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poor lateral control on a slow tow?
On Wed, 5 Jan 2011 09:23:29 -0800 (PST), Derek C
wrote: Gliders appear to get near to the stall during slow aerotows at much greater than their normal free flight stalling airspeeds. I would suggest that aerotowing must increase the wing loading in some way. I have to admit that I didn't bother to read all the 120+ postings about this topic, so please forgive me if the things that I'm going to post have already been mentioned in this thread. 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. Likeise, 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 alieron 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). One interesting fact: When Akaflieg Braunschweig flight-tested their SB-13 flying wing (with a back-swept wing), they encountered a nose-down momentum after lift-off that could not be recovered and usually lead to a crash immediately after lift-off. Explanation: The downwash of the tow plane (Robin Remorqeur) hit the inner part of the wing, decreasing its AoA (and lift) and therefore shifting the center of lift backwards due to the sweepback. Increasing the length of the tow rope helped. Greetings from a snowy Germany Andreas |
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