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By Genghis the Engineer
#1566748
This isn't a problem I'm trying to solve - just something I realised is a gap in my knowledge and I'd be interested.

Can anybody explain what defines the published winch launch and aerotow speeds for a sailplane?

G
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By Flyin'Dutch'
#1566780
IIRC it is structural considerations/calculations
By profchrisreed
#1566823
I typed a long answer earlier which disappeared! This is from memory of my reading, so ...

On a winch launch the wings need to generate lift to equal the weight of the glider + the downforce from the cable's pull. Max winch is set to keep the wing loading at a safe margin below the wing's structural limits. À weak link in the system should break if that safe loading is exceeded substantially, but before the wing is at risk of structural damage. I don't know how the margins are set, but you could reverse engineer them via the formula for calculating max winch after a glider is reweighed.

Max aerotow is a mystery to me, but I know from experience that when being towed near that max the controls require more force to operate than when the glider is in free flight at the same airspeed. Guess this might be because the glider is climbing rather than descending through the airmass, and thus the wing and tailplane are operating at a higher AoA, but I don't understand that relationship (you might though). As a further guess, either this speed is related to max manouvering speed, or that beyond that speed the controls require excessive force to control the aircraft.

A 15 min phone chat with the BGA Chief Technical Officer would probably give the information rather more reliably!
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By Sooty25
#1566867
Aerotow, from memory I seem to recall aerotow speed of a K13 behind a Condor was about 15-20knts above normal flight speed, 60-65knts seems to ring a bell, but was naturally dictated by what the poor old Condor would drag us up at! The book quotes a max aerotow speed of 76knts which is the same as the Rough air speed limit (Vra). Maybe this is where the aerotow speed is derived from.
By profchrisreed
#1566900
Sooty25 wrote:Aerotow, from memory I seem to recall aerotow speed of a K13 behind a Condor was about 15-20knts above normal flight speed, 60-65knts seems to ring a bell, but was naturally dictated by what the poor old Condor would drag us up at! The book quotes a max aerotow speed of 76knts which is the same as the Rough air speed limit (Vra). Maybe this is where the aerotow speed is derived from.


I don't think so. The Open Cirrus I used to own had max aerotow speed of 75kt and max rough air of 119kt, the same as Vne. This was why I wondered whether the difference in the wing's AoA while on tow played a part.
By oldbiggincfi
#1566917
Winch launch .
Isn't the top of the launch that is critical when excess speed gives to much lift that is restrained by the cable putting too much force on the tow hook .
Half way up the launch the glider is still able to climb reducing loads.
Being heavy handed will break the protective weak link , which is specific in breaking strength to glider type.
Having a good Winch Driver helps.
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By Sooty25
#1566928
Maybe as FD mentioned it is purely a structural limitation of the glider. I guess there must be a speed it which drag overcomes the strength of the gliders tow hook.
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By Flyin'Dutch'
#1566932
Sooty25 wrote:Maybe as FD mentioned it is purely a structural limitation of the glider. I guess there must be a speed it which drag overcomes the strength of the gliders tow hook.


There are other considerations; not every glide has a nose hook so they will need to be towed using the winch-hook. Controllability may be an issue. Ditto some gliders are very sensitive in pitch at higher speeds - notably those with an all flying tail.

Minimum and Maximum tow and winch launch speeds have been specified in the POH for the gliders I have flown.
By low&slow
#1566947
CS 22 sets the minimum maxima, not less than 125kmh for aerotowing, 110kmh for winching. The manufacturer then has to demonstrate that the glider can be flown under various launch scenarios up to their chosen maximum speeds as part of the certification process. I suspect that the manufacturers pick a typical speed & build it strong enough.

Aerotow forces are very low, overcoming inertia on a grass airfield is probably the greatest load on an aerotow hook. At the top of a winch launch the wings are producing about 2G, so the load on the winch hook is in the region of 1G.
By ChrisRowland
#1567543
IIRC the aerotow forces are with high loads at considerable angles. I suspect that in the Real World the loads on the tug would prevent the tug remaining in control. It's there to protect the structure. Tug upsets can happen without exceeding the limits.

The max winch launch speed seems to be the loading on the wings, maybe the tailplane, and the cable tension is part of the non lifting load. I did some studies and it looks as if the max speed is also the speed where the load on the cable reaches the cable limit, and the optimum load to give the best launch. The load can be very high near the top of the launch because the cable pull is down and with a C of G hook it's difficult to tell how much load you are pulling. It's possible to stall and spin off a winch launch.

Welch and Irving "New Soaring Pilot" is old but probably the most accessible source of information.
By spanloading
#1578642
I believe the max speed on a winch launch is determined by the top of the launch max gust case when the wing root bending moment is almost as severe as the max g pull-up at the max manoeuvre speed (Va) case. This is because there isn't any relief from wing inertia. I suspect the max aerotow speed is as much a control issue as structural.

Spanloading
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By Lockhaven
#1586281
Genghis the Engineer wrote:This isn't a problem I'm trying to solve - just something I realised is a gap in my knowledge and I'd be interested.

Can anybody explain what defines the published winch launch and aerotow speeds for a sailplane?

G


Airframe structural limits on the tow hook mounting, winch and aerotow lines also have a weak link fitted to break in the event of overload during a launch, they are colour coded to define the Kgs limit of the glider to be used.

For a more detailed and technical read of the effects of a winch launch there is this article from the BGA although mainly dealing with the winch launch scenario, towards the end of the article there are drawings explaining the aerodynamic effects.

https://www.bfgc.co.uk/Technical/BGAmanual.pdf