Martin wrote:
No, there are just two discussions going on and it seems some have problems distinguishing them. One is about the effect of gearing itself, that’s what I was talking about (I don’t know enough about starters, but I do remember enough of elementary physics). And another is about particular starters where difference in cracking speed obviously matters, you participate in that one. There is potentially a third one, but I’ll leave that one to vic.
Ok, I understand that. But I thought we were discussing this aircraft low speed (heavy weight) and high speed (low weight) aircraft starter that Peter descriped. All these lightweights have high crancking RPM then the direct drive one’s.
Thanks all for your answers, even if it goes way beyond my complete understanding :-)
Considering the starter, I had a Lamar which suffered fatal disconnection (750TT)
A Skytec was installed then, and I must say it is much more efficient. Hot start have become an easy game :-)
But, don’t know if there is any link, the battery died few flights after (but it was a eight years old Gill battery them).
PetitCessnaVoyageur wrote:
A Skytec was installed, and I must say it is much more efficient. Hot start have become an easy game :-)
It is more powerful, but likely less energy efficient. Efficiency does not actually matter much for things you run for just a few seconds. Unless they get really hot and break as a result 
The reliability issues with geared versus direct drive starters are typical for all kinds of vehicles where geared starters have been introduced.
Efficient in the way I can plan to leave the pump area in less than 1 hour (I do exaggerate :-)
Time efficient then 
I made a photo of some different aircraft starters.
From left to right: Sky-Tec NL-149, low weight, high crancking RPM, high power geared starter. The same starter is used for 12 and 24 Volt which is quite unique. Uses solenoid instead of bendix. Kelly Aerospace MZ-6222, PM starter with planetary gear, low weight, high crancking RPM, comes with bendix. Prestolite MZ-4204 , heavy weight, direct drive, low cracking RPM, low power uses bendix. Prestolite MHB-4016, geared heavy weight starter uses bendix.
Weak point on low weight starters is their power consumption. Aircraft must have proper electrical system to make good use of these. Pro is their high RPM which makes starting easy.
Weak point on planatary gear gearboxes are the gears, they break / explode.
Weak point on bendix use, modern bendix quality is quit poor. Bendix do split, fail to extend, fail to retract. Old bendix units are very good quality which typically easily make TBO.
Weak point on heavy weight starters is their power. Aircraft will be hard to start, especially during cold weather, cranking speed low.
PetitCessnaVoyageur wrote:
Considering the starter, I had a Lamar which suffered fatal disconnection (750TT)
Your starter shows the poor modern bendix. Then bendix breaks (splits). The teeth of the bendix don’t longer align with the starter gear. The top of the teeth of the bendix will be running on top of the teeth of the starter gear. This forces the starter to go down, and your housing to break as well.
@Peter: You didn’t misunderstand me, it was just a thinking error on my end. Of course at 0 volts it would be an issue. The drop seems to actually be to about 1/2 of the initial voltage at the stall torque, where also the current is highest of course, therefore maximal electrical power us drawn, which you were talking about? I was thinking about peak mechanical power. I am not sure wheter this can easily be determined or approximated from stall torque down, might depend on the individual power curve. And while I have a physics and electronics background my knowledge of electrical motors is actually rather basic, so I might be mistaken on all of that. Thanks for pointing it out, made things clearer!
And from the somewhat more practical side: Yes, the normal ones are quite weak from experience, while I always attributed bad starting behavior at low temps to the battery one with a higher nominal peak mechanical power rating would also draw a higher current, as long as the weaker battery, with lower idle and loaded voltage, can still supply that higher short term current the starter would supposedly work better with the weaker battery, if I am not missing something?
Maybe we could summarize the different functions (please, give any correction needed):
1) Charging the battery
In my case, this could be done through the Ground Power Receptacle. But this is not the best options, because it implies powering the essential bus (PFD, Comm/NAv1, AHRS, ADC), which is not good for a long time.
So, I should use for that purpose a BatteryMinder/BatteryTender/CTEK charger, which are smart chargers able to desulfurize and maintain the battery fully charge for a long time.
To do that, I could:
- take the battery out of the plane and connect it
- install a permanent socket, but how exactly ?
BatteryMinder FAA permanent installation
CTEK permanent Slot
I read somewhere that we should not connect directly on the battery, rather on the battery relay. Any opinion about that ?
2) Powering the avionics on the ground
This need a ground power supply, 25A-50A. They are sometimes able to switch to “charge mode”. Anyway, when plugged, they will try to charge my battery: is there a risk to overcharge it ? Should I take the battery out of the plane when I use this power source for a long time ? (annual for example)
Of course this won’t allow me to crank the engine.
BatteryMinder:
Bycan:
3) Starting the engine
This need a 24v battery combined with a charger, or Portable Starting Unit., such as the one produced by StartPack
They should also supply energy for the avionics, but for a limited amount of time.
4) What you bring when travelling
Obviously, one should consider light stuff to be carried on board.
BatteryMinder or CTEK chargers are light, it may be a good thing to have one ? Of course, it will take some time…
Do you also carry stuff like that ?
It would allow a ground start from 2*12v car batteries, which could be found easily. I read somewhere it could kill one of them. Do you confirm ?
If you are concerned about charging with the buses “live” you could pull the two CBs powering the buses when you are charging.
Using car batteries to start may work with your setup because the aircraft battery remains in the circuit. On mine the external power isolates the aircraft battery/master switch. The external power socket is on the rear fuselage, so the combination of cables is so long that there is not enough power to crank the engine fast enough top start.
