Propeller twist compensates for the speed difference along blades On a 20 degree Celsius day and at 2,500 RPM, that Cessna 172 propeller blade tip is traveling at a speed of 558 MPH (that’s Mach 0.72 – and you thought everything about a Skyhawk was slow!).ġ0” out from the hub, at the root of the propeller, the propeller is traveling at only 149 MPH. Warp Drive Props has a really cool propeller tip speed calculator that can be used to calculate that difference. This creates a pretty substantial speed difference between the tip and the root. In other words, on a Cessna 172, at 2,500 RPM, the tip is covering almost 4x the distance that the inboard part of the blade is on each revolution. When that propeller is spinning at 2,500 RPM, the tip (37.5” from the center of the hub) has to travel almost 20’ in the same amount of time that the inboard part of the propeller (10” from the center of the hub) has to travel 5’. The circumference that the tip of the propellers travels through each rotation is therefore 236”. The relationship between propeller RPM and the actual speed at which the airfoil moves through the air is a very simple yet dramatic concept that you may not have thought much about.Ī typical Cessna 172 propeller has a diameter of 75”. However, a propeller blade is subject to an interesting phenomena that is unique to the rotational movement not present with a traditional wing: the airfoil is not traveling at the same speed along its entire length. The production of lift by an airfoil is a complex topic that is better explained in an aerospace engineering course, but the explanations above are correct enough for our understanding as pilots. Newton also explains some of the thrust by pointing out that as the propeller moves vast quantities of air backwards along the longitudinal axis of the aircraft, the “equal and opposite” reaction is the resulting thrust. The higher air pressure behind (“under” the blade) wants to fill the low pressure void and “pushes” along the length of the blade, pushing the entire airplane forward. Again, this is just like what is found in most airfoil designs.Īs air moves over and around the blade, explanations provided by Bernoulli and Newton provide insight as to how thrust is created.īernoulli taught us that an area of low pressure develops in front of the propeller blade (over the “top” surface). The leading edge of the propeller is rounded and the trailing edge is a bit more pointed. The “bottom” of the blade (toward the cockpit) is relatively flat and the “top” of the blade (facing forward) is rounded just like the more familiar wing-type airfoil. Stand back for a moment while on the ramp, and you’ll easily see that each blade of a propeller has a shape similar to a wing, along the entire length of the blade from the hub to the tip. Thrust is produced by propeller blades because each blade is an airfoil, just like a wing. The main difference is that while the wing is oriented to produce lift (upwards force) meant to counteract weight (downward force), the propeller is oriented to produce thrust (forward force) to counteract drag (rearward force). Thrust is developed because as the propeller moves through the air, it produces a force – just like a wing. The twist of the blades compensates for this speed difference so as to produce uniform thrust along the length of the propeller blade.Ī propeller’s main job is to convert the torque produced by the engine into a linear force that propels (hence, “propeller”) the aircraft forward. Propeller blades are twisted because the blade tips travel faster than the center of the propeller. This twist along each propeller blade is, of course, very intentional and necessary to extract the most performance out of the propeller. Take a close look along the length of a propeller, from the tip towards the spinner, and you will likely notice that it looks like the propeller twists.
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