High wing anhedral

[dcoded]

Hey guys..


Recently attended an interview with an airline and got the following Q which I could not answer. Maybe you could help?

Why has the Pa28 Dihedral?
-For stability purposes
Ok, the C172 have high wings, why does it not have dihedral?
-Because its naturally stable. It has CG underneath..
Ok, then, but the BAE 146, its got anhedral angle, why does it so?
-Because they wanted to reduce some of the stability, thus anhedral angle
Ok, but look at the C172, it has not got anhedral angle, why does it not??

Now, here I could not continue.
The best thing I can come up with now after the interview is:
They wanted it to be stable, haha...
No but seriously.

Why?

[Mr.Vortex]

Hi,

I think becaues Bae 146 have a swept angle which help to damp out the bank
which high wing provide the similar effect too but also enhanced the unpleasant
dutch roll character. So we make it adhedral to avoid the dutchroll character
which can upset the passenger.

On the other hand, the C172 didn't have the swept wing so the roll damping is
not too much to cause the severe dutchroll.


Best regards

[sevenstrokeroll]

tough question

you should have countered with...does all your excellent training of aviation minutiae make up for your poor pay?

[mattpilot]

First of all, when the 172 question came up i would of countered with "but it does have a dihedral, albeit less than a low-wing" (assuming you accurately phrased the question u got), then added that while, as you correctly stated, highwing aircraft are more stable, the 172 also has struts that produce drag, which make up for some of the stability thus requiring a bit less dihedral.

The BAE 146 has an anhedral because a highwing & sweptwing combo produces an aircraft that is too stable. Oh and of course because it looks better... :)

[PyroTek]

I was under the impression that C172 has got dihedral - though, not much. :8

[NigelOnDraft]

Hinted at in above posts... Wing sweep back is strongly stable in roll, so high wing types need anhedral to reduce - it most clearly seen in the Harrier.

For the 146 however, it is so the engineers can reach the engines :D


Link (http://en.wikipedia.org/wiki/Dihedral_(aircraft)) SweepbackWing sweepback also increases dihedral effect. This is one reason for anhedral configuration on aircraft with high sweep angle, as well as on some airliners, even on low-wing aircraft such as Tu-134 and Tu-154.

[edit] Vertical position of the center of massThe center of mass, usually called the center of gravity or "CG", is the balance point of an aircraft. If suspended at this point and allowed to rotate, a body (aircraft) will be balanced. The front-to-back location of the CG is of primary importance for the general stability of the aircraft, but the vertical location has important effects as well.

The vertical location of the CG changes the amount of dihedral effect. As the "vertical CG" moves lower, dihedral effect increases. This is caused by the center of lift and drag being further above the CG and having a longer moment arm. So, the same forces (lift and drag) that change as sideslip changes produce a larger moment about the CG of the aircraft. This is sometimes referred to as the pendulum effect.[note 4]

An extreme example of the effect of vertical CG on dihedral effect is a paraglider. The dihedral effect created by the very low vertical CG more than compensates for the negative dihedral effect created by the strong anhedral[note 5] of the necessarily strongly downward curving wing.

[edit] Effects of too much dihedral effectA side effect of too much lateral stability, caused by excessive dihedral among other things, can be yaw-roll coupling (a tendency for an aircraft to dutch roll). This can be unpleasant to experience, or in extreme conditions it can lead to loss of control or can overstress an aircraft.

[BOAC]

so high wing types need anhedral to reduce - it most clearly seen in the Harrier. - well you learn something EVERY day - I always thought it was so the outriggers would reach the ground.................

[Machinbird]

If the 172 had no dihedral, where would you mount the fuel pickup ports in the wing tanks? You would have a high unusable fuel quantity.
The 172 does have dihedral and that level of stability enhances its general mission capability.

[mike-wsm]

Interesting question. But I'm not sure there is any need to produce a correct answer. In my experience of setting and marking exams there is a class of question where the marking sheet says 'any reasonable answer'. The examiner has to see whether the candidate is being honest and reasonable or is given to making wild guesses.

[dcoded]

Hey all.

As always, great answers guys. appreciate it!
Now its more clear. Its funny how fast you loose your knowledge if your not refreshing it constantly...

Thanks

[Tinstaafl]

This sort of question is like asking "How long is a piece of string?" The whole of the aircraft design has to be considered and how the effects of the parts interact. Some high wing types have anhedral, some dihedral and some are neutral. It depends on how all the factors interact to affect the stability and if there are other considerations (outriggers for example) that must be allowed for.

Some factors that can affect things include low CG, high keel surface, sweepback, desired amount of stability (or even instability), fin area and lots of others I could probably recall if I thought longer.

