Wing dihedral

Would anyone care to comment on the accuracy of this illustration of how
wing dihedral works from a 1981 Jeppesen Sanderson book.

http://makeashorterlink.com/?B25A35DCC

The accompanying statement reads:
"When an aircraft with dihedral rolls so that one wind is lower than the
other, the lower wing will have more effective lift than the raised wing
because it is not tilted from the horizontal as much. The imbalance in lift
tends to raise the lower wing and restore level flight."


Dallas

Give some thought to the components of lift, on both wings, and to which
direction lift acts. ;-)


"Dallas" wrote in message
nk.net...
Would anyone care to comment on the accuracy of this illustration of how
wing dihedral works from a 1981 Jeppesen Sanderson book.

http://makeashorterlink.com/?B25A35DCC

The accompanying statement reads:
"When an aircraft with dihedral rolls so that one wind is lower than the
other, the lower wing will have more effective lift than the raised wing
because it is not tilted from the horizontal as much. The imbalance in
lift
tends to raise the lower wing and restore level flight."


Dallas

"William Snow"
Give some thought to the components of lift, on both wings, and to which
direction lift acts. ;-)
http://makeashorterlink.com/?B25A35DCC

I have. I see no forces presented in this illustration that would change
the components of lift on the wings or it's direction that would cause the
aircraft to roll back to horizontal.

Why would a horizontal wing create "more effective lift" than a banked wing?

This thread began in another group and some interesting points were
discussed, but I honestly expected a few belly laughs here on the absurdity
of this book's explanation.


Dallas

I agree completely - it's about the slip created and how dihedral
responds to it.

Dallas wrote:
"William Snow"

Give some thought to the components of lift, on both wings, and to which
direction lift acts. ;-)

http://makeashorterlink.com/?B25A35DCC

I have. I see no forces presented in this illustration that would change
the components of lift on the wings or it's direction that would cause the
aircraft to roll back to horizontal.

Why would a horizontal wing create "more effective lift" than a banked wing?

"Dallas"

Why would a horizontal wing create "more effective lift" than a banked
wing?

Let's think of a set of wings with a dihedral angle of 10 degrees up from
horizontal, on both wings.

Now, think of the shadow the wings would make, if the sun were straight
overhead, while the plane is banked at 10 degrees. The wing that is up
would make a smaller shadow than if the plane were flying level. The wing
that is down would be making the largest shadow that is possible.

The size of the shadow is the only size that is important, because the lift
that is straight up (towards the sun, in our example) is the only lift that
will be important to the plane, as that is what is counteracting the force
of gravity. The fact that the down wing's shadow is larger, will make have
more effective area than the up wing, and will tend to bring that wing back
up.

While you are in level flight, the same thing will constantly be at work,
automatically trying to keep the plane level.
--
Jim in NC

It's a coherent description but I think it's inaccurate. The sum of the
lift vectors is now simply tilted from the vertical aircraft will
simply turn.

I agree that dihedral will have a stabilizing effect but it's not
because more of the wing low wing is parallel to the earth. It's
because the a/c slips towards the inside of the turn (controls being
neutral). The slip gives the low wing a higher angle of attack, thus
more lift and it will tend to right itself.

Conversely, a wing with dihedral will tend to bank if the aircraft is
yawed with the rudder. Left rudder, right skid, right wing has higher a
of a in relation to relative wind and the aircraft banks left.

Fold a hersey bar sized piece of paper to simulate such a wing. Then
slip and skid it and imagine the a of a on each panel.

Morgans wrote:
"Dallas"

Why would a horizontal wing create "more effective lift" than a banked

wing?

Let's think of a set of wings with a dihedral angle of 10 degrees up from
horizontal, on both wings.

Now, think of the shadow the wings would make, if the sun were straight
overhead, while the plane is banked at 10 degrees. The wing that is up
would make a smaller shadow than if the plane were flying level. The wing
that is down would be making the largest shadow that is possible.

The size of the shadow is the only size that is important, because the lift
that is straight up (towards the sun, in our example) is the only lift that
will be important to the plane, as that is what is counteracting the force
of gravity. The fact that the down wing's shadow is larger, will make have
more effective area than the up wing, and will tend to bring that wing back
up.

While you are in level flight, the same thing will constantly be at work,
automatically trying to keep the plane level.

