Takeoff distances

On Apr 13, 2:03 pm, "toad" wrote:
Ok, maybe a big block of salt :-)

In practice, my takeoff performance check is: If I'm not flying by X
distance down the runway, pull the release and land straight ahead.

If you are not flying how come you need to land?

"Kilo Charlie" wrote in message
...

"Gary Nuttall" wrote in message
...
And keep a note of how much fuel the tug has on board.
How heavy the glider pilot is. How clean the glider
wings are. What time of day it was. Outside Air Temperature,
pressure and moisture content. Local CAPE and Lifted
Index. Length of rope (and its elasticity). Power
setting of tug. What mood each of the pilots were
in. Stick position on ground run. Local thermal and
wave activity. All can have an effect on take-off
distance and climb rate.

There's so many variables that I'd be dubious of any
metrics developed beyond the fact that high altitude,
high temperature and heavy gliders do not make a good
combination.

Anybody who comes up with a set of explicit numbers
and sticks to them is likely to discover how often
theory doesn't work in practice!

Happy soaring
Gary Nuttall


Continues to amaze me at how much disdain glider pilots have for
quantitation. I suppose that may be what draws some of them into the
soaring in the first place. I also enjoy that aspect but think that
attitude has gotten many powered pilots into bad situations. To think
that we are immune to it because we don't have engines is naive.

Casey

I agree, Casy

It would be a big safety factor if we had takeoff performance charts
available. I've been involved in some extremely hazardous high density
altitude takeoffs where the tug was unable to provide adequate climb
performance.

Having enough information to decline the tow might save a few lives. It
seems like a major safety oversight that we don't have hard information on
this. If the STC for a tow hook doesn't require new performance charts, it
should. The glider tow chart should add glider gross weight and L/D to the
tugs TO performance chart and make 300FPM the minimum acceptable climb rate.

I have flown light airplanes in the high and hot western US all my life and
ALWAYS looked at the performance chart before takeoff. Many who didn't left
crumpled aluminum on mountainsides. I think charts would have to be
developed empirically from tests on a specific tug but once there were a few
data points, interpolation should be possible.

I also agree with the poster who selects a go-no-go point on the runway for
release if not airborne. Keep in mind that gliders have a high L/D in
ground effect and usually poor wheel brakes so leave a generous safety
margin for getting stopped after an aborted TO.

Bill Daniels

Also remember that density altitude will affect your height for a 180
back to runway.

i.e. two large men in a Grob, field elevation 3,700 ft msl, air temp
107F, rope break at 220 ft above field.

It was not enough altitude for at least one such senario. They plowed
into trees short of the runway.

Wayne Walker

On Apr 13, 11:00 am, "Bill Daniels" bildan@comcast-dot-net wrote:

snip

If the STC for a tow hook doesn't require new performance charts,
it should.

I don't believe that there would ever be another tow hook approved in
the US, if this was required by the FAA. The amount of testing and
analysis that would be required would break any club's budget.

I think charts would have to be
developed empirically from tests on a specific tug but once there were a few
data points, interpolation should be possible.

Interpolation (between data points ) or extrapolation (beyond the data
set ) ?

Interpolation to a different tug ? or to a different glider ?

You might be able to extrapolate to different gliders, but different
tugs could be vastly different, even if they were similar in major
specifications ( ie same horsepower, weight and wing loading). A
different prop size or having a constant speed prop would make a great
difference, for example.

If I had a specific tug in mind, I might come up with different tables
for single seat glass (with and without water), 2 seat glass and 2
seat trainer.

But I think it would take many-many flights to gather the data.

Todd Smith
3S

On Apr 13, 10:32 am, "Kilo Charlie" wrote:
Continues to amaze me at how much disdain glider pilots have for
quantitation. I suppose that may be what draws some of them into the
soaring in the first place.

Well, there's not much in glider flying that CAN be quantified with
any confidence. For example, you can quantify your still air L/D, but
on final glide, you have to estimate the lift/sink potential and that
is qualitative estimation (inspired guesswork).

