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Old Sep 27, 2002, 10:49 AM
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Uvalde, TX
Joined Sep 2002
105 Posts
Disappointed with my first electric sailplane...

I built Skybench Aerotech's electric version of their 2M Bird and flew it for the first time last week. I'm using an 8 cell, 500 MAH Sanyo Nicad pack, Jetti 110 ESC, 11x8 Graupner Cam folding prop, 4:1 inline reduction drive on a Graupner 400 motor, Hitec 555 receiver, HS-81 servos, a 10 amp fuse between the motor's positive lead and ESC, and Sermos connectors. I'm hoping somebody here can maybe give me a few suggestions.

First, I don't have a small/sensitive scale and cannot tell you exactly how much this airplane weighs. What I can tell you is that I built it as light as I possibly could (no epoxy anywhere, used CA sparingly during construction, PLENTY of sanding, Aerospan for covering and no extra weight for balancing). This is not my first sailplane.

This design has 636 square inches of wing area and a 78" span. Compared to my non-electric Gentle Lady and Sophisticated Lady, this airplane feels like a brick. I was expecting this. I was surprised how the light airframe suddenly became so heavy as soon as I put the motor and battery in it.

The plane climbs out faily decently, but is nowhere near approaching a 45 degree climb angle that I've read some sailplanes can do. It has to labor to gain altitude. At the end of the motor run, it's really "up there," though. That's when the trouble starts. The glide performance leaves a lot to be desired. It's not nose heavy and it doesn't fly with the nose down in a gentle descent; it simply kind of "mushes" straight and level while sinking. Even with plenty of altitude, I'm only getting power-off times of maybe several minutes.

Is this the kind of performance I can expect from my equipment? Or should things be much better? Really, I think I would have been better off simply sticking an .049 on the nose - at least the plane wouldn't have been so heavy. I was really expecting more from this plane.

I'm charging my battery pack at 1.5 amps (Astroflight 110 D) and it takes anywhere from 20-30 minutes using the fast charge rate. Is this normal? Seems like other people say they charge their packs in 15 minutes.

I've never used ESCs before or micro-receivers. After the ESC turns the motor off and directs the remaining power to the receiver and servos, how much flight time do I have left? Does the micro-receiver have enough range to make high-altitude flying safe? Even if this plane stays up after the motor quits, I'm worried about losing it.

Skybench Aerotech says their plane with this equipment weighed in at 32 oz. This seems overly optimistic to me. 40 oz or even a little more seems more realisitic. Do your planes weigh in close to what is advertised?

Any help you guys can provide would be greatly appreciated. I really want this electric "stuff" to work out!

Thanks!

Dave
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Old Sep 27, 2002, 11:17 AM
RIP Ric
Andy W's Avatar
Marietta, GA
Joined Jun 1999
43,312 Posts
First of all, 32oz would be accurate. Secondly, what type of battery do you have? 500mAh doesn't tell us much - there are several different size (and weight) cells in that capacity - they should be 500AR or 500A cells. If so, the pack would weigh about 5.5oz. Add that to the motor and GB (2oz) and prop/ESC and you're looking at adding no more than 10oz, hopefully as little as 8 or 9oz, to electrify a small glider like this. For comparison, competitive 2M 1/2A (speed 400 powered) 7-cell gliders usually come in under 20oz ready to fly.
Take the model to the post office (after hours! ) to weigh it - ready to fly, 32oz on 636 sq" would only be a 7oz/sq' loading - which should perform very well! Another question - which speed 400 motor do you have? The 6V would be the correct one for sports flying on 8 cells (competitive models would typically use a 4.8V or Rocket motor, on 7 cells).
45 degree climb is about as much as you want. Best climb rate is between 45 and 60 degrees, in most cases, no matter what kind of power system (i.e. how much money) you put into it! Vertical climbs with sailplanes are cute for impressing the crowds only!

I just checked out their specs on the web site and everything they claim is accurate. With the steps you took to build it light, it would not surprise me if your model came in at 30oz or so..

So, I'd say your climb is as good as it's going to get. As to gliding performance, any 2M glider with even a 10oz loading is going to glide EXTREMELY well (long flights even in dead air) - for comparison, my Filip 600 ARF weighs in at over 42 oz on only 592 sq", and I get easy 10 minutes flights from 800' or so in the lightest lift. I'd check your incidence and CG to improve glide performance.
..a
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Old Sep 27, 2002, 12:01 PM
MTT
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West Chester, Ohio
Joined Feb 2002
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Dave, I agree with Andy :

There must be something wrong with the CG or incidence, even at 42 oz, you still have only a wing loading of 9.5 oz/sqft, and it should glide very well !

For comparison : I have an electric powered Multiplex Alpina, which has a wing loading of 20 oz/sqft, and it glides and thermals very well. I also have an Cumulus e from Shredair, which has a wing loading of 12 oz/sqft, and that one is a real floater !

