Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilders

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Rienk, Sep 16, 2014.

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  1. Sep 16, 2014 #1

    Rienk

    Rienk

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    So, I've been reviewing the spreadsheet linked earlier (http://www.southampton.ac.uk/~jps7/D8 website/homebuild aircraft design spreadsheet.xls) and finding some issues that I don't understand.

    1) In the sheet 'As Drawn Performance' in cell H41, I recognize all of the variables except the 3600. Why is this there and where does it come from?In trying to use this spreadsheet, it shows that (H42) the range will be 1331 n.m. when it should really be 30% of that.

    2) On the same sheet, cell D21 has some sort of calculation for "empty weight" which makes no sense to me. Until it aligns with everything else in the spreadsheet, I'm instead using cell G19 from the 'WeightSheet'.

    3) On the 'Sizing Graph' the Wo figures don't make sense to me. In my spreadsheet, it keeps wanting to cross the lines at 1000 pounds, when everywhere else it computes to 1320 pounds (fortuitous for an LSA).I need some help on this one.

    4) The Tail Volume coefficients already input onto the 'Initial Sizing Inputs' sheet (D40 and D42) seem low to me. Are these what Raymer usually recommends?

    It's a bit disconcerting to know whether the spreadsheet is truly original, since someone has input all the data for a particular twin-engine design of some sort (presumably his "Safety Twin", but not having the books, I don't know). Not sure how to know if any of the cells were purposefully/inadvertently changed.

    All in all, the numbers come pretty close to what we calculated for one of our designs.

    Thanks in advance for any enlightenment offered.
     
  2. Sep 16, 2014 #2

    cluttonfred

    cluttonfred

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  3. Sep 16, 2014 #3

    Autodidact

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder

    I don't know for sure, but when converting from "hour" units to "seconds" units you divide by [60(minutes)*60(seconds)] = 3600... As in converting from mph to fps.

    Edit: Or is it multiply? :rolleyes:
     
    Last edited: Sep 16, 2014
  4. Sep 16, 2014 #4

    cluttonfred

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder

    I have also messed around with the spreadsheet a bit and I can't get the Wo figures on the "Sizing Graph" to cross at all. I am trying to figure a low-hp design like the cheap racers under discussion but I am doing something wrong, it seems.
     
  5. Sep 16, 2014 #5

    Rienk

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder

    I doubt you are doing anything wrong.
    I have been studying the spreadsheet a bit, and I'm pretty sure there are some problems with it.
     
  6. Sep 16, 2014 #6

    something-awful

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder

    For the terminally lazy like myself, excel has a convert function for hours (hr), minutes (mn) and seconds (sec): =convert(CELL,"hr","sec). I think it'll work for other units like kg-lbs-mph-m/s etc.

    As for that spreadsheet I took a quick look at it and struggled to make heads or tails out of it. Far be it from me to question the way he's put it together but as with all pre-made tools it may be forcing you to use some methods which either don't work best for you or don't quite match the plane you're designing.
     
  7. Sep 16, 2014 #7

    Hot Wings

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder

    It may not be the sheet. It may be us. I too don't have the book so I'm probably plugging in numbers that aren't what they should be. I tried numbers for the plane I'm working on, and numbers for a known quantity similar and I get weights 3 times what I expected and wing areas 30% less. Will try the link to the "original" sheet.
     
  8. Sep 16, 2014 #8

    Rienk

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder

    The original link is exactly the same as the one I started with.
    I understand the difference between f/s and mph, etc., and have no problem with units and conversions.
    But when different equations for the same numbers on his sheets are significantly off - there is an issue.

    For example, I plug in 'actual' numbers of components which total up to a realistic empty weight of 600 lbs, but then his spreadsheet gives an empty weight of over 830 lbs, based on some arbitrary numbers.

    And it seems that there is an assumption that the desired cruise speed can be had with only 65% power (thus an 80 hp engine is supposedly going to give a 827 nm range with only 165 lbs of fuel, for a full size two-seat LSA).
     
  9. Sep 16, 2014 #9

    Topaz

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder

    Please remember that the spreadsheet and associated book (Simplified Aircraft Design for Homebuilders) you're working with is an extremely simplified version of the full-blown methods shown in the larger college text, Aircraft Design: A Conceptual Approach. As such, it makes a lot of assumptions, and is really intended only for aircraft very like what's already out there. Anything even remotely unlike a C-150 or RV-x is going to be pushing the bounds of that spreadsheet's utility. Something very different, especially in terms of power-to-weight ratio, span loading, and empty weight fraction simply isn't going to work with that sheet.

    It's also critical to understand the methods behind the spreadsheet. At the very least you need the book that goes with it. It is not intended as a stand-alone airplane design tool that can be used without the associated text.

    I have both Simplified Aircraft Design for Homebuilders and the spreadsheet you're working with. I consider them very useful as a primer to the concepts Raymer uses. IMHO the book and spreadsheet are of only very marginal utility in actually designing an airplane. If you are really serious about designing an airplane, spend the cash on the full-blown Conceptual Approach. Even at $92, the cost is a very tiny fraction of the cost of any airplane you'll build.
     
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  10. Sep 16, 2014 #10

    Rienk

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder

    I have the Simplified book buried somewhere, from back when I was pretending I had the ability to design an airplane.

