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Question from a helicopter pilot . . .

  1. Apr 15, 2008 #1
    Hi.
    Please excuse me if this is just too banal for this site, but we have a long-standing "real-world" problem that we need answers for and I am smart enough to know that I need to ask others ( = you guys ) for help.
    We normally carry up to 19 passengers but, during training and occasionally for our Customers we have to carry freight in the passenger cabin. There is a load limit to our floor, and we have to secure whatever freight we carry against decelerative forces so that we don't kill people in the aircraft in the event of a crash. While it is simple to make sure that we don't exceed the floor loading limits (in this case 1500 daN per square metre) it is less easy to figure out how much extra stress we place on the floor by tensioning the straps which tie the freight down. An important detail is that we have to secure the freight against 4g forwards acceleration, and there are other criteria.
    I can provide more details to help with the calculation, but can anyone point me in the right direction to solve for this (or, better yet, do the sums for me)?
    Thanks in advance
    BOING
     
  2. jcsd
  3. Apr 15, 2008 #2

    Danger

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    Welcome to PF, Boing.
    It definitely is not a banal question. I'm not the one to answer it, but you're in the right place. Hang tight and some engineers will be with you shortly.
     
  4. Apr 15, 2008 #3
    Hiya
    I will add that I am a PhD student trying to do my day job, study and stay married . . .
    B
     
  5. Apr 15, 2008 #4
    Hello?
     
  6. Apr 15, 2008 #5
    Last edited: Apr 15, 2008
  7. Apr 15, 2008 #6
    Fantastic answer Cyrus
    But what do I tell the fork-lift driver?
    B
     
  8. Apr 15, 2008 #7
    Dont strap the hell out of it.
     
  9. Apr 15, 2008 #8
    Do you have specifically designed strong points to tie the straps to? If so do they have a rating? If not,to what are you securing the straps?

    Chris
     
  10. Apr 15, 2008 #9
    But my Engineerng Manager and my Licensing Authority want an answer. "Tell him not to pull so hard" will not cut it. B
     
  11. Apr 15, 2008 #10
    Ok, how heavy are the boxes?

    Why not get a bathroom scale and put a box on it and start strapping down. See how much the scale increases in weight to figure out how much the tension is increasing. Simple, cheap, easy. You can also calculate the tension this way. At least it gives you an 'idea' of whats going on. And you can see that x' clicks of tension on the straps increases the 'weight' by 'y' - much.

    Try it with different size boxes to see if x' clicks always increase the tension the same amount given any box shape or not. Then you can say I have, I dont know, 10 boxes, and each box will require 5 clicks. Then sum up the weight of the boxes plus that of the tension, and leave yourself a safety factor of 20%.

    Hint: if you screw up, the floor of your helicopter will start smiling back at you from the bending moment. It will become wavey all over the place. I am also going to assume the floor serves as a structual member of the helicopter because they have to cut weight every possible way in the design. I would NOT continously load the helicopter without knowing what the weight is. If the floor starts to deform, it can compromise the integrity of the structure.

    I would assume there is very little weight on the floor during flight due to the components above the helicopter, becasue thats being supported by the rotor. But when you land, the weight of everything above the helicopter will now be supported by the floor. So if you land and the floor folds like an accordion, I'd back off on the weight.
     
    Last edited: Apr 15, 2008
  12. Apr 15, 2008 #11

    Danger

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    I suspect not, since this is going in the passenger cabin, unless the seat hardpoints are being used (which is what I would first think of doing in that situation).
     
  13. Apr 15, 2008 #12
    Hi Chris. This is the issue - are the tiedown ponts adequate? How do we calculate this? B
     
  14. Apr 15, 2008 #13

    Danger

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    If you have actual tie-down hardpoints, that information should be in your operations manual.
     
  15. Apr 15, 2008 #14
    I have been doing this for about 20 years. I was hoping to get a better answer.
     
  16. Apr 15, 2008 #15

    Danger

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    There's no need to insult people who are trying to help you, particularly when you have provided very little information. You still haven't stated whether or not you have tie-down hardpoints, or are replacing the seats with straps bolted to the mounting bosses, or if you're just drilling holes in the floor. If you do have hardpoints, then your manual sure as hell should state what their capacity is.
    And I wouldn't think twice about the load limit; it's that -4 g tear strength that you need to worry about.
    (By the way, I started flying 33 years ago, so don't try to come off as the aged veteran.)
     
  17. Apr 15, 2008 #16
    Hello to everyone
    I would like to say that I do not wish to insult anyone. My original question was about how to calculate the extra stress on a floor as a result of the restraining straps.
    I have provided little information because there is no point, at this stage. That is all.
    "Danger", I have no interest in a pi$$ing contest, but I have been in continuous employment as a pilot for more than 30 years, just 20 in my current job.
     
  18. Apr 15, 2008 #17

    Danger

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    Alright, let's just call a truce.
    Still, though, there's no way to know the effect upon the floor unless we know whether or not hardpoints are involved. It makes a huge difference to the distribution of stresses.
     
  19. Apr 15, 2008 #18

    Astronuc

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    Hello Boing, it's a bit difficult to answer the question, but one would have to know the tension in the holddown straps, the number of holddowns, and the angle with respect to the vertical with the floor.

    Is there a procedure that your folks follow, and then what does that specify about tensioning? If the process involves different people or different geometries each time, then there could be a substantial variation in the tensions. One could estimate the maximum tension applied, but then one would have to measure the pull that each person involved could manage - and then there would be the issue of consistency and geometry.

    I think the person doing the tensioning is a key variable. What load can that person or persons apply?

    I think Cyrus gave a reasonable answer in terms of measuring the tension.

    Have you checked with the helicopter manufacturer with the respect to the load limit on the tie downs? Are there specs on the strapping?
     
    Last edited: Apr 15, 2008
  20. Apr 15, 2008 #19
    A single strap tied to the floor at each end will pull the item placed between the tie points against the floor with a force that is equal to twice the tension force in the strap. The load on the floor underneath the item will be distributed according to the area of contact (force/area=stress).

    Chris
     
  21. Apr 16, 2008 #20
    You really need to open up your POH and see what it says. If its made to strap things down inside, it probably tells you the maximum weight that can be strapped down at each station. If it does not say this, then you are probably rigging things in a way the helicopter was not deisgned for.
     
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