Inertial Starter

[ilya1502]

Are there any plans to implement a realistic inertial starter with the flywheel and clutch? When I choose it now nothing happens — it works the same as the direct drive.

[Piper_EEWL]

As far as I understand this is as realistic as it'll get. What is modeled here is a direct drive inertia starter where the starter motor drives the flywheel that is already connected to the crankshaft. So no clutch to be engaged as in the P-40 or the B-17 for example.

Whereas with the direct drive starter the starter motor drives the crankshaft directly without the flywheel in between.

[ilya1502]

As far as I understand this is as realistic as it'll get. What is modeled here is a direct drive inertia starter where the starter motor drives the flywheel that is already connected to the crankshaft.

Then what's the point in switching between them in the sim? They don't seem to make any difference.

[AKar]

This "direct drive inertial starter" is actually something I've been intending to find more about if I just had some time. It would be interesting to know how it is constructed in detail, and what are the claimed benefits.

I don't know much about radials, but something about electric motors and drives. At first glance, it is not too sensible modification: an electric motor must be properly dimensioned for the moment of inertia of the load (this is even more critical to many AC motors). A direct-drive starter must produce a relatively high torque, and quickly accelerate the engine to cranking speed. While the engine's moment of inertia and compression are reduced over a reduction gear, such a starter remains a very high current draw device. They depend on the engine reaching its cranking speed very quickly, with increasing back-EMF almost immediately limiting the current that rushes into the engine. In that sense, an added rotational mass would be the last thing one wants to add to a direct-drive starter.

I don't know what kind of motors the inertial starters used, but I'd expect them to draw much lower current when held back. This necessarily makes them of lower torque, but it is acceptable as they may take their time speeding up the smooth load of the inertial weight. The momentum of the wheel is then used to kick the engine over the compression. As I understand it, it essentially avoids the need for high-current, high-torque starter, but then again, this would be lost if the whole package is made fixed, I think.

An interesting piece of equipment, would be great to see some details.

-Esa

[T6flyer]

As far as I understand this is as realistic as it'll get. What is modeled here is a direct drive inertia starter where the starter motor drives the flywheel that is already connected to the crankshaft. So no clutch to be engaged as in the P-40 or the B-17 for example.

Whereas with the direct drive starter the starter motor drives the crankshaft directly without the flywheel in between.

Two airframes were used in the development of the A2A T-6, both being G models with the same starter system and therefore this was modelled as to reflect authenticity.

And as you asked so nicely, here are a few useful diagrams from the G Maintenance Manual:









Martin

[AKar]

The MM doesn't do too good job on describing the system, being quite superficial (of course, at least eyes-on experience on the airplane is typically implied in MMs). It speaks of direct-cranking starter on one revised paragraph, and inertia-type on the other, and has that schematic that still implies a separate meshing function in a little confusing way (when not knowing the system). The flight manual, on the other hand, explains that:


What is curious about this system is why the flywheel is retained, and how the clutch is constructed. In general, I can't think of any reason of keeping the flywheel when changing to direct cranking starter mechanism in an airplane modification, except for retaining the possibility for manual starting. However, if the original starter is retained as it is, and it is designed solely for use with inertial wheel, it becomes noteworthy that a series-wound motor (as these starters are) has a tendency to surge, or "run away", in speed unless a load is applied: with no load they have very poor speed regulation from back-EMF due to the low current, in turn due to low load, being shared by the armature and field windings. If the starter fails to engage when it is supposed to and is not designed to withstand this kind of situation, it may easily overspeed and get damaged. I don't know, maybe it is retained to avoid overspeeding the starter before it engages/in case it doesn't?

-Esa

[Scott - A2A]

Then what's the point in switching between them in the sim? They don't seem to make any difference.

Proper physics modeling isn't only about obvious things, but more in the subtlety. Watch and listen closely. The difference will become quite apparent. There are no compromises here - this is how the T-6 starter operates.

Scott.

[JoeS475]

I've only ever used the inertia starter which was selected by default. Tonight I will try the direct crank.

Which system do A2A's test aircraft use?

Edit: I'm assuming that with both starters, the prop control should still be all the way back? Wondering if one starter or the other requires that prop pitch specifically...

[Piper_EEWL]

Edit: I'm assuming that with both starters, the prop control should still be all the way back? Wondering if one starter or the other requires that prop pitch specifically...

The prop pitch has nothing to do with the starter. It is moved to fully coarse to scavenge all the oil. So with both starter types the shutdown procedure remains the same (at least as far as I understand )

Edit: fixed typo

[AKar]

Yes, it appears that the prop is put all-the-way coarse pitch during shut down to empty the hub from the oil, and to cover the otherwise exposed piston surface by moving cylinder all the way back. On startup, the prop control should remain at coarse to keep the oil from flowing into the prop hub immediately when the engine starts, and moved forward to fine pitch only after the engine has been started. The starter selection is not related.

-Esa

[Lufthansa 380]

I guess what Joe meant was if you still start the engine with the prop full back. I'd say yes, since that's pretty much standard on the T-6 and has nothing to do with the starting mechanism itself.

[JoeS475]

Thanks- Yeah that's what I was wondering. The oil being a factor is interesting. IIRC (Darryl can help me here), in the early Spitfires the prop was back due to counterweights or something (still unsure the starter type would affect it)... Of course that is a Merlin, not a radial. So much to try and understand lol!