From: fhage@virga.rap.ucar.edu (Frank Hage)
Subject: Re: Electric Motors/Gears/Props
Followup-To: Christopher Kristof <ck3i+@andrew.cmu.edu>
Reply-To: fhage@virga.rap.ucar.edu (Frank Hage)
Organization: Research Applications Program/NCAR, Boulder, CO
Date: Thu, 15 Oct 1992 20:37:56 GMT

>>For example, I'm using a "Master Airscrew Electric Flight Pak" [ .05
>>ferrite moror, (2.5:1) gearbox with folding 12 x 8 prop] system that
>>gives about 22 oz thrust on 7 cells. Motor, gearbox and prop weigh 8
>>oz. 1500 mAHr batteries (7 cells)  weigh in at about 12 oz. -  System
>>costs ~$25, mail order w/o batteries.

>So, does that leave you with only two ounces of "surplus" thrust?

Well, I've put the system in an airplane, where the wing provides the
vertical lift. My plane, servos, motor controller and radio weigh another
32 oz, so my craft is a long way from having vertical performance.

>Price is not much of a consideration as long as the per motor cost is
> roughly between one and two hundred dollars.

Samaurium Cobalt (and other rare element)  motors start at about $100
and go to over $1000. 

>Basically what I'm looking for is THE motor with the largest "surplus"
> thrust  when it's weight and power source are taked into account.

As is usual, there is no simple answer. The thrust one obtains out of any
electric motor is very dependant on what power source (battery probably) and
controller and wiring one uses. The resistances across even short segments of
heavy gauge wire becomes significant at the large currents these systems run at.
So, even if you could run a tether directly to a N-reactor, you would
have to worry about wire weights, etc. NiCad batteries are excellent
power sources because they have very low internal resistance.

Additionally, one can get 2-10 times the output from electric motors at the
expense of shortened motor lifetime (and fire). Permanent magnets lose
their "strength" at high temperatures (the magnetic moments realign) and
during vibration. This is primarily why cobalt motors are considered
"superior"; they are more resistant to these conditions and can operate
at higher current (and thus heat) levels.

Ultra high performance electric aircraft use the combination of
"0.35" or greater cobalt motors, 25+ NiCad cells in series, large
props and very short run times. The cost of owning and operating is Ultra
too.

>(For those not familiar with this thread, I plan to use
> the motors in a VTOL type robotic aircraft.)

I think what you want to do (for a reasonable price) is possible using
2 .05 astro flight cobalt motors, each running off 7-10 cell high
quality SCR battery packs, turning "oversized" props. These motors have
adjustable timing, so will run equally well in both directions.  You
may have to build your own props because of the opposite rotation
directions.

Hobby Lobby seems to have the best support for electric power around,
that I know of. I suppose it depend on who answers the phone. Their
catalog has motor/battery performance spec tables, which are very useful.
Beware, however, I've noticed serious errors (inconsistancies) in their 
figures,  and their experimental methods and controls are questionable.

I know of no other source of comparitive data. I'd love to hear of other sources.


-Frank Hage. National Center for Atmospheric Research. Boulder, Colorado.

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