| I find that a slowish plane needs about 20W/lb to keep it flying.
| Actually at about 20mph you only need theoretically about 3W/lb for a
| clean airframe.
When I think of clean airframes, I think of gliders. Not the Senior
Telemaster, which is what we're talking about here.
| However there are motor, and prop inefficiencies..
of course ...
| Thats a 15lb plane you have there..not sure a Telemaster is anything
| like that.
Doing some math (http://www.hobby-lobby.com/srtele-arf.htm gives the
weight of their plane at 10.5 lbs, and their battery pack should be
34.9 oz and the motor 14.4 oz, so the plane itself comes in at 7.4
lbs.) That's 7.4 lbs without any power plant at all.
(Though maybe you can make the Sr Telemaster kit lighter -- I've never
| And half throttle I have found to be about 1/4 the power of WOT.
Fair enough ...
| Power to stay up, as opposed to climb, is simply the models weight,
| times its speed, divided by its lift to drag ratio at that speed.
Simple ... once you know all these figures. Weight is easy, but the
rest will require some calculations or in-flight measurements.
| You can see that a low wing loading model - large and light, and
| therefore slow - with fairly clean aerodynamics , therefore high
| L/D..takes almost nothing to stay up really.
Perhaps 300 watts was too high, but it's more than `almost nothing'.
| I originally built that plane to carry a camera, but its too nice to
| hack it about..
| Now a senior telemaster I am fairly sure can be built at 6lb or so.
Is this an all-up weight or with the power plant?
| It probably needs 300W or more to get it to climb decently, but I doubt
| it needs more than 90W to stay there.
Even assuming 90 watts to stay there, a 3s1p 3000 mAh pack can't
provide that for an hour. To be more precise, assuming 100%
efficiency and an average voltage of 3.9 volts/cell, a 3s1p 3000 mAh
pack can provide 90 watts for only 23.4 minutes.
| On a 6lb model thats 360W. Lets say its running a 3s LIPO pack, thats
| 35amps..so I want a 3s 4000mAh pack roughly.
Well, from what I can find, you can't get a Senior Telemaster going
that lightly. Looks like 7.4 lbs without any power plant at all, at
least for the ARF. Maybe you can shave an ounce or two off that by
not having ailerons ...
I went ahead and ran all these figures through Motocalc myself --
sedate flight, 30 minute flights, 94" wingspan, 1330 sq. in. wing
area, 7.4 lbs w/o power plant, flat bottom wing, 2nd thinnest airfoil,
sea level, brushless only, direct drive or geared, any size prop, lipo
only, up to 8 cells, and it came up with me needing a lot more than a
3s 4000 mAh pack. In fact, the smallest configuration that came close
to an hour up (54 minutes) is 9x 3000 mAh cells (in a 3s3p)
configuration -- an Astro brushless 020 3T with a 5:1 gearbox (not
something I'd have really considered myself) and a 12x6 prop, and it
could maintain level flight at 76% throttle drawing 98 watts into the
motor, with an AUW of 148 oz.
(I don't know what assumptions Motocalc is making for the `dragginess'
of the airframe or anything like that (and it didn't ask), so we have
to treat this as a pretty serious approximation.)
So it sounds like I was off by a factor of about 2.7 too high, and you
were off by a factor of about 2.25 too low. So I guess you were
indeed closer -- but we were both pretty far off.
| Here's a suitable pack http://www.maxamps.com/index.html?lmd9014.919236
| at $75.
A 3s 4000 mAh pack for $75 is a good deal. I couldn't find it on that
site -- can you be specific about which pack it is? (They use frames,
so your link didn't seem to go directly to that pack.)
| Then we are just about on. However I assumed the $500 for for the bits
| that would break in a crash.
| 350W brushed motors are almost as expensive unless you pick up Ebay
Yup. Really, the motor/ESC isn't a good place to save money when you
start getting into the bigger planes.
| > With 25 cells to charge at once, you'll want a pretty serious charger,
| > but since we're on a pretty serious budget, let's say $100 for a
| > charger.
| Er. Three cells. Yor power calculations were way out.
Well, I used the cells I could find the cheapest -- per amp-hour, the
Emoli cells are even cheaper than what you found. (I wasn't getting
into the exact configuration of the cells.)
Roughly $100 (on ebay, but new/unused packs) gets you 7x 3000 mAh
emoli cells, vs. $75 getting you 3x 4000 mAh cells with your pack --
your cells are 30% more expensive. (Granted, the emoli cells don't
perform as well as some, but they're not too bad.) But for only 30%
more and not having to make your own pack, what you found may well be
| > And those Emoli cell are roughly half the price of anything else out
| > there -- but you'll have to build your own packs.
| No, they aren't.
No, apparantly not. The pack you found (but I can't find it on their
web site) is only 30% more per amp-hour.
| You just don't know how to calculate power requirements and where to
| buy LIPOS.
Apparantly we both have problems there ...
| Anyway, the bit of the post I replied to originally was referring to the
| cost of the batteries alone. I simply said you could easily get
| batteries for under $500. In fact I found a suitable pack for about $75...
I don't think so. Your pack is not sutiable, at least not without two
or so friends ... but yes, it appears that you can get just the
batteries for your Senior Telemaster to fly for an hour for under
Of course, Marty was being more practical, and you'll want at least
two packs of batteries, so double that cost ...
Doug McLaren, firstname.lastname@example.org
One way to stop a runaway horse is to bet on him.
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