Dihedral Design on Lateral Stability

[Refindaria]

Dear all,

How is it going? Anyone out there can explain to me why the dihedral design causes the the lower wing to have a larger angle of attack than the higher one during a bank?

Cheers. :ok:

[BOAC]

during a bank - sideslip?

[DaveReidUK]

Dihedral (aircraft) - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Dihedral_%28aircraft%29)

[Lightning Mate]

sideslip not bank:

http://i636.photobucket.com/albums/uu82/Lightning_29/DihedralEffectConverted_zpseaabb501.jpg

[dubbleyew eight]

me tinks allus you experts forgetting something.
why is the stall warning on the left wing??
nothing to do with sideslips that one.:=:=

[Refindaria]

Lightning Mate: Shouldn't the angle of attack be referenced by the relative wind direction (headwind) rather than from the side? That's what I don't understand.......:ugh:

[BOAC]

It is - 'Relative Airflow'.

[Lightning Mate]

What BOAC said. Relative wind is an amateur term.

me tinks allus you experts forgetting something.
why is the stall warning on
the left wing??
nothing to do with sideslips that one.:=:=

The angle of attack sensors on a transport aeroplane are on the front fuselage below the office windows.

Lightning Mate: Shouldn't the angle of attack be referenced by the relative wind
direction (headwind) rather than from the side? That's what I don't
understand.......:ugh:

My drawing shows the relative wind component due to sideslip!

:ugh::ugh:

[Busserday]

Geometry.
The vertical lift vector is larger in the case of the lower wing.

[tommoutrie]

Busser is right. In a steady turn, the Angle of attack is the same. The lift vector is perpendicular to the wing. So if there is dihedral, the vertical component of the lift vector is greater from the lower wing which tends to roll the aircraft back level. To keep most aircraft in a turn you have to hold a small amount of aileron - if you centralise the yoke the aircraft will tend to roll back to wings level. The angle of attack is significantly different as you roll into or out of a turn because the wings are moving up or down relative to the free stream and that will tend to oppose the roll but that effect is the same whether you have dihedral or not.

[tommoutrie]

I have no idea what lightenings diagram is showing.

[Lightning Mate]

To keep most aircraft in a turn you have to hold a small amount of aileron - if
you centralise the yoke the aircraft will tend to roll back to wings level. The
angle of attack is significantly different as you roll into or out of a turn
because the wings are moving up or down relative to the free stream and that
will tend to oppose the roll but that effect is the same whether you have
dihedral or not.

I've never flown an aeroplane which behaves like that. The latter part of the statement refers to aerodynamic roll damping.

If you have no idea what my diagram is showing then perhaps I'll leave it at that. All my students understood it perfectly well.

Both BOAC and I have given the answer.

[tommoutrie]

superb..

"I dont understand your diagram"

"well you're obviously thick"

"please explain it"

"no, you're too stupid"

Please can you explain the diagram with the yellow wedges and the blue arrows?
Its probably right, I just don't understand it.

[tommoutrie]

BOAC will explain it for me... Whats that pic all about BOAC?

[tommoutrie]

Lightening, if you say you have students you must be an instructor. Facilitate me mon brave...

[Lightning Mate]

Ok then.

I didn't mean to suggest you were thick. It's just that I grow tired of trying to explain dynamics on a thread such as this - it's easier in a classroom.

Yes - I was indeed an instructor, both as an RAF QFI and as a civilian. I am now retired.

So to the diagram.

The black arrow shows sideslip in a turn. Very little is required, but it is necessary for any dynamic stability attributes to work.

Therefore there will be a component of the relative airflow as shown by the blue arrows (very much exaggerated).

The yellow "wedges" show angle of attack, which is the angle between the relative airflow and the wing chordline. Because of dihedral the AoA of the lower wing is slightly higher than that of the upper.

Therefore if you look closely the two yellow angles are different.

