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-   -   How Much Motor Would A 3 Meter Glider Need? (http://www.rcuniverse.com/forum/rc-gliders-sailplanes-slope-soaring-112/10406608-how-much-motor-would-3-meter-glider-need.html)

invertedthoughts 03-17-2011 03:33 PM

How Much Motor Would A 3 Meter Glider Need?
 
Thinking about putting a power pod (either electric or glow) on my Robbe ASW19 3 meter glider. It weighs in at 4 pounds 6 ounces. Ihave an Esky Belt V2 heli with a brushless motor (450 size) 30 amp speed control and 3S 1800MAHlipo that I'm thinking about making a powerplant for the glider out of.


Would it have enough oomph to even get one climb into thermal territory with?

kaziki 03-17-2011 04:22 PM

RE: How Much Motor Would A 3 Meter Glider Need?
 
The motor might have enough power, but it "probably" is designed to develop it's power running at high speed. A sailplane needs a large prop that turns at relatively low rpm. If you provide any information you have about the motor, we may be able to give you better advice.

Roger

BMatthews 03-17-2011 07:47 PM

RE: How Much Motor Would A 3 Meter Glider Need?
 
Yep, heli motors typically have too high a Kv value to spin the big sort of props needed by sailplanes unless used with a gearbox.

In my experience with my own flying and learning from others back in the days when high power setups were not cheap and in many cases were not possible I learned that 30'ish watts per pound (W/lb) generates basically a decent "uphill glide" and no more than that. At 40 W/lb you get a shallow climb but you still want to be the patient sort. Some of my early brushed can motor models flew at that power level and a climb to around 500 feet would take a good minute and a half to get to altitude. As the power rises to 60'ish W/lb you get a solid and decently quick 40 to 45'ish degree climb angle and a decent speed during the climb. At around 70 W/lb you've got a spirited 60 degree climb angle. And at around 100W/lb you can just about go vertical for the whole climb if not not heavy handed on the controls. Over 120 W/lb it's all about how fast you want to go vertical.

With these numbers in mind weigh your model and try to guess at the weight difference of the motor and battery pack that would replace the radio gear battery and nose weight.

invertedthoughts 03-23-2011 08:28 AM

RE: How Much Motor Would A 3 Meter Glider Need?
 
Great, that gives me a good baseline, now I just need to figure out how to calculate the wattage of a motor. :D

BMatthews 03-23-2011 10:18 AM

RE: How Much Motor Would A 3 Meter Glider Need?
 
Watts is simply the battery voltage multiplied by the current drawn by the motor. If the maker's specs don't provide a max wattage then you can find it by multiplying the constant max current (not the "peak" current) by the voltage of the battery pack you intend to use. And for a 3 meter model I'd suggest you look at using a 4S pack to go with the roughly 350 to 400 watt motor you'll likely want to use. By using a higher voltage pack you keep the current requirement down to a reasonable level. A 4S pack voltage is 14.2 volts nominal. At the roughly 380 to 400 watts of power you'll likely want to use that's 380/14.2= ~27 amps. If you went with a 3S pack you'd need 380/11.1= ~34 amps.

The only issue with going to a higher operating voltage is that the motor will need to use a prop on the smaller size of the range for that motor so as not to over draw on the current and force a burnout issue. So keep that in mind and shop for a motor with a low enough Kv value to allow using a 4S pack. You'll likely want something in the Kv=700 to 800 range to allow for a decently sized 12 to 14 inch folding prop on such a model.

cowboynshadow 03-25-2011 07:53 AM

RE: How Much Motor Would A 3 Meter Glider Need?
 
I have a 3 meter Bird of Time I have flowen on a heavy duty high start. Got tired of the walking and the wind changes. I saw a guy put an O.S. 10 on a bird and made a pod on you tube. Expensive mistake.... way to underpowered. Wingspan was about 118" andthe weight isabout the same as yours. I would go with an O.S.15. myself. Remember.. ask 5 people and get five different answers. I try to get my info from people who have flowen exactly what I have with experience. I have been flying since 1964. Hope this helps.

aeajr 03-25-2011 05:46 PM

RE: How Much Motor Would A 3 Meter Glider Need?
 