    Since then I have become, as you say, serious about designing an airplane. Thus, I am spending the cash to pay engineers who supposedly know what they are doing :think:

    I studied Greek (and Hebrew) in college... I realize I do not have the time to learn another language using ancient and mysterious symbols :)
     
  11. Sep 16, 2014 #11

    Autodidact

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder

    The range equation contains L/D which in turn contains "q" the dynamic pressure, "q" is calculated with the velocity in fps, but the sfc is in hour units so it must be divided by 3600 to convert from lb/hour/hp to lb/sec/hp so that the units will match up. Since engine sfc is in the denominator, that means that the 3600 will be in the numerator. You've got to match the units. You know that of course, but it takes some backtracking through the formulas to see it. I doubt if there's anything wrong with the SS, maybe reading the book will clear things up?
     
    Last edited: Sep 16, 2014
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  12. Sep 16, 2014 #12

    nerobro

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder

    First, the spreadsheet isn't self explanatory. Second, if you don't own the book, you do not have license to use it. Third, the spreadsheet is very, very, hard to interpret outside the methods used in the book to size and analyze the performance of airplanes.

    The spreadsheet also forces some "fudge factor" into designs. And tries to account for rich glue joints, unusable fuel volumes, etc..
     
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  13. Sep 16, 2014 #13

    Matt G.

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder

    You guys would probably be best off writing your own analysis spreadsheet tailored to your design. This way, you are more likely to:

    a) know and understand the assumptions used in it;
    b) understand the limitations of the analysis;
    c) understand how the analysis methods are being executed;
    d) have an analysis tool that meets the needs of your specific design.

    It's kind of scary for me to watch you guys talk about plugging numbers in to a "black box" and then wondering what's happening without completely understanding everything it is doing.

    I, too, have Raymer's Conceptual Approach book, and it is just a starting point. Nothing more. It contains perhaps 5% of the information needed to completely design and analyze an aircraft. I currently have about 40 aircraft design/analysis, mechanical engineering, and other related books, not including any of my A&P school books, or any of the dozens of PDF NACA and other reports I've found here and there and saved, and I'm still not sure I have the complete picture yet.
     
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  14. Sep 16, 2014 #14

    Rienk

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder

    Uh, that's why I'm asking these questions?
    I actually understand the equations pretty well - can even understand the precedence and dependents in the formulas... but it's because I'm comparing to specific aircraft spreadsheets and data already generated that I'm even able to question the results of some of this spreadsheet. I'm not a designer, but I do understand math - which is why I'm puzzled when it doesn't add up.
     
  15. Sep 16, 2014 #15

    Hot Wings

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder

    Works for me! Spreadsheets from other people often take as much, or more, time to figure out and proof than building your own. This thread is just one more data point to prove that. The though of having an "all in one" sheet is quite compelling but it would be a lot of work.
     
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  16. Sep 17, 2014 #16

    Monty

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder

    I have both books. The method is iterative. You must guess, then update. The directions are on the spreadsheet. Get the book, read, understand, then use the spreadsheet. It's about as simple as it gets.......conservative too.

    BTHW, his numbers matched mine within a few tries, once I understood how the spreadsheet worked.

    And tail volume is always too small. It's not a real non dimensional number. Only gets you in the ball park....just like Raymer's notes on the spreadsheet say. You guys sure are quick with the rocks......
     
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  17. Sep 17, 2014 #17

    Rienk

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder

    Seriously guys, quite treating me like a moron.
    I understand the method is iterative - I've gone through those iterations.
    I can read the directions on the spreadsheet, I learned how to do that in kindergarten.
    I've read the book, though many moons ago.
    I've been studying the spreadsheet to try to understand where he was going with it... I find it ironic that on one hand you say it's "conservative", and then on the other say the "tail volume is always too small."
    It's not a real non-dimensional number? What is it then, a fake non-dimensional number?

    I was using what I would call fairly standard LSA type designs to see how it worked... there are numbers that don't agree by 20-30%. I don't know how having even one major design parameter off by that much is "in the ballpark", let alone several such parameters each being off by that much.
    On several attempts at different aircraft, I listed some of the things that were off the most (Wo, empty weight, range, and to a lesser extent - cruise, tail volume). I am simply trying to understand why the different results.
    The whole point of this thread was for me to understand how the spreadsheet works (better than darts).
    Sorry to malign this sacred cow.
     
  18. Sep 17, 2014 #18

    nerobro

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder


    What design are you having trouble with? I've always been able to "build" any conventional design I run across in the spreadsheet.
     
  19. Sep 17, 2014 #19

    something-awful

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder

    Funnily enough, a lot of the guys I've worked with spent a long time making conceptual design tools, the most recent two I can remember was one for sailplanes and one for hypersonic aircraft and they all said the same problem was taking into account all the variables of different configs, the GUI and trying to second guess the end user. These spreadsheets become real messy real quick unless you hide most of the complexity from the user, which is a nightmare when things don't add up.

    The conclusion always was to build tools to help with specialist applications and assume the user could make their own 'master' spreadsheet that worked for them.
     
  20. Sep 17, 2014 #20

    Rienk

    Rienk

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    Re: Questions about Raymer's "Simplified Aircraft Design" Spreadsheet for Homebuilder

    Years ago, I hired three aero interns, and one of their projects was to develop spreadsheets for as much of the basic design stuff as they could, with the occasional help of a professor... they couldn't even agree on what data to input first, let alone how to organize it and cross check it.
    We ended up with some pretty good stuff eventually (wish I still had it), but finding bugs in formulas is a bear - especially with units and conversions.
    I had hoped such tools were available by now; kind of like Airplane PDQ?
    Oh well...
     

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