Hope that helps.

edit: I have just noted that you hold a CPL.

[Capn Bloggs]

Stop teasing me, you lot. What diagram? :}

[keith williams]

If the problem is simply one of trying to understand the fact that dihedral causes the angle of attack of a dropped wing to become greater than that of the raised wing, a demonstration may be sufficient.

Take an A4 sheet of paper and fold it down the centre so that the two short edges come together. Now unfold it so that it looks like a pair of wings with dihedral. We will use this to represent our aircraft. It works best if you use a large dihedral angle (about 30 degrees).

Grasping the sheet at the tail end of the centre crease, hold it up about a foot in front of your face so that the crease is pointing directly at your right eye. Hold the paper so that it is in a non-banked attitude, and raise the front end of the crease to represent a nose up pitch attitude.

Close your left eye and look at the paper with your right eye. The view that you see is what the approaching air would see (assuming of course that air could see at all). You should be able to see the underside of both wings and the view of the left wing should be a mirror image of that of the right wing. This indicates that both wings are set at the same angle of attack.

Now rotate the front end of the crease so that it points at your (still closed) left eye and slightly drop the wing that is closest to your right eye. The view from your right eye now represents an aircraft that has dropped its left wing and is side slipping towards you. Once again the view that you have of each wing represents the view that the approaching air would have. You should be able to see more of the underside of the dropped wing and less of the underside of the raised wing. This means that the angle of attack of the dropped wing is greater than that of the raised wing.

If this difference is not obvious gradually increase the bank angle. You will eventually get a situation in which you can see the bottom of the dropped wing and the top of the raised wing.

The above process will not explain why the angles of attack change, but it should at least convince you that it is true.

To understand why the difference in angle of attack occurs we need to consider the relative airflow as being made up of two components. The first component is that due to forward motion of the aircraft. This component approaches the aircraft from directly ahead. The angle of attack between this component and each of the wings is identical.

The second component is the lateral airflow caused by the aircraft side slipping towards the dropped wing. As illustrated in Lightning Mate’s diagram, the dihedral angle causes the wingtip of the dropped wing to be higher than the root. The lateral airflow on this wing is from tip to root, so this means that the lateral airflow meets this dropped wing at a positive angle of attack. But for the raised wing the dihedral angle causes the wing root to be lower than the wingtip. The lateral airflow on this wing is from root to tip, so this causes the lateral airflow to meet the raised wing a lesser angle of attack.

The total angle of attack of each wing is the vector sum of the angles of attack of the two airflow components. The contribution made by the forward motion of the aircraft is the same for both wings. But the contribution made by the lateral (side slip) airflow is greater for the dropped wing that for the raised wing. The overall effect is that the dropped wing experiences a greater angle of attack than the raised wing.

[tommoutrie]

I've got an ATPL and an engineering degree. But thats not all that relevent, I simply don't think the response to the original posters question addresses what he or she asked. I think they want to know if, in a balanced turn, the angle of attack is the same for both wings.
I still don't understand the reply. Do you mean the angle of attack with respect to the airflow component parallel to the lateral axis as per your diagram? Or is this a representation of the angle of attack with respect to the free stream?
If its the free stream, then I don't agree and I don't agree with the parallax demonstration by Mr Williams either. The reason the parallax arguement doesn't work is the free stream presents itself to the wing from a direction which is parallel to the trajectory of the aircraft. So if you close one eye then look from a point half way down one wing you are assuming that the free stream is approaching both wings from one point. Which it clearly isn't.
By the way, I agree that a roll torque is applied if there is a sideslip and thats what gives stability in roll about the longditudinal axis but I dont think there is a difference in the angle of attack in a balanced, zero slip turn. I'm happy to be proved wrong but not by answering a different question.

[Lightning Mate]

So please educate the three of us who have posted.