[osmosis]

Mike-wsm's answer has merit and we hope is what the interviewers had in mind. But why hasn't anyone here mentioned wingflex? Apparent anhedral on the ground whilst dihedral in the air.

[gossetti]

Hi folks.

here 's a simple but tricky question.

how does dihedral works? the key word is : HOW?

Everybody has his own explanation and I am confused.

thank you.

[FE Hoppy]

AOA difference

[flyboyike]

PFM. Pure Bleeping Magic.

[safetypee]

The BAe146 anhedral is a misnomer. The wings are pre-stressed downwards when on the ground due to the structural and stressing design such that under level flight load there is no or minimal anhedral.

It’s not always the achievement of an answer that matters. It’s the way you deduce and present the result of the arguments.

[Dash8driver1312]

I must be seeing some kind of weird optical illusion in flight then, considering that the 146 anhedral appears very much the same whether parked, rotating, climbing, cruising or approaching...

One thinks you are confusing it with perhaps the B-52 with its much greater span and larger amount of wing flex...

[Uplinker]

To understand how dihedral works, imagine a dihedral pair of wings banked 10 degrees. The effective width of each wing is now different - one wing is closer to horizontal than the other. The more horizontal wing has a greater effective width and so a greater vertical lift vector than the one pointing upwards, and this unbalanced force rolls the aircraft back until each wing has the same effective width and the lift forces become equal. Thus the dihedral gives rise to an automatic stability acting to keep the aircraft level.


Sorry safteypee, you are sooo wrong about the 146!

[Oktas8]

Anhedral and dihedral have no effect unless the aircraft is slipping. Whether one wing appears more or less parallel to the horizon is irrelevant. Lift is a function of angle of attack and relative airflow, and has nothing to do with earth reference.

If an aircraft is slipping, anhedral or dihedral will cause the wings to have different (asymmetric) angles of attack. Therefore there will be asymmetric lift across the wingspan, which will generate a rolling moment to bank into the slip (anhedral) or away from the slip (dihedral). Hence it is called lateral stability (stability in roll).

(That's an oversimplification. In practice, as someone has already said, many factors affect stability.)

[junebug172]

http://my.fit.edu/~dkirk/3241/Lectures/Dihedral.ppt

[Uplinker]

The lift vector though is relevant. Lift is derived at 90 degrees to the lifting surface. With dihedral, the more horizontal wing's vector gives more force upwards, hence more lift, whereas the vector of the wing pointing up is more sideways, so less lift upwards. The imbalance causes a restoring force.

[Teddy Robinson]

Re BAe146 .. your statement is incorrect.

[Oktas8]

The lift vector though is relevant.

Indeed it is, if you're talking about overcoming weight. However, you are not. You are talking about a force couple used to roll the aircraft about its own CG.

Without slip, there is no lateral stability. With slip, an- and di-hedral function correctly whether the aircraft is erect, inverted, or anywhere in between. Granted, the different "vertical-ness" of the lift vectors may make the aircraft climb or descend, but this has nothing to do with rolling couples.

Consider a model aircraft that is tied to fly along a taut wire. It can roll but it cannot slip. Will dihedral tend to restore it to wings level, if disturbed in bank? For answer, consider that for every force that you note on the more horizontal wing, there will be an exactly equal force, providing an exactly equal rolling moment, on the other wing.

[Uplinker]

Consider a model aircraft that is tied to fly along a taut wire. It can roll but it cannot slip. Will dihedral tend to restore it to wings level, if disturbed in bank? For answer, consider that for every force that you note on the more horizontal wing, there will be an exactly equal force, providing an exactly equal rolling moment, on the other wing.


Yes, I see what you're saying, Oktas, although in your illustration, the wire is supporting the aircraft; taking the place of the lift. When the wings have to provide the lift as well, the imbalance of lift, (i.e the difference in the vertical component of the lift vectors), will surely provide a restoring force to roll the aircraft level? Your wire will prevent the high wing dropping due to having less vertical lift component than the more level wing, but in the real world the high wing would drop. I will have to get my head around this.

[Oktas8]

The wire is overcoming weight. It has no effect on roll, so if your explanation is correct, the model should be laterally stable despite there being no slip. Do not confuse TOTAL lift (acting in the vicinity of the CG to overcome weight) with LIFT ASYMMETRY (ignore weight, just focus on rotation in any axis about the CG.

Here's another example, FWIW.

Consider an aircraft at 70* angle of bank in a steady turn. With skilful rudder application, the pilot perfectly prevents slip. The aircraft has dihedral, so the lowered wing's lift vector has a greater vertical component than the raised wing's lift vector. Will the aircraft roll wings level without aileron input? No, there is no slip, therefore no asymmetric lift, therefore no rolling moment about the CG.