Todd Smith
3S

All airplanes already have performance charts in the POH. These charts
aren't custom developed for each serial number but for the test airplane and
copys provided for all identical production airplanes. If the airplane is
subsequently modified with a different engine or prop, modified performance
charts are always required as part of the STC. Tugs are no different.

For a tug, all that's needed is additional inputs for glider weight and L/D
(Both highly quantifiable). With these data, the tow combinations takeoff
distance and rate of climb can be accurately predicted.

It's a bit of work to develop these performance charts but the payoff is
saved lives and tugs. I can't see a rational reason not to do it. It's a
responsible thing to do.

I've developed expanded charts for airplanes I've owned. Piper, for
example, seems to think nobody flies their singles above 6000 feet since no
data are provided above that altitude. I extended my ROC charts to 18,000
feet by noting the performance over a dozen flight or so. I also noted the
takeoff distances on each takeoff and added that to the charts.

For a tug, all you have to do is keep notes on each tows performance and
plot the date later. Soon you have the needed chart.

Bill Daniels


"toad" wrote in message
oups.com...
On Apr 13, 11:00 am, "Bill Daniels" bildan@comcast-dot-net wrote:

snip

If the STC for a tow hook doesn't require new performance charts,
it should.

I don't believe that there would ever be another tow hook approved in
the US, if this was required by the FAA. The amount of testing and
analysis that would be required would break any club's budget.

I think charts would have to be
developed empirically from tests on a specific tug but once there were a
few
data points, interpolation should be possible.

Interpolation (between data points ) or extrapolation (beyond the data
set ) ?

Interpolation to a different tug ? or to a different glider ?

You might be able to extrapolate to different gliders, but different
tugs could be vastly different, even if they were similar in major
specifications ( ie same horsepower, weight and wing loading). A
different prop size or having a constant speed prop would make a great
difference, for example.

If I had a specific tug in mind, I might come up with different tables
for single seat glass (with and without water), 2 seat glass and 2
seat trainer.

But I think it would take many-many flights to gather the data.

Todd Smith
3S

Oh, I have no problem with a club collecting the data and making the
charts, but your post seems to state that those actions should be
required before the FAA approves the tug installation. The FAA
requirement if what I would object to.

And no, it wouldn't require data for each serial number, but for each
type of tug.

Todd

FreeFlight107 wrote:
Also remember that density altitude will affect your height for a 180
back to runway.

i.e. two large men in a Grob, field elevation 3,700 ft msl, air temp
107F, rope break at 220 ft above field.

It was not enough altitude for at least one such senario. They plowed
into trees short of the runway...

....having executed the 180 degree turn immediately, with optimum
bank, g-load, and airspeed control -- as always?

Do you have enough information to make that claim, that it was
simply a matter of density altitude? If you do, then perhaps we, or
at least you, already have all the metrics that are needed. Perhaps
you can share them with us, for future use?


Jack

Not as much as it disturbs me that there might be people
out there who will stake their lives against a set
of numbers in a book. The margins of safety are so
much tighter in a glider/sailplane than a powered aircraft
and a minor change in any one part of the equation
could have dramatic results.

Let's be clear here.....Some numbers you can be confident
in becuase they've been measured in a certain, calibrated
and controlled environment and as such I 'trust' them
- e.g. VNE/VDF are tested in still air with a new airframe.
Even then, I'm not going to fly right up to the limits!

The problem I have with calculating a takeoff run is
that there are just so many variables involved that
you cannot consistently and safely rely on the results.
As Todd's post suggests, work out a go/no go point
and if you're not airborne, release.

As a glider pilot I act within both my and the glider's
limits. I trust my instincts that if something doesn't
feel right, I abandon the launch while it's still safe
to do so. Maybe it's a US vs UK thing but here in
the UK we take personal responsibility for our actions.
If you're not sure that you have sufficient distance
to take-off then why would you trust a set of numbers
that say otherwise ? I think the concept of calculating
takeoff runs is actually quite interesting but the
sheer number of variables involved make it an impracticable
exercise.

