Here's a good rule of thumb for electrics. 50 watts per pound will
> fly like a Cub , 75 watts per pound will do some aerobatics , 100
> watts per pound will hover. This will vary of course , depending on
> the aircraft , but , it WILL put you in the ballpark.
> Hope this helps.
and 30W per pound will fly like a GWS tigermoth :-)> fly like a Cub , 75 watts per pound will do some aerobatics , 100
> watts per pound will hover. This will vary of course , depending on
> the aircraft , but , it WILL put you in the ballpark.
> Hope this helps.
Its not *quite* that simple tho. The wttas per pound does two things -
overcomes the drag, and any surplus gets to lifte the mass of teh
aircraft against gravity - a (in my day) O-level mechanics exercise in
calculation. So heres one for you all. How many watts does it take to
lift one pound at 1000 feet per minute in an ideal frictionless world?
The reason parkflyers get way with less than 50W/lb is that they fly SLOWLY.
First order approximations have drag and lift roughly relaed - i.e.
double the weight needs double teh lift which is double teh drag.
Similar maths from your O-level course will show you that the power
needed is proportional to drag times speed. So to keep a lb of plane in
the air at 30moph takes roughly twice as many watts as to keep one in
the air (assuiming it doesn't stall) at 15mph.
The original '50W/lb' rule was formulated for sport planes with stall
speeds in the upper teens and lo twenties, with overall
battery/ESC/motor/gearbox/prop efficiences around 50% or less.
However broadly your original tables are correct..but I'll amplify tehm
a little
30-40W/lb Slofly or parkfly, or other low wing loading scale plane (GWS
Tiger moths etc)
40-60W/lb Good basic trainer, or draggy WWI biplane. Also parkfl;y
warbirds like GWS.
60-80W/lb Warbirds or smooth aerobats, probably good enough to loop from
level. This is where picojets and the like tend to start operating.
Aklso entry level gutless ducted fans :-)
80-100W/lb. Fast models with almost unlimited vertiocal, and possibly
hover potential with efficient setups, but not fully 3D. minimum for
decnt ducted fan IMHO.
100W-130W/lb. Full 3D in the right plane with unlimited vertical. Ducted
fand now starts to look like a jet.
i have alos derived another formula, which isn't quite as useful as it
was with the advent of LiPoly, and that is that the weight of motor and
cells etc. and teh equivalent power delivered is such that without the
airframe,. a typical setup is capable of delivering 200W/lb. That means
that to get e.g. 100W/lb the airframe weight must be no more than equal
to the weight of the flightpack and motor.
For e.g. 66W/lb the airframe is twice the weight of the power train.
This is a useful rule of thumb for electric conversions.
> Ken Day
> >
>
>>Hi;
>>
>>A while ago, I looked up the "Watts per Pound" rules, trying to decide on a
>>motor/prop/cell count package for my plane, and I can't remember, are the
>>Watts counted as "Watts input" or "at the prop"? Can you help me out? >>
>>To put the question in a context, here's my application: I need 30oz. of
>>"motor stuff" to balance 40oz. of airframe (in the shape of a GeeBee) at the
>>proper center of gravity. I'd like rise-off-ground capability at least, so
>>by the guidelines, I think I'll need around 60W per pound, or 260W total -
>>and I'm assuming that's output.
>>
>>I'm thinking it's going to be an AXI 2820/12 with a 10x6 on twelve cells...
>>According to the manufacturer, that will give me 253W (close enough) at a
>>cost of 23A...
>>
>>Have any better ideas? I'd love to hear them. Especially if I can get
>>decent duration and power out of a geared, cheap can. Frankly, I'm >>guessing...
>>
>>Thanks. A bunch.
>>Dan.
>>
>>
>