Replies to This Discussion

Permalink Reply by Paul Saccani on April 27, 2015 at 5:57am

  Even though it was an April first post, it makes sense when you're talking about floats and skis, where you don't have a brake option.  May I make a suggestion when using this method?

You cut the rope/cord, so that you have a short piece attached to the aircraft, preferably by a hitch, and long piece attached to shore, also with a hitch.  Then use a sheet bend to attach the aircraft rope to the shore rope, as close to the aircraft as practical.

 

The breaking strain at the sheet bend will be approximately half that of the rope, and the hitches should be much stronger than this.  This way, you control where the break is going to happen.  Otherwise, you might have it break at the far end, and with the recoil, it may end up fouling control surfaces or even wrapping itself around the propeller.

 

Decent quality 6mm cord would beak at around 1,500 lbs at a sheet bend, so probably 5mm cord or less, tested for an appriopriate breaking strain at a sheet bend before hand is probably the way to go, otherwise you might find it too hard to break.

 

Cheers,

Paul

• ▶ Reply

Permalink Reply by Brad Rawls on April 27, 2015 at 8:00pm

Paul -

Just wondering why the sheet bend (my favorite Boy Scout knot) breaking strain is half of the rest of the rope?

Thanks - Brad

• ▶ Reply

Permalink Reply by Paul Saccani on April 28, 2015 at 4:49am

G'day Brad,

Most knots greatly reduce the breaking strain of rope. It's because of the bending radius and twists. If I recall correctly, some of the rather involved climbing knots can get to about 97% of the ropes breaking strain. But most knots are less than half. A double sheet bend, if combined with hitches to stop it slipping, will do close to 60%. But interestingly, if you don't do that, it will usually come undone at about 37%. Which could be handy in this application, if you wanted to reuse the rope.

Cheers,
Paul

• ▶ Reply

Permalink Reply by Bob Jones on April 29, 2015 at 12:28pm

ME and granny knots get along fine, the tail tie down is sharp enough to aid in cutting the rope, but thanks for all the cool suggestions 

• ▶ Reply

Permalink Reply by Paul Saccani on April 29, 2015 at 9:10pm

Yep, the plans tie down can do that, which is one reason some people choose not to use the plans tie down. ;) Anyway, you've got the break point controlled, which is the main thing. That's all I was worried about, someone trying the technique and getting the break at the shore end.

If they don't slip, granny knots are good for around 57%, but they almost always slip long before then.

I presume everyone is familiar with the practice of using a rope secured next to the cockpit, with slip knots at both ends, so you can do run ups conveniently, then simply pull the free ends of the rope into the cockpit when you want to go?

Cheers,
Paul

• ▶ Reply

Permalink Reply by David J. Beaulieu on May 1, 2015 at 11:33am

I am not - please describe the method/technique

Dave

• ▶ Reply