gliding back to your departure airport

"Harold" wrote in message
...
Well obviously, but I'm not talking about in the pattern area like the
Impossible Turn is. I'm talking about 10 minutes after departure at 7k
feet
where the departure airport, if you can make it, is the best landing
option.
Then for all intents and purposes its a 180 degree turn.


In that case you might want to use ft/nm as a better measurement.

jerry

"Harold" wrote in message
...
[...] If
my best glide is 85 KTAS and it loses 700 fpm at that speed, shouldn't I
be
guaranteed I can make it back if I climb at 84 KTAS and 701 fpm ?

Do you descend at 700fpm gliding at 85 knots (which you should reference as
*indicated* airspeed, not true) with the wings level? Or did you verify
that descent rate in a turn?

Several factors prevent the simplistic analysis you've made from being
valid:

* The turn itself increases descent rate

* You need to turn a net of closer to 270 degrees: 225 to get you on an
intercept course back to the runway, then another 45 degrees the other
direction to align yourself for touchdown.

* Typically you are climbing into a headwind; that becomes a tailwind
halfway through your turn and through the remainder of the descent. The
tailwind will either push you past the runway, or you need to steepen your
descent by increasing the descent rate. Either way, that interferes with
the basic "if I climb at such-and-such a rate, then instantly turn 180
degrees and descend at a different rate, can I make it back to the runway"
simplification. Assuming "no wind" conditions doesn't make sense, because
that assumption is almost never correct and the consequence is significant.

Don't forget the reaction time it takes to start the turn, and the time
spent at something other than best glide airspeed. For the vast majority of
pilots, a large proportion of the post-engine-failure flight will be done
quite a bit away from optimally.

If you have a hard time believing this, it's easy enough to experiment.
Find yourself a nice quiet airport where you can depart straight out. Climb
straight out to 2000' AGL, then cut the power. Wait a second or two (since
you won't be surprised by the power cut), then go ahead and start your turn
back to the runway. Note the altitude loss at the point at which you are
back aligned with the runway. This will give you the absolute *minimum*
altitude you might successfully attempt such a turn-back.

For extra credit, time the post-power-cut flight, noting your airspeed as
well. This will allow you to figure out how far you actually flew during
the descent, which will give you an idea of whether you'd have actually had
enough runway left to land on by the time you got all set up. For extra
accuracy, take someone along to keep track of the actual airspeed, or use a
GPS to track the experiment (to get distance directly, rather than depending
on speed over time).

Finally, keep in mind that not all airplanes have the characteristic yours
does. In fact, I'd say it's unusual to find an airplane that climbs and
descends at exactly the same airspeed and vertical speed. Especially
powerful aircraft will climb more steeply than they descend, while slower,
lower-powered airplanes will climb less steeply than they descend.

Pete

Due to the vertical lift loss during your more than 180 degree turn back to
the airport, you will loose more than 700fpm and your glide range will
decrease if you try to maintain your best glide speed during your turn. I
believe Barry Schiff wrote that this maneuver is best done at a fairly high
rate of turn which involves an approximate 45-50 degree bank to keep the
radius of turn small. This will enable you to then level the wings and then
obtain your best glide speed in the shortest time while getting you back to
the runway in the shortest distance. Not a maneuver for the non-proficient
or the startled and hesitant.
--
Jim Burns III

Remove "nospam" to reply

"Harold" wrote in message
...
If a small single engine plane can out-climb its engine-out glide ratio
from
take off through the top of climb point, wouldn't it follow that it can
always theoretically make it back to the departure airport in the event of
engine failure ? Assuming straight out departure, no wind, and the
altitude
loss in the 180 turnback is offset by the runway portion you didn't use.
If
my best glide is 85 KTAS and it loses 700 fpm at that speed, shouldn't I
be
guaranteed I can make it back if I climb at 84 KTAS and 701 fpm ?

On Tue, 21 Oct 2003 20:02:00 GMT, "Harold" wrote
in Message-Id: :

If a small single engine plane can out-climb its engine-out glide ratio from
take off through the top of climb point, wouldn't it follow that it can
always theoretically make it back to the departure airport in the event of
engine failure ? Assuming straight out departure, no wind, and the altitude
loss in the 180 turnback is offset by the runway portion you didn't use. If
my best glide is 85 KTAS and it loses 700 fpm at that speed, shouldn't I be
guaranteed I can make it back if I climb at 84 KTAS and 701 fpm ?


