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Guest Tom Goodrick

Aircraft pitches DOWN when in a turn & in turbulence.... help!

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Another issue i have is to do with aircraft stability in turbulence..Other aircraft i have downloaded by other people seem to cope easilty with this & maintain their altitude whilst making an Autopilot turn in Turbulent air. However an aircraft I'm working on DOES hold it's altitude & turn nicely when the air is STILL, but if it tries to make a turn in turbulence, it pitches down gradually, trying to fight to stay at the same altitude... Only when the plane levels off again can the autopilot raise the nose & get the plane back to the altitude it was set to.Again, it only happens during Turbulence... Still air, it is fine & level.What causes this & what do i need to edit in the .air or cgf files to make the plane cope better.As i said already, other planes by other people do seem to be able to cope with turning in turb, with the autopilot controlling the alt & the turn.??Can someone please advise me what to edit to correct the problem... Remember in STILL air the plane IS fine!!Al

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Sounds like you may be flying level with a lot of up elevator trim. In the turn it needs much more if the AP is going to hold the nose up.You need to setup the air file so that at cruise, the trim should be close to zero (giving you a lot of latitude for available trim). Secondly, ensure that elevator trim deflection limits aren't improperly constrained in the aircraft.cfg file (airplane geometry section).My guess is that the AP has no more up trim left to get you through the turn when there's a lot of downdrafts happening.Also, one should not be flying with AP on in turbulence, but I get your point. This certainly points out a problem that you didn't realize you had.In the .air file, record 1101, look for cmo - Pitch Moment Coeff at AoA=0 (trim os)= -nnnAdjust this factor to set trim at/near zero at cruise.Milton

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Thankyou Milton,I'll give it a try....What sort of settings do you reccommend for that param to fix the issue? & what other params, (if any), do i need to take into account to make the changes behave correctly.I have another post on here as well, concerning absolute speed limits... almost like a wall that i reach, when my groundspeed hits 2666kts, or there abouts.Do you have any ideas what causes this? I see other people saying that they can only get to M3.2 . Well i HAVE reached M5.03 (with the right winds) however this was still below the groundspeed mentioned above. (due to the strong headwind), on average a cruise can be anywhere between M4.25 - M4.8, however this is trans-hypersonic, & I want ot achieve true hypersonic.I'm aiming for a groundspeed of 3128kts = 36000mph (1 mile per sec)& faster, But I am stumped as to which param is holding back the accelleration.What in the .air file needs to be altered to loosen up these limits? Or have i found an actual limit with the sim?Look forward to your answerAl

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oops one zero to many there in the MPH, but you know what i mean :-)Al

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Speeds over .8M make my hands sweaty and the open cockpit gets really noisey. :-)I have no experience in aircraft modeling or FD's for other than prop aircraft. You should pose your question over at AVHISTORY where these things are best understood.http://www.avhistory.org/scripts/MegaBBS/f...w.asp?forumid=5Regarding the nose problem, if you are really into FD analysis, you need a good set of test gauges.Visit Herve's site for the AFSD test gauges and get some insight into what is really happening with that aircraft. From here with limited information, its not much more than speculation on my part.http://perso.wanadoo.fr/hsors/FS_Soft/fsairfile.htmlMilton

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I do not think it is reasonable of you to expect to design an aircraft in this sim that will fly above Mach 2 with any degree of realism. I spent several years at NASA designing hypersonic vehicles and studying their aerodynamics and flight mechanics. When the craft goes from Mach 2 to Mach 3, the nature of the flow field producing aerodynamic forces changes significantly. The center of pressure moves significantly creating a trim problem. Most aircraft must have special configuration details to handle this - eg- swing wings, canards or, as in the SR-71, a provision for rapidly pumping fuel between forward and rear tanks for balance.Above Mach 3 and certainly by Mach 5, the air interface is radically different. Air does not flow around the aircraft. It impacts the aircraft and is deflected. Very significant heating occurs on any sharp edges such as wings and control surfaces. The static trim on most vehicles requires an angle of attack on the order of 40 degrees. The craft starts literally body-surfing. Airfoils become ineffective.I don't think you want to mess with this. If you persist, anything can happen and it will probably have little relation to reality.I shudder to think what winds help you reach Mach 5!!!!! Incidently, the Mach number depends only on your true airspeed and the local ambient temperature. Groundspeed has nothing to do with it.

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