after 50 years of both I don't get my panties in a wad over them and found the most descriptive terms are best, it's like Starboard in right and Port is wrong I fly with Kids A lot and simple is best for most everyone, Bubsteve
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Old 01-24-2016, 05:53 PM
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Originally Posted by stevecooper View Post
I used to get all hung-up on term's for this and that, be it Boating or flying, after 50 years of both I don't get my panties in a wad over them and found the most descriptive terms are best, it's like Starboard in right and Port is wrong I fly with Kids A lot and simple is best for most everyone, Bubsteve
Boating and Flying have been part of my entire life ... and I agree !!

Boating : Champion Baltsail Regatta 2003 in my boat EOLA ... photo here is me passing the second placed !!



RC : Here's me readying for Southern area Scale Champs in UK with Nieuport 28 ... placed 8th.



Sorry about Nieuport photo quality ... scanned real photo .. was back in the late 80's !


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Old 01-26-2016, 05:49 PM
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Originally Posted by fhhuber View Post
I'm sorry... but that is the way that its explained in physics class...
In physics class they have the ability to compute the vertical lift component of a wing surface at any inclination to horizontal. If you do that for a 5 dihedral wing banked 5 so one wing surface is horizontal and the other inclined 10 you'll see the lift differential is very tiny.

For a plane finding itself banked while flying straight forward, this would very slowly regain the equilibrium of the plane. But it doesn't explain the quick roll factor of a 3-channel wing when rudder is applied.

That is all the yaw of the plane meaning that although the direction of travel is unchanged, the leading wing exposes its bottom to the airflow as you can see from the photo above. This is combined with the fact that the trailing wing shows the top of that wing to the airflow. Both wings then contribute to a rotational torque to the airframe which is very strong, not weak like the differential lift is.

The entire wing is transformed into an aileron because of the yaw and the plane banks decisively. You can easily roll a Champ, Slow Stick, EasyStar, Radian and many other planes with just the decisive application of rudder.

Do the math. Differential lift can't account for that.
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Old 01-26-2016, 09:40 PM
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But it was a simple enough explanation for the question that started the thread...

And the "I'm smarter than you" wasn't needed.
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