b_kimoun

Hi you all ! WarpD asking :"Ok, I just gotta ask... taking Aerodynamics in college or something? ". No, I am just trying to understnad flight mechanics and how an iron bird stands in equilibrium in the air and moves around its axes. I am using FS2004 for a long time and thought that it is time to go further to understand how an aircraft flies.Now concerning ronmarks's reply : "Re a comment that as an a/c accelerates flies faster?, the nose goes up. This is produced by the angle of the wing and tail." , I don't think you are right, since speed ( and thus q the dynamic pressure) varies equally for both surfaces so that the total pitching moment remains constant and does not change with change in speed.

Hi, but how would you explain the fact that the aircraft pitches up as one accelerates( increases thrust)? That is, how does the necessary up pitching moment build up?Thanks.

Hi & thanks alot, I wrote that even when I locate the engines and cg at the origin (0,0,0)(fs2004 aircraft), the pitching up at thrust increase is still there. There is no thrust arm with respect to the cg, so where does the pitching moment due thrust increase come from?

Hi all, Can you please explain to me why aircrafts pitch up as thrust is increased? As you all know, at level flight, when thrust is increased, the aircraft piches up and starts climbing at an increasing speed.Precisely, I would like to understand where does the pitching moment come from, because even when I locate the engines at (0,0,0) and cg at (0,0,0) ( reference being also (0,0,0)), the pitching up occurs at thrust increase. Moreover, I notice that all the aerodynamic equations dealing with longitudinal stability and neutral point I have seen, do not refer to any pitching moment due thrust or drag. Thank you very much.

Hi everybody! I wish to know if drag, thrust and weight longitudinal component (weight x sin(gamma) at climb and descent) contribute to pitching moment and how. Thank you very much.

Clear now, thak you so much.

Thank you all for responding to my question.But as for yours, M.Colin B., I find it difficult that Prop a/c Jet a/c react differently to elevator & power . Can you please say more about this difference? Because, I consider that jet engines or a prop engines both produce thrust.

Which one the two (or both) , the elevator or the throttle, controls the climb and descent of the aircraft ?

Sorry Wolfgang, frankly, I have no answer to your question. But keep in mind, that a trimmed level flight means Sum(Lifts) = 0, and Sum (pitching moments) = 0.In my above post, I mean that reducing power impacts the aircraft speed and consequently makes the dynamic pressure q decrease. q decreased means that the above equations are no more satisfied and the aircraft enters in a hunting process trying to recover the initial equilibrium situation, that is the condition that makes again Sum(Lifts) = 0, and Sum (pitching moments) = 0. If nothing is done, the hunting process continuous until crash (loss of altitude and the equivalent potential energy gained trying unsuccesfully to accelerate and maintain the aircraft at the trim speed).

Exactly. Once the trim speed is disturbed (let's say thrust reduced ), a stable aircraft will swing (in altitude, with loss in altitude) about the trim speed. That is, in the descent phase, it accelerates and starts to level off ( with loss of altitude of course) when approachnig the trim speed and at this stage a short ascent phase starts, then again a descent phase commences due to loss of speed etc. This means that one can not have again a trim situation unless he adjusts thrust to have the previous speed (the true trim speed), or adjusts the trimming surface position. The speed stability augmentation system that I wrote about above uses this last alternative.

This new topic is intended to respond to 2 messages posted both in "The trim bug...issue topic". The first message posted by :DJ : "Posted 22January 2011 - 04:16 PMDJ, on 30December 2010 - 08:28 PM, said:Afterreading all the comments here I have to jump in. What Ireally 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 powerback, the nose will drop to maintain the speed. In reverse, if pwr is increasedthe nose will go up. In all versions of FS and other sims the aircraft willtend to accelerate or decelerate vice nose going up or down. It is afondamental flaw in FS. I have beenflying for 25 years (air Force and Airlines) and while I was an Air Forceinstructor this was always a struggle to make student pilots understand thatconcept. I could continue for pages on the aerodynamics lessons but I thinkthis is enough. Happyflying in the New Year,"And the second message posted by audiohavoc supporting the above message :"Exactly! Itook to the controls of a real aircraft for the first time yesterday and thiswas one of the most important differences I noticed between FSX and realflying."-------------------------------------------------------------------------------------In fact, it appears that , as a matter of trim, not all aircrafts respond to speed change the way DJ and audiohavoc are suggesting. Only aircrafts equipped with speed trim system seem to have this ability.The following is what I read about this subject:The speed trimsystem is a speed stability augmentation system designed to improve flightcharacteristics during operations with a low gross weight, aft center ofgravity and high thrust when the autopilot is not engaged. The purpose of theSTS is to return the airplane to a trimmed speed by commanding the stabilizerin a direction opposite the speed change. The STS monitors inputs of stabilizerposition, thrust lever position, airspeed and vertical speed and then trims thestabilizer using the autopilot stabilizer trim. As the airplane speed increasesor decreases from the trimmed speed, the stabilizer is commanded in thedirection to return the airplane to the trimmed speed. This increases controlcolumn forces to force the airplane to return to the trimmed speed. As theairplane returns to the trimmed speed, the STS commanded stabilizer movement isremoved.

