Battery drain for startup

[Jenisch]

Watching training films about WWII American bombers, I notice they say to always use an external source of power to start the engines when avaible, to save the batteries.

As far as I know the engine would turn the dynamo, which would charge the battery. The drain could be so severe as to totally drain the battery if the engine didn't started at the first time or what?

[Jenisch]

AHHH, the inertia wheel! Guess I already found the answer to my question.

[tyrodtom]

Some WW2 aircraft had pneumatic/ shotgun cartridge starters, some had inertial starters, some had electric starters, and some used a hucks starter, and so on.

But even with a car, the starter uses a lot more current than the generator/ dynamo can supply, especially at low rpm.

But being a aircraft, it's not going to want to carry all the extra weight of a giant battery, that you'd need for a difficult start.
Aircraft are like that still, if it doesn't start right the first time, go look for a auxially power unit.

[Jenisch]

But even with a car, the starter uses a lot more current than the generator/ dynamo can supply, especially at low rpm.

But being a aircraft, it's not going to want to carry all the extra weight of a giant battery, that you'd need for a difficult start.
Aircraft are like that still, if it doesn't start right the first time, go look for a auxially power unit.

Yeah. And in the case of American bombers, they had big engines and a consequently big inertial wheel. The inertia wheel was already a system to overcome the weight problem, but even so it consumed a lot of current.

[gumbyk]

I believe that the tendency to use ground power for starting is to preserve battery life. Starting provides a big strain on the battery, and can cause batteries to get very hot which can be a potential fire source, so the less you can do it, the longer the battery lasts, and the safer the start will be.
The inertia wheel was to enable you to use a smaller motor, and lower current drain on the battery. It would consume a lot of current, but not as much as a motor that can overcome the engine's compression when cold. The inertia wheel also has the advantage that it can be hand-cranked, so the aircraft can be started even with a dead battery, and no ground power unit available.

[Jenisch]

BTW, the Bf 109 inertia wheel was always manual?

[Siegfried]

BTW, the Bf 109 inertia wheel was always manual?

The inertia starter had an electric motor that could be used to spin up the fly wheel however in order to spare the battery it was standard opperating procedure for the ground crew to manually charge the flywheel. I immagine the life for the battery could be greatly extended since it would experience no great current draws outside of starting. The inertia starter was a good idea, plenty of crew have been severly injured pulling a propellor around manually.

[gumbyk]

The inertia starter was a good idea, plenty of crew have been severly injured pulling a propellor around manually.

We generally have a couple of broken arms a year around here from people hand-starting vintage aircraft.

Didn't some models of Stearman only have a manual flywheel?

[tyrodtom]

I doubt that anyone has the muscle to hand prop a 1000+hp engine. Plus on about any WW2 aircraft, they're so high up you couldn't pull it through enough of a revolution to start it either.

The pictures you see of men turning the propellers are just doing it with the ignition switch off, to get the accumulated oil out of the bottom cylinders of a radial engine, or to draw fuel into the cylinders prior to switching on the ignition.
I do wonder at what HP level hand proping to start became impossible though.

[FLYBOYJ]

The inertia starter was a good idea, plenty of crew have been severly injured pulling a propellor around manually.

How do you think the crew would pull the prop manually? Are you referring to starting or to clearing hydraulic lock in a radial engine?

[gumbyk]

Tyrodtom,
Those incidents are around WW1 replicas Tiger Moths, and the like.
We have a hand-propping procedure for our CJ6, which is 285 HP, geared.

It would really depend on compression ratios on the engine, and gearing, but I wouldn't want to try it on anything much bigger than the CJ6. Just pulling the bigger engines through before start is enough work!

[evangilder]

And make SURE the mags are off before propping a hydro lock on a CJ6. We had someone nearby get seriously injured that way recently. His kneecap was in 27 pieces and it basically also shattered an arm.

[gumbyk]

Yeah I can understand how that happens. The mag switch operates backwards to most western types, so a quick glance looks like they are off when they are on.

Fortunately for me, it happened the other way around - trying to start it with the mags off.

[GregP]

On most radials, you need to turn the propeller through 8 to 12 blades before engaging the mags. That is a heavy drain on the battery. So, they saved the batteries when they could.

Even for an inline, you sometimes rotate a number of blades before engaging the mags.

For an Allison V-1710, you engage the starter which spins up the interia wheel. When it is spiniing at max speed, you hit the solenoid that pushes the starter dog fiorward to engage and turn the cranklshaft. Youn usually leave the mags ON, and the mixtuire is in auto lean while the throttle is open about 1 inch. You continue to crank until you start or lose power ... don't stop. If you play with the mixture yourself and miss it, you can get a backfire that blows out intake gaskets easily. When that happens, you have some 20+ hours of work to do, assuming you have the new gaskets. Most people use auto lean and have no issues.

For a radial, you need to turn the prop through some number of blades before engaging the mags. The blades vary with the number of blades on the prop and the radial in question ... but radials ALWAYS need to turn some before starting or they can backfire and blow out gaskets with extreme ease or hydraulic lock and require a complete rebuild.

So the power carts are there to provide auxilliary battery starting power to both save the on board batteries and to ensure a complete squadron launch when, to use the onboard batteries, might mean a partial launch due to run-down batteries. They wanted to give every aircraft a good chance to make the mission. That's all. You know, make sure it gets started. If gound power failed, they COULD use onboard batteries but, more likely, would troubleshoot the issue before doing that. No use to run down TWO sets of batteries.

In Germany, the Me 109 used the SAME inertia starter that we did. We supplied them through Switzerland to Germany. But THEY used crewmen to turn the inertia starter manually while we used a battery to spin it up. It added weight, but we did it anyway.

Some years back we restored a Messerschmitt Me 109E at Chino Aerro Traders and invited several former Luftwaffe Me 109 pilots to see the first flight.

The Me 109 was rolled out and the pilot got into the cockpit. The former German pilots were looking arounf for the "batman" when the pilot engaged the electric starter and started the DB 601. One of the former Luftwaffe Me 109 pilots was heard to say (in German), "My God, if we'd had those electric starters in WWII, we'd have won the war!" We were all quite amused at the comment after it was translated and after realizing the implications. They were all laughing at the electric starter ...

Oh, and I can pull an Allison prop through myself for a number of blades (and a Merlin, too). They have mostly very nearly identical compression ratios ... unless it is a Reno engine and then the compression rato is usually less due to increased boost. You usually can't get a WWII V-12 to run on less compression ratio than about 5.0 : 1 - 5.5 : 1, but some people try so they can go to higher boost before detonation. Doesn't usually work, occasionally it does.

[Shortround6]

Some WW II aircraft (like the P-39) also had electric landing gear, and/or flaps and perhaps an electric propeller. Getting the battery in a low state of charge and then taking off could cause real problems if there isn't enough battery left to operate the systems before the engine can recharge it. DC generators (not alternators) are also rather heavy and in order to save weight sometimes smaller sizes were used, like a 900 watt instead of a 1500 watt. The watts generated varied with the engine speed and sometimes a low cruise speed could not generate enough power to keep up with demand even during cruise let alone lowering landing gear and flaps.