Pitch Trim - too much tail downforce an high speeds?

[flyingeagle42]

Dear my fellow aircraft builders! 

 

I am in the early stages of a Learjet 45 (XR) project, currently working on the flight model and basic 2D cockpit.

The fuselage, wing sizes, positions, stabilizers are all build from a 3-view-image that I imported and measured within a CAD software. So all positions and sizes set in Plane Maker should be "quite good".

 

Area of interest is the horizontal stabilizer that is a moving surface to control the pitch trim and of course the magic center of gravity (cg). 

 

Facts:

• The Learjet 45 (LJ45) has a moving horizontal stabiizer to control the pitch trim
• I modeled that through the use of the appropriate settings in the control surfaces and made the hor. stab. a moving surface
• According to the FAA certificate the LJ45 has some "strange" hor. stab. max. deflections http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/880d41da2731227386257d5d006b9505/$FILE/T00008WI_Rev20.pdf
• i.e.: leading edge up trim = -0.3° and leading edge down trim = -9.5° -> that means NO positive incidence of the hor. stab. at all? I modeled that in Plane Maker the following way:
• set the incidence of the hor. stab. to -4.9° (middle position between -9.5° and -0.3°)
• set the control surface defllection up and down to 4.6° (so i can reach -0.3° and -9.5° with max. trim in both directions)
• set airfoil to symetric NACA009 (I have NO information about the hor. stab. airfoil of that bird)
• cg of gravity is set in the middle according the FAA cert. and AFM (middle of 1% MAC and 28% MAC)

Problem (or maybe this is not a problem, but just "normal"?)::

• when I go the a very fast speed (within the envelope!) e.g. 330 KIAS at 3.000 feet (.. and loose my licence ;-) the max pitch trim brings the hor. stab to -0.3% (nearly straight level, but no pos. incidence allowed) the hor. stab still produces too much DOWNFORCE that the nose of the aircraft goes UP and I can not trim the plane and must use elevator to hold it straight leveled.
• I can test more situation and bring the aircraft to exxtreme weight/balance situations WITHIN the flight envelope and weight / balance specs. that I can not trim the aircraft

 

Ideas / Questions:

• Is this a problem? Or is it "normal" that a aircarft is NOT trimmable within the flight envelope and weight / balance specs?
• Did I read the FAA spec. correctly? i.e. the incidence is always negative betwenn -0.3° and -9.5°
• I am really questioning the -0.3° max. UPTRIM... when I could set that to 0° or +1° all problems would be solved? But the cert. says -> NO only negative? ;-)

 

If anyone even has the time to take a testflight, I can send you a link to the current acf.

 

 

Your help and ideas are very welcome!

 

Edited November 12, 2014 by flyingeagle42