Part147
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Dihedral Vs Anhedral?

Thu Dec 25, 2008 12:33 pm

Hi All!

This question has been bothering me since I started my aviation career back in 1985, and I've never gotten a straight answer - or at least one I felt entirely comfortable with...  Yeah sure

"Explain why very large commercial passenger aircraft tend to use a dihedral wing configuration instead of an anhedral design?"

Any thoughts or ideas?...  bouncy 
It's better to ask a stupid question during training, rather than make a REALLY stupid mistake later on!
 
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Starlionblue
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RE: Dihedral Vs Anhedral?

Thu Dec 25, 2008 12:51 pm

This has been discussed in several threads.

Dihedral adds roll stability, which is needed in a low wing airliner. In simple terms, the dihedral wings makes the aircraft want to come back wings level if the aircraft starts to roll.

High winged transports and airliners tend to have anhedral since they are "too" stable (pendulum effect).
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
GST
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RE: Dihedral Vs Anhedral?

Thu Dec 25, 2008 1:15 pm

Additionally to what Starlionblue said is that low wing aircraft often have engines underslung that need to be clear of the ground. Dihedral offers a convenient way of raising the nacelles that bit higher from the tarmac without needing to extend your landing gear with all the problems that will then give you having to enlarge the bays and keep doors low enough for airstairs etc etc. If you look at the A380 for example, it has a huge ammount of stability purely from its swept wings, so the ammount of dihedral on the wings is very much overkill. It requires aerodynamic and avionic systems to loosen it up and make it less stable to fly.
 
metroliner
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RE: Dihedral Vs Anhedral?

Thu Dec 25, 2008 1:48 pm



Quoting GST (Reply 2):
If you look at the A380 for example, it has a huge ammount of stability purely from its swept wings, so the ammount of dihedral on the wings is very much overkill.

But it's mostly all in the wing root, where the moment is least. The rest of the wings look relatively normal, even drooping at the tips when it's on the ground and tanked up.

The drawings of the 787 Boeing used to release with the wingtips practically touching... now that's dihedral! Big grin

Any exceptions to this 'high wing-anhedral, low wing-dihedral' rule? Lots of Tupolevs have low wings with anhedral, so there's a start - what about high wing dihedral, though?

Merry X-mas,

Toni  Smile
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Mir
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RE: Dihedral Vs Anhedral?

Thu Dec 25, 2008 2:49 pm



Quoting Metroliner (Reply 3):
Any exceptions to this 'high wing-anhedral, low wing-dihedral' rule? Lots of Tupolevs have low wings with anhedral, so there's a start - what about high wing dihedral, though?

The Dash 8 has dihedral, and so do the Cessna high wings (piston singles and Caravan). I figure that with the stabilizing effects of swept wings, you're probably not likely to see dihedral on aircraft that have them, but with straight wings dihedral can be beneficial, or even necessary.

As I recall, if you do have a high-wing airplane, it needs less dihedral to achieve the same effect as on a low wing airplane, due to the fuselage interfering with the airflow more in the case of a low wing.

-Mir
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SEPilot
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RE: Dihedral Vs Anhedral?

Thu Dec 25, 2008 3:42 pm

Starlionblue summed it up nicely. To expand on his explanation, it is desirable in a conventionally controlled aircraft (non FBW) to have "natural stability"; i.e. the aircraft will tend to return to its trimmed attitude following any disruption. With bank this is accomplished by one of two means; dihedral, which has the effect of increasing the lift on the low wing when the plane is banked, or pendulum effect, which has the center of mass below the center of lift. Dihedral is actually aerodynamically less efficient, as the lift vectors from the two wings are to some extent fighting each other and thus losing some lift, and thereby creating more drag. But the alternative (high wings) has its own set of problems, chiefly structure for the landing gear. For civil applications where the plane will always land on a well-prepared runway and loading and unloading can be accomplished with lifts (and not requiring loading and unloading large vehicles) the structural penalties of a high wing outweigh the aerodynamic penalties of dihedral. This is not the case with birds; however; note that low winged birds are quite uncommon, at least on this planet. Military requirements are quite different, however. Loading tanks, large vehicles and guns into a conventional transport (even with the much argued about nose door) would be extremely difficult on an improvised runway near a battlefield, and so most military transports are high winged for that reason (bringing the fuselage closer to the ground) and also to keep the wings, control surfaces, and engines as high as possible to minimize damage from FOD on unprepared airstrips. You will note that most high winged large planes in fact have anhedral; this is because, like most good things, too much stability is not good. The anhedral is introduced to reduce control forces and maintain the right amount of stability.
I mentioned at the beginning of this epistle that this discussion did not apply to FBW. So now that all new aircraft have FBW (which can introduce whatever degree of stability it likes into the equation without requiring aerodynamic stability) why do new airliners still have it? Frankly, I'm not sure, but the only explanation I can come up with is wingtip and engine clearance during crosswind landings. This would explain the A380's wing shape, but not the 787's. It could be the 787's wing, being CFRP is just much more flexible than it would be if it were AL for the same strength, and it just bends more during flight. Since the pictures we see of the 787 with the wings gracefully curved up are during flight, this makes sense to me. This will cause the plane to be much smoother during turbulence, as the wing flexing will act as a kind of suspension and smooth out the bumps. I do not know this to be the case, I am just speculating.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
tdscanuck
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RE: Dihedral Vs Anhedral?

