Low-altitude flight

I seem to spend a lot of time scratching (due to incompetance), but I
rarely get below 500ft and almost never below 400ft. When scratching below
800ft coordination (yaw string) and airspeed (ASI) are alway high on my
agenda, and the workload goes up.

When mountain flying (much of flight can be within 500ft of surface), and
circling up the side of cliffs, not hitting the mountain and control
responsiveness also become important.

On the rare occasions when I mess up and find myself doing a ground level
turn, I do look down the wing in order to ensure that I dont stick the end
in a tree or the ground, I expect my rudder control and coordination is
maintained due to the many hours of scratching.

When doing some two seater xcountry flying recent, my complaint to all the
P2s was about their coordination.

I must try to work out about pivot heights. My wing always goes backwards.
What is the pivot height for a glider flying a 45 deg banked turn at 55
knots? A formula would be nice.

Rory

Rory,

Good points, the one I wanted to reinforce, .... you have to get both
speed and yaw seriously wrong to spin most gliders. For a start, at
least one wing needs to stall, so "lack of speed kills", ... not a small
error with the the rudder. A small speed error, plus rudder, yes.

Of course, good coordination all the time is better, but that can be
practiced at any height, why do it at 100', 5knts above stall, with a
good wind gradient? I am not sure I would thank an instructor that let me
try that.

Finally, if you are close enough to terain to need to check the wing tip,
I wonder if you are looking to see if anything is missing, or looking to
see the collision, or can you navigate the glider this way? I imagine it
is a "callibration check", takes a split second to complete, and will
not interfere with the aviation work load which always comes first.

David.

At 19:30 29 June 2009, Rory Oconor wrote:
I seem to spend a lot of time scratching (due to incompetance), but I
rarely get below 500ft and almost never below 400ft. When scratching
below
800ft coordination (yaw string) and airspeed (ASI) are alway high on my
agenda, and the workload goes up.

When mountain flying (much of flight can be within 500ft of surface),
and
circling up the side of cliffs, not hitting the mountain and control
responsiveness also become important.

On the rare occasions when I mess up and find myself doing a ground
level
turn, I do look down the wing in order to ensure that I dont stick the
end
in a tree or the ground, I expect my rudder control and coordination is
maintained due to the many hours of scratching.

When doing some two seater xcountry flying recent, my complaint to all
the
P2s was about their coordination.

I must try to work out about pivot heights. My wing always goes
backwards.
What is the pivot height for a glider flying a 45 deg banked turn at 55
knots? A formula would be nice.

Rory

On Jun 29, 1:30*pm, Rory Oconor wrote:
I seem to spend a lot of time scratching (due to incompetance), but I
rarely get below 500ft and almost never below 400ft. When scratching below
800ft coordination (yaw string) and airspeed (ASI) are alway high on my
agenda, and the workload goes up.

When mountain flying (much of flight can be within 500ft of surface), and
circling up the side of cliffs, not hitting the mountain and control
responsiveness also become important.

On the rare occasions when I mess up and find myself doing a ground level
turn, I do look down the wing in order to ensure that I dont stick the end
in a tree or the ground, I expect my rudder control and coordination is
maintained due to the many hours of scratching.

When doing some two seater xcountry flying recent, my complaint to all the
P2s was about their coordination.

I must try to work out about pivot heights. My wing always goes backwards..
*What is the pivot height for a glider flying a 45 deg banked turn at 55
knots? A formula would be nice.

Rory

My experience is that airspeed and coordination need to be monitored
when flying low. If those are within safe bounds, there's no need to
fear stall/spins. There are many optical illusions that can trick you
when flying below pivotal altitude - your instruments can be your best
friend.

Long ago a retired WWII Marine Corp Instructor Pilot taught me
something he had learned landing F4U Corsairs on straight deck
carriers. If you try, he said, you can train your brain to use a part
of your peripheral vision to monitor airspeed even while your gaze is
outside the cockpit.

At first, with your eyes out of the cockpit, all you can do is locate
the ASI but not read it. Still, that helps guide quick glances to the
instrument. Soon, those quick glances will not only pick up airspeed,
they will also see the trend. In time, it will seem as if you are
seeing the ASI full-time with your eyes outside and maybe you are.
Even with sunglasses, your pupils are very small in bright light
giving you what photographers call a large "depth of field". What
you are learning to do is focus your eyes on the "hyper-focal"
distance so both the distant horizon and the panel are just within the
"depth of field".

Try it while thermalling. Human brains are very plastic and can be
trained to do this. It's useful not only in landings but also when
the natural horizon isn't visible as in canyons or near a ridge.

Therefore, I respectfully add one more comment to your good analysis
on low-altitude maneuvering: There may be a strong desire to use
inside rudder to slow the perceived speed of the inside wing, and to
reduce the radius of turn (resulting in a skidding turn).

Due to your post, not only will I be more aware of this low-altitude
hazard, but my students will, too.

Raul Boerner

Hopefully Raul meant to say.. on the comment of a little inside rudder..

DON'T DO IT.. steep skidding turn with inside rudder = spin entry
BT

On Jun 30, 7:30*am, Rory Oconor wrote:
When mountain flying (much of flight can be within 500ft of surface), and
circling up the side of cliffs, not hitting the mountain and control
responsiveness also become important.

Absolutely. And pretty soon you're not letting the closeness of
terrain and changed view put you off normal coordinated flying.


I must try to work out about pivot heights. My wing always goes backwards..
*What is the pivot height for a glider flying a 45 deg banked turn at 55
knots? A formula would be nice.

I'm not sure it's important, but it's pretty trivial to work out.

The condition will be tan(bank_angle) = pivot_height / turn_radius

Rearranging: pivot_height = turn_radius * tan(bank_angle)

Now of course turn_radius is related to bank_angle by a=v^2/r
(centripedal force) and a=tan(bank_angle)*g (g = gravity = 9.8 m/s^2)
giving:

turn_radius = v^2 / (tan(bank_angle) * g)

Substituting:

pivot_height = (v^2 / (tan(bank_angle) * g)) * tan(bank_angle)
= v^2/g

That's pretty remarkable actually. The pivot height depends only on
speed, not on bank angle.

To convert to knots and feet:

pivot_height = speed^2 / 11.3

At 40 knots it is 142 ft.
At 55 knots it is 268 ft..
At 75 knots it is 498 ft.
At 100 knots is it 885 ft.
At 150 knots it is 1991 ft.