[keith williams]

If its the free stream, then I don't agree and I don't agree with the parallax demonstration by Mr Williams either. The reason the parallax arguement doesn't work is the free stream presents itself to the wing from a direction which is parallel to the trajectory of the aircraft. So if you close one eye then look from a point half way down one wing you are assuming that the free stream is approaching both wings from one point. Which it clearly isn't.

If you think that the effect is just a result of parallax then let's adjust the experiment.

keep both eyes open and position the paper so that it is side slipping towards your eyes. Now look directly ahead so that the distance between the two points at which your eyes are focused (one on each wing) is the same as the distance between your eyes. The line of sight from each eye is now parallel to the trajectory of the aircraft.

As before the angle of attack will be greater for the leading (dropped) wing. Taking it to extremes if you yaw the paper to give a side slip angle of 90 degrees you will see the bottom of the leading wing and the top of the trailing wing. This means that the leading wing will have a positive angle of attack and the trailing wing will have a negative angle of attack.

My only reason for suggesting that one eye be closed is that some people experience blurring and double vision keeping both eyes open.

In one of your earlier posts you have stated the following:

The lift vector is perpendicular to the wing. So if there is dihedral, the vertical component of the lift vector is greater from the lower wing which tends to roll the aircraft back level.

You appear to be saying that a vertical component of lift is required to roll the aircraft level. Well let’s consider an aircraft with no dihedral/anhedral in a 90 degree bank. If we apply aileron we will increase the lift on one wing and decrease the lift on the other. This difference in lift will exert a rolling moment. But all of this lift will be horizontal, so if your hypothesis is correct the fact that there is no vertical lift means that there will be no rolling moment. Do you really believe that aileron deflection will have no effect in this situation?

[Lightning Mate]

...some people experience blurring and double vision keeping both eyes open.

Get a lot of that mate!

[Clandestino]

Busser is right. In a steady turn, the Angle of attack is the same. Coordinated, not steady. One can perform steady slipping or skidding turn where it is not the same.

The lift vector is perpendicular to the wing.In coordinated turn. Not so if performing e.g. forward slip...

I've got an ATPL ...which you absolutely had to perform a couple of times with your experience.

To keep most aircraft in a turn you have to hold a small amount of aileron - if you centralise the yoke the aircraft will tend to roll back to wings level. Interesting... I have always flown exotic designs that don't display this behaviour, such as Cessna 172.

I have no idea what lightenings diagram is showing. I do. It shows B737 series 300 or higher in left bank and slipping to illustrate AoA difference when slipping the dihedralled wing. I've seen it first in my ATPL papers.

I think they want to know if, in a balanced turn, the angle of attack is the same for both wings.No. He asked:

Anyone out there can explain to me why the dihedral design causes the the lower wing to have a larger angle of attack than the higher one during a bank?...which is unexplainable as it's not bank but slip what causes dihedral effect to work.

[tommoutrie]

Chaps, in true PPrune fashion, there seems to be an unpleasant edge to these responses. Especially from Clandestino who seems to think its important to have a poke at my qualifications which wasn't a point I raised. But I find that edge really annoying.

I believe what the original poster was asking was to do with an aircraft in a steady state, coordinated, balanced turn. He wanted to know why the inner wing (that is the lower wing, the one on the inside of the turn) has a greater angle of attack than the outer wing and was asking why dihedral caused this. My suggestion is that dihedral doesn't cause this and unless your airspeed is very low indeed there is no discernable difference in the angle of attack of the two wings. It is the result of the very slightly different airspeed as the outer wing describes a larger turn radius but its only of theoretical interest until you are operating at very low airspeeds very close to the stall. At normal operating speeds it is of no concern at all.

