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Thread: Longeron tube thicknesses

  1. #1

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    Longeron tube thicknesses

    I've been doing a bit of research lately, exploring my options for a 2-4 place aircraft that would go on wheels, skis and (potentially) floats. I have Solidworks and have been using it now for several years, so I tend to run these types of things through it for study. So here's my question: What do you guys tend to use for lower longeron thickness for your PA-12/-12/-18 builds or re-builds?

    I have the Super Cub drawing USB stick, and also plans for the Wag Aero Sport Trainer and Sportsman 2+2 aircraft, and have notice that the longeron tubes tend to be 3/4 x 0.035" in size. So is that thick enough for float use, or would you go with the 0.049" wall thickness on an E-AB aircraft?

    For reference, Solidworks tells me this:

    3/4 x 0.035":
    # Per foot
    Density = 0.284 pounds per cubic inch
    Mass = 0.268 pounds
    Volume = 0.943 cubic inches
    Surface area = 54.067 square inches

    # 18' longeron
    Density = 0.284 pounds per cubic inch
    Mass = 4.816 pounds
    Volume = 16.982 cubic inches
    Surface area = 970.532 square inches


    3/4 x 0.049":
    # Per foot
    Density = 0.284 pounds per cubic inch
    Mass = 0.367 pounds
    Volume = 1.295 cubic inches
    Surface area = 53.070 square inches

    # 18' longeron
    Density = 0.284 pounds per cubic inch
    Mass = 6.610 pounds
    Volume = 23.309 cubic inches
    Surface area = 951.591 square inches


    7/8 x 0.035":
    # Per foot
    Density = 0.284 pounds per cubic inch
    Mass = 0.314 pounds
    Volume = 1.108 cubic inches
    Surface area = 63.519 square inches

    # 18' longeron
    Density = 0.284 pounds per cubic inch
    Mass = 5.658 pounds
    Volume = 19.950 cubic inches
    Surface area = 1140.206 square inches


    So it seems then that there's a ~40% increase in weight by going with the thicker-walled tubing, which translates to about 3.6 pounds more weight for two 18' lower longerons (for example). An alternative to that weight penalty would be to use short sections of inner sleeve reinforcement(s) only where needed.

    I've looked around in the forum archives a bit, but didn't find a whole lot of information on this question, so I thought I'd just start a thread. Any input would be appreciated.

    Thanks!

    TB

    EDIT: Added the 7/8 x 0.035" tubing mass data from Solidworks. Compared to the 3/4 x 0.035" tubing, this represents only a 17.4% increase in weight. So it's a definite improvement compared to going to a thicker-walled 3/4" tube.
    Last edited by tcbetka; 10-10-2019 at 11:56 AM. Reason: Added 7/8 x 0.035" tube mass data

  2. #2
    Mauleguy's Avatar
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    What I was told by an engineer once when my gear failed and I wanted stronger gear was to increase the diameter and leave the wall thickness the same. I went from 1 1/4" to 1 1/2" and have not failed a landing gear since. Some company's like Maule beef up the fuselage by doubling just a few key tubes in the fuselage and add a sheet metal plate below the large rear baggage door. I think you would need to know exactly where the stress is on floats and beef up those areas.
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  3. #3

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    Quote Originally Posted by Mauleguy View Post
    What I was told by an engineer once when my gear failed and I wanted stronger gear was to increase the diameter and leave the wall thickness the same. I went from 1 1/4" to 1 1/2" and have not failed a landing gear since. Some company's like Maule beef up the fuselage by doubling just a few key tubes in the fuselage and add a sheet metal plate below the large rear baggage door. I think you would need to know exactly where the stress is on floats and beef up those areas.
    That's a great idea. I thought of modeling the 7/8" tubing, but then didn't do it...so thanks for reminding me. I've edited my initial post to include mass data on the larger tubing.

