On the importance of weight.

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by bifft, Jul 14, 2019.

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  1. Jul 14, 2019 #1

    bifft

    bifft

    bifft

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    I've been thinking about the possibility of an aerobatic part 103 ultralight. Pretty much just using a 103 appendix 7 spreadsheet and drawing napkin sketches so far. Looks like assuming a 170 lb pilot and a 254 lb plane you need at least around 120 ft^2 for a double surface wing with 50% or more flaps.

    Doing it as a biplane lets you brace everything and get the spar bending loads low. Haven't done any structural calcs beyond just that. Liked the looks of this one enough to build it in x-plane as a test of the basic idea:

    [​IMG]
    Cute little thing. 16 ft span upper, 14 lower, 4 foot chord (including the part inside the fuse as wing area). Just assumed I can put a Verner 3V on it.

    Flys pretty good. With full span flaperons on all four wings roll rate is over 360 deg/second. Climb isn't spectacular, but about the same as the 152s I used to fly.

    Then, I go into the settings->Weight and Fuel. It defaults to half the max load. So put in all 5 gallons of gas, and a 255 lb pilot instead of the 150lber it defaulted to. Won't even climb out of ground effect. Start it in the air it won't maintain altitude on the full 40 hp much less the 34 "max continuous".

    So, if I'm going to build something for me to fly (instead of some hypothetical average pilot) I need more wing, less drag and probably more power. Doubt the weight budget would let me put a bigger engine on it.

    Back do the drawing board for this one. Was a fun little lesson for me, thought I'd share.
     
  2. Jul 14, 2019 #2

    BBerson

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    A thick monowing might get what you want. Like a 100% scale sized Extra 300S (single seat) that weighs only 254 lbs. (112 sq.ft.)
     
    Last edited: Jul 14, 2019
  3. Jul 14, 2019 #3

    Speedboat100

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    Flying wing might yield the tightest loops.
     

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  4. Jul 14, 2019 #4

    pictsidhe

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    I'm hoping to build a semi aerobatic 103.
    Why only semi?
    Well, there's only so much that you can do with a 103. Your wing area is about right, but you need more span. That's why it won't fly on 40hp. X-plane has a very optimistic roll rate. That's a pb/2V of about 0.5. I'm planning huge ailerons to get >0.15, hopefully. With 26' span, roll rate won't be fast. Flick rolls will be the way to roll it. But it will climb very quickly on 40hp with 26' and low drag.

    Edit, my last roll rate calculation was 5 seconds, that's with tapered wings and huge ailerons.
     
    Last edited: Jul 14, 2019
  5. Jul 14, 2019 #5

    Dana

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    I went through this exercise some years back, there's a thread about it here. My approach was a pusher biplane, sort of like a Curtiss Headless Pusher.

    Quicksilver made a few aerobatic "Supers" back in the day, though they probably weren't quite 103 legal.
     
  6. Jul 14, 2019 #6

    BJC

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    They demonstrated one in the airshow at Oshkosh.


    BJC
     
  7. Jul 14, 2019 #7

    bmcj

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    Over 360° per second roll rate? I find that number suspiciously enthusiastic.

    The wingtips would be traveling almost as fast around the longitudinal axis as the plane was traveling forward (maybe even faster, depending on the plane’s speed). The wingtip would have to scribe a circular arc at 40 mph (ignoring forward velocity). Part 103 max forward velocity is only 63 mph.
     
  8. Jul 14, 2019 #8

    bmcj

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    I’ve flown the ‘Super Quick’. Clipped wing MX with a Rotax 503 and some extra bracing. It does aerobatics, not definitely not a precision flyer. I also found that it was a little too close to tailheavy with a pilot as lightweight (at the time) as me.
     
    Last edited: Jul 15, 2019
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  9. Jul 14, 2019 #9

    radfordc

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    A CGS Hawk will fix your problem. It is designed to carry a "big man" and is mildly aerobatic. I did loops, rolls, and spins in mine with no problem. Whether or not it is 100% Part 103 compliant....who cares?
     
  10. Jul 14, 2019 #10

    TFF

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    I believe Ed Fishers Zipster biplane was 103 and aerobatic.

    The Polecat was designed by some who wanted to try and fly competition style aerobatics.
     
  11. Jul 15, 2019 #11

    Dana

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    Other aerobatic ultralights include the Maxair Rocket (a beefed up Drifter), something from Rotec based on their Rallye, and the Phantom. I don't know if any of them really meet the 254# weight limit.
     
