It seems to me that GA aircraft find it very hard to climb to anything like their operating ceiling with autopilot.
Case in point flying the Bonanza today. ATC (I know, I know) instructed me to climb to 15,000 ft. Shouldn’t be an issue as the stated ceiling is just over 18,000 ft especially as the weight was fairly minimal with fuel at about 40%. Once I got to about 9,000 ft I was down to about 500 vs and speed was dropping below 100kts. By the time I got to 12,000 ft vertical speed was down to 100 and I was in danger of stalling. No wonder, the AOA was insane, well into the red. I tried knocking off AP, dropping the the AOA to build up speed then re-engaged AP. Things ere better, but there was no way I was ever going to get to FL150.
Why is it that autopilot seems intent on these disproportionate angles of attack?
I flew the G36 the other day and I’m wondering if the mixture control is not working. I should be getting more power at altitude and I got the feeling that the mixture wasn’t working well, if at all.
First off in lighter vein: I hope you have an oxygen system in the Bonanza as it is typically dicey for a large percentage of the population to go above 12,000ft as hypoxia becomes a concern!
But more seriously: I believe it is generally understood that the base G36 as modelled is underpowered. The community project G36 is supposed to be much closer to the POH performance: this is what I have been flying almost from the beginning. To check the performance of this mod I just did a flight starting in the air at around 7,000 ft. (spawned in the air near a high-ish altitude airport). Set the altitude at 25,000ft. (!), engaged the AP with FLC at 100kts IAS. Set the power @ full throttle, auto mixture and prop fully fine @ 2,700RPM as required by the POH.
And it soon became clear that they improved the community mod performance a bit TOO much! Admittedly I was light (2 people on board, less than half tanks), but still…
Even at 20,000 ft. it was still climbing at a VERY healthy (and very unrealistic!) 200FPM. By this time the 100kts IAS was equating to 135kts TAS with an OAT of -24°C. For reference: the POH gives the following climb performance:
As a general comment: I find that the Bonanza’s AP is pretty good. A common issue is that people often tell it to do something the aircraft is not capable of and thus forces the AP to do something bad. In this case one should be sure that you are using FLC mode with the speed set at 100kts for best climb performance. With this setting you won’t get unrealistic AOA, the aircraft will just stop climbing once it hits its ceiling.
@ SlabsFly post is excellent! To elaborate a little on how an autopilot works.
First, no GA aircraft has autothrottle so the pilot is responsible for setting the correct throttle, propeller, and mixture. The autopilot uses whatever the power settings are.
There is a big difference between VS and FLC modes very important for GA aircraft.
When a new altitude is entered into the autopilot, the pilot can set a fixed vertical speed. The autopilot will change the aircraft’s pitch to achieve the vertical speed using the same performance settings. If the aircraft is flying at a particular speed, pitching the aircraft up will decrease the airspeed if no power settings are changed. The autopilot will continue to increase the trim until the VS target is reached NO MATTER WHAT THE AIRSPEED IS. Sometimes if the VS is set too high, the autopilot will continue to trim the aircraft until 100% up trim is reached irrespective of the stall speed. Downdrafts can cause problems when VS is used. If there is a 100 ft/min downdraft when VS is set to climb at 700 ft, the downdraft changes the VS speed to 600 ft making the autopilot trim the aircraft to return to the 700 ft VS. When the downdraft disappears, the VS will change to 800 ft due to the autopilot adjustment trim and the autopilot will adjust the trim to 700 ft VS. If a downdraft is too large, the autopilot will continue to trim until it reaches 100%.
IRL the VS mode can cause stalls as well. This isn’t a MSFS bug.
Using FLC the autopilot will adjust the trim to reach the desired airspeed no matter what the VS is. If an aircraft is flying at 150 kts and a new altitude is entered into the autopilot, the AP will pitch the aircraft to hold the FLC speed. The best rate of climb speed is often used. The fixed airspeed prevents stalling during the climb as long as the power settings are correct for the climb or descent.
Somehow it became general knowledge that sim pilots should only use VS and never use FLC. This is good advice for piloting aircraft that have an autothrottle (although stalls can still happen) but bad advice for aircraft without autothrottle.
