Bolts or Rivets

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by davedpilot, Sep 3, 2004.

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  1. Sep 3, 2004 #1
    A question for all:

    I have been totally redesigning my sport plane project and would like to hear some opinions from the group.

    The basic fuselage structure is designed with 1/8" al angle joined together with 1/8" al gussets. The structure will be skinned with .025 using pop rivets. My question is would it be better to bolt (or rivet) the structure together with flat head fasteners (or solid rivets) and have an open structure while working on all the systems? Or should the skins be riveted on through the angle and gussets all at one time? I'm not necessarily looking for the open structure and I realize bolting or riveting it together before skinning will add some weight.

    What do you guys think?
     
  2. Sep 10, 2004 #2

    pylon500

    pylon500

    pylon500

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    G'Day Dave,
    I remember reading your first post, then going to your website, and thinking;
    This all looks OK, I'll wait 'till he needs specifics..
    I liked the layout you chose, simple, typical and balanced, sort of BD-1~Pazmany~Sonex.
    However, in looking more closely at your CAD images, and reading the above, I'm beginning to think you may be over engineering a little.
    Your fuselage structure looks suitable for a more Semi-monocoque structure rather than dragging around an internal MECCANO set framework wrapped in fairly thick alloy sheet.
    This type of structure was used inside the BD-4 (a heavier 4 seat plane) and while not having the plans with me, I suspect the angles were more likely between 0.063" and 0.080" thick and the outer cladding was probably only 0.020".
    The Sonex is more in your weight range, and it's fuselage is basically a metal box, either 0.020" or 0.025" sheet with 0.025"~0.032" framing to support the sheet structure.
    The only real reinforcement required is around your cabin cutout.
    You can see some good construction shots of a Sonex at;
    http://sonex260.wheelsup.org/
    By comparison, the BD-1 (ne Grumman Yankee) had NO internal structure as it was a box made of aluminium honeycomb sheet, Bede's latest kit, the BD-17 Nugget uses the same technology;
    http://www.jimbede.com/bd17index.html
    I wanted to say this type of structure is not for the beginner, but in reality it is very simple, it's just different?
    Moving onto your wings, for a wing panel that's only about 11ft long, you look to have a lot of ribs in it?
    This may be necessary if you want to use a laminar section and thin sheeting, have a look at a Piper Cherrokee (PA-28) wing (the early square one) and find out the various thickness of the skins used.
    I see also your tank location, will it be built into the wing and covered by the wing skin?, maybe you could use the same tank concept as Piper used, (I know I did!)
    My belief was that tanks within wings cost weight, complexity and maintenance headaches!!
    Just a quick note on your ailerons, you can cut down on your parts count with some 'clever' building by making the whole aileron with one skin!
    I couldn't find a photo, so I've created one, this is the same method as used on the French Robin 2160's, a two seat aerobatic tourer with roots back to the Zodiac range.
    Arthur.
     

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  3. Sep 13, 2004 #3
    Pylon500

    Thanks for the input. I'm having to get all my internet access at work (need another modem) so I can't post any pics. The cockpit is built with the al angle and the rest with formed channel. I'll post a pic as soon as I can. I haven't been able to update the web page either. The thing doesn't look anything like it anymore. I've moved the fuel to a bladder in the fuselage because I want to be able to get the wings off easily. Changed to conventional gear, taken out the chute, moved the tail back a little, swept the v-stab, added some h-stab area and the canopy is a smaller one piece unit. I still need to work with someone on the analysis down the road a bit.

    Dave
     
  4. Sep 13, 2004 #4

    wsimpso1

    wsimpso1

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    Hey Davedpilot,

    Lets answer your first question.

    Use rivets everywhere you can, bolts only where you have to be able to take things apart for the annual inspection or to get from shop to airport and back.

    Bolted joints are heavier for two reasons:

    Bolts themselves to do any particular job weigh more than the rivets;

    The load is generally more concentrated with bolts, so the surrounding structure to funnel the loads to the bolts and distribute the load back out again will have to be beefier than with rivets.

    Pylon500,

    I must take exception to this term "over-engineer". I have three degrees in engineering and a wife working on her third, and between us a bunch of experience with many projects in eight different industries. I have never seen nor can I imagine a project where there has been and excess of good thought, planning, and analysis.

