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Zeus have previously provided us with links to aviation certifications standards (e.g. FAR 23.155, FAR 23.397).

I think Far 23.155 specifies the forces to provide maximum elevator control signal. But I do not find the same for rudder nor Aileron. Far 23.397 do specify higher values, but it seems like forces that might happen on more manual controls.

I just tried a simple test setup with person weight and sitting in a chair, and I am a 85 kg. male. I think it gets uncomfortable to press more than 45 kg or (100 lbs) with one foot. A smaller (female) person may not like that.

If you design a force sensor in a pedal for rudder, what is the design limits according to these standards? What should be considered max signal?

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FAR 23.143 used to limit the short-term maximum rudder pedal force to 150 lbs-f (670 N) and the maximum for long-term to 20 lbs-f (90 N). For gliders (JAR 22.143) these limits are 400 N and 100 N, respectively.

MIL-F-8785 B (§ 3.3.2.5) used to limit the rudder pedal force to maintain coordinated flight to 50 lbs-f (220 N). § 3.3.9.1 gave a maximum limit of 180 lbs-f (800 N) in case of unsymmetrical engine failure.

All these standards are no longer enforced, but they were the result of years of experience and still give good advice. I would expect that you will see much higher short-term loads, so your force sensor should cover the range from 0 to 300 lbs-f (1330 N) on a single pedal at least. Don't disregard dynamic loads and the force people are able to produce when under stress.

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  • $\begingroup$ Thank you for this information. This provide you information on the maximum loads an airplane MAY require the pilot to press on a pedal and structural design limits. I have been a glider pilot, and I remember that only small forces on pedal was required in any case. Regarding elevation control the standards require minimum forces, that is required for a max control signal. In this way the pilot get an ergonomically (proprioception) feed back on how hard the pilot try to influence the airplane. But as I see this, standards require no minimum forces for pedal and yaw control. $\endgroup$
    – Backflip
    Dec 13 '21 at 10:58
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    $\begingroup$ @Backflip Yes, there are also minimum forces. You need a positive gradient and MIL-F-8785 has a minimum friction of 1 lb. For practical purposes, you need to signal to the pilot when the structure is overloaded. Glider rudder cables are run in PE tubes to give the mechanism more friction. $\endgroup$ Dec 13 '21 at 11:20
  • $\begingroup$ Thanks. I found a readable copy of Mil-F-8785 here and will start to read :) archive.org/details/DTIC_AD0738625/mode/2up You seem to indicate, that you need friction force for the motion of the rudder. So there is no proportional relationship between force applied and "influence on airplane" (I hope my term makes sense). $\endgroup$
    – Backflip
    Dec 13 '21 at 11:49
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    $\begingroup$ @Backflip A bit of friction is helpful for damping. Generally, a linear relationship between cause and effect is desired and asked for in the regulations. $\endgroup$ Dec 13 '21 at 18:18
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The regulations prescribe forces that can comfortably be maintained during required periods of comtrol, by a representative pilot. But if the design is for an installation, it should be able to accomodate for the highest load that can be expected, without mechanical failure. An 85 kg person standiing on one leg can maintain an 85 dN load for a while, and even jump up and down on it.

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Above pic is from this answer, and depicts actual measurements from multiple test persons - what the human max forces are over time, per flight control. It is from prof. Gerlach’s uni book I held on to.

You can see the variation in force and endurance, particularly with the pedals. The force sensor must be able to handle the maximum force, times a safety factor of 2-3. The forces achievable over a couple of minutes would be of interest for the regulations, the impact forces at t=0 are much higher and it would not be good to have the sensor replaced after every kick.

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  • $\begingroup$ Thank you for this information. This will provide you information about maximum forces humans can deliver on pedal. Regarding elevation control the standards require minimum forces, that is required for a max control signal. In this way the pilot get an ergonomically (proprioception) feed back on how hard the pilot try to influence the airplane. But as I see this, standards require no minimum forces for pedal and yaw control. $\endgroup$
    – Backflip
    Dec 13 '21 at 10:52
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    $\begingroup$ The regulations are concerned with the right hand side of the graph: stay below the line of what humans can maintain comfortably. The mechanical set-up and force transducer should be able to handle the left hand side. Proprioceptive pedal feeling can be taken proportionally to the graphs, @ 10 mins pedal forces are about 3 times as high as yoke forces. The plots we took for simulator force control always showed higher static forces on the pedals. $\endgroup$
    – Koyovis
    Dec 13 '21 at 13:49
  • $\begingroup$ In a way the rudder signal comes from a difference in forces to the two pedals. But I guess that when pilot want about max effect, then the pilot do not press with the other foot. So a design signal (force) range for an "electronic pedal" could be 0 - 25 daN. And then of cause it shall be able to withstand higher values. $\endgroup$
    – Backflip
    Dec 13 '21 at 18:40

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