Polar with spoilers extended?

Tony Verhulst wrote:

My gosh, an honestly new idea (at least to me). In my book, that
qualifies as #7. It would not be my first choice, but if you allow
yourself get to the point where your best choices are behind you, the
270 degree turn from base to final could be the better of several bad
options.


My choice, given this situation. would be one or more figure 8's with
all turns toward the runway. In a 270, the first third of the turn is
heading away from the runway - not good down low if you hit unexpected
sink.

But would you prefer a figure 8 to a 270 in something with as low a roll
rate as an ASW22 or ASH25?


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

My choice, given this situation. would be one or more figure 8's with
all turns toward the runway. In a 270, the first third of the turn is
heading away from the runway - not good down low if you hit unexpected
sink.

But would you prefer a figure 8 to a 270 in something with as low a roll
rate as an ASW22 or ASH25?


Good point! I've not flown these gliders and maybe, in these, a 270 is
preferable - I don't know, you tell me :-). My glider is an LS6.

Tony V.

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.

I will be the first to agree that pilots die every year because they
are too slow on the approach. However I have not been able to find any
evidence that any of these were caused by the pilot intentinally
flying the approach slower than normal. In fact in nearly every case
I have examined it appears more likely that pilot was flying by sight
and feel and not paying any attention at all to the airspeed. Often
they are landing or flying downwind which gives the illusion of
airspeed, as does flying close to the ground. If fact the few pilots I
have been able to interview or read their description of the accident
described it as feeling like the controls went limp and had no effect.
The didn't make any mention of airspeed. It is not hard to find this
description in NTSB reports for power aircraft. I believe nearly all
Stall Spin Accidents are caused by the illusion of speed. The pilot
thinks he going fast so he doesn't look at the airspeed indicator and
he is not thinking about a stall or a spin.

Brian

On Oct 24, 8:44 pm, Brian wrote:
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.

I will be the first to agree that pilots die every year because they
are too slow on the approach. However I have not been able to find any
evidence that any of these were caused by the pilot intentinally
flying the approach slower than normal. In fact in nearly every case
I have examined it appears more likely that pilot was flying by sight
and feel and not paying any attention at all to the airspeed. Often
they are landing or flying downwind which gives the illusion of
airspeed, as does flying close to the ground. If fact the few pilots I
have been able to interview or read their description of the accident
described it as feeling like the controls went limp and had no effect.
The didn't make any mention of airspeed. It is not hard to find this
description in NTSB reports for power aircraft. I believe nearly all
Stall Spin Accidents are caused by the illusion of speed. The pilot
thinks he going fast so he doesn't look at the airspeed indicator and
he is not thinking about a stall or a spin.

Brian

I'm pretty sure that's not a true. I know of at least a couple of
accidents where the pilot was knowingly flying as slow as possible and/
or s-turning on final to get into a tight spot. Both were fatal.

I think the best way to think about this is in terms of energy
dissipation between wherever you start and some fixed touchdown point
(or stopping point if you are willing to plant the glider on the
ground and use the wheel brake too). Frankly if I am looking at a
finite distance to an unpleasent end of the field I will do whatever I
can to burn energy - before and after touchdown.

For starters I took a look at my ASW-27B factory polar. It turns out
that I will fly the same L/D spoilers closed at 37 knots (stall) as at
86 knots. This is basically the breakeven tradeoff between between
high induced and and high parasitic drag manuevers. If I add spoilers
I am adding a parasitic drag device which increases in effectiveness
with the square of velocity. That means that the breakeven speed for
the parasitic maneuver has to be lower than 86 knots. You don't need
to get to Vne to do better by going faster - even 70-75 knots is
probably better.

The ground effect argument has some merit I think, but keep in mind
that you can always stop the manuever before you get into ground
effect and bleeed off airspeed at 100' or so - you will still be
ahead. The altitude you consume slowing form the higher speed will be
roughly equal to the altitude you burned getting to the higher speed.

This is for still air. If I add headwind the breakeven airspeed for
speeding up versus slowing down goes up, but in most cases I'd be hard
pressed to believe that with spoilers out you will get a better energy
dissipation going slow than fast. At some point there will be a
crossover as headwind goes up. Just think of a headwind that is
greater than your stall speed to convince yourself.

Theory aside, I am convinced that as a practical matter making the
glider as draggy as possible and adding speed is almost always a more
practical and safe solution to slowing down and maneuvering at low
altitude. Stall/spin is a buzz-kill.

Lastl;y, I have done the parasitic drag maneuver down to touchdown in
a G-103 with spoilers closed and convinced myself that it is the
preferred method. I 'm guessing a Duo with the boards out is at least
as draggy as a G-103 clean.

My 2c.

9B

Tony Verhulst wrote:

Assuming the U.S., not likely.

You're assuming wrong.

Brian wrote:

I will be the first to agree that pilots die every year because they
are too slow on the approach. However I have not been able to find any
evidence that any of these were caused by the pilot intentinally
flying the approach slower than normal.

It's hard to tell after a fatal accident whether he was intentionally or
accidentally too slow... :-/ But if you're actually correct, well, then
you'll be the first.

Brian wrote:

However I have not been able to find any
evidence that any of these were caused by the pilot intentinally
flying the approach slower than normal.

After a fatal accident, it's usually hard to tell whether he was
intentionally or accidentally too slow... :-/

But if you're correct, well, then you'll be the first... :-/

Tony Verhulst wrote:

My choice, given this situation. would be one or more figure 8's with
all turns toward the runway. In a 270, the first third of the turn is
heading away from the runway - not good down low if you hit
unexpected sink.

But would you prefer a figure 8 to a 270 in something with as low a
roll rate as an ASW22 or ASH25?


Good point! I've not flown these gliders and maybe, in these, a 270 is
preferable - I don't know, you tell me :-). My glider is an LS6.

I was asking, not telling!

Apart from club two seaters, all my flying has been in 15m types,
currently a Libelle. I've had one ride in an ASH-25 and flown a Nimbus 3
once. In both cases the low roll rate and high inertia round all axes
struck me as being very different from my usual mounts. I raised the
issue in the hopes of getting a response from pilots who regularly fly
big wings.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

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

This is the kind of rigid thinking that kills people.

While I entirely concur that being too slow on final approach is dangerous, one
has to understand the variables. If you teach that the needle has to be ON the
yellow triangle, what happens to the pilot when he suddenly notices the glider
he is in doesn't have one. While under pressure in a difficult situation.

For every weight and configuration there is a stall speed.
For every wind condition there is a different factor to add to stall to get to a
safe approach speed.

Personally I would far rather fly with someone who knows exactly why he/she is
trying to maintain a particular speed, than someone who is a master at nailing
some arbitrary speed. Which happens to be perfect for one set of conditions.

As a Std Cirrus driver (pre serial 75) I can vouch for the wisdom of being at
the right speed. Too fast by even a small margin and you will float forever, and
PIO if you try to fly it on. Too slow and the handling gets very interesting if
there is turbulence. That right speed varies widely depending on the wind and
other circumstances.

Maybe we make it too complicated - our rule is you have to be able to decide an
appropriate approach speed, explain why you chose it, and then keep within 5km/h
of it.

John Smith wrote:
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.