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Small Plane
09-02-2005, 04:56 PM
I purchased a Potensky 150 watt outrunner during Hobby Lobby's (http://www.hobby-lobby.com) "Thanksgiving in August" sale, and need to build an appropriate airframe for it. This is my first "brushless" motor, although I have been flying electrics for 7 years. I do not know what would be the best airframe for it. From the hobby lobby website, it is not advertised as a "3D" type motor, so I am leaning toward the Skimmer 400, but would like some input from somebody that has run brushless outrunners on the other two airframes.

Here are the MOTOR specs from the hobby-lobby website:

POTM150 POTENSKY 150 WATT EXTERNAL ROTOR BRUSHLESS

NEW! For airplanes to 24 oz., 500 sq. in. wing area

1-1/4" (30mm) diameter, 1-5/16" (33.8mm) long (from prop back plate to firewall mounting surface), 6mm prop shaft, weight 1.6 oz. including the motor mount. 150 watt motor with 1-3/8" (36mm) spinner and 9.7x4.7 folding prop, also includes a nylon prop nut for use without spinner. Use with 6-10 Nicad or NiMh cells or 2-3 Li-poly cells and a Jeti Advance PLUS 18 Amp controller. Maximum efficiency of 82% at 6.7 amps. Can go to 15 amps for short bursts. Can also be used with APC propellers from 10x3.8 to 11x4.7.

I am trying to figure out the best airframe for it. I have three choices 1) Hobby Lobby, Skimmer 400 2) Herr Rally XP Sailplane 3) 3D Foamie Ultimate Biplane.

1) Specs on the Skimmer 400:
Easy to build balsa kit with clear plan builds in a few evenings. 59” wingspan, 30-3/4” long, 342 sq.in. wing area, 1.4 lbs, low drag Selig 3021 airfoil for great thermal soaring and even aerobatics! We’ve even climbed out inverted! Beginners can fly it, but experts will like its hot climbs on SPEED 400 motors.

2) Specs on Rally XP:
3 Channel R/C
Wingspan= 48.25"
Area= 247.6sq."
Weight= 8 to 11oz.
Wing Load= 4.6 to 6.41oz/sq.'
Direct Drive= 280 Class Power
Flight Time=
8 to 12 min using 7 cell 270 or 350mAh Nicad
Over 20 min using 7 cell 750mAh NIMH

3) 3D Blue Foam Ultimate:
Ulitmate 10-300 3D Aerobatic Park Flyer
Wingspan: 30"
Weight: 11-13 oz.
Power: GWS EPS300C "DS" Gearing (6.6:1) or HackerB20L/Maxon 4:1 Gearbox
Propellor: GWS 12x6 Slowflyer
Battery: 8 cell Sanyo AAA 720 Mah

My (limited knowledge) opinion is that the Skimmer 400 will work, but with not much improvement over a stock SP400. I think the motor is overkill on the Rally XP and would worry about structural integrity (motor rated for twice the airframe weight and wing area). The Foam Ultimate might be a pretty good match, as you want excess airframe to thrust ratio, and if you bag the folding 9.7x4.7 prop and put an APC 11x4.7 on it.

What do you brushless experts think?

Thanks in advance,

Small Plane

Matt Kirsch
09-04-2005, 02:01 AM
You've got a pretty good sense as to what will work and what won't there. Trust it. My personal choice would be the Ultimate, or a smaller "hotliner" type plane somewhere around the 1lb weight range.

One thing I would recommend is that you get a Wattmeter or similar tool to measure Amps when trying different props. I believe the motor is designed to draw 150 Watts with the included prop and a 3S LiPoly, and at that point, it's maxxed out. A bigger prop will only push it over the limit.