DH 88

Discussion in 'Classics' started by Atomic_Sheep, Nov 7, 2008.

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  1. Nov 7, 2008 #1

    Atomic_Sheep

    Atomic_Sheep

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    Hi guys, as the name of the thread suggests, I'm interested in the De Havilland 88 Comet. I fell in love with this machine the minute I saw it but the idea of building one for myself literally popped in to my head this morning (but I've wanted to fly for a very long time). I've got experience building model airplanes (RC and free flight), so obviously thats not exactly what one might call experience, but none the less.

    I would like to find out a few things about the plane (and yes I know there's really not all that much info available for it) but they are more general aviation questions I think. As a sidenote I would like to mention that apart from general external looks, I do not much care for reproductions of the original plane.

    1.) in terms of flyability, what do you think a plane such as the 88 would be like?

    2.) since it was historically of a wooden construction, what do you think would be the consequence of me building one from aluminium? Weight wise and strength wise? Obviously strength would be improved but weight?

    3.) linked to 2.) but since I mentioned I care little for exact reproductions, I obviously care little for the 170kw straight 6 Gipsy motors that were in the original aircraft. I want to fit into the original aircrafts engine (cowlings?) but I would prefer to have something modern powering mine. One would assume something more modern would have more power, better reliability and fuel consumption and as a result I would imagine that if I was to assume the answer to 2.) was that aluminium would make the aircraft weight a lot more, then would the newer engines be able to counter the weight increase? I have no idea of whats out there on the market powerplant wise so I really have no idea what can be or can't be done.

    P.S. I'm not entirely sure of which section this is supposed to go in, hopefully I'm in the right one, but admin is obviously free to move this thread to an appropriate section.
     
    Last edited: Nov 7, 2008
  2. Nov 7, 2008 #2

    PTAirco

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    As far as flyability goes - the original had a reputation for being downright nasty. High approach speed and not terribly benign stall behaviour. The test pilot was not particularly happy with it, but it was a one-purpose design; to go fast to Australia. With such a design something has to suffer and people accepted the faults that went along with it.

    But since you're not trying to design a perfect reproduction of it, I'm sure most of the faults could be designed out. I personally think wood wood be the material of choice for this, since you could at least keep the structure close to the original and have a lot of the design work already done for you. It did use clever techniques to get the most out of the material.

    Doing a look-alike in metal is a huge undertaking, though I am sure very rewarding - it is one of the most beautiful aircraft ever, in my opinion. Just don't spoil it with modern flat 4 or 6 engines -ugh... What about LOM's - perfect substiutes for Gypsy Sixes.
     
  3. Nov 7, 2008 #3

    Atomic_Sheep

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    Well I've read about tip stalls (whatever they are) and yes with the wings that it has it certainly would require high landing and takeoff speeds, but that in itself is not enough to put me off it. I guess that question was aimed more at the reliability of the frame and things like that. Since I have no flying experience apart form many many many hours in simulators, I can't say whether I'm going to enjoy this aircraft landing and take off wise but the fact that it was designed for speed certainly makes it a very attractive aircraft. The fact that test pilots dreaded it certainly has made me wonder though.

    Well I don't know how smart this may be but I was thinking of pretty much fathefully reproducing the airframe except in aluminium. So based on that I figured that since aluminium is stronger than wood, well then it would prove to be sufficient in the way of airframe strength (I have no knowledge of material attributes so perhaps tensile and all those other measurments of the wood were taken into account in relation to the design) but I'm hoping it will prove to not really be an issue since this plane was designed at a time when people had little knowledge of aircraft design and most likely would have overcompensated on the structural integrity.

    Other than these rather ignorant statements I was wondering whether you culd elaborate on your comment. Why would it be a huge undertaking?

    The two reasons why I wish to do this aircraft in aluminium is mainly because I don't trust wood especially since it warps (or can) over time and especially since all the glue joints (in my opinion weaken over time significantly as a result of the glue oxidising/hardening or whatever epoxy does over time).

