M20C elevator position in cruise

[PT20J]

I’m curious about something. The M20J has a 5 deg or so down elevator deflection when trimmed for cruise. I wonder if the shorter C does the same thing. Anyone looked back at your tail?

[Immelman]

I haven't looked back lately (E model), but it will depend on speed and CG.

Edited February 1, 2019 by Immelman

[Bryan]

My M20K is the same way, or maybe the elevator it commanding an up pitch about 5d; I’ll have to look next time I fly again but I have wondered the same thing... while at cruise.

[N201MKTurbo]

You can adjust that angle with the spring bungees in the tail. It turns out that the lowest drag position is that weird angle. They figured that out a long time ago at the factory.

[Hank]

This was discussed at length several years ago (2012? 2013?), complete with pictures. The elevator of my C droops slightly in cruise; I expect the amount of droop to vary with weight and altitude.

[PT20J]

The Mooney trim system adjusts the angle of incidence of the stabilizer as I'm sure everyone knows. The stabilizer incidence moves through a range of about 6 degrees for the M20J. It's not enough for the full trim range required, but it's impractical to get more range from the stabilizer -- it already takes a lot of rotations of the trim wheel just to get 6 degrees. The trim assist bungees bias the elevator to deflect along with the stabilizer movement to increase the effectiveness of the trim system.

The interesting question is why isn't the elevator faired with the stabilizer in cruise? Back in the early '90's, Lowell Foster (engineer at Mooney) told me that he thought this was a side effect of lengthening the fuselage for the F and that LoPresti had noticed it but calculated that it didn't contribute enough drag to make it worth the effort to fix it. But, Lowell's response to a lot of questions was that no one really knew since there had been so many management and staff changes in engineering over the years that the reason many things were the way they are had been lost.

So, if Lowell was right, the C elevator should be more closely faired than the F or J. It makes sense that lengthening the fuselage would cause a downward deflection in the elevator because the longer moment  arm would require less tail down force to create the same pitch moment. And I can see that the increase in drag would be negligible because the deflection is so small and offset by the lessened tail down force. Still, I've often wondered about it.

Skip

[N201MKTurbo]

1 hour ago, PT20J said:

The Mooney trim system adjusts the angle of incidence of the stabilizer as I'm sure everyone knows. The stabilizer incidence moves through a range of about 6 degrees for the M20J. It's not enough for the full trim range required, but it's impractical to get more range from the stabilizer -- it already takes a lot of rotations of the trim wheel just to get 6 degrees. The trim assist bungees bias the elevator to deflect along with the stabilizer movement to increase the effectiveness of the trim system.

The interesting question is why isn't the elevator faired with the stabilizer in cruise? Back in the early '90's, Lowell Foster (engineer at Mooney) told me that he thought this was a side effect of lengthening the fuselage for the F and that LoPresti had noticed it but calculated that it didn't contribute enough drag to make it worth the effort to fix it. But, Lowell's response to a lot of questions was that no one really knew since there had been so many management and staff changes in engineering over the years that the reason many things were the way they are had been lost.

So, if Lowell was right, the C elevator should be more closely faired than the F or J. It makes sense that lengthening the fuselage would cause a downward deflection in the elevator because the longer moment  arm would require less tail down force to create the same pitch moment. And I can see that the increase in drag would be negligible because the deflection is so small and offset by the lessened tail down force. Still, I've often wondered about it.

Skip

You can check the drag and speed and find the optimal setting by getting the plane to a stable cruise on a smooth day. Then run the trim nose down a bit and muscle the plane back to level (you are the bungee). Do this 1/4 turn of the trim wheel at a time and record the speed. Then do it nose up. If you can (it helps to have a helper) look back and note the relationship of the elevator to the horizontal stabilizer. By doing this you can find the optimal trim to elevator setting for the least drag. You will find it is pretty close to the factory setting.

[Yetti]

Has anyone experienced getting the nose down like you are riding a wave or getting it on a plane like a boat.

[carusoam]

There is a discussion recently of somebody experiencing the effects of mountain waves... very much surfing like...

Long Bodies also have the same elevator visual effect. They also have bigger tail feathers to some extent...

Best regards,

-a-

[Tom Johansen]

Based on visuals, my M20C flew with elevator down maybe 5 degrees. Many long cross country flights, altitude up to 16,000'. Its TAS was always decent so I didn't worry about it.

[Bryan]

Here is my M20K in cruise, level flight trimmed.

[carusoam]

Welcome aboard TJ.

Flying a C in the FLs would be cool!  16K’ is pretty close.

Best regards,

-a-

[orionflt]

Here is mine in cruise 

brian

[Andy95W]

I like the paint scheme- part original, part later style.

[PT20J]

2 hours ago, Bryan said:

Here is my M20K in cruise, level flight trimmed.

Always nice to have the thunderstorms behind you . This looks like the elevator is deflected up maybe 8-10 degrees. IIRC the K has a variable downspring like the long bodies. So, I wonder if all the models with the downspring trim in cruise with the elevator up. 

[PT20J]

2 hours ago, orionflt said:

Here is mine in cruise 

brian

Nice picture. Looks like the elevator is aligned pretty well with the stabilizer as expected. 

[ArtVandelay]

Here is my M20K in cruise, level flight trimmed.

