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Question - How much lift on a tied down RAA say 600kg aircraft with gusts or a storm.


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Example lets assume concert blocks buried into the ground that are not going anywhere. Not pegs hammered into the ground.

 

So for all the physics guys -what size rope / chains probably or how much load or strain is placed on the rope / wing tie down point with an aircraft in the open.

 

As we have all looked at pics with aircraft upside down that were tied down after a storm or heavy wind - what is the strain can be put on a RAA aircraft ropes.

Has it ever been calculated?

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If the aircraft weighs 600 kg then as a minimum your two wing tie downs would need to take at least 300 kg each to keep it on the ground. I think I read somewhere that tiedowns should be able to take 500kg.

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As a bit of a maths klutz, I just lock my brake on, tie down wing tips, tail and nose and make sure my control surfaces are secured against movement. I use those weight rated "luggage" straps with the friction clamp thingy for fast tightening/loosening and always tie off the loose end for added security & neatness.

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I will leave the tie down points on the aircraft and limits to someone else, or the mfr but on wind force :

 

assume worse cases : side wind onto flat plate of fuselage +tail :

force will be (wind squared) * 0.768 * (equiv area) Newtons

wind in meters per second. area in square meters, of course.... I dunno maybe 6 sqM at 50 m/s = 11kN

 

and if the wind gets front onto the wing, at high AoA, lift will be produced to whatever the maximum lift is at say 100 kts wind, maybe 4x the weight ? (thinking 4g Va speed at 100 kts)....like 2400kg (24kN ?) before the plane breaks ?

 

alright, lets assume 24kN is the biggest number, (we have 11kN and 24kN) assuming tie downs make with the anchors at 45 deg to horizontal, rope tension will be 34kN (or ~ 3400 kg)

 

about 1.5 cubic meters of concrete in the ground is required to hold the plane down.

 

Now, that's not what might really happen in the real world case scenario.

So that brings now in to sharp focus about where you might (or if you even could ) attach anchor points to the airplane and not tear it apart.

 

With solar panels, in some cases it is better that their bolt break and glass comes out, rather than overturn the entire structure holding them together.....especially if on top of a building.

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what is the strain can be put on a RAA aircraft ropes.

 

Let's see what the question is about.

 

It is not about what is the wind velocity that will lift an aircraft against its tie-downs. That's easy. It's the wind velocity that is equal to the stall speed of the aircraft in level flight. (Assuming the aircraft is tied down on level ground with the wind coming from head on. For those with slide rules, you can calculate the head-on wind vector of a wind blowing at an angle to the centre of gravity on the longitudinal axis of the aircraft.

 

The question is: What strain can be put on the ropes? The assumption is that the question relates to the amount of strain the ropes can accept before one of three things happens:

  1. The ropes break
  2. The ropes pull out the stakes in the ground
  3. The aircraft suffers damage at the tie-down hooks.

The answer to No. 1 depends on the ability of the ropes to resist the Stress of the lift force acting along the rope to the tie-down. If the Lift force exceed the Stress capability of the rope, the rope will break.

The answer to No 2. depends on the contact area of the stake with the ground, and the density of the ground. In some cases, the weight of additional material attached to the stake (concrete block).

The answer to No. 3 depends on the the design of the tie-down points and where they are on the located. If the tie-down points were located at the wingtips, then we would expect bending of the wing to occur when the G rating of the spar was passed. In other places (wing strut attach points, or undercarriage axle attach points the lift force would be much greater.

 

The argument is probably more theoretical than practical for those who do not live in hurricane or tornado areas. Consider that the fastest wind speed not related to tornadoes ever recorded was during the passage of Tropical Cyclone Olivia on 10 April 1996: an automatic weather station on Barrow Island, Australia, registered a maximum wind gust of 220.2 kn. During the cyclone, several extreme gusts of greater than 161 kn were recorded, with a maximum 5-minute mean speed of 95 kn. That means that a tied-down aircraft is not likely to have to deal with winds much greater than its normal cruise speed, or even grater than its Vne. And we all know that the Vne has a fudge factor adjustment.

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OME- pretty good .

 

I'll try and simplify this :

1) Find out from the manufacturer the appropriate tie down points and their limits

2) assume that for an LSA, you could see a sum of 3 tons over all ropes at highest likely windspeed (say 50 m/s)

 

In some cases (my own non aviation circles) its acceptable to have the concrete block anchors move by being dragged by the object. - IE there is a let-go force, where the anchors slide along the ground, which limits the peak load of whatever it is attached to, rather than tear the anchor out of whatever it is attached to.

 

There are several multiple screw anchor systems around. Most of those are good in shear by not in pullout / overturning moment.

