Jump to content

Tugging the Mooney


rotorman

Recommended Posts

Clarence is correct: The adjustment I showed above will lengthen or shorten the piston rod for the master cylinder to change the pedal angle. It is the only adjustment that I am aware of. I don't really know if it has any effect on your problem, but since replacing the brake cylinders seems to have initiated the issue, it's where I would start.

The rudder pedals have two connections: Item 4 for rudder control and item 5 for the brakes. The distance between where the two connect on the pedal is the same as the length of the arm on item 7. So, mechanically, the pedal and items 4, 5, and 7 form a parallelogram such that pushing on a pedal without rotating it will rotate the rudder torque tube without applying the brakes.

Another possible way things could have been messed up is if rudder pedal extensions were installed perhaps incorrectly.

Skip

Link to comment
Share on other sites

16 hours ago, PT20J said:

Clarence is correct: The adjustment I showed above will lengthen or shorten the piston rod for the master cylinder to change the pedal angle. It is the only adjustment that I am aware of. I don't really know if it has any effect on your problem, but since replacing the brake cylinders seems to have initiated the issue, it's where I would start.

The rudder pedals have two connections: Item 4 for rudder control and item 5 for the brakes. The distance between where the two connect on the pedal is the same as the length of the arm on item 7. So, mechanically, the pedal and items 4, 5, and 7 form a parallelogram such that pushing on a pedal without rotating it will rotate the rudder torque tube without applying the brakes.

Another possible way things could have been messed up is if rudder pedal extensions were installed perhaps incorrectly.

Skip

I have opened the panel and my installation is slightly different from yours. I should have taken a picture but I was short on time last night so I just wanted to get an initial look. Were your piston rod joins the bell-crank I have an additional threaded fitting pinned to the bell-crank that allows piston rod length adjustment. A quick look seemed to indicate that I can change the effective length of piston rod such that it would not affect the pedal position. But the question is, does the spring push the rod to a mechanical full release stop inside the piston? In other words. If the rod were unattached at the bell-crank end would the piston rod be pushed further out of the piston thereby introducing more free-play before the pressure increased to active the brake?

I plan to return today with the proper tools to change the rod length. There does not seem to be any other way I can introduce more free-play into the system. Everything else looks normal.

 

 

Link to comment
Share on other sites

Before changing anything, I’d have someone move the nose wheel with the tow bar while watching the mechanism to try to figure out what’s causing the coupling between the rudder and the brakes.

I’d especially watch the pivot point at bolt 8. The brake bell crank and rudder torque tube both pivot at this point. That’s the only obvious source of coupling that I can see. 

Skip
 

 

Link to comment
Share on other sites

On 12/25/2020 at 5:33 PM, PT20J said:

There is an adjustment to the lengths of the master cylinder piston rods. There is no description of it I could find in the S&MM for my M20J when I had to tweak mine to get the pedal angles right after installing the 3" pedal extensions. There should be some free play in the pedal before the brakes go solid.

If you remove the left exhaust cavity an examine the linkages it should be fairly obvious what's gone wrong.

IMG_2868.thumb.JPG.c3cdcc6ba0f7fd39905b178fe7ed8cdd.JPG

Looking at Skip's photo starting at the top/left which is the forward/left side of the airplane.  The condition is full right pedal and nosewheel full right. The black bell crank with the bolt and castellated nut, through other linkages, drives the nosewheel and rudder. That bolt/nut is currently forward of the brake master rod end.  At the bottom of the picture the condition is the opposite. Full left pedal has been applied and the bell crank has been pushed aft and passed the brake master rod end without interference. In my case there was interference and it nudged the brake rod aft enough to pressurize the master and apply the left brake. The contact came as a result of the master cylinder installer, instead of using a pin in the master rod end as in Skip's picture, used a bolt and a castellated nut which protruded enough to push the rod slight aft applying enough pressure to activate the left brake. The amount of movement was surprising small but apparently enough. As I said earlier I have very little free play in my brake pedals. The lousy picture shows my left side fix. A tiny witness mark can be seen on the black bell crank.  The right side also had interference but not as much. 

My fix was to remove the existing nuts and bolts and install a shorter bolts with a lock nuts. As soon as I can acquire the correct pins I will install them. 

Clarences diagram and Skip's photo were invaluable to me in understanding how this system works and to solve this mystery. A great lesson and a good outcome. Thanks.

   

Inked20201227_134518_LI.jpg

  • Like 4
  • Thanks 1
Link to comment
Share on other sites

  • 3 weeks later...
  • 1 month later...

One last follow up to this problem. To review, when I the nose wheel was turned to near max left or right, the corresponding brake would pressurize and lock that main wheel.

I started to notice that I had a pronounced vibration during takeoff and landing rolls. I speculated I had an out of round tire. I jacked the offending side and found I had a large flat spot on that tire. I am speculating that before I fixed the above problem, I made a crosswind landing that that resulted in significant application of pedal and the tire touched down with the brake applied.

Link to comment
Share on other sites

On 12/24/2020 at 5:10 PM, rotorman said:

No jacks available at the moment.  But why would turnin nose wheel left or right to the limit apply brake in the direction of the turn? Can someone elaborate on the logic?

Not saying it is, but when you turn the nose wheel. the pedals are of course moving too.

never mind as I kept reading I see you found and fixed it

Edited by A64Pilot
Link to comment
Share on other sites

4 hours ago, rotorman said:

One last follow up to this problem. To review, when I the nose wheel was turned to near max left or right, the corresponding brake would pressurize and lock that main wheel.

I started to notice that I had a pronounced vibration during takeoff and landing rolls. I speculated I had an out of round tire. I jacked the offending side and found I had a large flat spot on that tire. I am speculating that before I fixed the above problem, I made a crosswind landing that that resulted in significant application of pedal and the tire touched down with the brake applied.

Or possibly turning real tight during taxi.  You could have easily drug a tire.

Link to comment
Share on other sites

I had a similar problem. Mechanic found one brake was sticking a lil bit a quick brake check showed nothing. He then removed the wheel.....lo and behold an wheel bearing shim/spacer was on in the wrong place. There is a tiny round shim that goes on the axle I’m not sure wether it goes on the axle first or behind the axle nut but it was in the wrong place. This was on a 1965 C model.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.