Rudder-Aileron Interconnect

[Blue on Top]

3 hours ago, ArtVandelay said:

Lead counterweights balance the elevators? I wonder how much they weigh and why didn’t they use lighter springs?

That’s not in the early version of the POH.

Tom

@ArtVandelay  Tom:  Balance weights and springs are installed for two very different reasons.  Balance weights on the elevator are for aeroelastic bending (flutter) and springs are for flight stability.   Lead weights installed on the forward portions of the flight controls (elevators, rudder and ailerons) are to (more closely) statically balance the flight control surface.  Not all surfaces are 100% balanced (like the elevators in the manual above.  Thanks, @PT20J).  Let's look at the elevator (it's the easiest).  In turbulence (vertical in this case) the airplane will move up and down (both pitching and vertically), which causes the stabilizer to move up and down with the airframe.  The elevator on the other hand is free to move, controlled by the pilot.  When the stabilizer goes up, (if not balanced) the elevator in relationship will go down, causing the first excitation of the cycle.  The opposite is true on the down movement of the stabilizer.  And now we have the beginnings of a flutter cycle.

The springs on the other hand (I believe there are 2 sets for stability reasons on Mooneys).  The first is the lateral-directional springs, know as the rudder-aileron interconnect, which loosely ties rudder and aileron control position together.  The other is the down spring (newer airplanes), which pulls the elevator down in slower flight to give the airplane more static (speed) stability. 

[carusoam]

11 hours ago, Blue on Top said:

Part 25 airplanes also must get home after any failure plus any probable failure (those failures that occur more than 10^-5).  They also have to deal with a jammed flight control surface.  Part 23 (our Mooneys) don't have to.  As a not quite "equivalent", Part 23 airplanes must be more stabile.  For example, we have to fly home with any one bolt not installed in the flight control system.   IOW, disable the rudder and fly home on ailerons alone (btw, even with the rudder aileron interconnect, the rudder would not move).  Disable the aileron(s); fly home on rudder.  Disable the elevator, fly home on trim (if you haven't tried this you should … at altitude first).  Disable the pitch trim, fly home on elevator only.

Hope this helps!  -Ron

  

We have an example of how challenging it can be to fly home after a single failure...

Amelia had Marvin K. Mooney... and the electric trim stuck, and jammed... Marvin was an M20K...

Fortunately, she was not alone... and when finished MKM needed replacement....

 

Best regards,

-a-

[Blue on Top]

12 minutes ago, PT20J said:

The elevator weights mass balance the elevators and I don't see a weight listed in the service or parts manual. Interesting that the service manual calls for under balancing (trailing edge heavy)  all the control surfaces. Maybe Ron @Blue on Top can explain this.  -Skip

So, … weight is … weight, and weight is bad (takes away from useful load).  Weight away from the CG is doubly bad.   Now let's look at those elevator balance weights in particular (my last post explains a little of the aeroelastic (flutter) issue and why we balance the flight surfaces.  Remember that EVERYTHING in design is a compromise or tradeoff.  We pounce on those items that are not … an engine that has more HP per lb. of weight AND has a 4000 hrs. TBO  AND costs 50% of current.  I wish.  So on to the elevator balance weights.

We analyze the aeroelastic (flutter) modes to give us good characteristics with some static margin.  This is similar to how the aircraft aft CG is set for the entire airplane.  Note also that the balance weight is 2-4 times the weight of the actual elevator (its a geometric thing of balancing the moments around the hinge line (which is what your knife edges are in the real world).  So, if your elevator weighs 2 lbs., the balance weight might be 6 lbs. to balance that 2 lbs. (it has a shorter moment arm).  Now your 2 lbs. elevator weighs 8 lbs!  Another tradeoff is where to put that weight.  The balance area at the tip is ahead of the hinge line … trying to destabilize (flutter) the elevator.  This area also lightens the control force.

The easiest way for me to think about this is an arrow (of a bow and arrow).  The CG of the arrow must be ahead of the aerodynamic center, too.  If those 2 points are at the same location, the arrow will fly in any attitude (and very draggy).  As weight is added to the pointy end, the arrow will fly more stable (less wandering).  If the weight continues to be increased, the arrow will become more and more stable, but will travel less and less as the total weight increases. 

[N201MKTurbo]

30 minutes ago, Blue on Top said:

1 hour ago, Blue on Top said:

The springs on the other hand (I believe there are 2 sets for stability reasons on Mooneys).  The first is the lateral-directional springs, know as the rudder-aileron interconnect, which loosely ties rudder and aileron control position together.  The other is the down spring (newer airplanes), which pulls the elevator down in slower flight to give the airplane more static (speed) stability. 

