CAR-251: The Cheap Air Racer Discussion thread.

Discussion in 'Hangar Flying' started by nerobro, Sep 10, 2014.

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  1. Sep 20, 2014 #181

    Pops

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    I agree, the design should be as easy to built as possible, but in each type of construction there is really not much difference in skill level , just different, and we all have our gifts and deficits in ability's in each type of construction. The fact is, we can all learn and build our skill level in any of the construction types to construct a good safe aircraft. No one should let something new scare then away, but we all know it happens. Learning is what homebuilding aircraft is all about. Of course , I'm preaching to the choir.
    Keep on , keeping on, I'm enjoying this. Dan
     
  2. Sep 20, 2014 #182

    Matt G.

    Matt G.

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    If somebody can build the wooden structure, they can cover it with fabric. IMO, less skill is needed to do the fabric covering than the wooden construction.
     
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  3. Sep 20, 2014 #183

    nerobro

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    You're right. I know you're right. "they" don't know you're right. And that will turn people away.

    So, I took my books to dinner with me. Sadly, I did not do the integration to get the numbers spot on... So this is a slightly conservative number.

    Assuming the use of pretty bad wood (5ksi, that is hardware store 2x4 quality..) a 3" wide main spar can do the job. And that ignores the 1/16" ply skin that's 5ksi too.

    Now that I know it's the right ballpark... time to get into the details... That's going to take a lot of scrap paper...
     
  4. Sep 20, 2014 #184

    Topaz

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    How did you get the loads?
     
  5. Sep 20, 2014 #185

    nerobro

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    at 20g, and 550lbs, the spar will need to carry 11000lbs. (the weight of the wing is being ignored at the moment, until I can calculate the weight of the wing... )

    Each wing panel will need to carry 5500lbs (The lift that may come from the fuselage is also being ignored.)

    Because I didn't calculate the actual center of the wing, I used the midpoint of the wing to calculate the bending load. That worked out to 264,000 inch pounds of force. (the real number will be somewhat less...)

    When calculating how far a beam will bend, you need to know the bending stress at the edges, the formula for that is: b*d^3/12 = I (which is the unit used conventionally in the stress calculations) That spits out a number something like 62.5. Dividing the load by I gives you the PSI of stress at the edges of the beam at that load. In this case it works out to 4223psi. Which means the spar could be a good bit smaller.

    The sheer stress on the wing is still 5500lbs, which over a 6.3x3" spar works out to 291psi. That's a bit high, but there will be another spar, and the wing skins to help share that load.

    Here's hoping I got all of that right. Now.. i'm going to sit down and work out a "ballpark weight" for this wing. If that ends up being a workable weight, I'll go back and re-do the stresses properly to see how much more weight I can get out of the wing.
     
  6. Sep 20, 2014 #186

    Chris Young

    Chris Young

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    I = b*d^3/12 is the inertia of a solid rectangular section. Is your spar solid ? Yon probably can gain a lot of weight by having it built-up with caps and plywood webs.

    For the spanwise load distribution on the wing, you should use Schrenk's approximation to reduce the bending moment. (Look it up on Google, there is a NACA report about it)
    To calculate the bending moment at failure, the common method in France is to use Prager's method which accounts for the nonlinearity of the stress-strain curve of the wood in compression (which can be compared to a plasticity for a metallic beam, but only on the compressed side). I don't know of any English-language book that describes it, but there must surely be some. Otherwise try to get a copy of Vallat's "Résistance des Matériaux Appliquée à l'Aviation".
     
  7. Sep 20, 2014 #187

    nerobro

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    I know a built up spar would be much, much, much lighter. But a big block of wood is a heck of a lot simpler.. But if the spar is "one project" you're taking on at least a day's worth of work. Hopefully a sophisticated spar won't be needed.

    I will dig up Schrenks's approximation. And I'll look up Prager's method.

    So I've been sitting here with a spreadsheet, and I did the moment arms at 1' intervals across the wing. The bending moment ended up being 229000 inch pounds. That's a bigger difference than I was expecting, a full 15%. And then the lift distribution will take that a step further. This spar is shrinking...
     