    Engine wise I haven't begun to look into that yet at all. The engines you suggested I haven't heard of but I'll look into them at some stage for sure. I only included that part in order to emphasise the fact that I will be increasing the power of the engines and therefore enabling higher weights (resulting from the aluminium constrution) to be compensated.

    If you could elaborate on the sorts of things that I would be looking at in relation to this comment, that would be great also. But overall, structurally I would like to leave it as fathful as possible, it's the internals that will be modern, but if there's simple things that can be done that won't change the appearance of it, then why not, so definately open to suggestions.
     
    Last edited: Nov 7, 2008
  4. Nov 7, 2008 #4

    bmcj

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    Tom Wathen, owner of Flabob Airport in Riverside California has a very faithful flying replica of the DH-88. It was built for Tom by the late Bill Turner's Repeat Aircraft (also at Flabob). They made a trip to England to study and measure the original in order to build an authentic replica. It is currently being flown by Robin Reid to airshows around the country, but it will be coming back to Flabob once they install a belly camera so the pilot can see to land on smaller runways.

    Go to Flabob Airport Thomas Wathen Foundation for information on Flabob or the Wathen Foundation; http://www.dhmothclub.co.uk/pixbin/comet-x.JPG for a photo of the plane.

    Bruce.
     
  5. Nov 7, 2008 #5

    PTAirco

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    Replicating the airframe in metal rather than wood means you will will have to design a brand new airplane. All that would be common would be the outline, every structural piece would be designed from scratch. After you figure out the "How will I build it" you will need to do at least a basic stress analysis. And if you try to fix all the "hairy" aerodynamic quirks you basically need to figure out all that too, and do an aerodynamic analysis of the (new) aircraft.

    Not reason it can't be done, but it is a job and a half. Not something you could do without getting some engineer on your side or spending several years learning it all yourself. Which is what I did - I had all kinds of ambitious ideas at first, much to the amusement of other armchair experts and people who thought they knew better. What you need to find is people who really do know better and pick their brains and don't get too discouraged by reality.
     
  6. Nov 7, 2008 #6

    Atomic_Sheep

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    I envisage placing a camera (IR?) in the nose glass section of the aircraft with gyroscopes so that I will get an in-cockpit view of not only where I'm going on the ground but also once in the air. Gulfstream G550 and G650 have these cameras installed in their business jets. Turns out that statistically 70% of all aircraft accidents occur during landing (or something along these lines) and having such technology on board will certainly help in flying in poor conditions (if ever I decide to do so or if I simply stumble into bad weather by accident...) either way its a great safety feature that I plant to have and I don't see a reason why it can not double up as a nose cam for the above stated reason.

    And yes I've hear/seen their work. There's another restoration of Black Magic being carried out right now, so there should be another one of these birds in working order soon enough. I think the green one (don't know what that one is called) is also being restored also. The Royal Blue was supposedly lost in a fire after a crash.


    This is certainly a bitter disappointment :dis: especially since I'm not particulalry keen or perhaps not in a position on doing another university degree especially since it will take 3 more years of schooling, more debts and postpone construction 3 years. So I will have to do some serious thinking about this. I think engineering knowledge can only be useful but doing it just to build this plane, well I don't know. When I was at school I wanted to do aerospace engineering but my marks weren't high enough for that and my mate who was doing it dropped out after a year as he found it too difficult and time consuming and he DID get the marks.

    I do fail to see how aerodynamics will change if the shape will remain the same :ermm:
     
  7. Nov 7, 2008 #7

    djschwartz

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    If all you've flow are simulators and don't even know what tip stall means I would suggest for your own safety and happiness you choose something a little less daunting for your first project. This would be a major undertaking even for a very experienced team and a pilot with a lot of time in challenging aircraft. I knew Bill Turner when he was around and Robin Reid when I flew out of Reid-Hillview and even though I fly the Stevens Akro and have flow various other interesting birds from a Fokker DR-1 to a T-6 I would not put myself anywhere near their league.
     