The mathematics of turning back to the airport have been thoroughly
discussed in the newsgroup a while back. I suggest you do a
www.deja.com search for articles authored by John Lowry on the
subject. Here's an example:

http://groups.google.com/groups?selm...&output=gplain

From: John T. Lowry )
Subject: Min Turnaround Alt. on Single Engine Aircraft-Engine
Failure Question
Newsgroups: rec.aviation.piloting
Date: 1999/02/26


Dear Mike, Henrik, and All:
For the single engine-out return-to-airport maneuver, all the
various parameters (aircraft weight and flaps settings, runway length
and elevation, wind speed and direction) matter. But a crucial
performance number is, instead of just best glide ratio (which is
important once the turn is made) or minimum sink rate, the maximum
turn rate PER altitude lost, dTheta/dh. As close to (banked) stall as
possible. That rate is:

Max(dTheta/dh) =
-g*Rho*S*CLmax*sin(theta)*sqrt(cos^2(theta)+k^2)/(2*W*k)

where g is 32.2 ft/sec^2, Rho is density, S wing area, W weight, and

k = CD0/CLmax + CLmax/(Pi*e*A)

where CD0 is the parasite drag coefficient, e the airplane efficiency
factor, and A the wing aspect ratio. The optimum bank angle is just a
little (except for flamed-out jets) OVER 45 degrees and is given by

cos(phi_bta) = sqrt(2)*sqrt(1-k^2)/2

You'll find a full discussion in Chapter 9, Glide Performance, of my
forthcoming Performance of Light Aircraft published by AIAA.

John.

John T. Lowry, PhD
Flight Physics; Box 20919; Billings MT 59104
Voice: 406-248-2606

--------------------------------------------------------
Here's the formula for best glide

http://groups.google.com/groups?selm...&output=gplain

From: "John T. Lowry"
Subject: Formula for Vbg
Date: 1999/02/07
Message-ID: #1/1
References:

X-MimeOLE: Produced By Microsoft MimeOLE V4.72.2106.4
Organization: Montana Communications Network
Newsgroups: rec.aviation.piloting


Dear Phil, and All:
There is a fixed relationship between speed for best glide Vbg and
speed for minimum descent rate Vmd -- Vbg = 1.3161*Vmd -- but (since
you probably don't have Vmd) that won't help you much.
Vbg depends on the drag characteristics of the airplane, depending
on 1) W/sigma (W gross weight), 2) reference wing area S, 3)wing
aspect ratio A, 4)parasite drag coefficient CD0, and 5) airplane
efficiency factor e, according to
Vbg = sqrt(2*W/sigma*S)*(Pi*e*A*CD0)^-1/4

If you're willing to cut the engines and feather the props, to find
Vbg experimentally, here's a rough outline of the procedure. Go to
some nice high altitude and pick a vertical interval of pressure
altitudes, say for purposes of illustration from 14000 ft down to
13000 ft. Time repeated glides down through that interval and record
the product KCAS*delta_t, where delta_t is the time needed for the
glide. When you've found, by trial and error, the speed V which
maximizes that product, that speed is Vbg.

John

----------------------------------------------------

Larry Dighera wrote:



The mathematics of turning back to the airport have been thoroughly
discussed in the newsgroup a while back. I suggest you do a
www.deja.com search for articles authored by John Lowry on the
subject.

Yeah he's a genius. That's why he keeps wrecking aircraft.

Most gliders can do this and the pilots train to that standard..

Departure problems below 200ft AGL, tow plane power problems.. rope breaks
or tow hook failures.. and the idea is to land straight ahead as quickly as
possible and get stopped.

Above 200ft AGL (which most glider/tow combinations can get to about 3000ft
after start of take off roll), if the rope breaks, tow plane says.. GET
OFF!!.. the glider pilot can pitch down for airspeed and begin a turn back
to the departure runway.. land opposite the direction of take off and have
enough energy to roll back to the starting point.

A nice tow pilot will allow the tow to "Drift down wind the cross wind" on
climb out, so if something does happen the glider can turn into the wind
when returning to the runway.. turning away from the wind can push the
glider to far away (tailwind on base) and make returning to the runway more
difficult.

This maneuver is part of the practical test standards, though most DE's will
wait until 300ft or higher and most CFIGs will review the procedure on BFRs.

Our "Training glider" has a L/d of 23-1. Schweizer 2-33.

BT

"Harold" wrote in message
...
If a small single engine plane can out-climb its engine-out glide ratio
from
take off through the top of climb point, wouldn't it follow that it can
always theoretically make it back to the departure airport in the event of
engine failure ? Assuming straight out departure, no wind, and the
altitude
loss in the 180 turnback is offset by the runway portion you didn't use.
If
my best glide is 85 KTAS and it loses 700 fpm at that speed, shouldn't I
be
guaranteed I can make it back if I climb at 84 KTAS and 701 fpm ?

The glider has a couple of things going for it. Power planes are seldom over
12:1 glide ratio and it is achieved at a higher speed than the glider.
This means that the power plane covers about half the THEORETICAL glide
distance of the glider. Said loudly.

The speed ratio between the wind and the aircraft is a factor. A given wind
speed will be a higher percentage of the glider's best glide speed and will
result in a greater advantage to it's L/D downwind compared to a power plane
with the same wind. The wind also works to advantage for the glider's on tow
part by decreasing the distance that it covers on climb, compared to a power
plane at typical climb speeds.

Finally, for some ancient and illogical reason, power plane standard procedure
is to stay on center line of the runway for climbout. It's considered bad form
to put yourself in a safer position for a turn back to the field. The
exception is an IFR departure. They typically maintain runway heading. Lots
of luck making a turn back under IFR.

