Chromoly tube sizing

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Patrick Kevin

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Aug 6, 2019
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Hello. I am a new builder here. I am currently on the design stage of my first aircraft. I'm planning to build my fuselage using chromoly tubing but I don't know what size to use. I read several articles here that before selecting the tube size, a stress analysis must be done for the design. Where do I start with the calculations? thank you.
 

BoKu

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Welcome, Patrick!

The kind of stress analysis required for what you're thinking of is the sort of thing they teach in classes on engineering statics. They usually require calculus to sign up for statics, but I know for a fact that you don't really use much calculus; it's mostly elementary trig and algebra that you can learn or re-learn pretty quickly. The book I've heard most often recommended is "Stress without tears," so that's probably a pretty good place to start.

However, before starting your own design, I strongly encourage you to examine as many airplanes as you can in as much detail as you can, especially ones like the ones you envision for yourself. Visiting repair shops, salvage yards, and anywhere you can find broken or disassembled airplanes is the best way to see what they are like on the inside. That also gives you a good idea of how to do detail design of things like hinges and attachments. Until you are an experienced designer, the best way to stay out of trouble is to stay within the bounds of established practice for things like what tubing sizes to use where, and how to arrange them. There are some subtleties to it that you might not get from a purely theoretical approach.

--Bob K.
 
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wsimpso1

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As one who has embarked upon self-designed airplane construction, I have some advice:
  • Just building an airplane that already exists as plans and already has a bunch flying is a huge accomplishment.
  • Define the material set you love to work in, then commit to that path;
  • Define your mission for your airplane. Carrying capacity, landing speeds, cruise speeds, ete;
  • Then go looking at all of the existing airplane design with known good flight history and known good flight characteristics that fit your mission. Look hard at those designs and seriously consider building one of those...
Here is why. I possess a BSEME and MSEME with lots of structures and manufacturing and composites background. I have a college buddy who is a world class aerodynamicist. I still present the self-designed airplane as prima-facia evidence of my insanity. If I had just bought plans for a Cozy Mk IV and built it, I would have already flown it around this country a few times by now. The next airplane I build will likely be to the plans...

Now if you are determined to do this, you have a big educational gap to fill. This is not building a house with standard lumber and spacings. Designing the fuselage truss is not a matter of using some certain size tube. You have to define your airplane, get the loads that go into the fuselage from all of the flight loading and landing scenaria, then scheme out your truss, then analyze the truss to get the loads in each member of the truss, then select the tube sizes that suit. Usually tubes are selected based upon worst case compression load and buckling criteria, which has to do with both how big the compression is and how long the element is. You will get into Statics and Mechanics of Materials texts to get after these topics. Books on these topics are listed in the FAQ. We can help you... Just understand that designing your own airplane is a HUGE add onto building, and that is coming form a guy who already has the engineering knowledge in his pocket.

Billski
 

Charles_says

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Welcome, Patrick!

The kind of stress analysis required for what you're thinking of is the sort of thing they teach in classes on engineering statics. They usually require calculus to sign up for statics, but I know for a fact that you don't really use much calculus; it's mostly elementary trig and algebra that you can learn or re-learn pretty quickly. The book I've heard most often recommended is "Stress without tears," so that's probably a pretty good place to start.

However, before starting your own design, I strongly encourage you to examine as many airplanes as you can in as much detail as you can, especially ones like the ones you envision for yourself. Visiting repair shops, salvage yards, and anywhere you can find broken or disassembled airplanes is the best way to see what they are like on the inside. That also gives you a good idea of how to do detail design of things like hinges and attachments. Until you are an experienced designer, the best way to stay out of trouble is to stay within the bounds of established practice for things like what tubing sizes to use where, and how to arrange them. There are some subtleties to it that you might not get from a purely theoretical approach.

--Bob K.
Excellent advice! The "Stress without tears" is a good book to have.
Ive had one for the past 10 years, and sometimes still refer to it.
 

Aerowerx

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I would start by reading "Simplified Aircraft Design for Homebuilders" by Dan Raymer. That was enough to convince me that I did not want to design my own, at least for the first time around.

The math involved in basic truss analysis isn't that difficult. Poke around on google and you will find information on how to do it. In particular, search for PDF documents on aircraft design. To get you started "Construction of Tubular Steel Fuselages" (copywrited, so I will not provide a link).
 

Winginitt

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Join a local EAA chapter and talk to different members about the kind of flying you want to do. See if any of them have reccommendations or experience with existing designs that meet those needs. Once you decide what type of flying you wish to do, try to obtain some plans for a design similar to what you want. Then analyze that design for stress. Good place to start and practice by seeing the results and double checking your work.

Those plans may help you with your design....both negative and positive changes.Get the recommended books.
When done, resell the plans.Often, what you initially want changes as you learn more. Get a good camera, go to fly ins and take lots of pictures of details you see, and start a file on your computer.
 

pictsidhe

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As another insane person, I have to second Billski. Designing my project is way harder than just builing something already engineered. Rebuilding parts that don't work as planned is going to add more time.

For loads and basic design criteria, I am following FAR23. Get a 2016 or earlier FAR AMT from amazon/ebay for a few bucks. Read ALL of 23 rather than looking for parts you think you need. Take notes. There will be bits you haven't thought of. I'm building an ultralight, so I have no requirement to follow any standards. But my desire for something that flies well and isn't a deathtrap means I want to build on the experience of others, not repeat their mistakes...

