Date: 18 May 98 16:03:02
From: "Pierre Bertrand" <pierre31@total.net>
Organization: TotalNet Inc.
References: 1
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Karl Swartz wrote in article
> Would someone care to enlighten me on the aerodynamics of wing dihedral?
Dihedral is a powerful aerodynamic means of generating a roll from a
sideslip. Normal stability requires that wind coming from the left, for
instance, lifts the left wing. However, sweep is also a powerful means to
generate roll due to sideslip. It is easy to visualize this: as wind is
coming from the left, for instance, the left wing is essentially unswept
and generates more lift, therefore rolling the aircraft right.
The problem is that for modern swept wing aircraft, the sweep already
provides plenty of "effective dihedral" and the aircraft does not need any
dihedral at all. Actually anhedral would be required to reduce the "roll
due to sideslip" to what is the necessary minimum (too much sideslip
stability has its problem which i will discuss below).
Military cargo aircrafts which are required to be able to load and unload
cargo from unprepared fields without special ground equipment are driven to
a high wing configuration and therefore have the luxury of designing
anhedral into the wing.
Modern transport aircraft, however, are more efficient in a low wing
configuration (uninterupted cabin floor, underfloor cargo). This
configuration has one drawback: tip or engine ground strike. The dihedral
on modern high subsonic swept wing transport is actually driven by ground
clearance criteria and not by aerodynamics. My guess is that the 777 has
more dihedral than the 747 because it has a longer wing span (for tip
strike) or a much bigger fan (pod strike).
Too much dihedral is bad statically and dynamically. Statically, too much
dihedral requires more roll power to counterbalance the "roll due to
sideslip". This might force bigger ailerons and spoilerons than necessary
(i say "might" because they might be sized for coordinated roll rate).
Dynamically, and that is possibly the biggest problem, is the "dutch roll"
(a natural oscillation of the aircaft in flight from left to right, like a
speed skater, a "dutch" speed skater i guess...). Effective dihedral is
the "dutch roll"s engine, so to speak. The more the effective dihedral,
the more severe the Dutch Roll gets. All swept wing transport require a
"yaw damper", a device wich artificially, through rudder inputs, stiffens
up the aircraft to dampen out the Dutch Roll.
In fact, a Russian low wing swept transport, the Tu-134 "Crusty", has some
anhedral built-in. This was not too penalising for them on landing gear
length (and weight!) as the Tu-134 has the main gear in pods on the wing.
I don't know this for a fact, but i'm guessing that the high wing swept
transport have a much smaller Dutch Roll problem. Mind you, this might not
get rid of all of the Dutch Roll, but might allow them to have less
severity associated with the loss of of the Yaw Damper system allowing a
single channel architecture or a lower reliability and therefore lower cost
system.
I hope this helps,
Pierre Bertrand,
Aeronautical Engineer