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My even more summarized suggestion , based on all of above.

 

- ensure all ropes rated for at least 1000kg SWL. they'll be fine at 3x. and it is unlikely you will have all 3x on one rope

- Anchor any single anchor point should be good for 1.5 tons and any fixing device (D shackle, carabiner) >= 15kN

- do not 'over tension'. short lengths of slightly elongatable rope can assist.

 

- if there are long ropes employed, a towel or other heavy damping device over the rope, will tame the 'slingshot' if something gives way. at one end or in the middle.

 

*This is from my knowledge of radio masts and towers.... not about aircraft .*

 

Jabiru states for their COMPOSITE aircraft , up to FOUR tie down points. :

 

8.4.1 TIE-DOWN INSTRUCTIONS

The J120-C is equipped with 3 built-in tie-down points: 1 under the tail and another under each

wing at the top of the wing strut. To secure the aircraft ropes must be tied from each of these to

hard-points on the ground.

In very exposed conditions or where strong winds are predicted it is recommended that the

aircraft be secured by a fourth point at the nose leg. Pass a rope around the nose leg within the

nose leg housing and attach to a hard-point on the ground. Nose and tail ropes should not have

slack but must not be too tight.

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Answer: - much, much more than you would think possible. Several Avalon Airshows ago on the Saturday evening a wind storm went through the row of parked aircraft and inverted many. About 4 aircraft away from mine a Savannah was picked up and dumped inverted about a plane length behind its tied down position. It had 4 tie downs; each wing strut, nose and tail. I helped dismantle it on Sunday morning and one of the strut attachment plate assemblies had been pulled apart in tension. (Not rivets pulling out, but the plates torn apart). A Foxbat close by had been picked up, flown over the top of a Lancair barely touching its prop and then dropped tail down, bending the fuselage into a "J". It, too, had a severed strut attachment plate just below the spar. This plate is from memory about 38 X 6 mm 2021 flat. The break was clean, with no evidence of bending.

A Gazelle tied down 1 metre from my left wingtip was flown up and over the nose of my plane, landing inverted over the rope fence in front of the parked aircraft. It did not touch my wing.

I'll leave it to you to think about how much instantaneous lift it would have taken in these scenarios.

My aircraft was undamaged, the only explanation being that as a taildragger the wing angle of attack is above stall when the tail is down. All other aircraft were nosewheelers and the wings generated lift.

Somewhere I have photos of this event which I could scan and post if requested.

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Aircraft are NOT 600kg tied down......from 300 to 400 with fuel and gear.

Wing section defines the lift generated.

Depending on wind strength and direction, the wings can get a rocking motion happening putting "snap" loads on components. I've seen this work pegs out of the ground so personally (with a high lift wing) try to secure it to cables or blocks. I always fuel to full asap if needing to tie down to get as much weight in as possible.

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bushcaddy. sounds about right. Those numbers I computed before - they are real.... IE the LSA grade plane will need about 3 tons to hold it down at 50 ms wind.

 

I would highly recommend screw pegs, the ones you put in the ground with a T bar or hammer .

 

The light weight ones are certainly good for 500-1000kg each, depending on the soil. Wet areas you need to go deeper, but not much deeper.

 

I was tidying up a plane this arvo at YSCB and noticed the tie down kit- of tent pegs ! I almost laughed out loud considering the calcs I had done earlier in the day on this question.

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I like the cable along ground at airports. They seem to be anchored quite well, and the tension on cable pulls ropes taught with some spring if require. I don’t know if the claw set I have in cessna would help in high winds. I do have some alloy star posts and dee shackles to put in kit

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Clinton, the spring is certainly useful from a point of view of being able to tighten down to a specific tension.

example : XXX spring will have 25kg of tension when length is YY centimeters.

So, when the spring has expanded to whatever length, you have that tension. No guessing, cause we do not guess in this game...

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Clinton, the spring is certainly useful from a point of view of being able to tighten down to a specific tension.

example : XXX spring will have 25kg of tension when length is YY centimeters.

So, when the spring has expanded to whatever length, you have that tension. No guessing, cause we do not guess in this game...

I think he means the fixed horizontal cable tie down found at airports that you tie on to has the spring/flex inbuilt.

 

However I have camping ropes that I can use with the extentions springs you describe for tieing down if I choose them.

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Hi DU. yeah the cable is good if the anchors are good... my comments certainly relate to grass.

 

A screw peg about an inch diameter, up to 500mm long is good for something up to 500kg. for 500-1000kg need to go up to maybe 600-800 mm long. depends on the style. The pitch is high, so it doesnt take long to get them in. Don't use anything that is not rated... Optimising different pitch and diameter for different soils yields smaller pegs. like say, at the beach ! depends depends depends.

