Great Planes Spectra

Motors, speed controllers, batteries and other e-power stuff

Great Planes Spectra

Postby hohocc » Tue Jan 01, 2013 6:45 am

I've got an old (but low mileage!) great planes spectra that I'm freshening up for a return to the sky. Can't help but thinking that since it was built in the mid 90s that things have moved on a bit and I could do with some advice/education.
It's powered by a great planes goldfire RS550SH, according to a brief look on the interweb this is a bit of a dinosaur. A servo turns the power supply on and off to it. The prop is a folding 8x4.5 which I'd like to keep if possible so externally it looks like it always has.
The battery is a huge nicad 1400mah 8.4v weighing an awful lot!
On the tower hobbies site they show the current motor specified is this one:
http://www3.towerhobbies.com/cgi-bin/wt ... LXJ766&P=V
ie a great planes electrifly s600 7.2-8.4v ferrite motor, current 26-28a, power 165w, thrust 19-21oz and will turn an 8x4 prop at 11820 to 12300rpm on 8.4v.
The trouble with tower hobbies is of course the freight cost, I've looked on hobbyking to try to find an equivalent motor (which must be there somewhere) but am lost. Of course I also need to decide on a battery (life preferably), and I understand I'll need an ESC to add to my confusion.
So at this stage I can't get my head around the numbers and the terminology surrounding electric flight, I've taken a look at a couple of online calculators and not got very far with them so if anyone can provide advice as to where to look for info, or their take on which motor/battery/esc from hobbyking I could use I would be grateful!
And another question, as it is set up the motor is either on or off, with an esc does the power output become variable? And does the esc need a servo to control it, or is it controlled directly from the rx?
Also, currently there is the motor nicad and an rx nicad, can the whole shebang be powered by one life/lipo through the esc?
Thanks!
hohocc
 
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Re: Great Planes Spectra

Postby hohocc » Tue Jan 01, 2013 9:24 pm

After posting the initial question I took a look on another forum which had a number of posts regarding the Spectra, as a result of this (mis)information I've come up with the following, bearing in mind the Spectra is a 2m 1.1kgish powered glider and I need to climb to altitude (not necessarily vertically!) then switch off and glide:
http://www.hobbyking.com/hobbyking/stor ... duct=18969
http://www.hobbyking.com/hobbyking/stor ... duct=12919
http://www.hobbyking.com/hobbyking/stor ... duct=12921
And for Life batteries:
http://www.hobbyking.com/hobbyking/stor ... duct=14061
http://www.hobbyking.com/hobbyking/stor ... duct=14064
All opinions welcomed!
hohocc
 
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Re: Great Planes Spectra

Postby BillGriffiths100 » Wed Jan 02, 2013 11:44 am

http://www.hobbyking.com/hobbyking/stor ... duct=12921
Of the three looks your best bet for the motor don't forget you will need a brushless speed controller.
I recommend http://www.hobbyking.com/hobbyking/stor ... oller.html
Although it says it has a linear regulator to power the servos in fact it has a switch mode power supply in it, (at least the ones I have bought do).
Switching is much better than linear as it does not burn up energy reducing the voltage to the servos and has a much reduced failure rate.
Bear in mind if you use li-fe you don't get as much voltage as a li-po and hence the speed controller may not perform as expected.
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Re: Great Planes Spectra

Postby hohocc » Wed Jan 02, 2013 7:03 pm

Thanks, that's just the sort of info I was after!
Sounds like li-po might be the way to go. So far as the ESC I'd agonised over quite a few on HK, appreciate the guidance.
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Re: Great Planes Spectra

Postby hohocc » Mon Jan 28, 2013 7:49 am

Had the first flight after making a few changes today and am pretty pleased.
Ended up using a Turnigy 2830-11 1000kv 210 watt outrunner with a Turnigy 2200 3s lipo, and a 8x4.5 folding prop. Only flew for ten min or so as it needed some downthrust so have made that change and might be able to have round two tomorrow.
Climb in the new configuration was adequate, I may try an 8x6 folding prop on it as I have one, but my suspicion is this won't make it go much differently and will just load the electric bits up more, so there will be a few more flights sorting it out (might need a whisker of right thrust too) before it is "finished".
The c of g shown on the plans is at 33% chord, as it is today it's at 35%, and the lipo can go in 2 different positions with the rearward giving 39%, from reading on the web 40% sounds a good max rearward figure. Another job to look at is a change to lower the wing incidence, that and the cg change apparently improve the soaring ability considerably.
Weight went down by about 370 grams, roughly 13 ounces in old money so the new old Spectra is likely to be a considerably different animal from the old old Spectra.
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Re: Great Planes Spectra

Postby hohocc » Fri Feb 01, 2013 7:57 pm

Have had about half a dozen flights now, all going well so have moved the cg back to 39% with no ill effects, won't go any further aft than this.
I decided to try the 8x6 prop and what a difference it made! With the 8x4.5 it climbed just fine, but the extra pitch really helps a lot and now a short motor run of 20 to 30 seconds gets it to a couple of hundred feet which is more than enough performance for me. The ESC needs reprogramming to put the prop brake on as it windmills with no power on, likely creating considerable drag.
Next bit of experimentation is with the wing incidence reduction.
Anyway thanks very much to BillGriffiths100 for the guidance on selecting motors and ESCs, much appreciated and I'm very pleased with the results.
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