Polar with spoilers extended?

Tim Taylor wrote:
High on Final, Summary

Thanks to all that have given input so far. My original intent was to
do some modeling before starting the discussion, but this is RAS and
it has a life of it's own.

So here is the issue. You are high on final and full spoilers are to
enough; what do you do?

List of options so far:

1. Slip
2. "S" turns
3. Dive until intercepting normal angle for spoilers
4. Dive until near the ground, then decelerate
5. Slow down until intercepting normal angle for spoilers
6. 360 degree turn

Unfortunately I still don't have good data for what happens to the
polar as speed increases with the spoilers open. Condor was a good
suggestion, and I am working to see if I can get meaningful data from
it.

John Cochrane brought the discussion back to the real point which is
what would you use in the real world? It is interesting but not that
useful to discuss how you do this at your home airport with 2500 to
9000 feet of runway and know precisely the field elevation. When your
aircraft and your own safety are on the line in a real off-field, what
are you going to do?

What I do must be taken with a grain of salt, because I'm not an
instructor, pretty much learned how to deal with field landings by trial
and error, have 20 or 30 of them under my belt (5 to 10 in "small"
fields), and have yet to do any more damage than scuff up the underside
of the nose.

First, I *never* fly a normal pattern. I fly directly overhead high
enough to make one or more big lazy circles around the field at approach
speed, so I can look carefully for fences, wires, rocks, figure out
which way the field is sloped, get an idea of the actual wind direction,
potential for sink, get a picture of just how high I am above the field,
and pick the spot where I plan to touch down. When it becomes clear
that I won't be able to make another 360 (and I've found that pretty
easy to determine), I shift the circle as needed to approximate an
abbreviated downwind, base, and final, and will use spoilers, landing
flaps, slips, adjustments to the circle, etc., to get myself into the
field as best I can. I never let myself get out of reach or visibility
to the touch down point, and keep plenty of energy in case it becomes
clear that I need to make a last second shift to a different touch down
point (and that has happened more than once). Using this approach, I've
never found myself too high or low to land when I commit to final
approach...

Marc

Tim Taylor wrote:

So here is the issue. You are high on final and full spoilers aren't
enough; what do you do?

List of options so far:

1. Slip
2. "S" turns
3. Dive until intercepting normal angle for spoilers
4. Dive until near the ground, then decelerate
5. Slow down until intercepting normal angle for spoilers
6. 360 degree turn

Unfortunately I still don't have good data for what happens to the
polar as speed increases with the spoilers open. Condor was a good
suggestion, and I am working to see if I can get meaningful data from
it.

John Cochrane brought the discussion back to the real point which is
what would you use in the real world? It is interesting but not that
useful to discuss how you do this at your home airport with 2500 to
9000 feet of runway and know precisely the field elevation. When your
aircraft and your own safety are on the line in a real off-field, what
are you going to do?
Is there really a need
to be on the ground 10 seconds faster than using some of the other
techniques? Maybe only if a severe thunderstorm is approaching and
you must be on the ground now.

Fast approaching weather is a good reason for an "expedited landing",
and besides thunderstorms, there gust fronts, snow, rain, and blowing
dust. And also other reasons: there are a dozen gliders approaching at
high speed to finish a contest task; you want to land before sunset and
are still high; you want to land before the tow plane so you don't
interfere with the next tow (or have the next tow interfere with your
landing); airplanes are holding their takeoff until you land; to fit in
between the four airplanes circulating in the pattern doing incessant
touch and go's; getting out of the way before the skydivers exit the
jump plane. I've done it for all those reasons.

Regardless of the need to get down quickly, Option 3, as I use it, is
something I do on final after a normal pattern entry. It's not a "get
down quicker" technique.

snip

It is true that sink rate goes up with speed, but the
actual decent angle does not go up nearly as much. For my Ventus B at
45 knots descent rate is 122 ft/min while at 135 knots it is 894 ft/
min, but actual loss per nautical mile is 163 ft/ktm verses 397 ft/
ktm.

