Elevator down spring

[Guest]

I was just moving my elevator with the aft equipment bay open and heard a squeaky sound like a spring. I looked in the parts manual and found there are 2 bungee springs on the elevator control. Can someone explain the purpose?  Are they just for control feel, or is there an aerodynamic reason?

[1980Mooney]

43 minutes ago, Kerrville said:

I was just moving my elevator with the aft equipment bay open and heard a squeaky sound like a spring. I looked in the parts manual and found there are 2 bungee springs on the elevator control. Can someone explain the purpose?  Are they just for control feel, or is there an aerodynamic reason?

Which model? year?

[PT20J]

All the models through the M20J had "trim assist bungees" which are elevator centering springs with the center point variable according to the trim setting. The primary effect is to bias the elevator with trim changes so that the angle of incidence of the stabilizer doesn't have to change as much. A secondary effect would be to improve handling qualities. The bungees shouldn't squeak. They are supposed to be lubricated during annual inspection.

Starting with the M20K, Mooney changed the system to use a down spring with variable tension according to trim, and a bobweight. This was done apparently to improve handling qualities. 

 

[A64Pilot]

Purpose of elevator down springs and or bob weights or anti-servo tabs is to expand the CG envelope.

We used both springs and anti-servo tabs on the Thrush

Usually the aft CG limit is determined by when the control forces get too light, FAA requires a certain amount of push back in a flight control, as the CG moves aft those forces get lighter, at the stick force neutral point there is no push back and further aft than that and the controls if you let go of them would go full travel one way or the other. Airplanes must be positively stable in pitch.

There are several ways to add push back or “feel” to a system, springs is one way, with springs you’re fighting the spring of course.

Trivia but every helicopter I know of has no control “feel” so spring cartridges are added to the cyclic in both axis, spring force is all there is to  return the cyclic to center.

[skykrawler]

The down spring is a common solution to provide a heavier elevator control for pitch up when the aircraft approaches stall.  It provides a built-in nose down tendency.

 

[Guest]

Interesting. I’m just surprised that is necessary on a plane like the 20j I have. Many of the jets I have flown have all the control feel generated by springs, but I have not seen that in a light aircraft.  Expanding the cg envelope makes a lot of sense. Thanks all for the good explanations. 

[Guest]

Mine are squeaky though… perhaps it got missed at the last annual. 

[A64Pilot]

39 minutes ago, Kerrville said:

Interesting. I’m just surprised that is necessary on a plane like the 20j I have. Many of the jets I have flown have all the control feel generated by springs, but I have not seen that in a light aircraft.  Expanding the cg envelope makes a lot of sense. Thanks all for the good explanations. 

Just about every aircraft have something.

Personally I don’t like bob weights because they use gravity of course and in a neg G it’s forces reverse, but Cessna has used them with good success.

A servo tab of course lightens control forces, usually seen on aircraft with bigger control forces, a servo tab when you move the flight control moves opposite the control movement, an anti-servo tab moves in the same direction as the control surface adding force to move the control surface. I have seen control horns which reduce the forces and anti-servo tabs which add force used in the same flight control, seems to not be logical. I theorize as the aircraft evolved the CG envelop need to move aft.

Thrush is that way with both control horns and anti servo tabs on the elevator. 

[Ragsf15e]

12 minutes ago, A64Pilot said:

Just about every aircraft have something.

Personally I don’t like bob weights because they use gravity of course and in a neg G it’s forces reverse, but Cessna has used them with good success.

A servo tab of course lightens control forces, usually seen on aircraft with bigger control forces, a servo tab when you move the flight control moves opposite the control movement, an anti-servo tab moves in the same direction as the control surface adding force to move the control surface. I have seen control horns which reduce the forces and anti-servo tabs which add force used in the same flight control, seems to not be logical. I theorize as the aircraft evolved the CG envelop need to move aft.

Thrush is that way with both control horns and anti servo tabs on the elevator. 

Meridian is that way too.

[Hank]

1 hour ago, A64Pilot said:

A servo tab of course lightens control forces, usually seen on aircraft with bigger control forces, a servo tab when you move the flight control moves opposite the control movement, an anti-servo tab moves in the same direction as the control surface adding force to move the control surface. I have seen control horns which reduce the forces and anti-servo tabs which add force used in the same flight control, seems to not be logical. I theorize as the aircraft evolved the CG envelop need to move aft.

Thrush is that way with both control horns and anti servo tabs on the elevator. 

Servo tabs add drag. That's why our Mooneys don't have them.

[PT20J]

1 hour ago, Hank said:

Servo tabs add drag. That's why our Mooneys don't have them.

No, Mooneys don't have them because the control forces are not heavy enough to require them.

[PT20J]

I asked Bob Kromer why Mooney changed the gadgets in the elevator system between the J and K. It was before his time. He asked around and no one remembers past saying that they expected the the handling characteristics of the K to be similar to the J and were surprised that the stability, handling and stall characteristics were different. The change was to fix that.

A pilot senses what is called stick free stability when controlling an airplane. (There is also a stick fixed stability, but we don't fly with the stick held rigidly in place. Instead we allow the control surfaces to float to their trimmed position, so it is the stick free stability that concerns us). The normal sense of this is that to increase airspeed relative to trim speed requires a push and to decrease airspeed requires a pull. As the CG moves aft, the forces become less and at a point called the neutral point, they vanish. If the CG is moved farther aft, the forces reverse requiring a push to slow down and a pull to speed up. Adding a down spring (which is usually designed to have a constant force) allows the pilot to still have normal control feel with airspeed changes with the CG farther aft than without the spring. In a sense, it is making up for aerodynamic forces that have become too light. Also, the down spring can alter the control feel at stall by increasing the down stick force. The spring doesn't really affect the actual stability of the airplane, but it allows the pilot to control it normally in situations that would be problematic without the spring. A bobweight works the same way by pulling forward on the stick. But since it is a mass affected by g force, it has the added ability to adjust the stick force per g which is important in maneuvering flight. Too light a stick force per g can result in the pilot accidentally overstressing the airplane during abrupt maneuvers.

