My toe brakes

[wbpace]

Last weekend I was not up to working in the shop, so I drafted my design for toe brakes. I came across some plans for the brake pedal for the Acro II, adapted it to work with the Sonerai rudder pedals, and drew it all up in SolidWorks. That allowed me to fully model it, independently simulate motion of the rudder and brake pedals, check for interference between parts, and generally make sure it would all work. I’ve attached a picture. It is based upon the Matco MC-5 master cylinder (I scored two at a good price from Wicks).

Basically it is the standard S-II aft rudder pedal with the top tube (that makes up the adjustable part of the pedal) turned sideways to make a pivot for the brake. The height of that tube stays the same at 7-3/4 inches, and the brake pedal adds another 2-1/2 inches. This gives the 2.5:1 mechanical advantage that Matco says is needed on the cylinder piston. Spanning across the fuselage, the position of the pedal is at exactly the same place, with the same width of the part that touches your foot, so clearance with any front seat occupant will be the same.

Rudder pedal adjustment will be done where the cable assembly attaches to the pedal horn. The three-hole strap assembly that is specified in the plans is replaced by one with multiple horn attachment holes that will allow fairly easy adjustment.

My only problem is I cannot find any references for designing with 4130 tubes, so I am asking if there is anyone with tube design experience reading this who would be willing to take a look at a couple of my design decisions (specifically my choice of tube thickness for the brake and the tabs of the MC-5 attachments, and whether the gusset is appropriate). Also, is there any reason that the rudder pedal won’t fit? I’m happy to provide the SolidWorks files and details on what I need reviewed.

Once I get this all settled and proven to work, I’m thinking of writing a long-form article explaining how to make this for S.net, complete with my SolidWorks files, in case anyone else would be interested.

By the way, next to the magazine, the fact that SolidWorks is free through EAA is hands-down their best benefit. I don’t have the strongest CAD background in the world. Yet, starting from scratch, it only took me about 3 hours to first draw the master cylinder, and another 20-ish hours to do everything else. About 2/3’s of that time was pure learning curve, but is well worth the effort! I can highly recommend getting SolidWorks if you are doing any kind of mechanical design. I will be doing a lot more of this.

O’Bill

 

[mhflyit]

Unfortunately I can't help you Bill but well done!

 

[TejasNW]

By the way, next to the magazine, the fact that SolidWorks is free through EAA is hands-down their best benefit.

Great write up (would enjoy reading a full article.) You really caught my attention with the comment about SolidWorks being free through EAA. Tell me more about that please. I gave my laptop to a young man whom needed a computer for school; only to discover Fusion 360 does not work on my other computer.

I am about to pony up for an new desktop just so I can have access to a decent CAD program again. But if solid works is free...

 

[wbpace]

Tejas,

Start here: https://www.eaa.org/en/eaa/eaa-membership/eaa-member-benefits/solidworks-resource-center. From there you can start to find many different resources, including the link to download and get a license. The license is good for one year. Each year you must download the latest SW (aww, so sad! ) and a license to go with it.

There are also a couple of webinars in the Hints for Homebuilders area (maybe Hints is really the #2 benefit and SolidWorks is really #3; both are fantastic!).

Other than the training that SW provides internally and on their website, I found googling "solidworks <search critera>" will find a wealth of 3rd party videos and sites to explain how to do things.

Go have fun!

O'Bill

 

[acrojohn]

Bill,
Are you going to install slaved brakes in the front cockpit? I don't think you indicated. If so read on for some cautions related to the Acrosport brake design.

