View Full Version : Tie Down Ropes
06-16-2003, 05:17 AM
I recently read a article in Northern Pilot that talked about high winds and tie downs. They stated a 3/8 inch diameter rope should be sufficient for a tie-down, they also advised the use of cargo straps? I guess they only have so much printing space for their story I feel the choice of rope could take up a whole article in itself. I feel rope is fairly cheap and don’t have any trouble spending a couple of bucks more every year or two when I replace my ropes and upgrade to a rope larger diameter then 3/8 inch.
When checking the strength of different ropes the same diameter I have found a large variance in strengths. Knowing this I would think a person would need to know how much there wing can lift and then buy the proper strength rope for you aircraft and not use rope diameter as a criteria. Of course a T craft would not lift what a 206 or 185 would, so I would safely assume you could get buy with a smaller strength rope for the T craft a higher strength rope would be required for the Cessna..
I don’t think I would use a cargo strap on my aircraft it would only take a little movement and the strap could come loose.
The next question is how much can a rope weather before the UVs weaken it? Should I change my ropes every year or can I go longer?
What criteria should a pilot use when buying tie-down ropes?
06-16-2003, 08:38 AM
I saw the same article. So many people will tie a $100,000 airplane down with a fifty cent piece of rope.
I have had good luck using the "Dacron" style ropes that are often used on sail boats and docks. Not the Polyethylene type. The dacron stays flexable after having been wet unlike nylon and is very strong. Very little stretch with dacron but some may say is a disadvantage to disapate shock loads. I have not noticed the rapid deterioration either. The dacron is much higher in price but worth it. I use the 5/8 inch.
How about also discussing knots?
I personally don't use the locking knot that is soooo common. The one where you have a large loop at the top and the rope passes through it twice and then pulled to "lock it". I have seen these work loose many times as an airplane vibrates in the wind. The 185 next to mine in the storm wound up with 1 1/2 foot of slack after the knot slipped. The left main wheel was nearly airborne. This knot is supposed to be better as the rope won't break in the knot but I prefer to know that it stays tight. You can slide your hand up the rope under the knot and it will release most every time.
I just use the old multiple half hitchs for mine and made it thru the high winds OK. Thank the Lord the Atlee tie downs and heavy rear struts.
06-16-2003, 10:32 AM
This was discussed on another post, don't have the time right now to do a search, but you are both right, stronger the better, (not necessarily bigger) I don't use the typical knot either and I don't want the wing to move at all! I have switched from "1/2" gold line to kevlar reinforced 8mm climbing rope! stronger, lighter, and I can carry twice as much with me in half the space!
06-16-2003, 10:36 AM
I've found that dock lines in about a 25' length with a pre-spliced eye work well for tie down ropes. The 3/8" line according to the catalog has a breaking stength of 4500#. They are flexible, easy to use, light and don't take up much room. See http://www.samsonrope.com/home/newrecmarine/docklinecat.cfm and look at the 2-in-1 braid.
See also http://www.supercub.org/phpBB2/viewtopic.php?t=1453
06-16-2003, 10:47 AM
A knot in any rope will reduce the tensile strength of that rope. Some knots will reduce the tensile strength by 50%. Aside from the knot issue, is that of the size and configuration of the attach point of the rope. When a rope is passed over an object and makes a change of direction, i.e. 180 degrees, a radius of bend is effected. The tighter the radius of bend the greater the reduction in the tensile strength of the rope. Imagine a rope passed over a 36 inch diameter object and then imagine the same rope passed over a 1/4 inch object. If both are loaded equally the one passing over the smaller object will fail first. Carry it to the extreme and imagine it passed over a knife edge.
Tim, If you are on tires especially big tires keeping the wing from moving will be most difficult. I don't mind that the airplane is moving around a bit. I think it is more important to take the time and energy to turn the plane into the wind and kill the lift somehow e.g. spoilers. Once the lift is spoiled you are only dealing with the wind pushing against the plane a far less load than with the lift componet added. Most of the wind damage I have seen through the years could have been prevented. Yes, certainly there are exceptions to this. Have a good trip up. pak
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