Hi-Performance and Sailplanes RC hotliners, electric pylon racers, F5B, F5D, sailplanes and gliders

Determining Flight Time

Old 01-05-2019, 01:25 AM
  #1  
Captain Sid
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Default Determining Flight Time

Just getting back into the hobby, so a lot of this is new to me. This will be my first venture into electric sailplanes. Have built and flown lots of non-powered planes. My question concerns how to determine how many flights I can get. I plan on using the motor to get to altitude and not for general flying around. So we're talking 15-30 seconds of runtime at a time.
If I use a motor such as this one, http://www.headsuphobby.com/Firepowe...or_p_1613.html,
or this one,
https://hobbyking.com/en_us/turnigy-...___store=en_us,
with a 2000-2200 mAH lipo pack what can I expect? Is there a formula to determine how much runtime to expect? The new build is a Chrysalis 2m...
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Old 01-05-2019, 06:47 AM
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Wildflyer
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First thing I would say is to be careful not to over power a sailplane.
The Hobbyking motor at 750 watts is sort of like a 40 size nitro engine which I think everyone would say is too big for a 2 meter glider.
The first motor is plenty for sport flying, at 250 watts it still will haul a sailplane like yours pretty fast.

I have over powered a couple of sailplanes, they didn't glide worth a darn.

A 2000 mah battery should give you several launches, maybe 10 depending on prop choice.
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Old 01-05-2019, 03:47 PM
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Captain Sid
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Originally Posted by Wildflyer View Post
First thing I would say is to be careful not to over power a sailplane.
The Hobbyking motor at 750 watts is sort of like a 40 size nitro engine which I think everyone would say is too big for a 2 meter glider.
The first motor is plenty for sport flying, at 250 watts it still will haul a sailplane like yours pretty fast.

I have over powered a couple of sailplanes, they didn't glide worth a darn.

A 2000 mah battery should give you several launches, maybe 10 depending on prop choice.

I agree at 750 watts it's too much. Not sure how I missed that.
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Old 01-05-2019, 04:21 PM
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solentlife
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eCalc (online motor / prop program) and a Wattmeter are your friends .....

eCalc will tell you if the motor / prop / lipo / model combo are good match ... plus give you total mixed flight time ...

Wattmeter will of course give you exact direct readings from your motor setup to be able to calculate run time ... be sure prop matches motor etc.

Just please take care that you use 80% of stated capacity LiPo as the MAXIMUM energy drain ... do not push that LiPo down below 3.2V per cell.

Nigel
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Old 01-10-2019, 09:48 PM
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quorneng
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Captain Sid
I appreciate you intend to "climb and glide" and possibly with specific competition rules but is worth noting that a rapid climb is not a particularly efficient way of doing things either in converting battery electrical energy into mechanical Watts or converting those Watts into altitude.
Sailplanes need much less power to maintain height than a conventional powered design so there are duration benefits in climbing relatively slowly.or as I like to put it "powered assisted thermal hunting". Indeed the closer the glider is to its normal gliding flight the easier it is to spot when it enters an area of lift.
Whilst you might expect say ten 30 second near vertical rapid climbs, the same battery capacity expended at a minimum cruise power setting could keep the glider flying under power for getting on for an hour. As a by product if you don't need the power for a near vertical climb then the motor and the associated bits can be smaller and lighter. In addition the prop operates more efficiently so the actual climb performance at reduced power is a bit better than might be expected.
I know such "power assisted" flying can be considered cheating to a gliding purist but in the UK at least "easy" thermal days are not that common!
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Old 01-11-2019, 04:41 PM
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ron_van_sommeren
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Originally Posted by Wildflyer View Post
First thing I would say is to be careful not to over power a sailplane. ...
Plane mass and type are leading when selecting power(system).
Prettig weekend Ron
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Old 01-11-2019, 04:43 PM
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ron_van_sommeren
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Originally Posted by Captain Sid View Post
Just getting back into the hobby, so a lot of this is new to me. This will be my first venture into electric sailplanes. ...
Some well-structured reading for rainy/windy days, and some handy e-tools as well. Will save you, and us a lot of questions. Notably the 'what went wrong?' kind of questions Will also prevent you from burning up several controllers and/or motors and/or battery:
E-flight primer and tools

And pleasepleaseplease, do your RC equipment, wallet, ego, battery, controller, motor, house/garage/car a big favour ... get a watt-meter. It will more than pay for itself, will save you at least one fried motor and one fried controller. Will also help you finding optimal setup.

Without a watt-meter you are in the dark. Until something starts to glow
close out sale Hyperion Emeter II, with optical and electrical tach, servo tester, local&remote logging - RCG

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Old 01-12-2019, 09:06 PM
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Captain Sid
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Great information here and lots of reading. Looks like it's time to order a wattmeter....
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