airplane tie-down knots

Practical Knots
41

Having read a number of technical specs (most of which went over my head I admit) I think I’m beginning to understand the problem.

Aircraft wings have to be able to take 2 loadings - a positive flying load load when flying normally and an inverted flying load when upside down. For normal small aircraft (this is hugely different for acrobatic planes) the maximum permitted inverted flying load is less than half the normal load. If tie down lines are too taut then this load capacity can be exceeded because all of the force is pulling the wing in the inverted direction via the tie down and structural damage can result. Allowing slack lets the plane rise slightly and only when this upward movement is restricted by the tie down is there an inverted flight load but by now a good deal of the wind lift has been taken up by the aircraft weight - normal flight load. I think that makes sense. What is clear is that this is not the same as tying down eg cargo where no movement is wanted.

Barry

42

This argument can be used the same way to argue for pretension. As wind load increases, the inverted flight load is relieved by the normal flight load. As long as your initial preload is within what the wings can take, all is good.

In the pretension scenario, you have the added benefit of avoiding impact loading.

43

I disagree. If the line is pretensioned then the lift caused by the wind which would normally allow the aircraft to rise subjecting the wing to a normal flying load as it lifts the aircraft is turned into an inverted flying load because the tie down is stopping any upward movement of the aircraft and in effect pulling the wing down from the outset - the weight of the aircraft does not have any bearing on this, only the force of the lift generated against the tie down rope matters.

Impact loading is unlikely to be as important simply because of the aircraft weight - a very powerful wind sufficient to cause an impact load of any size is likely to damage the wing in any event (pilots are advised to add boards to the wings if a high wind is expected - these disrupt the airflow and reduce lift).

Barry

44

The wings generate the lift, not the body of the plane. When the wings pull up they negate the inverted flight load while increasing the restraint tension.

The only exception would be if the tie downs were placed near the tips of the wings, which doesn’t seem too common for the small craft we’ve been discussing.

http://www.j3-cub.com/forum/f79/tie-down-knot-17647/

45

Report on my visit to my flying club yesterday:
First I spoke to our recently retired Chief Engineer - he was adamant that tiedowns should NOT be taut and that is the main reason for using ground lines (14/16mm braid on braid pegged out along each aircraft parking line) to allow for pilots who like to yank their lines tight. This applies to both high and low wing aircraft.
The knots I found tying down mostly low wing aircraft were tied in lengths of all sorts of rope, (none of which were more than 6/8 feet long) were predominantly various combinations of HALF HITCHES - though I did find two Fisherman’s Bends, three clove hitches (around the ground lines). One bowline used as an eye to pass the working end through to pull down and secured with two Half Hitches.
So all in all quite disappointing.
Now back to the reasons NOT to use the Trucker’s Hitch or Versitackle - Apart from the fact that both would be hauled down taut (see Engineers comments above) the Versitackle uses at least three times more rope (x2), that rope is weakened by two knots and chaffed by the action of pulling on the end to tighten it (how many times could you use this before having to replace it!). The trucker’s hitch (if made using the bellringers knot) needs to be under constant tension which is unlikely as the aircraft is likely to move forward or back slightly in the wind.
Finally - anyone who leaves an aircraft out in a hurricane deserves all he gets!

Gordon

46

Most aircraft I have flown (AA, Piper, Cessna, Eurostar, Europa, C42) have stall speeds of an average of about 35-45 knots, so unless the ground speed of the wind over the wing of a tied down aircraft is in excess of this there will be no lift - but the wind against the whole aircraft could move it a little (hence the use of chocks). Another factor is that aircraft should always be parked into wind - so if you tie down into wind and go home you may return next morning to find you are parked off the wind!
But this is getting off topic a bit.

Gordon

47

I’m still not clear on the rationale on leaving lines loose from your previous post. That aside, the Versatackle can be employed such that it doesn’t use much line. See the right hand diagram here:
http://notableknotindex.webs.com/Versatackle.html

All knots weaken rope to some extent, including the ones you listed. There may be some wear due to the pulley simulation, but all rope eventually wears out (including if you leave it loose to chafe from wind whipping). Just inspect your ropes occasionally, or, as has been discussed in another thread, use short, separate sacrificial loops in the line that can be changed out (not that I do that last item myself).

There are several reasons one might have to leave a plane out in a storm, such as lack of warning, lack of pilots to fly craft, or simply because the plane isn’t airworthy at the time.

