Constant speed or constant attitude?

At 15:54 19 August 2003, Finbar wrote:

.....'There's a timing difference between the main
wing and the tail.
Entering lift there is a transition region where the
lift is growing
stronger as the glider moves forward. The main wing
will be about 15
feet ahead of the tail. As a result the main wing
will be at a higher
angle of attack than the tail during the transition
into the lift,
creating a pitch-up tendency until the aircraft has
gone through the
transition region. This would tend to offset the pitch-down
tendency
that's been discussed. I've personally flown aircraft
that pitched
down, others that pitched up. Flex-wing hang gliders
tend to pitch up
very strongly on entering lift. However, I used to
fly a rigid-wing
hang glider (flying wing) that pitched down. I never
could explain
the difference.


I can confirm that, at least in the Discus and Duo
Discus flying with mid to aft C of G , if you simply
cruise with a rigidly fixed elevator position - set
for a reasonable median cruise speed of your choice
- then the glider slowly pitches up and slows under
positive acceleration as you enter regions of lift
and pitches down and speeds up under reduced acceleration
as you enter regions of sink. I have often flown this
way in the last few years since reading about the technique
as an aside in Reichmann (seventh edition in English,
pages 64 and 133). He discusses it in connection with
methods of trying to optimize g loading in transitional
phases of flight between lift and sink and refers to
it as 'the near optimal solution of simply flying with
the controls locked'. I have often wondered why it
works when the Yates effect would at first sight tend
to have the opposite effect so thanks to Finbar for
the obsevation above.

Flying fixed elevator results in very nice gentle speed
variation without the divergence you get flying hands
off but it takes a surprising amount of concentration
to keep the elevator fixed. (Perhaps a little 'dead
man's handle' on the stick that temporarily fixed the
elevator control alone would help.) It works best
when the lift and sink are continually changing because
if there is a long period of steady lift or sink without
vertical acceleration from the airmass then the airspeed
tends to settle back at the cruise speed for the elevator
setting chosen. In those circumstances you need to
depart from the fixed elevator.

John Galloway