The amount of forward movement for a single complete rotation of the propeller is called the pitch of the propeller. Imagine the propeller screwing itself through a solid medium with no slippage. Source: Oliver Cleynen, Wikimedia Commons. The angle of attack near the hub is much steeper than at the tips. Turbine blades showing their twist for acheiving constant pitch. A 30% centerline results in something that looks like a Spitfire wing. A 50% centerline results in a symmetrical ellipse. The interactive drawing below shows what the wing (or blade) planform looks like when an arbitrary "centerline" is defined as a percent of chord. This thickness alignment requires distorting the ellipse, but the elliptical lift distribution remains unaffected.Ī typical NACA airfoil is thickest at about 30%–40% of the airfoil chord from the leading edge (the "chord" is a line drawn between the leading and trailing edges of the airfoil). In the case of a propeller, this effictively provides a straight bar of material of maximum thickness running along the length of the blade. In the case of a wing, one can manufacture a straight spar to fit along a straight line of maximum airfoil thickness. This is because the thickest part of the airfoil cross-section is aligned in a straight line along the length of the wing or blade, for greater strength. Propeller blade planformĭid you notice that the Spitfire wing in the picture above isn't a symmetrical ellipse? The trailing edge curves more than the leading edge. For the propeller, instead of making a crude simple flat blades like other designs I have seen, I decided to go all-out and engineer a propeller that uses real-world propeller design features, such as an elliptical blade planform and using multiple NACA airfoil profile transitions along the length of the blades. I already over-engineered the handle to be an ergonomic design, and I over-engineered the pull cord gear teeth as well. Sources: PickPik and Wikimedia Commons I want to make a pull-copterĪs I wrote in my article about ergonomic handle design, my objective is to 3-D print an over-engineered pull-copter, a toy that drives a propeller when you pull a cord through a handle, to make the propeller fly into the air. For example:Įlliptical planform blades (left) and truncated elliptical blades (right). (In the case of the Spitfire, the decision to give it an elliptical wing had less to do with induced drag and more to do with being roomier near the wing root than a straight tapered wing, allowing the aircraft guns to be completely internal.)While elliptical-wing aircraft are a thing of the past, modern propeller blades are still made with elliptical planforms, or approximately elliptical planforms, and sometimes the rounded end of the ellipse is truncated. Because the advantages to elliptical wings are often negated by other design considerations (such as wingtip washout to improve stall characteristics), it is more economical to fabricate tapered wings with straight edges, so this is how wings are designed nowadays. It is possible to create an elliptical lift distribution over the length of a wing without actually having an elliptical planform, by adjusting the airfoil shape and angle of attack. Another advantage to an ellipse is that the wing tip is quite small, which reduces drag from induced wingtip vortices. The optimal and most efficient wing planform shape to distribute lift and minimize induced drag, theoretically, is an ellipse. It is also inversely proportional to aspect ratio (the ratio of wing length to airfoil average chord length). Induced drag depends on the planform shape of the wing. Induced drag has nothing to do with the drag created by surface area, surface roughness, or thickness of the airfoil. Of all the kinds of drag that a wing or propeller blade experiences, induced drag is an unavoidable price for lift. However, with all curved edges, elliptical wings are expensive to construct. In the case of the Spitfire, the gentle taper of the ellipse near the wing root also provided more room to mount weapons internally than a straight-taper wing, while providing an overall thinner, low-drag cross-section. Elliptical wings have the most uniform theoretical distribution of lift and therefore the least induced drag. The most famous aircfaft with such wings is probably the Supermarine Spitfire fighter aircraft from World War II. You don't see aircraft with elliptical wings anymore.
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