Post by Doug McLaren|
| >
| > | 10S3P, 78amp (nominal) li-poly 7800mah 37 volt pack $873.60
| >
| > And in case somebody wasn't paying attention, this is exactly why we
| > don't see many large electrics yet.
| >
| > This battery pack puts out up to 2900 watts, or 3.8 HP.
| >
| > This would be comparable to an OS FX 1.6 engine, which puts out up to
| > 3.7 HP. And costs $270.
|
| However the FX puts it out at a not too useful RPM. To be fair that pack
| and a motor
A high quality, and expensive motor. For example, an Aveox 1415/1.5
with a 3.7:1 gear ratio and a 14x10 prop can use 2500 watts (36 volts
or so, 70 amps), giving a useful output of about 1700 watts, and costs
about $250 for the motor and $300 for an appropriate ESC. (Are the
ESCs really this expensive for larger motors?)
Yes. To date its been a narrow market with low volume sales. There is no
real reason they should be tho - the majority of the electronics is teh
same for a 10A ESC as a 100A one, you just need a lot more power FETS
stuck on teh back, and some way to drive them harder maybe added.
Post by Doug McLaren(Maybe my choice of motor is very poor -- I've never actually really
investigated putting together an electric plane this large before. At
least not once I realized the small fortune I'd spend on batteries
alone.)
| is probably nearer a 50cc motor.
Actually, that HP rating is at 9000 RPM. That's not that unreasonable
-- certainly, it's not some screaming 0.049 engine. I doubt you'd
need to go to 50 cc (3 ci) for similar performance.
It depends on what you want. If you want speed, you want high RPMs
and a small prop. If you want to do 3D, you want low RPMs and a big
prop. Electics make the latter easier to do.
Agreed. All out speed you will generally find a tuned pipe IC screamer
is still the best. But for 3D work teh electric is more controllable and
far more efficient overall as a a power train.
Post by Doug McLaren| > This battery pack will put out that much power for 6 minutes *at best*
| > (since it's a 10C discharge rate.)
Don't disregard this. If you're using this much power, it won't be a
very long flight.
| > And this is just the battery pack -- there's no ESC, no motor (with
| > this much money going inot just one battery pack, it's silly not to
| > get the best brushless motor you can find. And of course, it won't be
| > 100% efficient, reducing the actual power you get.) You're looking at
| > many hundreds of dollars more for that.
|
| No, but it will be approaching 90% overall.
Probably not even too close to 90%, even for a brushless. And even
less if you're running at the maximum output of the batteries. Run it
through Motocalc ... it's very upsetting. 70% seems more realistic,
and that's for a good motor.
Mmm. The point is that that sort of power train is mostly used at or
near max eficiency current, only peaking to full power for short bursts.
You use full power really only when going for max rate of climb (keep
that up and the model is out of sight soon) or absolute max speed. The
rest of the time its throttled back a lot.
So flight times are often surprisingly longer. Even EDF models with
lower weeight lithium packs suddenly don't need full power all the time.
Once you come down a few notches from flat out efficiency is better,
current its lower and suddenlt flight times ae much longer.
Post by Doug McLaren| How efficient is your glo prop?
As for props, at some level, props are props. Gearing your motor down
can give you more prop efficiency for slow flying, but you're still
losing energy at the prop, glow or electric.
That is a misleading statement. An ungeared motor putting teh prop tips
close to mach 1, and being strong enough at the root to cope with the
centrifugal and torque pulse loads imposed by an IC motor, and heavy
enough to act as a flywheel, is nowhere near as efficient as a thin
bladed larger one geared down to the correct RPM for your model.
This is teh MAJOR reason why people are surprised at how little E-power
they need to fly a given glo airframe.
Only in the case of >75mph models does teh glo engine start to match teh
overall prop efficiency of the geared electric.
Post by Doug McLaren| > And of course, you usually want at least two battery packs ... the
| > costs add up very fast. Some say that it all evens out in the end,
| > that you're essentially buying all your fuel up front. There's some
| > truth to that, but it still seems that large electrics still cost a
| > lot more, both up front and over all.
|
| Its true. Get into the big sizes and the cost does rocket.
|
| But you are not being totally fair here.
|
| There are more variables than you account for.
Like? I guess you do need to add a throttle servo to the glow setup
-- $10 or so.
