A Tale of Two Takeoffs

Many a tow pilot has wondered, "why are they all whining about 60 knot tows, when that's well above their stall speed, even ballasted?" The answer is that gliders following the towplane are in the downwash of the wing -- not the propwash, the downwash -- but the wingtips are not. So, the wingtips are operating at a larger angle of attack than the main wings. They will now stall first.
Even if the wingtip doesn't stall, the aileron does -- you get the aileron response of ragged edge of stall though the main wing is flying. That's why gliders get wallowy and hard to control on tow behind slow towplanes. This has caused crashes. That's also why it's especially a problem when the towplane takes off, gets up to speed, and then heads for the sky, losing speed, as quite a few tow pilots seem to do. Now the glider is slow, stuck in ground effect, well below the towplane, and deep in the downwash. Yes, airspeed indicators are off, but it's not that hard to be aware of the issue, and ask the first few pilots for a calibration. I think the larger problem is we all get used to towing dry, and the higher speeds just don't feel right.. It's a worse problem for standard class, since they're stall speed challenged to start with and many flaps typically allow a little more twist.

10.6.2.9 ...Towplanes will tow at 80 miles per hour (unless otherwise requested) in a pre- selected pattern to an altitude of 2000 feet AGL (or as specified by the CD).

Thanks, Chip for a great post.
We all find ourselves exceeding personal rules at times, and sweep them under the rug as "I got away with it."
Jim

What John said, even though he had to go and mention "whining" again.

For more discussion (any takers?), see "Glider Handling on Tow" on RAS a few years ago: https://groups.google.com/forum/#!searchin/rec.aviation.soaring/%22glider$20handling$20on$20tow%22/rec.aviation.soaring/rxCTVDJnKVc/Qn6gkHnP7EgJ

My number is 60 kts. dry and 70 kts. fully ballasted in a Standard Class glider. Below those numbers, I am VERY uncomfortable and less maneuverable. So add 5 kts. to allow for tow pilots who ease back on the stick after they've climbed up a few hundred feet.

Flapped 15M gliders have a bit more margin. Big ships seem to have still more but I can't speak from experience.

Chip Bearden
"JB"

Congrats on your 8 years of CFI work. I hope that wasn't meant to start a measuring contest, because it won't end well.

And everyone can always use a little more training. Anyone who thinks they don't need practice is the most dangerous one on the field.

I must admit the downwash discussion is one I'm not fluent on. I've been around a while, but it's certainly an interesting concept I don't know much about. Makes you wonder how high a position you'd have to be for it not to matter...especially when you consider actual climb gradient, compared to AoA and angular fluid dynamics from the tug wing back to a glider 200' back. I haven't seen it in any book, but the books have been updated a few times and it doesn't mean it doesn't exist.

That's true....seems ballasted standard class birds have gotta be the highest speed required on tow group. Can't speak much to open class. Not a lot of that on the east coast.

On Sat, 17 Jun 2017 19:57:22 -0700, Echo wrote:

I must admit the downwash discussion is one I'm not fluent on. I've been
around a while, but it's certainly an interesting concept I don't know
much about. Makes you wonder how high a position you'd have to be for it
not to matter...especially when you consider actual climb gradient,
compared to AoA and angular fluid dynamics from the tug wing back to a
glider 200' back. I haven't seen it in any book, but the books have been
updated a few times and it doesn't mean it doesn't exist.

Its even a bit worse than has been described: your outer wings and
ailerons are in the tow plane's wingtip vortex, which adds a still
further to the AOA difference due to the upward flaw in that part of the
vortex.

Low tow may be a better answer simply because it puts the whole glider
below the tugs downwash and tip vortices. It certainly feels smoother,
but doesn't sort out takeoff and initial climbout problems.

Side comment: the issue of wingtips extending beyond the tug's wing
downwash in pretty much unique to our sailplanes: all of the Allied troop
carrying gliders during WW2 had lower spans than their tugs (a Waco CG4
is smaller than a C47 - look it up if you don't believe me). The only
exceptions were the British Hamilcar (about 6 feet bigger span than the
Halifax tow plane and one or two of the German and Russian troop carriers
when towed by single engine aircraft and, of, course the Me 321 Gigant.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

Wow that's interesting stuff. I once went to a WW2 glider museum in Iron Mountain Michigan, but it was more about the manufacturing and stories and less about the design and flight characteristics. The only time I've ever noticed any kind of wash from a towplane is on pavement behind a CAP 182. 100' into the takeoff roll, the right wing always drops. It's a briefing item when we fly there. Spiraling slipstream of a tri gear vs taildragger.