48

The plane won’t take off but there is a force acting on the wing even at 5 mph which is what “lift” refers to - it’s just not enough to support the aircraft so there is no lift off.

This is a misunderstanding. The wings lift the aircraft they do not pull it up (you could perhaps say they push it up). The construction of the wing enables this upward force to lift the weight of the plane at take off. But the taut tie down converts this upward force on the wing to a downward force from the outset - which the wing is not designed to take to anything like the same extent. This is what inverted flying force means. There is no netting off involved - the force is only one way.

However if the aircraft has lifted off even to a small extent then the nett downward force acting on the wing as a result of a slack-at-start-but-now-taut tie down is the excess over the lift required to have actually raised the plane off the ground. In other words if the plane weighs in at 800lbs then the wing supports 800lbs when the plane rises as it is designed to do but if the lift is in excess of 800lbs then only this excess force acts to pull the wing in the “wrong” direction as a result of its being tied down by a line which has only now become taut.

Barry

49

i have flown a fair share of aircraft, an as far as i have found, firstly, wings don’t break easily. it’s probably a good idea to use ropes with a breaking strength that is less that the empty weight of your aircraft, but as far as not having tenison, i have never found this helpful. normal catagory aircraft are certified to at least -1g, and wing tie downs are usually somewhere within halfway out on the wing. factoring that with eliptical wing loading, the wings at the point of the tie down should easily be able to withstand half of the ariplane’s gross weight. then, if there is no slack, the airplane will have no momentum when it does jerk, meaning that the jerk will be lessened compared to if it accelerates and is then suddenly jerked taught. consider for instance jumping on a scale as opposed to standing on it. if you were to find the integral of your weight over time staning on the scale, it would be more that if you were jumping on it, but jumping on it is easily more detrimental to the scale, because upon every landing you are applying undue pressure. you don’t need to fully compress the undercarrage, but having any slack will easily permit the ariframe to jerk around, and this momentary stress is much more likely to damage the airframe than constant or at least less instantaniously variable stress. if you don’t have any tension, nothing can prevent the airframe from being abruptly stopped by the rope. the arguement that the slack people seem to be focused on is rotational stresses, where this would help, as the rope will be better angled to counteract the stress of the airframe rotating, but at that point you are letting the airframe develop more momentum when it does reach the point where the rope is taught. also, if the rope were taught, the friction between the wheels and the landing gear would be greater, so that friction would prevent excessive rotation. this is better for the rope, but not for the airplane. you will have better luck varying the type of rope you use than using a slack line to tie it down. of course, if you are applying this to a glider, don’t. they are by nature less rugged, and should be stored in a safer place.

50

To help you visualize what is really going on, I would think of it as a restraint change:

In normal flight, the body is the restraint point and each wing is a cantilever that flexes concave upward as it goes toward the tips.

When restrained to the ground, the wings still flex concave upward in the wind, but the tie down points become the new restraint point.

The only way the wings flex concave downward when restrained is, as I mentioned before, if the restraints are near the tips of the wings.

51

But this opinion is only that --albeit from one w/credentials to know…–,
and leaves us no wiser as to Why … ?! --a rationale, please!
And our pressing the issue should include pointing to the choice
of line --of stretchy nylon (in perhaps a stretchy construction : hard-laid)
vs. non-stretchy polyester (and braided).

unless the ground speed of the wind over the wing of a tied-down aircraft is in excess of [35-45 knots, there will be no effective lift]

As one can know reading my posts, I spend some time at Cape May
Point, New Jersey (USA); in the past 5 years there have been two
“hurricanes” that have come up along the coast, and have thus
given me concern about a family house there --some few blocks
from water (pre-hurricane water, that is!). I recall looking over
the www.Weather.gov “3-Day History” data for several places
along the coast from where Irene landed, in North Carolina,
on northwards ; I was puzzled that --and had great concern–
Irene sustained its “hurricane” rating --sustained winds >65knots
(75mph)-- and NONE of the sites’ data showed it much even
near that level, and only a few GUSTS rose to such speeds!?
Huh?! (And i.p. I was happy to find one military? air field giving
data on a small island near the Carolina point Irene touched
land, and thought it surely would be perfect for catching the
full fury. IIRC, top gusts there were in 70s –gusts.)