  8. Nov 7, 2008 #8

    Atomic_Sheep

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    I would think that by the time that the 88 is completed I would have had some flying experience (especially since I would imagine it's illegal to fly without a licence, and I would want to get an instrument rating for sure). As for simulators, I've mainly used them for fun, not as an educational tool simply because I hate computer games/simulators as they don't come close to giving you an engaging enough experience for me to consider them a learning tool. Other than learning navigation and other similar lessons they are completely useless when it comes to actual flying. The reason why I don't know what a tip stall is, proves my outlook on simulators as in the many many years of flying them, I've never had one. I've had plenty of normal stalls though.

    But now that I've thought about it... I'm guessing a tip stall is exactly the same as an ordinary stall but occurs when an aircraft is turning sharply (at low speeds) resulting in the inside wing losing lift and cuasing the plane the flip as the other wing still has lift. I'm guessing it's called a tip statll because it starts at the tip as the inside wing tip would be moving the slowest as its travelling the shortest distance in a cirlce. That's just a guess.
     
    Last edited: Nov 7, 2008
  9. Nov 7, 2008 #9

    Spodman

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    The original was built on a concrete form, yeilding the flowing lines compared to more boxy metal aircraft from then (and now). The construction was ply, then a layer of end-grain balsa then more ply. I would say the method that best approximates that is foam-core composite construction. dunno about the engines though.
     
  10. Nov 7, 2008 #10

    Atomic_Sheep

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    I've got a feeling I'll be designing one of these from scratch as I'm quite set on making it from aluminium. Since the design won't change very much (although I will have a look at aerodynamics and see what I can improve/tailor more to my requirements/liking etc) for example winglets (assuming I don't hate the way it looks with them), advanced props and things like that.

    Other than that, what are the books that would be a good read both on aerodynamics and airframe design? I found

    Amazon.com: Practical Stress Analysis for Design Engineers: Design & Analysis of Aerospace Vehicle Structures: Jean-Claude Flabel: Books

    this to be highly regarded but based on the reviews it's not an easy read.

    Any suggestions?
     
  11. Nov 7, 2008 #11

    Dana

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    There's nothing wrong with wooden construction. Yes, aluminum ia stronger but it's also heavier; the strength to weight ratio of wood is actually very good. As for longevity, there are thousands of older wooden aircraft out there that have been flying for years with proper care.

    And as already pointed out, it doesn't look like an airplane for a low time pilot.

    -Dana

    If you glue a piece of toast, butter side up, to your cat's back, and drop it from a high place, which way will it land?
     
  12. Nov 7, 2008 #12

    Atomic_Sheep

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    The more info I find out about its flight characteristics the more I start to understand the reasons of its various design aspects and the more I realise that this is exactly what I want and these are the exact reasons that prompted me to fall in love with it as all these issues, I think, were worked out on a subconcious level. So no offense to anyone, but I'm very much set on it and hence do not try to sway this view, as you are wasting your breath... once again no offense.

    Just wanted to throw this video in, to see them fly is definately a rare thing.

    http://au.youtube.com/watch?v=6JTxQPJt0VM

    EDIT: it's half scale
     
    Last edited: Nov 10, 2008
  13. Nov 7, 2008 #13

    djschwartz

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    "Tip stall" is when the tips of the wings stall before the roots. It is a function of the design of the wing and it turns out to be a common characteristic in a wing design for optimum efficiency with all other handling characteristics made secondary. With a wing designed such that it occurs, it will do so in any type of stall maneuver. My Stephens Akro has such a wing as do many aerobatic aircraft. it's great for doing snap rolls. It also mean the airplane will try to do a snap roll on its own any time it stalls. Not good for the inexperienced pilot.

    If you're serious about this here's what i would suggest as a plan:

    Get a private pilots license.
    Get a multi-engine rating.
    You absolutely must have these to fly a twin like the DH-88. An instrument rating is optional.

    Get taildragger qualified. Then find some one with an Extra 300L and get VERY comfortable with normal flight operations in it. Get a LOT of stall recovery practice until
    you can recover instinctively from all manner of stall entries.

