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 1, 2019 #1

    RCBinChicken

    RCBinChicken

    RCBinChicken

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    Hi all, after lots of lurking and reading threads pertaining to similar(ish) craft, I figured it was time to work up the nerve and show some of my designs, and find out exactly how bad they are. :p This is a (scruffy) 3D model of my ideas so far, and in the time it took me to model it I've already thought of a lot of tweaks and things to fix... but at this stage, when I know that I know very little, better to get an iteration out and get to work on a Mk.II pronto than shoot for perfection.

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    Notes and/or sort-of-specs:

    - Yes, I need to learn a real CAD package and stop using Sketchup. :D I'm hoping to pick up Fusion 360 over the next few weeks now polytechnic assignments have died down a little.

    - This is intended to be a VERY light, slow flyer, a Wainfan Facetmobile/Rowe UFO wannabe specced down in both weight and power to occupy the wing-loading 'ecological niche' of the Sandlin Bloop. Wing area is 15m^2 (6m span, root chord 4-4.5m, tip chord 1m) which will give me just under 2lb/ft^2 wing loading, the figure Mike Sandlin quotes for the Bloop if I can keep empty weight around 70kg/154lb. (I weigh about the same.)

    - Controls will be elevons and tip rudders, like the Facetmobile.

    - Construction uncertain, as I have a lot to learn on the different methods and their merits, but likely fabric-covered, either wood or aluminium tubing for structure, foam under fabric for shaping.

    - The shaping will be simplified to facilitate construction - not quite to the Facetmobile's degree, but I know that all the compound curves I've put into the model here will make my life a misery in practice!

    - Airfoil thickness, excluding the fuse bulge, is a roughly consistent 15% - though having read that the Facetmobile went right up to 18%, I might follow suit and go fully blended-wing-body on the next revision, to reduce parts count and wetted area a little more. The airfoils themselves are not something I know enough about to make a definite pick, but the root is going to be mostly flat-bottomed with a little reflex, thickest around 29-30% of chord, transitioning to a semi-symmetrical, more reflexed tip with a fair few degrees of washout.

    - Propulsion will be electric, a single brushless motor around the 20kW (25hp) mark, the same horsepower as the gas Moster 185 on the Bloop. Sandlin has estimated the Bloop cruises comfortably on a little under 11hp; I'm hoping my total drag will be non-trivially less than for a wire-braced biplane and thus for even lower cruise power requirements, but I'll err on the side of caution at this stage. Prop-diameter-wise, will be looking at 1.2-1.4m as that seems to be fairly tried-and-true for paramotor-class setups.

    - I'm aware that within my intended weight target, on electric, flight time will be abysmal, and I'm happy to accept that; 20min or even a little less is fine by me. I'll gain a little fuel fraction benefit just on the weight of my powerplant; the beefier motor systems I've looked at, for motor and ESC etc., clock in at 6-7kg total compared to the Moster 185's 13kg, so that's a few minutes' extra LiPos I can cram in at least! :)

    - Extremely rearward placement of the motor and prop is due to the aforementioned - the weight of the motor itself may be as little as 4kg, so its impact on CG placement will be negligible unless given a long moment arm, and I want the pilot pretty far forward for visibility at high AoA. Plus, a longer moment arm will enhance the stabilizing effect of a rear prop disc, so I may be able to cut down the wing-tip vert stabs a little and reduce their root bending loads.

    - The aspect I'm questioning a little at present is the high thrust line. My initial justifications for it were that, in my experience with RC model aircraft, (1) it gets the prop disc up out of the pressure changes of the wing wake, considerably reducing noise and vibration loads on the prop blades, which are often cited as a primary disadvantage of pusher layouts, and (2) the only way I'll be able to manage a prop strike is if I land inverted. :p At RC scale, I've only found the downward pitching moment significant/problematic with far higher thrust lines than what I have here, but I'm aware that not everything translates perfectly from RC to full-scale... My optimistic assumption is that the wing dihedral should move the vertical CG and center of drag higher, and the far-rearward motor location means I'd hopefully only have to give a couple of degrees' down thrust to put the line through the CG again and neuter the pitching moment; but opinions most welcome.

    - That fin-like extrusion of the fuselage, with the motor at the end, does look like it needs to be a looooooooot thicker. That's a definite for the Mk.II.

    - I have made no decisions regarding landing gear yet, as I'm too on-the-fence about the comparative pros and cons of every config I've seen so far.
     