(Yes, the raised wing is travelling faster in this case, but we shall say the aircraft is descending slightly also, putting the lowered wing at greater AoA. So each wing is generating precisely the same total lift - in this example!)

[Uplinker]

Yes, your turn example makes sense. I am just debating whether to go and dig out my books.....

[balsa model]

Anhedral and dihedral have no effect unless the aircraft is slipping.
Actually, they do have an effect even in a coordinated flight, although honestly I don't know whether it is a major or a purely "2nd order" effect:
With anhedral, the point of application of the combined lift vector is lower. Presumably closer to the CG, thus contributing to the reduction of lateral stability.

[Oktas8]

I must respectfully disagree Balsa.

If you are not slipping, then there is no lift asymmetry, and there will be no restoring tendency in the rolling sense.

But what about the pendulum effect, you ask? If you are not slipping (ball centred in the turn) then there is no pendulum effect, and the aircraft will not tend to return to wings level of its own accord.

After the slip develops, yes, what you say is correct.

[dubbleyew eight]

methinks the plot is being lost......

a wing does not have to be slipping for dihedral to have effect. what do you think corrects for the off centre pilot weight in a two seater aircraft?
the left wing is slightly more level than the right wing, has a longer effective span and generates just a tad more lift and thus makes the aeroplane fly straight(ish)

dihedral also has the effect of changing the angle of attack of each wing in a slip that is self correcting.

ligetti wrote that about 6 degrees of sweep back was equal to about a degree of dihedral. so in the BAE146 the anhedral is to reduce the stability.

not to kill an argument with facts entirely why do tailplanes occasionally have dihedral? :-)

[henra]

a wing does not have to be slipping for dihedral to have effect.


Absolutely!

You just need to think about the lift vectors when rolling a bit left or right.
With anhedral, the 'opposing' wing's Lift vector component in vertical direction will increase (Lift x cos(dihedral - roll)), whereas on the 'supporting' side it will decrease Lift x cos (dihedral + roll).
You get a self righting moment of the wing's lift. Then you need to consider the leverage of this stabilising force against the CG. If the CG is higher than the combined vertical position of where the lift is produced, these two moments will act against each other. If the postion where the Lift is applied is above the Cg the two moments will complement each other.
If you have anhedral, the effect will be inverse.
Therefore for a low wing aircraft you want to have rather dihedral if you don't want to go too aerobatic. With a low wing and no dihedral the aircraft will constantly try to roll off.
In a high wing design the leverage between Cg and the point where the Lift is applied will already cause a stabilising moment. This leads to increased aileron forces if you want to bank.
In order to avoid excessive self righting and thus excessive aileron forces for turning (which would increase drag and structural loads) high wing aircraft often get anhedral, especially bigger ones where the mentioned factors become significant.

[WeekendFlyer]

Well, there are some misconceptions going on here!

Lateral stability summary:

- Wing sweep, high wing, dihedral and the vertical tail are stabilising influences

- Low wing, anhedral and keel area are destabilising influences

- A laterally stable aircraft ALWAYS rolls away from sideslip

Lateral stability will only come into effect when there is sideslip present. This will happen whenever the lift force is not directly aligned with (and opposite to) the resultant of the weight force and any centrifugal force present. Weight ALWAYS acts vertically down, regardless of aircraft attitude, but lift will roll and pitch with the aircraft. Therefore if the aircraft has a non-zero roll angle, the lift force will be at an angle to the weight force and thus there will be a sideways component of lift. This causes the aircraft to fly slightly sideways as well as forwards, leading to sideslip.

Sideslip will cause the wing on that side to experience a higher AOA than the other wing, leading to a difference in lift that rolls the aircraft level.

[balsa model]

How about a hypothetical situation:
Dark night. The aircraft is not loaded symmetrically, so it WANTS to roll. We loose all gyros. We have a glider style slip indicator (a string on the centerline of the windshield, in the slipstream) and we religiously keep the string centered with rudder. So no sideslip. No lateral inputs (since we have no gyros). Are we going into a spiral, or does it depend on our anhedral / dihedral setup?
(BTW: I don't think pitch matters here.)

So I think I agree that all roll disturbance recovery and final stable equilibrium will involve some sideslip. I don't know whether this helped the OP, but it got me straightened out a bit. Thanks.:ok:

And to answer my own prior dilemma about 1st / 2nd order influence of relative position of center of lift w.r.t. CG: it must be 2nd order. The moment of unbalanced lift (in a slip) must be a much larger effect, considering typical wing spans.

