My Preposterous Fantasy Aircraft: The Luge

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Victor Bravo, Mar 7, 2019.

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  1. Mar 8, 2019 #41

    Tiger Tim

    Tiger Tim

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    I got the impression that's what was wanted. This stage:
    [​IMG]

    Not this stage:
    [​IMG]

    ...at least not yet.

    VB, am I close to the intent?
     
  2. Mar 8, 2019 #42

    Vigilant1

    Vigilant1

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    You've said you want a wingspan of 18' or less and a wing loading of approx 10 lb/sft, and you want to use an engine of 45 HP or less. The Hummelbird matches those critieria. You don't like the construction method it uses and its size (length?). But, with HP, wing size/span, and a MTOW matching what you want, whatever plane you could come up with will have approx the same performance as a Hummelbird (or, likely, worse, if the desire for simple construction adds drag (blockier fuselage, external struts/wires)). A semi-recumbent cockpit would reduce the frontal and wetted area compared to a Hummelbird with a bubble canopy, but the larger tail surfaces of the "luge" necessitated be the shorter tail arm would probably result in something close to a wash.

    So, a reality check would be to see if the Hummelbird gives the performance you'd consider acceptable, because higher performance probably isn't practical given your stated constraints/preferences. An engine of 30HP or less is probably not going to get the job done.
    +++++++++++++++++++++++
    Hummelbird: (specs per Wikipedia)
    Crew: 1
    Capacity: 230 lb (104.3 kg)
    Length: 13 ft 4.0 in (4.064 m)
    Wingspan: 18 ft 0 in (5.486 m)
    Wing area: 57.2 sq ft (5.31 m2)
    Airfoil: Clark Y (modified)
    Empty weight: 300 lb (136 kg)
    Gross weight: 530 lb (240 kg)
    Fuel capacity: 6 US gal (22.7 l)
    Powerplant: 1 × Volkswagen ½ VW engine 2-cyl. air-cooled horizontally opposed piston engine, 32 hp (24 kW) to 45 hp (33.6 kW)

    Performance

    Maximum speed: 126 kn; 233 km/h (145 mph)
    Cruise speed: 87 kn; 161 km/h (100 mph) to 115 mph (185.1 km/h)
    Stall speed: 33 kn; 61 km/h (38 mph)
    Range: 174 nmi; 322 km (200 mi)
    Service ceiling: 10,000 ft (3,000 m)
    g limits: 6
    Power/mass: .060 to .084 hp/lb (0.0986 to 0.138 kW/kg)
    Takeoff Distance 300 ft (91.4 m) (over 50 ft (15.2 m) obstacle) - 1,000 ft (304.8 m)
    Landing Roll - 800 ft (243.8 m)
     
    Last edited: Mar 8, 2019
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  3. Mar 8, 2019 #43

    Hot Wings

    Hot Wings

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    Don't forget folding, or removing, the fuselage just aft of the cockpit. There is nothing stopping us from using glider style automatic connect for elevator and rudder. A side hinged aft fuselage, especially on a nose gear, could be a very quick transition.
     
  4. Mar 8, 2019 #44

    RJW

    RJW

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    VB's requirement prompted this sketch. It's a bit bigger than suggested at 18 feet of span and 60 square feet of wing. It's a little longer than the Teenie Two (the fuse tub is 11 feet long). The seat is reclined at 45 degrees. Though it does look like a coffin I don't find that aesthetic unpleasing. And the fuse would be easy to pound together. It's all planar sheets.

    Recumbent.jpg

    Recumbent 2.jpg

    The problem is apparent. Mass-wise, are we like the pilot in the top picture (6 feet tall but slight)? Or are we more like the happy fella in the bottom pic (I'm more like this pilot)? I really like small airplanes but it's not too difficult to see why not many of the small designs are being built nowadays.
    Rob
     
  5. Mar 8, 2019 #45

    Victor Bravo

    Victor Bravo

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    Well I am a typical overweight American, but I sure don't have man-boobs like the golden guy in the second sketch!

    Tiger Tim I was hoping to start off with some people offering sketches and renderings, and then have the thread distill down to the next steps in terms of what would or would not actually work.

    I am already aware of most of the small single seat aircraft out there that are known to the public. I have seen a Hummel Ultra Cruiser in person, and waatched it fly, and without disparaging anyone the aircraft is not what I want to be flying around in. It's the op posite of what I am trying to create with this concept.