  8. Sep 20, 2014 #188

    Victor Bravo

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    nerobro, I think you may be over-estimating the level of simplicity that is really appropriate here. This is not an entry level ultralight, and it is not a Volksplane. The "Entry level" air racer does not need to appeal to the absolute beginner in aviation. Those are not the people we want to be involved in this racing class.

    We're discussing a racing airplane here, which by definition will (and should) require a pilot and team owner of average or above average airplane skills. While it is entirely conceivable that an expert pilot could be a hopelessly incompetent builder, I think it is important that you do not assume that ANY racing aircraft should be able to be constructed by a rank beginner in homebuilding, regardless of the pilot.
     
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  9. Sep 20, 2014 #189

    nerobro

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    I'm really most concerned with speed of build. More layers of "anything" add days to the build process.

    For instance, skinning in plywood means you can "just paint it."

    Fabric covering means gluing the fabric. Finding some UV blocking paint, painting it, painting it again.. and again.. and again.... Each time sucking up a day, or a large portion of a day. I want as few waiting steps as can be in the build process.
     
  10. Sep 20, 2014 #190

    Victor Bravo

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    All very good and appropriate thinking for the 21st Century Volksplane, or the next generation quick and dirty replacement for the Pereira Osprey 1 flying boat. But the cheap air racer HAS to be a little more complex and much more refined than your approach, in order to meet the weight requirement. There is a reason for why the Luciole and SD-1 and Chilton Monoplane are not built out of huge rectangular logs of wood with plywood thrown over them. Light airplanes powered by small engines NEED "more layers" of engineering.

    Unless you personally have experience doing fabric covering, please take some advice from those of us who have. It's truly not a difficult or intensive process on a small airplane. More importantly, new fabric systems like Stewart Systems and Oratex cut out a HUGE chunk of the time and waiting previously required by the Stits and Ceconite process.

    Also, with all respect and constructive intent, nobody "just paints" wood structures. Plywood sheeted surfaces are almost always covered with fabric, or coated with really thin silk-like fiberglass (3/4 oz./sq. ft.) and epoxy. There is good reason for this... you must stabilize the grain and keep it from expanding or contracting. Other than 5 coats of heavy "bar varnish" (not "spar varnish", I mean the stuff on top of the local drinking bar), you're definitely absolutely positively going to have serious issues with your wooden skins without fabric or glass over them.

    Again, with the utmost sincerity and no ill will or insult... in order for this idea to actually be viable as a real airplane, you are going to have to start approaching this with a little more refinement. Not because all airplanes need to be refined... the VP and several other airplanes do fine built out of clunky slabs of wood. But this airplane needs a much higher level combination of strength, rigidity, robustness and near-ultralight weight.

    For all the right reasons and best intent, you're wanting to design this with a grease pencil and chalk lines, when in reality it has to be done with a sharp, fine drafting pencil.

    Any of the trained and/or experienced airplane engineers can feel free to correct me on this if I'm wrong.
     
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  11. Sep 20, 2014 #191

    Matt G.

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    That's something you need to figure out very soon. Assuming your spar goes from tip to tip of your 18' tapered wing, it weighs about 48 lbs if it is made of Sitka Spruce. That is a LOT of weight for a 550 lb gross weight airplane.

    You mean "264,000 inch pounds of bending moment". A bending moment is a force multiplied by a distance. It is not a force. Units are important.

    It's actually going to need to be bigger because there's a serious error in your calculation. The formula for bending stress is f=Mc/I. f is the stress, M is the moment, and c is the distance from the neutral axis to the extreme fiber. You forgot the c, so your bending stress is actually about 13300 psi.

    The wing skin will react some bending loads, but not shear loads due to the lift distribution. It will react shear due to torsion, though. The shear stress you've calculated is the average, not the peak value, which is approximately 1.5x the average value.


    In my opinion, trying to shape one 18' long piece of wood that weighs 50 lbs is going to be more difficult than working with some smaller pieces and gluing them together. Why does the spar only need to be a day's worth of work? I agree with the others, you have latched on to 'simplicity' so hard that it is at the expense of everything else.

    And how do you know that? All of the beginner wooden glider enthusiasts I know (myself included) had the preconceived notion that fabric work is easier than woodwork, and speaking from experience, that's pretty much how it is.
     
