102" Purpose-Built for Electric Power SU-26 - RC Groups
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May 05, 2017, 02:12 PM
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102" Purpose-Built for Electric Power SU-26


102" Purpose-Built-Electric SU-26 with sheeted foam wings, stabs, and turtledeck, with high density foam and 1/16th inch balsa. It's covered in genuine Oracover and the ailerons can be cut to custom sizes for IMAC, 3D, or XA. 6800W of power and enough capacity for 5.5 to 6 minutes bring the plane to an AUW of 24.34 lbs RTF.

Covered weights:

Fuse + Rudder = 2165g
Battery Tray = 83g
Firewall = 101g
Motor Beam Top & Bottom = 103g
Wing 1 = 941g
Wing 2 = 944g
Wing Decals = 99g
Stab 1 = 236g
Stab 2 = 236g
Cowl = 528g
Canopy = 267g
Wing Tube = 169g
Stab Tube = 66g
CF Landing Gear = 197g
CF Tailwheel Assembly = 57g
KavanLite 5" Wheels = 147g (stock wheels are heavier)
Steel 6mm Axles & Hardware = 32g
Hardware = 39g
Triangle Stock and Doublers = 28g

Total Empty Weight = 6438g or 14.19 lbs
Power & Control Components for 6800W and 14S 5300mAh = 10.15 lbs
AUW = 24.34 lbs

This plane would be about the same weight with a DA-100L on ES pipes and about 500cc of fuel.

MAC * Wingspan = 1928 sq in wing area
Wing Loading using MAC * WS= 29.1 oz/sq ft
Wing Cube Loading using MAC * WS = 8.11 oz/cu ft
CG at 12% Static Margin = 8.0 inches at the wing root
Last edited by Aeroplayin; May 03, 2018 at 07:57 AM.
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Jun 07, 2017, 09:21 AM
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I built my own plane to [1] lower weight and [2] increase wing area so that I can still add the capacity weight needed to push the power I need while maintaining a respectable discharge rate.

At 6800W and 14S 6600mAh, I'm still at 8.5 oz/cu ft and 272 W/b power loading with a 1.9:1 thrust loading. The way I did this was to return to some of the more traditional and scale wing dimensions, and I can do that because of the construction material.

Built-up wings lose rigidity and strength at a certain size, and the only way to overcome this is to change the construction material or increase the weight by adding supports to the lateral D-box and diagonal axis since they not only bend along the spar line, but twist as well. This twisting is really worse if you're trying to fly precision or IMAC.

To test for this, have a friend hold the root firmly by the leading edge and trailing edge while you hole the tip by the leading edge and trailing edge and twist the wing.

IMO, this is why wings in a lot of ARFs today stay the same size while the rest of the plane gets longer and wider. With sheeted foam, a thicker and stronger wing of the same size can still be built at a lighter weight than built-up wings. Mine is lighter than a built up wing of the same size, and I can stand on my wing while it's supported only by the tip and root, and I'm about 182 lbs. There is also no sign of twisting along both diagonal axis.

The only sheeted foam wing I know of that was released from China for a giant scale ARF was severely overweight.

Something most of us have experienced with ARF wing design is how the wing span to fuse length ratio has changed. Instead of a more scale 89-90 percent ratio, 100% is the new normal, and there are some that have even exceeded this.

Accompanying this in new wing design is:
  • an increase in the root length to maintain area
  • a thinner airfoil to reduce drag
  • added area to the horizontal stab to compensate for shortening the wing
  • added SFGs to increase the outboard wing and aileron efficiency
  • added fuse area to increase yaw stability
  • and widening of the aileron and elevators to [a] add more control surface efficiency and [b] take better advantage of vectored thrusting at low 3D speeds
As far as vortex generators, Quique uses sharks teeth on the leading edge of the outboard half of the wing to counter separation. The more efficient sharks teeth design I've seen are actually shaped more like a real shark's tooth but pointing backwards and placed on the top of the wing before the spar line with the pointed tips slightly elevated. I've seen this design on racing wings on cars, where the elevation of the tips are controlled with servos to increase and decrease their effect when needed.

So the future of electric power in bigger airframe will probably require a little bit of a shift in the current ARF paradigm and back to more scale dimensions with sheeted foam, or other novel and creative building material. We'll see what happens next, but if we don't do something else, we may start to see airplanes that look like they have alligator arms for wings. Hasn't quite happened yet, but it may come to that.
Oct 21, 2017, 03:26 PM
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My friend Olivier from Switzerland flying it in a really nice Swiss meadow.

SU26, 6. Flight (6 min 46 sec)
Last edited by Aeroplayin; May 16, 2018 at 02:27 PM.
Oct 26, 2017, 09:43 PM
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Another video today.

Tom Kitt 102" SU-26 Electric (5 min 20 sec)
Nov 24, 2017, 08:34 AM
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Here are some numbers with the props I tried on the Scorpion SII-6540 155Kv motor, which is a Scorpion prototype. In the video above, the prop is the 26x12 Fiala Classic Beechwood E2. This is the prop we settled on for this motor and plane so far. I was also using the Hobbywing Platinum Pro V4 200A OPTO controllers with a 14S 5000mAh main pack in the video.

26x12 Fiala Classic Beechwood E2 (painted, which really measures 25.5" in length)
156g
51.95 min volts
127.2A peak
6610W peak
44.9 lbs of thrust @ 5 secs

26x12 PJN Xoar (painted)
149g
51.58
120.1A peak
6195W peak
37.6 lbs of thrust @ 5 secs

Xoar 27B 3D PJE (a gas beechwood prop with a scimitar shape that I lightened at the hub a bit, dropping the weight by about 12g)
158g
50.21min volts
120.90A peak
6208W peak
44.5 lbs of thrust @ 5 secs

I also tried the newer Fiala E3 version of the 26x12 Fiala electric beechwood prop and it was a complete disappointment. Physically, it measures a true 26" in length, the hub it wider and the shape of the blade is more traditional. There is no reason I see to go forward with this Fiala E3.

Here are some observations:
  • The throttle response was not bad on any of these props, and probably the best with the 27B.
  • The Watts per pound of thrust was better with the 27B gas Xoar, which was surprising.
  • There was no significant motor or battery heat with any of these configurations.
  • The airspeed with the 26x12 Classic E2 Fiala was best and the 27x10 was of course slower.
  • The voltage sag was significantly more with the 27B and I suspect that more battery capacity is required to alleviate this issue, which will certainly add more weight to the plane and increase wing and cube loading but increase RPM and power-to-weight.
Last edited by Aeroplayin; Nov 26, 2017 at 07:28 AM.
Feb 21, 2018, 10:53 PM
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Some new pics with the new spinner... a 5" Falcon white painted carbon spinner from NorthwestRC.
May 16, 2018, 02:20 PM
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Forgot to mention the new power system I'm trying. It pulls 55 pounds on my Rapala with a 28x12 PJN prop seen in the images above. The efficiency of the reduction gear system is so good that I'm getting this power from only 156A on 12S. With the motor and drive unit, the weight is the same as with the 1380g outrunner motor but with a $350 heli motor, and a prop that the 1380g outrunner could not manage.

Here's a video of someone else's setup for idea of what this can do.

Flitework TB100Z Zahnriemenantrieb (1 min 43 sec)
May 16, 2018, 02:25 PM
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Here's a demonstration of the wing strength. That's just a little over 150 lbs of sand on a damaged wing from a slight mishap.


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