    TB

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    On my J4 lookalike project I went up to 7/8 longerons and most diagonals went up a diameter.
    The bending strength and especially the compression/ buckling resistance is many times stronger with a diameter increase when compared to a wall thickness increase.
    Loaded areas as where loaded fitting, gear, ski, floats etc do a wall thickness increase as well. Tubes at the gear mounts are 1"x.095 wall for a short distance, far greater than you will find most anyone else using.
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  5. #5

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    With all things aviation, everything is a compromise. Weight is the enemy! Before upsizing anything, look at the history of that model airplane. If there have been failures, then increasing strength may be warranted, but when you do that, you are moving stresses to somewhere else. Most tubes in a fuselage have significant margins given the truss structure. Local reinforcements may be a good idea in high stress areas, but aside from gear and float fittings there really aren’t many areas that need reinforcement.


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    Right, that's pretty much the whole point of this thread...to find out if folks have found it necessary to increase the thickness of the lower longerons, due to putting the thing on floats. It may not be necessary, and there's no guarantee that I would do it. I haven't yet decided on which model aircraft to go with, so I just thought I would ask the question in a general fashion here in the forum. I've found one or two threads here in the forum archives, but there wasn't really much participation--and there was no real consensus on an outcome. But the threads I found weren't really specific to float applications, so I decided to create a thread specific to that purpose.

    TB

  7. #7
    www.SkupTech.com mike mcs repair's Avatar
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    .035" is fine...

    the ONLY place it "stretches" is the last few feet of the fuselage lower longerons, after MANY THOUSANDS OF HOURS OF OFF AIRPORT USE(which will not even be an issue on floats!)

    keep it simple and light... don't dream up problems that don't exist...

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    Good to know you've not seen any problems. I guess my thinking was that the slightly thicker wall thickness of the 0.049" longerons would mean greater resistance to wall compression and deformation. Since the float fittings are going to be welded on at a cluster, I wouldn't think the great resistance to bending deformation of the larger diameter tubing is really needed. I was wondering how significant the stresses trying to cave in the walls would be, right at the cluster.

    A week or two ago I ran across someone's site that was advertising PA-18 fuselages, and they mentioned that they used 3/4 x 0.049" lower longerons. I wish I could find it again now, but I can't seem to do so. But it made sense to me at the time I saw it, as an increased resistance to bending deflection in the longerons didn't seem to be as big of an issue as resistance to denting (wall compression) caused by the concentration of forces applied by the float fitting(s). Basically, the cluster prevents the longeron from deforming (flexing); but essentially predisposes it to "denting" at the float attach point.

    Anyway, I'd like to see some structural test data to shed light on that one way or the other. I wouldn't make any changes before investigating it further.

    Thanks for the posts guys.

    TB

  9. #9
    www.SkupTech.com mike mcs repair's Avatar
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    other than a BOLT ON float fitting, most are WELDED ON to the SIDE AND BOTTOM tubes... and a little to the longerons....

    you'd have to do some drastic damage to floats to bend the fuselage I bet... can't think of any I've seen

    concentrate on making it LIGHTER.... not heavier...

    so unless you have knowledge of an area that deforms in NORMAL SERVICE(not a crash/accident) don't "fix it"

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    I think you may have missed the part where I asked if there *was* a problem that requires a fix. I only mentioned the things I did as a means of providing additional context to my question. I’ve been an A&P since 1988, so I do understand the basics of aircraft design and construction. I’ve just never put floats on a tubed fuselage before, so I wanted to hear the thoughts of others in this regards. And like I said, there is definitely someone making and selling fuselage kits the feature 0.049” thick lower longerons. I’ve seen the details on their site myself—I just don’t remember which company it was.

    Anyway, thanks again for the discussion.

    TB

  11. #11

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    For what I see there is no direct problem with the longerons that you are working with. But for the local loads where the aft float fittings would be added should you choose to reinforce the area I would add a doubler along a short section of the lower longeron. I would not replace the full longeron and I would not cut it to insert an internal doubler.
    A 6ish" long .049 or .058 half diameter doubler with the ends tapered down to reduce the stress concentration would greatly increase the tearout loading for welded on brackets.
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  12. #12

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    Thanks for the tip. I'm going to go visit Javron next week, and will ask Jay for his thoughts as well. There are lots and lots of fabric-covered aircraft in the world, with lots of them on floats. So that tells me there likely isn't a problem, but since I couldn't find much about it in the forum archive, I thought I would just ask...

