Jumping a 24V Aircraft with 2 cars?

[Old Ironsides]

I got to thinking, if I got stranded in an airplane somewhere with a 24 volt system, how could I jump it?

Does this sound possible?

Have 2 cars running and 2 sets of cables ready,

hook the + and - of the cars batteries (in series to double the voltage),

take the remaining + terminal on one car and connect to the + terminal on the aircraft,

take the remaining - terminal on one car and connect to the - terminal on the aircraft,

Hit the starter on the plane!

Would different amperage alternators disturb the power or would it simply revert to the amperage of the alternator putting out the lowest amperage?

Thoughts?

[Herk]

As you note, the batteries must be connected in series (Battery A + to Battery B -). By doing this you raise the potential of the batteries to 24V. Make sure you disconnect the negative battery cables on each car FIRST (no point in having them running). Otherwise the car systems will see 24V and bad things will happen.

If it were me, and years ago it was, instead of having the FBO bring out a start-cart, see if they have an instructor that can give you a quick hand propping lesson. Valuable skill to have.

[SJTD]

Seems to me as long as the cars are isolated from each other, except for the cables of course, they could be running.

[bufurd]

If it's 2 12 volt batteries, very simple. You hook your cables up just like you would jumping a "car", positive to positive, negative to negative to charge one battery at a time. If you have 2 cars hook one on each battery hooked up normally, just don't have the cars touching each other or the plane. Seems odd to ignore the way the battery cables are hooked but I do it all the time jumping 24V equipment. Just gotta wrap your head around the fact that it's still 2 12 V batteries, pos to pos, neg to neg regardless of the cable hooked to post. WEAR SAFETY GLASSES WHENEVER YOU ARE JUMPING ANY BATTERY.

[tgreese]

Seems to me as long as the cars are isolated from each other, except for the cables of course, they could be running.

Yes, I'd think so. Don't let the cars touch. You'd better have some very long cables.

[Old Ironsides]

I have hand propped (with good success) Lycoming O-235s and O-320's. The Continental's are another story. I owned a C-185 for many years and never was able to do it successfully. I even printed an article about it from the 185 Club and tried their method. Had 2 different mechanics help me try it but with no success. Seems like most shy away from it. A backfire could take your fingers off.

Mine was a 12 volt system, which makes jumping easy. The downside to that is with an injected engine and 12V, you get 1-2 attempts in a hot start situation before the battery wont turn the prop. That's why they went to 24V in the mid '70s.

[SJTD]

I had a friend with a 6 Volt VW with a weak starter. (like all other 6v VW's)

He had two 6v batteries behind the seat connected to the car with jumper cables. To start he hooked them up in series for 12v then when it was running he rearranged the the cables back to parallel to keep them charged.

Maybe that would work on your Cub?

[Old Ironsides]

That sounds like a simple method. Problem is that the batteries are 24v. I've jumped with 2 12v batts from kushman electric cart and it seemed to work ok. But the backcountry is another story.

[Rod2]

I had a friend with a 6 Volt VW with a weak starter. (like all other 6v VW's)

He had two 6v batteries behind the seat connected to the car with jumper cables. To start he hooked them up in series for 12v then when it was running he rearranged the the cables back to parallel to keep them charged.

Me too, only it wasn't a friend--it was I. I still have a '66 bug, and before I changed it to 12v, I put another 6v in the back floor, rigged them up in series separated by a hand made spring loaded switch that I could reach between the seats to connect. As soon as the engine started, I released the switch and was back on 6v for driving.

[Old Ironsides]

Wouldn't double voltage eventually fry the starter and wiring? I would guess that since the starter was only used in short bursts the risk would be lower.

[tgreese]

Wouldn't double voltage eventually fry the starter and wiring? I would guess that since the starter was only used in short bursts the risk would be lower.

Eventually is the operational word here. The wires are in no danger, since the required wire diameter gets smaller as the voltage goes up (wire gauge depends on current). But the amount of heating in the starter increases with increased voltage. The torque of the motor is dependent mostly on the magnetic field strength, which is roughly dependent on current (force goes like di/dt). The speed of the motor will increase (slew rate goes like dv/dt).

Back in the day, many Willys Jeeps were converted from 6V to 12V while keeping the original starter. It was a little more risky to run the 6V starter, but they seemed to tolerate it well.

[joe]

Wouldn't double voltage eventually fry the starter and wiring? I would guess that since the starter was only used in short bursts the risk would be lower.

Yeah eventually but if the orig is in good condition and you keep the eng in tune to minimize cranking time it'll last quite awhile. Back in the day I don't know how many old Chev's I converted to 12v and it was pretty common to keep using the 6v stater till you needed to buy a 12v starter.