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Thread: Final year project

  1. #1
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    Unhappy Final year project

    Hi guys,

    I am pretty new to this forum but I was wondering if you could help me? I am a final year student at University studying Aerospace Engineering. As part of my final year project I am re-designing a microlight (ultralight) aircraft (lecturers design) using composites, the aircraft at present is all aluminium. Basically, we have decided to go down the route of the C42 Ikarus as far as the fuselage construction is concerned, i.e. a main load bearing aluminium boom and frame with an E-glass fuselage skin (this has been dictated to us). However the wing has been a pain. We have been told to look at all foam construction, just like Rutan's design. I noticed in one of these threads entitled "Spar-less wing" some posters talk about similar ideas. We were thinking of an all foam wing box (with the load bearing boom "sunk" into it) adhesively bonded and glassed. Then the wings, (not a monoplane but two distinct wings) attached to the foam wing box possibly adhesively bonded to the foam wing box using something like a scarf joint at the wing/wing box joint. The wings themselves being foam (acting as the shear web) glassed with S-glass. We calculated that each wing would be about 3.35 metres (10.99ft) each, this is the wing span (7.9m or 26ft) minus the wing box width divided by two. However, we are not sure if it is possible to hot-wire something of this size, nor are we sure that you can even get foam blocks of this size or whether this kind of design is feasible!? We were thinking that if it is not possible to hot wire cut a wing of this size then maybe it could be done in two sections then joined in a similar fashion to the wing/wing box junction using a scarf joint? Any info or a pointer in the right direction would be greatly appreciated, it is so difficult to find information of this sort anywhere!!!!

    Thanks in advance!

  2. #2
    Registered User PTAirco's Avatar
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    Re: Final year project

    Pictures pictures!
    "Aeronautical engineering is highly educated guessing, worked out to five decimal places. Fred Lindsley, Airspeed."

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    Moderator autoreply's Avatar
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    Re: Final year project

    Solid foam wings aren't practical above a certain size. Since foam volume (and mass) goes with the square of the chord (and linear with the span) it's mainly used in thin, long and relatively small wings like the Long-Ez.

    Aircraft Spruce is a well-known supplier of all kind of homebuilding stuff:
    POLYSTYRENE <br>LARGE CELL FOAM from Aircraft Spruce
    Where the thickest foam is 10" (25 cm). I don't think it's impossible to glue two layers together and 50 cm thickness is plenty for even the most volumious wings.

    Try using the search here for all kind of information and construction methods (and don't hesitate to ask). Especially Billski has made some very informative posts about pro en cons of different methods.

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    Re: Final year project

    For those of us in other countries, could you tell us your country's definition of a microlight? For instance, here in the USA, ultralights can only weigh 254 lbs empty. LSA's can gross up to 1320 lbs (actually, I think it's more arcane than that, but it's close.) But I'm guessing your limit is lower.

    You might try looking at links, etc. from forums or web sites on the Cozy, Velocity, etc. Or find one of the Rutan Aircraft manuals for putting one of those designs together. I saw one once and it had some pretty practical advice.

    However, unless the wing chord is small and the airfoil is not too fat, the foam is going to be heavy. Particularly if you use extruded foam instead of flimsy bead foam.

    Foam sections aren't very strong compared to almost any glue, so you don't have to scarf them. People usually use epoxy and microballons, as I recall. It's important not to let the epoxy get too thick as it will heat up, since styrofoam is such great insulation. Obviously it's best to join the foam first and have a continuous layup of the stronger parts.

    Jack Lambie has a book on composite construction, but I don't remember what's in it. And it's not new. But I'd guess it's worth the small risk of acquiring the book.

    A neat trick I've seen is that if you can control the speed of the foam cutting wire precisely enough, and there are no obstacles, then you can slow it down and cut by radiant heat alone. This means no wire sag and greater cutting precision. People do this by automated foam cutters, but I bet you could use a Tekoa style cutter, really good templates, and maybe some kind of clockwork brake and do the same thing.

    BTW, piano wire makes ok cutting wire if you don't abuse it. And if you do, it's cheap to replace. Maybe .020" (0.5mm). Even better, I've heard, is stranded control line wire. (For flying control line planes.) I'll bet stranded fishing leader wire works ok too.

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    Re: Final year project

    Autoreply,

    Thanks for getting back to me. How about this foam, it seems lighter still, at 1.6lb/ft^3 (25.63kg/m^3) Wicks Aircraft Supply

    Ir27,

    Thanks for the reply some good info there. The regulations state that, in a nutshell;

    a microlight aircraft is one designed to carry not more than two persons which has a Maximum Total Weight Authorised (MTWA) not exceeding 450kg or 992.1lb for a two seat landplane. A microlight must also have either a wing loading at maximum weight authorised not exceeding 25kg per square metre or a stalling speed at the maximum weight authorised not exceeding 35 knots (64.82kph) calibrated air speed (CAS).

    Any ideas where I might get hold of plans/construction details on Rutan's Long Ez? I have been looking with no success.

    Some spec of our microlight............

    Aerofoil: NACA 63(subscript 2) - 415
    Max.thickness/chord: 15%
    span (m): 7.92 (26ft)
    Area: 100ft^2


    We have been "directed" towards a alu fuselage boom/frame with non load bearing glass skin fuselage.

    And we have been told look at foam wings such as Rutan's design!

    I have Jack Lambie's book already, not much help I'm afraid.

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    Re: Final year project

    Assuming you're near the max of the weight class, how are you planning on dealing with the stall speed issues? At 10 lb/ft^2, you're at almost twice the permitted wing loading, so you'll presumably have to demonstrate stall speed. A coefficient of lift of 2.4 is not really likely on this type of aircraft...

