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Using the autopilot is an important thing when flying airliners.

Re: Getting started with Route Manager

Postby HHS » Thu Oct 13, 2011 2:05 pm

Hooray wrote in Thu Oct 13, 2011 12:59 pm:Regarding terrain following, you may want to read this (which still holds true as of 10/2011) : http://www.flightgear.org/forums/viewtopic.php?


If you are talking abut the terrain-following mode which the Tornado GR/F's in RL has and and the BAC TSR2 as well, then you might wanna try out the B1B here in FGFS.....(at least when it is still working- I didn't try it myself)
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Re: Getting started with Route Manager

Postby macnab » Thu Oct 13, 2011 3:00 pm

HHS, I tried both solutions. Neither work. With me anyway.
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Re: Getting started with Route Manager

Postby HHS » Thu Oct 13, 2011 3:02 pm

Then you have to wit until the main author fixed it....
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Re: Getting started with Route Manager

Postby macnab » Thu Oct 13, 2011 3:07 pm

Looks like it :-(

BTW I've never figured this out - how do I know if the author updates his xml code, and then where do I download his edited files?
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Re: Getting started with Route Manager

Postby Hooray » Thu Oct 13, 2011 4:06 pm

HHS wrote in Thu Oct 13, 2011 1:57 pm:I must admit I'm a bit confused about what people here understand under "VNAV".


To be honest, I am not confused at all ;-)

VNAV is, as I understood, and wikipedia also says, just an autopilot function which controls the aircraft vertical movements.

Wikipedia is only an "okay" information source here, the article is not particularly detailed at all: Yes, VNAV is "simply a way to assist a pilot in managing the altitude profile of a flight". But that isn't done so easily, as you'll see. The wikipedia article on FMS has a section on VNAV which is much more comprehensive and much better: http://en.wikipedia.org/wiki/Flight_man ... ystem#VNAV

HHS wrote:So a lot of aircraft including the 777 indeed doesn't simulate the right behavior, though it should be easy.

That's exactly your misconception, it's REALLY not so easy - to cite a boeing reference linked to by your wikipedia article:

http://www.ae.gatech.edu/people/jpclark ... Miller.pdf
  • VNAV is the vertical navigation flight profile
    which is the predicted flight trajectory of the
    airplane in the vertical plane as a function of
    distance along the horizontal flight path defined
    by the LNAV flight plan.
  • The flight profile reflects all speed and altitude
    restrictions specified in the guidance flight plan
    while honoring airplane operating limits.
  • VNAV computes guidance commands for the
    Autopilot or Flight Director and Autothrottle to
    follow the vertical profile.


Not even this Boeing file is particularly detailed.

FMS just controls how this vertical movements in cruise will look: fast climb rate, thrust and engine settings, speed...


One of the best and most comprehensive explanations on VNAV and FMS inter-operation is referenced in the previously mentioned jsbsim discussion, in the form of a 25 page PDF document, see here: http://www.davi.ws/avionics/TheAvionics ... Cap_15.pdf

To quote some excerpts:

15.2.2.3 Vertical Flight Planning
Waypoints can have associated speed, altitude, and time constraints. A waypoint speed constraint is
interpreted as a “cannot exceed” speed limit, which applies at the waypoint and all waypoints preceding
the waypoint if the waypoint is in the climb phase, or all waypoints after it if the waypoint is in the descent
phase. A waypoint altitude constraint can be of four types — “at,” “at or above,” “at or below,” or “between.”
A waypoint time constraint can be of three types — “at,” “after,” “before,” “after” and “before” types are
used for en route track-crossings and the “at” type is planned to be used for terminal area flow control.
Vertical flight planning consists of selection of speed, altitude, time constraints at waypoints (if required
or desired), cruise altitude selection, aircraft weight, forecast winds, temperatures, and destination baro-
metric pressure as well as altitude bands for planned use of aircraft anti-icing. A variety of optimized speed
schedules for the various flight phases are typically available. Several aircraft performance-related crew
selections may also be provided. All these selections affect the predicted aircraft trajectory and guidance.

15.2.3 Trajectory Predictions
Given the flight plan, the trajectory prediction function computes the predicted four-dimensional flight
profile (both lateral and vertical) of the aircraft within the specified flight plan constraints and aircraft
performance limitations, based on entered atmospheric data and the crew-selected modes of operation.
© 2001 by CRC Press LLC
The lateral path and predicted fuel, time, distance, altitude, and speed are obtained for each point in the
flight plan (waypoints as well as inserted vertical breakpoints such as speed change, cross-over, level off,
T/C, T/D points). The flight profile is continuously updated to account for nonforecasted conditions and
tactical diversions from the specified flight plan.