It's a shame we can't put a glider and towplane in a wind tunnel together....or at least some smoke/fluid testing released from the towplane wingtips. So much of aerodynamic "fact" isn't really known, but more speculated and generally accepted. Would be a pretty neat study to actually watch said downwash. I still have never noticed it in flight; It was explained differently to me many years ago, and it has always seemed to prove to be passing below me. If the glider keeps the towplane referenced on the horizon from high tow position, they're essentially level horizontal with each other. That means that the towplane is not dragging the glider through it's path, but rather the two are climbing together, a level horizontal plane going up. (Most gliders' drag in lbs at a tow speed is probably only 50ish pounds...so the tension on the rope wouldn't be much more in order to yield a climb at constant speed. Similar to the discussion on excess power required for climbs in the book Aerodynamics for Naval Aviators). If applying that theory of the horizontal plane climbing evenly, and towing at say 60kts and 500ft/min (for simple math), that's a climb gradient of 500ft/nm, or 5 degrees. So the relative wind is essentially from 5 degrees above the horizon, and downwash results in 5 degrees off below to the rear, or more depending on angle of incidence, etc. That, at 200, back, puts the theoretical downwash line a little over 15 feet below the level horizontal plane that the glider exists in with the towplane. So clearly in that theory, a weak towplane and a heavy glider has many issues beyond clearing the trees, but a strong towplane makes everything less of a factor.

Again that's just a theory, just like the perceived downwash one. My only point here is to say that all this stuff is theory, something from voodoo black magic aerodynamics. It would be truly interesting to hook up some smoke canisters across the wing of the tug and see where it all goes. Same with the tips.

I would love to hear the perspective of military guys here who have done many hours of aerial refueling...what they think about the presence of downwash and how much it affected their aircraft when sitting behind a tanker (with a much higher wing loading).

Jordan

This is a great discussion!

Before I retired from towing, my technique was to level off in ground
effect until reaching full towing speed (and note that the glider's
ground effect is deeper than the tug's due to the difference in span)
and then begin a gentle climb at a pitch rate which would preclude a
reduction in airspeed. Some glider pilots like to be towed at 75 KIAS
when fully loaded.

On 6/17/2017 7:32 PM, wrote:
What John said, even though he had to go and mention "whining" again.

For more discussion (any takers?), see "Glider Handling on Tow" on RAS a few years ago: https://groups.google.com/forum/#!searchin/rec.aviation.soaring/%22glider$20handling$20on$20tow%22/rec.aviation.soaring/rxCTVDJnKVc/Qn6gkHnP7EgJ

My number is 60 kts. dry and 70 kts. fully ballasted in a Standard Class glider. Below those numbers, I am VERY uncomfortable and less maneuverable. So add 5 kts. to allow for tow pilots who ease back on the stick after they've climbed up a few hundred feet.

Flapped 15M gliders have a bit more margin. Big ships seem to have still more but I can't speak from experience.

Chip Bearden
"JB"

--
Dan, 5J

On Friday, June 16, 2017 at 4:55:55 PM UTC-7, wrote:
I have a secret.

Well, like secrets in a Congressional Intelligence subcommittee, this one is probably known by several hundred people. But I haven’t written about it. Until now.

But first, the lead in. A few days ago on the last day at the Cordele nationals, I dropped a wing on takeoff. The ballast tanks were half full so although the tip didn’t go down hard, the water sloshed in that direction and the wing stayed down. My ASW 24 started to swing off to the right. I wasted no time in pulling the tow release. The glider continued to swing and I braked as I rolled off the asphalt runway into the grass and stopped..

No big deal. I’ve had a rule for many years that if the wing goes down early with ballast, I release. It's almost impossible to get the tip up again (more later). I’ve done it twice before--both times on grass--and it’s slightly embarrassing but uneventful except for blocking the launch. This time I actually rolled out of the way. Not much more than five minutes later, I had been retrieved with a golf cart, put back on the launch line, and was away without ever having to climb out of the cockpit.