Question from a helicopter pilot. .

[boing]

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

 

[Danger]

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.

 

[boing]

Hiya
I will add that I am a PhD student trying to do my day job, study and stay married . . .
B

 

[boing]

Hello?

 

[Cyrus]

Ta-da.

http://img294.imageshack.us/my.php?image=picfp8.png

G'day.

 

[boing]

Fantastic answer Cyrus
But what do I tell the fork-lift driver?
B

 

[Cyrus]

Dont strap the hell out of it.

 

[ctjen]

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

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

 

[boing]

But my Engineerng Manager and my Licensing Authority want an answer. "Tell him not to pull so hard" will not cut it. B

 

[Cyrus]

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 what's 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 don't 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 that's 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.

 

[Danger]

Do you have specifically designed strong points to tie the straps to?

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).

 

[boing]

Hi Chris. This is the issue - are the tiedown ponts adequate? How do we calculate this? B

 

[Danger]

If you have actual tie-down hardpoints, that information should be in your operations manual.

 

[boing]

I have been doing this for about 20 years. I was hoping to get a better answer.

 

[Danger]

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.)

 

[boing]

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.

 

[Danger]

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.

 

[Astronuc]

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)?

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?

 

[ctjen]

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.

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

 

[Cyrus]

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.

 

[Andy Resnick]

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

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.

 

[NoTime]

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.

 

[boing]

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
Boing

 

[boing]

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.

Boing

 

[yuiop]

Hi,

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.

 

[Danger]

Pilot or not, Kev, that was a good post with a lot of common sense.
I should point out here that I am not a chopper pilot (although I wouldn't mind learning). Strictly fixed-wing for me so far.

 

[mordechai9]

I'd just like to point out that I believe cyrus's free-body-diagram drawing is incorrect. Excuse me if I'm wrong, but:

Where's the normal force (the force of the floor pushing against the box)? Also, the sum of the forces equals zero in that frame. These (important) items appear to be missing from the analysis.

 

[Cyrus]

You are correct. It is missing. If you want, you can include it, it will equal the sum of these forces. So, Fnet is the normal force.

Now, I could draw the normal forces on there, but then that would make things messy and provide no insight to the problem at hand.

I was a bit sloppy.

 

[mordechai9]

You still did a good job man I don't mean to be a dick. Sorry I'm too lazy to post my own.

 

[Crazy Tosser]

I have been doing this for about 20 years. I was hoping to get a better answer.

Ive been flying for over 9000 years, first being on a chinese paper dragon

Take an image of the cabin and show it to us (preferrably with some cargo tied up in it)

 

[boing]

Hello everyone

At the risk of generating more vitriol, I am still struggling with this and I have managed to get more information from the Manufacturer. Can anyone (Moderator?) tell me how to add attachments? A proper Scientific solution to the everyday problem would be great.

Thanks in advance.

Boing

 

[Topher925]

Under the "Additional Options" section when you make a post, there is a button called "Manage Attachments". Click it and the rest is self explanatory.

In order to give you a good answer we will need the following:

1. geometry of the cabin
2. mass of the payload
3. geometry of the straps
4. tension in the straps
5. geometry of the load

This is only to determine the stress placed on the aircraft cabin. In order to determine if it is safe, well that's a much bigger problem.

 

[nucleus]

There is information on this in AC 43.13-1B Chapter 12. Available at
http://rgl.faa.gov/Regulatory_and_G...cular.nsf/0/E533BB05389C90E486256A54006E47B2?

If you cannot determine what the strength the strength of fittings provided by the manufacture you will have to do a test as outlined in the AC. An AC is not law but it is a good place to start if there is nothing in your structural manual.

It does not appear to me that you are familiar with this subject. There are many rules, such all material to be fire resistance, that apply. Personally I would recommend you obtain the services of an engineer familiar with it, otherwise you are leaving yourself open to legal suits.

 

[Gear300]

If the freight experiences 4g of acceleration, then the force is 4gm. The component of tension perpendicular to the floor along the straps is actually in the opposite direction of the force the freight exerts on the floor and should sum up to 4gm (as a vector component). The tension will also have a horizontal component. You'll have to take into account the friction with the floor, the direction of flight, and the angle of elevation for this. The horizontal force will exert, I'm thinking, shear stress, which you can cover up with good flooring. For a better answer, you might want to ask in the engineering thread.