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  2. Jun 1, 2019 #2

    BBerson

    BBerson

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    I don't follow tail-less much but I do remember the Dean Delta. It had a fatal crash on first high speed taxi.
    It was thought with the tall gear legs near the engine the thrust pushed the nose down so much the pilot couldn't lift the nose with back stick. But when he chopped the power at high speed and with no down thrust holding it on the runway it then lifted off steeply and stalled.
    Some reports in EAA Sport Aviation archives around 1960 or something.
     
  3. Jun 1, 2019 #3

    RCBinChicken

    RCBinChicken

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    Oo-er. I'll have a look into that one. I'd wanted the landing gear to be pretty short to begin with, but now I'm even keener. :(
     
  4. Jun 1, 2019 #4

    deskpilot

    deskpilot

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  5. Jun 1, 2019 #5

    JetProvost

    JetProvost

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    My 2cents worth as a retired engineer. Put the horse before the cart.

    1. Mission for craft.
    2. Power plant.........beyond turbine nothing since the Wright Brothers
    3. Impact Survivable passenger cabin
    4. With controls in neutral it should be stable regardless of power on/off
    5. Stall power on / off should drop nose not wing.
    6. Must have Ballistic Recovery system .
    7. Ease of construction to aid repairs later
    8. Ease of annual inspections (beware of lazy mechanics)
    9. Fire suppression system.
    10. Survive 20 years tied down outside in all weather
    11. Will insurance companies like it

    So called new planes are designed all the time, unfortunately there is nothing new about them. Too many emotions and egos 'involved.
    Get the important stuff right, then put Lipstick on the pig.
    With that said, Best of luck with a winner
     
    David Lewis and drstress like this.
  6. Jun 1, 2019 #6

    pictsidhe

    pictsidhe

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    A 6m span delta is going to have quite a lot of induced drag. Have you run any drag numbers yet?
    The important thing about thrustline is its height from cg. With the prop well behind the cg, you can angle it to get things right.
     
  7. Jun 1, 2019 #7

    Tench745

    Tench745

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    I'm no engineer or anything, but 70KG sounds extremely light for a man-carrying aircraft. Particularly one that will be fully-skinned. Curious to see where this goes, though.
     
  8. Jun 1, 2019 #8

    pictsidhe

    pictsidhe

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    Having the pilot way forward will make the cg very sensitive to his weight. With the limited cg range of a delta, this could be a PITA.

    Is this going to be a 103?
     
  9. Jun 2, 2019 #9

    Sockmonkey

    Sockmonkey

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    I have a few SU tips posted on another thread. They might be useful to you.
     
  10. Jun 2, 2019 #10

    Riggerrob

    Riggerrob

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    The OP is off to a good start with some impressive sketches.
    Now that the latest version of Sketchup makes dimensioning easier, it is a more serious drafting program.

    Now to learning from others’ mistakes:
    Dean’s Delta suffered two problems: landing gear and elevons.
    Dean’s Delta had simple tricycle gear that sat the airplane at a neutral angle of attack on the runway. This required raising the nose for takeoff.

    Dean’s mainwheels were just aft of the centre of gravity and his elevens were not much farther aft, creating a short moment arm that required massive amounts of elevon deflection to lift the nose wheel off the runway.
    Once the wheels lifted off, main wheels quit resisting rotation and she promptly did a tiny back loop!

    Landing gear creates problems for many delta wing designers. John Dyke and Mr. Verhees took the simplest approach by fixing landing gear length at the ideal angle of attack for tak-off and landing.

    Consider that deltas generate ridiculous amounts of induced drag at high angles of attack. This massive amount of drag translates into massive sink rates on all but the most powerful jet fighters. It was possible to fly Concorde so deep into that - low and slow - corner that it was impossible to power out of the massive rate of descent.
    The simplest solution is limiting/flattening angle of attack in the landing pattern. If wheels limit angle of attack to the best for take-off, you end up like the Dyke Delta pilot who reports “I simply add throttle and she flies herself off the runway.”
    This will require 3 or 4 landing gear legs. One to prevent the prop from digging in ..... one to prevent the trailing edge from dragging and a couple to prevent wing tips from dragging.
    Installing main wheels slightly aft (say 15 to 17 degrees) of the centre of gravity stabilizes yaw on the runway.
    Speaking of runways, deltas are never going to be STOL with the small engine specified by the OP, so forget about flying near the stall angle and plan on a runway at least 1,000’ (300 metres) long.