It sounds to me as if your CG is too far forward, and is compensated with too much incidence or elevator trim, and you are flying always on the edge of stall, or the wing partially stalled.

Michael
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Old Sep 27, 2002, 01:50 PM
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Concord, MA
Joined Jun 2002
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climb and power from speed 400

There is Volta's equarion: Volts x Amps = Watts. Watts is power. 700 and some to the horsepower. The rule of thumb for electric flight is; 30 watts per pound may fly (barely), 50 per pound flies "pretty good," 75 will give spirited performance, and 100 plus, straight up like a rocket. I suspect you are in the 40 watt per pound range. This is based on 10 amps at 9 volts and 2 lb airplane. I concur with the previous posts about c.g. and incidence. It should glide better. Eyeball the wing. Is it warped? Do you have to give it up elevator to fly?? Are you trying to fly it too slowly?

If you want to climb at a greater rate, lose some weight! I think a 60" glider with the motor you have, at 24 oz, would be about right. Or else get a speed 600 and bigger batteries. A Mag Mayhem at 2-2.5 to 1 would swing that prop, and you would need a speed control and batteries that coulds handle the increased amps, like 8 CP1300's and a 35 amp esc. This would increase the plane's weight to 40 oz, but you would now have 150 watts or so to pull it around. At 600 squares it should still glide well, just don't try to fly it at stall speed all the time!

Are you using the BEC feature of your speed control? Your post suggests yes. The remaining juice in the power battery will run the radio a long time. Just don't go hitting the throttle after the cutout a whole bunch of times. It will reset when the voltage bounces back after the motor shuts off. It's OK to use it on approach or a go around, probably. Otherwise you should have a half-hour if you leave the throttle alone after the motor cuts out.

I have a 2 meter Riser and a 480 geared motor. It probablyweighs more than yours. I run 10 cells, AE1400's and 1250 SCR's. I also have a Jeti 30-3 I have stuck on it. Climbs like heck with this set-up. If you really want performance and don't mind paying a little more, go brushless.

I also have a Spectra, with a Master Airscrew 3:1 geardrive and their 12-8 folding prop. This on 8 cells will climb at 45 degrees.

Most ESC's have an overload feature where they shut off in the event of a short. If this is the case with yours, you could lose the fuse. Make sure you use 16 guage wire at least, and 14 for a 600 motor; as short wires as possibe. The electric gliders you see climbing like heck are sometimes called hotliners. They aren't meant to be flown slowly, and there are always compromises when you are fiddling with heavier power systems, a more rigorous construction to survive imperfect landings, etc.

I like to go out and fly after work, and the time saved by not rolling out and in a high start is nice. I also like the faster planes and can fly them near my house without someone calling the cops.

The 049 would be lighter and more powerful than a speed 400 and 8 batteries, but think of all that castor oil on your plane!
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Old Sep 27, 2002, 04:07 PM
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in front of p c
Joined Aug 2002
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c/g

is got to be a c/g problem,i have seen the same plane with similiar power arrangement and it does very well, keep messing with the balance point,i have a sp 600 2metre that gained 20 oz to electrify,but it just glides better,harder to land though!!! dont get upset,that glider will do real well just needs to be set up better! fledge
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Old Sep 27, 2002, 04:23 PM
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Napoleon, Ohio
Joined Dec 1996
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My converted Sophisticated Lady (V-tail and E-power) has been flown with 8-600AE, 8-1700 Zagi and 7 CP1300 packs. The same motor/GB/Prop as you.

You're right that it doesn't climb at a 45 but more like a 20+ angle. I've found that the trick is to tag a thermal during the climb and let it do more of the work.

On a good day I can just about sit down and not even touch the sticks....just watch her slowly move around. On a bad day it's just like any other sailplane; watch it go up and come right back down.

BTW the heavier CP1300 pack actually gets better flights in most conditions. The 600AE's just don't add enough weight to penetrate wind.

Keep 'em hummin'

Ray
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Old Sep 27, 2002, 04:38 PM
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Leamington Spa. England
Joined Apr 2002
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Lots of good advice here for you to keep in mind.

As an experienced glider guider you probably expect this plane to just 'float'. It is quite a bit heavier than the glider version just as it would be if you ballasted the glider to make it fly faster in a wind.

With this in mind, fly it fast (about 25% faster) and see what happens. Then start playing with your trims. You have balanced it as shown, haven't you?

Been there, bought the shirt. Don't just give up.
Best of luck
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Old Sep 28, 2002, 09:07 AM
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Uvalde, TX
Joined Sep 2002
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Wow, thanks for all the help!

After having read everybody's replies, I'm convinced my airplane is nose-heavy. I've also been doing some research and will perform what was described as the "dive-test." I don't think a warped wing is the problem, as the plane appears to be very straight, but I will doublecheck.

Here's an article you may find interesting, I know I did:
http://www.modelaircraft.org/templates/ama/198_12.asp
I will keep you posted on how my next flight goes and (hopefully) the improvements that were made as weight was taken out of the nose.

