Screw calculation. Drawing, description

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Screw calculation. Tips for the home master

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To build propeller templates (side view) for a given blade shape (top view) and pitch, two main calculation methods are used: analytical and graphical. Usually, this is based on the following propeller characteristics: diameter, relative blade width and pitch. Since it is difficult to take into account the amount of slip in advance, they proceed from the geometric pitch of the screw.

Figure 1 shows that the tangent of the blade inclination angle (α) in any section is related to the pitch (Н) and radius (r) of the propeller at this point by the relation: tgα = H/2πr.

Rice. 1. Pitch of the propeller and development of the helix (click to enlarge): R - radius of the blade, r - radius of the section of the blade, H - pitch of the propeller, - circle described by the section of the propeller blade, 2πr - development of the circle on the plane, B - spiral described by blade, B' - the development of the spiral on the plane, α - the angle of elevation of the blade.

From the equation, you can determine the thickness of the blade in the side view (b) (Fig. 2), knowing its width in the top view (a). In the same section. located at a distance r from the axis of the screw, the equality is true: b = a*tgα.

Screw calculation
Rice. 2. Blade cross section: a - blade width when viewed from above, b - blade thickness when viewed from the side, a' - actual blade width, α - blade pitch angle.

Actual blade width (a₁) is determined from the ratio: a₁= a/cosα, or a₁= b/sinα. Thus, using the first two dependencies and Figure 1 and selecting the top view of the propeller and its pitch, it is possible to construct a propeller template in the side view.

The procedure for the analytical calculation of the screw

1. For the convenience of calculations, it is necessary to calculate the so-called reduced step (H₁): H₁ = H/2π.

2. A screw is built on graph paper in a top view.

3. Perpendicular to the axial line of the blade, segments are drawn that are 30, 40 ... (r) mm from the center.

4. Determine the tangent of the angle of inclination of each section located at a distance of 30, 40 ... (r) mm from the center:

tanα = H₁/r.

5. Determine the thickness of the screw when viewed from the side:

b = a*tga.

6. Parallel to the axial line of the blade in the top view, a straight line must be drawn, on which perpendiculars are restored, spaced from the center by 30, 40 ... (r) mm. They lay off the values of the screw thicknesses calculated for the corresponding sections.

7. The resulting points are connected by a smooth curve. (For convenience, the results must be entered in the table.)

In practice, many aircraft modellers, especially beginners, use the graphical method of constructing a propeller side view. Since both methods are based on the same dependencies, the graphical method has advantages only in greater clarity.

The order of the graphical calculation of the screw: 1. On graph paper, build a plan view of the screw (Fig. 3).

Screw calculation
Rice. 3. Building a screw template

2. Perpendicular to the axial line of the blade, draw segments spaced from the center by 30 ... 40, etc. mm.

3. Draw an auxiliary line parallel to the center line. From the center of the screw to it, restore the perpendicular height H₁.

Analytical calculation of screw templates H = 180, H₁ = 28,7

4. Connect the end of the perpendicular with inclined lines to points spaced at a distance of 30 ... 40 mm from the center.

5. On each inclined line it is necessary to build a right triangle. Its base will be equal to the width of the blade, and its height will be equal to its thickness when viewed from the side in the corresponding section.

6. Draw a straight line parallel to the axial line of the blade, on which restore perpendiculars that are 30 ... 40 mm from the center. Then, on them, set aside the screw thickness values determined from the figure for the corresponding sections.

7. Connect the resulting points with a smooth curve.

The table and figure 3 show examples of calculating a screw Ø 220 mm, pitch 180 mm for a 2,5 cm engine³.

Screw calculation

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