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K-model elevator rigging


geoffb

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The K doesn't have the interconnect between stabilizer and elevator that the E had, but does have the big block of lead on the control shaft that keeps the elevator full down at rest.  Have to admit I don't understand the mechanics of it yet.  But, what I do notice it that in cruise, at all weights and with as much CG change as I can muster, it's running with up elevator.  Thought maybe it was the AP not auto trimming correctly, but even if I hand fly and manually trim, level flight has some up elevator.  I assume this isn't correct and must be costing me speed.

This a rigging issue?  Reading the maint manual leaves me baffled.

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4 hours ago, geoffb said:

The K doesn't have the interconnect between stabilizer and elevator that the E had, but does have the big block of lead on the control shaft that keeps the elevator full down at rest.  Have to admit I don't understand the mechanics of it yet.  But, what I do notice it that in cruise, at all weights and with as much CG change as I can muster, it's running with up elevator.  Thought maybe it was the AP not auto trimming correctly, but even if I hand fly and manually trim, level flight has some up elevator.  I assume this isn't correct and must be costing me speed.

This a rigging issue?  Reading the maint manual leaves me baffled.

Normal. All models up to and including the J had "trim assist bungees" which are centering springs with the centering point attached to the trim system so that it changes with the trim setting. Originally they trimmed with the elevator aligned with the stabilizer. I haven't flown every model but I believe that all the "mid" bodies trim with the elevator slightly trailing edge down. For sure the M20J does this. The M20K began a succession of airframes with more weight forward and this changed the flight characteristics necessitating a change in the control system. The K and subsequent models have a variable downspring with tension adjusted according to trim and a bob weight. Bob weights are added to increase maneuvering stick forces (stick force per G) which is perceived by the pilot as greater stability. All the airplanes with this system trim with the elevator slightly trailing edge up. 

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14 minutes ago, geoffb said:

So built-in drag?

Not significantly. All that the small elevator deflection does is change the camber of the horizontal stabilizer. The air flows smoothly over it (like a wing) so there isn't a lot of drag increase. Trim drag is mainly induced drag. Some from the tail because it creates a tail down force (downward lift if you will) which creates vortices and drag, and more from the wing that must generate lift in addition to the weight to support the tail down force of the tail and this lift strengthens the vortices and creates more induced drag. Look at it this way: When Roy Lopresti was creating the "201" he was looking for the easiest ways to reduce drag. If the slight elevator deflection had been a significant issue, he would most certainly have fixed it.

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You can test all this yourself. Trim the plane and muscle the pitch with the yoke until the elevator is in trail and note the airspeed. Do this with elevator up and the elevator down . Take as many data points as you like. Plot the data and find the peak airspeed. You may be surprised to find the peak at the factory setting. They didn’t put all that mechanism in there by accident.

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1) Take pic

2) Post pic

3) Weight for gravity to adjust pic

4) Compare pic to other Mooneys

5) Discuss...

 

Expect the amount of down force needed varies with the WnB and the environment of the day...

Load the plane up with back seaters,  check the WnB to be in the back limit... then repeat steps 1-5 above...

What I believe you are witnessing... is all the flexibility required to fly a real plane, in a real world, with real variables of loading...   :)

Now you know why people like to fly with their tool box strapped down in the baggage compartment all the time...  the aerodynamic downforce is a touch draggier than a box of tools...

PP thoughts only, not a CFI...

Best regards,

-a-

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