Mooney elevators, Mine at rest are up a few degrees, 1966 E, saw a 1969 E yesterday on the ramp, at rest elevator is down...

[asaxet]

Silly question (maybe) but was there a change in design ???   rigging ???   TIA

 

 

[ShuRugal]

In my 64 C model, there are a set of bungee springs which mix stab trim position into the elevator pushrods in the tailcone. I always park with trim fully nose down to unload these springs, but the elevator still rests on them.

I think the later year models don't have this spring, and trim only effects the stab position?

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[Hank]

Our shortbodies should be pretty level when parked. Starting with the K, elevators droop a lot.

[N201MKTurbo]

It depends where you have the trim set when you shut down.

[Hank]

28 minutes ago, N201MKTurbo said:

It depends where you have the trim set when you shut down.

The trim wheel moves the horizontal stabilizer, the elevator itself should be pretty flat. Full up trim will make it look like Down Elevator, but it isn't. I often land with trim near the Takeoff mark, and find my tail is generally pretty flat. Like this.

[PT20J]

2 hours ago, N201MKTurbo said:

It depends where you have the trim set when you shut down.

This is correct. On all the models up through the J (The K and later have a bob weight and down spring) there are "trim assist bungees" that move the elevator as the trim changes the angle of incidence of the horizontal stabilizer. The elevator should be pretty much in trail with the trim in takeoff position on the ground, but there is some friction in the elevator system and if you move the elevator by its trailing edge, it will stay a bit up or down depending on where you release it.

Skip

[Prior owner]

1 hour ago, Hank said:

The trim wheel moves the horizontal stabilizer, the elevator itself should be pretty flat. Full up trim will make it look like Down Elevator, but it isn't. I often land with trim near the Takeoff mark, and find my tail is generally pretty flat. Like this.

A short body with that much up stabilizer trim should have the elevators much further up….with leading edge of stabilizer 3.5 degrees down, the elevators should be 10 degrees up, per the TCDS….(19 degrees if it is a converted D model) The plane in the photo looks to have the stabilizer at at least 2/3 up trim. His elevators look low to me- a bungee adjustment will fix it. This plane probably will not trim at 75-80 mph hands off with full flaps, as it should.

[markgrue]

I took some pictures of my D/C aircraft with the trim full down then take off then full up trim.  

Mark

[PT20J]

Looks right. If the airplane didn’t have the trim bungees, the stabilizer would have to move through a greater angle to effect the same trim range. The bungees also act as centering springs which improves the airplane’s airspeed stability.

Skip

[carusoam]

Fancy….

More trim comes with more apparent camber….

That Al Mooney guy must have been brilliant!

…

 

Why did the Long Bodies do away with the springs…. The elevator hangs from its hinges with the yoke pressed against the instrument panel…

Best regards,

-a-

[PT20J]

35 minutes ago, carusoam said:

Fancy….

More trim comes with more apparent camber….

That Al Mooney guy must have been brilliant!

…

 

Why did the Long Bodies do away with the springs…. The elevator hangs from its hinges with the yoke pressed against the instrument panel…

Best regards,

-a-

It took me a long time (and a lot of emails back and forth with Ron Blum) before I understood the bungees. There are lot’s of ways to build a trim system. The most common is the trim tab. Another way is the variable incidence stabilizer like a C-180. Or, you can do it all with bungees like the rudder trim on some Cessna singles. Mooney just used a combination of the latter two methods.

Probably no one remembers why the system was changed starting with the K. Bob Kromer told me that he was told that initial flight tests of the K (which were before Bob joined Mooney) showed undesireable characteristics, which apparently were unpredicted, and that required the change. But he didn’t recall anyone ever elaborating on what problem the were trying to solve. 

The K and all the long bodies have a bob weight and down spring. Bob weights are used to increase maneuvering stability by increasing stick force per g: The harder you pull back, the more g’s you pull and the more the bob weight pulls against you. The down spring might have been to improve low speed airspeed stability: at lower TAS, the elevators are less effective and you have to pull back more on the controls to change pitch which stretches the spring and it pulls against you.

