c172p
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c172p-team
FG's Cessna vs Real life.
Hi all, so yesterday I piloted a 1977 Cessna 172N, upgraded to 172P (Engine replaced), in real life.
Important Observations
- About the Cessna 172 in real life. Man, in real life, it is an absolute pleasure to fly it!
- Below 90 knots, it is extremely forgiving, you can make huge inputs and the thing barely responds.
- A 60 knots climb, it's extremely sharp, and your nose will be pointing to the sky. A shallow climb will be at 85 knots.
- The Cessna DOES NOT ROLL TO THE LEFT ALL THE FREAKING TIME. In fact, I didn't even notice that in real life.
- Seating position is way higher in real life, in FG it seems like you are hiding behind the instruments panel, in real life you can see almost the full nose of the aircraft.
- Tyres don't squeak like that, like someone is raping a chicken.
- It accelerates a lot quicker than it does in FG and stalls at 45 knots.
- The controls are a lot less sensitive overall. I strongly advise to have a look at Octals/Joshs PA-28 on how to implement proper inputs from the joystick/yoke.
Below some photos of the aircraft I flew in.
The good in FG's Cessna.
- Start procedure is spot on, just like in real life.
- Engine sound is ok enough.
- Engine startup is alright, but the aircraft shakes too much in FG.
- General dynamics are ok too.
you do know that you can adjust the trim to counteract the left movement, right? there is a trim tab on the rudder... and the seats are adjustable... if you turn on the craft save state, the seat position will be retained for the next flight(s)...
you do know that you can adjust the trim to counteract the left movement, right?
The rudder trim is optional, and the aircraft I flew in real life didn't have any sort of rudder trim.
and the seats are adjustable...
I am well aware of that and, in the Cessna, I need to adjust that all the time. Shouldn't the seat position be optimal by default? I can actually see a point where you need to adjust it once, in real life you do adjust the seat height once and then leave it there, assuming you are the only one flying the plane.
if you turn on the craft save state, the seat position will be retained for the next flight(s)...
Oh, I didn't know that! Thanks for sharing this information. Perhaps that should be the default?
Now, the seat adjustments, you mean the option in View > Adjust View?
@wkitty42 I tried the "adjust view height" option and it doesn't persist between sessions. However, I just learned about the Save/Load slots for states, that works fines! Thanks!
Now, the seat adjustments, you mean the option in View > Adjust View?
grep -i -e "seat" c172p-keyboard.xml ;)
reglnx@NeuX230:~/FGB/FlightGear-Common/Aircraft/c172p$ grep -i -e "seat" c172p-keyboard.xml
<desc>Reset to saved seat view</desc>
<desc>Seat Down</desc>
<desc>Seat Down</desc>
<desc>Seat Up</desc>
<desc>Seat Up</desc>
<desc>Seat Forward</desc>
<desc>Seat Forward</desc>
<desc>Seat Back</desc>
<desc>Seat Back</desc>
<desc>Seat Pitch Down</desc>
<desc>Seat Pitch Down</desc>
<desc>Seat Pitch Up</desc>
<desc>Seat Pitch Up</desc>
I fly C172SP in real life, 180HP. I agree with you, the FDM of FG 172P is far more from the real life.
reglnx@NeuX230:~/FGB/FlightGear-Common/Aircraft/c172p$ grep -i -e "seat" c172p-keyboard.xml
the goal was to see and show you that the craft has seat positioning within it... you can see the defined seat movement keys, doubled for upper and lower, as well as one (lower 'q' IIRC) to reset the position to the current saved seat position... so you don't need to use the sim's view positioning but you can to augment if desired...
Thank you @Megaf for the details, it is really appreciated. I've been taking a break from development for the last month or two but just today did some work on the fg1000 variant and want to get some of the existing issues that are close to finished wrapped up, fg1000, variant, new kap140, compositor upgrades.
I will review your observations and get back to you, if possible, for more details as we try to address these issues.
One quick question about the screeching tires. I assume you talking about the screech that happens when you press the brake too long? Do you know if you can apply enough brake to lock the tires up on the c172p in real life? I imagine that would be an extremely rare event, but is it possible to put the craft into a skid by slamming on the brakes or the parking brake? The sound your hearing is the tires being locked up, as in a skid on pavement or dirt (different sound). The problem is there is no feedback in the sim and the brakes are not analog. So it is not a perfect simulation of how it would work in reality. You don't hear that sound unless you have the wheels not turning but the aircraft is still moving across the ground or pavement, in other words, you don't hear that sound until you hold the brakes down long enough to start skidding. So I submit, if you even can "lock the brakes up" in real life, you would most certainly hear that sound. In the sim, when you hear that sound, you are moving across a surface without the wheels turning.
