project pattern
T
Redux
he trimming process for Yuri thus far has included: (1) some very mi- nor changes to wing and stabilizer incidence; to exactly center the con- trol surfaces; (2) setup of a rudder > aileron programmable mix (Pmix) to eliminate un- wanted roll coupling; (3) setup of a throttle > aileron Pmix to reduce changes in roll trim attributable to torque; (4) setup of a throttle > rudder Pmix to reduce changes in yaw trim attributable to spiral airflow.
Setting the CG
With the recommended CG for Yuri rang- ing from 200mm aft of the leading edge (windy conditions) to 215mm (calm condi- tions), readers may recall my preference to trim for a specific CG location, one that is suited for all weather conditions. In the first 20 flights or so, I experiment- ed with CG locations ranging from as far forward as 190mm to as far aft as 215mm. During those 20 flights, I completed the aforementioned trimming processes, which are essentially unaffected by CG location. I find it beneficial to complete several flights on a new plane before delving too deeply into trimming aspects that can be substan- tially affected by CG. With the CG at 205mm, Yuri exhibited a small tendency to pull to the canopy in up- lines, downlines, and knife edge flight. Pitch damping was very good, and yaw damping was marginal. At 210mm, downlines pulled slightly to the canopy, uplines and knife edge were very straight, but, I found Yuri to be too easily upset in yaw in turbulent con- ditions for my taste. With the CG at 190mm, Yuri was stable in yaw in turbulence, but the pitching to the canopy in vertical lines and knife edge flight was much more pro- nounced. I felt the best CG location was at 205mm, and at that location, I also felt the trim condition and flight
characteristics
could be easily improved. Knife edge concerns
A very minor characteristic I noticed with Yuri was a slight asymmetry in rudder “feel” and knife edge “power” when comparing sec- tions of rolling circles under positive G (slight up elevator) versus sections under negative G (slight down elevator). This is a very common characteristic on many de- signs, and is generally attributed to variable turbulence from the canopy bump at various angles of attack.
I often cringe when I hear conversations about knife edge trimming, and how it is (or isn’t) possible to have an airplane that will knife edge without pitching to the belly or pulling to the canopy. The source of my cringing is generally attributable to a couple of phrases; “the wing is unloaded in knife edge flight”, or, “the wing produces no lift in knife edge flight.”
Suggesting the lift component of the wing changes simply because the orientation of the plane has changed evidences a lack of understanding of basic flight. Going back to
34
by dave lockhart with algirdas ungulaitis You can reach Dave Lockhart via e-mail at
davel322@comcast.net
PHOTOGRAPHY: DAVE LOCKHART Text explains how altering fuselage shape with this small foam fin affected pitch coupling in knife edge flight.
a “one gravity” (1G) trim condition, when the airplane is trimmed for straight and level flight, the wing will produce exactly 1G of lift to negate the effect of gravity. The 1G trim condition is the result of the rela- tionship between the engine thrust line, wing incidence, stabilizer incidence, and center of gravity.
This relationship does not change simply because the airplane has been rolled 90 de- grees to a knife edge orientation. Please note that “knife edge” flight is a de- scriptive term, as the flight regime being ad- dressed is that which involves an angle of at- tack in yaw (beta) for the fuselage. Knife edge simply describes an orientation of the plane in which the wings are perpendicular to the ground, whereas trimming for beta is applicable any time rudder is being used. If the lift from the wing (trimmed for 1G) were the only influence in knife edge flight, all airplanes would pull toward the canopy (following the lift produced by the wing). The “ideal” fuselage shape would generate not only (side) lift for knife edge flight, but simultaneously generate the effectiveness of -1G (negative one gravity) to neatly cancel out the 1G of lift produced by the wing.
Propeller influences
The remaining complication is that the addition of beta also introduces P-factor, and the combination of P-factor and spiral airflow work together to impart right rud- der/pull and left rudder/pitch.
At this point, having outlined several fac- tors seemingly destined to divert the head- ing of the aircraft (in pitch); I will emphati- cally state it is entirely possible to trim an aircraft to have no pitch coupling tendencies in knife edge flight.
Assuming our aircraft has the ideal fuse- lage shape (generating side lift and -1G), it has been suggested that the “perfect” air- plane will have a minor pull to the canopy in right rudder knife edge, and an equally mi- nor pitch to the belly in left rudder knife
edge (caused by P-factor and spiral airflow). The amount of the biased right rudder/pull and left rudder/pitch is very dependent on airframe design; quite noticeable on some, and difficult to notice at all on others. While such a trim condition can be fully remedied with a counter-rotating propeller system (by eliminating the effects of P-factor and spiral airflow), it can also be addressed on single propeller aircraft.
When beta is introduced, the stabilizer op- posite the rudder input is effectively pushed into clean air, while the opposing stabilizer is partially blanked by turbulence from the fuselage. The dominance of the stabilizer half in clean air provides an opportunity to bias the incidence of the stabilizers such that pitch/pull behavior is introduced when the aircraft is in yawed flight. Increasing the incidence (leading edge higher, positive angle of attack) of the left stabilizer while decreasing the incidence of the right stabi- lizer an equal amount will result in no net change in the incidence of the stabilizer in normal (no beta) flight. However, when right rudder is used, the positive incidence of the left stabilizer will generate a minor pitching moment to the belly, opposing the minor pulling moment to the canopy caused by P-factor/spiral airflow. Similarly, with left rudder, the negative in- cidence of the right stabilizer will generate a minor pulling moment to the canopy, oppos- ing the minor pitching moment to the belly caused by P-factor/spiral airflow. Of course the amount of stabilizer inci- dence “twist” needs to be adjusted to exactly balance out the effects of P-factor/spiral air- flow. In my experience, I have used as much as 1.5 degrees of biased incidence in the sta- bilizers halves to cancel the effects of P-fac- tor/spiral airflow, and I have never noticed any adverse effects on roll trim or loop track- ing. While it may seem counter-productive to “twist” a perfectly good airplane, it is key to understand that the geometrically “straight” airplane is flying in a spiral vortex. Ulti-
APRIL 2014
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