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Old May 10, 2006, 03:19 PM
Glowfuel
Moscow Pennsylvania
Joined Jun 2004
66 Posts
Discussion
Great Planes Spectra ARF Sailplane

Test flew my new GP Spectra today. Everything is stock except the 12x8 folding prop and 3.5:1 MA gearbox.

Here is the situation:

1. Need full throttle to take off and the take off seems sluggish.

2. Controls are set per factory recommendations, but they seem very slow to turn the bird.

3. Having trouble gaining serious altitude (Note: The CG is 3 1/8" - what the plan recommends first flights at)

4. Going down wind seems better, but when going upwind, it porpoises very easily (up and down).

5. When slowed, it almost falls out of the sky. Like it stops trying to fly and wants to go into some sort of spiral. Hard to get out of.

Thinking there is something wrong with the CG, but need some input.

Thanks in advance........
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Old May 10, 2006, 04:55 PM
Registered User
Ontario, Canada
Joined Sep 2004
140 Posts
Can't help with the motor as I use brushless and thrust is not an issue.

Control will be slow if you fly slow. Sometime I push a bit before a turn to pick up some speed but mostly it will turn OK at normal cruise.

I think I bumped the CG back a bit but that is really a personal preference you tune in for yourself over time.

Upwind, downwind makes no difference to the plane. Airspeed is the same. Groundspeed is the only change.

I too get some porpoising hands off but not enough to bother me.

What I can say is that I did not like the stock decallage. I've ended up with the trailing edge of the wing shimmed up about 1/8".

Hope some of this may help.
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Old May 10, 2006, 05:29 PM
Glowfuel
Moscow Pennsylvania
Joined Jun 2004
66 Posts
Spectra

Bumbelbee:

Do you have the spectra?

Will try to move the CG aft a bit and retest.

Not sure if this will help cure the porpoising, but think it may help.

Anything else you can pass along will be appreciated, BB.

Thanks......
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Old May 10, 2006, 05:53 PM
Registered User
Central California
Joined Aug 2003
1,137 Posts
What battery?
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Old May 11, 2006, 06:27 AM
Registered User
London, UK
Joined Aug 2005
1,295 Posts
The plane is meant to be a graceful, traditional powered glider - not a aerobatic model

Beginners like me, absolutely love the "sluggish" take-off and response;
it is very good plane and the very flying characteristics you mention explain the popularity of the Spirit range
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Old May 11, 2006, 11:11 AM
Registered User
Joined Apr 2006
1,906 Posts
Quote:
Originally Posted by glowfuel
Will try to move the CG aft a bit and retest.

Not sure if this will help cure the porpoising, but think it may help.
A too far forward CG will cause porpoising.

R/E
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Old May 11, 2006, 11:28 AM
Registered User
Joined Apr 2006
1,906 Posts
Quote:
Originally Posted by glowfuel
Test flew my new GP Spectra today. Everything is stock except the 12x8 folding prop and 3.5:1 MA gearbox.

Here is the situation:

1. Need full throttle to take off and the take off seems sluggish.

2. Controls are set per factory recommendations, but they seem very slow to turn the bird.

3. Having trouble gaining serious altitude (Note: The CG is 3 1/8" - what the plan recommends first flights at)
You don't mention what battery pack you are using - # of cells and type. The stock motor is a "can 600"?

When you say "can't get serious altitude" - are you saying at first it climbs out OK, but eventually it levels off at full throttle? To me that sounds like a voltage drop on your pack from too high a current draw, if on a fresh pack. If a glider climbs out at 45 degrees on a fresh pack, it should continue on that angle until out of sight or until the battery starts to poop out.

Generally, I adjust my servo and control surface ends of the push rods for maximum travel - the furthest out hole on the servo arm, and the closest hole on the control horn end. (I do this after an initial test flight with the control surfaces moving at about suggested movement.) One must insure that doing that doesn't cause binding/stalling on the servo, however. Also a test flight will show if the amount of travel is too much for your style of flying. Probably best to move one hole at a time on one end of the rod and then do a test flight.

R/E
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Old May 11, 2006, 11:38 AM
Fly R/C writer
Redlands, Ca
Joined Dec 2004
1,915 Posts
If you are set up with the stock motor and battery recommendations, yes, your plane will perform fairly sluggish. this is because it is a very conservative set-up and the manufacturer didn't think that someone who is a novice should have a medium to high power set-up as this gets them in trouble. So, if you want to increase performance, you can change to a more powerful motor, or switch to lithium battery. Both will give you an increase in performance for the climb.