Hi,then yes, you agree, there is a residual moment ( x being not zero) when Cm_de = 0.Now why the other surfaces (flaps end spoilers) do not show also a residual moment when their Cm's is zero too ?Troublesome isn't it?

Hi dmaher !I don't think that zeroing the CL's (especially Table 404) makes your experiment possible since the aircraft is no more flyable. With the Cm's zeroed (as well as Table 473, but Cm_dt excepted), flight is however still possible. If the reference datum is (0,0,0) and aircraft cg is also (0,0,0) (in other words x = 0), I agree with you, the elevator (with its Cm=0) does not produce anymore a moment.But try a cg at, let's say, at (5,0,0). This new situation shows pitching moment when elevator is deflected. But the real problem is why the other surfaces (flaps and spoilers) do not show the same phenomena ? Because these are all identical surfaces that differ from one another only by their respective Cm's and CL's.

I agree even though I still find the behaviour of the couple (Cm_de * elevator angle and Cm_dt * trim angle) not really clear so far .Because all the aerodynamic litterature talks of : the elevator deflection angle to trim. The tab remains then a secondary aspect involved to overcome the elevator hinge moment and make the necessary elevator deflection possible without a necessay action from the pilot (ie hands-off). And then we can consider that the Cm expression is rather :Cm = Cmo + Cm(α) + Cm_de * elevator angle. I have said already in one of my previous posts that may be a real force feedback joystick is need to have this satisfied but still do not know if it is really the right answer to the issue of trim rised through this topic.Now concerning the equation Cmx = Cm + CL(α) * x . We agreed that CL(α) is the Table 404 ( wing-body-tail combination) of airfile and is with respect to a given origin. We can agree also that Cmx is simply Cm when CL(α) does not produce any moment (that is x = 0). Now if we agree that all lift components add to CL(α), that is, overall aircraft lift = CL(α) +Cl_de.elevator deflection+Cl_df. flaps deflection+Cl_ds. spoilers deflection, we can agree that concerning the pitching moment [overall lift.x] (x not zero), the components Cl_de.elevator deflection, Cl_df. flaps deflection and Cl_ds. spoilers deflection contribute to create piching moment even when their Cm's are zero. However, this verifies well with the elevator deflection, but the flaps and spoilers defections affect the overall lift but do not show any effect on pitching moment. I can conclude that the elevator lift applies where CL(α) applies, but flaps and spoiler lifts apply at the rotation point (that is at the model visual centre) whatever the value of x is. But this seems very strange to me. Because so far I have considered that since for each surface moments and lifts are separated through coefficients (Cm's and CL's), all the lift components apply at the same and unique point.

Hi, I am wondering of what Clwing you are talking about; this coefficient does not really exist in airfile. And in my experiment, these are the Cm's (and table 473) that I set to zero not the Cl's.The issue rised is then still pending : why ( offset x being different from zero) elevator can still produce a moment and the flaps and spoilers not, supposing that all lift components apply at the same point?

Hi, sure CL(α) ( table 404) and CM(α) (table 473) are for the comlete aircraft. But one has to know that the values that serve to build these curves are in cfg file.For example, if one tries to rebuild the aerodynamics of an aircraft using FsEdit, he has first to enter to this utility the type of data contained in the cfg file. The result will be new tables (1545 and 1546) that override tables 404 and 473 respectively. Plus an other record,1547, that serves to adjust for the centre of lift.