    I have seen projects managed to their very demise (bad management), many where engineering was cut off to early and then come to grief (bad management), and few where the basic assumptions and direction were faulty (bad engineering), but no over-engineering.

    I have also seen many cases of eyeball design without any analysis at all where they are seriously "Over Designed" or "Over Built", and I suspect that is what you really meant for Davedpilot. It does dound like his wings and fuselage are way oversized to me too, and thus serously in need of some analysis and real engineeering. Have fun. I look forward to the next step...

    Billski
     
  5. Sep 15, 2004 #5
    Hey Billski,

    One of the great things about being an amateur airplane designer is I do not really know what I can't do. However, being the manufacturing guy that I am, I do know what I can do to make this a very friendly build. Which brings us to CAD, I can parameterize an entire airplanes skin, and structure for that matter so I'm not too worried if I may have over-built a component. Like I have mentioned before, I will work with an analysis to optimize the structure for the intended mission.

    Man, it seems you got to be an aeronautical engineer AND a structural engineer to get it right. There are a lot of fine kit airplanes I could purchase, but what fun is that? I noticed you are building an airplane of your own design so you know what I am talking about.

    Dave
     
  6. Sep 15, 2004 #6

    wsimpso1

    wsimpso1

    wsimpso1

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    Davedpilot,

    I do not mean to be discouraging, but other folks will read this too, so I am going to write a bit here. An undersized structure will kill you, and an oversized one will carry little and perform poorly, and without adequate engineering work, you have no idea where you are in that range. Yeah, you do need aero and structural engineering to design a light structure that does the job.

    Lots of folks ask if this or that is adequate, or how to beef something up... So, here we go.

    Without doing the job from a zero base, you have to rely upon the monkey-see monkey-do school of airplane design. This can be effective, in regards to fuselage design and flying surface skins. The reason that it is possible is because structurally, these systems must be designed to survive other effects as much as the flight loads. Other effects are handling, construction, animal and insect contact, the inevitable slings and arrows of life with passengers, baggage, line personnel, and the occaisional airshow moron who thinks that your airplane is a writing desk or seat for an energetic and agitated child.

    A look at the typical Cessna or Piper will show that skins are usually the same thickness everywhere on the fuselage or tail, and only a couple of thicknesses are used in the wing skins. Rib spacing may change, but not a lot across a surface. The spars are another matter that I will get into below...

    Once you have decided upon the minimum skin thickness and the risks that you are willing to take on them, you have to figure out how close together the ribs, frames, and longerons have to be. The spacing of frames/longerons in fuselages and ribs in flying surfaces are dictated by membrane loadings in the skins - that is, the air is stationary inside and moving outside, and that applies a pressure difference on the skins, inducing bending moments and shear loads in the skins.

    You end up trading skin thickness for reinforcement spacing. Usually, you will be lighter with thinner skins and more frequent ribs/frames/longerons, but you again have to make that trade-off decision. There are books with formulae on this topic... Or you can repeat the monkey-see monkey-do approach on internal reinforcement, ribs, frames, and longeron spacing and relative beefiness, using similar speed/wing loading aircraft for reference...

    Now, one of the big troubles with the monkey-see, monkey-do approach is that you do the gambling that your bird is sufficiently similar to the ones you are looking at to use their design instead of doing your own design...

    As I said earlier, spars are another matter. Wings and tail carry considerable shear, torsion, and bending loads, and the sizes of the loads vary hugely based upon mass, g's you want to carry, and span. Spar design is heavily influenced by loads carried and the thickness of the surface where you want to build the spar. Thicker wings and tails cost little in terms of speed and allow big savings in terms of spar weights, as well as give you places to stick stuff like fuel, control runs, wiring and powersources for lights and strobes, etc.

    Now loads are big at the root and drop off rapidly as you go towards the tips, so this gives you a big opportunity to reduce mass by tapering spar caps and shear webs towards the tips.

    To give you an idea of relative weights, my efficiently designed and minimum epoxy content fiberglass structures for my 6g, FOS 2.0, two-seat, big baggage bay airplane has an 85 pound fuselage tub, 36 pound canopy roof and gullwing doors, 18 pound horizontal tail, and a 108 pound main wing spar. The skin thicknesses are all monkee-see, monkey-do to protect from the usual hazards. The spars and reinforcments are efficient designs. Imagine if they were not...