    If the BD-5 weren't so riddled with problems, design errors, and mechanical/structural complexities it would be the type of flying I want to do with this design exercise. I don't want to float around at 60-70 mph, I want to have something that feels a little more solid and is capable of 100-120 mph. The Moni with the clipped wing accomplished this almost exactly, but good used Moni's are not easy to find. I would absolutely love to buy a Moni and clip the wings, but I couldn't afford one right now and they don't come up for sale often anyway.

    Hot Wings is absolutely correct, I have looked at splitting the fuselage in two rather than the wing. That is definitely an option.
     
  6. Mar 8, 2019 #46

    Topaz

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    That's fine. Trouble is, those images are absolutely meaningless until you've done at least some minimal back-of-the-envelope calculations to determine some basic parameters of the airplane from the requirements. Without doing that, you might as well draw an SR-71 or 747 shape, since you have no more means of telling if those two shapes meet the needs of the requirements than any other, beyond statements including the words, "probably" and "I think...".

    This comes back to letting the requirements drive the design, not trying to draw an airplane separately from understanding the consequences of the requirements. All the latter gets you is pretty airplane pictures. The former gets you somewhere into reality. With not a ton of work, or by making comparisons to existing designs like Vigilant1 did above, you can get a pretty good notion of what wing loading, span, installed power, and MTOW will work with what you want for your design. Once you know those things, you can start doing some sketching of an airplane design that might actually work the way you want. Without that basic work, picking up a pencil and drawing is artwork, not engineering or even "design."
     
  7. Mar 8, 2019 #47

    Himat

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    Well, several airplanes have been designed to fit constrains set as hard limits at the start. The A4 Skyhawk should be able to carry one nuclear bomb and fit the carrier elevator without any wing fold. The Airbus A380 and probably several other airplanes have had span constrains. All soaring, racing and aerobatic competition planes are designed to fit the rules. Designing an airplane to fit the hangar is nothing new either, a jet fighter design pretty much have a hard limit (concrete) if they are to fit the standard NATO hardened shelters.
     
  8. Mar 8, 2019 #48

    Topaz

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    Of course. But you can't say you have that hard constraint without also talking about the intended installed power, stall speed, projected aircraft weight, range, etc. All of those were considered in the design of, say, the A4, and the final chosen configuration, wing area, aspect ratio, maximum gross weight, etc. was selected to meet all the requirements for those parameters as well as others. Even the A4 had a minimum acceptable climb rate requirement, along with the others.

    Again, my point is that you can't just say you want a particular arbitrary airplane and fit it into some arbitrary constraint. A span, weight, or length constraint is just one more parameter for the requirements and specifications list that has to be developed before any airplane can be designed. Even a "I just need it to fly" ultralight has to have some kind of basic climb requirement, along with the requirements imposed by Part 103. Saying, "I want a one-piece wing monoplane with no more than 18' span and a 20hp motor" is fine, until you find out that perhaps that results in an airplane that can't climb adequately on a mid-summer day with a real-size human on board or even "climb very well at all" on any given day. It might be fine, or it might not be fine, but there's two ways to know: The expensive way of "build it and find out", or the not-expensive way of running some numbers first. You can't do any kind of rational structural design without those numbers (and more) anyway, so you might as well get started early and save a bunch of rework later.
     
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  9. Mar 8, 2019 #49

    RJW

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    The constraints set here can't result in an airplane capable of performing the intended mission. 50 square feet of wing on 25 or 30 horsepower at 500 gross (almost certainly more given the requirements for industrial motor and real human pilot) just won't do it. A little math or looking at other similar aircraft makes this clear. But I think VB already knows this. After all he wants a preposterous fantasy aircraft.

    Rob
     
  10. Mar 9, 2019 #50

    Topaz

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    Very likely true. On my own project, I get a projected 800fpm climb under sea-level, standard conditions with 22hp installed power, 63ft[SUP]2[/SUP] wing area, and 550 lbs design take-off weight. At a density altitude of ~4,700', that climb performance reduces to just over 500fpm. Of course, the wing area on my project is spread over 35' of span, which is a "sweet-spot" for a homebuilt three-panel wing in composites. Even then, my climb performance numbers are probably slightly optimistic because of the depressing realities of small wooden propellers. I tried to account for that, but still.

