The issue
of autonomy is often asked by helicopter. How to calculate, how to increase it,
what are the variables to be taken into account. We will put in the simple case
of hovering to give some answers.
First calculate how
autonomy?
- Assume
that the model is equipped with a battery capacity C
- The
current consumed in stationary (measured at the outlet of pack) is equal to I
- It uses
80% of the capacity of the pack (to preserve) or 0.8 ° C
So the flight time in seconds is equal to
(0.8 * C / I) * 60
Example:
on my Eco8 I use a 3S pack of 4350 mA and consumes 16 amps I hover. Autonomy is
(0.8 * 4350/16) * 60 = 13 minutes.
Improve autonomy:
Given the
above calculation, means that immediately comes to mind is to increase the
autonomy to increase the capacity of the pack. But it is not alone and you can
also try to play the second parameter and decrease the intensity in flight.
Increase the capacity of the pack . Indeed seems to be the easiest way.
But beware if you increase the capacity of the pack increases the weight and
therefore we will need more power to hold in the air (see below). For example,
if I replace my battery 4350 mA 5000 mA a pack, I go from 352 to 396 grams. The
balance will be:
- Gain
(5000/4350 -1) * 13 = 2 minutes
- Loss of
about 30 seconds due to the increased weight
->
Balance gain 1 minute 30, passage 14 minutes 30 seconds
Only
limitations: the available space, and also in case we greatly increase the
capacity (and therefore weight) sizing the motor and controller as will have
more power to keep up.
Reduce the weight of the unit . Indeed, the power required to
maintain a hover is approximately proportional to the mass of the helicopter.
If I take my Eco8 weighs 1kg and 400 consumes 16 amps 3S, so in about 10.5
volts, the power consumption is 170 watts which is 120 watts / kg. We find this
value on many helicopters, knowing that the power consumption depends largely
on the speed of rotation. So we just feel that if I make 100 grams on the
weight of my Eco8 I will consume 12 watts less, so amps 1.15 and less. My
autonomy becomes:
(0.8 *
4350 / 14.85) * 60 -> 14 minutes of flight. What makes a gain of one minute.
The only
problem is to make 100 grams ... Weight gain was possible when flying in NiCd,
NiMH could enter. NiMH and could go into lipos. But now that almost everyone
uses lipos, the possible gains (same capacity) are low. If you want to make a
record of autonomy can lighten up the model: Engine light (but attention must
be able to dissipate heat), lightweight fitted radio, etc ...
Reduce the speed of the
motor:
The power
consumption is highly dependent on the rotor speed. Lower speed rotor (flywheel
not higher) can therefore greatly reduce the intensity. On my Eco8 example, the
power that I mentioned above corresponds to a rotor speed of 1600 rev / min. If
I lower the speed to 1200 rev / min I'll go to a consumption of about 13 amps.
Flight time:
(0.8 * 3500/13) * 60 = 16 minutes.
Consideration:
a helicopter less vivid, less pleasant to drive, less reactive. In passing we
note that the power consumed under these conditions is of the order of 100
watts / kg in these conditions.
Increase the tension:
If we
increase the voltage, power consumption equal intensity output pack will
decrease in the same proportion. So if I spend 3S 4S, an increase of 25% in
blood, I will reduce the intensity of 25%. At the same time I have increased
the weight of the pack to 25% (remaining capacity iso pack). The calculation is
the same as that above.
Result of
an increase in voltage, the motor will run faster. So we will put less gas if
you stay at the same speed. Or better to decrease the size of the pinion to
return to the same rotational speed of the rotor. Warning though, if you double
the voltage you will have roughly halve the size of the pinion. It will not
necessarily be possible, may be will it take to change engine and take a lower
Kv.
Decrease
the size of the pinion has an advantage over the fact of reducing gases. If I
reduce gas I lower the voltage in the coil and so I return to the same
intensity as when I was in 3S. If I reduce the size of the gear motor will spin
faster (for the same rotor speed), so I put more gas, and hence the voltage in
the coil is higher. But as the power consumption is the same as with 3S early
(since the rotor speed is the same and it is in both cases stationary), the
current in the motor will be lower (reminder: P = U * I). Thus, the losses in
the motor related to the Joule effect (heating losses of the coil: power loss =
I ² * R with Ri Internal resistance of the motor) will decrease. And therefore
the engine performance will be better, and therefore the autonomy will
increase.
To the smallest sprocket and
gas can 100%:
This is a
consequence of the foregoing: smallest sprocket and gas 100% give us a maximum
voltage in the engine (basically the output voltage of pack) and an intensity
minimum. Which reduces heating losses of the motor winding.
Put an engine with better performance:
Solution
difficult to implement. In fact it is very difficult to know the exact
performance of an engine manufacturer data are often missing or incorrect.
Moreover, from the moment we chose Kv and engine ground returns are more or
less equivalent. Can be avoided by prudence engines really too low cost (low
quality magnets, etc ...) and focus motors with low Io (because it will run at
low intensity, so Io will lose more power than the heating of the coil).
To summarize, several solutions can be combined:
Increase
the capacity of the pack (simple though not optimal)
- Increase
the voltage of the pack (by reducing the number of sprocket teeth)
- Have the
rotor speed as low as possible, with 100% gas and the smallest sprocket
possible.
- Lighten
up the chopper (knowing that lipos with the loss of autonomy due to the
increase in mass is lower than the gain of autonomy due to the increase of
voltage)
The most
natural and simple of course, is the ability to play the pack. The description
of the alternatives is mainly there to reflect the operation of the propulsion
chain. But it is not entirely theoretical, I have known pilots who had a very
good autonomy Eco8 having put a large pack voltage and very low capacity, a
small gear, a low speed and a light engine (as possible because it reduces the
intensity can use a smaller engine).
Keywords:
helicopter télécommandé, helicoptere radiocommande, helicoptere RC