((Also puzzling : weather.gov asks for , or ,
yet using the 08212 zip for Cape May Pt. gets one “15NM NE
of Lewes” --in the ocean!-- information (as though the postal
service might maybe deliver out there, to the fishes?) !? ))

I had similar searches for our recent Sandy, and the highest
GUST was in 60s, IIRC (Atlantic City, Washington D.C. --didn’t
check a NYCity-area site). Not to lessen the terrible effect
that that massive (>850mi diameter) storm has wrought,
but it is a puzzling obervation vis-a-vis “hurricane” definition.

A friend who is more intimate with such weather aspects
said something to the effect that wind speeds are measured
at upper altitudes … . (News media, OTOH, lustily grab for
the highest, most frightening figures they can spew --wind
chill, e.g., peak gust estimates, as though such effects are
commonly felt.)

So, back to Gordon’s point : it would be a significant event
to generate such winds. But, OTOH, we do feel some bursts
of wind in non-uncommon, not-occupying-vast-radar-real-estate,
thunderstorms.

–dl*

52

also in response to the winds in question
L = (1/2) d v^2 s CL
L = Lift
d = density of the air. This will change due to altitude. These values can be found in a I.C.A.O. Standard Atmosphere Table.
v = velocity of an aircraft expressed in feet per second
s = the wing area of an aircraft in square feet
CL = Coefficient of lift , which is determined by the type of airfoil and angle of attack.
note that CL and s are fixed, and d is variable, but not by much. so, lift is really just dependant on airspeed squared. seems how critical angle of attack is only dependant on airspeed because airspeed determines the direction of the relative wind, a tied down wing is not going to be stalled, and even should it be stalled, not all lift goes away in a stall. the coeficient of lift just decreases, as opposed to increasing, with aoa, and aoa doesn’t change. so although it may require winds of 35 or 45 knots to lift an airplane, there is still lift at slower wind speeds. also, above, it is noteworthy that there is extremely low air pressure in a hurricane, so the density of the air is less, so in hurricane winds the lift force is decreased by density altitude problems.

53

I’m a pilot and a sailor. Ive just come back from a ten-day glider camp, where after looking at and pondering about the various tie-down knots, I decided to do some research, which brought me to this thread.

I see there is no consensus on how to tie down an aircraft. My personal preference is to use a round-turn and two half-hitches on both ends of the tie-down rope, with the rope tight, but not excessively so. I then secure the bitter end of the rope to the standing part with another couple of half-hitches, to avoid any flogging of the underside of the wing.

A couple of observations about the discussions.

  1. Stalling speed is weight-dependent, so the stalling speed consider for a tied-down aircraft is significantly less than the normal flight stalling speeds. The ratio of stalling speeds is equal to the square root of the ratio of the weights.

A 172N at a gross weight of 2300 lb has a flaps-up stalling speed of 50 kts. An empty one (no fuel) weighs about 1400 lb, so the stalling speed would be 39 kts.

  1. The discussion about inverted and normal flight loads on the wing misses an important point, which is best illustrated by considering the glider winch-launching case. This is the critical case for wing stress calculations, since at the top of the launch, the stress from the winch cable is essentially vertical. So the wing has to generate enough lift to equal the weight of the glider plus the force applied by the cable, which in the worst case, can equal the weight of the glider.

Similarly, if the wind picks up a tied-down airplane, the wing is generating lift equal to the weight of the airplane, plus the tension in the tie-down ropes.

54

I think you’re trying to argue that pre-tensioning the ropes is bad? I’ll tell you that it doesn’t matter as far as the plane actually getting away, assuming 1) knot shaking is irrelevant and 2) we can ignoring shock loads. (let’s come back to those assumptions)

The rope (degraded by knots) can hold a certain amount of tension, and whether pre-tensioned or not, at the point the rope is about to break, the weight plus tension will equal the total lift force, or said otherwise the tension will equal the lift minus weight (let’s call lift minus weight net lift). Let’s ignore wheel/suspension compression force for now. If the lift force is strong enough to overcome the weight and supportable tension combined, then the rope will break. Period, pre-tensioned or not.

Now let’s come back to those two assumptions. If the plane lifts off the ground because the rope is slack or it stretches under tension, then the knots might see unsteady load, your plane will get josteled about, and the ropes will see shock loads as the plane jerks upwards and reaches the end of the slack. None of that sounds at all good to me, so I don’t like your plan if it were my plane.

