800XP Question

[IFLYHI]

Hi all,

I've noticed something odd while flying both a Honneywell 800XP and a Proline 850XP.

When the engine synch is turned on (fan or turbine), both N1s quickly drop about 1.5 percent. However, nothing else (N2 and fuel flow) changes. If the synch is turned back off, the N1s returns to its previous value.

So my questions are; Why does this happen? Is this somehow caused by the DEEC? Is the N1 reading accurate with the synch on or off?

Any ideas?

Greg

 

[Grizz]

It's the flux capacitor kicking in.

 

[Archie Bunker]

I always thought it was the #7 Fetzer valve...but I could be wrong.

 

[charter dog]

Just a theory here, but the N1 change upon selection of eng synch may be related to the fact that the 731-5 (including the -5BR) family of engines have N1 compensators. A brief explanation of the purpose of N1 compensation:

The fan speed which produces a given amount of thrust varies somewhat from engine to engine. N1 compensators are installed to compensate for this difference. Without compensation, one engine may require a higher indicated N1 than the other to achieve equal (assumed) thrust. Compensation value is determined in test cell runs at the factory. Each DEEC is programmed with the compensation value for it's respective engine and controls the engine N1 ( by controlling the N2) so as to achieve the programmed thrust value. With eng synch OFF, as during T/O and landing, the compensated value is displayed on the N1 indicators. The actual RPM will be different than the indicated RPM according to the compensation value for each engine. Engine synch is accomplished by a synch circuit connected through the DEECs (the DEECs receive uncompensated N1 signal) by comparing the raw, uncompensated RPM of each engine (either N1 or N2) in order to synchronize the selected rotating groups by altering one of the engine's N1 or N2 (as selected) and achieve the most comfortable vibration characteristics.

The XP I used to fly did the same thing. My theory is that when the synch is selected on, the N1 guages now reflect the raw, uncompensated value. This is what would interest you for the purpose of RPM synchronization, where the compensated value displayed for takeoff is more useful in achieving an assumed value of thrust to meet certified takeoff performance.

If the compensation value of either engine is signifigantly different than that of the other, the master engine N2, Wf (fuel flow) and ITT will remain stable while the slave engine will shift slightly to match either the N1s or N2s according to the synch switch position. N2, Wf and ITT must change to reflect any ACTUAL change in N1. You are most likely seeing the difference between compensated and actual N1 when you switch the synch on. If no shift in N2, Wf and ITT takes place, it probably means that your engines have nearly equal values of N1 compensators installed. So you see only the difference between compensated and uncompensated N1 value. Less equally compensated engines may, when RPMs are matched, exhibit noticably unequal thrust. The plane I currently fly requires a notable rudder trim offset when N1s are synched. Oh, well...

The 800XP is a pretty good ship. Enjoy her!

Best,

 

[IFLYHI]

Thanks, that makes some sense. I'll see what Simuflite's opinion is when I go next week.

Right now I'm flying an 850XP with Proline and paperless cockpit. I think I can learn to like it

 

[Bandit60]

Thanks, that makes some sense. I'll see what Simuflite's opinion is when I go next week.

Right now I'm flying an 850XP with Proline and paperless cockpit. I think I can learn to like it

WOW your flying an 850XP. Are you a test pilot for Ratheon? Because the 850XP just received certification this past month. If you are flying a 800xp with winglets that came from Aviation partners then you are flying a 800xp2.

 

[h25b]

I always thought it was the #7 Fetzer valve...but I could be wrong.

That ain't 'nothin some 30 weight ball bearings can't fix...

(I think that was the line in the movie "Fletch" ???)

 

[IFLYHI]

No, It's an 850XP. The company I fly for took delivery of the first one about a month and a half ago. Its sweeeeet.

 

[HawkerDOG]

True FlyHI! the 850XP is a flying machine. We have two 800XP's with aviation partner's winglets. I love them!!
As for the compensator question, CharterDog hit it on the head although Stimuflight probably won't know the answer. Ask for Allan Simmons, he will tell you the strait up on it...

 

[TORCH67]

Hawkerdog are you in Casper?