From: kvk@questor.sw.stratus.com (Ken Koellner)
Newsgroups: rec.models.rc
Subject: Re: Electric Sailplane questions
Date: 14 Apr 92 17:43:45 GMT
Organization: Stratus Computer, Software Engineering

In article <1992Apr14.154121.18957@news.larc.nasa.gov> otten@icase.edu (John Otten) writes:
>(questions about electric sailplane selection).

First, raise your budget to a little over $200 if you want to do it
right.

Second, get a 1991 certified modern radio system.  They start at about
$120 mail-order for a four channel radio with three servors and lots
of hardware, nicads and tx/rx charger.  If you fly where noone else
flies and you really want to fly with drycells, you could keep your
old radio but it probably is better to invest in a real radio.  It is
not worth upgrading your radio to nicads.  The cost of the rx and tx
packs plus a charger would be the better part of the cost of a whole
new modern system.

Third, select a ship.  The Electra is a decent ship.  It's easy to
build.  It flies easily.  The delux kit comes with a motor, prop,
spinner and switch harness (includes Tamiya connectors, fuse and
holder and switch).  Some other ships you can consider are the
Astro-flight Challenger, the Airtronics Eclipse, and the Spectra (by
Great Planes or Goldberg, I forget).  The Spectra comes only with
motor, prop, wiring, etc.  The Eclipse comes with or without.  The
Challenger comes without a motor.  These all have slightly less
flat-bottomed airfoils that should penatrate better and have a wider
speed range.  The electra tends to float a lot and fly real slow.  The
kits with a motor are around $60-70.  Without a motor they are around
$30-40.  The motors that come with these ships are your basic 05
ferrite electric.  These motor cost about $10-15 and perform
acceptably.  A very good electric motor would run $70-$100.

As far a motor controls, there are several ways to do it.

The best but most expensive is a ESC (Electronic Speed Control).  A
decent one costs about $70.  You don't use a servo on throtle channel
with one of these.  It has one wire that plugs into the reciever, one
high-current wire that plugs into the motor, and another for the motor
battery.  It lets you vary the throttle from off to full with little
current loss by doing high speed switching.  Some of these will let
you use your motor battery)to run your rx through a circuit called a
BEC so you don't need the rx pack.  Many people don't think doing this
with airplanes is a good idea though.  If your motor battery
castrophically fails (eg shorts and catches fire, it can happen), your
rx will be dead and you will have an uncontrolable, dangerous missle
in the air above your head.  There are cheaper ESCs that only allow
on/off, they are about $40 and probably are not better than a server
and switch unless you want to risk the BEC.  You can get a radio with
a special electric sailplane flight-pack that includes an ESC.  The
problem with these is that you lose flexibility if you want to use the
radio with another type of ship.

The alternative to an ESC is to use a server and a switch to turn the
motor on and off.  The normal way is to use a third servo to turn the
motor switch on and off.  You could alternatively link it to another
channel so that it flicked the switch only at full deflection of one
of the two control surfaces.  This saves the weight of an extra server
but makes the linkages more complex and less reliable.  A servo only
wieghs 1.5oz so don't bother.  The third servo method is the easiest
and cheapest and by all means an acceptable way to go.

Battery packs have gotten real sophisticated in the last few years
with the boom in electric car racing.  First, don't get SCE packs.
They cost big-bucks and can't be fast-charged.  You may not want to
bother with and SCR packs either.  They cost semi-big-bucks and you
probably won't notice the difference.  Get a 1200-1500mH fast-charging
packs, either 6 or 7 cells.  The bigger the mAH rating the longer they
will last in the air.  They cost about $18-25.  You also need a
charger.  A cheapo timer-operated one will do.  You run it off your
car battery.



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