    TB

  13. #13
    skywagon8a's Avatar
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    Quote Originally Posted by tcbetka View Post
    ..EDIT: Added the 7/8 x 0.035" tubing mass data from Solidworks. Compared to the 3/4 x 0.035" tubing, this represents only a 17.4% increase in weight. So it's a definite improvement compared to going to a thicker-walled 3/4" tube.
    Only 17.4% weight increase will be roughly 17 pounds which will forever be part of the empty weight on that airplane. Not a good idea unless empty weight is not important to you.

    Quote Originally Posted by tcbetka View Post
    ..I’ve just never put floats on a tubed fuselage before, so I wanted to hear the thoughts of others in this regards.
    Take a look at pictures of almost any plane of any type of construction which is fitted with floats. They will have three struts connecting the floats to the fuselage. All three struts attach to the fuselage at a cluster of tubing or a concentrated structural location. There will be two struts which are attached to the floats where the two spreader bars are also attached to the floats. These two struts meet at the fuselage connecting and attaching at the main wheeled landing gear location. These two struts absorb the bulk of the landing loads, transferring these loads from the fuselage at the exact same location as the wheeled landing gear. The third float strut attaches to the fuselage at a tubing cluster which has vertical tubes connecting elsewhere in the fuselage. In the case of other types of construction there will be bulkheads or other types of concentrated structure at this attachment location for distributing the loads.
    The two struts carry the bulk of the landing loads of the airplane into the floats.
    The third strut fixes the angle which the floats are aligned with the airplane and carries a small portion of the landing loads into the floats.
    The bulk of the landing loads passes from the airplane through it's main wheeled landing gear location into the floats.
    Notice in all of the installations that the diagonal strut joins the fuselage at a main wheeled gear attach point. This diagonal strut attaches to the rear spreader bar location of the floats for airplanes which have tail wheels. This diagonal strut attaches to the front spreader bar location of the floats for airplanes which have nose wheels.

    On type certificated airplanes it is the nose wheel planes which have their aft fuselages reinforced for float installations. The ones which have tail wheels are considered strong enough.
    N1PA
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  14. #14

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    Um...huh? Take a look at the data I posted from Solidworks:

    3/4 x 0.035":
    # Per foot
    Density = 0.284 pounds per cubic inch
    Mass = 0.268 pounds
    Volume = 0.943 cubic inches
    Surface area = 54.067 square inches

    # 18' longeron
    Density = 0.284 pounds per cubic inch
    Mass = 4.816 pounds
    Volume = 16.982 cubic inches
    Surface area = 970.532 square inches


    3/4 x 0.049":
    # Per foot
    Density = 0.284 pounds per cubic inch
    Mass = 0.367 pounds
    Volume = 1.295 cubic inches
    Surface area = 53.070 square inches

    # 18' longeron
    Density = 0.284 pounds per cubic inch
    Mass = 6.610 pounds
    Volume = 23.309 cubic inches
    Surface area = 951.591 square inches


    So if a guy would *only* replace the two lower 0.035" thick longerons with those of 0.049" thickness, it's only a difference of about 3.6 pounds.

    2 * (6.610 - 4.816) = 3.588

    Therefore I don't follow your math here--where are you getting the 17 pound number?

    TB

  15. #15
    Steve Pierce's Avatar
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    I don't see a need for larger bottom longerons. Later fuselages have a few more diagonals where I have seen people bend the tail and the Atlee Dodge H or Univair X in the tail has fixed that.
    Steve Pierce

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  16. #16
    skywagon8a's Avatar
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    Quote Originally Posted by tcbetka View Post
    Um...huh? Take a look at the data I posted from Solidworks:

    Therefore I don't follow your math here--where are you getting the 17 pound number?

    TB
    I was thinking the whole fuselage, sorry.

    Take a look at the rear fittings of a Citabria. They are a 6" long piece of 1/2" aluminum which is held to the bottom longerons at a cluster with only two thin stainless steel straps and 1/4" bolts. Not nearly as beefy looking as the weld on fittings which are being done on Cubs.
    N1PA
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  17. #17

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    Quote Originally Posted by skywagon8a View Post
    I was thinking the whole fuselage, sorry.

