On Wednesday, November 6, 2013 10:20:06 AM UTC-6, John Carlyle wrote:
Thanks for the clear explanation, Steve. Let me summarize: The root cause of poor handling on tow is the tow plane down wash producing either differential lift if you’re not centered on the tow plane, or tip stalling if you are. Having the tow plane go faster gets you further above the stall speed-wise, and reduces your AoA because the down wash moves up. Since turbulence interaction is causing the problem, it implies that you could combat a slow tow plane pilot who refuses to speed up by flying below his down wash. But comments in a previous thread indicate that those who tried that maneuver still had handling problems. Any idea why? -John, Q3

Yep, pretty much. And since the wake moves around, you are never centered in it.

Faster gets you lower AOA. Deck angle also goes down a bit more due to reduced downwash angle from the towplane wake. Below the wake, the air will be also getting pushed down, similar to how above the wake, it will be getting pulled down. So if you do get below the wake when being towed slow, you are still going to feel precariously nose high. And sometimes, pilot "gain" is heightened to the point where the pilot can drive what seems to be an instability (PIO). Us humans are good at being able to accidentally couple with a natural frequency and drive it. I know I have done it.

I do think it could be a worhtwhile program for someone to study. I do think that a longer rope will get you further away from downwash influences, if you are flying high tow. If you are flying low tow, the longer rope will likely mean that you have to go well below the altitude of the towplane to get below the wake. And you will still end up close to the wake and downwash, so a longer rope for low tow would likely provide no benefit.

It is also possible that the if glider being pulled too slowly by the short wing towplane can get up above the wake, the poor handling could be resolved and leave you with only a very nose high deck angle. Of course, it is very much counter to your thinking to be nose high, buffetting, and struggling with roll control to then pull further back on the stick while still hooked to the towplane.

This has mainly shown up (please correct me if I am wrong here) on newer Std Class planes with less wing incidence and less airfoil section camber in their wings. This improves high speed performance, but gives a more nose high attitude at low speed. And nose high at low speed on tow requires a lower tow position to be able to see the towplane and keep him safe while he is pulling you to altitude. So, you get closer to the wake, drop into it or start out in it when the towplane lifts off, and get into the controlability issues of low tow speed, particularly behind short winged tugs with more energetic wakes.

Steve