Glider Handling on Tow

Thanks for this idea. Three out of the four gliders have the ability to show ground speed, as well as an independent IAS from their SN-10s to cross-check their ASIs. Getting a ground speed value from the tow plane to compare to the tow plane ASI as well as the glider ground speed during flight is also doable. Not sure when we can do this, but I intend to try it!

-John, Q3

On Tuesday, November 12, 2013 10:45:33 AM UTC-5, soarin wrote:
[snip}
The next relatively calm morning put a handheld gps displaying ground speed
in the tow plane where the tow pilot can easily see it while towing. If
his indicated airspeed is notably different than what he sees for gps
ground speed, you have good grounds for a discussion. However if your
indicated airspeed was notably different then his gps ground speed, the
discussion may be go differently.
[snip]

Mike,
You said that poor handling during low speed towing is mainly felt by newer standard class racing ships. This is probably correct if we accept Andreas’ explanation that downwash interaction (requiring higher AOA which leads to tip stalling) is the root cause of the poor handling problem, as flaps would allow a lower AOA than with a standard ship. However, a look at section 4 of several pilot handbooks showed that the 15/18 m ships have higher minimum tow speeds:

Ship, dry min tow, wet min tow
D2, 54 kt, 65 kt
LS-8, 54 kt, 65 kt
Duo, 54 kt solo, 65 kt dual
V2, 65 kt, 70 kt
ASW-27, 65 kt, 70 kt

Do you know what the 15/18 m pilots experience when being towed below 65 kt?

-John, Q3

On Tuesday, November 5, 2013 1:32:35 PM UTC-5, Mike the Strike wrote:
The Discus 2 exhibits the same behavior. It's a problem mostly seen in newer unflapped racing ships because of the angle of incidence of the wing. This results in a nose-up attitude at low speeds and on aerotow this results in a downward pull on the nose from the towrope. This down-pull has to be counteracted by up elevator. With ballast, I run out of up elevator around 60 knots or so, and I have had my glider sink into low tow a couple of times with a slow tug and have had to release a couple more. In free flight, the glider will stall at its placarded speed and handles fine. Tows below 60 knots have very poor aileron and elevator control.

There may be other things going on, but I suspect the relative angle of the towrope to the fuselage on tow is responsible. Flapped ships alter their pitch and hardly suffer from this effect.

Mike
Discus 2b

On Tuesday, November 5, 2013 11:10:33 AM UTC-7, John Carlyle wrote:
A question for any aerodynamicists out the why does low aero tow speed adversely affect the handling of a glider so drastically?



Occasionally I’ve received an aero tow in my LS-8 (dry) at 55 kt (minimum recommended aero tow speed is 54 kt). The plane is heavy and unresponsive at that speed; it’s an extremely stressful experience! But once off tow the same plane handles like a dream at speeds down into the upper 30 kt range.



Before anyone asks, yes, I do immediately ask the tow pilot for 10 kt more speed - right now! I’d just like to understand what the root aerodynamic cause of the poor handling might be.



-John, Q3

Not sure how different the experience of the flapped ships is, but the biggest complainers behind our Pawnees seem to be the D2 and LS8. From a wingtip video, I eyeballed the towrope angle and thought that its vector was below the fuselage center line. however, the down-wash theory is also certainly plausible. The dynamics of aerotow are certainly not simple and the problem could be a combination of several effects.

Mike