The trim bug in MSFlight Simulator

[b_kimoun 0]

Hi all,I don't know if you have already noticed that an aircraft at trim ( hands off) in MSFS does not show any elevator deflection . Normally, the trim tab developes the necessary moment that deflects the elevator surface (by the way, trim tab and elevator surface rotate in opposite direction) and maintains the elevator at the deflection angle that trims the aircraft so that the pilot does not have anymore to pull or push the yoke (hands off) to maintain level flight. In other words, it is the pitching moment Cm_de*elevator deflection that trims the aircraft and not the Cm_dt*trim tab deflection. The latter serves only as a substitution to the moment that the pilot exerts when dealing with the yoke to assure the trimming deflection of elevator.As well explained by the aerodynamic litterature, the trim tab (Cm_dt*trim tab deflection) serves to counter the aerodynamic force around the hinge( ie the hinge moment) of the elevator so that the latter can deflect by a certain angle. The role of the trim tab is then the same as pushing or pulling manually the elevator yoke. It appears unfortunately that MSFS makes a confusion between elevator hinge moment and elevator pitching moment.Any comment about this bug?

[UlfB 7]

Bug? No bug with my FSX. Trim and the animation of elevator deflection works perfectly well.I don't know why you don't see any deflection. Maybe you have to check the display setting Advanced animation in FSX?

[b_kimoun 0]

Bug? No bug with my FSX. Trim and the animation of elevator deflection works perfectly well.I don't know why you don't see any deflection. Maybe you have to check the display setting Advanced animation in FSX?

Hi, are you sure that your aircrafts (Cessna for instance) show a deflected elevator when trimmed or when AP engaged and autopilot altitude set ? I am not sure, in all cases only the trim tab deflects, and as I explained in my previous post, it is not only an animation problem but also this does not comply with the aircraft longitudinal stability rules. The elevator deflection to trim means a deflection of the elevator that involves both an elevator pitching moment (Cm_de*elevator deflection) and an elevator down force(Cl_de*elevator deflection).As a major bad consequence of this bug, If you keep the weight of aircraft constant (only payload, let's say aircraft with zero fuel and Illimitted fuel checked) and you move the cg location , let's say between 6% MAC and 36% MAC, the angle of attack alpha does not change (remains the same) and this violates the aircraft aerodynamics!

[Paul J 349]

FSX is this way because it's a sixty-dollar flight simulation aimed at the general public. Do you really think we should expect all the vagaries of Air Canada's two million dollar Airbus simulator?

[Guest wims]

Remember that the flight model has nothing to do with the graphics in FSX. There might be graphical imperfections that has nothing to do with the flight dynamics.

[Ron Attwood 3,469]

FSX is this way because it's a sixty-dollar flight simulation aimed at the general public. Do you really think we should expect all the vagaries of Air Canada's two million dollar Airbus simulator?

Er...$60?! I got my Gold edition for $35. Not only that, what's a 'vagary' and why shouldn't we have them? Now! :(

[magentajim 10]

What?!?! A bug in Flight Simulator!!! Outrageous! You're kidding-right?

[vgbaron 3,698]

Not intending to be rude but generally, who cares? That's like complaining that a rivet is 1/2 in out of place. Other than to a purist, it makes no difference.Vic

[cmpbellsjc 2,963]

Any comment about this bug?

As they say on the Monday Night Football pregame show:C'mon Man

[scoob 18]

Remember that the flight model has nothing to do with the graphics in FSX.

correct! exterior animations are tied to 'triggers' as defined by the model designer. looking at animations to see if the flight dynamics are correct is not appropriate.--

[scoob 18]

... this violates the aircraft aerodynamics!

the FS flight dynamics are mostly a 'black box' where the majority of knowlege about how the game simulates flight dynamics has been found by trial-and-error. the 'nuts and bolts' specifics have never been shared by MS for many stated reasons; the main one being the information in the 'box' cannot be shared as it includes details gained from the aircraft manufacturers simulated planes (ie, boeing, cessna, etc) who do not want the propriety information shared.oveall the game is more right than wrong. the detail in the dynamics is one of the reasons FS is not popular like other computer games ... planes are not 'easy' to fly. this leads to the questions: will MS dumb down the flight dynamics for FLIGHT to make the aircraft easy to fly.--

[some1 65]

It's hard to name it just a bug, it's an incorrect (or overly simplified) implementation of the whole trim system in FS series. Not the only thing that is incorrect, to speak the truth ;)Some of the consequences of this implementation are that:-at maximum stick deflection you can still achieve greater control effect by overtrimming. This virtually allows you to fly most aircraft in any situation completely ignoring CoG values and the limits that developers put in aircraft FM. -triming on force feedback joysticks doesn't work correctly.A workaround is possbile, but requires creating custom trim system for the plane, or for the whole sim (like in payware FSforce, or my helitrim gauge). Most developers don't do that, as it is not simple and not very important to most simmers.