15.2.3.2 Vertical Profile
The fundamental basis for the trajectory predictor is the numerical integration of the aircraft energy
balance equations including variable weight, speed, and altitude. Several forms of the energy balance
equation are used to accommodate unrestricted climb/descent, fixed gradient climb/descent, speed
change, and level flight. The integration steps are constrained by flight plan-imposed altitude and speed
restrictions as well as aircraft performance limitations such as speed and buffet limits, maximum altitudes,
and thrust limits. The data that drives the energy balance equations come from the airframe/engine-
dependent thrust, fuel flow, drag, and speed schedule models stored in the performance data base. Special
construction problems are encountered for certain leg types such as an altitude-terminated leg because
the terminator has a floating location. The location is dependent upon where the trajectory integration
computes the termination of the leg. This also determines the starting point for the next leg.
The trajectory is predicted based on profile integration steps — the smaller the integration step the
more accurate the computed trajectory. For each step the aircraft’s vertical speed, horizontal speed,
distance traveled, time used, altitude change, and fuel burned is determined based on the projected
aircraft target speed, wind, drag, and engine thrust for the required maneuver. The aircraft’s vertical state
is computed for the end of the step and the next step is initialized with those values. Termination of an
integration step can occur when a new maneuver type must be used due to encountering an altitude or
speed constraint, flight phase change, or special segments such as turn transitions where finer integration
steps may be prudent. The vertical profile is comprised of the following maneuver types: unrestricted
ascending and descending segments, restricted ascending and descending segments, level segments, and
speed change segments. Several forms of the energy balance equation are used depending on the maneuver
type for a given segment of the vertical profile. Assumptions for the thrust parameter are maneuver type
and flight phase dependent.

15.2.5.2 Vertical guidance
The vertical guidance function provides commands of pitch, pitch rate, and thrust control to the param-
eters of target speeds, target thrusts, target altitudes, and target vertical speeds (some FMS provide only
the targets depending on the flight management/flight control architecture of the particular aircraft).
Much like the lateral guidance function, the vertical guidance function provides dynamic guidance
parameters for the active vertical leg to provide the crew with vertical situation awareness. Unlike the
lateral guidance parameters, the vertical guidance parameters are somewhat flight phase dependent.
Vertical guidance is based on the vertical profile computed by the trajectory prediction function as
described in a previous section as well as performance algorithms driven by data from the performance
data base.

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Re: Getting started with Route Manager

Postby HHS » Thu Oct 13, 2011 4:40 pm

Hooray,

First part of VNAV is the control of vertical movement of the aircraft.
Second and more important part of that is "to predict and optimize the vertical path. Guidance includes control of the pitch axis and control of the throttle." and the other fancy stuff it can do today to optimize the flight - but only in combination of a FMS/FMC

The 777 and the Citation X doesn't even use the VNAV in their simplest way by just targeting the altitude or using FLC-mode. (We even don't have a correct FLC-Mode implemented yet!!)
Even if we have a realistic FMS- it would be useless without a correct Autopilot.
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Re: Getting started with Route Manager

Postby HHS » Thu Oct 13, 2011 5:08 pm

From the 737 which explains the whole thing a bit more:

The first basic FMC on the 737 was introduced 1979. The 737-200Adv was fitted with SP177 autopilot with integrated PDCS/FMC. It indeed has a VNAV-Button.

But: "The PDCS was developed jointly by Boeing and Lear Seigler in the late 1970's. It enabled EPR and ASI bugs to be set by the computer and advise on the optimum flight level, all for best fuel economy."

Please notice, that it just advised- it never controlled yet. This came 1982: "By 1982 the autothrottle had been devised and thrust levers could be automatically driven to the values specified by the PDCS."

But this is wasn't still the FMS we know today and has been described by Hooray.
"The true FMC was introduced with the 737-300 in 1984 this kept the performance database and functions but also added a navigation database which interacts with the autopilot & flight director, autothrottle and IRSs. The integrated system is known as the Flight Management System (FMS) of which the FMC is just one component."

Quotes from: http://www.b737.org.uk/fmc.htm

So a modern airliners has following system.
- an autopilot
- a flightdirector
- autothrottle
- IRS/GPS
- a FMC (Flight Management Control) and together with a perfomance database and functions, navigation database

And they do all interact together which makes the FlightManagementSystem.

http://www.biggles-software.com/software/757_tech/flight_management_navigation/vnav.htm describes VNAV more simple.
"VNAV controls the path and speed to comply with waypoint crossing constraints."
Those contraints can be min/max altitude, speed limits. So before we can think about a perfomance database, we should make it the simple way as described.
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Re: Getting started with Route Manager

Postby Hooray » Thu Oct 13, 2011 6:41 pm

Okay, so what you were trying to say is that we need to have a full autopilot/PID controller configuration first, right?

We could add new sections on VNAV to the wiki: http://wiki.flightgear.org/Howto:_Design_an_autopilot

What aircraft would you suggest? About a year ago, zakalawe suggested the use of the 777: viewtopic.php?f=46&t=8366
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Re: Getting started with Route Manager

Postby HHS » Thu Oct 13, 2011 7:32 pm

Hooray wrote in Thu Oct 13, 2011 6:41 pm:Okay, so what you were trying to say is that we need to have a full autopilot/PID controller configuration first, right?

We could add new sections on VNAV to the wiki: http://wiki.flightgear.org/Howto:_Design_an_autopilot

What aircraft would you suggest? About a year ago, zakalawe suggested the use of the 777: http://flightgear.org/forums/viewtopic.php?f=46&t=8366


Exactly, I would also take the 777 is in the base package and has currently the best developed AP though. ThorstenB has put a lot of time in it, and we have an user here who knows the real one.
As I understood ThorstenB has it somewhere on his To-Do-List....
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Re: Getting started with Route Manager

Postby Hooray » Sat Oct 15, 2011 4:00 pm

Please don't send support requests by PM, instead post your questions on the forum so that all users can contribute and benefit
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