My J is the opposite of yours when I’m solo, did you have 2 up front?

 

Tom

[cliffy]

 

As discussed a while back most short bodies cruise with the elevator a few degrees nose down. Mine does also. CG will vary it some. Bill Wheat told me that the bungees lower the trim drag over more stabilizer angle for the same effect. 

As shown below the elevator "set" angle is set with the stabilizer nose down 3 1/2 degrees and adjusting the bungees so the elevator sits at 19 degrees and this is done with the FWD end of the bungees at the rod end (not the spring case in the back). 

"Elevator Trim Assist Unit With stabilizer set at 3½° negative setting to the thrust line, adjust trim assist unit (740044) for elevator up angle of 19° ± ½° at the zero spring travel position."

The above from the TCDS BUT note- Up to S/N 690001

 

Now here's an interesting anomaly-

Further down on the TCDS for subsequent S/Ns all the angles change  I have printed the TCDS but the formatting didn't hold. 

Early short bodies

Elevator up 24 down 10 1/2   Stabilizer up 1 to 2 1/2 down 4 1/2 to 5 Bungee setting 19 degrees up

Later short bodies

Elevator  up 22 down 22, stabilizer  up 1/2 to 1 down 5 1/4 to 5 3/4 Bungee setting 10 degrees

What changed in the short bodies? I don't know and no one I have talked with will say anything. I seem to recall way back in my memory (but its fuzzy) that the FAA dinged them on elevator control force at stall ( not enough down force at 10 1/2 degrees to guarantee stall recovery. But I can't say for sure. 

Now my question is which settings are faster? 

BTW, even though the bungees are different part numbers. I have the factory drawings on them and the appear to be identical except for a couple minor changes.  TCDS cut below.

 

Control Surface Movements (Aircraft with serial numbers up to 690001) Wing Flaps .................................... T.O. Position .... Down ........ 15° ± 1° ....................................................... Landing ............ Down ........ 33° + 0°, -2° Aileron..............................Up ....... 12½° to 17° ...... Down ........ 8° ± 1° Aileron static position............................................... Down ........ 0° to 2° Elevator ...........................Up ....... 24  1° ............ Down ....... 10½° ± 1 Rudder ............................ Left ....... 23° to 24 ......... Right ........ 23° to 24° Stabilizer (L.E.) ................Up ....... 1 to 2½°............ Down ....... 4½ + 5 Elevator Trim Assist Unit With stabilizer set at 3½° negative setting to the thrust line, adjust trim assist unit (740044) for elevator up angle of 19° ± ½° at the zero spring travel position.

(Aircraft with serial numbers 690001-700091, 20-0001 and up) Wing Flaps .................................... T.O. Position .... Down ........ 15° ± 1° ....................................................... Landing ............ Down ........ 33° + 0°, -2° Aileron..............................Up ....... 12½° to 17° ...... Down ........ 8° ± 1° Aileron static position............................................... Down ........ 0° to 2° Elevator ...........................Up ....... 22  2° ............ Down ....... 22° ± 2 Stabilizer (L.E.) ................Up ....... ½° to 1 ............ Down ....... 5¼ to 5¾ Rudder ............................ Left ....... 23 to 24° ......... Right ........ 23° to 24° Elevator Trim Assist Unit With stabilizer set at 3½ negative setting to the thrust line, adjust trim assist bungees (740188) for elevator position of 10° ± 1 at the zero spring travel position of the bungees. Leveling means Edge of skin splice over aft fuselage radio access panel. Spirit level is used to level.

[Hank]

For my 1970-C, in cruise the elevator counterweight is about half exposed above the horizontal stabilizer. Photos from my digital camera were posted back in 2012? or so

[Bryan]

6 hours ago, ArtVandelay said:

My J is the opposite of yours when I’m solo, did you have 2 up front?

 

Tom

Single pilot, full fuel.

[orionflt]

Here is another angle from the same flight. 3 people on board. Half tanks. 

Brian

[PT20J]

9 hours ago, cliffy said:

Now my question is which settings are faster? 

Interesting question. I doubt it makes much difference. Trim drag is primarily the incremental induced drag caused by the wing flying at a higher angle of attack to support the tail down force, and, to a lesser extent, the induced drag of the tailplane creating the tail down force.  Near the trim point, any small elevator deflection necessary to generate the tail down force should cause negligible parasite drag.

Anyone know why the K and later models switched from the trim assist bungees to a variable downspring? I'm guessing that as the speeds increased, the stick force gradient with airspeed was insufficient to give good stick "feel" around the trim point at higher airspeeds which would be the usual reason for adding a downspring, but I don't know for certain. From Bryan's picture, this change seems to have caused the elevator trim position to shift from slightly nose down to slightly nose up.

[carusoam]

Adding to your observation PT...

The Long Bodies don’t seem to have the springs.... the elevator sits full down while on the ground....

Best regards,

-a-

[PT20J]

11 hours ago, carusoam said:

The Long Bodies don’t seem to have the springs.... the elevator sits full down while on the ground.

Yep, that’s the down spring that started with the K. 

[DonMuncy]

Isn't the position of the elevators totally a function of weight and speed. I would think that a few knots difference in cruising speed would change the elevator position enough, you would have a hard time drawing any real conclusions about them.