SD-8 8amps, 2.9lb Alternator $545 with regulator, 8 amps (at 13.8v) at 2700 engine rpm, 7amps at 2300 engine rpm, but engine must be turning 1500 rpm before 2amps are produced. Link
BC410 20 to 40 amps, 5.75lb Alternator $500, regulator $180. 32 amps (at 14.4v) at 2700 engine rpm, 29 amps at 2300 engine rpm and 15 amps at 1500 engine rpm. Link
Is an SD-8 alternator sufficiently powerful?
The answer used to be yes, no question. Many aircraft have been built with only a B&C SD-8 for electrical power and have been run quite happily without any need to charge the battery externally. However, now I?m not so sure. As we all now like to run more and more electrical gizmos it is possible to use up most of an SD-8?s output and leave very little to charge the battery. The SD-8 produces very little useable energy on the ground, if you fly many short flights it is possible an SD-8 may not be able to replenish your battery.
Let?s consider an aircraft with a radio, transponder, EFIS/engine monitor, GPS and fuel injection. The fuel injection system is potentially significant as the boost pump can take 4amps, that?s more than half the SD-8?s output. If the boost pump has to be run all the time or is inadvertently left on there is only 3 amps for everything else. The electronics can take 2 amps and a master solenoid takes nearly an amp. That means there is almost nothing left for the battery.
The next size up alternator from B&C is the is the BC410, a 20 to 40 amp unit. From the spec sheet it looks like the BC410 produces some power at 1000rpm, so there is no battery drain on the ground and some power left over in flight for the battery, even if (LED) strobes, nav lights and a landing are installed. It is nearly 3lb heavier (double the weight) and $135 more expensive, but what is it worth not to run out of battery power?
Recently I was several hours away from home flying with some friends. We were flying short trips at low level in hot temperatures. Everyone was suffering some fuel vapour problems, so we routinely ran with the boost pump on continuously. Those with fuel injection were also cranking a lot to get started. The consequence was that those with an SD-8 were not replenishing their batteries. One aircraft ran out of power when we were starting up to fly home. Fortunately we had a battery charger and access to a power point very close to the ramp. After a short delay there was sufficient charge to crank the engine. We were not seriously inconvenienced this time and all made it home on the day we planned, but we were fortunate.
Building aircraft is always a compromise. I?m now of the opinion that for RVs with ability to comfortably fly 1000 miles in a day a 20amp alternator (and an autopilot) is a required item.
Pete
BC410 20 to 40 amps, 5.75lb Alternator $500, regulator $180. 32 amps (at 14.4v) at 2700 engine rpm, 29 amps at 2300 engine rpm and 15 amps at 1500 engine rpm. Link
Is an SD-8 alternator sufficiently powerful?
The answer used to be yes, no question. Many aircraft have been built with only a B&C SD-8 for electrical power and have been run quite happily without any need to charge the battery externally. However, now I?m not so sure. As we all now like to run more and more electrical gizmos it is possible to use up most of an SD-8?s output and leave very little to charge the battery. The SD-8 produces very little useable energy on the ground, if you fly many short flights it is possible an SD-8 may not be able to replenish your battery.
Let?s consider an aircraft with a radio, transponder, EFIS/engine monitor, GPS and fuel injection. The fuel injection system is potentially significant as the boost pump can take 4amps, that?s more than half the SD-8?s output. If the boost pump has to be run all the time or is inadvertently left on there is only 3 amps for everything else. The electronics can take 2 amps and a master solenoid takes nearly an amp. That means there is almost nothing left for the battery.
The next size up alternator from B&C is the is the BC410, a 20 to 40 amp unit. From the spec sheet it looks like the BC410 produces some power at 1000rpm, so there is no battery drain on the ground and some power left over in flight for the battery, even if (LED) strobes, nav lights and a landing are installed. It is nearly 3lb heavier (double the weight) and $135 more expensive, but what is it worth not to run out of battery power?
Recently I was several hours away from home flying with some friends. We were flying short trips at low level in hot temperatures. Everyone was suffering some fuel vapour problems, so we routinely ran with the boost pump on continuously. Those with fuel injection were also cranking a lot to get started. The consequence was that those with an SD-8 were not replenishing their batteries. One aircraft ran out of power when we were starting up to fly home. Fortunately we had a battery charger and access to a power point very close to the ramp. After a short delay there was sufficient charge to crank the engine. We were not seriously inconvenienced this time and all made it home on the day we planned, but we were fortunate.
Building aircraft is always a compromise. I?m now of the opinion that for RVs with ability to comfortably fly 1000 miles in a day a 20amp alternator (and an autopilot) is a required item.
Pete
