Stability of sim vs RW 172

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Oracle427
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Stability of sim vs RW 172

Post by Oracle427 »

Today I went for a flight on a beautiful day and made some observations about the stability of the aircraft in the roll axis. I see so many topics about people saying that aircraft are designed to be stable and that dihedral makes aircraft like the 172 so stable that they will remain level or roll back to level after any small change in bank angle.

I then see all sorts of discussion about sensitivity and null zones or modifying files to get to some totally hands off behavior that just isn't found in the real world.

So what I found was that the aircraft would no stay level when I took my hands off the yoke for more than 2-3 seconds while in cruise.
Within 2-3 seconds I would roll to one side, but generally to the left. I'd say 75% of the time the aircraft began to roll left in cruise. I was at 75% power and later at 65% power all trimmed out in pitch.

If I took no action, the aircraft would continue to slowly continue roll in the direction of the initial roll and generally within 15 seconds I would find myself in a 15+ degree bank. I took over each time at this point and leveled out the aircraft. Sometimes the roll would occur faster due to thermals or other turbulence.

I tried to "set it up" to be as stable as possible in level flight and it would generally stay put, until the smallest disturbance in the air would initiate a roll and then it would repeat the behavior from above.

So at least the 172 requires hands-on the controls even when trimmed out for "hands-off" flight. The only way to get truly hands-off flight is by using the autopilot. I'm 99.9% sure the 182 behaves exactly the same way, but I didn't fly that one today.

The thing is that it takes the most minute pressures on the controls to correct for these uncommanded rolls or pitch changes, which is why it is a good idea to leave the controls set for maximum sensitivity and no null zone to allow one to make these minute inputs. Increasing null zones has the effect of magnifying the required input and decreasing sensitivity has the effect of delaying the response to the user's input.
Flight Simmer since 1983. PP ASEL IR Tailwheel
N28021 1979 Super Viking 17-30A

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AKar
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Re: Stability of sim vs RW 172

Post by AKar »

My...favorites if you can say so, are surprisingly common incidents where pilots of aircraft such as commuter turboprops mistakenly believed they had the airplane under autopilot but for some reason or another failed to confirm, leaving the airplane into no one's control. More than one degree per second roll rates are usually observed, and changes in heading of up to 90° in around 30 seconds are not unheard of when the airplane goes on its own.

I agree to your point to the fullest.

-Esa

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Piper_EEWL
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Re: Stability of sim vs RW 172

Post by Piper_EEWL »

Very well put. And I think a difference in real world flying and the sim is that in the sim always a movement of the control unit is required and in the rw it’s mostly pressure on the yoke/stick to keep the plane flying straight. I think one is less inclined to notice the inputs given in real life if you don’t pay very close attention like you did on that test run of yours.

Happy flying
B377&COTS, J3 Cub, B-17G, Spitfire, P-40, P-51D, C172, C182, Pa28, Pa24, T-6 Texan, L-049&COTS, Bonanza V35B

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Oracle427
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Re: Stability of sim vs RW 172

Post by Oracle427 »

Pressure equals movement both in real world and simulator. The difference being that our joysticks do not have the resolution the simulate the minute movements we make. The movements/pressure we apply must be "calibrated" to our equipment to get the response we desire.

I think another of the big differences is that aircraft yokes and sticks don't self center when released. One can dial in a slight full input and the yoke will more or less stay there. I'm not sure if there are any aircraft with cable or pushrod linked controls that will self center in roll.
Flight Simmer since 1983. PP ASEL IR Tailwheel
N28021 1979 Super Viking 17-30A

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AKar
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Re: Stability of sim vs RW 172

Post by AKar »

Oracle427 wrote:I think another of the big differences is that aircraft yokes and sticks don't self center when released. One can dial in a slight full input and the yoke will more or less stay there. I'm not sure if there are any aircraft with cable or pushrod linked controls that will self center in roll.
Yeah, in reversible flight controls there are actually two distinct concepts: stick free and stick fixed. With most PC controllers, they are forced to be the same, but in reality they are not. How the 'translation' is done depends on the simulator. At one extreme, one could plot a direct relationship in between controller position and the corresponding flight control surface position, but that would result in completely unrealistic feel of the control forces, and would make a proper simulation of an aerodynamic trim tab impossible. Perhaps surprisingly, this is an approximation of how the FSX/P3D FDE seemingly works by default.

Yet, there is the fact that real mechanical flight controls stick a little, and carry just a bit of hysteresis. Just like your car may not perfectly center if you simply let go of the wheel. If we note a slight sticking with our controllers, the position of which we can read in very good accuracy, we simply apply some null zones to discount this situation.

-Esa

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