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    Re: Final year project

    edit
    Last edited by Ulsterman_UK; April 19th, 2010 at 07:23 PM. Reason: rethink

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    Re: Final year project

    I can't tell what kind of foam that is. Usually expanded means bead foam, which is pretty weak stuff. You've got a lot of volume in that wing. If you used 2.2 lb foam, I think you'd have close to 100 lbs of foam in it. Perhaps solid foam for the front and cut ribs in the back? You could overlay the foam with fiberglass tape and epoxy, say, and cut away in between.

    I haven't perused it, but look around for the Open EZ project. Also, I'm sure there are many build logs for this sort of construction.
    Here's one:
    Marc Zeitlin's Cozy MKIV
    I used to know this guy, and he seemed pretty careful about things. Lots of details in various places on his site. Lots of construction pictures too.

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    Re: Final year project

    Addaon,

    To work with the stall speed issue?
    Thinking maybe double slotted flaps? Leading edge slats? Wing tips (Hoerner style)? Flaperons? All ideas I can look at. I appreciate what you are saying but I have a report to write up, this is the direction I was told to go, however the report isn't the final report, its an interim one so I can iterate I guess at a later stage.

    Ir27,

    Again thanks for your input, its greatly appreciated. Could you explain to me what you mean. Sorry I have never built anything of this nature before, and composites are a fairly new subject to me. Do you mean form the whole wing out of foam, and then basically cut out a lot of it from the back (from which position?) leaving only ribs? Would this not weaken the structure? When you say tape.....is this instead of the glass fibre cloth/weave skin? Can the skin still be laid wet over just ribs rather than a solid form? You see another issue I had due to the fact that I have been told aluminium fuselage frame and all foam wing is how to "join" the two. The fuselage skin is obviously non load bearing, therefore the wing has to somehow connected to the load bearing boom or aluminium frame.
    Even with internet pictures it is still hard to visualise if I have never physically worked on this sort of thing before!!

    Once again thanks!

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    Re: Final year project

    Addaon,

    Can you tell me where you got that 2.4 lift coefficient from?

  11. #11
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    Re: Final year project

    weight = CL * 0.5 * rho * v^2 * S

    Let me google that for you
    The Lift Equation

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    Re: Final year project

    You've got the right idea on the ribs. It would be weaker, but if you don't cut, it will be stronger and heavier than it needs to be. By tape, I mean fiberglass which has been formed into a nice, convenient tape form. The edges don't unravel. For a rib, it might be good to use a layer of unidirectional tape. You can probably find some kind of tapes at Aircraft Spruce, Soller Composites (a small outfit, I think, but I've used them), etc. Not sure, but it might be a good idea to use a light woven tape to keep the other one from splitting. You would then have to cover with fabric, of course. I would lay down the tapes and THEN cut out between the tapes. Or maybe just make the front part, then glue ribs, etc. to the back. Might be easier to get required shear web in there that way.

    Something to keep in mind is that although I've worked with fiberglass and epoxy, I haven't cut out ribs the way I say above. It's just an idea. I'm sure it would work except I haven't thought of how to deal with shear web stuff, which I'm guessing might be necessary if you have only the foam in front. On the other hand, you could afford to use denser foam, and maybe that would take care of it. Maybe even glue a layer of really dense foam into the blank beforehand, at the thick part of the airfoil.

    At the moment, I don't have any bright ideas on how to connect the wing and that aluminum tube. You might consult Strojnik's books, if you can find copies within your budget. The titles all start with "Laminar Aircraft....", and I'm pretty sure the info relevant to your questions is mostly in one of them. Strojnik liked the aluminum tube with non-structural fairing methods and built several aircraft this way. If you have access to old copies of Sport Aviation, he had a couple of articles in there, years ago. I think he used plywood bonded to aluminum, but you have to be VERY careful and thorough when epoxying stuff to aluminum. Funny chemicals, deionized water, etc.

    Quote Originally Posted by Ulsterman_UK View Post
    Addaon,

    Ir27,

    Again thanks for your input, its greatly appreciated. Could you explain to me what you mean. Sorry I have never built anything of this nature before, and composites are a fairly new subject to me. Do you mean form the whole wing out of foam, and then basically cut out a lot of it from the back (from which position?) leaving only ribs? Would this not weaken the structure? When you say tape.....is this instead of the glass fibre cloth/weave skin? Can the skin still be laid wet over just ribs rather than a solid form?
    snip

    Once again thanks!

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    Re: Final year project

    Addaon,

    Thanks buddy, I know the formula


    Cl = W/0.5*rho*V^2*S

    where W = weight (of aircraft) = lift in steady level flight
    where S = wing area

    or you could use W/S = wing loading (w)

    (Making assumptions of course)

    I wondered how you came to the figure 2.4 for the Cl, but we realise what the issue is now, oops a school boy error to do with mass and weight A factor of 10 out, if you get me

    Thanks again dude.



    Ir27,

    Excellent, thanks for getting back to me Ir27. That's all really useful information, I'll look more into it however I don't have a great deal of time on my hands to get this interim report written up but I'll certainly consider this in my final report! The info about the tape is handy, I didn't know about it of course so that's another material I can look at! As for the books I'll have a dig around the University library and see what I can come up with!!


    Thanks!

  14. #14
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    Re: Final year project

    Yep, that's why I use google calculator for these things... it checks units for me automatically.

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    Re: Final year project

    Just a brainfart.

    If you have a Rutan-style torsion box (before the wing) one could build a conventional (female mold, sandwich skin) wing skin after the spar. Normally making female molds is a major issue. By going to a straight wing panel (from say 50&#37; chord) you can save a considerable amount of work.

    Requires some major work in aero though.

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