The shaking "too much", is that the short shake at start-uo that was recently added?
The Cessna DOES NOT ROLL TO THE LEFT ALL THE FREAKING TIME
Are you referring to the pull to the left on take off while still on the ground, or early on in the air, or always? Can you be really exact in the description of this effect or lack of?
Thanks @Megaf for this detailed report.
Part of your remarks are hard to fulfill, the aircraft's FDM is the result of compromises. Moreover, coupling between controls and effects often results in a change somewhere which breaks something elsewhere. However, if you find better compromises which does not break anything else which already works, please tell us.
JSBSim imperfections may also add to this, useless to say that this subject is very difficult and we (basic modellers) cannot change this. To submit improvements or remarks, we have to be certain that there is a flaw and accurately report it to the developers.
About the roll to the left: I think that you mean the roll at low speed and full throttle in the air, typically at climbing. Not "All the freaking time", as you wrote. Obviously, it is due to the propeller torque. I checked several times by power calculations, this torque is correct in JSBSim. It inevitably exists in real life and must be compensated. A difficulty is that is varies with on the propeller torque, which itself depends on the engine regime, throttle, aircraft airspeed. In real life, this trimming is either done by the the mechanics (preset compromise after tuning on the ground) or by a controllable aileron trim, if available. In FG (without aileron trim control), it is a compromise too. Currently in the c172p, this setting is such as the aircraft is approximately balanced close to 110 - 115 kt, 2300 - 2400 RPM (I don't remember exactly, but in typical cruise conditions). It can be changed on the fly in the Internal properties, but not saved. If you want to change this trim preset value and keep it saved, you can in c172p-main.xml, line about 594:
<controls>
<flight>
<aileron-trim type="double">0.022</aileron-trim>
About the general behavior (control sensitivity): often reported in simulation, mostly for light aircraft, very hard to fix. Part of it is the result of a feeling due to our joysticks. Springs are often harder than in real life, and we have no force feedback. For example, if we abruptly loosen the JS, it too quickly reaches back to zero. This gives a false feeling. Also, one should keep in mind that the JS sensitivity for each axis must be such as it can reach the max deflection (full effect), e.g. the ailerons must be powerful enough to hold a forward slip or a landing with strong crosswind (crossed controls); the elevator must enable to reach the stall AoA at landing. This for a given stick deflection (0 - 100%), often mechanically short. In the FDMs, these controls are generally linear. They can be made softer near the center by a quadratic or power 3 response in the JS binding file, but at the expense of a steeper response at high deflections. This can also be done and saved in the FDM file, but not so easily. I do not like this too much because it is a kind of trick, a workaround.
Once more, if you find better compromises which do not break any thing else, please submit your proposals such we can test them. You are in the best position to test this if you pilot a c172.
For the brakes do you have rudder pedals (progressive effect) or do you use the keyboard by short touching?
@wlbragg welcome back :)
The shaking "too much", is that the short shake at start-uo that was recently added?
When you start the engines, in real life it is much smoother, the aircraft moves just a little opposite to the spin of the engine. It's not your head that shakes, it is the aircraft that rolls slightly, very slightly.
I assume you talking about the screech that happens when you press the brake too long?
Yep, and when turning sharply. In all fairness, you might be correct here in leaving that as feedback. In real life, you have a very different feeling. And the brakes are digital, so you have a valid point.
Are you referring to the pull to the left on take off while still on the ground, or early on in the air, or always? Can you be really exact in the description of this effect or lack of?
In sim the Cessna has, in flight, a constant roll tendency cause by the rotation of the propeller, rolling in the opposite direction the propeller is turning. To counteract that, we use the rudder. In sim, this is too strong, so strong that even on the ground it makes the aircraft turns. I surely don't have a lot of experience, I have exactly one flight in real life, but I didn't notice any turning on the ground nor in the air. I flew it myself, with full controls, for around 40 minutes and barely had to touch the rudder pedals. @Octal450 your input would be very much appreciated in here, about constant roll tendency, I understand the PA-28 has a similar engine and propeller.
