Ground launches and weak links

On Monday, December 3, 2012 5:12:22 PM UTC-7, Dave Springford wrote:
The Canadian Air Cadet manual for winch and auto launch of 2-33's specifies the weak link should be 2 x max gross weight and this gives 2080 lbs.



We recently winch launched a 2-33 at a neighbouring club and used a 1925 lb rope for the weak link.



For a 1-26, I would use the same (2x) rule of thumb.



With the weak link at this level, you can not put more than 2g on the airframe.

I feel a cold knot forming in my gut. A 1925 Lb-F weak link can put far more than 2G on the airframe. PLEASE read some winch launch guidance.

On Monday, December 3, 2012 5:12:22 PM UTC-7, Dave Springford wrote:
snip

With the weak link at this level, you can not put more than 2g on the airframe.


Well Actually it is 3g since you have add the weight of the glider to it. Still a 2-33 will easily handle 3g's. If the tow speed is flown below V/a then you have two safeties in place to prevent overloading the air frame.

Brian
CFIIG/ASEL

I don't think most here have constructed a V-N diagram or know the basic actual limit loads as dictated by the BGCH of 1962. In the case of this document, Schweizer and Breigleb literally "Wrote the book".

Prior to JAR 22, the 24 fps gust was the prime driver for these gliders, and when using the mandatory design point Vg, typically gives a limit load of over +5gs. The the 2-22 also has a 69 Mph max winch speed. Are you saying that somehow, hundreds of similar gliders flying over 60 years have been doing so with a serious undetected flawed loads analysis?

If you are not an experienced degreed Aero Eng who has done V-N diagrams and perused the above certification requirements(I have) perhaps you should not worry about this as an issue.

Side remarks concerning the Blanik are unwarranted. LET did extensive testing and knew in the 60's they had a fatigue problem exactly where the Blanik wing failure occured. The eastern block design philosphy of "safe life" and abundance of cold war trained expert mechanics influenced how they handled this known problem. Have you seen the extraodinary detail and background on the loads analysis in the LET service/repair manual? Even so, post accident limitations indicate they were more concerned about aerobatic time than number of winch launches. Since the accident report has never been released, we will never know if poor execution of the wing rebuild on this aerobatic trainer just 800 hours prior was a factor. Based on the work I saw come out of the LET factory about that time I have my suspicions.

If this is not enough to allay concerns of winching the 2-33, consider that wing failures of high wing, strut braced aircraft are extremely rare. The strut and its adjoining attachments are sized for down bending driven compression and the resultant local buckling, and have a huge margin in an upbending case. Also, it is unlikely as I mentioned earlier, that winch tows loads are as severe on the Schweizers, the large moment arm between the hook location and the CG means you will run out of elevator a lot sooner than with a CG hook. Try ground launching a 1-26 and you will see what I mean.

If you are not an experienced degreed Aero Eng who has done V-N diagrams and perused the above certification requirements(I have) perhaps you should not worry about this as an issue.

I am, I have, and I do worry.

Look, I wish the 2-33's weren't 50 years old. I wish they were great winch gliders and would last forever but none of that is true. Winch launch puts a lot of stress on an airframe. Stress metal enough and it breaks. We seem to keep learning that the hard way. Wishful thinking doesn't help.

This particular old metal glider has a number of red flags right out of the factory - unspecified weak link, dubious Vw speed and no true CG hook among them. After a half century of hard service they need to be treated with respect like the antiques they are.

Do yourselves a favor. The next time you are at the airport take a hard look at your 2-33. Specifically, look on top of the right wing D-tube in the area where the strut attaches. I've seen half a dozen with patched fatigue cracks there - saw one last weekend. Yes, it was being winch launched.

Why the right wing? Go sit in the front cockpit and open the spoiler half way like students are taught to land. Now try to use full left aileron like you were trying to prevent the glider from tipping over onto the right wing. If a student is going to drop a wing, likely it will be the right one because left aileron is blocked by the student's leg which is trapped by the spoiler handle.

If the glider has a "CG hook" grafted onto the right side of the fuselage, get down and carefully look at it. You'll notice it hangs below the skid so it's probably been ground off. So much for Schweizer giving much thought to winch launch.

