This post is a summary of what is going on with the X-Plane 10 starters. I am working on a more comprehensive X-Plane 10 aircraft update check-list that will include starters.
How the Starters Used to Work
In X-Plane 9.70 and 10.05r1, the starter motor applies a constant torque to the engine to increase its RPM. As you motor the starter without adding fuel and spark, the engine’s drag will increase (due to its higher RPM – pretty much all engines move air when turned and thus have higher drag at higher RPM) and eventually you’ll hit an equilibrium: the torque of the starter exactly matches the drag of the engine and you sit there at a constant RPM. This RPM can be quite high because the starter motor can deliver its torque at any RPM.
The torque of the starter is decided by X-Plane using a formula known only to Austin and a few highly trained monkeys that have secure access to the X-Plane source code. The starter “ratio” you set in Plane-Maker is a scaling factor on that ratio. A scaling factor of 2.0 doubles the torque, and a scaling factor of 0.5 halves it.
Note that the default torque (1.0 scaling factor) varies by engine type and engine size! The justification by this is that if you don’t want to have to deal with starters, you set the starter ratio to 1.0 and X-Plane’s “natural” choice is strong enough to start your engine. In practice, the default torque is surprisingly high for jet engines, but at least they start!
How the Starters Work in X-Plane 10.10 Beta 11
Pay no attention to X-Plane 10.10 beta 11 – the changes for RC1 are quite significant.
How the Starters Work in x-plane 10.10 rc1 (coming soon!)
X-Plane 10.10 rc1 is also a torque-based model, with the starter putting out constant torque. There are two key changes:
- The torque is expressed as a ratio of maximum engine torque, not a ratio of an arbitrary default torque. This change the numbering scheme! When you open your aircraft in 10.10 you’ll see the new numbering scheme. For example, the default torque in X-Plane 9 for a jet engine was 50% of engine max torque (see above that jets started really fast), so if you set a starter ratio of 0.6, the net result was 30% of max engine torque. In X-plane 10.10 you will just see 0.3. In other words, what used to be a ‘secret scaling factor’ is now baked in to your starter value.
- The starter now has a maximum design RPM; past this RPM its torque will fall off – very very quickly for electric starters, less so for bleed-air starters. The default will be 100% of engine RPM, so for old planes the starter will continue to motor up to equilibrium. You can turn this number down to model the real world: real starters generally can’t put out their maximum torque up to really high RPMs.
For piston engines with electric starters, I recommend setting the maximum RPM very low and the torque fairly high; a real electric starter for a piston engine is going to put out a ton of torque to get your engine to turn over, but it just can’t run that fast.
For turbines, make sure your design RPM is well above your fuel introduction point. From what a number of people have told me, the starter can often be motored quite a bit past the fuel introduction point.
The net of all of this is: no change from 10.05 for existing aircraft, but the potential to fix a number of weird starter behaviors in 10.10 by limiting RPM. (One advantage of the RPM limit is that you can increase the starter torque without getting an absurdly high terminal RPM.)