high tow vs low tow

On 2/24/2019 8:27 PM, Steve Koerner wrote:
As a US pilot, I've flown low tow a few times but mostly just high tow as
that is the convention here. I'm wondering what all of the trade-offs are
by the two methods? Might it be time to reconsider this?

As pointed out on another thread, low tow would seem to be safer against
the problem of glider kiting up to overpower the tugs up elevator. I'm
wondering if it's ever possible for a low tow glider to go too low and
overpower the tugs down elevator?

Having performed low tows only by way of demonstrating 'em during BFRs
(remember them?), the aerospace engineer in me has little doubt that 'all that
glider mass back there' *could* 'cause alarming things to happen to the tug'
if Joe Glider Pilot is either seriously asleep at the stick or intentionally
and unilaterally plays Joe Test Pilot.

That said, my visual take on low tow is that 'well before' a tug-upsetting low
tow position is likely to be reached, Joe Glider Pilot will almost certainly
find the visual picture so alarming that he'll take corrective action. Clearly
that's not the case with high tow...

So, OZ glider pilots, how do you folks manage to kill your tow pilots? Too
much beer?

Bob W.

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(Other than landing on tow)
High tow makes the most sense to me, even in Australia.
Fly the glider!
Jim

It only makes sense if you like towplane crashes. When a glider kites on high tow down low there is no time for the tow pilot to recover.
Had this been a low tow, the likely hood is very strong that the pilot would have survived. In Au, we only do low tow training, and I’ve never seen
any problems or even the slightest upset of the tow plane on low tow. Some years ago we used to go to high tow to release, but even that was scrapped.
It needs to be remembered that low tow means just below the slipstream. C of G hooks make high tow even more dangerous.

On Monday, February 25, 2019 at 3:07:36 AM UTC-5, Charlie Quebec wrote:
It only makes sense if you like towplane crashes. When a glider kites on high tow down low there is no time for the tow pilot to recover.
Had this been a low tow, the likely hood is very strong that the pilot would have survived. In Au, we only do low tow training, and I’ve never seen
any problems or even the slightest upset of the tow plane on low tow. Some years ago we used to go to high tow to release, but even that was scrapped.
It needs to be remembered that low tow means just below the slipstream. C of G hooks make high tow even more dangerous.

I realize that you come here chiefly to wind people up, but let's just set that aside for a moment.

If you have a shred of actual evidence for the relative safety of low tow, cite it. The guys who studied the problem and test flew upset scenarios at altitude don't agree with you.

Chris Rollings:

"The belief that low-tow significantly reduces the risk of sling-shot tug
upsets is mistaken. The trigger is if the glider pitches up to about 30
degrees above the line of the rope. Being in low-tow when that happens
simple makes the sunsequent event take about half a second longer - not
enough extra time to greatly increase the chance of releasing before the
critical point."

Critical point: it's the acceleration of the kiting glider that does the tow pilot in. The towplane is suddenly slowed, probably below 1g stall speed, accompanied by loss of elevator authority, then it's pitched over and put into a deep negative angle of attack. Take away: low tow doesn't and can't prevent this from happening. The glider pilot has the task of preventing this from happening. See figure 2 here https://members.gliding.co.uk/librar...owing-booklet/

Rope length is far more important to tow (in)stability than either tow position or hook location. Anyone can test. Try a 150' rope and a 225' rope. Difference is obvious and dramatic. In contrast, the difference between high and low tow is pretty subtle as far as control pressure goes and I honestly feel no difference in stability. We use 225' because it's the longest our tow pilots can safely manage on final approach (road crossing). I have on rare occasion towed on even longer ropes (275') and felt that was over doing things.

Nose vs CG hook is another myth. It would take a lot of work to assemble the data, but anecdotally we already know: the vast majority of tug upsets and upset/accidents involve nose hook equipped gliders. Most CG hook gliders (e.g. ASW-20s and such) tow very nicely.

T8

On Monday, 25 February 2019 15:28:09 UTC+2, Tango Eight wrote:
Chris Rollings:
Being in low-tow when that happens
simple makes the sunsequent event take about half a second longer - not
enough extra time to greatly increase the chance of releasing before the
critical point."

So low tow does offer more time to react to a tug upset.
From the info at hand it appears that a tug upset occurs over a duration of about 3 to 4 seconds. An additional 0.5 seconds on 4 seconds is a 12.5% increase.
Why throw a free 12.5% additional safety margin away?

On Thursday, February 28, 2019 at 12:45:49 AM UTC-5, Surge wrote:
On Monday, 25 February 2019 15:28:09 UTC+2, Tango Eight wrote:
Chris Rollings:
Being in low-tow when that happens
simple makes the sunsequent event take about half a second longer - not
enough extra time to greatly increase the chance of releasing before the
critical point."

So low tow does offer more time to react to a tug upset.
From the info at hand it appears that a tug upset occurs over a duration of about 3 to 4 seconds. An additional 0.5 seconds on 4 seconds is a 12.5% increase.
Why throw a free 12.5% additional safety margin away?

I think we have two distinct types of events to consider, that differ in their causes, effects and kinetics.

In the case of "kiting" accidents, what does the towplane in is the *pitch of the glider*, not the pitch of the tow rope. It's the acceleration of the glider that produces the dangerous deceleration of the tow plane. We think that beyond a critical (glider) pitch of about 30 degrees, the situation is not recoverable. I don't think low tow buys you much in this scenario.

In "distraction" events, the glider remains below the critical pitch, the situation remains recoverable much longer, the problem is caused when the glider drifts so far out of position that the angle of the rope becomes a problem for the tug. Or it may turn into a kiting event.

Here's the order of priorities for prevention of both types of problems:

1. PIC that maintains situational awareness and positive control, all the time.
2. Rope of reasonable length. We find that 200 - 225' works well. Longer gets to be a problem for recovery at our airport.
3. Tow position probably improves margin in distraction scenario.

I have reservations about using low tow at our club because a PTT below 200' means something is going to get bent with high probability (it's roughly a ten second window of time between able to land straight ahead and able to make a safe 180). On a 225' rope, low tow is about 30 feet lower than high tow, why throw away 15%? :-)

best,
Evan

Put simply, kiting accidents just don’t happen on low tow. The only time I’ve ever seen a tug upset was a glider that had to use high tow
fortunately, the glider released immediately and landed ahead. Again, to be simple, the tug is at all times visible to the glider pilot, it cannot disappear
under the nose, a feature unique to low tow. As for the small height gain, I have the opposite view, being lower gives a better chance of straight ahead landings.
I can’t recall a single accident caused by an upset in low tow. I stay as low as possible after lift off, and wait for the tug to climb above me and stay there.
As my glider has winch hook only, I would not risk the tug pilots life by doing high tows. All I’m seeing is spurious arguments in favour of high tow, that put the risk on the tow pilot not the glider pilot.
I’d rather crash myself than kill an thuggish, but that’s just me.