----[Please read http://ercoupers.com/disclaimer.htm before following any advice in this forum.]----
Dr. R. Beeman wrote:
>>>>> If you land an
Ercoupe on the mains in a crosswind and wait "...until the coupe
itself swivels straight down the runway..." please tell me specifically what force will
cause the aircraft to "swivel straight down the runway"?
The answer is there is no magical force available to an
Ercoupe or a spam can for that matter. A spam can uses rudder to counteract the
crab just before touchdown, but a rudderless Ercoupe must use the runway itself
to counteract the crab, for if the Ercoupe pilot tries to take out the crab
before touching down, he'll simply fly away from the centerline.
The only counterforce to a crosswind
crab that a rudderless model of the Ercoupe possesses is the force exerted
through the nose wheel touching and counteracting the cocked main gear. This is
what the POH calls for and this is all there is. There is no
automatic swivel straight down the runway and if you wait for it before
planting your nose wheel you'll be cutting daisies.<<<<<
Doc,
Please review Chapter 17 of Stick and Rudder,
which in my copy starts on page 312.
Yes, there IS an automatic swivel,
straight down the runway.
The force that rotates the airplane to
line up with the direction of travel is exerted through friction with the
ground.
The friction vector comes at an angle
along the direction of motion. It can be represented by two vectors at
right angles, one along the longitudinal axis of the aircraft and one directly
sideways.
|\
| \
| \
| \
|
\
|
\
|
\
|
\
-------à
When touching down in the crab (whether
the nose gear touches at the same time or not), there is a side load on the
tires – they are not lined up with their motion.
The tires do start to rotate and almost
instantly stop providing resistance along the vector of the aircraft’s longitudinal
axis. But, the frictional push is at an angle and part of the force may
be vector diagrammed as being from the side of the aircraft.
It is that side vector which pushes
sideways on the main gear – and the main gear is behind the aircraft’s
center of gravity. So, there’s a sideways push on the rear of the
aircraft.
But, if the nose gear has already touched,
any sideways vector on that is almost instantly relieved because the nose gear
will turn and align itself with the direction of motion. So, there’s
no sideways push vector on the front of the aircraft. (This is why we
must not hold the yoke firmly when landing – a fingertips only grip will
allow the nose gear to turn to line up with its travel across the ground.)
Summary: There’s a sideways push
to the rear of the aircraft but no sideways push on the front of the
aircraft. So, the aircraft rotates.
When the aircraft is in line with the
direction of motion, all sideways vectors disappear.
Side notes: the mass of the aircraft
is concentrated near the center of gravity. The structures far from the
center are light weight. So, the aircraft automatically rotates
quickly. The sideways force vector is relieved quickly and is not very
violent. An egg in a saucer in your lap will probably stay there. (Please
use hard boiled eggs for your personal testing.) J
If this isn’t adequate, I’ll
try to hand draw, scan and send a proper vector diagram of the forces involved
later. (I’ve got to get my daughter up and get her to school.)
Ed Burkhead
http://edburkhead.com
ed -at-
edburkhead???.com (change
-at- to @ and remove "???")
============================================================================== To leave this forum go to: http://ercoupers.com/lists.htm