I would not put a glow motor on a glider. If nothing else, too messy.  But they are pretty light and cheap.

I would go electric.  You can restart in the air and you can make multiple climbs.  No goo on the plane and almost no noise.

Figure 75 watts/pound should get you about a 45 degree climb. 100 will get you about a 70 degree climb and 125 might take you straight up.  Assumes you are using the right prop. 

You say the glider is 4.5 pounds.  Is that as a glider or with the power system?  

invertedthoughts 03-25-2011 09:45 PM

RE: How Much Motor Would A 3 Meter Glider Need?
 
4.5 lbs. and that's pure non-powered glider, no artificial thrust added.

BMatthews 03-26-2011 09:09 AM

RE: How Much Motor Would A 3 Meter Glider Need?
 
That's 72 oz. And it's also very heavy compared to the original designer's versions which, if I recall correctly, came out in the low to mid 40's. If it's that heavy mostly due to required nose weight you will want to mount the motor on the nose and perhaps extend it at the same time so the motor and battery pack weight can replace the nose weight and keep any raise in the overall weight to a minimum.

aeajr 03-26-2011 09:55 AM

RE: How Much Motor Would A 3 Meter Glider Need?
 

Quote:

ORIGINAL: BMatthews

That's 72 oz. And it's also very heavy compared to the original designer's versions which, if I recall correctly, came out in the low to mid 40's. If it's that heavy mostly due to required nose weight you will want to mount the motor on the nose and perhaps extend it at the same time so the motor and battery pack weight can replace the nose weight and keep any raise in the overall weight to a minimum.
That is the best way to do it, if you have a hunk of lead in the nose.

invertedthoughts 03-26-2011 02:07 PM

RE: How Much Motor Would A 3 Meter Glider Need?
 
I actually have 14 oz. of lead in the nose, there was no ballast plug whatsoever in the nose from the manufacturer.

aeajr 03-26-2011 04:00 PM

RE: How Much Motor Would A 3 Meter Glider Need?
 


Then the objective should be to let the motor and battery be positioned such that they replace lead to minimize weight gain.  </p>

BMatthews 03-26-2011 07:36 PM

RE: How Much Motor Would A 3 Meter Glider Need?
 
I'd even go so far as to make the nose a bit longer so that the motor and battery can replace ALL of the 14 oz of lead. These days the power needed to fly your model just won't add up to a total of 14 oz. Such is the advantage of technical progress. So to get more out of your electric conversion lengthen the nose by about 2 inches so the motor can be more to the front. And then shift the radio gear around to put the battery pack right behind the motor and put the relatively light radio control components in under the forward portion of the wing or just behind the battery pack which should sit in the nose right behind the motor and ESC.

aeajr 03-27-2011 05:09 AM

RE: How Much Motor Would A 3 Meter Glider Need?
 
how about showing us a photo of the current fuse layout with servos and open spaces under the wing. 

cowboynshadow 03-27-2011 09:30 AM

RE: How Much Motor Would A 3 Meter Glider Need?
 
Again... as a few posting above. You see how the electric gets complicated. Modeling is becoming a lost art with people buying the ART and not knowing how to build.... period. The electric world has confused everyone. Evenhobby shop owners. Get different answers from everyone. Gas was always used. Windex cleans them up very nice with a wipe down after flying or each flight. It called taking care of your stuff. Its really not a bug mess or problem.Noise.... Real airplens make noise, race cars make noise. If you want a quiet hobby,,, play cards. Models used to have beautiful paint jobs and monokote jobs, detailed cockpits etc.Where did that go??? The say gas engines are heavy, but as soon as you start with electicthe combersome batteries come into play. What size will fit into the plane? Heavy? Now I have to hack up the aircraft to fit a motor... Build a pod. Most sailplanes, if balanced correctly get way up anyway and most have spoilers on them. Weight is not a great concern unless you are going to thurmal contests and then you are looking at a different aircraft to begin with.. Take a little time, forget the trouble, get a gas engine and have fun. You can also fly longeron the reciever battery and use a smaller battery. Don't have to worrry about the charging equipment. The expense of gas? Go put a gallon in your car? I have been flying gas for years, (1964)tried electric.... wow what a difference!!! used the electric for a sinker at the lake and caught a trout. Good for fishing! Have a great day.. P.S Either way you go...... electric or gas.. best of luck and have as much fun as you can.

invertedthoughts 04-06-2011 08:13 AM

RE: How Much Motor Would A 3 Meter Glider Need?
 
