Aileron Push-Pull Control Tube

[Blue on Top]

The aileron push-pull control tube is a great example to look at the tradeoffs in materials, tolerances, costs, weights, certification, manufacturing, etc. and how each of these is related to each other.  I probably won't think of everything but here's a conversation starter.

1) weight - aluminum, composite then steel, but I think that they would all be fairly close. 

2) cost - steel, aluminum then composite.

3) manufacturing - steel/aluminum then composite

4) Stiffness - composite, steel then aluminum

The problem is they all relate to each other and all have very strong positives and negatives.  Here's some random thoughts.  Steel produces the smallest diameter.  Flexibility (buckling) is both good and bad. For metallic tubes, it allows less binding (and resulting lower control force); for composite (very stiff), alignment has to be perfect or it won't move.  To back this, the 3 hinges on the aileron are rarely in perfect alignment, but when installed they move easily because the aluminum aileron flexes slightly to allow for it.  A composite aileron does not (an M10 learning experience).  Certification would be a large expense for composite as the material and processes would need to be qualified and tested.  BUT (and this is where OEMs today are missing it) if one could find an off-the-shelf, commercially-produced tube that would work, end pieces could be designed to mate with the tube and the current attach points.  Each tube assembly would need to be strength tested, but that is A LOT less work than certifying an original design.

The match has been lit.

Go! 

[bradp]

How about the control surfaces of the Mooney 301 wing?  Is that a lesson in original design? If I recall it had a 10% stub aileron, a positive and negative spoileron through a cavity in the midwing, and nearly full span Fowler flaps.  The thing is just pretty to look at.  Wonder what the control surface engineering was (if they stuck with tubes).  Same as the TBM? 

[Blue on Top]

7 minutes ago, bradp said:

If I recall it had a 10% stub aileron, a positive and negative spoileron through a cavity in the midwing, and nearly full span Fowler flaps.  Wonder what the control surface engineering was (if they stuck with tubes).  Same as the TBM? 

It was a 90/10 split on flaps/aileron span.  A spoiler has very little (actually negative) control force.  Spoilers need to be held down.  

[Yetti]

You can buy straight carbon fiber tubes and just bolt on ends.  Problem with the Mooney is there are some custom bends.  Now you have to custom make the carbon fiber tubes.  More expensive.

[carusoam]

What is in my plane?

I thought my M20C got some parts welded / filleted... and rust had a tendency to speckle through the yellow paint...

 

Control tubes have a tendency to break, when they get out of rig... as some long body Mooney has proven... broke a nose gear control tube...(?)

I haven’t read anything regarding loss of control of the flight surfaces, that I can reference...

Best regards,

-a-

[N201MKTurbo]

In flight the aileron control tubes are always in tension, so they aren’t actually push/pull tubes, but pull tubes. The only long push/pull tubes are in the tail. They are  larger diameter to keep them from buckling when pushed.

The only time the aileron tubes would be in compression would be when flying inverted.

[1964-M20E]

3 hours ago, N201MKTurbo said:

In flight the aileron control tubes are always in tension, so they aren’t actually push/pull tubes, but pull tubes. The only long push/pull tubes are in the tail. They are  larger diameter to keep them from buckling when pushed.

The only time the aileron tubes would be in compression would be when flying inverted.

Interesting analysis.  I didn't think about this last time I went inverted in the Mooney and was hanging from the seatbelt.

 

Seriously I wonder how the aileron tubes would hold up during inverted flight they are long tubes and would be in compression?  I guess you could always increase the diameter and wall thickness of the tubes.  I think they are currently designed to be about 1/2" or 5/8" diameter.  Increase to 3/4" and adjust the phenolic blocks accordingly.

 

 

[N201MKTurbo]

20 minutes ago, 1964-M20E said:

Interesting analysis.  I didn't think about this last time I went inverted in the Mooney and was hanging from the seatbelt.

 

Seriously I wonder how the aileron tubes would hold up during inverted flight they are long tubes and would be in compression?  I guess you could always increase the diameter and wall thickness of the tubes.  I think they are currently designed to be about 1/2" or 5/8" diameter.  Increase to 3/4" and adjust the phenolic blocks accordingly.

 

 

I personally avoid inverted flight in my Mooney. Any time it was upside down was a positive G maneuver. And I haven't done any of them in the last 30 years.

[PT20J]

3 hours ago, N201MKTurbo said:

In flight the aileron control tubes are always in tension, so they aren’t actually push/pull tubes, but pull tubes. The only long push/pull tubes are in the tail. They are  larger diameter to keep them from buckling when pushed.

The only time the aileron tubes would be in compression would be when flying inverted.

Help me understand this: When one side pulls, does not the opposite side push?

And, I don't understand what inverted flight has to do with it. Are you are referring to the aerodynamic force on the aileron which causes them to want to rise up in flight? But this force puts the tubes in compression.

Skip

[1964-M20E]

When sitting on the ground the ailerons droop some putting the tubes in very light compression.  As you begin to move forward the air pressure under the wing forces the aileron up putting both tubes are in tension.  When making a turn one tube will be in compression and one in tension.  If I recall correctly the working of these tubes and the aerodynamics of the situation.

The tube in tension (deflecting the aileron down) would see more force than the one in compression (deflecting the aileron up).  This is due to the higher pressure under the wing than above the wing.

OK anyone wish to shoot holes in my analysis you are welcome to.

[Andy95W]

Skip's right.

Because of the bellcrank just forward of the aileron linkage, up-force on the aileron translates to the long aileron control tubes as compression.

[N201MKTurbo]

12 minutes ago, PT20J said:

Help me understand this: When one side pulls, does not the opposite side push?