My main ride is an Acrosport II and I am building a Sonerai I. I also plan to use the same rudder pedal design that I have in my Acro, but they are the simpler Christen Eagle pedals with a single .063 4130 tab on each rear brake pedal to actuate the cylinders. The Acro design uses a .063 x 3/4 piece of channel stock extended on a length of .049 x 3/4 4130 tube. My cylinders are a Cleveland design and the geometry with the Eagle pedals was different, so I had to bottom out the cylinder actuating rod adjustment and then raise the .063 tab slightly to position the pedal further forward so my toes weren’t interfering with the pedals. The Eagle tab is a triangular shape to better take the loads imposed on it. Your extended strap looks a bit frail and may fail much like the extended tube on the Acro brake pedals did. See Acro newsletters
What I want to bring to your attention are some issues with the Acrosport II brake system related to front brake pedal mechanism design. The design calls for a brake actuating rod to slave the rear brake pedals and cylinders when the front brake pedals are utilized. With this design you can NOT use a simple set of return springs on the front pedals to keep the pedals upright. The issue is that in some operations with a spring configuration and with operation by the rear pilot, the opposite front pedal from the one being depressed will move backwards against a weak spring or strong operation of one of the rear pedals/brake and then suddenly kick back fully while the associated brake pedal goes forward and over center of it arc of movement effectively locking that front rudder pedal in a rearward collapsed position while forward collapsed brake pedal/brake actuating linkage rod holds it locked down. Please note, this can happen even when no one is in the front cockpit! I hope I explained this well enough.
This situation can be resolved using a continuous cable linkage and pulleys to complete the rudder pedal circuit around the front of the fuselage forcing the front and rear rudder pedals to operate in unison. This design is incorporated into the Sonerai II tricycle gear configuration. My planes are taildraggers and don’t require a nose wheel steering mechanism or complete rudder pedal/cable circuit. While return springs are a common feature on taildraggers, they are not compatible with the Acro rudder/brake pedal system.
Luckily, this over-center lock-up situation occurred during my building and hangar flying tests. It was needless to say, quite a surprise and I was able to repeat the hazardous result easily. That got me thinking. Point #2 - I was going to modify my plane, but then I thought, “Do I really want brakes in the front cockpit?” I do a lot of Young Eagle flying and I always warn passengers not to rest their feet on the pedals. Still, my wife is a big offender. She has likely dented the floor in my truck from mashing the imaginary brakes on the passenger side. If she was in the plane it would be a bad day. At least I have a chance to over-power an errant rudder input. If the brakes are inadvertently applied you’re likely in for an off-road (runway) experience. With tandem seating there is nothing you can observe or do. I would think long and hard over the utility of brakes in the front cockpit. I took them out of my Acro before I ever flew it. I still use a simple set of springs to return the rudder pedals to the neutral position and eliminated the weight of extra pulleys, cables, linkage rods, and front brake pedals from my taildragger.
The attached picture shows the original system on my Acro. This is an Acro rear rudder pedal and the Eagle brake pedal. The rod linkages were removed and the triangular tabs were cut off and raised about 3/4 of an inch to move the brake pedal forward. There is a lot more beef here, but it is a heavier plane. Note: the original Sonerai 1 brake pedals had issues with these extended unsupported tabs.

Good Luck,
John

 

[wbpace]

Thanks for the feedback, John! I took your input and made several modifications.

The Acro design uses a .063 x 3/4 piece of channel stock extended on a length of .049 x 3/4 4130 tube.

I saw that and agree it looks flimsy. But it wouldn't work anyway since my MC (Master Cylinder) uses a clevis on top.

Your extended strap looks a bit frail and may fail much like the extended tube on the Acro brake pedals did.

This is exactly the concern I had and is one of the two things I was hoping to get some opinions on. (The other is whether I need to use 3/4" or .058 tubes for the rudder pedal and bottom torque tubes rather than the 5/8 .032 tubes from the standard S-II plans.)

So I moved the MC to center behind the brake pedal vertical tube and, per your suggestion, made a big tab out of it. I also shortened the mount at the base of the MC, and made them trapezoidal to increase the weld bead length. Together, the two bottom tabs now equal the weld bead length of the top tab. There's little clearance here, so it might need some strategic filing to get to fit. Also, I spec'd .090 for all of the tabs, but you said you used .065 for your tab, so if that is working for you, then I can look at using the thinner material (easier to weld on an already complex assembly).

There are other design considerations for me. First, according to Matco, the brake pedal needs a minimum 2.5:1 mechanical advantage (meaning if the top connection tab is 1” from the pedal axis, then the pedal needs to be 2.5” tall. I don’t want the total rudder/brake pedal combination to be much higher than 11” (I think) so that the rudder pedal will touch the ball of my foot/shoe without having to lift my foot too much. (But maybe I am wrongheaded here and it really should be taller?) Altogether, that 2.5:1 ratio constrains how long I can make the center-to-center distance between pedal axis and piston.

A third constraint is that the MC attaches to the rudder torque tube, not to the fuselage. This is so the brake pedal does not move every time I press on the rudder. Again, this is per the Matco papers I’ve read, and it makes sense to me.

All this combined means my changes puts the MC’s reservoir closer to the pilot's foot. I’m interested in whether anyone thinks this will be a problem.

The design calls for a brake actuating rod to slave the rear brake pedals and cylinders when the front brake pedals are utilized.

I am not going to link the brakes to the front pedals for all of the reasons you mention. Having anyone up front will be relatively rare, and a taildragger qualified pilot even rarer. So for safety, complexity and especially weight considerations, this was never in my plans.

Again, thanks for the feedback, and I’m still interested in any and all opinions. Also, if anyone can offer vertical measurements (heel to rudder to brake pedal) that works well for them, I would be grateful. Below are some more pictures of the modified design.

O’Bill

 

[oahupilot]

The design you have looks like it will not allow for full rudder deflection, your MC will hit the floor board if you have one. If you do not have a floor board then it wont matter. The other issue is to use that brake set up you have to actively hold the rudder in place while using your toes. I built a similar setup in my plane and found it to be less then ideal.