So let's say I want to buy from Hobbyking, is there a motor/ESC combo that can be suggested from what they have available that will at least get me close?



invertedthoughts 04-06-2011 08:20 AM

RE: How Much Motor Would A 3 Meter Glider Need?
 

Quote:

ORIGINAL: aeajr

how about showing us a photo of the current fuse layout with servos and open spaces under the wing.

Check this link for pics:


http://www.rcuniverse.com/forum/m_10361980/tm.htm

BMatthews 04-06-2011 10:23 AM

RE: How Much Motor Would A 3 Meter Glider Need?
 
Well, I'll have to withdraw my suggestion to lengthen the nose. I'd forgotten that we were dealing with a molded fuselage when I suggested that option.

Figuring a final electric weight of around 4.5 lbs that means you want 4.5lb x 75 watts/lb=337. So say a 400 to 450 watt capable motor so you're not running too close to the ragged edge for power handling. Besides, you need the nose weight anyhow. And to turn a bigger folding prop on a 3S or 4S battery pack you'll need a Kv value for the motor of around 700 to 900. The lower value if you want to run a 4S pack and the higher if you want a 3S pack.

The current needed to get 350 watts from a 3S pack is 350 watts/11.2volts= 31 amps. Not all that high but high enough. The higher voltage of a 4S pack would reduce the current to 350/14.3=25 amps. If you don't know it watts=volts x amps. But given that it's likely that the model weight will come in closer to 5 lbs once the whole conversion is done I'd suggest you want to run more like 35 amps with the 3S pack and 28 to 30 out of the 4S pack. These numbers now make the 4S pack a better alternative.

These current values are not at all outlandish by today's standards. But if you're not used to the subtle needs of installing high current systems in terms of wiring, connectors and other issues then I'd suggest you go with the 4S pack option and keep the required current value lower. It makes the rest of the installation a bit easier to get right enough without lots of jumping through extra hoops picking out big wire and finding the best high current connectors and stuff like that.

To run this motor we typically underrate the ESC and battery pack. So if you want to pull 30 amps out of a 4S pack you want to get an ESC that is 45 to 50 or more amp capable just so there's no over heating surprises. And do try to put the ESC out in a spot where some air can flow past it.

Same with the battery pack. At 30 amps you'd only need a 1500 mah 20C pack. The "C" rating is how many times the 1500ma rating you can draw before the pack is being pushed hard. 1500 mah = 1/5 amp-hr. And 20 times that value is 30 amps. But the pack won't live a good long and healthy life pulling the limit too often. And the useable motor run time would be a paltry 1.5Ah/30A x 60 min/hr= 3 min of full power running. Obviously not much power duration to work with. A better option would be a 2500 or 3000 mah 4S pack with a rating of 25C. Such packs will be loafing to delivering 30 amps and you'll get a nice long lifespan from them. Also at 30 amps draw a 2500 mah pack will give you 2.5 amp-hr/30 amps x 60 min/hr = 5 mins of full power. A 3000 pack would be 6 minutes. Now that doesn't seem like much but if you only use the power to climb to altitude and then glide around then 6 minutes would easily give you 2 climbs to a great height and likely 3 climbs. Obviously if you want more than just go for a 3500 or 4000 mah pack.

I've gone the route of writing this out based on the idea of the old story "if you give a man a fish you feed him for a day. If you teach a man to fish you feed him for a lifetime". With what is written here you should be in good shape to shop through the Hobby King website and pick out any number of motors, ESC's and battery packs that will work.


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