Mooney Trim System

[201er]

I understand that the trim pitches the entire tail. But what I'm curious about is why it also moves the elevator and how this takes place in flight. Changing the trim on the ground will not only pitch the tail but also move the elevator (much like the trim on a simlpe airplane without any tabs or real trimming). My guess is that the reason it moves the elevator is to provide stick feedback to the pilot rather than actually setting the position of the elevator for trim purposes. Otherwise it wouldn't make sense to change the angle of the horizontal stabilizer all together. But in flight, is the airflow strong enough to keep the elevator level and simlpy provide force feedback to the pilot or does the elevator stay in the same position as seen on the ground at that trim setting? In that case, what's the point of moving the entire tail rather than just using a spring to hold the stick where you want it?

What else can you tell me about the Mooney trim system? How come they didn't make some sort of rudder or aileron trim to balance the plane with uneven loading?

[Skywarrior]

Bravos have electric rudder trim.

[Cruiser]

Efficiency? Maybe the trim tabs create drag in the airstream?

[Bolter]

The way I understand the Mooney's unique trim system is that the entire tail is moved so as to move the angle of attack of the horizontal stabilizer such that the pitch forces are neutral.  The elevator surfaces are used to input a pitch up or down from the neutral position you have created.  

Keep in mind that the horizontal stabilizer is a wing, just a small one.  The lift it creates is meant to balance the pitching moment of the plane around its CG.  The lift or downforce is dependent on the angle of attack and air speed.  For a given airspeed at a constant altitude, there is only one angle of the stabilizer that will create a zero pitch condition.  The trim system lets you set the stabilizer for that condition, rather than create a new drag inducing force to counteract the existing forces.  

Other planes use a drag inducing trim tab to apply a force to the elevator such that it holds a position that gives the impression of neutral forces at the controls (ex: Bonanza).  In reality the trim tab is forcing down the elevator, and the horizontal stabilizer is creating a pitching force in the opposite direction.  Drag, drag, drag.  

Some planes (ex: small Cherokees) use stabilitators so that the elevator and stabilizer function are in one adjustable wing.  It still uses drag inducing tabs to hold the surface at the angle that creates zero pitching forces.  As an interesting experiement, you can fly a Cherokee with a maximum aft CG and get more speed from the plane.  The reason is that you require less trim in that condition, so the plane has less drag.

FOr my 201 with the 200 HP engine, lack of a rudder trim is not surprising.  The forces are not that big, and the plane is balanced out perfectly at speed.  For the higher power Mooney's, I don't know what the rudder forces are like.  If anyone wants to offer a free ride in an Acclaim or Ovation so I can find out in the name of science, let me know.

-dan

[201er]

But it appears that the elevator also comes in play while trimming the Mooney tail. How does that come into play and what drag effect does that create?

[N601RX]

Quote: 201er

What else can you tell me about the Mooney trim system? How come they didn't make some sort of rudder or aileron trim to balance the plane with uneven loading?

[kortopates]

Quote: tomcullen

.....

 As an interesting experiement, you can fly a Cherokee with a maximum aft CG and get more speed from the plane.  The reason is that you require less trim in that condition, so the plane has less drag.

.....

Dan, good explanation overall. But one correction or clarification because not everyone may understand your reference to less trim is meant as less lift. All airplanes will improve their performance at maximum aft CG due to less drag but not really less trim. The further the CG moves back the less downward lift the tail has to produce to main level or desired attitude. The less lift results in proportionately less induced drag as a byproduct of lift.

[Bolter]

Quote: kortopates

[jlunseth]

This is an interesting question, hadn't thought about it.  I think in the K, the elevator just hangs there and is brought into alignment with the horizontal stabilizer by air flow unless the pilot uses the yoke to move the elevator.  What makes me say that is that the elevator on the K hangs down (yoke falls full forward) when the aircraft is at rest, but comes up into alignment with the horiz. stab. when the engine is started and there is an airflow over the tail.  I believe (going from memory here) the elevator on the J stays level with the horiz. stabilizer when the aircraft is at rest on the ground.

[Bolter]

Quote: 201er

But it appears that the elevator also comes in play while trimming the Mooney tail. How does that come into play and what drag effect does that create?

[201er]

Quote: jlunseth

This is an interesting question, hadn't thought about it.  I think in the K, the elevator just hangs there and is brought into alignment with the horizontal stabilizer by air flow unless the pilot uses the yoke to move the elevator.  What makes me say that is that the elevator on the K hangs down (yoke falls full forward) when the aircraft is at rest, but comes up into alignment with the horiz. stab. when the engine is started and there is an airflow over the tail.  I believe (going from memory here) the elevator on the J stays level with the horiz. stabilizer when the aircraft is at rest on the ground.