This is not a good comparison, because these numbers are for a "clean"
glider, where the major drag at 45 knots is *induced* drag (which
reduces as speed increases), at 135 knots the major drag is *parasitic*,
and you've gone to negative flaps to reduce drag!

With the gear and spoilers out, landing flap selected, the drag will
increase more rapidly with speed than for your example, as the drag is
significantly parasitic to begin with.

The other thing we don't mention is the average pilot going to
handle the decision making process better at higher speeds and less
time? At stable speeds it takes about 11 seconds to lose 300 feet at
135 knots with the spoilers out verses 22 seconds at 45 knots.

These numbers way off: the Ventus (spoilers out) has a 800 fpm sink rate
at 45 knots (seems too high), and it is only 1600 fpm at 135 knots
(seems too low)? Three times the speed and only double the descent rate?
Even clean, the descent rate increased a factor of 7.


Ok, lets try a hypothetical (well maybe not, been there done that ;-)
off-field landing.

snip


Slip?

Yes, when I had the Std Cirrus (I practiced slips a lot, because it has
poor spoilers); no, with the ASW 20 C (slips work fine, however); maybe,
in my ASH 26 E (I don't practice them much).

"S" turns?

Never, at that altitude.

Dive until intercepting normal angle for spoilers?

Yes, in Ka-6E (got to love those divebrakes!); Yes, in the ASW 20 C (got
to love those 40 deg landing flaps!); yes, in the ASH 26 E.

Dive until near the ground, and then decelerate?

No, Std Cirrus; no, Ka-6e (it will be on glide path well before it gets
near the ground); probably won't need to with the 20 C; might be what
happens with the ASH 26 E, with it's 8.3 psf wing loading.

Slow down until intercepting normal angle for spoilers?

Never.

360 degree turn?

Never.

snip
Dive until intercepting normal angle for spoilers?

Maybe, but can you dive, lose the altitude, decelerate and get it on
the ground for a tail wheel first, full stall landing?

As others have pointed out - bad idea. I might be able to manage it the
Ka-6e with it's huge spoilers and light wing loading (5 psf); the
Cirrus, 20, and 26 would be put on the ground as soon as possible with
full air and wheel brakes and some forward stick to give maximum
traction. I think the 20 would stop the soonest of these three.

snip

Summary
Each situation is different, but I think we should focus on teaching
techniques that are robust and give that average pilot the best
opportunity to have a good outcome in an off-field landing. I know of
very few off-field landings that start at 1000 feet at the end of down
wind.

The off-field landing "technique" of turning final at 800' instead of
400' is "robust", in my opinion, and should be one of the things we
teach. A lot (majority?) of bad outcomes during an off-airport landing
have "too low" when starting the landing pattern as a major factor.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly
* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

Tim Taylor wrote:


The situation: You are 70 miles from home over unfamiliar territory
(read not sure of exact elevation of the terrain below, your altimeter
is useless now). You have gone for a Cu over a dry lake bed and it
doesn't work. You have selected a landing site in the lake bed that
is about 350 feet long and 100 feet wide that looks safe to land.
There are tree stumps and other object in other parts of the lakebed.
There are no obstructions on the ends of the site so you can do a
normal approach. The winds are 15 to 20mph out of the south so you
are landing from the north to south. As you drop lower you make a
rectangular pattern over the site checking for any missed obstacles.
The downwind is fast with the tailwind, as you turn base you estimate
you are 400 feet. Your adrenaline is pumping as you prepare for a
fairly technical landing. You want to keep it close so that you don't
end up short back into the wind and you turn base a little too soon.
You are on short final about 350 feet, but about 100 feet over full
spoilers decent. What do you do?


Tim,

We've already screwed up a bunch of things to get ourselves here, but
then we may do that from time to time, so:

S-turns.

