The trim bug in MSFlight Simulator

[LAdamson 240]

But, do you think that it is safe to continue flying aircrafts(even as desktop sim) that do not trim as in real world?

The process I go through in trimming is exactly the same as real world, in regards to using a joystick with a hat switch for elevator and aileron trim. Sight, and the joystick's centering spring have replaced "feel" from airloads on the control surfaces. My mind fills in the gap to a degree, which creates a sensation of "feel", although it has it's limits.There are always compromises though. For instance, when simulating an "active" auto-pilot for roll..........do the ailerons move, when you move the joystick/ yoke,.... or not? In real life, the ailerons must move with the joystick, but in reality, you'll also feel the resistance of the servo motor......until you put enough pressure to override the clutch/servo motor or disconnect the A/P. If you design the "simulated" aircraft to have the ailerons remain still as the stick is moved right or left, it will give a visual indication, and a sense of feel, that you're fighting against a force. However, different individuals have mixed feelings about the solution. Yet, without force feedback, it's a compromise, one way or the other.L.Adamson

[DJ 0]

After reading all the comments here I have to jump in.What I really don't like in FS is that in real flying a plane is trimmed for a speed, therefore if the plane is trimmed straight and level and you bring the power back, the nose will drop to maintain the speed. In reverse, if pwr is increased the nose will go up. In all versions of FS and other sims the aircraft will tend to accelerate or decelerate vice nose going up or down. It is a fondamental flaw in FS.I have been flying for 25 years (air Force and Airlines) and while I was an Air Force instructor this was always a struggle to make student pilots understand that concept. I could continue for pages on the aerodynamics lessons but I think this is enough.Happy flying in the New Year,Dan

[Brett_Henderson 5]

After reading all the comments here I have to jump in.What I really don't like in FS is that in real flying a plane is trimmed for a speed, therefore if the plane is trimmed straight and level and you bring the power back, the nose will drop to maintain the speed. In reverse, if pwr is increased the nose will go up. In all versions of FS and other sims the aircraft will tend to accelerate or decelerate vice nose going up or down. It is a fondamental flaw in FS.I have been flying for 25 years (air Force and Airlines) and while I was an Air Force instructor this was always a struggle to make student pilots understand that concept. I could continue for pages on the aerodynamics lessons but I think this is enough.Happy flying in the New Year,Dan

While it's not completely accurate, it does come close. I've never tested it specifically (because it always seemed to work well enough), but I just did, for this post.I took the default C172 up to 3500msl (aprox 2500agl from my test airport).. leveled out at 2400RPM and trimmed for ~105kias ... I waited for everything to stabilize, and then pushed the throttle all the way in. It did accelerate at first, but didn't take long to settle into a ~105kias 400ftm climb. Same thing in reverse. A power reduction to 2100RPM did cause decceleration at first, but it too settled into a ~105kias, 700ftm descent... (albeit after an unrealistic "dive" to reclaim the airspeed)Obviously, this "delay" makes it difficult to teach aerodynamic/piloting fundementals about pitching(trimming) for an airspeed, but during the course of realistic simming, it's pretty accuarate.. ala had I not been "testing" the C172, and was transitioning from cruise to climb.. I'd have just set climb-power; pitched for cruise-climb airspeed, and trimmed to maintain that airspeed.. it would all have happened before the "flaw" would have shown itself.

[mgh 89]

Perfectly right mgh ! But what bothers me exactly is that the trim process (Cm_dt*tab deflection) in MSFS supplies the pitching moment like the elevator deflection does (Cm_de*elevator deflection) but not the elevator lift component (Cl_de*elevator deflection) which is completely ignored.In other words, once the trim process enters in action to replace the elevator effect, the pitching moment component (Cm_dt*trim tab deflection) is there, but there is no more the lift component that makes this moment possible(moment=lift *arm) like in the case of elevator deflected.I think that Microsoft could add a Cl_dt coefficient to the trim tab so that at trim condition, both the lift and moment components are there to provide the same effect as elevator deflection.It appears that FSFORCE (www.fs-force.com) has some kind of solution to this issue. Have a look at the picture below.

I don't understand what you are saying. There is a Cl_de entry in the .air file. It is -0.37255859375 for the default FSX Cessna 172SP and -0.2890625 for the FSX default B747-400. It appears to be in either Table 1101 or 1539.

[dmaher 0]

edit...Lol, I'll have to think much longer before I post on flight models...I've just confused myself :blush:But I’ll ask this.In the FS flight model...Does trim act on the elevator? (...real-life)Or, does trim act on the 'single body' system as a whole? (...simulation)I've always assumed it was the latter...now I'm not sure.Because there is no CLDt...AFAIK not even in the books.It makes it look more like the elevator and CLDe does this work.