    You really have to design each spar for its loading and size. Failing to do so means that you either have a spar that is way oversized and adds a lot of wieght, or is undersized and can collapse in flight.

    Designing your own airplane does require determining your aero loads and then doing some structural design work. By the time you get done with it, fabricate your tooling, work out your processes, build, and then debug the bird, well, you may have been much better off with someone else's proven design... And we have not even touched upon performance, stability or control, all of which matter too.

    I have chosen to design and build my own bird. I have three engineering degrees, the backing of a noted aerodynamicist, a whole shelf of engineering texts (that I know well) on these subjects, and I am a "build it" type of guy who likes building tools and parts and working out processes. My path is not for many, but it is terrific for me and a few others.

    Now, you erstwhile airplane designers have fun, recognize that it is quite the intellectual pursuit to cover the bases and design a practical bird. Eyeball sizing may work for building choppers and one-use machines, but for man rated airplanes, you have to get serious.

    Billski
     
  7. Sep 15, 2004 #7
    Hello Billski,

    Discouraging? Are you kidding! It seems you're trying to stifle the creative talent looming out there. How can you say, "get serious" without knowing the history or research into a project. Once again I will say I am not an engineer, I'm a tool designer & manufacturing guy. I only have ideas that may make building an airplane less time demanding and expensive, if only for myself. Aluminum is not expensive, and I have plenty of experience working with the tools and materials necessary to challenge this endeavor. Of course I hope to be successful, if not, I will be proven wrong. Hey, I may find an airplane that fits my needs and buy it.

    Now, as far as monkey-see, monkey-do... you've got to start somewhere and this isn't new technology. There is so much information being openly shared on the net and with folks who welcome new ventures, I'm positive I don't need a degree in aerodynamics to have a good performing airplane.

    Monkey (Davedpilot)
     
  8. Sep 15, 2004 #8

    orion

    orion

    orion

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    I couldn't have said it better myself, and since I'm not all that much a fan of typing, I'm sure glad Billski went to the detail he did.

    I would like to add a few thoughts. First, in dealing with homebuilt aircraft, especially those that will end up being built by the general public, the designer must take all the above into consideration, as well as issues that deal with the skill of the average builder. We would like to assume that all builders will follow all the instructions to a high level of detail and build like craftsmen (and many of course do) but in the real world this is not the case. As such, it is sometimes a good practice to try to look forward a bit and actually try to account for future deficiencies. This includes the items Billski mentioned above, on top of the issues of building quality.

    The question then becomes, "How much to account for?" - and no, I do not have a pat answer here, it just depends on the part, the process and of course, the material.

    Second, it takes some amount of experience to decide when a specific amount of engineering result is sufficient. Classically, engineers are taught to design to a specific level of accuracy. I have often found though that it is very important to ask whether designing within a "gnat's ass" (I like that technical term) of a material's strength property is good practice. Usually, for the above reason and others, it is not good practice to do so and generally, it is actually unadvisable to do so.

    Having been a part of the Civil Air Patrol some time back, and having since then been a part of several crash studies, I often seen evidence that suggests that too much effort was spent on designing to a precise degree of accuracy however, maybe not using the right criteria. In virtually every airplane out there, an extra fifty pounds of weight in the right places would make a huge difference in a structure's crash stability and occupant survivability in the event of a mishap.

    Throughout our education we're taught that an airframe must be as stucturally efficient (translated: light) as possible. Taken to an extreme, the airplane often becomes no more than a fairing around the seats, with only marginal capability and protection at other than ideal circumstances.

    To do this correctly and safely then, the process of deciding when enough is enough takes a good amount of experience. No text or lecture is going to give you all the answers and no matter how many books you have on a shelf, if you don't have the right base education and practical experience (where you are guided by those who know) it will be very difficult for you to even understand whether you are selecting the right criteria or making the right decisions.