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  12. Sep 20, 2014 #192

    nerobro

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    The main spar is only 8' long. It may end up 10' in the end. And it's tapered.

    You're right on the bending moment, I wrote poorly. :)

    I was only calculating for I, not the whole kit and caboodle. I"ll go re-check my book.

    I have latched on to simplicity, if i'm not going to stick to simplicity with religious fervor, what will make this plane any different from say the SD-1? I may need to get fancy but I hope not.

    Doing fabric still takes time.
     
  13. Sep 20, 2014 #193

    Hot Wings

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    I've very scientifically broken airplane design down into discrete phases: :gig:

    Napkin phase. Just about anything that will write on a napkin with out tearing it is acceptable.

    Note book sketching. A .7mm 2H lead pencil and a BIG eraser are my preferred tools.

    Graph paper and first pass calculations. Move up to .5mm 4H pencils (color is added to the mix) and a nice sharp eraser.

    Excel and CAD is next. At this point the fun is over and it gets to be WORK. :tired:
     
    Last edited: Sep 20, 2014
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  14. Sep 20, 2014 #194

    nerobro

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    Quite. Designs don't go from ideas to complete in one revision. the wings... are at least 3 or 4 revisions from complete. gimme some time, we're un-rusting some math skills, and then crushing my clever ideas. :) We'll see how many survive.
     
  15. Sep 20, 2014 #195

    RJW

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    Who said anything about 220 pound iron motors?

    Rob
     
  16. Sep 22, 2014 #196

    Rienk

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    I'm catching up, so maybe someone's said this...
    Look at Oratex covering - it is very light weight and relatively simple to install.
     
  17. Sep 22, 2014 #197

    Pops

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    Not much of anything worth while is quick and easy, that's why the instant generation doesn't get a lot done. Even assembling the quick built aircraft kits is not quick and easy. Dan
     
  18. Sep 22, 2014 #198

    nerobro

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    Adding another kind of surface, just isn't in the cards. It's been suggested that this will be fiberglass covered (because that's what people do.) it's been suggested that some surfaces will be fabric covered. (because that fits in some logic..)I

    There's a lot of good arguments for both.

    If the plane was going to get even a cosmetic skinning of fiberglass, you're adding several days worth of work... And if one is going to bother with fiberglass, why not just copy KR wings? "if you're bothering, you might as well." Foam is cheaper than plywood.. and opens up a whole slew of other options. But you're also talking about a bunch of new materials, specialist suppliers, and intimidating techniques.

    I'd need to look in to the prices, but I strongly suspect ~any~ fabric covering method is going to cost more than a few square feet of 1/16" ply. Also, fabric breaks the shell that I'm depending on to handle torsional stresses on the wing.

    For now, the plan is just plain paint. Breaking out the "other stuff" waits for me failing the first pass of figuring out the weights of the plane. Simple first. If simple fails, I'll need to make hard decisions on where I can increase complexity.

    ... I'll do that after my bike ride tonight. (I'm trying to be a ideal FAA human more than the "well.. that's what we use nowdays")
     
  19. Sep 22, 2014 #199

    Apollo

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    While I'm in complete agreement with Victor Bravo's and Matt G's posts, I can lend some moral support to the above statement. Finding the simplest solution that meets the requirements has always been my design approach. In fact, I have a sign above my desk with the following quotes (as inspiration).

    “Simplicity is the ultimate sophistication.” – Leonardo Da Vinci

    “Everything should be kept as simple as possible, but no simpler.” – Albert Einstein

    “Making the simple complicated is commonplace; making the complicated simple, that's creativity.” – Charles Mingus

    “Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.” – Antoinede Saint-Exupery

    “Any intelligent fool can make things bigger, more complex and more violent. It takes a touch of genius to move in the opposite direction.” – E. F. Schumacher


    I respect these sentiments but they don't negate the need for more sophistication in your design approach. Simplicity is an admirable goal but must be balanced with other requirements. We now return to our regularly scheduled beating.
     
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  20. Sep 22, 2014 #200

    nerobro

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    Thanks. :) We're stretching how clever I can be. Hopefully.. it works out. If not, this thing quickly approaches a KR, and if it's a KR, it might as well BE a KR.

    Hopefully I have enough time tonight to add up some weights.
     
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