19-03-2011, 09:07 AM
When building my planes I always have a bit of struggle with deciding which servo to use, so I thought I would start this thread so that we could all share our thoughts and knowledge on the issue.
With my current project an E-flite AT6 Texan, I stuggled even more than usual as this is my first model that I have built which has flaps and retracts, plus an unusal dual setup using 2 servos for the elevator.
To add to my dilema we had a couple of aircraft recently meet with disaster at the flying field as a result of servo issues and this was playing on my mind. One was stripped plastic gears resulting in the loss elevator control and another was cut servo wires. Both resulting in a total loss of the aircraft, this served as a big reminder how important these little items are.
With the Texan having 9 servos, ailerons 2, flaps 2, retracts 2, elevator 2 & rudder 1 this necessitaed the use of a separate UBEC as I have learnt from my researching that an ESC with an internal BEC may have a stated BEC current rating of 2.5A or 3A but this is not to be trusted.
As a rule of thumb an aircraft using 4 or more full size servos should have a separate UBEC. If you are using a micro or minature servos, these draw less current therefore the above rule needs to adjusted accordingly. Another thing to remember is that digitals use more current than analogue for a servo of the same size.
Given the number of servos I need and the all up flying weight of my plane in mind I didn't want to go for the overkill when selecting the appropriate servos. My retracts are H/King electrics so that decision was taken care of. The build manual called for a minature servo of 17g in weight, a torque of 2.5kg-cm and a speed of 0.17sec/60deg @ 6V. I was torn between using metal gear and plastic gear with the events at the field recently, metal maybe reliable but are not as smooth as plastic and the backlash is greater with metal. Also a metal servo with the required torque and speed are much heavier than the plastic geared servos. In the end I compromised: ailerons, flaps & rudder using plastic (Hitec HS-81 analogue) and elevator using metal (Corona/HK DS-929MG digital), both are within the weight spec but the metals have a lower torque at 2.2kg-cm but I felt this was OK as there are two servos on the elevator instead of the usual one.
Now that the servos were selected I had one more thing to sort out - the 2 servos controlling the elevator (one for the port side and the other for the starboard) requires one of them to rotate in the reverse direction. This requires either rewiring the servo internally (see the YouTube clip below) or the purchasing of a servo reversing lead, both of these options are not so straight forward as the rewiring of a small servo requires some skill and the reversing lead is not readily available from the LHS. I intend on attempting to rewire the servo with some help from a person experienced at soldering small components as the servo cost $6 and a reversing lead costs $26 if available plus it adds weight. I am assuming that rewiring a digital servo can be done similar to the analogue ones only time will tell.
Some usefull web sites I found are below:
General servo specs 7 info
http://www.servodatabase.com/
http://fatlion.com/sailplanes/servos.html
Reversing of servos
http://www.youtube.com/watch?v=EK_FdRTenl0
Happy Flying
Chris M
With my current project an E-flite AT6 Texan, I stuggled even more than usual as this is my first model that I have built which has flaps and retracts, plus an unusal dual setup using 2 servos for the elevator.
To add to my dilema we had a couple of aircraft recently meet with disaster at the flying field as a result of servo issues and this was playing on my mind. One was stripped plastic gears resulting in the loss elevator control and another was cut servo wires. Both resulting in a total loss of the aircraft, this served as a big reminder how important these little items are.
With the Texan having 9 servos, ailerons 2, flaps 2, retracts 2, elevator 2 & rudder 1 this necessitaed the use of a separate UBEC as I have learnt from my researching that an ESC with an internal BEC may have a stated BEC current rating of 2.5A or 3A but this is not to be trusted.
As a rule of thumb an aircraft using 4 or more full size servos should have a separate UBEC. If you are using a micro or minature servos, these draw less current therefore the above rule needs to adjusted accordingly. Another thing to remember is that digitals use more current than analogue for a servo of the same size.
Given the number of servos I need and the all up flying weight of my plane in mind I didn't want to go for the overkill when selecting the appropriate servos. My retracts are H/King electrics so that decision was taken care of. The build manual called for a minature servo of 17g in weight, a torque of 2.5kg-cm and a speed of 0.17sec/60deg @ 6V. I was torn between using metal gear and plastic gear with the events at the field recently, metal maybe reliable but are not as smooth as plastic and the backlash is greater with metal. Also a metal servo with the required torque and speed are much heavier than the plastic geared servos. In the end I compromised: ailerons, flaps & rudder using plastic (Hitec HS-81 analogue) and elevator using metal (Corona/HK DS-929MG digital), both are within the weight spec but the metals have a lower torque at 2.2kg-cm but I felt this was OK as there are two servos on the elevator instead of the usual one.
Now that the servos were selected I had one more thing to sort out - the 2 servos controlling the elevator (one for the port side and the other for the starboard) requires one of them to rotate in the reverse direction. This requires either rewiring the servo internally (see the YouTube clip below) or the purchasing of a servo reversing lead, both of these options are not so straight forward as the rewiring of a small servo requires some skill and the reversing lead is not readily available from the LHS. I intend on attempting to rewire the servo with some help from a person experienced at soldering small components as the servo cost $6 and a reversing lead costs $26 if available plus it adds weight. I am assuming that rewiring a digital servo can be done similar to the analogue ones only time will tell.
Some usefull web sites I found are below:
General servo specs 7 info
http://www.servodatabase.com/
http://fatlion.com/sailplanes/servos.html
Reversing of servos
http://www.youtube.com/watch?v=EK_FdRTenl0
Happy Flying
Chris M