I don't think it's really reasonable to add the cost of chargers and
starters and such to the prices when making comparisons.
Well, what kind of kit do you need to take to the field for a glo motor.
- spare gloplug
- glo starter battery
- electric starter.
- fuel pump
- bag of baby wipes
The field battery of course is commoon for both sorts.
Additional model cost include
- throttle servo
- hefty engine mount
- maybe custom muffler for the plane
- fuel tank and plumbing
- fuel proofer and generally strong firewall construction.
And of course, fuel. Not irrelevant in a big model by any means.
Post by Doug McLaren| > The `sweet spot' for electrics is still the Speed 400 and smaller
| > sizes. Once you get larger, the price goes WAY up. If you double the
| > power, you basically double the price. (Compare this to glow, where
| > if you double the power, you add maybe 20% to the cost.)
|
| Actually I'd slightl;y take issue, and say that the sweet spot is
| 100-500W, whicgh is speed 400 up to roughly 40 sized glo
| equivalents.
A 0.40 sized glow engine can do a lot more than 500 watts. The OS FX
0.40 is rated at 1.36 hp -- 1033 watts. Yes, it's at a higher rpm
than most need, but still, you're going to have a hard time getting
similar performance out of an electric system that peaks out at 500
watts.
Well I disagree. For reasons stated above. Execpt when you DO need that
extra RPM.
Post by Doug McLarenEspecially when the power system weights more than the glow
power system.
Its actually fairly close. Especially with a custom designed E-plane
with all the beefed up structure to take engine vibraion removed, along
with the fuel, tank, heavy prop and engine mount, muffler and servo.
Post by Doug McLarenAnd I believe that power rating doesn't take into
account the energy lost in the motor (and gearbox, if there is one) --
but the BHP rating of a glow/gas engine does. (Of course, IC engines
are only like 25% efficient at best, but that's another matter
entirely.)
As I said, the gains on the prop outweigh the efficiency losses in the
box, or people wouldn't be using them.
I am not trying to maintain teh E-planes are xcheaper at higher powers,
bu they are not as expensive, or heavy, when costed out over a season or
two, as you are making out.
They also have some advantages in terms of reliability and ease of
starting and very predictable throttle response that make them ideal for
aerobats and multi-engined models.
That is where the cost is worth bearing. Bigger aerobats, and
multi-engined models.
Also in the sub .25 type of sport model, where the costs are comparable.
Only a big single engined sport or speed plane - 40 and up - is it
really a bit enthisiastic to go electric.
Post by Doug McLaren| In teh smaller planes, the radio gear has to be light, and that gest
| expensive, but planes around the 15-25 glo equivalent are not too
| expensive to electrify and not too small to need specialised gear.
No, it's not too bad, but you'll still end up spending hundreds of
dollars on batteries. It would take many many flights to run even
$100 worth of fuel through a 0.25 glow engine plane.
Well, I have spent $70 on a pack that will do 200W, about equivalent to
.19 power wise by my estimation. Certainly .15 power. I can. and will,
split that amongst several different models. I cannot reasonably unbolt
an engine and swap it amongst two or three models in the same day.
But I can only fly one plane at a time, and two such packs as that will
allow me almost continuous back to back flying with short breaks in
between on an unlimited number of models.
Because the pack is light, and powerful, for general purpose sport
models I can spend very little on the motors - cheap inefficient ones do
just fine for average flying. Sure I have a 200W brushless setup that
cost in excess of $100, but hat is not what I have to fly all the time,
Thats reserved for planes that need to go straight up!
Post by Doug McLarenI've never found my (e-power) batteries to last (the internal
resistances just seem to go up too much after that) more than a year
or two, and I spend far more on batteries now than glow fuel. Of
course, I also fly the electrics more than my glow planes ...
Again, there is a positive feedback mechanism. With Nicads, to fly at
all, we needed to thrash the packs and motors just to fly at all. I've
got some 2 year old packs that haven't been thrashed too much, that are
still fine.
All the signs are that the lithiums will be up there after two years as
well.
The key thing is to rememebr they are going to die of old age anyway
whether you use them or not, and simply use them to the full, by
splitting them amongst several models. You do NOT need to buy one pack
per model. I've got one 100W pack and one 200W pack. That, plus a
charger, is $200. Those will power ALL my planes built and on the board
- about 7 in all. So that works out at $30 per plane or less.