How does this not make sense? I am not making this stuff up. I am just restating aircraft and wing aerodymincs and autopilot operation.
No matter if throttle is at max or lower, using VS the autopilot will trim the aircraft to climb VS rate of climb. As trim is increased to reach VS setting, airspeed decreases even at full throttle. Using FLC, the airspeed is fixed and the autopilot sets the pitch to maintain the FLC speed setting.
It is possible to stall any aircraft at full throttle. Using VS can cause airspeed to decrease too much and stall the aircraft at any power setting. When this happens, the aircraft may suddenly bank to the left or right and enter into a spin and if not corrected eventually crash. Many people see this as “the autopilot is trying to kill me!”
If this doesn’t make sense, during your next flight enable the autopilot, enter an altitude to climb to, enable VS and set it at +1000 ft or +1500 ft. Then watch the airspeed drop until VS setting is reached.
The point is that autothrottle has absolutely nothing to do with VS or FLC.
Autothrottle or manual throttle, the engine is operating at max or max climb power in both cases.
Yes, I am missing the point. Autothrottle has everything to do with VS and FLC because Autothrottle is constantly adjusting the power, not the pilot. If the aircraft needs to climb, then the autopilot adjusts the power depending on VS mode (autopilot sets the power to maintain VS rate) or FLC mode (autopilot adjusts the power to maintain the FLC speed). Isn’t this how an autothrottle and autopilot work together?
I am also missing the point about the engine(s) operating at max power. Many aircraft can climb without engine(s) at full power. Many twin engine aircraft can climb with only one operating engine.
You don’t understanding how an autothrottle operates during climb.
During climb it maintains max climb power.
It doesn’t reduce power, regardless if it’s VS or FLC.
When controlling the power manually, it’s you who’s maintaining max climb power.
Of course. You can also climb at cruise power if you want (although it doesn’t make sense to climb at less that than max climb power), but that’s not how an autothrottle system works during climb.
Climb (VS or FLC) = max climb power
Basically all twin engined aircraft can climb with only one operating engine.
No. AP and AT are two separate systems. AP doesn’t control power.
If VS is set too low, the airspeed will increase, but AT will still maintain max climb power.
I think the reason for that is that the FLC never worked when the game came out. I haven’t tried with the Garmin NXi mod to see if that has changed tbh.
Even the stock Asobo G36 should be out performing the published numbers in your scenario, the most likely issue is either insufficiently leaned mixture or you picked up some ice if flying through clouds.
In the FAA handbook, regarding the Vertical Speed Autopilot mode, it says the following:
‘You must be very careful to specify an appropriate vertical speed, as the aircraft will fly itself into a stall if you command the autopilot to climb at a rate greater than the aircraft’s powerplant(s) is/
are capable of supporting.’
… which is what you’re seeing. As you get nearer the aircraft’s service ceiling, the lower the vertical speed it can maintain at maximum power. It seems there is still a performance discrepancy which is most likely the fault of the simulator aircraft model, but the effect itself can and does happen.
I just repeated the exercise with the mod removed, so using the sim G36. And the stock G36 consistently outperformed the modded G36 by 150fpm at the lower levels reducing to an extra 50fpm at 20,000 ft. With the same power settings and weight etc.:
the stock G36 did 950fpm vs 800fpm modded at 12,000
the stock G36 did 600fpm vs 450fpm modded at 16,000
the stock G36 did 450fpm vs 350fpm modded at 18,000
the stock G36 did 250fpm vs 200fpm modded at 20,000
Okay. I see the engines power change and the airspeed changes during climbs and descents. If the autopilot is not controlling the power and I’m not controlling the power as I don’t change it during flight, then who or what is controlling the power?
Now I’m more confused. I don’t touch the throttle at all. But the engine thrust and airspeed change. The ONLY thing I am doing is changing the altitude in the autopilot and enable VS or FLC. If the autopilot controls the AutoThrottle, then the engine power changes because I changed the altitude. But if the autopilot doesn’t control the AutoThrottle… FADEC? And if I want to control the engine power during a climb, when I disconnect the AutoThrottle, it somehow switches back on and overrides my engine settings.