    Then, while you at all this and trying to design your project, build yourself one of Van's designs. This will give you some experience with aluminum construction and also give you an example of a good design to get ideas from. If you're really capable of the project you envision, you should easily be able to afford his kit and be able to knock out one of his designs in a year to so. And once it's complete it will be worth more than you have in it (not counting for labor, of course) so you will then be able to sell it when you're ready to buy the expensive parts for yours (engines, props, etc). In the mean time you will have something to fly to keep you skills up. And you'll need them

    Dave
     
  14. Nov 7, 2008 #14

    Dan Thomas

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    "Tip Stall" is a really common RC term and doesn't mean a lot in the real airplane world. Most are designed to have a stall start at or near the wing root at the trailing edge and work outward and forward from there. If the stall is not equal on both sides, the loss of lift is also unequal and a wing will drop. The tips don't have to be stalled at all for this to happen.
    The DH88 had elliptical wings, whose stall patterns start at midspan rather than at the root. Can get nasty.
    A local guy was building a DH88, full-scale relica, using the Gipsy Queens, until he died about 12 years ago. Was doing a really good job of it. It was even going to carry the full fuel load. I think the project went to Australia.
    Wood is stronger for its weight than steel. Engineers have been using it for a long time. The deHavilland Mosquito fighter-bomber was built of it. And those guys knew as much about aircraft design for their times as they possibly could. Just look at some of the airplanes they came up with.
    I fly a wooden airplane, stronger than most aluminum airplanes. Wood just needs to be properly sealed and hangared or it'll rot. That's one reason why my next airplane is an aluminum airplane; the other is that good wood is getting scarce and expensive.
    The DH88, in my opinion, would kill the average private pilot in very short order, even if he was able to finish the project. It's not an easy design to either duplicate or fly. Those fixed-pitch props, along with the tiny tail, would make controlling it extraordinarily tough if an engine quit.

    Dan
     
  15. Nov 7, 2008 #15

    orion

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    A couple of people have asked for clarification on this so here's a few numbers (the higher the specific strength, the better). Like composites, wood does not have a yield strength so for purposes of this comparison all values are based on ultimate properties. Also, over the years I've also ran across a few folks who use wood's Modulus of Rupture as the ultimate strength value since actual tensile and compression properties (parallel to grain) are at times difficult to find. This however is improper since the "MoR" tends to be as much as 30% higher than the material's actual axial strength. Since it's a standard in the airplane industry, the values below are based on Sitka Spruce at 12% moisture content (kiln dried). Other woods will have nearly identical specific values since wood strength is very closely proportional to its density.

    4130 steel - Normalized
    Ultimate strength - 90 ksi Density - .3 pci Specific Strength - 300

    4130 steel - More common temper for structural applications
    Ultimate strength - 125 ksi Density - .3 pci Specific Strength - 416

    2024-T3 Aluminum
    Ultimate strength - 70 ksi Density - .1 pci Specific Strength - 700

    6061-T6 Aluminum
    Ultimate strength - 45 ksi Density - .1 pci Specific Strength - 450

    Sitka Spruce - Tension properties
    Ultimate strength - 8.6 ksi Density - .0145 pci Specific Strength - 551

    Sitka Spruce - Compression properties
    Ultimate strength - 5.6 ksi Density - .0145 pci Specific Strength - 386

    The conclusion in this is simply that despite being relatively weak, properly designed, a wood structure is no slouch in delivering good performance for weight. And of course this is why wood has been so popular and successful in the industry.

    However, when comparing airplane strength and weight, it is of paramount importance to understand that a particular structure is designed to the designer's goals. Saying that a wood airplane will be stronger than an aluminum one does not make sense since each will have equal strength if designed to a particular set of goals. Furthermore, given the specific values and other unique design requirements of the materials (shear strength, bearing strength, etc.), the aluminum airplane will in the end be the winner on the basis of strength and final weight, regardless of the scale of application.
     