You subtract reaction time, reconfiguration time, screw around trying to get
the thing to run, and it is very, very unlikely that a power plane will get
back to the runway at any time during their climb out. Unless it is a long
runway and you started from the end.




In article afjlb.63635$La.24804@fed1read02, "BTIZ"
wrote:
Most gliders can do this and the pilots train to that standard..

Departure problems below 200ft AGL, tow plane power problems.. rope breaks
or tow hook failures.. and the idea is to land straight ahead as quickly as
possible and get stopped.

Above 200ft AGL (which most glider/tow combinations can get to about 3000ft
after start of take off roll), if the rope breaks, tow plane says.. GET
OFF!!.. the glider pilot can pitch down for airspeed and begin a turn back
to the departure runway.. land opposite the direction of take off and have
enough energy to roll back to the starting point.

A nice tow pilot will allow the tow to "Drift down wind the cross wind" on
climb out, so if something does happen the glider can turn into the wind
when returning to the runway.. turning away from the wind can push the
glider to far away (tailwind on base) and make returning to the runway more
difficult.

This maneuver is part of the practical test standards, though most DE's will
wait until 300ft or higher and most CFIGs will review the procedure on BFRs.

Our "Training glider" has a L/d of 23-1. Schweizer 2-33.

BT

"Harold" wrote in message
. ..
If a small single engine plane can out-climb its engine-out glide ratio
from
take off through the top of climb point, wouldn't it follow that it can
always theoretically make it back to the departure airport in the event of
engine failure ? Assuming straight out departure, no wind, and the
altitude
loss in the 180 turnback is offset by the runway portion you didn't use.
If
my best glide is 85 KTAS and it loses 700 fpm at that speed, shouldn't I
be
guaranteed I can make it back if I climb at 84 KTAS and 701 fpm ?

dennis wrote:
snip
You subtract reaction time, reconfiguration time, screw around trying to get
the thing to run, and it is very, very unlikely that a power plane will get
back to the runway at any time during their climb out. Unless it is a long
runway and you started from the end.

After getting my private, I was wondering about this, particularly
because the airport I usually fly out of (PDK) is surrounded by
development that leaves no place, at any time of day, to even dream of
setting down safely in the event of an engine failure.

Not getting answers that satisfied me, I went out and did some tests
myself, at altitude. Used GPS, a partner to log altitudes and
waypoints, etc. Came to the conclusion that if everything went
*perfect*, and you *knew* it was going to happen, it would take 500' agl
to make it back, in a Warrior.

But what I did was pick the brains of my friends who were glider pilots,
worked on Vms turns, high bank angle turns, popping 10 degrees of flap
for the turn and popping it out for the glide, etc., etc. After
practicing all that, and knowing what was coming, 500' was the best I
could do. Which to me means 800'-1000' in real life, if you practiced
it a lot.

It was an eye-opener for me to see how little margin for error I have
operating out of PDK. It has changed my standard departure. I climb at
Vx to pattern altitude, just to gain the most altitude while I'm still
within reach of the airport boundaries. The one good thing about PDK is
it has 4 runways aligned 3 different ways, and lots of taxiways and ramp
space between them, so just getting back to the airport itself you have
a better chance of putting it down safely, if not on a runway.

Plus the crash trucks don't have as far to go. :-)

--
David Hill
david at hillREMOVETHISfamily.org
Sautee-Nacoochee, GA, USA

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David Hill writes:

After getting my private, I was wondering about this, particularly
because the airport I usually fly out of (PDK) is surrounded by
development that leaves no place, at any time of day, to even dream of
setting down safely in the event of an engine failure.

Confirm this with someone who knows better, but from what I've heard,
you need only about 20 ft of deceleration to have a chance of
surviving a landing in a Cherokee/172/Musketeer-class aircraft. That
suggests that setting down in a developed area (an unoccupied part,
preferably) might be survivable.

To take a real-world example, an instructor taking a sightseeing
flight out of Buttonville (near Toronto) had an engine failure over
solid development, so she set the plane down deliberately in a grove
of small trees on the front lawn of the IBM plant. The trees smashed
up the plane nicely, but in doing so, they dissipated enough energy
that she and her passengers walked away. Here's the story (with
photo):

http://www.thestar.com/NASApp/cs/Con...l=968793972154

I read afterwards that she went back to work later that day.


All the best,


David

David Hill wrote:
dennis wrote:
snip


After getting my private, I was wondering about this, particularly
because the airport I usually fly out of (PDK) is surrounded by
development that leaves no place, at any time of day, to even dream of
setting down safely in the event of an engine failure.

I would think, in a "172" or similar class airplane, if you have your
seatbelts and shoulder harnesses on, flaps down, minimum controllable
airspeed, and *maintain control*, you should be able to land on (or
into) almost anything and survive with minor injury. I know of two
landings on top of houses that were both "walk aways".

Another engine fail on takeoff landing here was at night into a
park full of mature oak trees. Front seat guys were seriously banged
up, but they also were not wearing seat belts. Three rear seat pax,
also unbelted, received minor injuries.

Tom Pappano, PP-ASEL-IA