Aeroconversion brake performance?

[Onex107]

In my tri gear Onex, Aerovee, I had the same problem with needing both hands to hold the brake during a runup. I added a single foot brake centered in front of the rudder pedals. It's made of welded alum. angle and tubing with a rod connected to the bottom of the brake lever. I can do a wide open runup without a problem. If you want pictures email me at flyvfr@comcast.net.

[13brv3]

A single foot pedal would be fine for run-ups, but way less than ideal for use during roll out, particularly in a tailwheel aircraft. No word from O'keefe yet. Spruce has no stock on the Hegar, and Hegar's web page appears to be from March 2009.

Out of curiosity, doesn't anyone happen to know the bore of the master cyl that Sonex supplies?

Rusty

[Kai]

Rusty,

See my posting of March 17. The diameter of the MCP master cylinder supplied by Sonex is 3/4’’. It is a pity the manufacturer can’t supply anything smaller- it would have made life so much easier!

Kai

[13brv3]

I missed that Kai. MCP seems to offer only the 3/4" and a reduced power 7/8". They seem to be mostly targeted to Kart racing, and I guess maybe they never want 1/2".

Ordered the brakes from Sonex, and a handgrip type master from Spruce. The master is a drop ship item, but I've ordered other Hegar masters in the past, and it only took a couple days. O'Keefe didn't have them in stock either. Sadly, I probably have plenty of time waiting for the return of the Dynon, probably mid April.

Rusty

[DCASonex]

If going to all the trouble of making a special one of a kind single hydraulic actuator for the brakes, why not just make two toe pedals, each with one cylinder to operate one brake. That alone doubles the force applied to each brake by each cylinder as compared to one cylinder displacing its force over twice the piston surface area.

That may also one one of the reasons O'Keeffe brakes out perform Sonex brakes. Sonex uses two pistons pressing on one side of each brake disk (trying to push the wheel off the axle) While O'Keeffe bakes are full floating calipers with a single piston clamping the brake disk from both sides, thus the area of one piston applies same force to two pads, one on each side. The force of one cylinder operating both Sonex brakes is distributed over the surface of four pistons, while with toe brakes, one cylinder's force is applied only to only one piston. I have found the standard O'Keeffe cylinders well matched to the toe bakes on my Sonex TD with a CAMit 3300 (127HP). They can hold back a full power run-up but.--- I once had a stray air current, at our hilltop airport, lift the tail and almost invert my plane when doing a full power run-up. I now only do that with tail tied to something very secure. Holding at the 1,500 RPM recommended for mag check requires very little force.

David A.

[13brv3]

For a hydraulic system like this, you need pressure, and fluid transfer. For disc brakes with the correct tubing and no air bubbles, there is essentially no fluid transfer so that can be ignored leaving only pressure. When we push on the master, we want to minimize our force, by maximizing the pressure. This is done with a smaller piston.

Say you push with 10 lbs of force on the master cylinder. If you have a larger piston with 2 square inches of area, then you generate 5 psi of pressure (10/2). If you have a smaller piston with 1 sq in of area, now you generate 10 psi (10/1). For the master, a smaller piston gives you more pressure for a given amount of force.

Since we aren't really displacing any significant amount of fluid with a proper brake setup, that one small piston can operate one or two brakes with the same force. Having dual masters gives you differential pressure, but no increase in brake force assuming the piston size isn't changed.

On the caliper end, we want to maximize force with the pressure we have available. If you have 10 psi of pressure available, you want the most piston area you can get (10 x total sq inches). For the caliper, more piston area (larger pistons, or more pistons) is stronger.

Of course there's a whole other layer of brake performance when you consider pads. Brake pad surface area and compound can make the difference between working well, and not working, even when the rest of the system stays the same. Nothing is ever simple.

Rusty

[Murray Parr]

Rusty is correct. Having one master cylinder supplying each slave cylinder independantly will reduce the pressure if the master cylinder size stays the same. I went with 1/2" hegar cylinders and copied the setup and geometry of the vans toe brakes. I haven't tried them yet however, the geometry sure feels right. Wasn't too hard to manage as I just removed the rudder pedals and did it all on the bench.

[Kai]

Murray,

I might be a little slow on the uptake, and I think I understand what you’re saying- but just to make sure:

SYSTEM REBUILD:
I want to keep the existing calipers from Sonex. Presently they are both operated with the single ø3/4’’ master from MCP (supplied by Sonex). I would like to get rid of the MCP since both holding power during runup and braking power during rollout is unsatisfactory.

SCENARIO #1:
I could replace the ø3/4’’ MCP master with one single ø1/2’’ stick mounted hand operated Hegar master. This will increase fluid pressure in the system and consequently also the fluid force on the Sonex puck pistons. Hence much better braking. Correct??

OR:

SCENARIO #2:
Differential braking is a safety item. Like David suggests, I could replace the single master ø3/4’’ MCP with two pcs ø1/2’’ masters from Hegar, ONE DEDICATED MASTER FOR EACH CALIPER. This will also increase fluid pressure in the system. Hence better braking, but not as much as scenario #1. Correct??

And of course the the milion dollar question: will this dual ø1/2’’ master setup provide any overall better braking than the single ø3/4’’ setup presently in the plane????

Much obliged for any comments!
Thx
Kai

[13brv3]

A smaller master cylinder bore will give more brake pressure "for the same force on the piston". If MCP made a 1/2" master that was a drop in replacement for the 3/4" you have, then it would absolutely improve braking. It makes me start to wonder how hard it would really be to make a custom master cylinder.

When you start considering changing from the tall handle Sonex likes, to a stick grip, or foot pedals, you are also changing the amount of leverage you have to push on the master cylinder. You'd have to look at the geometry of the actuation to determine if you gain or lose leverage compared to what you have now. Also consider that you probably have more force available from your foot than you have in grip strength, so a toe brake may gain performance simply because of the extra force you can apply.

Rusty

[lpaaruule]

This is what I did, it's not perfect, but I've been able to keep from rolling at full throttle:

viewtopic.php?f=39&t=2961&start=30#p45973

One recent observation is that the brake pads are wearing at an angle that suggests the rotors are flexing due to the increased pressure. I still occasionally think about switching to a traditional caliper design.