On Tuesday, November 5, 2013 4:36:45 PM UTC-6, John Carlyle wrote:
I did a search as Erik suggested. After reading a lot of posts, I found a very good explanation by Andreas Maurer, which I partially quote below. His explanation seems to account well for why the glider handling is so poor at slow tow plane speeds. But for me, several things are still unexplained: 1. Why does increasing the tow plane speed cause the poor glider handling to go away? 2. If the answer to (1) is that the down wash and wing tip vortices are now further below the glider, then why doesn't simply moving up higher above the wake of the slow tow plane remove the poor handling? -John, Q3 =========== The following was written by Andreas Maurer, and posted on RAS 1/5/11: The main factor for the seemingly odd flying characteristics behind the tow plane is the downwash of the latter. Let me explain: The downwash has a significant angle (the air is deflected downwards behind the tow plane's wing to up to four degrees!), but due to the larger span of the glider it only affects the inner part of the glider's wing. Therefore, if the glider if lying laterally displaced, only one wing is affected by the downwash of the tow plane - four degrees of AoA difference between left and right wing need a lot of aileron to correct. Likewise, if the glider is flying straight behind the tow plane, the downwash *decreases* the AoA of the affected inner part of the wing. Getting the nose up by pulling back will restore the lift of the inner part of the glider's wing, but now the outer parts of the wing have a much higher AoA than they have in free flight. Voila, meet the the conditions for poor aileron efficiency (high AoA!) and tip stall. The downwash is reduced by - wingloading of the tow plane - wing span of the tow plane In other words: The more a tow plane looks like a motorglider (say, a Dimona, or Katana Extreme), the less the flight characteristics of the glider are affected. Anyone who has ever been towed behind a motorglider or a microlight will testify that problems like poor lateral control or running out of elevator don't exist there, despite a far slower tow (55 kts compared to a typical 70-75 kts behind a typical tow plane like Reorqeur or Pawnee).

Back to seriousness. Ever looked and the con-trials behind an airline in flight? It is very narrow right at the plane, and spreads out as the plane moves away. The wake and downwash field are moving down and spreading out. The con-trail spreads out as it is marking a limit of the wake. All the air above the actual wake and downwash field is also pulled down by the downwash field behind the wing. Some very large distance away, this downward pull of air will be negligible, but in the amount of altitude you can rise above the wake on tow without seriously upsetting your tug, the downwash will still be about the same. So, getting above the wake may solve some of the awful handling because you aren't being trounced around, but you will still be very nose high and feel very uneasy. Interesting videos show the wake actually gets narrower for some distance behind the wing, and then spreads back out. So, there probably is some effect of the wing of the sailplane extending beyond the wake of short wing towplanes.

Why does increasing the towplane speed help? Two things. First, many things aerodynamic happen with the square of the speed. 10% increase in speed gives a bit over 20% increase in stall margin. Second, when the towplanes wing doesn't have to work as hard to keep it in the air, the size and intensity of the wake and downwash field decrease. So does the downwash angle. So, by adding speed, the downwash angle is reduced, your required angle of attack is reduced, your margins and comfort levels all go up. And by the way, the wake will actually be higher where you are at the end of the same length tow rope, as it won't be angling down as much as it comes off the wing of the towplane.

As Andreas said, long wing towplanes don't create large angular downwash fields when flying at 55 knots. Pawnees and the like do.

Also worthy of note, I believe the downwash field increases in angle but decreases in velocity as a function of horizontal distance from the wing. So, the length of rope you chose to hang yourself with might also affect slow tow speed handling. Hint: I would try longer for better handling at the same tow speed. Gets you further horizontally and vertically from the wake..

Well, that was more than just two cents worth!

Steve