@dany93 Hi :)
Obviously, it is due to the propeller torque. I checked several times by power calculations, this torque is correct in JSBSim. It inevitably exists in real life and must be compensated. A difficulty is that is varies with on the propeller torque, which itself depends on the engine regime, throttle, aircraft airspeed.
I understand that. The thing is, in sim, without using the rudders, FULL AILERON might not be able to compensate for it and, that is just plain wrong. In real life, not only the effect is much weaker, but a lot less input is required. In the sim often times I see myself applying nearly half rudder trim to compensate. IMHO, if the effect was reduced to half of what it is now, it would be better already.
About the general behavior (control sensitivity)...
I agree! It's very hard to adjust that. I just got a new joystick today, a T16000M. Before, I was trying in the Extreme 3D Pro. Will do more testing and get back to you.
@tonghuix Do you fly the Cessna 172P in real life?
In sim the Cessna has, in flight, a constant roll tendency cause by the rotation of the propeller, rolling in the opposite direction the propeller is turning. To counteract that, we use the rudder.
No. Propeller torque (roll tendency) is to be counteracted by the ailerons (or an aileron offset). The rudder is for other effects (see below). And, as I wrote in my previous message, the aircraft roll tendency is neutral at cruise speed.
In sim, this is too strong, so strong that even on the ground it makes the aircraft turns.
This is normal and well known. I remember on an ultralight IRL, at taxing before takeoff, we would have been on the left out of the runway with no rudder correction.
Asymmetric Thrust Explained Left-Turning Tendencies Explained: Why Your Plane Pulls Left During Takeoff
.... I didn't notice any turning on the ground nor in the air. I flew it myself, with full controls, for around 40 minutes and barely had to touch the rudder pedals
You might well be the only one to notice no turning tendencies on a single-engine light aircraft....
The thing is, in sim, without using the rudders, FULL AILERON might not be able to compensate for it and, that is just plain wrong. In real life, not only the effect is much weaker, but a lot less input is required. In the sim often times I see myself applying nearly half rudder trim to compensate.
I did tests (once more...)
Wind = 0 (or at least no crosswind), 180 hp, Pilot alone, Taxiing, then climbing 70-80 kt (full throttle).
- Taxiing before takeoff: rudder 0.15 max (decreasing with airspeed)
- Climbing after takeoff:
- No rudder correction (rudder = 0, bad way, slip ball off center!) : aileron 0.15 - 0.2 (0.24 briefly).
- Rudder 0.08 (for slip ball centered): aileron 0.05 - 0.1.
Far from full aileron or half rudder. Even for the ailerons with the "bad" way of climbing with no rudder correction. I don't know where the problem is, but something seems not correct in your controls, your observations?
Which does not mean that the FDM accurately renders the real behavior, but the effects are realistic, observed and reported in real life. The only thing, I admit that the controls probably need a bit more sensitivity than the real one. But I did my best and couldn't find a better compromise. Either that's proper to simulation, FG?... difficult to fix. Or that's me, but please in this case, I suggest to those not pleased (@tonghuix ?) to propose a better FDM.
.... I didn't notice any turning on the ground nor in the air. I flew it myself, with full controls, for around 40 minutes and barely had to touch the rudder pedals
You might well be the only one to notice no turning tendencies on a single-engine light aircraft....
this is probably due to the craft being already well tuned...
Propeller torque (roll tendency) is to be counteracted by the ailerons (or an aileron offset)
Just pointing out, in the real plane, the aileron yoke is neutral or near neutral. I do not really ever recall needing separate aileron input. Simply pushing right rudder 's roll moment is enough to counteract MOST of the tendency.
Kind Regards, Josh
@dany93 Are we flying the same airplane? You are the first ever I see who says the roll is not strong, and the first ever I see who says the tendency is neutral in cruise.
I've seen lots of people giving up on the cessna because of what I stated.
@Megaf Yep, I plan to write a test report recently. We, with my instructor, tested this project compared with other FS platform (P3D and X-Plane), and also real C172.