Hi, if you want to try the same experiment, make sure that you first delete or set to zero (in airfile) all the pitching moment sources except for Cm_dt (this means no Table 473 or values set to zero, Cm0 =0, all mach factors set to 0 etc.). You will then notice that the only pitching sources that remain are Cl(alpha). x and Cm_dt.Trim tab deflection. It is however the no effect of Cl_df and Cl_ds on the equation Cl(alpha). x = Cm_dt.Trim tab deflection that seems strange to me. Because I considere that Cl_de alone affecting (when x is not zero) the above equation makes a sense. More explicitly, the extra elevator lift Cl_de.elevator deflection adds and substracts from Cl(alpha) and the above equation turns out to be : (Cl(alpha) + Cl_de.elevator deflection).x = Cm_dt.Trim tab deflection.I expected that this equation is finally : (Cl(alpha) + Cl_de.elevator deflection+Cl_df. flaps deflection+Cl_ds. spoilers deflection).x = Cm_dt.Trim tab deflection but the experiment shows that it is not the case even though the flaps and spoilers lifts add and substract from Cl(alpha) like the elevator deflection does. Really a problem !

This confirms that airfile and aircraft.cfg are linked and work together. The data suppressed from aircraft.cfg re-appearing when loading aircraft means that the airfile supplies back (through MSFS) to cfg files the missing and same data used for its building(the airfile). But this does not mean that the airfile builds the aircraft.cfg for use by MSFS. It means that the data suppressed is already in the airfile, and that MSFS has simply set priority to read it from the cfg file. I don't think then that MSFF rebuilds the airfile. It is however possible that some specific data may not be in both files, in other words, that are either specific to airfile or to cfg files.Also, we have to know which data ( if any) in aircarft.cfg override the data coming from airfile or vice versa.Note that for FsEdit, you have just to built your airfile using an fs9 aircraft.cfg, then after manage to copy the new coefficients and tables (computed bu FsEdit) to the fsx aircraft files.

I am sure, I simplified the airfile and the cfg file so that the aircaft shows only 2 pitching moment sources : Cl(alpha). x (x being the distance between cg and model visual centre) and Cm_dt.Trim tab deflection . The AP then has only to adjust trim tab deflection for trim so that : Cl(alpha). x = Cm_dt.Trim tab deflection .I could check that when x = 0 (cg at the model visual centre), the trim tab deflection sets to zero . Further, the latter changes ( becoming positive or negative) as x changes (aft or fwd of visual centre) . This verifies well the former equation, Cl(alpha). x = Cm_dt.Trim tab deflection . Deploying flaps or spoilers (remember Cm_df =0 and Cm_ds =0) affects alpha but does not show any effect on the equation above, whatever x is.But as for the elevator (with Cm_de = 0 as well), when deflected, alpha is affected too, and a new pitching moment shows up when x is different from zero.How come the lifting component from elevator produces pitching moment (at x not zero) but the lifting components from flaps and spoilers do not ?

Sure It does not. When loaded by MSFS, the data in the airfile remain inchanged whatever you put in cfg file. But some factors in the cfg file (like elevator effectiveness for instance) are used by MSFS to tune data coming from airfile (I agree then with n4gix).As I wrote before, changing data in the airfile is possible only by using the MS utility FsEdit ( and may be other softwares) . FsEdit works with FS9 airfiles but one can use it for fsx airfiles.

Hi mgh ,As I have written it many times here, many data of the cfg files serve only to build the airfile and thus changing them has an effect only when building the airfile.Just try the FSEDIT ( of FS 2004). This MSFS utility permits to twick the airfile by changing entries in the cfg file. Once you have the airfile twicked, making a further change, for instance, in wing apex will just lead to a displacement of the mean chord. That is MSFS will just see the MAC displaced with respect to the cg location, and this is why we notice the cg gauge reading(% MAC) changing consequently. Then, by moving the Wing Apex, you can place the 25% MAC location wherever you want, but without changing the wing centre of lift. The wing centre of lift is already set by the airfile. The best thing to do then is to place, once and for all, the Wing apex at the right place (using the correct and real world data) then build the airfile ( manually or using any related software). To check that the MAC is really at the right place , have , for instance, your cg at , let's say, 3 feet forward of the Main Gear and look how much % MAC it reads. It must read the same % MAC as in real world. If not, change again the Wing apex position and rebuild the airfile.Now about the issue I rised about zeroing the coefficients Cm_de, Cm_df, and Cm_ds. As I wrote above, elevator continues producing pitching moment while flaps and spoilers not. I have thought so far that for elevator, the only source of pitchin moment in the coefficient Cm_de. Do you have an idea ?

