RPM increasing above 100% at altitude

[Keith Smith]

With XP 10.11, leveling off at FL250, with EGT and torque below redline, I ran into a case where the RPM would start creeping above 100%. I was first alerted to it when I the audible pitch of the ambient noise started to increase. I zoomed in on the RPM gauge and saw it heading above redline. I had to back the power off considerably, back down to around 180kias to keep it from creeping back up above redline.

I verified that it wasn't a governor failure (at least not within X-Plane's core system). Also verified both engines were showing the same behavior.

 

My suspicion is that the min/max prop pitch setting isn't quite right. I'm going to do a test flight now, outputting those values on screen to see if perhaps we're running up against the limit of the governor and it's not able to coarsen the blades any more...effectively becoming a fixed pitch prop.

 

Or, am I missing something obvious?

 

You can see this happen on my existing video: 

 

At 43:15 I make the comment about having to pull the power back, with the working theory about the blade pitch issue. If you rewind to the 40:30 mark and watch it for 2mins 45secs, you'll hear the pitch slowly increasing between the shots. You can even see the RPM's winding up very slowly in some of the shots.

 

I'm off to go flight test this more carefully in the mean time.

[Keith Smith]

Ok, finished flight test. At FL250, by around 260KTAS, the prop pitches hit 37.40 and 37.90 respectively and don't move after that. As the speed increases slowly to 280KTAS, the prop RPM slowly increases from 1600 to around 1680'ish and the RPM indicator shows > 100%.

 

So, as suspected, the prop governor is hitting its stop.

 

Either the aircraft is overpowered, the governor needs more travel on the blades, or this is a plane which requires the power to be pulled back significantly once it's in cruise, even at its service ceiling (which seems odd).

[cessna729]

....the prop RPM slowly increases from 1600 to around 1680'ish and the RPM indicator shows > 100%.

So, as suspected, the prop governor is hitting its stop.

 

Either the aircraft is overpowered, the governor needs more travel on the blades, or this is a plane which requires the power to be pulled back significantly once it's in cruise, even at its service ceiling (which seems odd).

Guess maybe it depends on which TPE331 engine variant Javier has modeled as to wether the aircraft is overpowered? I believe (ref: Type Acceptance Report TAR 96/12), the original Jetstream 3100 series had TPE 331-10Uxx-5xxH Variant rated at 940 shp, and the Jetstream 3200 series had the newer TPE 331-12Uxx-70xH Variant rated at 1010 shp (exact figure varies with mod state). That's quite a bit more power.

 

A very good source of info on the TPE331 is the Pilot Tips document.

cessna729.

 

p.s. I havn't had this much fun with a new "sim" aircarft in a long time.

 

[cruster]

I've mentioned this a couple of times. With the RPM lever at "Flight" (100%), the prop RPM % will slowly climb up to 103% (the white triangle), which is the lower limit of the governor, IIRC from the manual. So far, I haven't had a problem with anything breaking in flights of one to two hours, but I wonder if that will eventually happen. 

[Japo32]

No.. for now now.. but in a future update I will make the break of the engine as the cockpit decal limitations is set. I didn't do it because I was going to change some things inside the plane first. In this 1.01 won't come.. but in next one if I see everything is stable I will introduce that new failure. So better for now fly correctly to learn how to do it when the RPM % failure comes.

[Keith Smith]

Javier, the upcoming failure mode is great, but is it normal to have to back the power off so much in cruise to avoid hitting the prop governor limit?

 

If an engine is rated for xxxx hp, is that at sea level and on a standard day? I notice a critical altitude has been set of 15k...meaning it can produce xxxx hp all the way up to 15k, then THEN it starts to degrade. Unless the engine is supercharged/turbocharged, I'm not sure why a critical alt of > 0 has been set.

[Japo32]

First I used the most powerful engine model of the Jetstream32. and after some testings I found that was the best configuration to maintain almost all the time near the 100%. You can say to Plane Maker, "do this" that he will do "that". Even if you notice at iddle it is not at 72% RPM, where the manuals says that in Taxi position. I didn't find a good manner to do that. I could manipulate for the 72% the throttles so the iddle would be 72% but the plane would have push so any time someone would release parking brakes the plane would move.

There is a parameter for iddle and max RPM and more or less it respect it but yes.. it is true.. when going to flight level it increases to 103%. I want that increase because can be produced. If the mark is there, and it says in the caution limits, it is possible.

 

Even the temperature was always going farther than 650º when I didn't program it. Now I control that (if you set the TTL computer)

 

So yes.. you have to relax throttles at cruise flight... and take care in take offs because also it goes to 103% but as soon as you climb it goes back to 100%