First attempt at design for electric delta pusher

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by RCBinChicken, Jun 1, 2019.

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  1. Jun 5, 2019 #21

    Sockmonkey

    Sockmonkey

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    That's pretty much what I'm saying.
    Batteries in front, but with an empty place ahead of the pilot's feet for a landing viewport.
     
  2. Jun 27, 2019 #22

    RCBinChicken

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    Apologies to all for lack of replies to input, I'm an engineering student and the end-of-term assignments and exams have swallowed up all time for the last few weeks... Now it's quietened down a little, I can finally go through and respond properly to everyone! Thanks to all of you for chipping in so many suggestions, it's given me a lot more of an idea of where to focus/what to fix.


    Calculating usable drag estimates is not within my current skillset, I'll have to work my way up to that one :) but yes, I did assume that induced drag would be the single largest issue with my chosen layout. What pushed me into the "okay this is maybe worth trying anyway" zone was reading an interview with Barnaby Wainfan in T.W.I.T.T, where he was asked a fair few questions about drag on the Facetmobile, which has (I think!) a lower AR than my design and still achieved impressive performance on a smallish powerplant. I would tend (possibly naively/incorrectly) to attribute that to the favourable wetted aspect ratio, like the oft-cited (by starry-eyed delta fangirls like myself) comparison between the B-47 Stratojet and the Avro Vulcan, which had widely different AR as one would expect, but similar WAR and thus similar endurance (when adjusted for other factors.) The Bloop-esque super-low wing loading should also help keep induced drag lower than it might otherwise be.

    And yes, RE: thrustline, that's a big part of why I wanted to stick the motor so far back - glad to know I wasn't too far off-base with that.

    You're absolutely right to be skeptical, and I may well fall short of that goal. 70kg is ~154lb, and I'm using the later Bloops (~204lb) as my initial feasibility benchmark, my bet being that I should be able to save some weight on a simpler, or at least more compact overall structure. If my final weight (batteries but no pilot) falls anywhere in the 160-210lb range I'll be well satisfied.

    100% with you here. I'll go with gear fixed at ideal AoA as suggested. Sounds like it will solve a lot of headaches.

    Good point also on the turbulence cloaking issue. I'm hoping that the very-rearward prop will avoid this too.

    The model aircraft design I've been playing with variations-of for most of my time in the hobby, coincidentally, is essentially a VariViggen without the canard. I only found the VV a few months back and was quite tickled to find my silly-looking model had a bona-fide real-life counterpart! :p

    Oh heck no! :) BEST case scenario, if this thing works, it might provide inspiration or a useful case study for people wanting to build similar designs.

    But your point on pilot weight is entirely valid, and I'm thinking maybe this is another area where I should look to the Facetmobile, i.e. just scoot the pilot back as suggested and solve the high-AoA visibility issue with some see-through floors.

    That sounds good. Did you mean extend rearwards horizontally behind TE, or downwards making them more like wingtip plates? Or both?

    I'll look out for those books. Ta for the recommends!

    I'm going to basically learn about every construction method I can, as I'm by no means sure which will suit the design best. I am heavily drawing inspiration from the Facetmobile in a lot of respects, but not yet sure if its construction method will prove ideal in my case.

    I considered canards initially, but after reading everything I could get my hands on, I suspect they wouldn't offer much benefit for my target flight envelope - I won't have the raw engine power for flying around at the ultra-high-AoA regimes where their interactions with a delta planform really shine.

    As above, increasingly leaning that way, and Riggerrob and Aeroworx seem to agree with you. Also if worst comes to worst, that way I won't have half my own weight in batteries trying to fly forward THROUGH me if I get in a crash. :)
     
    Last edited: Jun 27, 2019
  3. Jun 27, 2019 #23

    RCBinChicken

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    Aaaaannnd having just seen in another thread that the Facetmobile's designer is a member of this forum, I really hope my own references to it are not embarrassingly factually incorrect. My sincere apologies to Mr. Wainfan should this prove to be the case! :eek:
     
  4. Jun 27, 2019 #24

    pictsidhe

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    Here's the wikipedia page on lift induced drag, now you can ballpark it! I usually calculate induced power
    Pi= Di*V/prop_efficiency
    That tells me how much engine power is sucked up by the induced drag.

