Evolutionary design
In contrast to many new F3B and F3F model developments that are focusing more on
single tasks such as launch height or speed, our development target for the
Precision 2 was to get a very well balanced all-round design. However, since F3B
and F3F competitions are often won in strong conditions, every endeavour has
been made to improve top speed performance whilst at the same time slightly
improving launch height on the F3B norm winch. Looking at the well-proven design
of the Pike Precision, one has to admit that this is very hard to achieve.
Coming from this successful design, all design decisions have been put to test.
Adding up all minor improvements led to an overall performance gain that
justifies the expenses for the production of a new F3B/F model.
During this evolutionary development process the same design principals and
strategies as described in the Pike
Precision development article where used. The following article therefore
mainly focuses on the evolution from the Pike Precision to the Pike Precision 2.
General layout & wing design
Following the concept of a weighting function to guide the development process,
the Pike Precision weighting function was adjusted slightly towards better high
speed performance:
To achieve this goal, the key design decision was
to reduce wing area. The Pike Precision is often described by pilots to be a
“lifty” design. This results mainly from the relative large wing: the plane
needs to be flown at a higher take off mass to reach a similar wing loading to
most competitors. With the limited power of a standard F3B winch it becomes a
challenge to reach competitive launch heights, especially in low wind
situations. With the reduced wing area the “ballast efficiency” (meaning the
wing loading increase per mass unit) increases. The lower take off mass improves
the “winch power per mass unit ratio” and thus helps to improve launch height in
low wind situations. The newly optimized wing has 57.7dm2 area and spans 2.98m.
The dihedral of three degrees per side ensures easy, vice free handling in slow
speeds turns. The wing planform was optimized for minimum induced drag and a
perfectly constant relative flap chord of 24% all along the wing span. Taper
ratio was chosen slightly more conservative compared to the Pike Precision wing,
to improve handling qualities at low Reynolds number/high lift situations e.g.
slow speed handling.
According to the weighting function I developed
eight airfoils along the span to match lift- and speed requirements and to
guarantee safe handling characteristics near maximum lift. The use of high
modulus carbon fibers for the spar allowed the airfoil thickness to be slightly
reduced. It is now in the range of 8.1% to 7.4% relative thickness which helps
to lower pressure drag. Compared to the Precision airfoil design, the very good
high lift performance could be preserved. At the same time I managed to improve
the performance at high speed/low lift at the expense of a little more airfoil
drag at slow speed/high lift conditions. This compromise appears to be justified
since at slow speeds lift dependent drag (induced drag) is the dominating drag
source.
Team Austria at World Championship F3B 2017
In the middle World Champion F3B 2017 Bernhard Flixeder with his Precision2
Tails & fuselage design
As demonstrated on the Pike Precision, the slim fuselage update helped to
improve high speed performance considerably. Since the slim fuselage represents
the limit of what is possible with today’s RC gear and construction techniques,
the slim fuselage design was kept with minor modifications.
Also, the two piece V-tail of the Pike Precision was used again because of the
simple construction and the very well proven, safe airfoil design. Even in
extreme flight conditions that might occur during recovery from cuts at the
slope or critical situations during winch launching, stalling of the tails is
rarely to be seen. The reduced wing area of the Precision 2 offers the chance to
decrease the v-tail opening angle and still have sufficient longitudinal
stability and static margin. At the same time the decreased V-tail angle
improves directional stability and provides favorable yaw damping
characteristics. Furthermore the rudder power is increased which results in nice
harmony of the controls around all three axis.
.jpg)
.JPG)
.JPG)
.JPG)
.JPG)
.JPG)
.JPG)
.JPG)
.JPG)
.JPG)
.JPG)
.JPG)
.JPG)
.JPG)
.JPG)
