Wing dihedral

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