But a little thought and commonsense will yield a suitable outcome. The good pegs have rating for different soils.

 

like all pegs, keeping them in shear will maximise hold, so that means low angle ropes, but of course you may be limited on total footprint.

 

For permanent installs there are plates you can bury with multiple rods that are driven with a small electric jackhammer type tool.

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Hi Guys, If I could add my small piece. I am no mathamatician, but, 20 years ago one of my aircraft tied down with 1/2 inch nylon rope from the top of each wing strutt anchored to two 12" square concrete blocks, 18"s long buried into the ground, pulled those two blocks clean out of the ground and proceeded to cartwheel down the strip 30 mtrs, crossing a fence and hitting a clump of trees, smashing four feet off one wing, breaking the nose leg, and one blade of the prop, all with the two blocks still attached. No one was on site during this event, so we dont have any idea of wind speed, but must have had a lot of force.

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Hi dt.

 

My calcs are one thing, but your example is excellent and adds weight . (clearly not enough, pun intended) .

 

well yeah my calcs say 1.5 cu meters of concrete total. So I am unsurprised.....

 

We would have to get the advice of the aircraft designer as to what the limits are and where the max stress points are for a particular design.

 

assuming the struts, wing, cabin is a perfect braced box, (it wont be, there will be some deflection) it most likely comes down to the max moment applied at the wing-strut join. My feeling is that because there is less area beyond the strut, the moment on the wing near the strut attachment wont be too high.

 

But it does represent a high G loading event, of sorts, if the big wind came through and the blocks didnt move.

 

In that case, you would want some telltale of the peak tension on the rope , so that a judgement could be made of whether a structural inspection was required....

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However well you tie your aircraft down won't help when you get a major storm hit. The wind gusts can come from any direction so having the aircraft pointed into the prevailing wind won't help at all. Tornadoes often accompany major storms so if your aircraft is hit by one of those either the tie down points will be ripped from the aircraft, the ropes will break or the anchor will be ripped from the ground. Then of course the aircraft will be thrown somewhere and be considerably damaged or written off. Hangars will suffer the same fate. This happened at Archerfield a few years ago. Most aircraft outside were severely damaged and some in hangars as well. The only way for an aircraft to escape this sort of weather event is for it not to be there at the time.

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As a bit of a maths klutz, I just lock my brake on, tie down wing tips, tail and nose and make sure my control surfaces are secured against movement. I use those weight rated "luggage" straps with the friction clamp thingy for fast tightening/loosening and always tie off the loose end for added security & neatness.

Suggest you get a loop sewn into the ends of those straps and put the ground end through that loop before anchoring to the ground. That way if there is any slipage it will not let go.

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I recall a lift destroyer that was round? section (about6") rubber Foam? or such and was tied along the top of each wing length wise. Also put gust locks on elevators and rudder. Nev

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Tied downs need special attention to get the best results. If possible have the wing tie down ropes at about 45 degree angle so that the aircrft is not pulling the anchor out of the ground, it is pulling partially sideways. Also have it about 45 degrees to the wing cord so that they are anchored against side winds. By having a 45 degree angle tension on the ground point it is less likely to be pulled out of the ground. There are specifications for concrete tie downs so that they will stay put. Have the tail tie down in line with the fuselage about 1-2 meters behind the aircraft. That will allow the tail to lift off the ground without putting too much strain on the tie down point. I use straps for tie down because they are easily adjusted and they have some give in them (see my previous comment). There should be no tension on the tie downs nor should there be any slack. Slack will allow the aircraft to lift and suddenly come to a halt when the strap goes into tension. That increases the load on the ground anchor point. Also, chocks will not help in a high wind. The aircraft will lift and move and could possible jump the chock.

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Kgwilson is correct on the damage that can be inflicted by tornado-like events embedded in storms - which are a totally different event, to just constant strong winds.

Often, "mini-tornadoes" accompany sudden and violent storms, which are usually associated with the approach, or passing, of strong cold fronts. Mini-tornadoes can also suddenly develop from severe thunderstorm conditions in Summer.

 

I have seen the result of many mini-tornado events. They are usually confined to narrow strips, generally ranging from a couple of hundred metres wide, to half a kilometre wide, and rarely more than about 2 or 3 kms long.

I have seen numbers of substantial farm sheds, and farm machinery, utterly destroyed by these events. I have seen violent destruction of heavy native vegetation, including large trees.

 

The worst mini-tornado result I have ever seen, was South of Higginsville, W.A. in July 1975. Whilst living on my gold mine at Higginsville, I endured a very frightening July night with a very strong and violent cold front passing through, with sustained winds in the region of 80-110kmh, all night.

