oriol
Well-Known Member
Hi everyone!
Electric propulsion seem to be an interesting option for a motorglider. Here below is an example with a few numbers. The idea is to upgrade from a basic glider to a motorglider; like transforming a Goat into a Zigolo. The MTOW would be 225 kg, roughly based on the Zigolo specs, and the glide coefficient is 10/1 for a gliding speed is of 55km/h.
With those values it is possible to calculate the power required for sustained flight;
power = work/t = m*g*h/s = 225*9,81*1/s
To find time I calculate the horizontal speed in m/s;
55*10/36=15,2778m/s
Then if the gliding ratio is 10/1, the gliding angle is tan^-1(1/10)=5,71º
The horizontal speed is then 15,2778*cons(5,71º)=15,202m/s
The time spend to travel 10 horaizontal meters, and 1 vertical meter is;
x=v*t -> t=x/v = 10/15,202 = 0,6578 seg
Then Power = 225*9,81*1/0,06578 = 3355,46 watts
This would be the energy required for steady flight.
Ideally as a motorglider I would like to fly to at least 500 meter height, cut the engine and perform ridge soaring.
Then to reach 500 meters height the time would be;
x = vt = 500 m
t = 500/15,202 = 328.9045 seg,
328,9045/60 = 5,48 min
The power required to achieve 500 meters height is;
225*9,81*500/328.9045 = 3355,5 Watts
From the available off the shelf batteries, I pick one from a reliable manufactuer used in UAVs; a Tattu Pro 12 S.
The battery price is 979E, weights 6,3 Kgs, has 22000mAh, 44,44 nominal Volts, the maximum rate of discharge is 25C.
I found out first, the power output of the battery per hour;
P = V*I = 44,44*22 = 977,68 Watts per hour,
977,68 Watts < 3355,5 Watts, the power required for reaching 500 meters,
I found out the maximum output of the battery;
22Ah*25 = 550 Amps
Then I divide, Amps per hour per the maximum ouput, to get the battery autonomy?
22/550 = 1/25 = 0,04 hours,
60 min*0,04 = 2,4 minutes.
The maximum output in Kw is 44,44v*550A = 24200 Watts,
With two batteries I can get enough power for reaching 500 meters in around 5 minutes.
The cost of the two batteries plus an engine etc... is similar to the cost of a parafan engine used in the Zigolo. The parafan engine can run for hours, not minutes. A second hand parafan engine in good conditions is not hard to find, for half the cost of a new one. The ICE engine seem to beat in both cost and performances.
One thing that troubles me, is how to calculate battery endurance, without exceeding the battery safety limits?
In theory a battery, can not be discharged beyond 80%. By using the nominal tension and the Amps per hour, provided by the manufacturer, are we considering the rate of discharge to be above 80% of battery capacity?
I am afraid that manufacturers exagerate their batteries capacity, or drain them beyond the limit, for marketing claims? Just like many aircraft manufacturers, claims absurd stall speeds, for being competitive with the rest, who also invent stall speeds...
Nominal Tension and amperage are average values, the battery voltage and current drops as the battery is discharged. I asume, but I am not sure, that those average values are Ok for the calculations?
Electraflyer offers electric kits with lithium polimer batteries for electric conversions.
It is surprising that with a similar power output calculated on the example above they are able to achieve 1 hour authonomy with their batteries?!
Given the low power output, I assume they run their engines with moderate wind etc.
As always, any input might be appreciated!
Cheers,
Oriol
Electric propulsion seem to be an interesting option for a motorglider. Here below is an example with a few numbers. The idea is to upgrade from a basic glider to a motorglider; like transforming a Goat into a Zigolo. The MTOW would be 225 kg, roughly based on the Zigolo specs, and the glide coefficient is 10/1 for a gliding speed is of 55km/h.
With those values it is possible to calculate the power required for sustained flight;
power = work/t = m*g*h/s = 225*9,81*1/s
To find time I calculate the horizontal speed in m/s;
55*10/36=15,2778m/s
Then if the gliding ratio is 10/1, the gliding angle is tan^-1(1/10)=5,71º
The horizontal speed is then 15,2778*cons(5,71º)=15,202m/s
The time spend to travel 10 horaizontal meters, and 1 vertical meter is;
x=v*t -> t=x/v = 10/15,202 = 0,6578 seg
Then Power = 225*9,81*1/0,06578 = 3355,46 watts
This would be the energy required for steady flight.
Ideally as a motorglider I would like to fly to at least 500 meter height, cut the engine and perform ridge soaring.
Then to reach 500 meters height the time would be;
x = vt = 500 m
t = 500/15,202 = 328.9045 seg,
328,9045/60 = 5,48 min
The power required to achieve 500 meters height is;
225*9,81*500/328.9045 = 3355,5 Watts
From the available off the shelf batteries, I pick one from a reliable manufactuer used in UAVs; a Tattu Pro 12 S.
The battery price is 979E, weights 6,3 Kgs, has 22000mAh, 44,44 nominal Volts, the maximum rate of discharge is 25C.
I found out first, the power output of the battery per hour;
P = V*I = 44,44*22 = 977,68 Watts per hour,
977,68 Watts < 3355,5 Watts, the power required for reaching 500 meters,
I found out the maximum output of the battery;
22Ah*25 = 550 Amps
Then I divide, Amps per hour per the maximum ouput, to get the battery autonomy?
22/550 = 1/25 = 0,04 hours,
60 min*0,04 = 2,4 minutes.
The maximum output in Kw is 44,44v*550A = 24200 Watts,
With two batteries I can get enough power for reaching 500 meters in around 5 minutes.
The cost of the two batteries plus an engine etc... is similar to the cost of a parafan engine used in the Zigolo. The parafan engine can run for hours, not minutes. A second hand parafan engine in good conditions is not hard to find, for half the cost of a new one. The ICE engine seem to beat in both cost and performances.
One thing that troubles me, is how to calculate battery endurance, without exceeding the battery safety limits?
In theory a battery, can not be discharged beyond 80%. By using the nominal tension and the Amps per hour, provided by the manufacturer, are we considering the rate of discharge to be above 80% of battery capacity?
I am afraid that manufacturers exagerate their batteries capacity, or drain them beyond the limit, for marketing claims? Just like many aircraft manufacturers, claims absurd stall speeds, for being competitive with the rest, who also invent stall speeds...
Nominal Tension and amperage are average values, the battery voltage and current drops as the battery is discharged. I asume, but I am not sure, that those average values are Ok for the calculations?
Electraflyer offers electric kits with lithium polimer batteries for electric conversions.
It is surprising that with a similar power output calculated on the example above they are able to achieve 1 hour authonomy with their batteries?!
Given the low power output, I assume they run their engines with moderate wind etc.
As always, any input might be appreciated!
Cheers,
Oriol
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