On Monday, December 3, 2012 8:09:39 PM UTC-8, Bill D wrote:
If you are not an experienced degreed Aero Eng who has done V-N diagrams and perused the above certification requirements(I have) perhaps you should not worry about this as an issue. I am, I have, and I do worry. Look, I wish the 2-33's weren't 50 years old. I wish they were great winch gliders and would last forever but none of that is true. Winch launch puts a lot of stress on an airframe. Stress metal enough and it breaks. We seem to keep learning that the hard way. Wishful thinking doesn't help. This particular old metal glider has a number of red flags right out of the factory - unspecified weak link, dubious Vw speed and no true CG hook among them. After a half century of hard service they need to be treated with respect like the antiques they are. Do yourselves a favor. The next time you are at the airport take a hard look at your 2-33. Specifically, look on top of the right wing D-tube in the area where the strut attaches. I've seen half a dozen with patched fatigue cracks there - saw one last weekend. Yes, it was being winch launched. Why the right wing? Go sit in the front cockpit and open the spoiler half way like students are taught to land. Now try to use full left aileron like you were trying to prevent the glider from tipping over onto the right wing. If a student is going to drop a wing, likely it will be the right one because left aileron is blocked by the student's leg which is trapped by the spoiler handle. If the glider has a "CG hook" grafted onto the right side of the fuselage, get down and carefully look at it. You'll notice it hangs below the skid so it's probably been ground off. So much for Schweizer giving much thought to winch launch.

I seriously doubt from your tone that you actually know what you are talking about, as you are adding a bunch of ancedotal observations which have little to do with the original subject at hand. What is the primary difference between the BGCH and the JAR22 wrt to gust load criteria? Why are you using load factors less than 5 g's? When did I ever imply that the Schweizer line is known for good winch launch aircraft?

Nothing lasts forever, and a majority of the training fleet is antiques. With the proper weak link, or design in case of the 2-33 (Limited elevator authority, forward hook location, etc) you should not be stressing these aircraft any where near the limit load, so I don't see the design issue there. There are all kinds of design problems with the 2-33, from ergonomics to tail design, etc, but most of them are still an issue with aero tow. Having said that, there is no doubt that sooner or later, abuse and poor maintenance like you describe will cause an accident. Until that time, I won't have any qualms in ground launching a Schweizer. How many aircraft manufactures can boast only a single AD and a handfull of minor S/B's across an ENTIRE line of aircraft spanning 70 years? BTW, that AD takes care of the valid tow hook issue you mention. As you must know the AD requires a preflight inspection. Damaged parts on both "CG" and forward hooks can be very serious.. I just bought new parts for the 2 ships on my field.

Bottom line, the extreme majority of accidents in GA (98%) are not caused by primary structural failure. Almost all of those were caused by pilot error, and that is where we need to focus. So I would apply my concerns by proper maintenance (Replace bad parts, fix cracks in your LE, etc) As added protection, it should be easy to come up with a weak link that is practical, that limits wing load to something like 1.8 to 2,5g's.

Unlike the Blanik or other gliders, the 2-33 wing is not a stressed skin design, so you can do your own simple stress and fatigue analysis of the lower spar cap just outbd of the strut. I would neglect effective cap area of the skin, use a Kt of an open hole, apply the appropriate S/N cut off and assume full stress reversal. Les at K&L should have all the applicable dwgs and material callouts. It is possible he could even share with you, where the lower margins are wrt winch launching loads.

Look forward to your results...

aerodyne

On Dec 3, 4:56*pm, Brian wrote:
With the weak link at this level, you can not put more than 2g on the airframe.
Well Actually it is 3g since you have add the weight of the glider to it.

disclaimer type="I am not an aeronautical engineer; might not know
what I am talking about"

Won't it actually be even more that 3g? Something tells me that
lifting and non-lifting parts should not be treated in the same way
here.

/disclaimer

Bart

On Sunday, December 2, 2012 9:28:45 PM UTC-8, Bart wrote:
All,



After some research I came to a conclusion that in the USA, there is

no legal requirement to use a weak link during a ground launch,

regardless of the rope/wire strength.



First question: Am I missing something? Is there something like a

ground launch version of 14 CFR 91.309?



Second question: If the glider's POH does not offer any guidance (say,

1-26), what would be an appropriate range of rope breaking strength?



Thanks!

Bart

Just curious. Have there been ANY catastrophic wing failures of ground launched Schweizers? We auto-tow our club Schweizers twice a year. We break weak links now and then. We get from 1000 to 1200 feet of launch from 1800 feet of 5/16" poly rope. Mostly, we have a lot of fun.

All,

Thank you for all the information, opinions and warnings.

Bill - while I tend to trust the manufacturer, your point is taken. I
will be paying close attention to airspeed. (This is not to imply that
I usually do not!)

Bart