  16. Nov 7, 2008 #16

    Dan Thomas

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    Something to remember, though: Ultimate strengths for aluminum can be considerably higher than yield strengths, and since we don't want the airplane to take a permanent deformation when we stress it (or more accurately, strain it) we'd use yield in calculating strength. At least I would. Wood, with its yield and ultimate being so close together, needs a bit of margin when doing the figures but it would still be pretty good.

    Aluminum is easily available and might be cheaper from outfits that sell to industry instead of aviation. Same specs. Aluminum doesn't care much about heat or cold or rain. But it takes a different skill set than working with wood and that scares a few folks off, but they could learn those skills with a bit of work. Riveting is noisy and can ruin the neighborhood relations. Sharp edges will cut you up some. I hate using gloves. Aluminum airplanes tend to be cold and noisy inside unless they're heavily insulated, making them as heavy as a wooden airplane. My wooden Jodel is fairly quiet and warm compared to a metal airplane.
    Composites scare me. I developed allergies to epoxies way back in the early '70s and have to stay away from that stuff. The composite airplane we had here suffered various problems in the cold, including windshield cracking (it's glued to the shell) when it got cold and contracted more than the fiberglass. I've wondered about the heat buildup in the outer skin against the foam when in the hot sun and what that might do to the sandwich. I've seen boat fiberglass bust loose in the sun.

    I wish there was an ideal material that covered all the bases. Isn't that what they call unobtainium?

    Dan
     
  17. Nov 7, 2008 #17

    orion

    orion

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    You're right - working with metal one must take into account both, the ultimate and yield strengths for proper design. But the benefit of the metal is that its popular use has created a very dependable database of properties and processes, allowing the structural assembly to reach a fairly high level of optimization.

    The problem with organic materials is that the properties will depend on a number of factors so the published values should be considered as close for rough sizing, but for design purposes, a bit more conservativeness is often necessary.

    Regarding composites, have you examined some of the more modern materials? The older systems had a fairly high occurrence of sensitivity, usually due to one or two particular chemicals in the catalyst. I ran into the sensitivity issue about fifteen years ago testing a product from Jeffco. Just a bit of Acetone on my skin with a tiny bit of the resin in it, instantly resulted in a rash. But no other system that I've worked with since then has instigated the same reaction.

    But composites do need a specific bit of experience in the design and in the fabrication, the lack of which can lead to the problems you describe. Some of the newer kits have gotten better but it's still up to the builder to do a good job in the construction.

    And yes, unobtanium is the stuff that has excellent strength and weighs nothing. The really high grade stuff is actually buoyant, almost negating the need for wings. Good stuff, if you can find it.
     
  18. Nov 8, 2008 #18

    Atomic_Sheep

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    I'll definately get all the qualifications needed to fly these things, as for van's design idea, I was astounded at the prices of the kits, obviously engines aren't included and they cost a fiar bit more than the kit itself, but that is definately an interesting consideration. (not that I can afford them right now but none the less very much attainable in a short/ish space of time, but as for the 88, well, if there's a mean, theres a way, I'm a firm believer that if you want something bad enough you'll definately get it and the only reason why this project will never get off the ground is if I find out that I didn't want it bad enough and only time will tell.)


    That's pretty much what I thought and as for the remainder of your comments, well I agree with them also. I felt that you couldn't really compare the two materials in simple terms and really had to compare them based on their utilisation, so that was a very good point that was raised. As for composite materials, well I just don't want to go down that road at all. I'm looking for tried trusted reliable methods. I wish to steer clear of wood because of its maintanance. I can see the benefits; quieter cabin etc, don't need a degree to notice just how much louder a steel shed is when you slam the door, to that of a wooden one. But essentially I don't like maintanance work. I like the Russian phylosophy of building things. Make it simple and rugged. Not saying I want to go stone age simple but what I mean by this is I want to use tried and trusted methods and materials that attain an end result that I desire i.e. an aircraft that I don't need to store in a humidor, constantly inspect for various tree munching dwellers and checking and rechecking every joint on the airlplane. Don't get me wrong, I'm an extremely anal retentive person. If I start to do maintanance work then I will spend an enormous amount of time on something that other people would just glance over. But an aluminium construction I think would give me a longer time interval between inspections. So thats another reason. Anyway enough jibber jabber on my behalf. Time is short so might as well get down to it.