All ropes stretch a little when tensioned, and the more they are tensioned, the more they will stretch. If the rope is pre-tensioned, then there will be no increased stress on the rope, until it stretches more. Assuming the tires are made of steel, this won’t happen until the plane lifts off the ground. The only way then to get 100% of the rope strength before getting any lift off is to pre-tension to 100% of strength. Well that’s obviously not smart either. Probably better is to a) degrade rope strength by 50% because the knots weaken it. This then should be 5x your expected worst case net lift expectations (so rope rating should be 10x worst case net lift force), then pretension at 1x worst case net lift (so 10% of rope rating). Then the plane will never lift off the ground even an inch and your rope will never come near failure. Adjust this math to suit the safety factors that you’re comfortable with (I gave you no safety factor for lift-off other than “worst case” for you to determine with whatever safety margin you want. I only included an ultimate failure safety factor)

Presumably the tires squish significantly too though. This actually doesn’t really change things much. Sure before lift, rope tension is wasted fighting tire tension, but right at the point of lift off, the tire force becomes zero and we’re right back to the same equation, net lift equals supportable rope tension. Again, the pre-tensioning conclusion works out about the same. Going without pre-tensioning sound wrong to me.

This by the way is not significantly different from the logic used for pre-tensioning bolts (which also stretch when loaded), and bolts are often pre-tensioned nearly to failure based on the same logic.

55

Tex,

Thanks for your detailed and interesting response. A lot of food for thought there.

No, I wasn’t arguing that pre-tensioned ropes are bad. I was just commenting on the fact that the wind-speed necessary to pick up an empty airplane (with slack tie-downs) is lower than the POH flaps-up stalling speed and that when the wind generates enough lift to pick up the airplane, the stress on the structure includes the airplane weight and the tension in the tie-down ropes.

I presume that this case is considered during the design of the wing structure.

Here’s a Blanik I used to fly whose tie-downs didn’t survive hurricane force winds:

http://i30.photobucket.com/albums/c309/india42/Screen%20Shot%202015-08-10%20at%2014.37.19%20_zps5lb9ibaa.png

56
the stress on the structure includes the airplane weight and the tension in the tie-down ropes.

Right, but this makes it sound as if the pre-tensioning of the lines is adding stress. An implicit part of my point is that that is only true so long the net lift is less than initial tension applied, and then that stress is only equal to the initial tension applied, which is controllable and hopefully not itself enough to damage the plane (if it was you would have broke it when tying it down). Once the lift force goes beyond that, one should not think about it as the tension adding stress, but as the lift force increasing tension, which adds stress. You cannot limit this lift force or reactive tension by adjusting initial tension (other than to limit shock loading).

Are the tie down points capable of handling point loads of the net lift of 90mph wind? You’re right, you would hope it was engineered for that. I have no idea.

57

Some further thought does say that the suspension issue is not entirely trivial and I shouldn’t have disregarded that element quite so quickly. My liftoff condition was right, but only if it gets that far. If you tie down with a force equal to max expected net lift T, and if the suspension travel is long and the rope travel is short, the ropes might actually get tensioned to nearly the total lift force, so T+W, W=weight (suspension still pushing up with force of full weight so tension must counter full lift) So this in fact could make the tension stress significantly bigger as than it needs to be.

The solution to this is to use dynamic rope, and then still tension them to 1 X the worst case net lift. Since the ropes will be very elastic, the stretch to accommodate the suspension as the plane lifts a little won’t develop much extra force in the rope. You will reach exactly the liftoff threshold with the rope tension still just barely over the max expected net lift, holding the plane on the ground, again avoiding rope breakage, and alleviating unneeded stress on the tie down points.

While this reduces excess tie-down stress and still maintains the lift-off limit, it will allow the plane to bounce a little within the suspension travel before reaching that limit.

Let’s say though you are using static ropes, then you want to decide at what point in the suspension travel you reach a rope tension of T (more accurately now L-W+S – lift, weight, suspension force–, but given the rope is pretty hard, anyway basically where does the rope run out of slack). You probably want this to be above neutral suspension loading to reduce excess suspension up-force compensation, so in fact for static rope, your strategy might be pretty good. Still more stress than dynamic rope, but less bouncing too actually. And maybe the engineer mentioned earlier knows what he’s saying.

Definitely I overlooked the importance of suspension on first try.