In this situation I want more time: to achieve the necessary descent
while flying the ship in the way I most frequently fly it--this is not
the time for something completely different--even though I can
_probably_ do a "360" from that height with a "clean" wing. _Know_ what
your configuration is. Gear? Spoilers? If there is any cross-wind, turn
into the wind initially, using anything from a 45 to 120 degree turn
depending on conditions, but I'll be more comfortable with 60 to 80
degrees; keep the touchdown area in sight; control speed carefully; do
not hurry the process; continually assess drift, obstructions, and
condition of roll-out space as your vantage point changes while crossing
the extended runway centerline.

Do not continue maneuvering below a safe altitude. The last 100 feet
should look as normal as any you've ever done. If not you've been in too
much of a hurry to get to the runway. Just relax and fly around until
you get to where everything looks about right again--really, thats what
we always do, isn't it? That's why we must be able fly the pattern very
comfortably without reference to the altimeter.

When airliners need more spacing on final approach, something similar to
this method is surprisingly effective without disturbing the customers
too much.

One more note: if every approach we fly at our home field looks just the
same as every other one, we may not be learning enough. Mix it up a
little. Where I fly that's rarely a problem, though, and I think that's
good for us in the long run.


Jack

On Oct 21, 12:13 pm, Tim Taylor wrote:
[...] It is purely an academic exercise from a safety
discussion we had about what are the best steps to follow if you are
high on final. I am trying to look at the difference between several
suggested techniques if full spoilers are not enough.


Since I was a participant in the original discussion, I
feel obliged to throw in my $0.02 even though the thread has
been hijacked. Truth in advertising: while I am a CFI-G,
the vast majority of my dual given has been in powered
airplanes.

First, Tim's method works for him and probably for many
other pilots. This does not mean that it works for all
pilots, or even most pilots. By analogy, some maneuvers might
be a piece-of-cake for a proficient aerobatic
pilot (that would not be me) but deadly for others. The
low airspeed drag-it-in kind of approach that some
have advocated for power planes falls into this category.
One of the skills an instructor must bring to the table
is the ability to figure out what approach is best for the student
and teach that.

The aerodynamics of Tim's maneuver - slow down and
get on the "back side" of the polar - mean that the plane
has lost both potential and kinetic energy. The two methods
lose comparable amounts of potential energy, so the loss of
kinetic energy is significant. Loss of kinetic energy
also means loss of maneuverability in all axes, due to
reduced airflow over the control surfaces. So, a glider
that has slowed and hits big sink will take longer to
recover than one with a higher airspeed, due to
reduced elevator effectiveness.

Also, the slower glider is just a few knots above stall, so
a rather small wind shear (headwind-to-tailwind) will
lead to a stall and more loss of altitude. For this reason alone I
would be uncomfortable teaching it to new pilots.

Frightening! That you would slow down to decrease
forward motion. What happens with downdrafts or wind
shear after you have given up the option for altitude
that speed gives you.

Forward slip in glass gliders won't get you much descent;
S-turns might eat up a good bit, but the high-parasitic
drag approach is a much more valuable tool.

Get about 4000ft agl near the pattern, open full spoilers,
and push over to about 70-80kts. When you've burnt
off 1000ft, lift the nose to the horizon until speed
drops to best l/d and then close the spoilers. You
will see that this is not a ballistic maneuver and
that it is completely controllable. I'm not sure a
speed curve for full divebrakes is needed; you can
eyeball this and make it come out right. Either find
an instructor who can demonstrate for you, or else
do it several times at altitude and when comfortable
practice it at lower altitude and on final. In the
latter situation you might do just a few seconds to
see how entry and recovery look and behave. There
is more probability of undershooting than overshooting,
in my experience, but you'll be aware that these are
about to happen before they become a serious problem.

I do these on BFRs routinely into a 2400ft strip.
Remember, you can break this off at any time, so you
don't have to give up options.


At 18:18 21 October 2007, Tim Taylor wrote:

LOL, thanks. It is purely an academic exercise from
a safety
discussion we had about what are the best steps to
follow if you are
high on final. I am trying to look at the difference
between several
suggested techniques if full spoilers are not enough.

