Chapter 16 - Section 6

Rudder Stops and Brakes

I read the plans and searched through the archives on this subject and the more I read, the more confused I got. Regardless, here's what I gathered:

• The rudder starts to deflect from the moment you step on the pedals and will stop when you are hard on the brakes. The total deflection, according to the plans should be less than 4.5" measured at the bottom of the rudder (or 22^o to 26^o).

• A 0.7" of over travel (sleeve clearance) is needed at the wing tip when full rudder is deployed and hard on the brakes. 

That means (at least to me), sometime during that rudder movement (from 0 to 4.5"), the master cylinder for the brakes will be engaged. 'When' the brake engages depends on the total cable movement and master cylinder displacement when brakes are applied. Confusing enough? Here's my take on it...

Rudder movement (0 to 4.5")

= rudder movement before brake engagement + max. master cylinder displacement when brakes are applied.

Determining Rudder Cable Travel

Rudder movement (4.5") translates to rudder cable travel, which in turn equates to rudder pedal travel. Therefore my first task was to measure the equivalent rudder cable travel. I stuck a piece of masking tape on the rudder cable at the conduit opening into the wing. Then I put a wedge of wood at the rudder to 4.5" deflection per plan. I measured the distance between the masking tape to the conduit - that's my cable travel for 4.5" rudder. My rudder cable travels 1 1/8" total with 24^o rudder deflection. I was actually quite surprised by that short amount of cable travel. That also means I will have very little pedal travel as well...hmmm...

Just for documentation sake, I took a measurement for rudder travel incrementally as follows:

As shown above, I will have an average of 2.04" of cable travel for a full 7.5" of rudder swing - leaving ~.5" of over travel clearance. That almost seems to be a waste not to use all that rudder authority, but the designer says...

Determining the Master Cylinder Displacement

My first look at the Matco's master cylinder (MC-4) had me worried because its shaft displacement is 1 1/2 inches - that's a lot more than my cable travel limitation (i.e. 4.5" rudder swing above). I asked a few fliers and I got answers from 'not sure' to '1 1/2"'. One specific data point was from the archives per Marc Zeitlin stating that its somewhere between 1/4" and a max of 1/2". Now, that's more like it...but I know I need to take the measurement for my set up.

So I made a couple L brackets and bolted the master cylinder to a 2x4. and hooked up all the nylaflow tubing I got from Matco. Then I filled the reservoir with aircraft brake fluid and bled the brakes. I tried just about all the methods suggested in the archives to get the bubbles out - that was a major pain . Anyway, it took me 2 days to figure out my brake displacement was 3/16" (i.e. from first brake engagement to locking the wheels - obviously not at high speed and Susann was doing the pushing). My measurements seem to agree with Marc's estimates though.

'Free' Rudder Swing vs Cylinder Displacement

My underlying concern in this set up is the rudder swing vs brake cylinder displacement. Since the rudder swing, say 7.5" (or 2.04" of cable travel) is the limiting factor, its becomes a give and take situation. The more 'free' rudder swing you have, the less cylinder displacement you'll get or vice versa. The question becomes - how much 'free' rudder swing is appropriate to handle the aircraft's rated cross wind condition? I posted this question to a few experienced flyers and the consensus seems to indicate 3" 'free' rudder swing is probably adequate - with good piloting skill. Well, that counts me out...I decided to set out the following criteria:

- minimum 'free' rudder swing of 3.5" to 4";

- minimum brake cylinder travel of .75".

With the help of my Excel spreadsheet calculations, I came up with the following table:

In summary, with the above settings, I will have the following:

- Rudder pedal will be set at 17.5^o aft from vertical (at rest);

- Rudder pedal travel will be 1.02" before contacting the master cylinder. It equates to 4.26" of free rudder travel;

- Once rudder pedal made contact, it will have an additional .725" for cylinder travel. At the end of this travel, the rudder swing will be at 7.5" max position.

Initial Rudder Calculation

You may recall that I am using the hanging rudder pedal design (Chapter 13) and my pedal arms have 6 holes down its sides. The top hole goes through the hanging tube while the lowest hole is used for the foot pedal (picture left). A special 'plug' will be made through the third and fourth hole (from top) to mount the master cylinder shaft and rudder cable respectively. Since the rudder cables will be linked directly to the rudder pedal arms, the outermost arms (per Velocity's design & picture left) will be eliminated.  

In summary:

• My pedal neutral position will sit at 17.5^o aft from vertical. I can certainly re-adjust them a bit forward to 12.9^o per plan; 

• The pedal will engage the brakes after 17.9^o or 1.02" cable travel (1.76" of foot pedal forward travel). The pedal will be at vertical position at this point.

• Once engaged (brake), I will have 17.5^o or .725" of cylinder displacement (also 1.76" at foot pedal forward travel) achieve hard on brakes plus some. I'll be in much bigger trouble if I reach that....

[Hindsight] Actual rudder attachment hardware is discussed in Chapter 13 Section 7.

[Hindsight] I found a discussion from the forum that if I change the thimble attachment arrangement at the horn, using a bushing through the hole instead of hooking the thimble through the hole, I can get another .75" of cable travel. Neat idea, I may implement that later on.