 

[wbpace]

Oahu, you have a good eye. Very true, the MC will go below the plane of the floor, so I plan to either not put a floor in that section, or alternately, make a cut-out.

I did some analysis (trigonometry) of pedal travel (picking the cable attachment holes on the rudder horn and rudder pedal horn that would create the most extreme pedal travel) and came up with the worst case fore-and-aft pedal travel. A roughly two inch long slot will accommodate the MC falling below the floor. More if I have to tilt the neutral position forward due to my leg length.

(Details: my math shows cable travel to be about 1-5/8" fore and aft, or 3-1/4" total. That's with the rudder hitting the elevator. The result is 120 degrees travel is possible, but I doubt anyone really configures it that way.)

Your observation about "actively hold the rudder in place while using your toes" is something I am also concerned about, and was hoping for feedback like yours. That begs the obvious question: is it better to tie the MC bottom to the fuselage crossmember instead? What have some of you folks with toe brakes done?

O'Bill

 

[acrojohn]

Bill,
I gave you some bum information. The brake tab welded to the vertical brake tube pedal is .080, not .063. I'm in the hangar now taking some pictures of Acrosport and Eagle pedals. I'll also get some dimensions for a later post.
Your rudder tab needs more height and support. Could you make the pedal more like a triangle and integrate some bushing stock into the inside vertical to attach the rudder cable to? When I post the Eagle pedals you will see a basic, elegant solution. Unfortunately, a totally different design, but easy to fabricate.
John

 

[acrojohn]

Bill,
I have attached pictures of the different pedal spare parts I have in my hangar. I also included Acrosport II Plans dimensions. I added the pictures of the Christen Eagle pedals I have in my Acrosport. This design could be adapted, but after some thought, I saw that you’d have to alter the orientation of the vertical Z leg to get the bushing closer to the run of the rudder cables – not a big deal, just different. The Acro Rudder/Brake Pedal picture is very adaptable. I flipped the brake pedal upside down to hide the original brake arm and I mocked up a piece of tape to show how a horn could be configured. Arm dimensions for the rudder cable attach point and arms for the brake horn would have to be set for your specific application. The Sonerai 1 plans show an overall height of 6”, but no specific height for the rudder cable attach point. It is approximately 5”, but any dimension would have to be coordinated with the specific to the rudder horn geometry. Just food for thought. Good luck.
John

 

[acrojohn]

another picture:

 

[acrojohn]

Last picture - Christen Eagle pedals on Acrosport II. Keep in mind these are heavier airplanes and the design could be scaled down a bit for a Sonerai II. Throws definitely need to be compatible with the Sonerai II which has a responsive rudder.

 

[wbpace]

That's helpful intel, John. That is the page of Acro plans that have, but its only a grainy picture someone posted on the net. Your pictures are much, much clearer to read. Nevertheless I should note I was inspired by, not directly copying the design.

Two points are worth mentioning. The position of the tab near the top of the brake pedal, as you show, will not provide the 2.5:1 mechanical advantage that Matco specifies. That's one reason why my tab is near the bottom.

Also, the S-II plans show two attachment points on the rudder pedal horn (I presume you pick one to set the sensitivity of the rudder, just like on the rudder horn itself, which gives you a lot of options to dial this in), and it specifies a rudder pedal height of 7-3/4". In particular I want to maintain this height, so the brake pedal goes above it. That's the other reason for my bottom tab.

Having said all that, I am taking all this input into account. One of the great things about SolidWorks is it will do a Finite Element Analysis to reveal on stresses on a part, so I can see where there may be problems. It's really simple to set up and it is really cool! SW even provides the mechanical properties of a lot of materials, so I just have to tell it everything is 4130 normalized steel. The EAA/Standard version will only do it on a part, not an assembly, but I also have access to SW at work (Professional version), which I will run this through once I an close to finalizing. The Profession version will let me do an analysis of the entire assembly.

Once through all that, I will be able to have high confidence that this will hold together well in service, and will share that information with everyone.

The issue of where to attach the MC base, to the torque tube or to the fuselage (Oahu's point about holding rudder in place while using your toes), is the major concern I think I have yet to think through. That's an ergonomic issue, not a structural one, so pireps are more helpful on that than any software!

O'Bill

 

[patmc]

O'Bill,
Your design for your rudder/break assembly has a design flaw that you would find after you tried to use the breaks (right or left) when the same rudder peddle is fully applied. The design you show has the base of the break cylinder mounted to the torque tube of the rudder pedal. You will find that the breaks will be very hard to apply with full rudder input. One of the replies to your post shows the installation of an Acro sport break assembly with the better design. The break cylinder is mounted to the structure separate from the rudder pedal. That makes the break pedal easier to apply when the rudder us fully applied. If you would like, give me a call @ 708-989 0267.