greenflash

Hi all,

I just thought I'd check with the forum about re-powering a World Models Blue Bird 2M electric glider.
This is a nicely built balsa ARF, 2M polyhedral, 2 piece wing with a steel joiner. I'm almost done assembling it.

By the way, I paid $59 + shipping, and they are right now $59 with a 15 amp brushed ESC included. (See the Airborne Models overstock site) See pic attached. Pretty nice 2M for the price.

It was powered by a 550 brushed can motor and gearbox. I could stick with it, but the motor/gearbox combo weighs a whopping 9.3 oz. It pains me to have that much weight for only about 125 watts! (8.4v x 15 amps draw)

The airframe and other hardware weighs about 19 oz. The manual says flying weight is 39 oz, so I guess they were counting on the 7-cell NiMH 1800 battery weighing almost 11 oz.

I've read that exceeding 120 watts/lb on a lightly built floater is not advisable and I guess I'd agree. The included prop is a 10" folder.
With a 1800-2200 3s lipo, the plane will weigh no more than 32 oz.

So I own two Hobbypartz motors I could use; an Exceed 480, max 250 watts, or an Exceed 450, max 175 watts. Both are 800-950 Kv which should work fine with the 10 " prop.

Would climb rate be OK at 87.5 watts per lb (the Exceed 450)? The Exceed 480 would be 125 watts/lb.

One note: The downside with these older gliders designed around heavy can motors seems to be; when you replace the 8-10 oz brushed can with a 3-4 oz brushless outrunner, they end up tail heavy. I could try bigger lipos, but my largest is 2200 3s. I have an 10 year old Wattage Whisper 2M that has a very wimpy direct drive 550 brushed, and it will be the next candidate for brushless.

Thanks!

flexsphincter

im hoping you get some comments for im in the same possition with an aspire ep with the can 550 motor,,,i own a park 450 brushless and was thinking of buying a 10x8 prop for it ,,, it would be nice to get some ones blessing first.

greenflash

Actually I think I answered my own question. 87 watts per pound is not enough, and real world numbers would be lower.
I think the Exceed 480 will do just fine, and the extra nose weight will be welcome. Probably still not enough weight.
It's a pretty nice looking plane, especially for the price. I think it's much lighter than my 2001 vintage Wattage Whisper, which is also a 2M.
Oh well, onward with mounting it; probably going with HS-81 servos.
I think as these sailplanes usually use a 10-11 inch folding prop, a motor with Kv less than 1000 is best.

BMatthews

Actually 87 watts per lb would turn this model into a rocket. It won't go vertical with that much but it'll climb at a good 50 to 60 degree angle at a good clip. And for a model of this sort do you really need anymore than that? Somewhere around 100 watts per lb is what it takes to climb pure vertical without slowing down so much that it falls out of the climb.

To aid with the nose weight issue if you can set up the motor to put the mount bracket on the rear and mount the motor to the front face of the nose and make up a cowl to fit over it then you'll gain a lot of room to put the battery pack and RC components in the nose and reduce any added nose weight. The ease of mounting the brushless motors in this way compared to the old can motors makes this very doable.

I would not sweat too much about the prop. Just use whatever is needed by the motor to pull the amps needed to get to the watts of power. The motors you have running on 3S may find that it's a bit much for a 10 inch prop. In particular if the prop is a fairly high pitch such as would be used on a geared motor. This is where a wattmeter comes in handy for selecting the prop to motor combination to avoid running over powered and risking burning out the motor.

flexsphincter

well after my post last night i put the 450 eflite park motor on the aspire ep with a 9" prop and today took it to the field,,,well it wasnt avery good match up the plane struggled to climb and only climbed when i was able to turn back iinto the wind. I need a bigger motor

BMatthews

Or you just need to prop the motor so that it draws the amount of current it needs to achieve the proper power to weight ratio. Looking at the notes on the 450 motor at E-Flite it would appear that a 9 inch prop is on the small side for this motor. Try a 10x6 prop. If you don't have access to a wattmeter then ground run the motor with that prop for a minute and then shut off the power and hold the motor for another minute. If the motor does not become too warm to hold onto comfortably then it would be safe to fly with a 10x6. If it's stone cold or barely gets warm move up to an 11x4 or 11x5 prop and try the heating test again.

Brushless motors are a bit odd in that they need to be loaded down by a big enough prop to force the motor to draw the power you want them to run at.