From the mail bag…
An astute reader commented that meeting the new Sport Pilot (MOSAIC) stall speed might not be the performance killer I thought it would probably be. In fact, he said, it might only reduce the top end by a few miles per hour. Well, I ran the numbers and he’s right. So from this point forward, we’ll plan on a stall speed of no more than 68 mph (59 kt).
Since the regulation is for clean stall speed (no flaps), this makes the task of sizing the wing easier. We can design the flaps later, and we might find that a simpler plain flap could yield a reasonable stall speed without having to go with the single-slotted flap originally planned.
Moving forward, we’ll design to this new stall speed requirement. It’s not as simple as just resizing the wing and moving on, since that extra wing area will weigh something, and that weight in turn needs more fuel to carry it through the air, so we need to run the numbers through the sizing process again. The following figures have been updated:
performance
stall speed (maximum)
lift-to-drag ratio (preliminary)
weight fractions
fuel fraction
empty weight fraction
wing & power loading
wing loading
power loading
weight
fuel weight
empty weight
gross weight
Wing sizing is simpler because we don’t have to deal with flaps for now. We’ll assume a 3D lift coefficient of 1.36 (more on this in an upcoming post).
$$\scriptsize S_{ref}=\frac{L}{qC_{L_{max}}} $$
$$\scriptsize S_{ref}=\frac{1.08 \cdot 1,952 \text{ lb}}{11.82 \text{ lb/ft}^{2} \cdot 1.36 }$$
$$\scriptsize S_{ref} = 131 \text{ ft}^{2}$$
So we’ve grown the wing by 10 square feet. Next time we’ll tackle the wing’s geometry.
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