David Wing

(see more photos at end of page)

Objectives

• slow-flying, robust, transportable, easy-build wing for the local park
  • slow: large wing area, low weight
  • robust: 1-piece and pusher
  • transportable: flat
  • easy-build: KF airfoil and cut-out electronics)

Design

• largely as per original 16" David design in http://www.rcgroups.com/forums/showthread.php?t=1173701

• revised plans David16.svg

• cut-offs on LE tips and aileron tips like Assassin
• increase width of ailerons to compensate for later propellor cut-out (see plans)
• substitute 3mm CF tube in place of (what looks like 3x1mm CF strip) since this is what I have
• wider cut-out to accommodate 5" prop (prop still in same place) and remove all depron behind the spar (in front of prop) to make quieter
• servos, RX and ESC all forward near LE to get CG right (leave enough room for LE tape)
• 1mm CF rod cut into LE to make nose and LE more robust
• 1mm CF rod for pushrods
• cover with 50mm packing tape - clear tape on top and bottom from KF step to LE, heavy clear tape on LE, yellow on top TE to KF step, red on bottom TE to step.
• motor mount:
  • motor plate is a rectangle of ply
  • screw motor to plate (left-over small servo mounting screws)
  • 3mm depron or EPP fillets on top and bottom in front of motor mount to support necessary depth of motor plate.
• electronics hatch
  • top and bottom plates cut from 2 litre milk bottle clear plastic
  • bottom hatch glued onto depron around hatch hole (UHU Por contact glue) and later with packing tape as part of covering
  • top hatch hinged at front with packing tape (similar to ailerons), back of hatch (liftable) held down by a patch of packing tape on both sides.

Weight Budget

• wing (with tape) = 25g
• battery (180mAh 2s) = 13g
• servos - HK251 (2.5g) x 2 + linkages = 8g
• RX 6 ch Orange w.o. case and sockets clipped off = 4g
• ESC (HK 8A)w.o. heatshrink = 8g
• motor (HK AP05 3000kv 5.4g) + 5x3" prop = 8g (prop later cut down to 3.75 x 3"

Actual Weights

• completed airframe = 29g (with hatches, tape covering and nose piece)
• final weight with battery = 66g (later added 2.5g (5c) on nose to tame down)
• balance at approx 20% MAC (later pushed it closer to 15% MAC)

Build Sequence

• cut-out and glue (UHU Por) 3 layers of Depron
• modify electronics wiring and solder up everything, then test (including motor direction) and centre servos
• verify balance and cut servo holes (within cable reach outward from hatch) and main hatch hole
• install servos and bottom hatch cover and electronics (no tape lining?)

Ideas (mostly unused)

• as described (one of the options) in http://www.rcgroups.com/forums/showthread.php?t=1173701

• plans attached KFmDavidWing.pdf

• plans measurements (imperial) KFmDavidMeasurements.pdf

• use the 16", 20" or 24" version?
  • want to stick to original 3mm depron and KFm4 airfoil (3 layers)
  • but wing thickness will affect robustness and slowness of flight characteristics

    Wingspan (")	thickness	MAC (mm)	wing area(in2)	loading(oz/ft2)	loading (oz/ft3)
    16	6.2%	146	86	3.9	5.0
    20	4.9%	183	134	2.5	2.6
    24	4.1%	219	194	1.7	1.5

  • original 16" version uses a 5g motor capable of driving a 3x2 prop, but my 5g motor can drive a 5x3 prop, so can drive a bigger wing area?
  • may need to make bigger purely to accommodate the larger propellor (prop space consumes more than 1/3 of wingspan on 16"!)
  • 24" (being 1.5x each dimension) should give 0.66x the stall speed (for same weight)
  • experience of 24" builders is that it still flies well even though only 3x3mm depron thickness airfoil
  • but with 24", three layers of 3mm depron doesn't come close to the required minimum 6% KFm4 airfoil thickness (is 4.9% for 20" wingspan)
  • if 24", maybe need the KFm6 airfoil, which has another 2 layers of depron to get the thickness near LE
  • benchmark against Battlewing: 0.83m (32.7") wingspan, AUW=265g, avg chord 220mm and 13.6% thickness
• modifications
  • consider wing-tip cut-offs similar to Assassin(?) to get increased stability without winglets
  • consider very small winglets to help protect ailerons
  • consider step taper at wingtips towards 100% of chord to provide a winglet effect (and strengthen tip)
• alternatives
  • 24" fold-over full airfoil wing - but this really complicates the build! Maybe as a second version if I like the first
  • single 3mm flat wing with CF spar with electronics glued on top (without KF airfoil this may need winglets!)
• Add top and bottom 3mm balsa reinforcement plates to increase wing thickness at the motor mount (may be more solid than 3mm depron)
• widen elevons to get better control at low speeds (better at high alpha?)
  • would need to shorten fixed wing area, or extend step location to maintain at 50%
• electronics compartment options
  • separate cutouts for battery, ESC, and RX (cut battery last to get CoG)
    • perhaps a larger hole for battery to allow larger battery sometimes (small sideways at front, large lengthways at back)
  • a single large cutout for electronics, Velcro to fix, hatch cover, plastic floor

Photos

Top (note flat plastic hatch cover - retained by sticky-tape)

Bottom (again, a flat plastic hatch cover - non-removable)

With hatch open

Close-up of hatch contents

Close-up of motor area

Close-up of nose reinforcement (from 13mm reticulation tubing); also note 1mm CF rod reinforcement of leading edge

Cutoff tip (in the style of Assassin) to assist with stability without winglets/fins

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