As for the porpoising, I tend to agree with the other guys about moving the balance back a tad. The more neutral the elevator trim, the more steady the flight path. But, don't move it back so far as to cause the plane to be pitch sensitive. So I would start with moving it back 1/8th inch at a time and then fly it to see how you like it. Each time you fly it with a different CG, be prepared to change the trim a bit.

As for the ultimate in climb and performance, and I don't know how much money you want to spend, but if it were me, I would install a brushless outrunner motor with a 3-cell 2000mAH lipo pack, and that should take you to the moon. The set up will be lighter than stock and more powerful by almost double. Not cheap, but if you want power....

Mike Lee
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Old May 11, 2006, 01:39 PM
Less Whinning, More Flying
Sailhigh's Avatar
SF Bay Area, USA
Joined Feb 2003
1,733 Posts
I just got the Spectra as a prize in the club raffle. I'm planning to finish it stock and use the stock motor/prop to begin with. This is my first electric plane, so I read everything I could on electrics. Seems like a lot of beginners aren't satisfied with the performance of stock set ups. Experts online (and locally) attribute that to imporper, or a complete lack of, break-in for brushed motors. Apparently between a 10-20% performance increase can be gained by that. This is something that R/C car racers had learned long time ago. Again, I'm no expert on the subject. Research online for the various break-in techniques. I'll be trying the water break-in myself. Thanks for the advice on the cg and decalage change.
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Old May 11, 2006, 01:48 PM
Registered User
Central California
Joined Aug 2003
1,137 Posts
A good pack would be 7 cells at 1900 mah. Better would be 8 cells at 1900 or 2000 mah or a 3s2100mah li-po.

I had to add about 1/8 oz. of weight to the tail.
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Old May 11, 2006, 04:59 PM
Glowfuel
Moscow Pennsylvania
Joined Jun 2004
66 Posts
Thanks to all for their posts.

Am running the standard 7 cell 2100 mah nich pack.

The more I think about it, the plane does seem a bit nose heavy.
Especially on take off, where it sort of struggles a bit, until it gets going.
Will move the CG back 1/8" at a time and retest.

Am using the stock motor with the MA 3.5:1 gearbox and 12x8 prop.

Any other advice will be greatly appreciated.......
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Old May 11, 2006, 07:02 PM
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Joined Apr 2006
1,906 Posts
I would think that an 8 cell pack would provide more power, but I have no idea what the stock motor really is, to attempt to look at the setup in MotoCalc.

Don't want to push too many amps through it.

R/E
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Last edited by rud-elev; May 11, 2006 at 08:41 PM.
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Old May 11, 2006, 07:10 PM
Let it Be
USA, WI, Oshkosh
Joined Jul 2004
308 Posts
I've had 2 Spectra ARF's. The first was stock, and quite sluggish. I was disappointed, and sold it.

Later, I regretted I sold it, and picked up another. It is a beautiful looking plane. This time, I took no chances, and swapped out the motor/ESC for a Jeti Phasor 30/3 brushless, 11X7 prop, and appropriate ESC. I can climb to altitude in 20 seconds, at about a 60 degree angle. I'm running a 10 cell 1700 mah Ni-CAD pack, and will try a 3 cell 1500 MAH Li po pack this year, if the weather ever permits here in Wisconsin.

What a difference the brushless motor makes! This is now my favorite powered sailplane. Take the plunge and go brushless.
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Old May 11, 2006, 08:42 PM
Fly R/C writer
Redlands, Ca
Joined Dec 2004
1,915 Posts
to Sailhigh

Having done R/C cars at the championship level, ( I have three national titles), I can tell you that getting 10 to 20% better performance out of the Mabuchi type motor isn't going to happen. You might 5%, as this water break-in only seats the brushes to the comm. What will do even better is if you can advance the timing of the motor. If I remember right, the stock set up is direct drive? Yes/no? If so, advancing the timing will do a lot. If not and there is a gear reduction drive, then the motor may have to run in reverse to drive the prop forward, and that would mean you have to retard the timing, which is actually advancing in reverse. But I will suggest you do one thing before anything else. Run the motor with a battery and nothing else. Listen to it and then reverse the polarity. The motor should run at the same speed both directions. If so, you have a motor with neutral timing. If not, then the motor timing has already been advanced and that's all you're going to get. But there is no 20% left in that motor. Pipe dream.

Mike Lee
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Old May 13, 2006, 12:36 PM
Glowfuel
Moscow Pennsylvania
Joined Jun 2004
66 Posts
Spectra

http://www2.towerhobbies.com/cgi-bin/wti0095p?FVPROFIL=++&FVSEARCH=gpma2343&FVPROFIL=++

(Hyper link is not working. Copy and paste it into your address bar.)

The above is the motor I am using with 7 cells, 2100 mah.

Will this motor handle 8 cells?

What are some other options for power, other than brushless?
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