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Problem with Aerosoft DC-8

T

TrevorS

Messages
19
Country
southafrica
I am experiencing a lack of altitude hold at altitudes above 35,000 feet. Up to and including FL350 the plane is rock solid in cruise. At FL370/FL390 the airplane begins to oscillate in altitude hold.
Not sure if this is a flight dynamics problem or a programming problem. What could cause this to happen and how does one go about trying to rectify it ? I have contacted the developer at Aerosoft but it has been eight days now and no response from him.
Any ideas?

Trevor
 
WarpD

WarpD

Messages
2,077
Country
us-ohio
True... but we don't do any type of support... even modifying flight dynamics for commercial products from vendors...
 
hairyspin

hairyspin

Resource contributor
Messages
3,253
Country
unitedkingdom
You’re quite sure you’re not going above service ceiling for the payload? In any event, I see a thread in the link Heretic gave you discussing that very issue.
 
Roy Holmes

Roy Holmes

Resource contributor
Messages
1,803
Country
us-virginia
It is basically an autopilot damping issue. It is caused by reduced aerodynamic pitching effect in relation to the airplanes inertia at high altitudes. The autopilot should be able to handle it, higher speeds would help somewhat. It is not a flight dynamics problem as such, the answer should lie in the autopilot constants.
Roy
 
T

TrevorS

Messages
19
Country
southafrica
Thanks for responding Roy. Increasing the speed from M0.8 to M0.84 at FL370 got the plane stable again. Obviously at this altitude the plane was very light.
Is there any tweaking of flight dynamics that can sort this out or has it to do solely with the programming of the autopilot?
Thanks for your assistance
Regards
Trevor
 
Roy Holmes

Roy Holmes

Resource contributor
Messages
1,803
Country
us-virginia
Increasing the airspeed increased the ability of the aerodynamics to overcome inertia, so you could increase the relevant coefficients and maintain the lower speed. However, what you do applies at all heights. I think you can vary the autopilot coefficients versus altitude to some extent, which would more closely address the problem.
Roy
 
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