The effect whereby dihedral will give a restoring couple is as follows. When an aircraft tips from wings-level into a bank, this causes the flight path to start to curve. To keep the aircraft's nose aligned with the actual direction of the flight path and relative wind at any given moment, the aircraft's yaw rate must increase. Until this happens, the aircraft's nose will point toward the outside of the turn in relation to the actual direction of the flight path and relative wind at any given moment. The aircraft's yaw polar inertia in yaw will tend to cause the aircraft's heading to remain constant as the flight path starts to curve. This is a sideslip. The resulting sideways component in the relative wind will interact with the aircraft's dihedral geometry to create a difference in angle-of-attack between the two wings, which will create a roll torque in the direction of a restoring couple.

But this effect has nothing to do with a steady state, coordinated, balanced turn. And if it does, I'm happy to hear about it because I haven't before.

Except that I'm tired of the underlying abusive tone on PPrune from contributors like Clandestino. I just don't care enough to carry on being interested in trying to contribute to intelligent debate to have someone have a pop at my ATPL - what does it even mean, perform it twice?

[Volume]

It is indeed a little more complicated than what pilot students are told (even ATPL)...
To make it simple, for a wing the angle of attack at a certain "rib" (aerodynamically, nor structural), so at any section with a rob chord parallel to the longitudinal axis of the aircraft and perpendicular to the wing plane, is defined by the angle between the local arflow projected into the rib plane and the rib chord. Any flow component perpendicular to the "rib" does not influence AOA.
Local airflow is a combination of the forward speed of the aircraft (the free stream component parallel to the longitudinal axis), the vertical speed of the aircraft (in the plane of symetry) and the lateral speed of the aircraft due to dideslip (perpendicular ro the plane of symmetry) and any local speed due the rotation of the aircraft. In a steady turn this means a pitching and a yawing rotation. It is different in any location of the aircraft, resulting in locally different AOA. Different "rib" orientation due to Dihedral/Anhedral will have an influence as well.
To make it more clear, a 90° angled V-Tail on an aircraft in a steady 45° bank turn mens one stabilizer is vertical, one is horizontal. As the Aircraft rotates around an vertical axis perpendicular to the ground in a turn, only the outer stabilizer (the vertical one) is affected by this rotation, the innner stabilizer (the horizontal one) is not affected at all. On the other hand, vertical speed (as indicated on the VS indicator) will only affect the inner (horizontal) stabilizer, while the outer (vertical) one will not be affected.

Its all about trigonometry...

[pattern_is_full]

I think the wikipedia drawing is slightly better than Lightning Mate's (no offence meant, professor) in that it shows the three-dimensional aspect of what goes on:

http://upload.wikimedia.org/wikipedia/commons/d/df/SK_Dihedral_3.png

A side-slip means the aircraft axis is slightly twisted relatively to the flight path, and it is that twist, or divergence from parallel between the relative wind and the airplane centerline, that (if the wing has a dihedral angle) presents the lowered wing to the air in such a way as to increase angle of attack to the oncoming (forward movement) airstream, while reducing AoA for the raised wing.

[Flash131]

I previously posted this explanation elsewhere on this forum. Perhaps not the classic explanation of dihedral effect, but it works for me:

Consider the flow over an unswept, untapered wing with no washout. Let’s imagine a streamline (the path followed by an air 'particle') starting at some point on the leading edge of the starboard wing. Let’s further assume that the point is roughly mid span, so we can ignore ‘end-effects’. If the aircraft is slipping neither left nor right, the streamline will have no spanwise component; if we are looking down from on top of the aircraft, the streamline will appear aligned fore and aft.

Now consider what happens if the aircraft slips to the left. Our right wing streamline will now meet the wing trailing edge further outboard than it did with no slip. If the wing has dihedral, the trailing edge ‘seen’ by the streamline will therefore be higher (relative to the leading edge) than when there was no slip. So, in a left slip, the wing surface profile followed by our streamline will appear to be ‘nose-down’ relative to the profile it followed with no slip.The right wing streamline therefore sees a reduction in effective incidence. The equivalent streamline on the left wing leaves the trailing edge further inboard and therefore sees an increase in effective incidence. So the right wing lift decreases and the left wing lift increases, rolling the aircraft such as to oppose the slip.