    Take a look at the rear fittings of a Citabria. They are a 6" long piece of 1/2" aluminum which is held to the bottom longerons at a cluster with only two thin stainless steel straps and 1/4" bolts. Not nearly as beefy looking as the weld on fittings which are being done on Cubs.
    Ah, that makes sense...no sweat.

    I don't know anyone with a Citabria on floats--in fact we don't have too many float-equipped aircraft in the Green Bay area, not that I've seen anyway. There might be one or two I see in a year, just flying by. I have helped remove/install floats on the C-180 I used for my float rating, back in 1988, and I do remember seeing the fittings installed at bulkheads...which of course makes sense. However I've never really seen (close-up) the weld-on fittings on a tubed fuselage. Someone mentioned in the thread that the fittings basically have "fingers" (my interpretation of their post) that distribute the load to other tubes, besides the longeron. I was thinking that the loads were being applied more to *only* the longeron--so if it is indeed spread across other tubes at the cluster, then the potential for crush deformation (ie; denting) the longeron wouldn't be as significant.

    At least I think that's the take-home message here.

    TB

  18. #18
    skywagon8a's Avatar
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    The various Piper tube planes fitting is a thick aluminum plate which is bent to conform to the outside of the longeron. This thick plate serves as a cushion distributing the compression loads along the longeron eliminating any need for increased wall thickness. It has added pieces which are used to capture the upper end of the strut holding it in line with the center of the tubing cluster. This aligns the load with the other tubes in that cluster thus distributing those loads throughout the fuselage. This original style fitting is held to the longeron and vertical tube with three 3/16" U bolts. These bolts are only there to keep the strut aligned with the center of the longeron and to keep the floats from falling off the plane in flight.
    The weld on float fitting is an aftermarket design which simplifies the addition of floats at a later date. This type of fitting distributes the loads into the walls of all of the tubes which make up the cluster.

    To sum up what I've said, there is no need to use thicker walled tubing for this purpose.
    N1PA
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  19. #19

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    Quote Originally Posted by skywagon8a View Post
    The various Piper tube planes fitting is a thick aluminum plate which is bent to conform to the outside of the longeron. This thick plate serves as a cushion distributing the compression loads along the longeron eliminating any need for increased wall thickness. It has added pieces which are used to capture the upper end of the strut holding it in line with the center of the tubing cluster. This aligns the load with the other tubes in that cluster thus distributing those loads throughout the fuselage. This original style fitting is held to the longeron and vertical tube with three 3/16" U bolts. These bolts are only there to keep the strut aligned with the center of the longeron and to keep the floats from falling off the plane in flight.
    The weld on float fitting is an aftermarket design which simplifies the addition of floats at a later date. This type of fitting distributes the loads into the walls of all of the tubes which make up the cluster.

    To sum up what I've said, there is no need to use thicker walled tubing for this purpose.
    That's great information, and very helpful...so thanks!

    I just visited Javron a couple of days ago, and he's indeed using 3/4 x 0.035" for the PA-12/PA-18 lower longerons. He's using 7/8 x 0.035" for the lower longerons on the 4-place he's working on--it's the same one he had at OSH this year. What a beast! And what a great shop Jay has--it's incredible to see all the aircraft he's working on, and what a super nice guy Jay is. He has what appeared to be the perfect set-up there in the woods of Minnesota...I was very envious of his shop environment. He's out in the country with a big-time aircraft fabrication facility. Absolutely amazing!

    Thanks for all the input in this thread guys. Very helpful and educational.

    TB

  20. #20
    Steve Pierce's Avatar
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    Here is a welded on float fitting on a PA18.
    Click image for larger version. 

Name:	102_5756.jpg 
Views:	148 
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ID:	45179
    Steve Pierce

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  21. #21
    skywagon8a's Avatar
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    Quote Originally Posted by Steve Pierce View Post
    Here is a welded on float fitting on a PA18.
    Click image for larger version. 

Name:	102_5756.jpg 
Views:	148 
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ID:	45179
    What's that extra X bracing for behind the seat cross tube?
    N1PA

  22. #22

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    Wow--that's a BEEFY fitting!

    TB

  23. #23
    Steve Pierce's Avatar
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    Quote Originally Posted by skywagon8a View Post
    What's that extra X bracing for behind the seat cross tube?
    180 lb baggage STC.
    Steve Pierce

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