On Thursday, May 21, 2020 at 7:27:39 AM UTC-7, Hightime wrote:
On Sunday, May 17, 2020 at 4:48:22 PM UTC-6, Tom BravoMike wrote:
I know there is a thread going on about and around the latest tragic accident from 5-9-2020 and I preferred to start a new thread as I felt uncomfortable going into a dry technical discussion in the context. So the issue is:

I would like to know more about the aerodynamics of the towplane in all those accident situations (and I mean above certain altitude minimum): Statistically, do they crash being stalled or in a deep dive? What is the usual reaction to the tail being pulled up - does the towpilot try to overrule the pulling glider and he gets into a stall? He cannot dive until the rope breaks. Recovering from a stall he has to get out of a dive anyway. So it seems that pushing down into a dive instead of trying to pull up would result in a smaller loss of altitude (and cause the rope to break earlier), rather than in the sequence: struggle to overrule the pulling glider - stalling (loss of height) - pulling out of a dive (loss of height).

Just thinking loud, curious if any research has been done and if so, to what conclusions? Does anybody know?

Dont forget people Towing is not that dangerous and there are plenty of Happy towpilots out there and not just happy helicopter pilots



The loss in airspeed during a kiting incident must be significant. Chris Rollings notes that a 'marked deceleration' was observed by the towpilot in his report from kiting tests in 1978 and 1982, but focusses on the pitch down. I've had two kiting incidents as an aerotow pilot, both above 2000agl, when the glider didn't release as desired. In both cases it felt like a mild aerobatic maneuver, not alarming to me due to the safe altitude, and my attention was so focussed on the pitch-down that I didn't notice any airspeed decrease, although it probably occurred. In those experiences, the kiting did not seem at all dangerous to me, at altitude, and might even be a good thing for all aerotow pilots to experience, at altitude and intentionally, as part of training. It would be best if the towpilot was comfortable with mild aerobatics before trying it however. Chris Rollings did not mention any danger in his kiting tests. Such intentional kiting might build up a better body of knowledge, and would increase the awareness of tow and glider pilots to the danger.

Reducing the towrope breaking tension seems desirable. I don't know the reason for the FAA 80% rule, maybe that should be reduced. Most glider manufacturers only specify a maximum breaking tension.

Another interesting consideration is the time taken for rope tension to rise to breaking tension. The tension in the rope will probably rise gradually during kiting, probably over a few seconds. The rise in tension is probably non-linear, and at some angle, but its easier to do a calculation if the rope tension is assumed to rise linearly and to act directly backwards. If the rope tension builds up linearly to 1800lbs (a typical rope break strength) and then breaks, a Pawnee with full tanks (say weighing 1800lbs) will decelerate linearly, reaching a maximum deceleration of 32ft/sec2 ie about 20kts per second, at the instant the rope breaks. The longer it takes for the rope tension to build up to breaking, the greater the total Pawnee airspeed loss will be, since the rope forces on the Pawnee will be the same but last for a longer time. Calculation shows the Pawnee total airspeed loss varies as t^2, where t is the time taken for the rope tension to rise to the breaking point. If the rope breaks in 1 sec, the Pawnee in this example would slow by about 10kts by the time the rope breaks. If the rope takes 2 secs to break, the Pawnee airspeed loss would be about 40kts. In a non-kiting rope-break, eg due to a slack line suddenly snatching tight, the time t would be very short, maybe 0.1sec, and the airspeed loss probably unnoticable.

The full analysis should consider both pitchdown and airspeed loss, and perhaps negative g effect on the pilot, and then the height needed to recover. This might be too complicated, and be too dependent on particular towplane types. Maybe intentional practice at altitude might give practical data more quickly.