Skip

[David Lloyd]

I've got a set of pictures somewhere that look just like those markgrue. They were taken to illustrate the vertical tail compared to the hangar siding at full up, take off and full down.  I had forgotten how the bungees cause the elevator to move so much.

[ArtVandelay]

Is there an SB somewhere that allows you to remove the bungees?
I remember seeing a Mooney on the ramp that had elevators that freely moved.

[cliffy]

13 hours ago, ArtVandelay said:

Is there an SB somewhere that allows you to remove the bungees?
I remember seeing a Mooney on the ramp that had elevators that freely moved.

No not on the ones that came with them They are integral to the aircraft stability certification. 

You may have been looking at a later model.

NOW the question I have never had an answer to is?

Why did the elevator throws change on the 69 and on short bodies ? From 24 up 10 down (68 and earlier) to 22 each way (69 and on)? 

[carusoam]

There was plenty of evolution going on related to the tail structure and set points… slowly over time and models…

Starting with…

1) wood structure of the M20A…

2) M20B had the short metal rudder, with the short rudder throw…

3) M20C had the short rudder, but improved the rudder throw… then improved again to get the long rudder….

4) Then Somebody also adorned it with a stinger tail, and a funky button hook on the top….  

5) Then adjusting the elevator throws a couple of degrees from being centered….  I’m familiar with the change… but don’t know the reasoning…

Expect that it has something to do with….  The phrase… “you can never have too much trim in a Mooney….”

I heard this during my less than ideal, non-Mooney specific transition training….

Very often, during the landing phase… with only two front seat passengers… the trim would be pretty close to max up….

I have never used the max down trim position even when loaded heavily toward the rear, and going super fast, near Vne….

 

How does that sound for a technical conversation set up?  Could just be evolution… it took that long to get there…

With the M20R… I never bump into the full up trim limit… I’m extra cautious of this, because the electric trim driving the system gets one hand on the wheel monitoring that it doesn’t hit the stops…

PP thoughts only, not a mechanic…

Best regards,

-a-

[cliffy]

Seems strange that they changed it for 69 and on but didn't require retrofit to earlier  IF there was a pitch stability issue they were trying to address (which is the only reason I can think of for the change). Pitch stability issue somewhere at high AOA angles needing more down force?

Why the change and why not retroactive

Aeroengineers your phone is ringing

[Prior owner]

9 hours ago, cliffy said:

Why did the elevator throws change on the 69 and on short bodies ? From 24 up 10 down (68 and earlier) to 22 each way (69 and on)? 

Better spin recovery??

[cliffy]

I've only got a vague memory of this issue from 40+ years ago but I seem to remember something about the FAA wanting to reevaluate the short body for either stall recovery way out at the corners of what was required or spin recovery at the same parameters (out at the ends of the required spectrum). I don't have a clear memory of it but it was something along those lines. I've asked for 15 years of anyone I could find if they had any idea why but all I got was crickets, Even at the factory when Bill Wheat was there and others, no one could "remember" why. 

[PT20J]

According to the TCDS, the change was made with M20C S/N 690001. Along with the change in elevator movement, the stabilizer incidence and trim bungee part number and setting changed. May be related to the increase in Vne and Vno that occurred at the same time. Maybe earlier models were running out of nose down trim at higher speeds.

Skip

[Hank]

From your photos, it looks like things began changing in 68, and were finalized in 69. Thankfully my C is a 1970 model. 

[cliffy]

I got the bungee blue print for both bungees right from Mooney when I was there and I haven't found any difference in them  Haven't looked at them for several years I'd have to find them in my papers somewhere.  

If nose down trim was the culprit I would think there would be a tremendous amount of trim drag then as the speed went up. I'm  more apt to think in terms of low speed issues.   

I wonder how much fwd shift in C of P there is as speed increases with our airfoil? Not a aero engineer so I have now idea how to figure it. 

[SheryLoewen]

It depends where the trim is

[Prior owner]

FYI, when I bought my plane, it had bungees from an F model installed. They are a different length, and it greatly affected the position of the elevator in different trim positions. I discovered this when I was unable to rig the elevators per the TCDS.

Here are the wrong ones I removed:

 

 

[carusoam]

Great details with pic Coyote!

Go MS!

Best regards,

-a-