    Design wise, where do I start? This day and age everything is designed on computers and this is what I want to do also. Use computers to do as much design and testing as possible. I don't want to have to build 15 different wings, attach those cables to them and do stress testing. Ideally, I want to do everything on the comp and then transfer all the parts to a CNC, get all the parts cut out, stick em all together and away you go.

    The other issue is how long would it take to do? Obviously depends on abilities and resources but some approximate figures would be nice.

    The other issue I have not decided on is the depth of my development. I can go the easier route and not change a thing appearance wise and hence leave the aerodynamics as is. Do the structural design (in aluminium), plug in some industry standard engines and away you go. Or I can go the full redevlopment. Tweak aerodynamics, rework the structure, design my own engines (since I now see the reason for the use of the straight six in the original and since there has been a lot of tweaks to car engines which I'm not sure have been implemented in prop planes e.g. direct injection and perhaps supercharge the engines) and the list goes on. I'm not saying that the comprehensive latter method can't be done as I know it can, but the question is, how long and how much more expensive will it be (taking into consideration my desire to do as much on the computer as possible). Like I said and would like to emphasise, I would prefer to do it on computer, cut out the parts, stick em together and go.

    And finally, I've asked this already but what books would you guys recommend?

    P.S.

    I would just like to summarise what I want from the plane:

    - speed (but not at the expense of reliability, but that would suggest reworking the aerodynamics I would imagine even a little)
    - reliability (so that would mean overengineering everything and toning it down a notch)
    - weight (linked to speed... as low as possible but not compromising reliability)
     
    Last edited: Nov 8, 2008
  19. Nov 8, 2008 #19

    PTAirco

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    A lot of of well-meant advice here may come across as discouraging, but don't look at it that way. What you propose is a very,very ambitious thing, for somebody with no design or building experience, never mind the flying experience. But reaching for the stars may get you somewhere eventually....

    Re engines - nothing except inverted inline engines will give the aircraft the proper appearance and character. "Plugging in" industry standard engines ? Industry standard is a Lycoming or Continental - it would be pure sacrilege to put those into a DH88! The only engines that come to mind other Gypsys are Fairchild Rangers and Walter LOMs . The Rangers can be gotten fairly cheaply but require quite a bit of inside knowledge and parts such as main bearings are becoming impossible to find. The LOM engines seem to be in production sporadically and hard to find sometimes. If you are shocked by the cost of kit aircraft, have you checked the cost of aircraft engines lately?

    As for books - if you are starting at square one as I believe you may well be (no disrespect intended) read "The Aeroplane Structure" by Kermode . There is no better primer on structures to give somebody with zero aero-engineering knowledge an insight as to what is involved. Perhaps after you read anything and everything on Applied Mechanics you can find.
    I am happy to see googelk books has it:

    Aeroplane Structure - Google Book Search

    Hope that link works, if not search for it.
     
  20. Nov 8, 2008 #20

    Atomic_Sheep

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    I don't understand how horizontally opposed engines would fit into it in the first place. But reading up on the disadvantages of inline 6's well... weight is supposedly what killed them off. But at the end of the day, not much other than an inline 6 (or smaller) would fit into it.

    I'm not discouraged in the least. And no offense taken, I know I'm a noob and I know I have a mountain to climb and I know that I'm ambitious.

    Generally speaking I think I know everything about the way planes are made as I've seen plenty of cut away pictures and such. Unfortunately observing the correct way of doing something doesn't automatically mean you fully understand the REASONS behind what you observe and hence that effects reproductability of those ideas in the correct context if you know what I'm talking about... so essentially... bring on the recommended books. And yes I'll check it out.
     

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