Brake load in cockpit 2

[berkshire]

I am trying to size a brake cable for a footbrake on an experimental sailplane. Single seat, single landing wheel.
I am tring to figure out what load a pilot CAN exert, arching his back and shoving as hard as he/she can. Since the aircraft is a tail dragger design, if the pilot over does it he/she is likely to put the aircraft up on its nose. The other scenario which bother me more, is wet grass, where the pilot locks the wheel which then skids, and the pilot not realizing this, just shoves harder and harder.
FAR 23 is not very helpful on this: it says: ""Sec. 23.405 Secondary control system.
Secondary controls, such as wheel brakes, spoilers, and tab controls, must be designed for the maximum forces that a pilot is likely to apply to those controls.""
All I can find is rearward stick loads of 167lbs in " 23.397 , and I am thinking that a pilot hauling back on the stick must be producing similar loads to a guy shoving on the brakes. If I double that number to 334lbs would this be adequate?
I am not considering G loads such as the 9g forward loading here, I am considering if it gets that high , the pilot has already hit something.
B.E.













The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[GregLocock]

FSAE specify 2000N, I'd regard that as a minimum. Basically you can't ever allow the brake system to bend or break under any conceivable pedal pressure.

FAA seems happy with 375 pounds for light sports planes.

Cheers

Greg Locock


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[verymadmac]

It is the same as the rudder load, but as the pilot can apply the load with both feet, we get 400 lb total.

 

[MintJulep]

Every sailplane I ever flew had the wheel brake actuated by hand. Integrated with the spoiler control.

Either the last portion of stroke, or a squeeze handle.

 

[berkshire]

Mint,
That was the old school thinking,the newer sailplanes now have heel brakes operated by a slide bar on the rudder adjusting pedals.
B.E.
Mac
I had looked at the rudder loads, and had missed the point that the pilot can push on both at the same time.
B.E.

Based on this I will use a 3/32" size cable rather than 1/16".

Thanks for you help gentlemen.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[rb1957]

is this a sport plane (where weight is critical) or a recreational glider (where safety is more important) ?

how about setting up a test rig, what can you push in a semi-reclined posture in a cramped cockpit ?

 

[berkshire]

rb1957
It is a recreational glider, with of course a semi reclining cockpit. I guess I can set up a load cell on the rudder pedals when we get that far, Right now I was looking for a first cut ( so to speak ) on that brake cable.
3/32" cable has a breaking strain of between 930 and 1100 lbs depending on the type, so I will try that first and see if anybody can break it. If they do I will be surprised, but strange things have a way of happening around aircraft.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[GregLocock]

Daft question, there is no lever ratio involved?

A strong male can bench press what 150% ? of their own weight, you'll probably find the limit is the seat strength.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[rb1957]

"make it stout, out of what we know about"

 

[RPstress]

A long time ago we apprentices were building a pedal cart. We used bathroom scales to measure how hard we could push when seated on the floor with our backs resting on a bench leg. The most was about 22 stone (308 lb). This was with one foot.

 

[berkshire]

RP,
So what did you get with both feet?
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[YoungTurk]

When I was a young lad, I could leg press about 500 pounds. I once saw a future NFL lineman exceed 1000 on the same device, which seems a good parallel to your situation in terms of ergonomics.

http://en.wikipedia.org/wiki/Leg_press

Apparently "Former US Secretary of State and septuagenarian Madeleine Albright claims she is able to leg press in excess of 400 pounds." If I were you, I would not let Madeleine Albright fly my sail plane

I do not work with controls, but we size for 250 lb limit, 375 ultimate on "steppable" surfaces for part 25 interiors per our company standard.

 

[berkshire]

YoungTurk (Mechanical)
Well now you have me worried about the brake cable, the brake pedal assembly, the seat back and half of the cockpit.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[ambrick]

For what its worth, the ergonomics chart I have states the max force a ‘strong man’ can exert on a foot pedal is 336 lb (with knee angle of 105-110 degree), 420 lb (knee angle 120 degree), and 449 lb (knee angle 135-155 degree). This is for someone sitting straight up (ie. Hip angle of 95 degree) which doesn’t sound like your situation so use with caution. These values are for instantaneous max force so I would consider them ultimate. This reference doesn’t specify what would happen if you used both feet.

Reference Diffrient, Niles, et al. Humanscale 4/5/6, October, 1981