Item by item discussion of analysis
Item 7: A baseline choice of engine was made based upon engine power found in the Sea Wind and the Lake Renegade. The tentative choice is the Lycoming IO-540 at 300 HP. This is a widely used engine with output varying from 260 to 300 HP, depending upon specific model. Preliminary drag calculations were made using rule of thumb estimates and verified that 300 HP was the right ballpark for 200 MPH maximum speed. More refined calculations using Raymer’s Chapter 12 equations were later made and are discussed below but wing size had to be estimated before the more detailed drag calculations could be done, since the refined drag equations need dimensional data which did not yet exist.
Items 8-11: Once weight is estimated, the wing sizing is mainly influenced by the stall speed and the maximum lift coefficient. Based on characteristics of competitors’ designs, stall speed was selected to be 63 knots clean and 51 knots with flaps deployed. A maximum value of lift coefficient of 2 was selected as typical and fairly readily achievable. To accomplish these lift characteristics at low drag, the NASA Natural Laminar Flow NLF-0215F airfoil was chosen, employing 20% chord slotted flaps. A moderate aspect ratio of 6 was chosen based on expected glide performance and an untapered wing was initially selected to keep the wing span as short as possible to simplify beaching in restricted areas and improve docking possibilities, though docking a low wing amphibian is not often done due to minimal vertical clearance. The speed of the design is well below critical Mach number so sweep is unnecessary. And twist of 2-3 degrees to prevent tip stall is likely, but does not figure into the early stages of preliminary design calculations. As shown in
Table 3 below, a wing area of 188 square feet is required to achieve the target stall speeds at a lift coefficient of 2.08.
Table 3
A top view diagram of the physical configuration is shown in
Figure 3 with dimensions listed in feet. This clarifies the dimensions used at this stage of the analysis. All are subject to change and are easily modified.
Figure 3
With these dimensions defined, it is now possible to complete the task of making the more refined drag estimation. The induced drag of the lifting surface is proportional to lift coefficient squared and is calculated as a function of aircraft speed as shown in
Table 4.
Table 4
The zero lift drag is then estimated using Raymer’s set of equations given in Chapter 12, known as the component build up method. The drag of each component is a function of its skin friction coefficient, its wetted area (surface area exposed to the moving air), and interference generated by the proximity of other components of the configuration. Skin friction coefficients are an easily calculated function of Reynolds number. Wetted areas of the various components were obtained from the solid model generated in the CAD software. Catia is the aerospace industry standard for CAD works and was used to create this model, from which several views are presented later. Interference factors are judgment calls for which there is no specific calculation equation. Detailed calculations and the results are shown below in Tables 5-7 and
Figure 4.
Table 5
Table 6
Total drag is the sum of induced drag
(Table 4) and zero lift drag
(Table 6). It was noted during editing that Table 7 and Figure 4 are incorrectly show total drag and total power required that are too low, and they will be fixed.
Table 7
Figure 4
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