Martin,
A symmetrical airfoil, having no camber, will produce lift centered
consistently at nearly the same chord position throughout the 'normal' range
of attack angles.
An airfoil with some camber will typically have a shifting forward of the
center of lift (CL) as alpha increases.
As alpha increases, this usually places more of the foil behind the CL
causing a nose down (negative) torque moment about the CL.
Efforts are usually pursued during modern airfoil design (NLF-0115) to
minimize this coefficient of moment, the Cm.
It seems that our esteemed designer has as well.
Even if we had symmetrical foils, we would still be down-trimming excess
lift off the wing as airspeed increases.
Since the down-trim at speed is required, we can deduce that the CL shift
aft and/or the negative Cm during this time is well tamed.
Else the fwd CG effect would require up-trim as the CL progressed aft.
Or high negative Cm would require up-trim as well.
If your trim is 'normal', the required trim settings should graph nearly
linear across your airspeeds, and trim should be downward at speeds over the
hump at 70(?).
Conceivably these effects can be in balanced at most times, reducing the
need for trimming.
This is what cruise flaps can help with.
By trimming full span flaperons upward as speed increases, the CL shift is
reduced along w/ reducing alpha and negative Cm.
This also allows for more favorable streamline attitude of the fuselage and
propeller thrustline when at speed.
Put the flaps down and increase the negative Cm, and also shift CL aft.
Sometimes these nearly balance, resulting in little trim change.
Aerodynamicists aren't paid enough!
Also; Builders insurance is offered by AvemCo through the EAA if you are a
member.
I am not a member yet.
errata-
I am still curious if there are other customers w/ fast-build tailplanes w/
protruding pip pin covers.
Maybe mine will reduce my trim load. hahahhaha
Nic- xs145
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