Fixed-wing performance model docs - update #3029
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| The minimum sink rate can be written as: | ||
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| $$\dot{h}_{min} = \sqrt{2mg\over{\rho F}} f(c_A, c_W)$$ | ||
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| where $\rho$ is the air density, F is the wing area and $f(c_A, c_W)$ is a function of the polars. | ||
| It's easy to see that the minimum sink rate scales with the square root of the weight ratio. | ||
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| From this equation we see that the minimum sink rate scales with the square root of the weight ratio. |
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FYI It is "easy to see", but what some see as easy is not as obvious to others. My training tells me to avoid suggesting things are easy.
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@hamishwillee I think I have read too many papers which suggest that something is "easy to see". I assume it's a way to try and make people feel dumb. I don't have a problem removing it. To me this section is for someone who is into the PX4 code and wants to double check calculations.
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That's precisely it - we don't want to make people feel dumb. I do think this is fairly easy to see, but it isn't a good habit to get into :-)
| If changes in weight and air density are not taken into account, altitude and airspeed tracking will likely deteriorate in the case where the configuration (air density and weight) deviate significantly from the configuration at which the vehicle was tuned. | ||
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| In section [Specify the Weight of the Vehicle](../config_fw/weight_and_altitude_tuning.md#specify-the-weight-of-the-vehicle) and [Compensation for Air Density](../config_fw/weight_and_altitude_tuning.md#compensation-for-air-density) you will find the parameters you need to adjust for weight and density compensation to take place. | ||
| In the [Notes](../config_fw/weight_and_altitude_tuning.md#notes-to-the-derivation-of-weight-and-density-compensation) section the curious reader can find more details on theory of each compensation. | ||
| ## Vehicle Weight Compensation |
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FYI This section is how I would (ideally) like all instructional sections - this is what you need to change and how.
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No flaws found |
| ### Specify a Service Ceiling | ||
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| In PX4 the service ceiling [FW_SERVICE_CEIL](../advanced_config/parameter_reference.md#FW_SERVICE_CEIL) specifies the altitude in standard atmospheric conditions at which the vehicle is still able to achieve a maximum climb rate of 0.5 m/s at maximum throttle and weight equal to [WEIGHT_BASE](../advanced_config/parameter_reference.md#WEIGHT_BASE). | ||
| By default this parameter is disabled and no compensation will take place. | ||
| This parameter needs to be estimated experimentally. It is always better to set a conservative value (lower value) than an optimistic value. | ||
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| This parameter needs to be determined experimentally. | ||
| It is always better to set a conservative value (lower value) than an optimistic value. |
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From "Effect of Density on Maximum Climb Rate"
where is the air density at sea level in the standard atmosphere and K is a scaling factor which determines the slope of the function. Rather than trying to identify this constants, the usual practice in aviation is to specify a service ceiling altitude at which the vehicle is still able to achieve a minimum specified climb rate
So my understanding is that while this scales by density ratio the scaling factor is hard to work out. So instead we set a service ceiling in which we know that the plane will fly OK. Is that right?
So what does the service ceiling do? I mean if I set this to 200m does this mean the vehicle is capped to that height?
Is there a rule of thumb about how you go about determining this? Can you suggest one? I.e. can you do some single maneuver and work it out from your logs?
| If the initial tuning is not done in standard sea level conditions then [FW_T_SINK_MIN](../advanced_config/parameter_reference.md#FW_T_SINK_MIN) needs to be multiplied with correction factor P: | ||
| The minimum sink rate is set in [FW_T_SINK_MIN](../advanced_config/parameter_reference.md#FW_T_SINK_MIN). | ||
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| If the [Basic TECS tuning](../config_fw/position_tuning_guide_fixedwing.md#tecs-tuning-altitude-and-airspeed) was not done in standard sea level conditions then the [FW_T_SINK_MIN](../advanced_config/parameter_reference.md#FW_T_SINK_MIN) parameter must be modified by multiplying with correction factor $P$ (where $\rho$ is the air density during tuning): | ||
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| $$P = \sqrt{\rho\over{\rho_{sealevel}}}$$ |
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So the density has to be known at tuning time right? Doesn't that mean you'll have to re-tune to set this because you probably didn't measure it?
Should we suggest us 1.225 kg/m^3 for the sea level value. How do you go about measuring the density when you're tuning? Can we get this from any of our on-board sensors?
How sensitive is vehicle behaviour to getting this wrong?
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@RomanBapst I'm going to stop chasing you on these.
This follows on from #2842 which I merged too quickly. I'm going to merge this to fix some mostly minor things. FYI @RomanBapst @sfuhrer