Thu Dec 25, 2008 7:09 pm



Quoting Starlionblue (Reply 1):
High winged transports and airliners tend to have anhedral since they are "too" stable (pendulum effect).



Quoting SEPilot (Reply 5):
With bank this is accomplished by one of two means; dihedral, which has the effect of increasing the lift on the low wing when the plane is banked, or pendulum effect, which has the center of mass below the center of lift.

Be a little careful about pendulum effect...it's a lot more complicated than it sounds. I had my rear end handed to me by FredT (and I deserved it) in a very long thread about what's actually going on with "pendulum effect". In a nutshell, it's not actually the same physics as a pendulum at all.

http://www.airliners.net/aviation-fo.../read.main/228245/?threadid=228245

Quoting SEPilot (Reply 5):
I mentioned at the beginning of this epistle that this discussion did not apply to FBW. So now that all new aircraft have FBW (which can introduce whatever degree of stability it likes into the equation without requiring aerodynamic stability) why do new airliners still have it?

All current FBW airliners have a direct mode...the primary flight control computers can completely give up the ghost and they'll just command the flight control surfaces directly from the controls. That basically means they have to be aerodynamically stable. If you're using FBW to provide stability, you don't have that fallback option, which is probably a hefty certification hurdle.

Tom.
 
wingscrubber
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RE: Dihedral Vs Anhedral?

Thu Dec 25, 2008 9:56 pm

Low winged a/c, CofG is above the rolling axis which is unstable, imagine balancing a marble on top of an inverted bowl... Dihedral wings help achieve nuetral stability, imagine a marble in the concave side of the bowl. Reverse applies for high winged anhedral monoplanes.
Mid-winged monoplanes can afford a straight wing - the rolling axis and CofG are the same.
Older bi-planes like the sopwith camel had a dihedral lower wing and a straight or perhaps slightly anhedral upper wing...
The aircraft stability anhedral/dihedral arguement is all said and done really, doesn't get more complicated than that. You design for the required purpose and stability your aircraft needs.
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SEPilot
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RE: Dihedral Vs Anhedral?

Fri Dec 26, 2008 2:14 pm



Quoting Tdscanuck (Reply 6):
All current FBW airliners have a direct mode...the primary flight control computers can completely give up the ghost and they'll just command the flight control surfaces directly from the controls. That basically means they have to be aerodynamically stable. If you're using FBW to provide stability, you don't have that fallback option, which is probably a hefty certification hurdle.

Good point; I actually thought of that after posting the above epistle.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
FredT
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RE: Dihedral Vs Anhedral?

Tue Dec 30, 2008 12:02 am



Quoting Wingscrubber (Reply 7):
Low winged a/c, CofG is above the rolling axis which is unstable, imagine balancing a marble on top of an inverted bowl... Dihedral wings help achieve nuetral stability, imagine a marble in the concave side of the bowl. Reverse applies for high winged anhedral monoplanes.

I suggest actually going through the trouble of reading the linked thread.  Smile

It's a long one, I know, but in there are hidden nuggets of information (complete with pictures with lots of arrows!  Wink) which does tell you why the above is not a true representation of reality. In your defense, it is a well distributed explanation model which appears very correct until you start looking at it in depth.

Regards,
/Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
Part147
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RE: Dihedral Vs Anhedral?

Wed Jan 07, 2009 2:39 pm

Thanks!

Great, detailed replies... have helped me a lot!

I'm sorry I didn't think to search for earlier threads on this subject... really appreciate the answers you've given here and also from earlier replies given in other threads.
It's better to ask a stupid question during training, rather than make a REALLY stupid mistake later on!
 
prebennorholm
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RE: Dihedral Vs Anhedral?

Thu Jan 08, 2009 12:35 am



Quoting Metroliner (Reply 3):
Quoting GST (Reply 2):
If you look at the A380 for example, it has a huge ammount of stability purely from its swept wings, so the ammount of dihedral on the wings is very much overkill.

But it's mostly all in the wing root, where the moment is least. The rest of the wings look relatively normal, even drooping at the tips when it's on the ground and tanked up.

Dear Metroliner, the very high dihedral at the A380 wing root has only practical reasons.

The wing has to be placed relatively lower on the fuselage because the wing center box has to pass under the lower deck floor, and the the lower deck floor on this two deck fuselage is relatively somewhat lower than on a single desk plane.

The A380 also sits relatively low on its landing gear which is possible because the fuselage is relatively short. Tail strike isn't much of an issue. Short landing gear legs save a lot of weight, not only because the gear legs are lighter, but mainly because they have a shorter moment arm so their supporting structures can be made lighter.

But still the #2 and #3 engines need a generous ground clearance to avoid FOD.

That dictates the unusual "seagull shape dihedral" of the A380 wing.

The opposite is very visual on the B777. The B777 has probably the relatively highest cabin floor of all airliners, and its wing center box is placed right under the floor, as it is on all low winged airliners. The fuselage is relatively long, at least on the stretched versions, so a tall landing gear is needed anyway to avoid tail strike. It has no need for a seagull shaped wing, even if it has the largest engines of any airliner.

The B737 has something like the same problem with engine ground crearance as the A380, but it is solved in another way. Oval engine intake and engines placed high on the wings. Both have a slight drag penalty. In addition the high placed engines restricts flap configuration. The advantages are simplicity and lighter engine support structures.
Always keep your number of landings equal to your number of take-offs

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