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Question from a helicopter pilot . . .

  1. Apr 15, 2008 #1
    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
  2. jcsd
  3. Apr 15, 2008 #2


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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
    I will add that I am a PhD student trying to do my day job, study and stay married . . .
  5. Apr 15, 2008 #4
  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?
  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?

  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


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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


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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


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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


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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


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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).

  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.
  22. Apr 16, 2008 #21

    Andy Resnick

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    I think part of the difficulty people are having in giving 'useful' answers is that the original question is unclear. For example:

    What is a 'daN'? I do not understand that unit.

    How exactly is the freight stapped down? How many straps are involved? What is the geometry of the tie-down? As others mention, is it simply attached to the floor via, say, an eyebolt? Are there any reinforced mounts that are involved? This detail is crucial, because stress is a force *per area*, and concentrating the load at a point contact is vary different than spreading it out over some area. Also the mounting scheme could provide different lateral loads vs. a pulling load.

    Lastly, there is no provided specification (other than 1500 daN/m^2, which I don't understand). It may be difficult to locate, but safe load limits are always specified in writing somewhere.
  23. Apr 16, 2008 #22


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    To keep the box from sliding then
    For a configuration as shown in Cyrus's image then this applies.
    If you use floor clamps at the bottom of the box then you only need enough tension to keep the strap from stretching enough to allow the box to jump the height of the clamp.
    This assumes that the box itself is strong enough to retain its contents under a 4g force.
  24. Apr 16, 2008 #23
    Thanks to everyone for their time, and I apologise if I offended anyone with my clumsiness.
    I will try to draft a more sensible question and provide more detail, but information from the helicopter manufacturer on this subject is sparse, which is what prompted my question here. At the end of the day what I need to come up with is a "rule-of-thumb" that can be posted on a wall and every loader can be expected to understand.
    At the risk of over-using metaphors, it feels like I am reinventing the wheel, but someone has shifted the goalposts, so what we have been doing "forever" is now under scrutiny and I have to defend it (or not).
    There is no useful information in the Flight Manual or the various Maintenance Manuals and whatever I learn from here will eventually be written into the Operations Manual.
    I will gather the details I have and re-post.
    Thanks again
  25. Apr 16, 2008 #24
    And another thing . . . (please remember I am not a Scientist)

    Am I wrong to use daN (decanewtons) as a unit of Force? My understanding is that it is an accepted SI unit which replaced kgf, but some respondents have suggested otherwise (e.g. "I do not understand that unit")

    Anything to educate.

  26. Apr 16, 2008 #25

    Might I suggest you use the seat belt mounting points as the starting point in your calculations? They are designed to restrain the weight of an average human in the event of a crash. If the seat belt attachment points are mounted on the seats and the seats have removed to accommodate the cargo, then the seat mounting points will have been designed to take the weight of a (more than?) average passenger in the event of crash. After all, loose passengers and seats will kill you just as surely as loose cargo. The cargo should be secured by as many seat mounting points as would be required to secure the equivalent weight in passengers. You probably do that anyway to ensure correct load distribution for COG considerations anyway. Information on the load limits of seat belts, seat belt mounting points and seat mounting points should be easy to obtain and probably specified by law.

    Another consideration is that strapping directly to the floor is probably putting unnecessary undue stress on the floor when you are trying mainly to prevent lateral motion of the cargo. Consideration should be given to using cargo netting and securing the load laterally from the front and back. That would reduce the amount of vertical tension required in the vertical straps. If you use the seat belt or seat mounting points you can probably obtain exact information on the load limits that will provide exact data to put in your report. I am not a pilot so if all this is completely wrong...sorry.
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