    Is it impossible for an amateur to do this? Of course not - several have done so. However only the conservative ones will know when and where to ask questions. But don't be surprised if you don't get the answer you are after. It is virtually impossible to ask whether your spar is good enough without the engineer looking at the entire wing. It is impossible to tell you whether your cockpt is safe enough without looking at the anticipated flight envelope and analyzing the entire fuselage for the flight and possible crash loads.

    In short, either be ready to spend long, long hours doing research, or be ready to spend a few dollars having someone experienced do the analysis for you. There really is no shortcut or cookbook method that will make this easy.
     
  9. Sep 15, 2004 #9

    wsimpso1

    wsimpso1

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    Davedpilot,

    I feel that if I am writing in a Forum like this, I have to write to everyone. We have people writing into this forum and treating the topic like house construction, you know, where we use 2x4's on 16 inch centers or 2x6's on 24 inch centers, headers less than 32 inches are...

    Now, monkey-see monkey-do works on a bunch of stuff. If you find out what skin thickness comparable airplanes (wing loading and speed) are working with for fuselage skin, wing skin, and control surfaces, and you also findout what rib/ frame/ longeron spacings they use, you will have what is probably a pretty safe estimate for what you should use. If in doubt, tighten the spacing a little, but understand that skin thickness and spacing are related...

    Now if you are a tooling and manufacturing guy, you have a leg up on most of the rest of those building sheet metal airplanes. A shear, brake and block forming of ribs and frames is about as sophisticated as most tin bashers get. They bend skins from flat stock and attach them to various reinforcements to set shape. You may be able to integrate ribs/ frames/ longerons into the skins, make these reinforcements more simply, or come up with other ways of getting to a good structure with a minimum of fuss. These will help out a bunch. And the reinforcements that are used out there can also serve as monkey-see monkey-do basis for your structure.

    You will still need to either figure out how to do analysis of spars or find help on doing them, and there are books on the topic...

    Now if you really want the strong cage around the cabin, it is done in lots of airplanes, including the North American AT-6 Texan, the whole Mooney single engine family, and the Glastar. But do it like the AT-6 and Glastar, with thin skins tacked on the outside of the cabin, and then go monocoque from just behind the seats through the tailcone, and save some weight. The other way is to go beefy structure at the cabin, and then go to thinner and/or smaller stock once aft of the seats.

    Pazmany's book is excellent, and he covers Monkey-see Monkey-do on stability (tail sizing and CG placement) and on control surface sizing as well.

    Let's talk some more. e-mail works too, and I don't have to talk in such big general terms.

    Billski
     
  10. Sep 17, 2004 #10

    pylon500

    pylon500

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    G'Day Davedpilot, Hey don't dispare, we 're actually behind you.:D
    You will come to find that there is no such thing as the ideal aeroplane, everything is a compromise between a heap of factors. :confused:
    But only you can decide where to draw the lines. :rolleyes:
    As you are a toolmaker, I think we are all confident that your 'hands on' work shall be top rate, as a plans reader and designer we assume you can hold a series of three dimensional concepts in your head at any one time.
    With these abilities, all you need now is a bit of exposure to what other designers have done to balance the compromises of what their end goal was.
    My history doesn't include a great deal of academic achievement, but more a lot of hands on experience starting with designing and building flying models (under the supervsion of my father, a noted modeller) thence to the RAAF to become an Aircraft Sheetmetal Worker (which was later changed to Aerospace Structural Fitter, which sounds a bit better than 'metal basher') where I worked on C130 and DHC4-Caribou.
    While doing this I learnt to fly sailplanes, so was exposed to high performance, high precision, lightweight structures, all of which led me to believe that only engineering genius' could design and build a light aircraft. :(
    Then I saw ultralights! :eek:
    Once I got over the shock of the level of (or lack of) technology involved, I started designing and building my own aircraft.
    Now with more than 25 years involvement with general aviation maintenance and rebuild, I've started to get a 'feel' for how airplane designers think, or even sometimes how I believe they don't! :rolleyes:
    Obviously in my work of repairing aircraft I don't just go rivetting lumps of aluminium over damage or cracks, but study repair manuals and regularly employ the advice of aeronautical engineers to bring the plane back to original strength.
    I guess that makes me the Monkey!! :p:
    I do however have to reign in these engineers sometimes to avoid ending up with repairs that are impracticable, a small bracket on the side of a rib does not have to be strong enough to lift up the whole aircraft!! :wail:
    Hey Billski, that's where I got my "over-engineered", which to me was the same as as being "over-designed" as opposed to being "overbuilt"
    Symantics I guess. :confused:
    Anyhow, keep at it Dave, and we'll keep trying to help.:ban:
    Arthur.
     
