About 20 years ago I was doing an aerotow from the
front seat of a Janus at Lasham with Derek Piggot in
the back during a cross country course. We were just
off the deck within the airfield waiting for the tug
to start to climb and I heard Derek say '60 knots'.
Later I asked him why he said that out loud and he
told me that he always made a note of when the speed
reached 60 knots because he reckoned that at that speed
he could pull up and turn back without loss of height.

John Galloway


At 14:00 30 October 2003, Martin Gregorie wrote:
On Thu, 30 Oct 2003 10:11:18 +0000, Robert Ehrlich
wrote:

Bob Johnson wrote:

As long as we're sitting around the campfire and also
to show you I can
go both ways, I 'member a time only a couple of years
ago that I
experienced the dreaded aerotow line break at 200
ft and 60 kt over the
outbound fence.

I looked out front and it wasn't too exiting, so I
gingerly turned
ninety to the left and the scenery looked some better,
but not the
greatest, so I REALlY gingerly gave it another ninety
to the left and
was really impressed this time as I found I was perfectly
lined up with
the takeoff runway. And I recall I hadn't lost too
much of my original
60 kt. Will wonders never cease!!

I thought 'OK now God, you've made it possible for
me to do this little
magic trick perfectly the first time I tried it in
front of all my
friends and hangar bums, give me your hand again and
let's try just one
more.'

So I pulled spoilers, checked gear down and rolled
up to my exact
takeoff spot.

And as I popped the canopy, my good friend who shall
remain nameless,
said 'S--t!, you've gone and lost us another Tost
ring, somebody go back
to the hangar and see if they can find another tow
rope'. It was his
turn to tow, so I pushed back. Nobody else said a
word.

Safety lecture from a dummy follows:

I don't remember to this day why I ninetied to the
left. During the
previous year's biannual when Juan Batch pulled the
plug on me over his
outbound fence, turning right was the correct choice
because in that
direction lay the wind, which blows one back over
the airport. This
improves the scenery like you wouldn't believe.

When I tried the trick for real solo, the wind lay
to my left. I'd like
to think it was instinct. But I believe it was a coin
toss.

Anyway, thank God. And Juan.

It Depends

BJ


This raises the interesting question of the height
loss during a 180 degrees
turn in a glider or an airplane with a dead engine.
I recently had a dicsussion
about that with a friend who is a power pilot and on
this occasion made again a
small computation I had already made on this matter.
As I never have seen
these results elsewhere, I think it may useful to show
that here. Assume
you fly your turn wit an angle of attack which correspond
to the speed V
when flying straight and wings level, and that the
vertical sink speed
in the same conditions wuold be Vz, then during this
180 degrees turn
flown with a bank angle phi, the height loss is pi*V*Vz/(g*sin(ph
i)*cos(phi)),
and the turn is flown at speed V/sqrt(cos(phi)). The
optimum (minimal
height loss) is when sin(phi)*cos(phi) is maximum,
i.e. phi = 45 degrees,
and the product V*Vz is minimum. A glance on a typical
glider polar will
show that this last thing is obtained with V just below
min sink speed, but
as it is not easy to find how many below, let's assume
the turn is done
at min sink speed, this is not very far from the optimum.
For a typical
glider with min sink of .6 m/s at 80 km/h (22.2 m/s)
the height loss is
8.5 m, for a typical airplane with min sink of 3 m/s
at 120 km/h (33.3 m/s)
the height loss is 64 m. This explains why the 180
degrees turn back to
the runway over the outbound fence succeeds in a glider
but not in a power
plane.

In the case mentioned above, the speed (60kt) was far
over the optimum,
however the result is as expected not catastrophic.
Assuming a bank angle
of 45 degrees, the equivalent speed in straight flight
would be multiplied
by 1.18, this gives 26 m/s or 93 km/h. Assuming the
sink speed is 1 m/s in these
conditions, we get a height loss of 16.6m. This is
for a poor glider (L/D =
26 at 93 km/h).

Thanks for that. A most informative calculation and
certainly matches
my most recent relevant experience.

The last time I was having a supervised aero-tow refresher
in our
Puchacz I was doing a running commentary for the instructors
benefit
and as soon as I said '400 ft - no problem now from
a rope break' BANG
as he pulled the release. We had a touch over 60 kts
and as soon as I
saw the rope go I pulled a 45 degree banked 180, keeping
the 60 kts
just as Bob described, and was amazed at how easily
we got in over the
fence. In fact, once I'd rolled out it looked like
a normal approach,
so I opened the brakes and did a typical Puchacz approach
and landing.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :