Need advice on ultralight idea

Discussion in 'The light stuff area' started by GeneG, Jul 6, 2019.

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  1. Jul 6, 2019 #21

    pictsidhe

    pictsidhe

    pictsidhe

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    220sqft of 0.020 is 61lb. That's only the wing skins. The fuselage skin is going to be about the same area. That would leave 13lb for luxuries like spars, bulkheads, empennage, undercarriage, hardware etc after your 120lb motor and batteries.
     
  2. Jul 6, 2019 #22

    poormansairforce

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    That 13 lbs is for the ribs.....:)

    Once again we are reminded why ULs tend to be truss and fabric.
     
  3. Jul 6, 2019 #23

    GeneG

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    Not sure where my error is, however the BOM specifies 3 4x12 sheets of .020 which totals 41.04 pounds and 1 4x12 .040 which is 21.36 pounds.

    That would yield 68.40 pounds total not including waste.

    Since I am adding 40 square feet of wing times 4 times .286 I would add 34.2 pounds total.
    that would bring the sheet weight to 102.6 pounds.

    My airframe would be 254 pounds minus the 120 for powerplant leaving 134 pounds.
    That doesn't include the angle, and does not include waste. After sheet weight leaves 21.4 pounds.

    I could opt for a BRS to gain a little, and possible reduce the rear wing skins to .016 reducing the wing weight by about 6.6 pounds?

    Worst come to worst I can reduce the motors to about 10 pounds each saving 30 pounds, and pare back my packs by 30 pounds leaving me with only 20 minutes flight time.

    Possibly I should aim for a LSA but I prefer to try for far103. The same airframe equipped with additional batteries may make a reasonable LSA or european etc. mission bird?
     
    Last edited: Jul 6, 2019
  4. Jul 6, 2019 #24

    Rockiedog2

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    The Hummel UL that TFF mentioned runs a Half VW which weighs about 80#, not the 160 that I believe you are using in your projections.
    The Zenith 701 uses 016 6061T6 for all the skins even the leading edges. I don’t know for sure but suspect the Himmel UL does too. At any rate, I suspect you’re gonna exceed 103, but good luck
     
  5. Jul 6, 2019 #25

    GeneG

    GeneG

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    My benchmark weight is based on the teenie with a full vw.
    Using .016 for all skins would give me about 15 pounds.
    I may consider oratex for the upper and lower skins on the wings.
     
  6. Jul 6, 2019 #26

    blane.c

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  7. Jul 6, 2019 #27

    pictsidhe

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    You can't size the structural parts of an ultralight by eye. Doing it with maths is not exactly simple, but essential for a 103 to meet weight and have everything strong enough. Start throwing 'extra strength' around and it will be an overweight turkey, or, it will need other parts undersized to meet weight.
    You should start by deciding the planform and surface sizes. That takes some aerodynamics. You can cheat somewhat or even a lot there and use dimensions or rules of thumb that have worked for others. Wing area from the 103 appendix, empennage from tail volume. Control surface size from what others have used. Cockpit from a mockup if you can't measure one up.
    Now, calculate the maximum loads on all the major parts. that's more aerodynamics.
    Only now can you size the structural parts, after choosing dimensions, while pure bending, tension and compression isn't too tricky, but buckling of thin shells can get tedious. A tin 103 will be limited by buckling unless you can oversize the spars and stringers to take the loads.
    Iterate the above steps until you have something that doesn't look like a glass turkey...
    That's why there are hardly any stressed skin 103s, it's far easier to analyse a fabric covered skeleton. Get some of the above maths wrong, you fly a turkey, or waddle a penguin. Get other parts wrong, you become Icarus.
    Plagiarism research is highly recommended. Acquire plans and see how others have built something that actually flew. The Ultracruiser is the only tin 103 for which you can get plans. Maybe there were other tin 103s, but I don't know of them.

    How much wing area are you planning on? You need to cover both sides. For our ~15% aerofoils, we need slightly over double the wing area in sheeting. A 4x12 is 48 sqft and about 13.3lb. 3 of those is enough for a little under 70sqft of wing area. You will need around 110sqft with flaps/flaperons. Call it 5 sheets. Add ribs, add a fuselage which will be several more sheets. Now you still need formers, stringers, spars, gear.
     