Skip

[Hank]

1 hour ago, PT20J said:

No, Mooneys don't have them because the control forces are not heavy enough to require them.

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???

[Pinecone]

Trim tab is different than a servo or anti servo tab.  But you can also combine a trim tab and servo/anti servo tab.

And in some aircraft, the controls connect to the servo tab, which moves the control surface.  There is no direct connection from the control stick/yoke to the control surface.

In the A-10, if you lose hydraulics, you would go into Manual Reversion, which operated this way for the ailerons.  The stick controlled the former trim tab as a servo tab, as there as NO WAY you could move the ailerons by muscle force directly.

[Guest]

Where do I access the spring to lube it, and what should it be lubed with?

[PT20J]

17 hours ago, Kerrville said:

Where do I access the spring to lube it, and what should it be lubed with?

You can see the spring cartridges and attach points standing behind the tail and looking into the tail cone where they attach to the elevators. It's easy to see what's going on if you move the elevator up and down while watching the mechanism. I usually shoot some LPS2 into the cartridges and onto the attach points. The service manual calls for MIL-L-7870 (which would now be MIL-PRF-7870), but any light machine oil will do.

 

[N201MKTurbo]

I thought about the spring bungees in the tail, and did some test flights. I came to the conclusion that the spring bungees are a speed mod.

Let me explain...

If you mis-trim your plane and compensate with the yoke, then record the TAS, you will see that the plane goes fastest at the properly trimmed position. This doesn't have the elevator in trail with the horizontal stabilizer. It is not intuitive that this out of trail position would have the least drag. I think the engineers at Mooney did the same tests and added the bungees so the trimmed position was at the least drag combination of horizontal stabilizer angle of attack and elevator angle to get the needed down force in cruise.

[Guest]

May not be the spring. The sound is coming from just forward of the aft equipment bay. I am going to need to open it up and have someone move the elevator to track it down…

[PT20J]

4 hours ago, N201MKTurbo said:

I thought about the spring bungees in the tail, and did some test flights. I came to the conclusion that the spring bungees are a speed mod.

Let me explain...

If you mis-trim your plane and compensate with the yoke, then record the TAS, you will see that the plane goes fastest at the properly trimmed position. This doesn't have the elevator in trail with the horizontal stabilizer. It is not intuitive that this out of trail position would have the least drag. I think the engineers at Mooney did the same tests and added the bungees so the trimmed position was at the least drag combination of horizontal stabilizer angle of attack and elevator angle to get the needed down force in cruise.

The lowest drag would be with the elevator aligned with the stabilizer and it seems that the original design does that. When the fuselage was lengthened it threw things off. My J, for instance trims with the elevator slightly trailing edge down. 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. Lengthening the fuselage again and changing the trim system resulted in the long bodies trimming with the elevator slightly trailing edge UP.

The purpose of any trim system is to remove stick force. This can be done by changing the incidence of the horizontal stabilizer, using a trim tab to bias the elevator, or using springs. The tab by itself is powerful enough because it taps into aerodynamic force. Springs alone can be troublesome because springs strong enough for cruise speed are too strong at low speeds and make the controls too heavy for landing. Changing the stabilizer incidence may require too much trim wheel rotation with the gearing necessary to make the trim wheel easy to turn. So, it is not uncommon to use a combination of springs and variable incidence. I believe the Piper Cub does that also.

Skip

[A64Pilot]

5 hours ago, N201MKTurbo said:

I thought about the spring bungees in the tail, and did some test flights. I came to the conclusion that the spring bungees are a speed mod.

Let me explain...

If you mis-trim your plane and compensate with the yoke, then record the TAS, you will see that the plane goes fastest at the properly trimmed position. This doesn't have the elevator in trail with the horizontal stabilizer. It is not intuitive that this out of trail position would have the least drag. I think the engineers at Mooney did the same tests and added the bungees so the trimmed position was at the least drag combination of horizontal stabilizer angle of attack and elevator angle to get the needed down force in cruise.

When trimmed the springs have no effect, or said another way when hands off the springs are centered, their function is to add spring force to return the elevator to center.

In truth all my experience is with elevator down force springs, not centering springs like a Mooney, but I’m sure the springs are there to increase control pressure the further it’s moved from center

Most aircraft have a fully symmetrical airfoil on the horizontal so at zero angle of attack there is no lift, as almost every aircraft is designed so that in level flight the tail is generating negative lift the elevator is normally down slightly, the further aft you move the CG the less down elevator there is

[N201MKTurbo]

30 minutes ago, Kerrville said:

May not be the spring. The sound is coming from just forward of the aft equipment bay. I am going to need to open it up and have someone move the elevator to track it down…

Once it is opened up, you can move the push/pull rods yourself while underneath the plane. you don't need a helper.

[Guest]

Thanks good info. Is it easier to look from the bottom up or from the top down. (I imagine there are inspection panels under the carpet in the cargo area. 

[Fly Boomer]

20 minutes ago, Kerrville said:

Thanks good info. Is it easier to look from the bottom up or from the top down. (I imagine there are inspection panels under the carpet in the cargo area. 

I would think it would always be easier from the bottom up.  Different Mooneys had different arrangements of belly panels.  The older models apparently have a zillion fasteners, while the newer models have a one-piece belly with far fewer fasteners.  Either way, I expect bottom up is going to be easier.

[Guest]

I got the zillion…

[Fly Boomer]

1 minute ago, Kerrville said:

I got the zillion…

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.