Sizing Considerations for ETex Propulsion Systems

I have recently written a basic approach to sizing Limited Motor Run SAM competition models with an aditional comment on sizing for the ETex event. Click here

The Giant power system, motor, gearbox and ESC will be taken from my 1050 sq inch Stardust Special. That model has seen some success but it is not obvious that the system will work or be optimum in an airplane 2 1/2 times the size. The system consists of a Neu 1107/6D motor with a Maxon 4.4:1 planetary gearbox feeding into a Cermark 2.33:1 spur gear reduction via a custom made adaptor plate. The overall gear ratio is 10.5:1 and in the Stardust Special it turns a Freudenthaler 23 x 12 prop specially modified into a 24 x 22 via a custom made middle part. It uses a 2 parallel x 6 series Sanyo N1000SCR NiCad cells.

The Giant is a whole different kettle of fish being 2 1/2 bigger. Nevertheless this motor system seems like it will do the job.

I did an analyis using Motocalc evaluating the performance with various battery and propeller options.

The first thing is to examine the different battery alternatives for a variety of wing loadings. The lowest allowable wing loading of 8 oz / sq ft. requires the Giant to weigh a minimum of 159 ounces. I can come close to this with a LiPo battery but the battery I have is a Polyquest 4000 mah two-cell and that requires a minimum weight of 161 oz. (although I ran the analysis at 158 oz).

NiCad alternatives start with 24 N1000 SCR cells but the Giant weight with this pack is 173 oz and 8.7 oz / sq ft. but the rule allows more battery at this weight so this is not a good match, more cells are appropriate and that will drive the weight higher.

32, 33 and two different 36 cell alternatives were evaluated.

Surprisingly, the lightest configuration, that with the LiPo battery, did not yield the highest flight time and I don't know why. However, it is relatively easy to check it out as I have the hardware and can test it. Note it needs a 27 x 22 prop to achieve the best time. More on this later.

The high time configuration is the 4 x 8 cell NiCad battery with a 26 x 22 prop. I have two 2 x 6 cell packs with these cells so if I make two 2 x 2 cell packs I can wire the 2 + 6 to yield the 8 cell packs. This configuration weighs 185 ounces and the wing loading is 9.5 oz / sq ft. This wing loading is right in Champion Jack HIner's sweet spot as it allows you to range the sky in search of thermals even on windy days.

Now both of these solutions require a bigger prop than I currently have, or can buy so I plan to make another set of middle pieces so I can evaluate prop various prop sizes and pitches using the basic set of Freudenthaler 23 x 12 blades. But these props are not perfect because the twist of the Freudenthaler blades don't match that required for the bigger props. An example is shown here for the 24 x 22 prop I have used the last two years.

But it seems to work so I will continue to use this approach. Rudy Freudenthaler does list a larger prop on his website but the cost is over $130 for one set of blades. I guess I will stick with my 23 x 12 blades.

Dave