  8. Jul 7, 2019 #28

    blane.c

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    The top two airfoils at this site are specifically designed for ultralight, not suggesting you should use them but you can see they are 18% thick airfoils compared to the 4412 being a 12% thick airfoil. Just food for thought.

    http://airfoiltools.com/search/list?page=u&no=0
     
  9. Jul 7, 2019 #29

    pictsidhe

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    Doppelposten.
     
    Last edited: Jul 7, 2019
  10. Jul 7, 2019 #30

    pictsidhe

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    I'm a very big fan of thick aerofoils on 103s. Look at 15%+ I'm currently going to use 21%. The Zenith E-ABs use 18%.
     
  11. Jul 7, 2019 #31

    radfordc

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    For an ultralight I would worry about the capability to do a go around the same as I would for a glider.
     
  12. Jul 7, 2019 #32

    BBerson

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    Gliders can sometimes cut in line for landing in accordance with the glider right away rule. Ultralights must yield to all other traffic. So depends on the field and type of traffic.
     
  13. Jul 7, 2019 #33

    GeneG

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    That is a fairly comprehensive presentation.
    I give the following comments:
    The motors he used are predecessors to my alternative motors.
    Like him I would be forced to rewind them for my use, however I would use a delta configuration and run them at higher voltages to increase torque and reduce current flow and heating.
    The battery technology today has at least 1/3 higher storage density, and higher safe discharge rate yielding much better cycle life.
    The current motors are available water cooled enabling longer maximum power settings.
    Using these I can expect at least 150 pounds of thrust static without redrives.

    Thanks for the link!
     
  14. Jul 7, 2019 #34

    GeneG

    GeneG

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    Using the smaller packs, I should be good for 20 minutes with 20% reserves. Full power will be available anytime up to that.
    Using the larger packs, I can expect about 45 minutes with 20% reserve.

    Thanks
     
  15. Jul 7, 2019 #35

    GeneG

    GeneG

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    Those are my requests for the forum. I did say I am open and lacking experience I am counting on you for this kind of feedback.
     
  16. Jul 7, 2019 #36

    Rockiedog2

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    if you use fabric on the wing you're gonna have to add drag, antidrag braces of some sort, right?
    and there goes your weight advantage using oratex. Most likely...I don't know exactly what got in mind
     
    Last edited: Jul 7, 2019
  17. Jul 7, 2019 #37

    GeneG

    GeneG

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    I am looking at least 100 square feet but it depends on the foil chosen. I quoted the total sheetage for a teenie and added 4 times for the additional span which should cover ribs and spar extentions. If you look at the Zenith ch750 wing detail you will notice that the outer 2 feet or so of the main spar is simply a thin web with 2 bends and no angles which is rivited to the main spar. the teenie had 53 square feet.

    The spars for a teenie (or a Zenith ch701) should support the loads. I would hire an engineer to design the airframe structure.
     
  18. Jul 7, 2019 #38

    GeneG

    GeneG

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    That could be done like the preceptor pup wing.
    I might add that I am loath to have to resort to this option.
    Thanks
     
    Last edited: Jul 7, 2019
  19. Jul 7, 2019 #39

    pictsidhe

    pictsidhe

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    Motor torque depends on the winding ampere-turns and magnet strength. Rewinding just changes the voltage needed for maximum torque. It won't give you extra without increasing the thermal load on the motor.
    Halve the number of turns you can get to use double the cross section of wire and double the current for the same resistive power loss. But your voltage is halved for the same rpm:
    Half voltage, double current, same power, same torque, same efficiency, same thermal loading on the motor. Lunch price is the same as yesterday!

    The e-lazair is something you may want to think about building. Electric planes are short on energy. Assuming fixed weight, wing area and Cd0, L/D varies with the square root of aspect ratio. That make sit proportional to the span. Double the span, you can fly twice as far, though at a lower speed. this makes a large span very, very attractive for electric aircraft. As long as you aren't in a hurry...
     
  20. Jul 7, 2019 #40

    GeneG

    GeneG

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    My primary choice is a true induction motor that is water cooled. The magnetism is therefore induced into the rotor.

    The second choice is a permenent magnet motor. The use of delta verses y windings changes the angular attraction of the rotor and the therefore affects the rotor speed vs torque available given the input current.


    Span is good except I am facing a weight limit that I wish was more generous.

    Thanks.
     
    Last edited: Jul 7, 2019

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