Ultralight struts/cantilever/additional weight

Discussion in 'The light stuff area' started by MrHopkins, Jan 14, 2020.

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  1. Jan 20, 2020 #61

    Riggerrob

    Riggerrob

    Riggerrob

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    The basic problem is the logarithmic curve of loads on the spar: insignificant at the top, but steep at the root. Traditional strut-braced wings tend to be lighter, but external structure reduces top speed.
    If you look at the World War 2 vintage Horsa glider, you will see mostly cantilever wings with a small diagonal strut far onboard.
    Struts allow for a much simpler wing spar, maybe even a constant chord spar. The disadvantage is that plank spars are always heavier than tapered spars. In order to be strong enough at the root, they end up with too much strength/mass/weight near the tips.

    There are a variety of ways to taper spars: lightening holes near the tips, varying chord, varying spar cap depth, nesting multiple spar caps, laminating extra layers to the front and rear, etc.
    With enough internal taper, you no longer need external struts.
     
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  2. Jan 20, 2020 #62

    lr27

    lr27

    lr27

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    Part 103 allows you to use some kind of gadget to keep the speed down, as long as the pilot can't turn it off while flying.
     
  3. Jan 20, 2020 #63

    lr27

    lr27

    lr27

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    BTW, there are some airfoils that are designed to perform well above a certain lift coefficient and have high drag below a certain lift coefficient. For instance, the Momfoil series. (See Mom, it CAN'T go too fast! I think this nomenclature may have been inspired by the Mom's hatch on submarines.) I'm not sure a Momfoil would be enough by itself, though. I suppose it depends on how much power is used,and other design details.
     
  4. Jan 20, 2020 #64

    crusty old aviator

    crusty old aviator

    crusty old aviator

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    In the 80’s & 90’s, ultralight manufacturers would go to a finer pitch prop (or adjust them to a finer pitch if they had one of Dale’s props) if the top end speed exceeded 100 kmh (62 mph), and that’s how the top speed is limited on the Ultracruiser. Wing airfoil selection should be focused mainly on the lowest possible stall speed and stall characteristics.
     
  5. Jan 20, 2020 #65

    pictsidhe

    pictsidhe

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    I know you jest, but...
    With a suitable lower surface, a momfoil is quite possible. Airfoils with a lot of camber have their lowest drag at highish Cl. Modding one of those to create higher drag at low Cl is the way to a momfoil.
     
  6. Jan 21, 2020 #66

    lr27

    lr27

    lr27

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    I wasn't jesting about the existence of a series of airfoils called Momfoil. You can look up coordinates and Xfoil plots. So if you came up with your own, the decent thing to do would be to pick a name like nannyfoil or grandmafoil.
     
  7. Jan 21, 2020 #67

    MrHopkins

    MrHopkins

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    Thanks everyone. To answer the questions from several posts:

    Ultralight in this case refers to US, part 103.

    I definitely considered Hummel's Ultracruiser. Hands down that is the most attractive ultralight I am aware of. The award winning one that looks like it was chromed is stunning. I decieded I will prefer looking down without obstructions vs alternitives.

    My bona fides are I got a degree in Aeronautics. Knocked out a few ratings in a 172 and Decathlon for credit. Due to requirements to be a full time student an extra year, despite already completing the courses required, I got all the airframe and about half of the powerplant of A&P. I am very comfortable with fabrication of whatever material but definitely aluminum is my go to That was 10 years ago and I am way out of practice.

    All that info above is me trying to sound qualified. Which is debatable.

    I put off building too long for no good reason so Im making myself do the things I tell people i'll do.

    I've built (assembled) a motor w/ a chain drive that spins 11 props on the trailing edge of a wing. 5 on left, 1 centerline, 5 right. The goal is lowering landing speeds/increasing AoA limits. Took a lot of adjusting but it seems effective so far.

    Secondary goal is to be able to fly over the wife's horse as quietly as possible.

    Third goal is attaching a recumbant bicycle for use in taxi, takeoff, post landing, and to get me the last mile to work. which is as close as I can be airborne wirhout being shot down or tackled.

    I'm already at 218 pounds. Unbuilt yet is another 90 lbs of airframe + whatever im going to use as instruments + recharging system when not at home . So farther over the weight limit than I think I can reduce but im working on it .
     

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