    For parasitic drag, the appendix in far103.7 should put you in the ballpark.

    Ballpark being the operative word.

    With some fairly simple calculations, you can get an idea of whether your magnificient idea is going to soar like an eagle, or fly like a turkey.

    Apologies to Barnaby if I'm not entirely correct: The Facetmobile is competive on total drag for a few reasons. The big one is that the lower AR enables many structural components to be reduced in size. It's root thickness of over 3' needs way less spar material than a conventional wing that is under a foot thick. The ~1" tubes used to create the shape double as the spars. Being a geodetic structure, is noticeably lacking in heavy lumps of metal. It's also a BWB (blended wing body), so the fuselage weight and drag are gone. Almost all the surfaces are lifting. The exceptions are the prop and fins. Getting rid off those is tricky, though not impossible.
    There's a report out there on the facetmobile.

    I did investigate building a Fauxetmobile-103, but decided that the AR was too low to work well within the constraints of 103, and I don't think I'm smart enough to stretch it out enough sideways.
     
    Last edited: Jun 27, 2019
  5. Jun 27, 2019 #25

    David Lewis

    David Lewis

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    You could use a variable sweep canard. This would also help landing and takeoff.
     
  6. Jul 11, 2019 #26

    Speedboat100

    Speedboat100

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    Or just adjust the geometry.The CG can be just there where the pilot is.
     

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  7. Jul 13, 2019 at 11:38 PM #27

    Doggzilla

    Doggzilla

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    This is completely feasible but has some minor issues.

    This is no more impossible than a powered harness. It’s simply thickening the wing and placing the pilot inside.

    https://en.m.wikipedia.org/wiki/Swedish_Aerosport_Mosquito

    To keep it simple the pilot and battery should hang from a harness within the aircraft so that they are always at the proper CG, just like a weight shifter. A two person harness will do just fine. Controls would only need to be a tube to push yourself within the aircraft as weight shift. Turning would be superior to a hang glider due to the superior proximity to the centerline.

    As for the flight time, 100lbs of standard lithium batteries is about 6-7kwh (16lbs per kWh from Chevy Volt lithium batteries). At 30% this would be an hour, but it takes nearly no thrust to keep a hang glider airborne. If you pushed a twin harness to it’s limit you could get at least 2 hours. Very likely more.
     
  8. Jul 13, 2019 at 11:45 PM #28

    Doggzilla

    Doggzilla

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    Also, there is the issue of getting in and out of such aircraft. They are not difficult to pick up, so using a bottom door that you pull over you like a backpack is completely feasible.

    And yes, I have flown hang gliders before.
     
  9. Jul 14, 2019 at 3:24 AM #29

    Sockmonkey

    Sockmonkey

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    Sketchup gets more cooperative once you get the hang of it's quirks and limitations.
    [​IMG]
     
  10. Jul 14, 2019 at 7:20 PM #30

    Speedboat100

    Speedboat100

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    How about having at least 4 x 5 KW engines and smaller dia props in the tail ?
     
  11. Jul 14, 2019 at 9:04 PM #31

    Doggzilla

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    Probably some regulatory reason not to. As well as avoiding other unwanted negative attention.
     
  12. Jul 15, 2019 at 8:23 AM #32

    Speedboat100

    Speedboat100

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    NEGATIVE ?

    Isn't any attention a good attention ? You get the thrust line lower if you have more motors closer to the trailing edge.
     
  13. Jul 15, 2019 at 9:50 AM #33

    Doggzilla

    Doggzilla

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    Yes, skirting the rules so obviously attracts nit pickers who then go on crusades to find petty reasons to file complaints. Whether or not they actually exist or not.
     

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