And, I don't understand what inverted flight has to do with it. Are you are referring to the aerodynamic force on the aileron which causes them to want to rise up in flight? But this force puts the tubes in compression.

Skip

The wings produce lift, lets say 5 Lbs/sq ft. The ailerons are part of the wing and produce lift. They are about 5 sq ft each, so they produce about 125 Lbs of up force each. Most of that up force is supported by the hinges. They produce a moment around the hinges which exerts a pulling force on the control arm on the bottom of the aileron. Both ailerons make about the same force. The aileron control system keeps them from flapping all the way up. Those forces are connected together at the center bell crank in the belly of the plane. When you move the yoke you move that pivot point back and forth which pulls one down and allows the other to rise. All the while they are both pulling against each other. This is why there is never any slop in the ailerons no matter how sloppy your rod ends are. The only place slop matters is between the yoke and the center pivot in the floor.

[Andy95W]

16 minutes ago, N201MKTurbo said:

 All the while they are both pulling against each other. 

 

It sure looks like they are in compression, because of the bellcranks in the wings, based upon the parts Manual picture I posted above.

[N201MKTurbo]

2 minutes ago, Andy95W said:

It sure looks like they are in compression, because of the bellcranks in the wings, based upon the parts Manual picture I posted above.

The drawing is misleading. Item 17 actual goes the other way. To the right in the drawing.

[PT20J]

25 minutes ago, Andy95W said:

It sure looks like they are in compression, because of the bellcranks in the wings, based upon the parts Manual picture I posted above.

 

16 minutes ago, N201MKTurbo said:

The drawing is misleading. Item 17 actual goes the other way. To the right in the drawing.

Believe @Andy95W is right. The bellcrank out by the aileron reverses the direction (changes a pull on the tube connecting to the aileron horn to a push on the long tube). Here's a drawing from my M20J IPC. I drew arrows showing the motions for left aileron up. So, if the stick is fixed, the aileron up force puts the long tube in compression.

[N201MKTurbo]

4 minutes ago, PT20J said:

 

Believe @Andy95W is right. The bellcrank out by the aileron reverses the direction (changes a pull on the tube connecting to the aileron horn to a push on the long tube). Here's a drawing from my M20J IPC. I drew arrows showing the motions for left aileron up. So, if the stick is fixed, the aileron up force puts the long tube in compression.

The arrow at 24 is correct for raising the aileron, but you have the arrow at 19 going the opposite way. How can that be the two rods are essentially connected together.

It is easy to tell. Next time you have the belly off, lift an aileron and see if its tube goes in or out. 

[PT20J]

4 minutes ago, N201MKTurbo said:

The arrow at 24 is correct for raising the aileron, but you have the arrow at 19 going the opposite way. How can that be the two rods are essentially connected together.

It is easy to tell. Next time you have the belly off, lift an aileron and see if its tube goes in or out. 

It's a little hard to visualize the motion the way the M20J illustration is drawn. It's clearer in the earlier-posted drawing. But if you follow ALL the arrows, you'll see that it has to work that way for the left aileron to go up when the wheel is turned to the left. The flexing of the tubes under compression (and the slight eccentric motion of the bellcrank) cause the push-pull tubes to rub on the guide blocks (which is why they have to be kept lubricated).

Skip

[Andy95W]

Skip and Rich- I'll be out at my airplane this afternoon and I'll take off the inspection panel at the bellcrank to see for sure.

The old IPC sure looks like the bellcrank reverses the force based on the pivot point location.

[N201MKTurbo]

1 hour ago, Andy95W said:

Skip and Rich- I'll be out at my airplane this afternoon and I'll take off the inspection panel at the bellcrank to see for sure.

The old IPC sure looks like the bellcrank reverses the force based on the pivot point location.

Wow, three drawings and three different orientations. 

[Andy95W]

Well, at least on my '64 M20C, raising the aileron puts the long pushrod in compression.  Attached are a picture and the IPC reference that most closely approximates the actual.  (I'm 99% sure all are the same, M20 through M20V).

[Andy95W]

Wow, that was a lot of effort and discussion and doesn't really change anything!

[Blue on Top]

31 minutes ago, Andy95W said:

Wow, that was a lot of effort and discussion and doesn't really change anything!

MS is a great way to use an afternoon.  

So, ummmmmmm, as politically correctly and non-offensive as I can be (which regretfully is definitely not my strong suit).  The original assumption is … ummmmmmmm … flawed.  The numbers and calculations are a little flawed, too.  Wing loading on a MTOW M20J is a little under 17 lbs./sq. ft. (not 5).  Using 5 sq. ft. of area, the "up" load would be 85 lbs. at 1G (not 125).  BUT, in reality, there is a pressure distribution on the wing (the highest pressure differential is ~30% chord and very low at the trailing edge.  So, long story short, if the aileron pushrod were disconnected in flight, the aileron would float up some but not up vertically.  It will float up a little from "trail".  Bonus note: your airplane will fly fine with only one aileron … don't try this at home … or the airport. 

Bottom Line: All flight control system tubes are push-pull tubes.  The design/sizing criteria is failure in buckling (compression).  If they were always in tension, we'd use a single cable; it's a lot lighter and less expensive.

[carusoam]

You guys are good!

Thanks for sharing the detailed pics, drawings, and mark-up!!!

Afternoons...? I need to try that. 

Best regards,

-a-

[eman1200]

I personally avoid inverted flight in my Mooney. Any time it was upside down was a positive G maneuver. And I haven't done any of them in the last 30 years.

I haven’t done inverted flight in the mooney since transition training.