[N1125Y 22]

.... ha .... not realy undesrstand all the high chet chut that goes on here .... but it's interesting :(

[ryanbatc 12,914]

But, do you think that it is safe to continue flying aircrafts(even as desktop sim) that do not trim as in real world?

Yes. Honestly your posts sorta read "FSX sucks, long live Xplane"imho.... XP has a laundry list of faulty things as well :(

[Glynn 405]

But, do you think that it is safe to continue flying aircrafts(even as desktop sim) that do not trim as in real world?

HelloWhy not?,I have never crashed a desk yet :(

[Brett_Henderson 5]

An interesting addition to this discussion; that I can't believe hasn't been brought up yet; is how a real, Mooney M20 trims.It's pretty close to a real-world version of MSFS trim.. there is no trim tab.. trim is accomplished by moving the entire "tail" up or down, form a pivot point in the empanage. Very similar to how MSFS moves the entire elevator deflection range up or down.

[LAdamson 240]

An interesting addition to this discussion; that I can't believe hasn't been brought up yet; is how a real, Mooney M20 trims.It's pretty close to a real-world version of MSFS trim.. there is no trim tab.. trim is accomplished by moving the entire "tail" up or down, form a pivot point in the empanage. Very similar to how MSFS moves the entire elevator deflection range up or down.

I thought about it, just didn't get to it.... :( And then there is the Piper Cub, where the horizontal stab uses a jack screw........kind of like passenger jets. As I remember, Cessna 180's had something like that too, but it's from memory, and I don't have the time to check for facts. At least trim tabs, are not the only way...edit: Just double checked on the Cessna 180. It's a jackscrew for trim....too.L.Adamson

[yellowjack 28]

I thought about it, just didn't get to it.... :( And then there is the Piper Cub, where the horizontal stab uses a jack screw........kind of like passenger jets. As I remember, Cessna 180's had something like that too, but it's from memory, and I don't have the time to check for facts. At least trim tabs, are not the only way...edit: Just double checked on the Cessna 180. It's a jackscrew for trim....too.L.Adamson

IIRC the jackscrews you refer to are ground adjustable only and are used to set up the basic rigging angles of the aircraft by ground engineers. The pilot don't get to move them.

[LAdamson 240]

IIRC the jackscrews you refer to are ground adjustable only and are used to set up the basic rigging angles of the aircraft by ground engineers. The pilot don't get to move them.

These jackscrews are pilot operated for elevator trim. They pivet the leading edge up & down. There are no trim tabs on the elevators.See this 2002 reply on the subject from Larry N. He owned a Cub, & specifically mentions the jack screw. 3rd replyhttp://forums.flightsim.com/vbfs/showthread.php?10622-The-mechanical-side-of-trimming"A third mechanism on aircraft such as the Piper Cub and Super Cub is a jack screw arrangement that actually moves the leading edge of the horizontal stabilizer up and down from the cockpit trim control. I understand that many jets use a similar mechanism.A fourth mechanism, such as on the Mooney, is a movable tail section (yes the entire empennage), which moves vertically with the trim control."L.Adamson

[b_kimoun 0]

I don't understand what you are saying. There is a Cl_de entry in the .air file. It is -0.37255859375 for the default FSX Cessna 172SP and -0.2890625 for the FSX default B747-400. It appears to be in either Table 1101 or 1539.

Hi mgh and happy new year !Yes there exists a Cl_de in the airfile for elevator, but the trimming tab does not have a lift coefficient (let me call it Cl_dt), while it has a pitching moment coefficient Cm_dt.Let's agree first that real world trimming process involves a tail pitching moment component as well as a tail lift component. But as far as I understand MSFS, the latter shows that at trim (hands off), only a pitching moment from tail is involved, that is the tab pitching moment Cm_dt-tab deflection. There is a free utility called AFSD (Advanced Flight Simulator Data, Website: http://pagesperso-orange.fr/hsors/FS_Soft/afsd.html) that can monitore almost all the internal variables values of FS. You can check using this utility that at trim , the elevator shows zero deflection, thus zero elevator pitching moment and zero elevator lift, meaning that : For lift: aircraft weight+ wing lift=0 ( then 0 tail lift component), and for pitching moment: Cm0+wing_lift moment(that is Table473)+Cm_dt* trim tab deflection=0. ( no pitching moment from elevator). If there were at least a lifting component from trim tab ie Cl_dt* trim tab deflection , things could look somehow logic ( since tail moment= tail lift *tail arm).Normally, and according to the aerodynamic equations I could see in books, the moment equation : Cm0+wing_lift moment(that is Table473)+Cm_dt* trim tab deflection=0 must not include the term Cm_dt* trim tab deflection, but the term Cm_de* elevator deflection, since really the trim is assured by the elevator deflection(moment and lift) and not by the trim tab which is just a secondary aerodynamic element that permits the required deflection of elevator and trim with hands off.Or may be all the problem comes from the type of joystick used and how centering is performed, because may be a zero resistance stick does not means necessarilly a stick at zero deflection. Has anyone here tried different types of joysticks (mechanical, force feed back etc.) and seen differences in trimming?