In the morning, I drove down the Coolgardie-Norseman Road to Norseman, and about 10kms S of Higginsville, I encountered the result of a mini-tornado that had crossed the highway - that was more like an American, full-size tornado.

The area was native W.A. Goldfields vegetation, with substantial amounts of trees - Gimlet, Goldfields Blackbutt, and other large trees such as Salmon Gums, up to 20-25M high.

 

A mini-tornado had developed in the front during the night, and it had cut a path of destruction through the trees and native vegetation, that resembled a scene similar to two bulldozers dragging a heavy clearing chain through the countryside.

Large trees were completely twisted off at heights of a metre to two metres off the ground, and virtually all the vegetation was flattened over a width of about 400-500 metres, and for a total distance of about 2 kms.

 

I have never ever encountered such frightening destruction and the power in a tornado-like event, before or since - but this destructive event indicated what is possible when the right weather conditions prevail.

As KGW says, you need to be alert to such conditions possibly developing, and ensure you and your aircraft are well away from these particularly destructive event/s.

 

A farmer friends machinery shed, after a mini-tornado hit -

 

 

SHED-Collapse-Noble.jpg.f135556de4837a827116a5e1e46d01a7.jpg

 

And of course, if you want the evidence of what a full-size tornado can do to aircraft, check out the American tornado that destroyed almost 2/3rds of the U.S.'s SAC heavy bomber fleet of Convair B-36 Peacemakers, at Carswell AFB in 1952.

The B-36's had a wingspan of 70M and were physically bigger than B747's - but the tornado threw dozens of them into piles like kids toys.

 

A total of 83 of these massive bombers were damaged in the tornado, around 30 of them suffering severe damage that necessitated months of major repairs.

The USAAF no longer risk storm damage to valuable aircraft, they fly them to areas away from the predicted storm paths.

 

https://theaviationgeekclub.com/remembering-the-1952-carswell-afb-tornado-that-damaged-two-thirds-of-sacs-b-36-force/

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We had some pretty strong winds last week in Melbourne - many large gum trees down, power outages etc. The media reported gusts of 124km/hr (~67 knots).

 

Our local airfield (Lilydale) has many GA aircraft on tiedowns (including a few ligher ones like C150's, Tomahawk etc), but no RAAus registered aircraft on tiedowns that I have ever noticed... even "cheap" Jabirus are all in (expensive-to-rent) hangers. I have seen a couple of older Jabiru J120's on tiedowns and covered up at Coldstream a few km's away, curious to know how they faired in the recent storms.

 

Has anyone had good experiences keeping RAAus registered aircraft outdoors in windy locales?

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Answer: - much, much more than you would think possible. Several Avalon Airshows ago on the Saturday evening a wind storm went through the row of parked aircraft and inverted many. About 4 aircraft away from mine a Savannah was picked up and dumped inverted about a plane length behind its tied down position. It had 4 tie downs; each wing strut, nose and tail. I helped dismantle it on Sunday morning and one of the strut attachment plate assemblies had been pulled apart in tension. (Not rivets pulling out, but the plates torn apart). A Foxbat close by had been picked up, flown over the top of a Lancair barely touching its prop and then dropped tail down, bending the fuselage into a "J". It, too, had a severed strut attachment plate just below the spar. This plate is from memory about 38 X 6 mm 2021 flat. The break was clean, with no evidence of bending.

A Gazelle tied down 1 metre from my left wingtip was flown up and over the nose of my plane, landing inverted over the rope fence in front of the parked aircraft. It did not touch my wing.

I'll leave it to you to think about how much instantaneous lift it would have taken in these scenarios.

My aircraft was undamaged, the only explanation being that as a taildragger the wing angle of attack is above stall when the tail is down. All other aircraft were nosewheelers and the wings generated lift.

Somewhere I have photos of this event which I could scan and post if requested.

Reality bites eh. i fly a storch muster and had a very severe wind storm event when parked up at my home field overnight while i was away the aircraft was wheel chocked and tied down into wind. The pilots who were present at the time were not game to try and relocate her because of the conditions. On arrival next morning i found her standding on tippy toes with the landing gear legs at almost full extension as when flying, the chocks (one piece fwd and aft ) were to each side of the wheels which had lifted gradually up and inboard. While counting myself very lucky at the time. On reviewing what happened i am convinced that your assesment of the aircrafts attitude being held under the stall angle of attack is the real reason it survived without damage. Ps their were no other aircraft tied down at that time,

cheers Mick

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Some good information given above.

I would also recoment using nylon rope of the correct strength. Nylon ropes have some give and stretch properties that avoid shock loads and last well in UV light.

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