[carusoam]

Dan,

I can assure you what you see outside the plane while it is parked is not exactly the same while it is flying.  The yoke pressure of the Ovation is very heavy while at rest, but quickly neutralizes when moving.  Yes, the geometry changes while the trim is operated on the ground.  

The electric rudder trim is somewhat of a gimmick.  I don't have long climbs to tire my rudder pedals.  It is a nice to have.

The A/P takes care of the roll axis trim.

The most important thing is that the plane is basically in good rig to begin with.

As far as the tail being on backwards, ours are on straight.  It's the other guys that have made a small miscalculation...

Best regards,

-a-

[PTK]

Quote: tomcullen

The way I understand the Mooney's unique trim system is that the entire tail is moved so as to move the angle of attack of the horizontal stabilizer such that the pitch forces are neutral.  The elevator surfaces are used to input a pitch up or down from the neutral position you have created.  

Keep in mind that the horizontal stabilizer is a wing, just a small one.  The lift it creates is meant to balance the pitching moment of the plane around its CG.  The lift or downforce is dependent on the angle of attack and air speed.  For a given airspeed at a constant altitude, there is only one angle of the stabilizer that will create a zero pitch condition.  The trim system lets you set the stabilizer for that condition, rather than create a new drag inducing force to counteract the existing forces.  

Other planes use a drag inducing trim tab to apply a force to the elevator such that it holds a position that gives the impression of neutral forces at the controls (ex: Bonanza).  In reality the trim tab is forcing down the elevator, and the horizontal stabilizer is creating a pitching force in the opposite direction.  Drag, drag, drag.  

Some planes (ex: small Cherokees) use stabilitators so that the elevator and stabilizer function are in one adjustable wing.  It still uses drag inducing tabs to hold the surface at the angle that creates zero pitching forces.  As an interesting experiement, you can fly a Cherokee with a maximum aft CG and get more speed from the plane.  The reason is that you require less trim in that condition, so the plane has less drag.

FOr my 201 with the 200 HP engine, lack of a rudder trim is not surprising.  The forces are not that big, and the plane is balanced out perfectly at speed.  For the higher power Mooney's, I don't know what the rudder forces are like.  If anyone wants to offer a free ride in an Acclaim or Ovation so I can find out in the name of science, let me know.

-dan

[takair]

The older planes work different than the new ones.  Older Mooneys have a spring/bungee that couples the elevator to the trim system.  For example, in take-off configuration, an older Mooney will actually have the elevator trailing edge up....significantly.  Newer ones are not coupled and will dangle.  When airloads are applied the older Mooney's elevator tend to remain where the bungee moves it while the newer will tend to streamline.  My 64 E does not completely streamline even in cruise.  With full down trim it barely streamlines.  I suspect the bungee was to provide a form of artificial feel.  Many other aircraft use weights or springs as well.  I seem to recall piper having a  large bob weight on the stabilator.  I suspect the newer long bodies have been optimized such that they no longer need the bungee which results in a pretty heavy feel.  I think they did away with it somewhere in the 201 series. 

[201er]

Quote: takair

The older planes work different than the new ones.  Older Mooneys have a spring/bungee that couples the elevator to the trim system.  For example, in take-off configuration, an older Mooney will actually have the elevator trailing edge up....significantly.  Newer ones are not coupled and will dangle.  When airloads are applied the older Mooney's elevator tend to remain where the bungee moves it while the newer will tend to streamline.  My 64 E does not completely streamline even in cruise.  With full down trim it barely streamlines.  I suspect the bungee was to provide a form of artificial feel.  Many other aircraft use weights or springs as well.  I seem to recall piper having a  large bob weight on the stabilator.  I suspect the newer long bodies have been optimized such that they no longer need the bungee which results in a pretty heavy feel.  I think they did away with it somewhere in the 201 series. 

[eaglebkh]

Not sure where the cut-over was.  I suspect it was slightly different when they went from the short body to the F/201.  It seems that the 231 actaully has an elevator downspring and bobweight.  In any case, the maintenance manual contains rigging instructions for both the stabilizer and elevator.  Not sure what it is for the 201.  On the short bodies you can actually see the bungees from the outside, where the elevators meet.  They are part of the elevator pushrod system and each elevator has one.  When you move the elevator you will see the mechanism.  Can't recall the 201 geometry.  In my E model, I can look back and just see the elevaotr counter weight, so I can tell that it does not streamline in flight.  I suspect it adds some drag, but that's the way the rigging works out.  That is one reason that the aircraft picks up a little speed with aft CG, less downforce (drag) and less drag from more streamlined elevator.