    I would consider these climb numbers about the minimum realistic values one should set for specifications on a real-world airplane. You wouldn't get this performance with an 18' span wing, probably regardless of area. Certainly not on 50 ft[SUP]2[/SUP] area wing. If an 18' span one-panel wing (or anything close to it) is a requirement, then one seriously needs to start thinking about a more-powerful engine. That's not a shot against anyone. It's just physics.

    You can do these calculations without ever drawing more than a broad, roughly-scale "napkin" sketch of an overall configuration. Going farther than that without doing them is probably a waste of time, IMHO, if you're really trying to design an airplane.
     
    Last edited: Mar 9, 2019
  11. Mar 9, 2019 #51

    12notes

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    The Ultra Cruiser (this is the first mention in this thread) and the Hummelbird (mentioned several times) are different aircraft. The Ultra Cruiser was designed to fit part 103, has almost twice the wing area and a bit over half the wing loading, has a 95 mph VNE, which is slower than the cruise speed of the Hummelbird, and is designed for a completely different mission. The Hummelbird is E-AB only, has a VNE of 145mph, and misses all of the part 103 targets by a good margin. Judging one by looking at the other isn't a fair comparison.

    I suggested the Hummelbird wing because the aircraft meets most of your design criteria.The wing is 18' long. It has a wing loading of 10 lbs/sq.ft. It cruises between 100-120mph, on 30-45hp. It has a reclined, but not quite laid back seating position. The fuselage is not built on a jig, it's built on two 2x4s bolted together in a T shape, slightly less than 10' long. Before you bolt the spar in, it's less than 30" wide. If you don't have room to build that fuselage, I'm not sure you'd have room to build any fuselage.

    I'm not trying to change your mind on what you want, just clearing up what seems to be a misconception of the construction method. I understand you'd like a different seating position and fuselage construction method.

    You mentioned wanting to keep the fuselage parts to 12 foot lengths, but for an 18 foot wingspan, the one piece wing requirement means you need 18 foot lengths for the spar. Or at 12' spar and two 3' wing extensions, which is really a 3 piece wing at that point.

    What exactly is the wingspan limit that you can fit without disassembly?
     
  12. Mar 9, 2019 #52

    pictsidhe

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    We're perverts?
     
  13. Mar 9, 2019 #53

    radfordc

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    Rans did a similar lifting body design....the S-11 Pursuit. It didn't seem to be a success for whatever reason. https://en.wikipedia.org/wiki/Rans_S-11_Pursuit

    https://doc8643.com/aircraft/PURS
     
  14. Mar 9, 2019 #54

    Victor Bravo

    Victor Bravo

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    If I remember correctly, Randy Schlitter **** near got killed and burned to a crisp in the S-11. "Not a success" is kind of an understatement. I do not know the details of that accident but I do remember reading that it was really serious.
     
  15. Mar 9, 2019 #55

    TFF

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    There has been lots of threads on the S-11 on HBA. Was Schlitter’s fire after it hit the ground? It was the 2 stroke I think that he crashed. The 912 ones did better. It had a funky stall where the fuselage would keep flying when the wings stopped. I don’t think he wanted to sell something like that to the general population.
     
  16. Mar 10, 2019 #56

    Aesquire

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    The "Kelly" Johnson sketch posted by Tiger Tim had assumptions built in, specifically, the Allison engine and a thousand hp per engine.

    The results of that sketch, with half a century of hindsight, had several problems, some could have been fixed with little ( relative ) effort, like the poor cockpit layout. Others would have needed a lot more work, like the thick airfoil that limited Mach performance. Still others needed developing, and failed to get it, like the cooling system that turned oil to treacle and intake air so cold the lead fell out of the fuel.

    Enormous money and time was wasted on designing a second generation successor to the P-38, which had silly design requirements like a powered turret, ( based on a romantic notion that was disproved in blood by the British Boulton Paul Defiant ) and engines never available, or never developed.

    In hindsight, a laminar flow wing, redesigned cockpit, cooling system and dive brakes would have been a faster, better, and actually shipped before the war was over airplane.

    But that is not what the customer ordered.
     
  17. Mar 19, 2019 #57

    Himat

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    A configuration that might work for a very small airplane. Joined tandem/biplane wings, engine up front, V or T tail. The pilot is to sit between the wings with his but at the rear lover wing leading edge and feet under the front high wing. To keep weight down, junk that industrial engine and fit a two stroke or Wankel.
    tw32_6.jpg
    Model and picture by "Macboffin" posted on www.rcgroups.com
     

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