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Build of a Steve Bage EDF MIG17 enlarged to 143% Chapter 1
By a happy accident at a Prontoprint facility, Steve Bage’s 17” span MIG17 was
printed as ‘A3 fit to page’ and a 27” version was borne. I had not before
contemplated a scale model but the excellence of the design impressed and I
succumbed to the challenge of producing a free flight electric EDF scale jet,
having never before been tempted by the ‘scale’ discipline. The complexity of the design prompted me to engage Belair
to laser cut 4 off kits for the Peterborough MFC as I was doubtful of my
ability to produce the plethora of intricate formers and wing ribs that
Steve’s design requires. Steve was happy to supply a 143% .dxf file that Belair
needed to laser cut the kits. This was a good decision as the quality of the
cutting was first class. However, the components require some adjustment as
the jigging slots were designed for the smaller 17” model. This is not a
build for the faint hearted and will require some modification of the Belair
supplied components. As supplied
the Belair hardware appear as ……..
Sticking
bits together result in….
These
assemble as……….
Discussion The plan calls for cutting the fuselage in half at the F7
junction to get access to the EDF. I was nervous about doing this , so I cut
the fore and aft dorsal K1A member to achieve the same. Having witnessed the state of a colleagues paper inlet and
outlet ducts in damp weather, I am not going to use paper. I have ordered
some clear acrylic from [email protected]
to act as formers and I intend to plagiarise the gummed brown paper technique
to build the ducting, described in the Free Flight Forum papers 2011. This
can be damproofed using Johnsons Klear. The parts assemble easily bearing in mind that the jig
slots were oversize having used sheet thickness as per the 17” original. The
method of wing retention is unique and esoteric, needs careful fitting but
works well. I did however CA in a couple of magnets at the root rib. Weight achieved
Airframe
Fuselage (90% complete) 39.2gm Stab 4.1gm Fin
5.0gm Wing right
11.0gm Wing left
10.9gm TOTAL
70.2gm For a complete wood only airframe 78gm Allowing 20% for covering and finish 93.6gm Assume 100gms for safety and self kidding Power system
AEO 6000 kv 44mm EDF with brushless outrunner 36gm Kpaero Mk1 E30 controller with 2mm gold connectors 2gm
Suppliers performance figures
Voltage Current Thrust Energy (volts)
(amps) (gm) ( 10 20.7 328 207 11.1 23.7 374 263 12.6 28.5 436 359 These figures are outrageous and I knocked out a test rig
to see what the unit was really capable of. I used an E30 kpaero controller
with a ‘wait to start’ feature achieved with a push button to the right of
the controller.
44 mm AEO
EDF test rig
44mm AEO EDF
on test Voltage Current Energy (volts) (amps) ( 7.4 11.2 82.8 11.1 18.4 204 The
suppliers figures are generous but not that far off. Assuming 1.8 gm/Watt of thrust a 2 cell pack will give
149gm thrust. A 3 cell pack will give over 363 gms of thrust ! (Way too
much) (Pay not too much attention to static
thrust figures, a decent forward speed works wonders as does good inlet and outlet ducting. At model
sizes its not a science but an art). A Hyperion G3 2 cell 550 mah is 65 x 18 x 15mm and will
fit nicely within the battery bay and weigh 32gm for 13.7 amps (25C). A Hyperion G3 3 cell 850 mah is 73.5 x 25.8 x 18.6mm and
will require mods to the battery hatch to fit. It will weigh 69gm and offer
21amps (25C) The 2 cell might be underpowered but nevertheless it’s the
one I am going for. I will team this with a 19gm basic Robotbirds ESC capable
of 25amps continuous. This recognises I might be wrong on the required thrust
allowing a 3s battery change to get out of trouble. For a 2s battery I could
fit a 12amp ESC and lose 6gm………not worth the effort. Hence power unit weight will be :- EDF 36gm Kpaero controller 2gm Loom, connectors, switches 15gm Total power unit weight 85gm Total model weight 185gm Thrust on 2s 149gm
(Is this enough?) Going 3 cell will be overkill and will add 37 gm to the
AUW, probably destroy the balance and put the AUW to 222gm. Try the 2s , it
might be OK. Look out for Chapter 2 when I have made the ducting and
installed the power unit. |