Once you have loads, then you can do stress analysis
 

Pops

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Designing is far more work that building the aircraft. Not matter how much you know, it will still be a learning process.
 

Charles_says

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I would start by reading "Simplified Aircraft Design for Homebuilders" by Dan Raymer. That was enough to convince me that I did not want to design my own, at least for the first time around.

The math involved in basic truss analysis isn't that difficult. Poke around on google and you will find information on how to do it. In particular, search for PDF documents on aircraft design. To get you started "Construction of Tubular Steel Fuselages" (copywrited, so I will not provide a link).
Copywrited (Copyrighted) does not encompass forwarding a link, which is OK under
fair use act. If the author posted it online, it is there for people to see. Linking does not violate copyright laws. Copying the item, and reposting somewhere in it's entirety, does.

I found this link...you can buy a copy here:
https://buildandfly.shop/product/construction-of-tubular-steel-fuselages/
 
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Charles_says

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Um yes that is perfectly legal.
You are not violating the authors copyright,
if the copyright is being violated, it is upon the copying and reposting the work.
Sometimes the rights to the work, are licensed to others and reposting is perfectly legal. You may see the words "Reproduced with permission" that means they have given someone the rights to its use.
The only time copyright laws are enforced, is when the author files a complaint,
and the complaint goes to a court.
Usually cessation of the posting is enough to quell the complaint.
There are no "Copyright Police" searching for violations.

I am not advocating copyright infringement, as I do respect the rights of others.
 

Charles_says

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But I was respecting the wishes of the HBA moderators, who take a dim view of posting copyrighted material.
A link isn't copyrighted. Copying the material for any use other than time shifting is.
in other words you can keep copy of a work for reading at a later date.
but you may not repost it. see prev post
 

BoKu

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The issue isn't that the content is copyrighted. The issue is that it appears that copyrighted content is being pirated, and the author is getting cheated.
 

Aerowerx

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The issue isn't that the content is copyrighted. The issue is that it appears that copyrighted content is being pirated, and the author is getting cheated.
And I do not wish to support such activity, so I did not post a link to the PDF of that book!
 

Topaz

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But I was respecting the wishes of the HBA moderators, who take a dim view of posting copyrighted material.
Charles_Says is entirely correct, with a caveat. Linking to material that has been posted by the copyright owner (or other legitimate rights-holder, within the scope of their license to the copyright-owner) is entirely legal, provided the material is posted publicly and is not on a restricted portion of their site.

Posting a link to something the legitimate rights-holder has posted publicly: Okay! Fire away!
Posting a link to something the legitimate rights-holder has posted to a restricted portion of their site: Please don't.
Posting a link to something posted by someone other than the legitimate rights-holder: Please don't.
Copying/reproducing/scanning/photographing a file, image, plans, tune, or other media, to which you don't own the rights, and then posting it here or anywhere else on your own accord: Please don't.

Licensing a copy of plans from the legitimate rights-holder generally does not include the right to post, display, or link to that material to third parties, except within the boundaries allowed by the Fair Use Doctrine.

Clear as vodka, right? ;)
 

wktaylor

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A good document to have...
MIL-T-6736 [latest Rev AMS-T-6736B] Tubing, Chrome-Molybdenum (4130 or 8630) Steel, Seamless and Welded...

Types:
Type I - Round
Type II - Rectangular or square
Type III - Streamline
Type IV - Oval

Physical conditions:
(A) - Annealed
(N) - Normalized or stress relieved
(HT-125) - Heat treated to a minimum tensile strength of 125,000 psi
(HT-150) - Heat treated to a minimum tensile strength of 150,000 psi
(HT-180) - Heat treated to a minimum tensile strength of 180,000 psi

AMS 6360 Steel Tubing, Seamless, 0.95Cr - 0.20Mo (0.28 - 0.33C) (SAE 4130) Normalized or Stress Relieved
AMS 6361 Steel Tubing, Seamless, 0.95Cr - 0.20Mo (0.28 - 0.33C) (SAE 4130) 125 KSI (862 MPa) Tensile Strength
AMS 6362 Steel Tubing, Seamless, 0.95Cr - 0.20Mo (0.28 - 0.33C) (SAE 4130) 150 KSI (1034 MPa) Tensile Strength

MS33529 Tolerances - Seamless Steel Tubing, Aircraft Airframe, Round
MS33530 Tolerances - Welded Carbon and Alloy Steel Tubing
MS33532 Square and Rectangular Tubing - Carbon Steel and Alloy Steel, 0.35 Carbon, Maximum
MS33534 Standard Dimensions for Streamline and Oval Tubular Shapes
AND10102 Tubing - Standard Dimensions for Round Seamless Alloy Steel
 

Charles_says

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Charles_Says is entirely correct, with a caveat.

Posting a link to something posted by someone other than the legitimate rights-holder: Please don't.
Copying/reproducing/scanning/photographing a file, image, plans, tune, or other media, to which you don't own the rights, and then posting it here or anywhere else on your own accord: Please don't.
Clear as vodka, right? ;)
Absolutely right!
Most people wouldn't know a free use posting on any forum,
let alone something copyrighted. Copyright isn't something you send away for.
Depending on the country, just producing something (anything) gives you rights to
copying prohibition.

Therefore the guidelines set down by the Mods is absolutely correct.

I was a member of an international club and website owner, for that club.
The international rules for copyright vary per country, as far Laws go.
Best not to post anything you're not sure of.
 
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