Post by Jef on Sept 13, 2015 19:58:59 GMT
Have been making gliders fly using electric power for a few years now, and feel there are things that I have learnt that others may find useful.
Watts.
These are the measurement of units of energy flow, ie One Joule per Second is One Watt. The more energy that it takes to get a glider to a set height in a set number of seconds, the more current flows or the greater the voltage, thus as Watts are the product of Volts and Amps the more Watts are required/produced.
How many Watts (W) are sufficient for a given weight of model?
Many modellers use rules of thumb such as 100W per pound (lb), others say 200W per kg, and I aim for 300W/kg to give more power than necessary.
The problem is greater than this simplified answer may suggest, as other factors play their part.
Yes, the figures help you plan ESC and Motor and Battery pack sizing, but what about motor efficiency and prop selection?
To answer the question fully would involve a vast number of words that nobody would really be interested in reading.
Volts
Using LiPo packs, we normally use 3S1P form batteries which give a maximum voltage of 12.6V and a nominal voltage of 11.1V
Motor speed... KV or kv is a theoretical figure quoted by motor manufacturers. Another rule of thumb, 1000kv is good for gliders. Some motors are used with gearboxes, which reduce the propshaft speed to sensible figures. Sometimes 4-cell packs are used to increase the voltage - to increase the revs, and if a smaller prop is used, this can be done without an increase in current. I would possibly do this if I had a motor that was nearly over current as it stood, but the performance needed improving further to achieve the climb rate, so might go from a 13x6.5 down to an 11x6 and run at 33% greater voltage. One has to be careful not to exceed the maximum revs for any prop used though, and carbon propellers are very advisable if using theoretical revs (kv x max batt pack voltage) of greater than 13,230 (3 x 4.2 x 1050).
Prop Sizes
One school of thought is that the prop size should be as large as possible to increase the effective prop swept area. As the fuselage is sitting in the middle of this area, the larger the diameter. The less the percentage of the ineffective area. Some say that the pitch should always be at least 50% of the diameter, ie 14 x 7 is OK, 15 x 6 is a paint stirrer!
Watts.
These are the measurement of units of energy flow, ie One Joule per Second is One Watt. The more energy that it takes to get a glider to a set height in a set number of seconds, the more current flows or the greater the voltage, thus as Watts are the product of Volts and Amps the more Watts are required/produced.
How many Watts (W) are sufficient for a given weight of model?
Many modellers use rules of thumb such as 100W per pound (lb), others say 200W per kg, and I aim for 300W/kg to give more power than necessary.
The problem is greater than this simplified answer may suggest, as other factors play their part.
Yes, the figures help you plan ESC and Motor and Battery pack sizing, but what about motor efficiency and prop selection?
To answer the question fully would involve a vast number of words that nobody would really be interested in reading.
Volts
Using LiPo packs, we normally use 3S1P form batteries which give a maximum voltage of 12.6V and a nominal voltage of 11.1V
Motor speed... KV or kv is a theoretical figure quoted by motor manufacturers. Another rule of thumb, 1000kv is good for gliders. Some motors are used with gearboxes, which reduce the propshaft speed to sensible figures. Sometimes 4-cell packs are used to increase the voltage - to increase the revs, and if a smaller prop is used, this can be done without an increase in current. I would possibly do this if I had a motor that was nearly over current as it stood, but the performance needed improving further to achieve the climb rate, so might go from a 13x6.5 down to an 11x6 and run at 33% greater voltage. One has to be careful not to exceed the maximum revs for any prop used though, and carbon propellers are very advisable if using theoretical revs (kv x max batt pack voltage) of greater than 13,230 (3 x 4.2 x 1050).
Prop Sizes
One school of thought is that the prop size should be as large as possible to increase the effective prop swept area. As the fuselage is sitting in the middle of this area, the larger the diameter. The less the percentage of the ineffective area. Some say that the pitch should always be at least 50% of the diameter, ie 14 x 7 is OK, 15 x 6 is a paint stirrer!
.