    Last edited: Sep 17, 2004
  11. Sep 18, 2004 #11

    AVI

    AVI

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    Hey, Davedpilot

    I'm with you too. Don't despair! Sometimes these over-educated folks are prone to telling others why things can't be done. Some of them find it difficult to "think outside the box," and are quick to discourage amateurs from original design. "Monkey see, monkey do" is a great education in itself, provided one does extensive research and is, on the other hand, not adverse to seeking advice, if and when prudent, from these same, well-intentioned engineers.

    These guys mean well. They're good guys, so don't despair! What you're attempting to build is not rocket science, but do exercise caution. Don't try to reinvent the wheel too quickly.
     
    Last edited: Sep 18, 2004
  12. Sep 20, 2004 #12
    Thanks for all the comments guys. I'm not in despair... really, sorry for sounding that way. I know this is more of a hobby than a goal to bring this to fruition. I've been designing R/C aircraft since I was knee high to a grasshopper, not that there is any comparison of course. I always thought it would be cool to build an airplane from scratch and what better satisfaction than to build from your own ideas, know what I mean? Anyway, if I ever get far enough to actually have this project looked at, and blessed, by one of our resident engineers; that's when the fun starts. In the mean time, my brother who has just been diagnosed with cancer has his Sonex for sale. I may finish it while absorbing as much of how they do it as I can.


    Have a nice day


    Dave
     
  13. Feb 26, 2005 #13

    PTAirco

    PTAirco

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    Re. Eyeball design

    I am in total argeement with Dave. This is not aimed at anyone personally,but I'd go further:building an airplane without doing some basic maths is downright stupid!

    It doesn't need to be all that involved either but anyone who thinks he can eyeball a spar for size is suffering from serious delusions and he will soon improve the genepool by removing himself from the population (hopefully without taking anyone with him).

    I am not a trained engineer but I have been reading everything on aircraft design published from the 1920's to the present day and I feel confident that I have amassed enough knowledge by now to produce a resonably efficient and safe structure. Not to mention going through the paper exercise of design inumerable times (I am building my own design two-seat biplane). But it was a long and hard road, it would really help, if anyone contemplates doing this, to get friendly with a 'real' engineer. It would have saved me 5 years.

    There is a world of difference between a skilled craftsman and an engineer; I am not saying one is more knowledgeable or skilled than the other but the fields are totally different. (I wish I was a better craftsman...)

    I feel it is perfectly possible for a non-engineer to learn this stuff, but it is not easy. If you want to scare yourself, give your ideas to a real engineer to do some very basic sums; you will be amazed how making even the slightest changes to your design can produce wild changes in stresses and load distributions.

    I would recommend getting a free demo version of something like Nastran and learning how to use it. It was an eye-opener as far as optimising a structure goes. It will tell you if you produce something like an unworkable truss structure for example, by simply refusing to give you a result! It is frustrating to learn, but worth it and will give you a good insight how to optimise a structure. On my biplane for example, I reduced the flying wire loads from an ultimate load of nearly 8000 lbs in my initial estimate (yes it's a heavy thing!) to about 6000, simply by careful shifting around of components, and the spars went from 138 lbs to 105. A quarter inch difference in spar depth can mean a safe airplane or your coffin in some cases, if you cut it really fine.

    I cannot believe anyone would trust his life an 'eyeballed' design; there are places where it is appropriate, (no, I didn't analyze my floorboards for example; I simply jumped up and down on them gently...) but not
    with spars, landing gears, flying wires, tails etc.

    It is not that hard to give yourself a little peace of mind; find someone who can do some sums for you. Or at least proof load your aeroplane with a carefully worked out load schedule (call the engineer again...), I have seen some pictures of proof tests that were a joke. We've all seen pictures of a couple of guys standing on the wings of an airplane with the caption something like :"... and look how strong it is.."

    It will still be'your' design, but it is always worth getting an engineer to criticise your structure constructively.
     

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