There is something really strange happening here. For kicks I decided to try using only the optional rudder trim and no other input to take off down the runway. Setting the rudder trim to .120 allowed me to roll down the runway at full throttle. At the lower speed it started drifting left slightly and then as the speed increased it started drifting to the right. Point being, it took literally a tiny amount of rudder to counteract the full throttle torque. So little in fact I was able to use only rudder trim. So I submit, there is a big discrepancy in descriptions of how this issue affects users, why? From my perspective and on my hardware, there is no issue at all.
@tonghuix I will be really interested to see your report.
@tonghuix Thanks so much for that. That will greatly help with the clarifications. May I suggest you fly in the sim the same engine (120 vs 180) you fly in real life and the same location. I've been testing with the same engine, same airport and same weather conditions.
@wlbragg I will make a video and upload it. Will use fair weather which isn't supposed to have strong winds.
@wlbragg @dany93 @tonghuix https://youtu.be/drLXaidstww
So you can clearly see the amount of right aileron I need to make the Cessna stop rolling. You can also see that a bit of right rudder will stop it rolling.
Wow, I don't get that at all. I get slight left pulling on the ground until the airspeed is enough to allow the rudder to correct. I need to hook up my peddles or use my desktop though to really test this more. I'm on my new laptop and I have no independent rudder control.
OK, using auto-coordination I exhibited something a little closer to what your seeing, but not near as much.
I'll try to record this to show you what I get. But currently my controls are in disarray and it is difficult to cleanly control the aircraft.
Are you using auto coordination by chance?
Are you using auto coordination by chance?
Nope. And again, I'm not the only one experiencing that. And just to confirm, I used "Fair Weather" with "Basic Weather".
I don't have to do such large actions, neither. It seems that, some times, you counteract too lately, then have to over react.
One thing: if you have the "Rudder Trim Option" activated, the rudder trim is at 0 per default (you have to adjust it yourself). I set it at 0.02 in c172p-main.xml, but it is at this value only if you start with "Rudder Trim Option" OFF. Important for the following.
I checked the balance at (some) cruise airspeed. 180 hp, default load (pilot alone, 1876 lbs), Trims: aileron-trim 0.022 (= default) rudder-trim 0.02 (= default, only if started with "Rudder Trim Option" OFF). Or set it in the Internal Properties.
Balanced (at least well enough) for: rpm 2250, airspeed-kt 104, elevator-trim 0.064
@Megaf after much more testing (while I try to get the JS configured), I am in agreement with @dany93 It is definitely related to your rudder trim and for sure you need that 0.02 or more to make it behave more closely to what your expecting. Try experimenting with the optional rudder trim and see if that make sense to you.
When we test it, we forgot to record a video, but good news I keep the written record.
@wlbragg rudder trim is 0... I tested in 3 different systems and 4 versions of FlightGear, all show the same behaviour ¯\_(ツ)_/¯
What versions? I just checked 2020.4 and starting without rudder trim option active /surface-position/surface-pos-norm is set to 0.0199... As soon as the rudder trim option is activated /surface-position/surface-pos-norm is set to 0.0. Regardless, does setting /surface-position/surface-pos-norm to 0.2 help make it feel more like the real thing to you?
I'm checking now, but I think any released fgdata version will not have this logic programmed? I just checked 2020.3.9 and it had only .02 regardless of trim option being active. This c172p version is whatever is in fgdata next.
@wlbragg wrote
rudder trim and for sure you need that 0.02 or more to make it behave more closely to what your expecting.
0.02 (preset in c172p-main.xml) is a compromise between pilot alone and pilot + copilot. Yes, 0.02 to 0.03 or 0.04 is more appropriate with the pilot alone (which is default load).
But this must not be an issue. For tests, it can easily been tuned with the Rudder trim option ON, and with this option OFF, it can be changed in c172p-main.xml at preference for a default value at start. Also changeable in the Internal Properties in-sim.
But this must not be an issue. For tests, it can easily been tuned with the Rudder trim option ON
Yeah. that's why I want to make sure @Megaf test with the trim option. Because, as he stated
rudder trim is 0... I tested in 3 different systems and 4 versions of FlightGear, all show the same behaviour
So that does explain why it's pulling so much for him, if he is not changing that value.
Hello please. see the changes by @HHS81 to the c182s. They have made the aircraft FAR more realistic to fly and better, perhaps the same can be done to the c172, it seems they use a similar type of aero modelling.
Kind Regards, Josh
Not only that- while investigating the c182s yaw issue I found c172 specific coefficients from the same source as the c182s uses. I can bring them up here on weekend.
