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9 Answers

Dihedral and Anhedral

Asked by: 7894 views
Aerodynamics

Is there ever a benefit to designing an aircraft with anhedral (where the wings droop)?  The small airplanes  like the Cherokee and Cessna have dihedral.  I'm just curious.

9 Answers



  1. Jason Schappert on Nov 27, 2010

    In small aircraft dihedral is what makes the aircraft want to resume level flight if or after approaching a roll, stall, spin etc…
    Ever heard of “let go and the airplane will fix it’s self” This stability is due in part because of the dihedral design.
    It all comes down to stability.
     
    There are larger aircraft with anhedral
    for example the Tupolev TU-154B
    Jason
     
     
     

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  2. Wesley Beard on Nov 27, 2010

    Why does the Tupolev TU-154B have anhedral?  What makes it different from the small airplanes?

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  3. Jim Foley on Nov 28, 2010

    I’ve always wondered the same.  Look at the AV-8B Harrier, you can’t miss the anhedral.

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  4. Best Answer


    Brian on Nov 28, 2010

    Your larger aircraft (C-17, B-52, etc) have anhedral because without it there would be enough roll control available to the pilot. The unfortunate downfall of a massive delta wing is an incredibly laterally stable airplane. The anhedral is done to counter this.
     
    As for the harrier and other fighters like the F-22, I can only speculate it being for combat maneuverability. In the case of the harrier it may be necessary for adequate control in a hover. Though as I said, this is mere speculation.
     
    What it all boils down to is dihedral and anhedral impact the aircrafts lateral stability. Finally a bit of useless information; a high wing design is equal to about 5 degrees of dihedral. So just moving the wing from under the airplane to on top of it is just the same as giving it 5 degrees of dihedral. Hence low wings Piper having noticeably more dihedral than their Cessna buddies.

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  5. Paul Tocknell on Nov 28, 2010

    When all else fails, there is always Wikipedia:

    http://en.wikipedia.org/wiki/Anhedral#Anhedral

    Here’s what I learned.  Sometimes when designing fighter aircraft, you don’t want stability you want manueverablity.  Also, sometimes when you have high mounted wings, the fuselage acts as a pendulum and creates stablity so you wouldn’t want the roll stability provided by dihedral because the airplane would be too stable. 

     

     

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  6. Wesley Beard on Nov 28, 2010

    Qt:  Brian “Your larger aircraft (C-17, B-52, etc) have anhedral because without it there would be enough roll control available to the pilot”
     
    I think you meant to say that there would not be enough roll control.  The airplane is too stable about the lateral axis.  It is always a question for designers of more stability or more maneuverability.

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  7. Brian on Nov 28, 2010

    Yes Wesley, that is what I meant. While your correcting me I shall correct one other thing: “massive delta wing ” should have been “massive high winged delta aircraft.” 
     
    Ever notice how just about all airliners have a low wing configuration but seemingly no dihedral? Yet any low wing GA airplane (with an elliptical or square wing as opposed to a delta wing) has significant dihedral? It is because of the contributors to lateral stability. The following is a list of things that increase static lateral stability:
     

    Delta wing (wing sweepback)
    Dihedral
    Vertical wing position
    Vertical tail

     
    Source: Flight Theory for Pilots
     
    Finally, Wesley you just said something incredibly important that I wish to repeat. In aircraft design it is nearly always a question of aircraft stability or aircraft control (also called maneuverability). You must sacrifice one for the other.
     

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  8. Steve Pomroy on Dec 16, 2010

    Hi Wesley.
    Everything the folks have said here regarding stability v. maneuverability is valid.  It is indeed always a tradeoff, requiring a compromise in the design.  And, as noted, the bias of teh compromise will depend on teh aircraft’s design mission.  Trainers tend to be more stable (at teh expense of maneuverability), aerobatic aircraft (including figters) tend to be less stable (and they get improved maneuverability as a result).  But there is one important point regarding anhedral/dihedral that seems to have been missed, and I’d like to add it in for you.
    Dihedral aircraft are prone to a dynamic stability response known as “Dutch Roll”. This is a rolling/yawing oscillation which, in extreme cases, can lead to control difficulties.  Even when the oscillations are small, they can promote airsickness in passengers and disrupt targeting in military aircraft.
    Dutch Roll is aggravated by swept wings and high vertical tails – both of which increase lateral stability.  Further, at high altitudes, there is less damping of the oscillations due to the reduced air density.  So high-speed and high-altitude aircraft are often prone to Dutch Roll.  There are two approaches to eliminating (or at least reducing) Dutch Roll.  The first is to install a yaw damper.  A yaw damper is a form of stability-augmentation — it uses roll/yaw gyros to sense dutch roll, and then automatically deflects the rudder to reduce/eliminate the motion.  Many transport category aircraft have yaw dampers.
    The second approach is to reduce dihedral, even to the point of having anhedral.  Many (not all) aircraft that have anhedral, especially those in the transport category, have it to combat Dutch Roll.
    I hope this helps.
    Cheers,
    Steve
    http://www.flightwriter.com

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  9. Jon on May 22, 2011

    Harrier Jump Jets have Anhedral..I would say check into the design and learn why they decided to use it.

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