Take a look at the NASA item in the following threads. It should be apparent
what happens. L1L2 therefore L1 wing rises until L1=L2. I can not believe
all of this discussion has ensued. This is a fundamental of aircraft design.

"Dallas" wrote in message
ink.net...

"William Snow"
Give some thought to the components of lift, on both wings, and to which
direction lift acts. ;-)
http://makeashorterlink.com/?B25A35DCC

I have. I see no forces presented in this illustration that would change
the components of lift on the wings or it's direction that would cause the
aircraft to roll back to horizontal.

Why would a horizontal wing create "more effective lift" than a banked
wing?

This thread began in another group and some interesting points were
discussed, but I honestly expected a few belly laughs here on the
absurdity
of this book's explanation.


Dallas

On Thu, 16 Mar 2006 at 05:56:35 in message
. net, Dallas
wrote:

Why would a horizontal wing create "more effective lift" than a banked wing?

I suppose the thinking is that the lower wing has more effective span
than the upper more raised wing.

However that is not wholly convincing, in any case if the only thing
that happened was a slight roll and nothing else then the lift would not
change. The lift vector would incline however and that would tend to
push the aircraft sideways. The lose of truly vertical lift at right
angles to the wing would also cause the aircraft to sink and the AoA to
increase initially before other things would happen. The initial yaw
displacement may be followed by a yaw rate which could be said to
increase the speed of the outer wing relative to the inner and increase
the roll! That is the possible beginning of a spiral dive!

Try this. Consider an aircraft rolled slightly and nothing else. If no
control inputs are made then the inclined lift and vertical weight will
tend to cause the aircraft to start a side slip as suggested above.

If the side slip continues then the lower wing will have a higher angle
of attack than the upper. Get a strip of card as a wing put dihedral on
it and look at it from various directions. A correcting roll couple is
then produced. Other things then come into play as well, like yaw
stability.

The power of dihedral can be demonstrated with rudder and elevator only
controls on a radio controlled model. I know; I used to fly one. With
plenty of dihedral apply say left rudder. The skid to the right so
produced results in a left roll and a subsequent side slip depending on
the yaw stability.. However that effect is likely to be small. Maintain
the rudder and pull back on the elevator and a nice turn results. Might
be uncomfortable for passengers but if you are controlling from the
ground who cares! :-) Quite steep turns are easily possible.

Incidentally sweep-back can have the same effect as dihedral and may
make a delta too stable in roll. Too stable? Add anhedral or turn down
the tips?

I have probably missed out several other effects but I just wanted to
indicate that all sorts of effects may come into play and it depends on
a number of factors which way it all goes. What about Dutch Roll for
example?


--
David CL Francis

"When an aircraft with dihedral rolls so that one wind is lower than the
other, the lower wing will have more effective lift than the raised wing
because it is not tilted from the horizontal as much. The imbalance in lift
tends to raise the lower wing and restore level flight."

I used to think that, but it was pointed out that the issue isn't
"raising the wing" as "rotating the aircraft" and that the higher wing
still has a moment that will tend to rotate the aircraft.

Jose
--
Money: what you need when you run out of brains.
for Email, make the obvious change in the address.

On Wed, 15 Mar 2006 23:13:03 GMT, Jose
wrote:

"When an aircraft with dihedral rolls so that one wind is lower than the
other, the lower wing will have more effective lift than the raised wing
because it is not tilted from the horizontal as much. The imbalance in lift
tends to raise the lower wing and restore level flight."

I used to think that, but it was pointed out that the issue isn't
"raising the wing" as "rotating the aircraft" and that the higher wing
still has a moment that will tend to rotate the aircraft.

Try both. The lower wing is trying to come back up as it has move
vertical component of lift than the other wing. The higher wing has
more lift in the direction of the lower wing so the plane will start
to turn that direction at the same time.

If you don't believe me, Get in the back seat of a Bonanza on a day
with lots of thermals. It doesn't have to be a V-tail, the F33 will
do the same thing. As the seat is behind the center of lift you get
some strange and exaggerated sensations and a high barf factor.

OTOH it makes the plane quite stable for the pilot even though it has
a wing loading on par, or slightly less than some Cherokees.

Cherokees which have a lot of dihedral also do pretty well on the
bumps and do not have quite the barf factor of the Bo's rear seat.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

Jose