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About pitch stability

A

almorro

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12
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france
I would like to ask about pitch stability in fs2004.It seems to me that this is not taken into consideration in fs2004 and may be in all the other versions.I noticed that any aircraft trimmed using autopilot ( that is the aircraft made steady at a certain stable altitude) is no more stable in that position (trim position) when the autopilot is disabled. When AP is disabled, any pitch disturbance, using for instance a brief input from the elevator, will cause the aircraft either to climb untill stall or to descend untill crash.In other other words, it seems to me that all the aircraft of fs2004 are longitudinally unstable.
According to my readings, an aircraft is longitudinally stable when the cg is fwd of the neutral point of the entire aircraft.

Thanks and regards.
 
Hi almorro,

yes, the aircraft is statically stable, if the cg is infront of the aircraft neutral point. However, it is not correct, that the aircraft in FS2004 or FSX are longitudinally unstable. If you would have an unstable aircraft in the pitch axis, you could barely control it, depending on the static margin (neutral point - cg distance as a percentage of the mean aerodynamic chord). You might be able to control an aircraft with a slight negative static margin, but you would have a hard time.

The characteristics of an unstable pitch axis is, that the aircraft wouldnt have the tendency to return to its equilibrium condition. Say you trim the aircraft and you can fly hands of. Now you do a short pitch up control input (this could also be produced by a gust) and you can see that the aircraft has the tendency to return to that condition, by nosing over itself. This is due to a negative cm_alpha derivative. That means if you increase the angle of attack of the aircraft, you will get a bigger negative pitching moment, which results in a nose down. I have seen this behaviour for all FS aircraft I know of.

Coming back to your problem with the AP. I just had a quick check in FS2009 and was using my Realair Scout. I removed all weather condition, in order to remove effects from wind and gusts, and trimmed the aircraft fully nose down (I did that in order to see the pitch trim changes by the AP). The aircraft was very hard to control then, because I needed a constant backstick input. Then I selected the AP and set Altitude Hold at a certain altitude and the AP moved the pitch trim until the aircraft was flying straight and level at that altitude. Once the airspeed stabilized and also the pitch trim remained static, I switched of the AP and the aircraft was perfectly flying straight and level without touching the stick. Also a rapid short pitch stick input produced a nose down motion after releasing the stick pressure, as expected.

So I don't know what you are experiencing. I guess it depends on if the AP is really setting the pitch trim or if it is playing around with the control surfaces itself. Also you have to make sure that the airspeed is stable. If you deselect the AP during an acceleration for example, the aircraft will climp as soon as you switch of the AP. Depending on your previous pitch trim setting, this could mean a significant nose up condition.

Hopefully this all helps you a little.
 
Thanks a lot Peter,

But I still do not understand why my aircrafts do not show longitudinal stability. I would like you to tell me If I can tweak the airfile and aircraft.cfg to achieve that stability and how to proceed in order to meet the requirement : a negative cm_alpha derivative at all cg's in front of NP.

The moments coefficients in the airfile I know are:
Cm_de, Cm_dh, Cm_dt, Cm_df, Cm_ds, Cm_dT, Cm_q, Cm_adot,Cm vs alpha table(473). Does one have to adjust all these to achieve stability? And how about the lift coefficients (Cl_adot Lift,Cl_q,Cl_de,Cl_dh,Cl_df,Cl_ds) and lift vs alpha (table 404) since at trim all forces on the aircraft must be equal to zero?

I have also noticed that the aircraft visual model rotates (pitches)always around the visual center and not around the cg as in real world aircrafts. Does this matter for longitudinal stability?

Thanks again Peter.
 
Unfortunately, I am pretty new to MSFS Development so I can not really tell you anything about the airfile in FS2004. There might be others which are in a better position.

However, I have a pretty strong flight dynamics and simulation background, so I can answer questions in that area. :)

The main driver for pitch stability is the cm vs alpha derivative and it should be negative for all cg's infront of the neutral point. This derivative is dependent on the cg position itself, but I dont know if you can model that in the airfile. However, if you can tweak the cm vs alpha derivative, make these numbers negative.

All the other derivatives are not really primarily for stability. They are more related to control power (cm_de) or pitch damping (cm_q, cm_alphadot). I would assume that cm_dT is the pitch moment change due to a change in Thrust. All of these, beside cm vs alpha have just a little, if any, effect on static stability. Dynamic stability is another story. Here you have to consider (cm_q and cm_alphadot) derivatives.

All the Cl forces have also little effect on static pitch stability, since I think all lift values are related to the cg of the aircraft and therefore not split up in tail, wing, body etc. where you would have to calculate the moments separately.

So start to tune you cm vs alpha derivative and make the numbers negative and play around with them. If the values are to low, the negative pithing moment due to an increase of AOA might be too low.
 
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