[Dan Winterland]

?

http://upload.wikimedia.org/wikipedia/commons/thumb/9/96/Turan_Air_Tu-154.jpg/800px-Turan_Air_Tu-154.jpg

[WeekendFlyer]

Had a long think about Balsa's scenario:

If you maintain zero sideslip with rudder, the relative airflow will always come from straight on. However, the laeral mass imbalance will cause a wing to drop because he weight vector and lift vector will have a lateral distace between them, causing a couple which will roll the aircraft.

So you are now rolling but keeping sideslip zero, which will prevent the dihedral effect from happening. As you roll, the lift vector will point away from vertical, which should cause sideslip except you are using rudder to point the nose back into the relative airflow. Therefore as the roll continues, the nose points more and more into the turn.

This would lead to a spiral dive with increasing roll angle, eventually ending up with a near vertical dive. Not a nice thing to happen at night if you have lost your AI!

Following this thought experiment, my thinking is that if you lose gyros at night, it is essential to let sideslip do what it wants to do. that way a lateral mass imbalance would lead simply to a steady heading sideslip, which is way more manageable than a spiral dive!

[Uplinker]

Well, just for my own revision I went and dug out my ATPL notes and, yes it does say that dihedral works because a roll causes a side slip which changes the AoA vectors which increases the lift on the lower wing. :ok:

I find that hard to visualize, which is why I think of the correcting roll coming from the difference in the vertical lift vector between the two wings - because in the real airplane in the real air, we are after all talking about overcoming weight. My (incorrect) theory assumes a vertical movement of the aircraft to provide the correcting roll whereas the actual restoring force is created by a sideways movement.

Okta's thought experiment with the wire prevented sideways and vertical movement of the aircraft, which then stops the dihedral working.

Cheers!

[barit1]

Benny Howard, the hero of the 1935 National Air Races, related this story to me in 1969. His production high-wing cabin aircraft (DGA-8 thru DGA-12, all similar save for the engine choice) had no dihedral and had no need for it - EXCEPT for an optical illusion that made the semi-elliptical wings appear drooped when viewed from behind. He thought this was unattractive and might have hurt sales.

So when he revised the design to accommodate another pax, designated DGA-15, he lengthened the wing struts a few mm to raise the wingtips maybe 2 cm. This made the wing LOOK straight, although it had no effect on flying qualities.

PS - probably the worst-kept secret of the day was that his DGA designation stood for "Damned Good Airplane"

[Matari]

Most beautiful combination of anhedral / dihedral ever:
http://www.world-war-2-planes.com/images/vought_f4_corsair_small.jpg

[WeekendFlyer]

Dan,

like the photo and the "?". A picture speaks a thousand words.

Explanation:

You can see clearly the slight anhedral (which is laterally destabilising). However, remember that aircraft also has:

- T-tail
- Rear side-mounted engines
- Swept wing

all of which are laterally stabilising design features. If you added dihedral as well you would either have an aircraft with a roll-rate that would make a Cessna 152 look like a world-class aerobatic aircraft, or you would need massive ailerons and probably roll spoilers as well, neither of which are particularly desirable design outcomes in a commercial jet... :)

[Radu Poenaru]

Hey guys..
-Because they wanted to reduce some of the stability, thus anhedral angle
Ok, but look at the C172, it has not got anhedral angle, why does it not??


And still nobody answered it:

"Reducing lateral stability makes the airplane more maneuverable,etc (good thing) but if you reduce it too much w.r.t. directional stability the airplane will become spiral-prone (bad thing). I suspect C172 designers felt they had enough maneuverability without adding an anhedral and an extra risk of spinning"

[barit1]

A note about dihedral on this series: The original "ragwing" Cessna 170 (http://en.wikipedia.org/wiki/Cessna_170)had no dihedral - nor did the 2-place 140 that preceded it. The first metal-wing-skin model, the 170A, also had no dihedral.

But the 170B - which introduced the Fowler "barn door" flaps - also introduced the dihedral seen on all subsequent 172 models. (It may be that it was the flaps that led Cessna to add dihedral, but that's speculation on my part.)

A personal note as an old CFI: If a student receives his initial training in a 172, he is likely to be a bit spoiled by the excellent stability of the ship, and may have a bit of trouble when first flying the 150/152; overcontrolling can be an issue. Conversely, if he starts in a 152, he'll have less problem transitioning to the 172.

[Flash131]

Perhaps the easiest way to visualise the effect of geometric dihedral on a slipping aircraft is to consider the flow over a wing that is both unswept and untapered.

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 the influence of 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 aircraft, 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.