X-Plane Flight Simulator:
Examples of New Tech Stuff
for Propeller Heads

Turbines

There are now two types of turboprop engines in X-Plane: Free-Turbine and Fixed Shaft. Here is how they work, in reality (and in X-Plane)

The Fixed-Shaft turboprop engine is pretty simple: a turbine spins at insane RPM as fuel is burned in front of it, and that turbine is geared down (a LOT) and a transmission goes right from the turbine to the prop. This means the prop is hard-locked to the turbine. Whatever percentage of maximum RPM the prop turns, the N1 turns as well. A perfect lock between prop and turbine. This type of engine is the most efficient since there are no losses in the transmission from the turbine to the prop. You get the efficiency of highly compressed, enclosed combustion, and since this turbine drives a prop, you get the high efficiency of a prop as well (more efficient than a jet since the prop takes a bigger bite of air than a jet, and only accelerates that air a little, unlike a jet, which grabs only a little bit of air and accelerates it a lot. You get the most propulsive efficiency by grabbing a LOT of air and pushing it back a LITTLE. This is what props do (not jets) and is why props are more efficent at low altitudes. They waste less energy shoving air back really fast. So, the fixed-shaft turboprop engine with its big prop, enclosed conmbustion, and (nearly) zero-loss transmission is a shoe-in for the most efficient engine. This describes the Garret engines used in the powerful, efficient MU-2 and Cessna 441. Unlike the Cessann Citation Biz-Jet costing much more, the Cessna 441 turboprop can fly unrefuelled from LA to NYC. The low-end Citation has to stop for fuel on the trip, making it considerably slower than the turboprop going cross-nation.

The second turboprop engine type simulated is the Free-Turbine type, demonstrated in the velnerable Pratt & Whitney PT-6. This engine works differently than the Fixed-Shaft Garret. This engine has a compressor that is not even connected to the prop! Instead, the compressor blows air over a turbine that is connected to the prop, but there's a gap between the two sections. Thus, the air blowing over the turbine spins the prop. Since there is no direct connection between the compressor (N1) and the prop, you can start a PT-6 engine while holding the prop still in your hands (not recommended). The compressor spins right up while the prop is held motionless. Our advice to you: If you let the prop go to let it start spinning, DON'T go back to grab it again. The PT-6's lose efficiency in this "air clutch" arrangement, though, so they burn fuel faster then the Garrets. Most people still like them though because they ALWAYS start, they ALWAYS run, they are easy to start and fly, and you simply don't get any headaches using them After all, they were designed to pump oil down pipelines from Alaska, where they would run for weeks at a time without anyone ever supervising or maintaining them.

Anyway, both engine types are now simulated, each with their own quirks intact. Check out the turboprop engine options in Plane-Maker to select the engine that is right for your airplane.

Electrics:

Another improvement for FAA-certification is the electrical system. Now, we have generators (which CHARGE the battery from the engine) as well as INVERTERS (which convert the DC juice from the battery into AC for certain flight instruments). So, if you want your ADF, RMI, autopilot, and electric artificial horizon to work, turn on the inverters if your plane has the switches! While they were at it, X-Plane added gyro-modelling (both vacuum and electric) for the artificial horizon and DG/HSI/ADF/RMI. Gyros not only spin down correctly, but spin up over time as well. Kill your vacuum system and your vacuum artificial horizon and DG will gradually die out. Leave off your generators (if you have the switches on the panel) and watch what happens to your electric artificail horizon as your batteries gradually die! As the votlage slowly wanes, so does your electric artificial horizon (becoming sluggish and finally stopping) and any other electric instruments (HSI card, for example) When the voltage finally gets down to zero.. lights out! If you have the "flagged" artificial horizon, then a little red failure flag will come out of hiding and display itself when the voltage gets too low.

Another improvement for FAA-certification is the autopilot. Airspeed, autothrottle, VVI, and heading modes work as before, but here are some differences: When you are coming in at an ILS, you will push the HNAV and VNAV buttons and they will lite up in RED, meaning that they are WAITING TO CAPTURE. You can be in altitude hold mode, VVI hold mode, heading hold mode, or just hand-flying. It makes no difference. All that matters is that those HNAV and VNAV modes are waiting for their first sniff of localizer and glideslope, and as soon as they get it (the localizer and glideslope needles come in from the edge) the autopilot snatches control away from the previous mode and follows the ILS home. This is the way it works in the real plane, and the way it works in X-Plane. The autopilot is just plain better at tracking. Also, we have added a "level-change" function to the autopilot as used in large airliners. Here's how it works: you are established on speed and altitude at cruise... dial in a new altitude and hit the level-change auopilot button. The autopilot will throw in or take out a bit of thrrottle and pitch the nose to hold a constant airspeed, resulting in a climb or descent at a constant airspeed until you get to the new altitude. This is how big planes do it. Also we added another new autopilot function: Pitch Sync. Hold this button to disengage all autopilot pitch functions except glideslope capture. Then, when you release the button, the autopilot holds your pitch attitude until you hit the localizer, some other autopilot mode, or the ground, whichever comes first. Control of pitch sync is toggled by some new autopilot buttons, or more, realistically, via a joystick button now in the joystick button list.