    Dean’s other problem was that when he chopped power (to abort the take-off) turbulent backwash from the propeller cloaked elevons reducing their effectiveness.

    This is why several deltas (e.g. Rutan Vari-Viggen) have vertical fences or vertical stabilizers immediately outboard of their pusher propellers. Those fences/fins limit spanwise travel of turbulence, smoothing airflow over elevons.
     
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  11. Jun 2, 2019 #11

    Riggerrob

    Riggerrob

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    ———————————————————-

    If you hope to sell plans for your Mark II, you should probably design for an SAE 95th percentage male
    pilot weighing 100 kg. The challenge is also making it balance with a 40kg, 5th percentile female pilot. Maybe mount your batteries on sliding rails.

    Too many homebuilts are tailored for tiny designers (e.g. Ralph Mong of Mong Sport Biplane) and are difficult to scale up. Apparently Curtiss Pitts lengthened the fuselage - on the second version of Pitts Special biplane plans - because most builders were already adding extra leg room.
     
  12. Jun 2, 2019 #12

    Riggerrob

    Riggerrob

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    On the subject of yaw stability: the farther aft your vertical fins, the more stable.
    I would extend vertical fins so that they overlap the wings’ trailing edges.
    Yes, that increases bending moments on wing tip structure, but will help damp yaw osscilations.
     
  13. Jun 2, 2019 #13

    Riggerrob

    Riggerrob

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    Be cautious about adding too much wing dihedral.

    Sharply swept leading edges already contribute a lot to yaw and roll stability. So much that many sharply-swept wings need negative dihedral for reasonable roll/yaw stabling.
    Look at all the delta jet fighters (e.g. Convair and Mirage) with slight negative dihedral.
    When you look at Verhees Delta from the front, you notice negative dihedral and pilot reports mention odd reactions to rudder inputs raising wing tips.
     
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  14. Jun 2, 2019 #14

    Aerowerx

    Aerowerx

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    A bit of philosophy from an "arm chair tailless designer".

    The problems with the Dean Delta were mentioned. This should be taken as NOT implying that the design won't work, but as an indication of potential problems. Take these in mind and don't repeat other's mistakes. Get a copy of "Landing Gear Design for Light Aircraft, Vol. 1" by Ladislao Pazmany (Vol 2 was never published).

    Get a copy of Nickel's "Tailless Aircraft in Theory and Practice".

    Learn about flat wrap and Conical Lofting unless, of course, you are building a Facet Mobile like aircraft or doing wet lay-up fiberglass over foam.

    Consider putting some fixed canard surfaces near the nose, like on the Saab Viggen, but smaller. I don't have it in front of me, but have a paper around here that says it can do nice things to a delta wing design, if placed properly.
     
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  15. Jun 2, 2019 #15

    David Lewis

    David Lewis

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    You can provide removable weights under the seat, or increase the horizontal tail volume. Subsonic deltas have some advantages in compactness and structural efficiency, but energy efficiency and L/D are low.
     
  16. Jun 2, 2019 #16

    282ex

    282ex

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    I run an electric aviation discussion group on FB (https://facebook.com/groups/electricaviation), we're excited to see where Horton ends up. Horton will be at Airventure. Might be worth a trip to get some flying wing tips... I'm pretty stoked about this design and they have plans for electric conversion down the road.

    Best of luck
    Erik
     
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  17. Jun 3, 2019 #17

    Sockmonkey

    Sockmonkey

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    As it's an electric, I'd stick the batteries in the nose and/or wing leading edge and scoot the pilot back slightly so variable pilot weight won't change the CG much.
     
  18. Jun 4, 2019 #18

    Aerowerx

    Aerowerx

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    Design the plane for the desired flight characteristics without the pilot then find where the center of mass is. Put the pilot at that location! Problem solved. Pilot weight will then not affect the static margin at all!
     
  19. Jun 4, 2019 #19

    pictsidhe

    pictsidhe

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    The cg will need to be in a location determined by the planform and your desire to have the thing flyable. Around 10-20% in front of the neutral point for a delta.
     
  20. Jun 4, 2019 #20

    pictsidhe

    pictsidhe

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    The cg is fairly easy to move on an electric, batteries are heavy. High voltage D.C. is dangerous stuff, though. With high inertia in yaw being beneficial for spin and tumble stability, the nose and tail are the best places to have them.
     

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