MSFT-1

MSFT-1

Well Known Member
I use my RV-10 to commute. This means it sits outside on a tie-down a few days per week. Here in the Washington DC area we get thunderstorms fairly often in the late afternoon. I have never had a problem, but it has started me thinking about tie-downs.

The airfield tiedowns have permanent anchors about 50' apart for the wings. This means that the tiedown lines are nearly horizontal.

So the question is, what is the ideal distance apart for tiedowns for an airplane. Should the tiedown angle be nearly vertical? 45 degrees? More?

Does it matter?

I have a set of Bear Claw tiedowns I can use if it will make my airplane more secure.

thoughts?
 
most secure would be small angle outward to around 45 degrees from straight down for single tiedown per wing. i would think the tiedown anchor would be best if it is wider than the wing so the tiedowns are straight 45 degrees away form the body of the plane, in line with the wing spar

if you wanted to be really secure two tiedowns per wing, apx 90 degrees from each other would physically try to lock the tiedown ring in one position

the current position is pretty poor, if the tiedowns are nearly horizontal that allows the airplane to be lifted up a fair bit and then dropped. I think you'd want to avoid that
 
It is just a classic force vector. You have to apply more force the steeper the angle for the same vertical holding force. Two of the things you're trying to accomplish with a tiedown are for it not to fly away vertically, and not to get pushed around horizontally. The horizontal movement is part of why you might rather not have perfectly vertical tiedowns. Except of course when tied in soil with stakes, which need more horizontal component to keep them from pulling out.
So with your steep angle state, you've well solved the horizontal part, but unless you're willing to tie them pretty tight, it can be picked up vertically a bit easier, while putting more total force (horizontal) on the tiedown rings in the wing. There, that is my hand waving theory for today :).
 
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The two extremes are vertical, which will keep your plane on the ground rigidly and not allow any rocking in the wind, and (almost) horizontal, which will keep the plane in position laterally but will allow the plane to move around forward, backward, up, down, rocking, etc. Any other position will be a tradeoff between how much "give" you want the plane to have and how rigidly you want it held in position. There isn't really an "ideal" answer for that.

Personally, when tying into a concreted-in tiedown, I want my tiedowns to be loaded vertically so there's no twisting moments on the bolt that goes into the bottom of my wing. Then I tie down with a rope that has some stretch to it.
 
Of course, the FAA would like to help...

...by publishing this Advisory Circular...

http://rgl.faa.gov/Regulatory_and_G...rcular.nsf/list/AC 20-35C/$FILE/Pages1-10.pdf

http://rgl.faa.gov/Regulatory_and_G...cular.nsf/list/AC 20-35C/$FILE/Pages11-20.pdf

Page 7 is the closest to the angle of ropes question.

The thing I see the most at airports are the wing ropes being at 90 degrees to the fuselage. Since it's a three point system, the wing ropes really should be angled forward, just like the picture on page 7.

Only if there is a cable tie down can you get the rope to be truly vertical.
 
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Ideally you'd like them to be at some angle, like 60 degrees, to the ground (30* off vertical). And they ought to be oriented so that the wing ones are forward and outboard of the wings, and the tail is directly aft of the plane.

That helps prevent lateral or fore/aft motion.

Tie the ropes as tightly as you can, and double the ropes if possible. What will cause the plane to get loose is rope that can let motion occur, so don't accept that. You don't want the plane to start rocking or surging in a dynamic motion because it'll fail something.

Don't use straps or nylon. Use polyester rope or better yet, one of the high-tech marine double-braid lines. They are stronger and stiffer (less stretch) than nylon rope.

You can get the marine ropes from www.apsltd.com, www.westmarine.com or www.defender.com.

If you use Amsteel, which has excellent strength, weight and stiffless characteristics, don't use the single-braid version. It's too slippery and doesn't hold knots that well. But the double-braid version, with a polyester cover, is excellent.

Now this only matters if it's really going to blow. If the location only gets small winds, like under 30 kts, it almost doesn't matter.
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I agree with most stuff already noted. The wing ties should be angled either in or out and a bit forward, and the tail straight back. If it's a nosewheel airplane, then don't put too much pull on the tail tiedown, or you will decrease the weight on the nose and possibly end up with a "bucking horse." I also have a set of chocks that is shock-corded together on one side that I use for my nosewheel. The chocks help keep the plane in place if the ropes stretch a bit, or you weren't quite perfect in setting up your angles. If the nose "lightens" up a bit, the chocks pull together slightly so that there is never any "wiggle room" between the chocks and the wheel.

cheers,
greg
 
45 degrees is optimum. At 45 degrees, the holding strength of the line is 70% in both the vertical and horizontal directions of what a vertical or horizontal line would have. The tie down has to keep the plane from lifting vertically, and from being pushed laterally at the same time.
 
Tie-downs are so important and this is an important thread. Many good points about angles and the effects on rings, etc. Back in the 80s and early 90s I flew a Phantom U/L. It survived TWO tornadoes because of proper tie-down. The most dramatic was while it was in the hangar. I tied down even there (and still do.) A tornado took out the entire row of hangars, and the planes that were not tied were just--gone. Of the seven in the row, my u/l was sitting there with very minor damage. Three planes were reparable. The winds totally removed the hangars, stretched my tie-down ropes (nylon) by about 1/3rd, and placed a hangar wall UNDER my Phantom. I was flying within two days.

When possible, I tie with pull from three directions. There is little choice of angle at most places, but 30-45 degrees seems ideal. I KNOW it works!

Bob Kelly
 
Kinda funny, but he's wrong.

What happens is that if the plane's pointed into the wind, it develops lift and can start moving, usually bouncing. Then something breaks and over it goes.

If it's side to the wind, it moves enough to establish equilibrium and then is stable.

Back a while we had a bunch of airplanes get blown over here, and I examined them after the wind had decreased to about 60-70 mph with higher gusts. My profession was an aerospace stress analyst, so how they failed was of particular interest to me, and since I had my plane tied out on that field, it was personal.

One other lesson is to park at the most upwind location. Guess why.
 
hanger tie down

The most dramatic was while it was in the hangar. I tied down even there (and still do.) In my old airport we had 2 hanger tie down incidents. the first guy tied his plane to the "tent" (aka cover-it) hanger directly. The windstorm took them both together into the woods. The second, same set-up tied his plane to the ground. The wind put the hanger 100 feet into the woods and left the plane undamaged. FWIW. -Ben
 
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