Originally Posted by
mithrandir
I am confused.... having a high stab.. above the "Drag" center will result in pulling the nose up no? so how does increasing the wing incidence help?
I typically trim my 40% planes to be as close to neutral as possible.. and then just a little RCH fwd CG of that... and very little (non really) pull to canopy in vertical up or down...
think about this.. the stab is producing in level unaccelerated flight mere ounces of lift... and likely a tenth of that in drag.... (I assume the drag from the tail, above the aircraft center is creating a moment and that is what you think is causeing the pull to canopy?)... this can't be a dominant force...
Pattern planes are assymetrical... and that causes a lot of s'prising flow phenomonon... but it seems that moving the CG aft thusly reducing the decalage between the wing and tail would go a long way....(Or rather reducing the incidence and putting the CG where it will trim out)
**********On second thought, if you increase the incidence on the wing and retrim the elevator, now the thrust line with respect to the wing is negative.....
can you comment on downline pull/push compares between now and before?
IMHO, stab height effect on pitch is not a big driver.... If I can find some old CFD studies I did.. I will interrogate them for pitch effects in level flight vs tail location...
I was also able to measure yaw coupled pitch vs tail location in the CFD study
To Flyincajun... I don't understand how wing incidence affects Proverse/Adverse roll... I can see how dihedral or wing height/location on fuselage would affect this... but not
incidence...
One thing i've learned is that aerodynamics and trimming of a pattern plane are not as straightforward and intuitive as we'd like them to be. Thorough testing and experience are very important; sometimes i made some changes that i was very confident will bring great improvements, only to discover they made things worse or didn't have any impact at all. Some other changes that i thought will only bring small benefits (like repositioning the stab) actually had surprising effects and brought unexpected improvements in different areas of the flight envelope.
When i tested the model in the spring with 0.7deg wing incidence and stab in the initial position, i noticed it pulled to the canopy on the vertical uplines. It took 2% of down elevator at full throttle to keep the upline perfectly straight. Increasing wing incidence means you need to add down trim to keep horizontal flight (assuming CG is in the same position). This down trim is also carried on the verticals, eliminating the pull to canopy. I eventually got rid of the mix mentioned above by increasing the wing incidence to 1.1deg; overall the model was flying pretty good so i kept this setup for the rest of the year.
I also experimented with -0.5deg downthrust, but it had little effect on the verticals. The differences were small, but in general i thought that 0 downthrust works slightly better.
I recommend reading Bryan's "Triangulation trimming" article, his methods are well known in the pattern community and work very well.
I certainly didn't expect that repositioning the stab will have such a big influence on the vertical uplines. Based on my previous experience, after moving the stab lower and going back to 0.7 deg wing incidence, i added the same 2% mix (down elevator at full throttle). I went testing and after trimming the model for horizontal flight, i pulled up for a vertical upline. To my surprise, the model pulled to the gear, so i landed, removed the mix and vertical uplines were perfect after that. Maybe placing the stab lower, in the area influenced by the wing downwash has something to do with it.
The proverse roll coupling experienced after adding the longer fuselage strakes also disappeared so i'm back to 0 mix now. Downlines also improved slightly, requiring 1% less mix. As mentioned earlier, the uplines are 0 mix too, so the only mix i need now is for left rudder knife edge. That's because i have the rudder servo in the tail and i couldn't move the CG as far forward as i wanted, but i plan to move the servo in the cockpit area (behind the wing tube) over the winter.
Anyway, there are many pattern designs out there, some similar in appearance, others significantly different. I'm aware others may have different results, but that's my experience with this model.