In the speed 400 sized cans, which are good enough for scale and modest
planes cost is less than $50 per motor/ESC/box combo, and the 200W 480
class motors are not much more. Even the 480 class brushless is only
$120 or so. So typically each model is costing me between $80 and $150
for the power train capital cost.
Contrast that with an IC engine, servo, tank, mount etc. etc. and add
fuel costs and its not very different.
In general with two fully charged packs charged at home before flying, I
can expect 40 minutes of continuous flight of the packs inclding general
stuff like getting them packed up and checked out prior to flinging in
the air, and the landing approach etc etc at low power. Then a 20 minute
hiatus to recharge the packs, followed by 20 minutes flight/forty
minutes wait AT THE VERY WORST WITH A FLAT PACK which they seldomn are.
Its more like 15 minutes flight 10 minutes wait in practice. The
amount of time glo planes seem to spend being started and fiddled with
makes it very little worse. If you have three packs and two chargers its
pretty much constinuous flying no breaks. Or indeed three packs and one
charger will allow you to take 45 minutes of continuous power to the
field ready to go.
More than most glo pilots except the chopper boys seem to actually spend
in the air anyway.
Apples and oranges. Unless you MUST have a 40 powered or bigger plane,
electrivc is cheap, and convenient. Why have a 40 powerd plane? Because
that is the 'sweet spot' for IC. That's where its at its cheapest
really. Slightly smaller is the sweet spot costwise for electrics.
Once you have enough packs, you don't need to buy more for new models.
Just unplug them, charge, and use on the next plane...onbce you have
enough packs, you don't even need to field charger them...pre-charge
them all, and fly them till they run out and go home. If you want
different power levels, parallel or series up packs (fully charged of
course to make sure discharge times are matched) and you can save on
buying another pack.
People on a tight budget have even been known to swap receivers, motors
and ESC's between planes at the field. Unplug a few connections and
unscrew two or three bolts, and away you go...
Of course all this requires a change in the way of doing things and
thinking. If you 'think glo' and do the sums you will never go electric.
You have to 'think flight times and planes' and work out an overall
budget for the type of flying you do...but in many cases, if you DO do
this, you will find its not half so bad as the critics make out. How
many planes do you take to the field anyway?
Post by Doug McLaren| And you don't need a brushless motor to enjoy kithoum - the excess
| power and duration means rather les efficient (and much cheaper)
| motors still deliver good performance.
When you're spending $900 per battery pack, it just makes good sense
to spend a few hundred on the best motor you can get. Getting a
better motor will save you money on the battery pack, because you can
use a smaller (and cheaper) battery pack and get the same performance.
For a $20 8 cell NiCd pack it's not a big deal. But for a $900 battery
pack ...
I don't see that at all. I run a $70 pack and a $18 gearbox on a $7
motor. All in $50 plane. Split amongst several models its not a silly
thing to do.
Look at the cost breakdown of a speed 400 plane.
Kit cost $50
Glue, paint, covering, odd bits - $15
Motor $7
Gearbox $15
Prop $5
Receiver $50
4 x feather servos $80
Thats $222 already ...
Now add ESC, at maybe $25, and we have a $250 plane without the battery.
Since as I said the battery is costing (because its shared between three
models at least only about $11 per plane), its a $262 plane at this point.
Now if I drop the $25 ESC and add in a brushless+ESC at maybe $100 I am
adding $75 to the plane cost. A 30% hike in cost.
Besides which, that model is a scale jobby, and will potter around doing
mild aerobatics for the best part of 40 minutes on the one charge.
I simply don't need to spend $75 to extend that to one hour.
Ive done the calculations over and over, and the way that makes sense is
to buy lithium batteries, and share them between models. Motors I
cosndeier more disposable, and cheaper than, servos. ONLY if I want the
last ounce of performance will I bother forking out for a brushless.
I mean, not every plane you have runs a tuned pipe high performance
motor, does it?
Stop thinking 'one plane, two packs' and start thinking 'oow packs,
infinite planes' or whatever, and suddenly the cost benefit equations
stand on their heads.
Post by Doug McLaren(For larger motors, doubling the power rating doesn't seem to double
the cost. A brushless motor setup with 0.40 performance doesn't cost
that much more than one with 0.09 performance.)
| If the price was right...
That's my beef. If I could get Li-poly cells for like 20% of the
current cost, I'd go all electric tomorrow.