Low-altitude flight

Dan,

Thank you for your insightful post on low-altitude maneuvering.

Just yesterday during an instructional flight, my student entered the
pattern too low (especially for someone with low experience). After
some prodding from Mr. Backseater, the student started looking outside
better. After recognizing that we were low, the student acknowledged
that we could not fly a normal traffic pattern. Instead, the pattern
would have to be abbreviated.

The student turned base to final at 100 feet AGL with a rather steep
bank, influenced by her changed depth perception and altered sense of
speed during this unplanned low-altitude maneuvering. Indeed, the
inside wing seemed as if it was racing ahead of the terrain.
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

Brings to mind this amusing quip of unknown author:


1. Try to stay in the middle of the air.
2. Do not go near the edges of it.
3. The edges of the air can be recognized by the appearance of ground,
buildings, sea, trees and interstellar space. It is much more
difficult
to fly there.

wrote:

Indeed, the
inside wing seemed as if it was racing ahead of the terrain.
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

I don't know what you teach your students. In a turn I'm looking at the
horizon, the yaw string and possibly the ASI, but certainly not at the
wingtip.

On 29 Jun., 09:23, John Smith wrote:
wrote:
Indeed, the
inside wing seemed as if it was racing ahead of the terrain.
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

I don't know what you teach your students. In a turn I'm looking at the
horizon, the yaw string and possibly the ASI, but certainly not at the
wingtip.

Dan has touched upon something very, very important here. Something I
am convinced can help explain many of those “unexplainable” stall-spin
accidents when down low.

However it is important to pay some attention to how our brain handles
information. There is the conscious part (logical thinking) and the
“reptilian brain” (instincts, automatic and very quick reactions) The
reptile brain is the one that will have your hand up to protect your
eyes when someone throws something at you, long before you had time to
“think” about it

But the reptile brain was never intended for flying, and it WILL make
wrong judgments!

The problem is that the reptile brain will act on input by means of
sending very direct commands to our arms and legs, totally bypassing
the logical thinking part. So even if you are thinking “I'm looking at
the horizon, the yaw string and possibly the ASI” your reptile brain
may very well be reacting in an instinctive way to the input from the
peripheral sight saying “wow that wingtip moves fast forward over the
ground” –Incidentally the reptile brain reacts extra strongly to input
from the edges of our field of sight. Probably those that noticed that
predator out there early lived longer and got to be our forefathers.
Unfortunately, when flying a glider it may have the opposite effect.


Be safe out there.

Lars Peder
DG-600 EE, Denmark

"Lars P. Hansen" wrote:
However it is important to pay some attention to how our brain handles
information. There is the conscious part (logical thinking) and the
"reptilian brain" [...]

Um, shouldn't that be "mammalian brain?" Or more likely you mean there is
the frontal lobe and the parietal lobe?

But the reptile brain was never intended for flying, and it WILL make
wrong judgments!

I thought birds were technically considered to be reptiles? So is the
information on this link wrong?:

http://www.ucmp.berkeley.edu/diapsids/avians.html

Dan has touched upon something very, very important here. Something I
am convinced can help explain many of those “unexplainable” stall-spin
accidents when down low.

While I don't disagree with this analysis- or the original post, I
wonder how many other 'unexplainable' stall-spin accidents were a
result of training where primary students were either flat out taught
to lead turns with the rudder, or even taught that it's OK to lead
with the rudder (a previous religious debate here...
http://tinyurl.com/nrgsh7 and http://tinyurl.com/kk8xdp ), a practice
which I personally despise for many reasons.

In the case of the recent event that prompted this useful discussion,
and in quite a rare situation, we may someday be able to actually ask
the PIC what happened, and about his previous training. I for one
would *really* like to know if he falls into the lead with rudder
crowd. Q: Will a ship spin without rudder input? Q: Will a ship turn
without rudder input?

-Paul

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.

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.