Wing dihedral

Originally Posted by Jose

Ignoring gravity for a moment, you can rotate the diagram any
which way, and the lift vectors will always be normal to the wing, and
will always have a net zero =torque= (and it's torque that would, by
this explanation, return the aircraft to level).

The point I was making is that this is not true. You can rotate the diagram any which way, but the lift vectors won't be the same. I think confusion is caused because you're looking at the diagram head-on, but the plane doesn't fly through the air that way. With dihedral, the AOA on each wing is only equal when they're level.

"Chris Wells" wrote in message
...
The point I was making is that this is not true. You can rotate the
diagram any which way, but the lift vectors won't be the same. I think
confusion is caused because you're looking at the diagram head-on, but
the plane doesn't fly through the air that way.

It doesn't? I normally make every effort to fly my airplane through the air
"head-on". Granted, it's possible to fly through the air in a wide range of
attitudes, but "head-on" is the most efficient and is what most pilots use
for normal, straight-and-level flight.

With dihedral, the AOA
on each wing is only equal when they're level.

Only because of the resulting slip. Which is what several people have been
trying to point out already, including Jose (in his latest post).

Ignoring gravity (as Jose suggested), if you take an airplane flying
straight and level and bank it, all that changes is the net lift vector, and
all that changing that will do is change the direction of flight. It will
do nothing to return the airplane's attitude back to straight and level.

It's only when you reintroduce gravity into the equation that rolling the
airplane will *also* result in a change in the resulting vertical component
of the lift, resulting in a descent, resulting in a slip (ta da!) that
changes the actual lift on each wing.

Which is what the people pointing out the flaw in the usual presentation of
dihedral diagrams have been saying all along (including Jose, now).

Pete

It doesn't? I normally make every effort to fly my airplane through the air
"head-on". Granted, it's possible to fly through the air in a wide range of
attitudes, but "head-on" is the most efficient and is what most pilots use
for normal, straight-and-level flight.

Your fuselage may be "head-on", but your wings aren't. The relative wind is coming from below. It's impossible to fly straight & level while the wing is head-on - you need lift to maintain level flight.

Only because of the resulting slip. Which is what several people have been
trying to point out already, including Jose (in his latest post).

Yes, and I am pointing out that this is not the main correcting force. Slip is a secondary factor, and takes a finite amount of time to develop.

Ignoring gravity (as Jose suggested), if you take an airplane flying
straight and level and bank it, all that changes is the net lift vector, and
all that changing that will do is change the direction of flight. It will
do nothing to return the airplane's attitude back to straight and level.

No, the AOA of the higher wing will be lower, due to the change in angle of the relative wind, as I have been saying all along. Besides, ignoring gravity pretty much changes everything in the equation.

You can draw the angles on a piece of paper, to help visualize it. If the angle of the wings is different (i.e. dihedral) then the AOA of the upper wing will get lower, and the AOA of the low wing will increase the closer it gets to horizontal. Past horizontal, the low wing will start to develop less lift as well, but the high wing will lose it faster.

No, the AOA of the higher wing will be lower, due to the change in
angle of the relative wind, as I have been saying all along.

Consider an airplane with 45 degree dihedral. When it is 45 degrees in
bank, one wing is horizontal and the other is vertical. To keep the
airplane from sinking, the horizontal wing will need to provide
significantly more lift, since it's the only wing supporting the plane.
But as you said above:

It's impossible to fly straight & level
while the wing is head-on...

you need lift. This means the horizontal wing will need a greater AOA.
How do you get a greater AOA on a wing that is horizontal due to bank?
Rudder, for one thing. You have to keep the nose up... sideways since
you are tilted.

Well, gee, that's a slip.

Jose
--
Nothing takes longer than a shortcut.
for Email, make the obvious change in the address.

How do you get a greater AOA on a wing that is horizontal due to bank?

It's already there, as I have pointed out repeatedly. Honestly, if no one is even reading my posts, than I'll leave this discussion for when folks are interested.

Here's another way to think about it.

Consider straight and level flight, including gravity, for an aircraft
with dihedral. Consider the "lift" vectors of each of the wings. (They
are not pointed up, they are actually pointed slightly inward due to the
dihedral).

Now, instead of rolling the airplane, swing the entire earth arround so
that the earth (and its gravity) is coming from the right somewhere.
The lift vectors on the wings do NOT change! NO rolling moment is
produced...

until...

....the airplane starts getting dragged to the right by the newly
misplaced earth. Of course, it's being dragged sideways against the
air. ONLY THEN will the AOA change for the wings, directly due to the
sideways slipping of the airplane.

Jose
--
Nothing takes longer than a shortcut.
for Email, make the obvious change in the address.