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Tasmanian

Cruise power - what am I doing wrong?

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Cruising over Panama at 20,000ft, total weight 10,500lb or close to. Going from the manual cruise table, at this height and weight, recommended cruise settings are 96% prop speed (check), 72% torque (check), 271 lb/hr fuel flow (check), giving a speed of 203 kts.

 

But I'm getting 175 kts and I'm not sure I can reach my destination at this speed. Any help?

 

(The flaps are up, the gear is up, the ball is centred, system is Windows 64-bit and XP10.25.)

Edited by Tasmanian

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Wayne - nope, I know the difference between IAS, CAS, TAS, and GS. Also, the manual's cruise table lists both IAS and TAS.

 

Ralf - I'm pretty sure I checked for ice, by looking through the failure datarefs with DataRefEditor. But I might be imagining it - maybe it was ice.

 

I don't expect an X-Plane aircraft to perform 100% to the real aircraft's figures. 90% accuracy is pretty good. But 175 vs 200kts is significant enough that I thought I had made a mistake. Also I thought the tables in the manual were for the XP10 MU-2, not the real one, and in that case I'd expect them to be much more accurate.

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My interest in flying by the numbers didn't last long :)

 

I did notice something interesting. The MU-2 has full-span spoilers for roll control, and aileron trim tabs on the back of the flaps for roll trim. But the autopilot can only control the spoilers, so in cruise normally one of the spoilers will be extended slightly as the autopilot holds the aircraft level. By applying roll trim in the direction of the deflected spoiler, the aileron trim surface takes over from the spoiler and the autopilot will retract the spoiler. Possibly this results in more efficient flight? I haven't tested it.

 

And this would explain the 'AIL TRIM SELECT' lights, non-functional in v1.5, which I imagine either tell the pilot that the autopilot is applying roll trim, or tell the pilot they need to apply roll trim manually to get most efficient flight. (Alternatively, make sure the yokes are visible, and apply roll trim in the direction of whichever yoke horn is lower. If the yokes are level, the roll spoilers are both retracted.)

 

Apart from anything else, if the aircraft is trimmed properly under autopilot, it won't try to spiral dramatically when the autopilot is disconnected.

 

Oh. Is this related to the 'TRIM' button on the autopilot control panel? The manual doesn't mention it...

 

Sent from my desktop PC using TapAKeyboard

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One of the things I found with the MU-2 was that, depending on wind speed and direction, I could get an additional 20-30 KIAS by unwinding the left trim that was dialed in for takeoff.

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And this would explain the 'AIL TRIM SELECT' lights, non-functional in v1.5, which I imagine either tell the pilot that the autopilot is applying roll trim, or tell the pilot they need to apply roll trim manually to get most efficient flight.

 

 

Not quite. Its true the autopilot is mechanically linked to the yoke/spoiler cables so the AP moves the spoilers.  The aileron trim tabs on the flaps though......one on the left and one one the right are independently controlled, each side having its own electric motor...... so the trim surfaces are not connected mechanically.  The are interconnected electrically so when one trim tab goes up, the other goes down as expected.  In the event of a single trim motor failure on one side, that renders the "interconnection network" inoperative for both motors.  The trim aileron select switch then allows isolation of the good motor so you can still get roll trim functionality, it just will not not be as effective.  This kind of thing today would be handled with smart, digital electronics, but the MU2 is a pretty old bird.  The aileron trim switch is; therefore, used under the category: "abnormal procedures".   Also, in such a case, with trim on one side only, you'd induce drag on one side only and therefore get a yaw that would need to be countered with some yaw trim. 

 

Because I don't simulate failures on the MU2 to that level with 1.5, that is not simulated.  

 

As far as flight efficiency,  you CAN have a deflected spoiler and deflected trim countering each other to produce a level flying aircaft; however, you would induce large amounts of drag in such cases.  You want as little "surface" protruding into the free airstream as possible to maintain level flight and that is best accomplished by adjusting trim in order to keep the spoilers as flat as possible (which means the yoke should be as level as possible).  You definitely need to adjust trim for any speed change or power change.  When I'm flying on AP, I routinely adjust the roll trim as needed to keep the yoke level.

 

TomK

Edited by tkyler
  • Upvote 1

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...When I'm flying on AP, I routinely adjust the roll trim as needed to keep the yoke level.

 

TomK

 

I've never really paid attention to the yoke, mostly just trimming to get the ball in the middle (so I'm not slipping). In my current flight, I trimmed a little more right than I would have normally, and indeed the yoke is truly level. The ball is slightly on the right lubber line in the inclinometer, though. I'm sure there is probably a data ref I can display that would tell me slip angle but, honestly, a GS of 220 into a headwind of 40kts at FL100, and a fuel burn of only 480 lbs/hr...I'm not going to worry too much.

 

 

edit: slew to slip

Edited by cruster

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Thanks for the explanation Tom, that's very interesting. Perhaps the AIL TRIM SELECT system will have a use when the MU-2 is extended to be a trim motor maintenance management simulator... hey, how inspired are you by the Accusim persistant wear-and-tear maintenancey bits in FSX?

 

Cruster the problem isn't the aircraft slipping - it's not, it's flying wings level and no slip. The problem is it's the roll spoiler which is keeping it level, when it could instead be the aileron-surface trim, and the trim produces less drag.

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hey, how inspired are you by the Accusim persistant wear-and-tear maintenancey bits in FSX?

 

 

 

Certainly I like the idea of incorporating those kinds of features into my simulations, but like everything else, its in a queue of "things to do" that sits lower in the priority list than other stuff in the queue.  We'll get there.

 

TomK

Edited by tkyler

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