  12. Jul 15, 2019 #12

    Aesquire

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    Get rid of the fuselage, sweep the wings a bit, and make it a pusher with elevons. A beefed up Easy Riser. Roll rate won't be horrible, but knife edge and inverted maneuvers may be difficult.
     
  13. Jul 15, 2019 #13

    Victor Bravo

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    Carbon fiber derivative of the Backstrom Plank, with a symmetrical airfoil, like many successful aerobatic R/C models. This will give you a pretty good chance at meeting 254 pounds while still be ing aerobatic. The wing will have to be thick enough to keep the stall speed within whatever calculated or measured limits. But the thick wing allows a reduction in the spar weight, so you're killing two birds with one stone. One central wheel, sailplane style, will help save significant weight by eliminating wheels, tires, and landing gear legs. The seat bolted right to the front of the spar will save a lot of weight in terms of required fuselage structure. The Polini 36 HP engine will deliver a high thrust to weight. Yes you will have to reflex the control surfaces to create some sort of pitch stability, but that has been demonstrated a thousand times wiwth R/C models. And you'll have a fully aerobatic airplane in the process.

    Calling Sockmonkey, please report to the white courtesy phone....
     
  14. Jul 15, 2019 #14

    BJC

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    How would you control pitch under sustained negative g?


    BJC
     
  15. Jul 15, 2019 #15

    Aerowerx

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    Maybe good for loops, but flying wings are not suitable for inverted or knife edge flight.

    Because of the low pitching moment of the wing and wash-out, required for stability, that doesn't work when inverted.
     
  16. Jul 15, 2019 #16

    pictsidhe

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    Forward stick will reflex the wing for inverted flight.
    I've seen a Fauvel at an airshow. Loops? No, flips! I'll see if I can dig out the horrible video I shot, though youtube should turn up decent quality footage.
     
  17. Jul 15, 2019 #17

    Aesquire

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    Big elevons. But you make my point, unlike a plank the Easy Riser biplane has stagger and the top & bottom wings at different angles. That's for pitch stability with a weight shift craft with no elevons.

    An aerobatic version would need to use the same tricks as Victor Bravo suggests, where reflex can be entirely in the elevons with a symmetric airfoil, and eliminate the decalage.

    The advantage of a biplane is weight reduction by using the wire braced truss, and span reduction plus high aspect ratio for the required wing area to meet the stall speed rule.

    The downsides are high drag from the wire bracing & a loss of efficiency from biplane interference.

    However, top speed isn't a goal in pt103, and you might be able to avoid expensive composites. Speed of building, with aluminum tubing & riveted gussets, is a plus. Balanced somewhat by fabric covering time. ( but all you need for paint is UV blocking sealing coat. )
     
    Last edited: Jul 15, 2019
  18. Jul 15, 2019 #18

    Aesquire

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    To be clear, my point on inverted flight. Knife edge stuff with flying wings, mono or biplane, is... Not on.

    I don't think you can get Pitts or Extra performance with pt103 weight, but barrel rolls and loops, sure! Even 2 point rolls. ;)
     
  19. Jul 15, 2019 #19

    BJC

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    Especially weight shift pitch control configurations, like the Easy Riser.


    BJC
     
  20. Jul 16, 2019 #20

    bifft

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    Thanks for the other aerobatic 103's for research/inspiration. My "goal" for this (aside from something to sketch during boring meetings, or to play airplane designer on while on call so I can't get to the airport) is:
    Aerobatic enough that a sufficiently skilled pilot could do a IAC primary routine without falling out of the bottom of the box. (Part 103 demands low power and high drag, so it would be an exercise in energy retention). Not looking to win, or even really compete, just setting a lower bar for "aerobatic enough".
    Strong enough to not break in the hands of someone who is still working on becoming that sufficiently skilled pilot.
    Can carry a big guy (250lbs is what I usually use) in Utah (field elevation 5000 MSL or so) on the kind of summer days you want to fly open cockpit (ground temperature 80-100).

    Not sure if all three of these can realistically be met. It doesn't need inverted or knife edge capability, but does need to be able to loop. In X-plane with this model loops are hard. Dive for speed, then either get accelerated stall on the way up if you pull too hard, or run out of speed before going over the top if you don't pull hard enough.

    And of course, if we're going to be dreaming, give it one person folding wings so it fits in a shipping container.

    My build thoughts are riveted aluminum D-section with fabric covering, but that's just what I'm used to. Want to learn to weld, so maybe a welded steel fuse.
     

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