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PostPosted: Sun Nov 12, 2017 7:25 pm 
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Location: Typically hanging around N07, 12N, KLDJ, KCDW
I was doing touch and go landings today at KCDW in our C182T and thought I'd share a little something about what sets sim handling apart from RL.

The lack of control forces are one of the biggest problems I see with simulations. This lack of feedback likely gives rise to questions about the accuracy of the simulations with some saying that there too much sensitivity in the simulated controls, particularly when it comes to pitch.

When we use our controls at home, we have no feedback control pressures to tell us that we are fighting against the trimmed state of the aircraft. We are also able to make excessive control inputs that would require a lot of strength in the real machine.

Today was a case in point for me. On my third touch and go, I was trimmed just a tiny bit nose up when I retracted the flaps and added in full power. The aircraft promptly rotated upward and I knew right then that I was not trimmed where I wanted. I was fine for now with the nose pitched up about 15 degrees, climbing at 1600 FPM and making 80 KIAS. Airspeed was bleeding off and I started to push forward on the yoke. First I pushed lightly, then I pushed very firmly as I realized I simply do not have the strength to overcome the out of trim condition I found myself in. I immediately trimmed a little nose down and everything was back to normal. The yoke felt like it was set in cement! In the sim, I know I could apply full forward input and the simulated aircraft would happily respond to my command. IRL, I imagine that might well require 50+ lbs of force in an awkward position. This is one airplane where you do not want to be late in dealing with a runaway electric trim, or missing the trim position check pre-takeoff checklist item!

As it stands, the simulation cannot represent the pitch control "heaviness" of an aircraft like the 182. The RL aircraft is nimble and the controls are very effective if you apply the force required to manipulate them. In the sim we can see the nimbleness of the aircraft and as a result of the lack of feedback we can easily overcontrol it if we aren't careful about the way we operate the stick or yoke.


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PostPosted: Tue Nov 14, 2017 4:42 pm 
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Oracle427 wrote:
[...] then I pushed very firmly as I realized I simply do not have the strength to overcome the out of trim condition I found myself in [...] The yoke felt like it was set in cement! [...] This is one airplane where you do not want to be late in dealing with a runaway electric trim, or missing the trim position check pre-takeoff checklist item!
Interesting cautionary tale Oracle: and, yeah, your account immediately made me think about a fatal C182S crash here in the UK which led to a redesign of the KAP 140 autopilot some 15 years ago (discussed here a bit).
Oracle427 wrote:
I immediately trimmed a little nose down and everything was back to normal.
Good response though in your case.

With my hardware setup, the absence of a physical trim wheel means that I'm limited to the equivalent of a manual electric trim rate which would probably be a bit slow for a situation such as this. I'm guessing that in the case you describe, your instinct was to go straight for the trim wheel?

Nick


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PostPosted: Tue Nov 14, 2017 5:54 pm 
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Location: Typically hanging around N07, 12N, KLDJ, KCDW
This wasn't a situation where a stall was imminent. I certainly don't mean to make it sound like the airspeed was going to drop critically low had a done nothing. The situation was such that I wanted to cruise climb, but the aircraft was behaving like it was trying to climb at Vy or even Vx. I was surprised at just how much force was required to get the nose down to pitch for 80KIAS. I imagine the control forces would have been much weaker if the engine was not at pull throttle and max RPM.

So for a balked landing or a touch and go, it is important to check the trim and/or be ready to promptly fix it!

In the 172 one can easily overpower the trim by pushing or pulling the yoke and then hold it there before fixing the trim. In the 182T, I think one would quickly get exhausted from the physical effort if they do not adjust the trim.

The manual electric trim movement rate is the same as in the sim. I could have used it in this situation, but I always prefer to operate the trim wheel manually except for making minor adjustments while in cruise. The trim condition was about one spin off from where I needed it to be.

Years ago during my training, I did cause my KAP140 trim to runaway by keeping my hands on the yoke and "helping" the autopilot. The instructor watched and waited for me to get the surprise and to see what I would do. It's the last time I'll ever do that! :)


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PostPosted: Tue Nov 14, 2017 6:53 pm 
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Oracle427 wrote:
This wasn't a situation where a stall was imminent. I certainly don't mean to make it sound like the airspeed was going to drop critically low had a done nothing.
Yeah, hope you forgive me for selectively quoting your post for dramatic effect! :mrgreen: However, I guess the truth is that being physically unable to overcome an out-of-trim situation is a potential hazard to life, which—as you say—can't be adequately reproduced in a PC sim. (At least not with common-or-garden hardware.)

I recall years ago reading about a parachuting club which operated a PC-6 Porter. Their practice was that every jumper on boarding the aircraft would slap the pilot on the shoulder and say "trim checked?" or words to that effect. (Actually, I may have made up the slapping part!) I gather that taking-off in an out-of-trim configuration in the PC-6 is particularly deadly, and quite a few examples seem to have prominent placards as a result.

I guess that finding some cunning programming way to actually reduce elevator effectiveness would be the the only way to simulate this within the context of a PC-based flight sim with standard hardware controls.

Nick


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PostPosted: Tue Nov 14, 2017 7:09 pm 
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Location: Typically hanging around N07, 12N, KLDJ, KCDW
Nick M wrote:
Oracle427 wrote:
I guess that finding some cunning programming way to actually reduce elevator effectiveness would be the the only way to simulate this within the context of a PC-based flight sim with standard hardware controls.


Yes, that and the sim can ask a few questions to build an appropriate profile. i.e. Been to the gym lately? :)

Both the 172 and 182 checklists I am looking at mention checking/setting the trim three times prior to takeoff.


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