Originally posted by Tilt
If the pilot has separate pitch control (rather than pitch control from a governer driven by rpm setting) is it really a constant speed prop? isn't it really a variable pitch prop?
The difference is between a pilot that has to balance throttle and pitch (ignoring mixture) to arrive at optimum rpm and boost whilst ensuring that neither exceed limits and a pilot that sets rpm and then varies throttle to set boost confident that neither will exceed max setting whilst always able to reach it.
If you have a direct pitch control, instead of an RPM control, then it's a variable pitch prop instead of a constant speed prop. I think the only AC we have that used this sort of system was the Bf-109. All the rest had a constant-speed.
Constand speed props set engine RPM to a given point, and through either oil or an electric motor the prop pitch is changed to maintain that RPM. Hence the reason over-speeding a prop can be a VERY bad thing for your engine. Variable pitch props only controlled prop pitch, not engine revvs, which made flying with this prop a lot like playing a three-handed piano. Adjusting pitch meant the engine speed would change, so you had to fiddle with the throttle and mixture to avoid over-revving it.
As for the under-bottom stall, it does happen. However it only happens when you crank in a lot of rudder at very low airspeed and rather high bank angles. Usually in AH the tendency is for the aircraft to stall on the left wing (right wing for the bass-ackwards engines). So you can dial in some rudder to increase turn speed without getting into a stall. However, I've never seen or had this happen to me and I do use a twisty stick. Then again, I don't like playing the low-speed stall-fight games some guys prefer. I've used some rudder to delay an oncoming stall in a turn more than once during practice. But I've never had an under-bottom stall occur. It might be a difference between AH modeling and the way a real aircraft handles. I'll check it in AH2 to see if this characteristic is present.
Striaga: The prop pitch in any aircraft is limited by a physical stop inside the prop dome. You can not get the prop angle below that stop, it's impossible without a feathering system. Also, when you increase airspeed with a low RPM setting, eventually you'll hit the point where the prop begins to drive the engine. When this happens you have a very finite amount of time to either firewall the prop lever or start dumping speed. 30 seconds was the limitation in most engines for this to happen, typically at 3,050 RPM and no higher. If you failed to take any action the crankshaft main bearing would melt, ruining the engine.
Now if you set 2,000 RPM with 48" MAN and proceeded to dive, the prop angle is set too high. As you pass a given airspeed, the engine/prop is no longer generating enough thrust to offset the increase in speed. So the prop starts turning faster (driving the engine with the prop shaft) while the governor tries reducing the prop angle. Once the adjustment plate inside the prop dome hits the fixed stop on the cam
the prop becomes fixed-pitch. All WW2 era props except the Curtiss Electric had/have a mechanical stop. The governor can not adjust the prop angle any lower, and your engine speed starts climbing. This is why you saw the RPM gage climbing after setting 2k RPM and diving.
Try it in any constant-speed prop aircraft that can move faster than 200 knots. Pull the RPM back to 1,500 and dive the aircraft until you see the RPM climb. It can and will happen. Constant speed props have mechanical limits that most people never see simply because they aren't that dumb. No one wants to have their crank sieze up or plow a hole in the ground. Take the time and actually read that article I posted, along with several others Deak has done on props.
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Flakbait [Delta6]
Delta Six's Flight SchoolPut the P-61B in Aces High