[mgh 89]

I believe that Cm_de and Cm_dt are hinge moment coefficients and so have nothing to do with the overall trim moments but are used. calculate control forces for force-beedback sticks. Moments have to be moments about an axis. If they were overall moments their would have to be about the centre of gravity which varies so the coefficients would have to vary as the centre of gravity moved. The standard references on flight dynamics refer only to the tailplane/elevator lift and not moment.Flight simulator is just that - a simulator. Given the need for it to work with a non force-feedback sticks it isn't possible to model the effects of trim tabs. In effect, Flight Simulator calculates tailplane lift using Cl_de * (elevator_deflection + Constant(?) * trimtab_deflection) This is a perfectly adequate solution as it provides the required tailplane lift for trim. It doesn't really matter what the individual deflections are as long as the aircraft is trimmed with zero stick force.. As has been pointed out, there are other methods of trimming an aircraft than by using trim tab - these include changing tailplance incidence and using adjustable springs. Flight Simulator has to cope with these and so it's sensible to have a generic method. It isn't a bug - it's a realistic compromise. After all, flight Simulator desn't model pistons going up-and-down, or compressors and turbines rotating with combustion chambers between them.

[b_kimoun 0]

I believe that Cm_de and Cm_dt are hinge moment coefficients and so have nothing to do with the overall trim moments but are used. calculate control forces for force-beedback sticks. Moments have to be moments about an axis. If they were overall moments their would have to be about the centre of gravity which varies so the coefficients would have to vary as the centre of gravity moved. The standard references on flight dynamics refer only to the tailplane/elevator lift and not moment.Flight simulator is just that - a simulator. Given the need for it to work with a non force-feedback sticks it isn't possible to model the effects of trim tabs. In effect, Flight Simulator calculates tailplane lift using Cl_de * (elevator_deflection + Constant(?) * trimtab_deflection) This is a perfectly adequate solution as it provides the required tailplane lift for trim. It doesn't really matter what the individual deflections are as long as the aircraft is trimmed with zero stick force.. As has been pointed out, there are other methods of trimming an aircraft than by using trim tab - these include changing tailplance incidence and using adjustable springs. Flight Simulator has to cope with these and so it's sensible to have a generic method. It isn't a bug - it's a realistic compromise. After all, flight Simulator desn't model pistons going up-and-down, or compressors and turbines rotating with combustion chambers between them.

Or may be all the problem of trimming I am talking about comes frome somewhere else. May be it is a question of joystick?That is , may be MSFS deals differently with the types of joysticks used, and thus trims differently (involving or not an elevator deflection)depending on the joystick type involved? Because, I think the centering of a spring controlled joystick must be different from that of a force feedback joystick. For the first case(spring joystick), a joystick released at trim, means no spring resistance and a joystick at null position (rest position) corresponding then to a zero deflection of joystick/elevator. While for the second case (force feedback), the no resistance position (trim) of joystick corresponds to a given position (not necessarilly zero position) within the range of movement of the joystickstick permitting then a certain elevator deflection at trim.In a more explicit manner, may be, MSFS involves the following 3 pitching moment quantities when dealing with trimming process: Cm_de*delta_e + Cm_hinge_e*delta_e + Cm_dt*delta_t (delta_e being the elevator deflection, delta_t being the trim tab deflection, and Cm_hinge_e*delta_e being the hinge moment about hinge axis of elevator);CASE #1: A spring joystick: At trim , joystick at neutral position, delta_e=0, then only the component Cm_dt*delta_t is different from zero and serves to balance the overall pitching moment aircraft and bring it to zero at trim.CASE #2: A force feedback joystick: at trim, the no resistance position of joystick leads to a delta_e not 0 , so that Cm_hinge_e*delta_e + Cm_dt*delta_t = 0 , and thus the elevator Cm_de*delta_e becomes the trimming moment in the same way Cm_dt*delta_t is for CASE #1.