 

Can you write about the elevator bungees? I have my thoughts about them, would like to hear yours.

[PT20J]

38 minutes ago, Blue on Top said:

So, … weight is … weight, and weight is bad (takes away from useful load).  Weight away from the CG is doubly bad.   Now let's look at those elevator balance weights in particular (my last post explains a little of the aeroelastic (flutter) issue and why we balance the flight surfaces.  Remember that EVERYTHING in design is a compromise or tradeoff.  We pounce on those items that are not … an engine that has more HP per lb. of weight AND has a 4000 hrs. TBO  AND costs 50% of current.  I wish.  So on to the elevator balance weights.

We analyze the aeroelastic (flutter) modes to give us good characteristics with some static margin.  This is similar to how the aircraft aft CG is set for the entire airplane.  Note also that the balance weight is 2-4 times the weight of the actual elevator (its a geometric thing of balancing the moments around the hinge line (which is what your knife edges are in the real world).  So, if your elevator weighs 2 lbs., the balance weight might be 6 lbs. to balance that 2 lbs. (it has a shorter moment arm).  Now your 2 lbs. elevator weighs 8 lbs!  Another tradeoff is where to put that weight.  The balance area at the tip is ahead of the hinge line … trying to destabilize (flutter) the elevator.  This area also lightens the control force.

The easiest way for me to think about this is an arrow (of a bow and arrow).  The CG of the arrow must be ahead of the aerodynamic center, too.  If those 2 points are at the same location, the arrow will fly in any attitude (and very draggy).  As weight is added to the pointy end, the arrow will fly more stable (less wandering).  If the weight continues to be increased, the arrow will become more and more stable, but will travel less and less as the total weight increases. 

OK, I think you are saying that since the Mooney has fairly small and rigid structures, and not much aerodynamic balance on the control surfaces, that it isn't necessary to balance the CG of the control surfaces all the way up to the hinge line to have adequate flutter margin. Is this right?

As for arrows: I wonder if the Plains Indians figured out how to optimize them? Seems like the shaft length is fixed by bow geometry so the variables are feathers (bigger = more drag) and arrow head (heavier = more mass to accelerate and be pulled off course by gravity). Arrow optimization would have been important when dinner depended on it.

Skip

[PT20J]

8 minutes ago, N201MKTurbo said:

Can you write about the elevator bungees? I have my thoughts about them, would like to hear yours.

@Blue on Top @N201MKTurbo Might be good to start a new topic for this 

Skip

[ArtVandelay]

Tom, which model and date POH are you looking at? I have a copy of the M20J POH dated 11/15/77 which has the notation.

 

This is the same section from online version?:

Edit: found it, they moved it up front from the back.

 

 

 

Tom

[ArtVandelay]

So on my J, the elevators resist movement as if attach to bungees which means they’ll center themselves. I have see Mooneys that don’t have this and their elevators hang down. When did they make that change?


Tom

[ArtVandelay]

This is the same section from online version?:



Tom

Never mind, found it, it was towards the back instead of the front:



Tom

[PT20J]

1 hour ago, ArtVandelay said:

So on my J, the elevators resist movement as if attach to bungees which means they’ll center themselves. I have see Mooneys that don’t have this and their elevators hang down. When did they make that change?


Tom

Tom, the change was made with the M20K. The K and all subsequent models ditched the bungees and use a variable downspring (variable tension with trim position, i.e., stabilizer incidence) and a bobweight. Also, a fixed tab was added to the trailing edge of the elevator set at 7-degrees down (which would make the trailing edge of the elevator tend to go up in flight).

Skip

[Blue on Top]

2 hours ago, N201MKTurbo said:

Can you write about the elevator bungees? I have my thoughts about them, would like to hear yours.

@N201MKTurbo @ArtVandelay @PT20J  and I know @carusoam will find it without me saying so ….

I'll @blueontop start another thread called "Elevators - balances, springs and bungees" to get answers  those questions that have been posted here … and get the discussion out of "Rudder-Aileron Interconnect".

[skykrawler]

Seems like the aileron - rudder interconnect rigging (attach point to the aileron link) could be the source of hands off control heavy wing.  Given that one is holding the ball centered a few pounds of force on the aileron link would induce a roll.

Those springs are sporty.  Had them disconnected this annual for cleaning up corrosion on the rudder push rod.