[takair]

Quote: eaglebkh

Not sure where the cut-over was.  I suspect it was slightly different when they went from the short body to the F/201.  It seems that the 231 actaully has an elevator downspring and bobweight.  In any case, the maintenance manual contains rigging instructions for both the stabilizer and elevator.  Not sure what it is for the 201.  On the short bodies you can actually see the bungees from the outside, where the elevators meet.  They are part of the elevator pushrod system and each elevator has one.  When you move the elevator you will see the mechanism.  Can't recall the 201 geometry.  In my E model, I can look back and just see the elevaotr counter weight, so I can tell that it does not streamline in flight.  I suspect it adds some drag, but that's the way the rigging works out.  That is one reason that the aircraft picks up a little speed with aft CG, less downforce (drag) and less drag from more streamlined elevator.

[201er]

Quote: eaglebkh

Not sure where the cut-over was.  I suspect it was slightly different when they went from the short body to the F/201.  It seems that the 231 actaully has an elevator downspring and bobweight.  In any case, the maintenance manual contains rigging instructions for both the stabilizer and elevator.  Not sure what it is for the 201.  On the short bodies you can actually see the bungees from the outside, where the elevators meet.  They are part of the elevator pushrod system and each elevator has one.  When you move the elevator you will see the mechanism.  Can't recall the 201 geometry.  In my E model, I can look back and just see the elevaotr counter weight, so I can tell that it does not streamline in flight.  I suspect it adds some drag, but that's the way the rigging works out.  That is one reason that the aircraft picks up a little speed with aft CG, less downforce (drag) and less drag from more streamlined elevator.

[jetdriven]

our elevator counterweight still is displaced above the end of the stabilizer at 200 KIAS.

[takair]

Quote: 201er

[Bolter]

I propose an experiment.  For those of you who have in-flight pictures, make a survey and see if you can find any images where the elevators are not "streamlined" with the horizontal stabilizer. 

Not a perfect solution to this question, however it could be interesting.  I looked at a few pics of mine and some photo galleries on this site that had planes in flight, and was confounded by poor resolution images, or the sneaking suspicion that the deflection will be too small to clearly see.  Cycling the control surfaces on the ground, it looks like a long traveling surface, but how often do we give maximum control inputs?  And of course, unless you are the pilot in the shot, you can't say for sure that the plane was trimmed or in level flight, either.

Maybe someone who regularly flies in formation can make a note to have a passenger watch for this?

-dan

[takair]

If you can't see the tail from the cockpit, you could always mark the yoke shaft in cruise with a black sharpie.  When on the ground just match the trim and black mark and see where the elevator sits. 

[201er]

I don't think that would work because there is airflow pressure against the springs/bungees in the elevator trim system. This is why I can't figure out if the elevator deflection by trimming on the ground is cancelled out by airflow in the air or not.

[takair]

It should work.  The linkage between the elevator and control wheel shaft is essentially solid. The bungee just biases the position. If you set the trim to the same position you have in cruise and then look at where the mark is, you will know how much the airload moves it.  If you then line up the mark and look at the elevator you will know exactly where it is in flight.  The only thing it won't account for is slop in the system, which should me minimal. I do know that on my E model, the airload does not completely overcome teh spring force, which is significant. 

[jetdriven]

This is an easy one. From the pilots seats you can see both stabilizer tips, and every time I have ever looked, the elevator counterbalances are around one thickness above the horizontal stabilizer. A little more, like 1.5 thicknesses if at forward limit of CG.

Here are some air race photos, taken with a 400MM lens and a Canon EOS digital SLR. The first two are at 160 KIAS and the 3rd is 200 KIAS. I think all 3 show the elevator displaced. CG is in middle of range.

Quote: tomcullen

I propose an experiment.  For those of you who have in-flight pictures, make a survey and see if you can find any images where the elevators are not "streamlined" with the horizontal stabilizer. 

Not a perfect solution to this question, however it could be interesting.  I looked at a few pics of mine and some photo galleries on this site that had planes in flight, and was confounded by poor resolution images, or the sneaking suspicion that the deflection will be too small to clearly see.  Cycling the control surfaces on the ground, it looks like a long traveling surface, but how often do we give maximum control inputs?  And of course, unless you are the pilot in the shot, you can't say for sure that the plane was trimmed or in level flight, either.

Maybe someone who regularly flies in formation can make a note to have a passenger watch for this?

-dan