[b_kimoun 0]

It's hard to name it just a bug, it's an incorrect (or overly simplified) implementation of the whole trim system in FS series. Not the only thing that is incorrect, to speak the truth ;)Some of the consequences of this implementation are that:-at maximum stick deflection you can still achieve greater control effect by overtrimming. This virtually allows you to fly most aircraft in any situation completely ignoring CoG values and the limits that developers put in aircraft FM. -triming on force feedback joysticks doesn't work correctly.A workaround is possbile, but requires creating custom trim system for the plane, or for the whole sim (like in payware FSforce, or my helitrim gauge). Most developers don't do that, as it is not simple and not very important to most simmers.

Very objective as an answer to my post, thanks alot some1.We have all to agree here then that MSFS is first of all a game that aimes to popularize flying experience through simulation but without using a thorough and very accurate aerodynamic model of aircrafts.Those who do not agree with me can check the following(Nb:Mach and ground effects no considered):1- Lift in MSFS: Cl= TBL404 value+Cl_flaps*flaps deflection+Cl_htail*incidence+Cl_de*elevator deflection+Clspoilers*deflection if we have htail incidence set to zero+a clean configuration(no flaps, no spoilers)+a trim condition (or AP enganged), this expression becomes: Cl= TBL404 value, (elevator deflection value returns to zero when Yoke is released and trim is achieved or AP engaged). This is of course wrong because there must be a trimming lift from the elevator !2- Piching moment in MSFS: Cm=Cm0+TBL473+Cm_flaps*flapsdeflection+Cm_htail*incidence+Cm_de*elevator deflection+Cm_spoilers*deflection+Cm_gear+Cm_thrust if we have htail incidence set to zero+a clean configuration(no flaps, no spoilers, gear up)+a trim condition (or AP enganged), this expression becomes: Cm=Cm0+TBL473+Cm_dt*trim tab deflection+Cm_thrust=0 No Cm_de*elevator deflection (it returns to zero) but Cm_dt*trim tab deflection , 2 totally different things ! As I said before Cm_dt*trim tab deflection serves to overcome the hinge moment of elevator (due to aerodynamic forces that tend to maintain the elevator in the air stream) so that elevator deflects without a need for a manual effort and makes possible the trimming moment Cm_dt*trim tab deflection .But I think that this issue can be resolved by linking (through programming of course) the trim tab deflection to elevator deflection, or to the htail incidence (for the case of large aircrafts that trim with THS, trimming horizontal stabilizer).

[b_kimoun 0]

Very objective as an answer to my post, thanks alot some1.We have all to agree here then that MSFS is first of all a game that aimes to popularize flying experience through simulation but without using a thorough and very accurate aerodynamic model of aircrafts.Those who do not agree with me can check the following(Nb:Mach and ground effects no considered):1- Lift in MSFS: Cl= TBL404 value+Cl_flaps*flaps deflection+Cl_htail*incidence+Cl_de*elevator deflection+Clspoilers*deflection if we have htail incidence set to zero+a clean configuration(no flaps, no spoilers)+a trim condition (or AP enganged), this expression becomes: Cl= TBL404 value, (elevator deflection value returns to zero when Yoke is released and trim is achieved or AP engaged). This is of course wrong because there must be a trimming lift from the elevator !2- Piching moment in MSFS: Cm=Cm0+TBL473+Cm_flaps*flapsdeflection+Cm_htail*incidence+Cm_de*elevator deflection+Cm_spoilers*deflection+Cm_gear+Cm_thrust if we have htail incidence set to zero+a clean configuration(no flaps, no spoilers, gear up)+a trim condition (or AP enganged), this expression becomes: Cm=Cm0+TBL473+Cm_dt*trim tab deflection+Cm_thrust=0 No Cm_de*elevator deflection (it returns to zero) but Cm_dt*trim tab deflection , 2 totally different things ! As I said before Cm_dt*trim tab deflection serves to overcome the hinge moment of elevator (due to aerodynamic forces that tend to maintain the elevator in the air stream) so that elevator deflects without a need for a manual effort and makes possible the trimming moment Cm_dt*trim tab deflection .But I think that this issue can be resolved by linking (through programming of course) the trim tab deflection to elevator deflection, or to the htail incidence (for the case of large aircrafts that trim with THS, trimming horizontal stabilizer).

Please read:As I said before Cm_dt*trim tab deflection serves to overcome the hinge moment of elevator (due to aerodynamic forces that tend to maintain the elevator in the air stream) so that elevator deflects without a need for a manual effort and makes possible the trimming moment Cm_de*elevator deflection (not Cm_dt*trim tab deflection .

[LAdamson 240]

The short & sweet version, as I must get to work. MSFS does simulate effects of moving loads/weight. However, it's limited, and I certainly wouldn't want to use a desktop model, as a test for the full version. Same applies to X-Plane, which uses an entirely different system to project flight. As an example, X-Plane has an awful time in determining the pitching properties of an aircraft, when flaps are applied. Is is up or down, and how much force to resist? These desktop programs are just not powerful enough to project actual values.L.Adamson