Dave
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Old Sep 28, 2002, 12:51 PM
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Cambridge, MA USA
Joined May 2001
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Re: Wow, thanks for all the help!

Quote:
Originally posted by houdini
Here's an article you may find interesting, I know I did:
http://www.modelaircraft.org/templates/ama/198_12.asp
I'd hate to say it, but that article has little or no scientific basis behind it. CG should not be set at some magical airfoil-specific CG location or for minimum sink, as the article describes. Instead, CG should be set for pitch stability, pitch response to thermals, and for elevator control response.

The Dive Test is a very methodical way to set the CG for desirable pitch properties regardless of the airplane or airfoil type. See one of the articles at www.polecataero.com for a pictorial/text guide on how to do it.
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Old Sep 28, 2002, 01:29 PM
MTT
I care about rising air !
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West Chester, Ohio
Joined Feb 2002
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Dive test

I agree about the dive test !
Although can't make any comparisons to the method described in the above mentioned article, until now I have used the dive test, and it has worked very well for me.

What is true is that manufacturers spec's for the CG always are very conservative, om all my sailplanes, I ended up with the CG a good deal further aft then the manufacturers spec.

Michael
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Old Sep 28, 2002, 02:13 PM
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yeah, you know when I bought my first sail plane, it wouldn't fly either.
It's called the "Wind Cruiser". And the damned thing was just too heavy and there was no WAY it would fly. Even my most experienced flying friends couldn't get that piece of crap off the ground. I'm pretty pissed off about wasting 100 bucks on that piece of garbage.
I think it was made by Wattage. I don't know if anyone else has had problems with them.
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Old Jan 09, 2003, 03:32 PM
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Canada, AB, Lethbridge
Joined Jun 2002
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Hi houdini,

I have just completed an electric conversion of the 2M Lil Bird, I would be interested in contacting you. My e-mail is hog_bipe@yahoo.com
Cheers
Mike
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Old Jan 09, 2003, 05:26 PM
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Palmdale, CA
Joined Oct 2000
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Houdini, your first experiences with e-power would have had me ecstatic, compared to the sluggish stuff I started with!
A 2M plane is a tad large for 400, but you have the right parts to get the performance you want.
Do try the dive test... it works just fine pointing out the limits of what you can handle.
Be cautious going more aft on the c.g. once you've
found one you like. It doesn't take much more aft position to make a plane tuck, which if not stopped in time results in the wingtips meeting above the airplane!
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Old Jan 09, 2003, 05:40 PM
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Palmdale, CA
Joined Oct 2000
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Technical articles in the AMA magazine tend to be not very good.I recall one which emphasized the ONLY way to get a K&B 28 motor started. Complicated procedure that took about 10 minutes each time.
This would be new to us that race them. And start them in seconds.
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Old Jan 09, 2003, 06:21 PM
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The 'Wack, BC, Canada
Joined Oct 2002
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Houdini. Lots of very good advice here. Do the dive test and don't give up. Also I've found in the 3 electric models that I've done that the prop is always a very critical component. Just the diameter and pitch numbers don't tell the whole story. Play with a few and I'm pretty sure you'll find out a lot. And the best for one model isn't always the best for another.

For judging performance I'd not a fan of "climb angles". It's easy to have a model that looks happy pointed up at 45 degrees or whatever and it's barely mushing along. Instead try to judge the climb rate. Time how long it takes to get to a comfortable thermal height. If you want to do some comparisons that are a little more scientific stick a little plastic bar on your antenna. When the wingspan of the model matches the bar width at roughly the same height hit or look at the stopwatch. It's still crude but it's a much better way to judge climb performance than just guessing or looking at the fuselage angle.

But if you think about it if you're getting up to highstart territory in one minute (not unreasonable at all) then that's pretty good.

And last but not least......

If everything you do still results in the model looking mushy when trying to hang a little then try what I've done to two sailplanes with good results. Add a turbulator strip about 15% back from the leading edge. Use two layers of 1/8 wide automotive trim tape. Both these models suffered from poor slow speed range mushiness and a very soft stall. Adding the turbulators greatly improved the slower speed range and the approach to the stall became much more sharp but still with good warning in a good way. I'll probably never know why these two models did this but I do know that they were having severe bubble separation that was resulting in a very mushy loss of lift and lots of drag.

But I'd save the turbulator option to last. Trim the model and learn to let it have it's speed then evaluate the low speed range last.

PS; You can sneak the CG back quite a ways if you're good on the sticks but I've found that it gets hard to fly if the trim is 3 click critical. This is where one click up from neutral and the model mushes, one down and it wants to tuck or go very fast. I like to set my sailplanes up with about a 7 to 9 click range between terminal mush and tuck under. That seems to me like a good compromise for a rear CG for efficiency and a flyable model. But YMMV. Set it to what you feel comfy at.
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