Hi you all,First of all, concerning the experiments I am trying on MSFS, I put "AP on and Altitude Hold on" in order to have at all stages of the experiment an automatically trimmed situation (Lifts sum = 0, and Pitching moments sum = 0).I insist once more that even though Cm_de = 0 (besides all the multipliers and factors) , the elevator deflection (Cl_de not zero of course) still produces pitching moment when there is a distance (x) between cg point and model visual centre.On the contrary, the same experiment performed on flaps ( that is Cm_df =0, and Cl_df different from 0) shows that at all cg locations, and from flaps 1 ° to full flaps, flaps do not produce any pitching moment unless Cm_df is not zero. More, I did the same thing on spoilers ( Cm_ds = 0 and Cl_ds not zero) and the results were the same as for the flaps. Then the elevator seems to behave differently, its lift seems to apply at a different point, as compared to flaps and spoilers.I use AFSD to monitore the different values. You can try these experiments. Use the keyboard (instead of a joystick), and for the case of elevator, try a full up then a full down. You will notice that the trimming moment( Cm_dt* trim tab deflection) shows each time a different value. As for the cases of flaps and spoilers, the trimming moment remains constant whatever the flaps or spoiler status are.

Hi mgh ,That's correct, and nobody can disagree about what you wrote about the pitching moment expression. I stick however to the evidence that the fixed point is the model visual centre. The empty cg position can not serve as a fixed reference point. It is just a particular cg position corresponding to the empty situation of the aircraft.Let 's now have a look at the point where the elevator lift acts. I performed the following experiment and noticed something a bit strange ( with AP engaged and Altitude hold on) :- I made Cm_de = 0 as well as all factors (multipliers that affect Cm_de, like mach factor etc.) = 0. I also put Cm0 =0 and removed the TBL 473 (so that the change in alpha does not complicate my experiment). In other words I modified the airfile so that the elevator cannot produce a pitching moment. But despite that, I noticed that, unless the cg is at the visual centre, both elevator up and elevator down produce pitching moment. Does this mean that the expression CM(α) + x * CL(α) remains however incomplete, and then the complete one is : CM(α) + x * [CL(α)+ CL_de*elevator deflection)] ? I don't think so however, because the aircraft weight has not changed during the experiment. In other words, [CL(α1)+ CL_de*elevator deflection)] = [CL(α2)+ CL_de*elevator deflection)] =[CL(α)+ CL_de*elevator deflection) ( α for elevator at rest, α1 for elevator up, and α2 for elevator down).

Hi mgh,I spent enough time studying the pitching moment formula CMx = CM(α) + x * CL(α) and can say now that this is what MSFS uses in dealing with the piching moment issue. Then , for sure, MSFS involves Table 473 + x * CL(α) as a matter of longitudinal stability and neutral point location.I must say however, that the term x * CL(α) is some how a hidden pitching moment component due to the fact that the pivot is not always the cg point. Note that when cg coincides with the model visual centre, this terme disappears (is null, obviously because x = 0).

To mgh exclusively :You wrote: "This means it is the point about which there is no change in pitching moment arising from a change in angle of attack." My way of saying the same thing : At neutral point, the pitching moment is independent of alpha.You wrote :"At the neutral point (x = h) dCMx/dα is equal to zero".My way of saying the same thing :. At neutral point, the slope of the pitching moment curve Cm(alpha) is zero. Then we agree so far, but I can not follow you further when you involve CL(α) which is I guess TBL 404, a lift curve. Can you please be more explicit ?You agree that the only pitching moment curve (pitching moment vs alpha) in MSFS is TBL 473 (with positive slope) ?But since we agreed already that the pivot point is the model visual centre and not the cg location, may be this situation involves a new pitching moment component dependent of alpha.