Elevator down spring

[A64Pilot]

1 minute ago, Fly Boomer said:

Maybe someone who has done it will chime in, and make a suggestion.  The one-piece is enough trouble to take off, so I suspect the "zillion" will be tedious.

It’s not bad at all not in my opinion, he probably only needs the big one pulled.

Now back in the day before rechargeable drills I bet it was a pain with just a screwdriver.

[PT20J]

If you set the trim where it is in cruise (quite a bit more nose down than the takeoff position) and hold the elevator in the position it sits in cruise, you’ll note that there is quite a bit of spring tension. That force opposes the hinge moment so that the stick force is neutral  without having to increase the incidence of the tail farther. That’s why they are called “trim assist bungees.”

[A64Pilot]

8 minutes ago, PT20J said:

If you set the trim where it is in cruise (quite a bit more nose down than the takeoff position) and hold the elevator in the position it sits in cruise, you’ll note that there is quite a bit of spring tension. That force opposes the hinge moment so that the stick force is neutral  without having to increase the incidence of the tail farther. That’s why they are called “trim assist bungees.”

I guess you would have to load the spring for it to be effective, if left centered it would be “soft” when centered and that wouldn’t pass.

As I said I’m used to springs that pull the stick forward only so that in any position but full down they are pulling down, they work because the trim tab fights the spring so when trimmed the forces balance, just apparently like a Mooney does.

The adjustable angle of incidence tail is unusual though, but aerodynamic trim force will exceed any spring force, it has to, in order to work.

[PT20J]

Another interesting hangar experiment (on Js and earlier models) is to run the trim all the way nose down and observe the elevator position, and then repeat with the trim all the way nose up. 

[Pinecone]

16 hours ago, PT20J said:

A Mooney engineer once told me that Lopresti calculated that it didn’t make enough difference to bother fixing which would have required an expensive redesign of the empennage to change the incidence of the stabilizer.

Except our stab is variable incidence. 

[PT20J]

2 minutes ago, Pinecone said:

Except our stab is variable incidence. 

But apparently the “center point” needed to be changed.

[Pinecone]

How would that be?   If I trim, I change the incidence of the stab.   The elevator will be under the influence of the airflow and the springs.

Unless the spring "center" is a bit off.  Not sure what it would be a huge deal to change that.

[skykrawler]

On 12/5/2023 at 12:08 AM, Hank said:

The 172s I learned to fly in all had trim tabs. Surely being lighter, slower and lower powered than my Mooney, their control forces are not higher without the trim tabs???

You missed the point.  Mooney's trim with stabilizer, don't need a servo tab to add control power.    Cessna's trim with the tab which introduces force for trimmed flight.

I've only looked closely at a 1980K.   The trailing edge of the elevator is extended about an inch more than a J by an extra piece of sheet metal.  I presume to provide additional authority for the power plant installation affect on CG.

[PT20J]

34 minutes ago, Pinecone said:

How would that be?   If I trim, I change the incidence of the stab.   The elevator will be under the influence of the airflow and the springs.

Unless the spring "center" is a bit off.  Not sure what it would be a huge deal to change that.

The springs and incidence change work in concert to trim out the stick force. But, the range of airspeeds and flap settings over which the trim functions is important also. In the M20J the elevator is trailing edge down when in trim regardless of airspeed. To get the same down force with the elevator in trail, the angle of incidence would need to be increased and the spring center point adjusted. But if this were done without changing the physical angle of mounting of the stabilizer to the empennage, it would affect the trim range.

[Hank]

18 hours ago, PT20J said:

Another interesting hangar experiment (on Js and earlier models) is to run the trim all the way nose down and observe the elevator position, and then repeat with the trim all the way nose up. 

This is fun to do with a new owner! The looks on their faces . . . .

4 hours ago, skykrawler said:

You missed the point.  Mooney's trim with stabilizer, don't need a servo tab to add control power.    Cessna's trim with the tab which introduces force for trimmed flight.

I'm aware of both of these. I guess I missed the point you were trying to make about the relationship between trim tabs and stick forces. Never mind.

[Guest]

I found it!  Thankfully it was just under an inspection panel. I don’t know what happened, but one of the screws holding on the inspection panel started rubbing on the elevator push tube. I removed it a will get a shorter screw. I don’t know why it would start doing that though. I just changed a battery, maybe a weight change?  Who knows, but no rubbing scraping noise makes me happy.  Still wonder what caused it. 

[Fly Boomer]

3 minutes ago, Kerrville said:

I found it!  Thankfully it was just under an inspection panel. I don’t know what happened, but one of the screws holding on the inspection panel started rubbing on the elevator push tube. I removed it a will get a shorter screw. I don’t know why it would start doing that though. I just changed a battery, maybe a weight change?  Who knows, but no rubbing scraping noise makes me happy.  Still wonder what caused it. 

Excellent!  We love success stories.

[PT20J]

1 hour ago, Kerrville said:

I found it!  Thankfully it was just under an inspection panel. I don’t know what happened, but one of the screws holding on the inspection panel started rubbing on the elevator push tube. I removed it a will get a shorter screw. I don’t know why it would start doing that though. I just changed a battery, maybe a weight change?  Who knows, but no rubbing scraping noise makes me happy.  Still wonder what caused it. 

This happens during maintenance when the wrong length screws are used. You might want to check the aileron tube inspection panels also. There is at least one where a long screw will rub on a tube. It doesn’t always make a noise. 

[Guest]

The screws were all the same length. The rear screw needs to be just a hair shorter. Or perhaps they used all the wrong screws. I will check the aileron tubes now too. Thanks. I’d hate to have a 15 cent screw destroy a flight control tube. 

[Pinecone]

8 hours ago, PT20J said:

The springs and incidence change work in concert to trim out the stick force. But, the range of airspeeds and flap settings over which the trim functions is important also. In the M20J the elevator is trailing edge down when in trim regardless of airspeed. To get the same down force with the elevator in trail, the angle of incidence would need to be increased and the spring center point adjusted. But if this were done without changing the physical angle of mounting of the stabilizer to the empennage, it would affect the trim range.

Yes, but that would only effect the ends of the trim range.  Which is not where the trim is during cruise.