Note that the above argument is independent of the attitude of the aircraft or the direction of gravity.

[BARKINGMAD]

Back in the dawn of history, our excellent non-commissioned aerodynamics instructor used to say the 4 ways to ensure lateral stability were - the 2 Chinamen and the 2 Welshmen.

Hi Fin and Hi Wing, Dai Hedral and Sweep Bach!!

This was before the digital computer took all the fun out of aircraft design and flying! Sweet memories, and a great way to remember when it came to the exams.

Apologies to our Eastern and Taff colleagues, this was NOT a racist rant.

However, it must have involved sideslip of some sort, and I'm alarmed at how quickly AoA and the position of probes etc crept into the discussion...............!

Back to basics anyone?? :rolleyes:

[tommoutrie]

none of that addresses the original posters question.

He is asking about the angle of attack during a stable, balanced, banked turn.

Not yawing, not slipping, not skidding, just turning.

Can't be that hard.

[keith williams]

The original question was

why the dihedral design causes the the lower wing to have a larger angle of attack than the higher one during a bank?

It included absolutely nothing about

during a stable, balanced, banked turn.

Not yawing, not slipping, not skidding, just turning.

You introduced all of those additional conditions.

[tommoutrie]

When your aeroplane is banked is it normally side slipping, skidding, or in a balanced turn? What's the normal, everyday situation? So if someone asks about angle of attack during a bank do you think it's sensible to assume, given that he's not mentioned slipping or skidding, that we are talking about a steady state, balanced turn.

What question do you think the op was asking?

[JAKL]

Everyone knows it is because the lower wing is closer to the ground and gets more ground effect :D

[AerocatS2A]

The OP was asking about dihedral. You made assumptions about his question that makes his question irrelevant. The others here are making assumptions, or adding information, that keeps his question relevant. I think the second set of assumptions are more reasonable, don't you? Or do you really think the OP will be satisfied with the answer "it doesn't"? Don't you think he is after information about dihedral, and that providing situations where dihedral is relevant is a better way to answer?

[keith williams]

So if someone asks about angle of attack during a bank do you think it's sensible to assume, given that he's not mentioned slipping or skidding, that we are talking about a steady state, balanced turn.

But the OP did not ask about bank angle. He/she asked

why the dihedral design causes the the lower wing to have a larger angle of attack than the higher one during a bank?

He/she was clearly attempting to discuss the situation in which the dihedral design causes the lower wing to have a larger angle of attack than the higher one.

Instead of answering this question, you decided to add further conditions.

What question do you think the op was asking?

The majority of the posters in this thread appear to have had no difficulty in interpreting the question in the manner intended by the OP. Only you appear to have applied a different interpretation. In doing this you have made the question irrelevant.

Who does that suggest has got the wrong interpretation?

[BARKINGMAD]

Oooooooh dear!

If this is the standard of "instruction" or the current way of "imparting knowledge", then gawd help us all.

A simple posting on a simple aerodynamic quality descends into a personal and adverserial series of criticisms of others attempts to answer the OP.

Tommoutrie, I hope you are not a training pilot or ground instructor of any type as I can see the lessons being excruciating and counter-productive.

If that's personal/adverserial then so be it, I hold my hand up and admit it, but I can hardly seee how your contributions are helping.

Maybe stay your hand on this one and allow the discussion to proceed productively? :*

[DaveReidUK]

The majority of the posters in this thread appear to have had no difficulty in interpreting the question in the manner intended by the OP.You mean the OP who gave up 30 posts ago, after complaining that he/she didn't understand the responses they had received so far?

So arguing about the intent of the original question is somewhat academic - we'll probably never know what was meant ...

[tommoutrie]

Yup. Same as the hand flying thread.
bloody trolls
I'm out

[flyboyike]

Trolls? These are the very acme of all professional aviation.