Gary


At 14:36 13 April 2007, Kilo Charlie wrote:

'Gary Nuttall' wrote in message
...
And keep a note of how much fuel the tug has on board.
How heavy the glider pilot is. How clean the glider
wings are. What time of day it was. Outside Air
Temperature,
pressure and moisture content. Local CAPE and Lifted
Index. Length of rope (and its elasticity). Power
setting of tug. What mood each of the pilots were
in. Stick position on ground run. Local thermal
and
wave activity. All can have an effect on take-off
distance and climb rate.

There's so many variables that I'd be dubious of any
metrics developed beyond the fact that high altitude,
high temperature and heavy gliders do not make a good
combination.

Anybody who comes up with a set of explicit numbers
and sticks to them is likely to discover how often
theory doesn't work in practice!

Happy soaring
Gary Nuttall


Continues to amaze me at how much disdain glider pilots
have for
quantitation. I suppose that may be what draws some
of them into the
soaring in the first place. I also enjoy that aspect
but think that
attitude has gotten many powered pilots into bad situations.
To think that
we are immune to it because we don't have engines is
naive.

Casey

On Apr 13, 2:09 pm, Gary Nuttall
wrote:
Not as much as it disturbs me that there might be people
out there who will stake their lives against a set
of numbers in a book. The margins of safety are so
much tighter in a glider/sailplane than a powered aircraft
and a minor change in any one part of the equation
could have dramatic results.

Let's be clear here.....Some numbers you can be confident
in becuase they've been measured in a certain, calibrated
and controlled environment and as such I 'trust' them
- e.g. VNE/VDF are tested in still air with a new airframe.
Even then, I'm not going to fly right up to the limits!

The problem I have with calculating a takeoff run is
that there are just so many variables involved that
you cannot consistently and safely rely on the results.
As Todd's post suggests, work out a go/no go point
and if you're not airborne, release.

As a glider pilot I act within both my and the glider's
limits. I trust my instincts that if something doesn't
feel right, I abandon the launch while it's still safe
to do so. Maybe it's a US vs UK thing but here in
the UK we take personal responsibility for our actions.
If you're not sure that you have sufficient distance
to take-off then why would you trust a set of numbers
that say otherwise ? I think the concept of calculating
takeoff runs is actually quite interesting but the
sheer number of variables involved make it an impracticable
exercise.

Gary

At 14:36 13 April 2007, Kilo Charlie wrote:





'Gary Nuttall' wrote in message
...
And keep a note of how much fuel the tug has on board.
How heavy the glider pilot is. How clean the glider
wings are. What time of day it was. Outside Air
Temperature,
pressure and moisture content. Local CAPE and Lifted
Index. Length of rope (and its elasticity). Power
setting of tug. What mood each of the pilots were
in. Stick position on ground run. Local thermal
and
wave activity. All can have an effect on take-off
distance and climb rate.

There's so many variables that I'd be dubious of any
metrics developed beyond the fact that high altitude,
high temperature and heavy gliders do not make a good
combination.

Anybody who comes up with a set of explicit numbers
and sticks to them is likely to discover how often
theory doesn't work in practice!

Happy soaring
Gary Nuttall

Continues to amaze me at how much disdain glider pilots
have for
quantitation. I suppose that may be what draws some
of them into the
soaring in the first place. I also enjoy that aspect
but think that
attitude has gotten many powered pilots into bad situations.
To think that
we are immune to it because we don't have engines is
naive.

Casey- Hide quoted text -

- Show quoted text -

The process of acquiring and analyzing the data will establish if it
is reliable or not. If it is not reliable there will be a large
scatter for the same set of conditions. The fundamental set of
parameters include, but are not limited to:
1. Tow plane POH take-off distance (this takes into account density
altitude and type of runway)
2. Glider total weight
3. Glider L/D
4. Wind
All of the other things mentioned are very minor compared to the above
factors, and will be represented as a scatter in the predicted take-
off distance. This can be accounted for with a safety margin, which
MUST be added to any POH calculation anyway. Exactly what are you
proposing as an alternative, launching without regard to available
data? This task is not on the level of a molecular chemical reaction
dynamics calculation. Personally, I would feel much more comfortable
having the data if I were the one being towed. If someone will collect
the data I would be happy to help them analyze it, something I do for
a living.

Tom