My list of preferences is:
1. Full spoilers
2. add forward slip
3. add 'S' turns

I have used the technique of slowing down to minimize
forward speed,
increase sink and decrease glide angle. Others have
suggested
increasing speed to increase drag. I am not a big
fan of this
technique because I feel it minimizes options for the
pilot and is
susceptible to pilot error that can end up in over
shooting the LZ.
Last years article in soaring I believe confirms my
feeling that this
is a technique that should not be held up as one of
the primary
techniques that should be used. I am working on developing
models to
asses each in terms of effectiveness, time required,
safety and
options left to the pilot.

Tim

On 22 Oct 2007 16:12:47 GMT, Nyal Williams
wrote:

Forward slip in glass gliders won't get you much descent;

I'm not sure about that - the glass gliders that I have tried forward
slips with usually got really huge descent rates.

A few examples:
ASK-21, G-103, ASW-24: Sideslip very effective
DG-300, DG-505: Sideslip extremely effective
AS22-2: Sideslip pretty effective






Bye
Andreas

On Oct 22, 10:12 am, Nyal Williams
wrote:
Frightening! That you would slow down to decrease
forward motion. What happens with downdrafts or wind
shear after you have given up the option for altitude
that speed gives you.

snip

Frightening? Really? It actually works very well with a bit of head
wind. Backcountry power pilots occasionally use this technique as
well.

It could be "Frightening" on a normal approach but remember the
context. A Downdraft or Windshear would be welcomed as the whole point
is to lose altitude. It does requires some expertise in slow flight
and stall awareness. But then glider pilots or at least soaring pilots
are supposed to be the experts at slow flight. Once you get down
close to a normal approach angle simply accelerate (which will bleed
off some more altitude) to your normal approach speed and fly the
remaining part of the approach normally. In fact it is necessary that
at about 200 feet AGL or higher that you do accelerate to a normal
approach speed so that you will have enough energy to flare with.

I would strongly recommend practicing it with an instructor and in the
specific airplane before having to use it. Usually with gliders there
are other, as good, or better options to this technique.



Brian

Brian wrote:

Frightening? Really? It actually works very well with a bit of head
wind. Backcountry power pilots occasionally use this technique as
well.

It's a completely different thing in a power plane. (Although I wouldn't
recommend it with power planes, either.)

A short look at a typical glider polar is all that is needed to
understand why your "technique" is a no-no. If you continue using it,
then it's only a question of time that we'll hear about you in the news.

On Oct 22, 3:33 pm, John Smith wrote:
Brian wrote:
Frightening? Really? It actually works very well with a bit of head
wind. Backcountry power pilots occasionally use this technique as
well.

It's a completely different thing in a power plane. (Although I wouldn't
recommend it with power planes, either.)

A short look at a typical glider polar is all that is needed to
understand why your "technique" is a no-no. If you continue using it,
then it's only a question of time that we'll hear about you in the news.

Looking at a polar is exactly why it works. It is called Speed to fly.
It really only works well when you have some headwind. It does work
somewhat in calm conditions but is really not very effective. It
probably doesn't work at all in a tailwind condition.

As noted gliders usually have a better ways of dealing with being
high. And since most people aren't excessivly high with a headwind it
does have limited use in gliders. There are really only two things
that can go wrong with using the technique and both should be easily
controllable. These are a Stall/Spin or continuing the slow approach
to too low of altitude to recover back to a normal approach speed.

Personally I seldom use it, The High Parasitic Drag approach and/or
Slips described above is usually more effective in a wider range of
conditions.

Brian

Brian wrote:

Looking at a polar is exactly why it works. It is called Speed to fly.
It really only works well when you have some headwind. It does work
somewhat in calm conditions but is really not very effective. It
probably doesn't work at all in a tailwind condition.

First I thought that you were pulling our legs, but it seems you're
actually serious.

Every year a couple of pilots die because they are too slow on approach.
Where I fly, a student will fail his checkride big time if he's only one
knot below the yellow triangle on final. *Especially* with a headwind.
If you don't understand this, I *strongly* recommend you talk to a
knowledgeble instructor.