I'm building a simple 3d model of one and because reasons I won't be able to give it a variable blade pitch so I need to find a typical value. My google-fu has failed me so far on this.
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$\begingroup$ A Radial air turbine? A radial turbine in a turboshaft, or a propeller-like Ram Air Turbine such as deployed by the A320 if all power fails? $\endgroup$– KoyovisAug 10, 2017 at 3:42
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$\begingroup$ @Koyovis sorry meant to say ram air turbine $\endgroup$– AidanAug 10, 2017 at 4:15
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1 Answer
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Ram Air Turbines have variable pitch which is set according to demanded load and airspeed, and the controller keeps the rotational speed as high as possible for maximum efficiency. Typical operational RPM of a RAT is between 5500 and 7700 rpm, at speeds between 125 - 375 knots. Let's take the averages: 6600 RPM @ 250 knots = 700 rad/s @ 130 m/s.
At a disk diameter of 1 m and a spinner diameter of 20 cm, lets look at velocity triangles at the blade root, centre and tip:
- Blade root: R = 0.1 m, rotational speed $V_{rot}$ = 0.1 * 700 = 70 m/s. With incoming air at 130 m/s, this creates a local velocity pitch angle of arctan (130/70) = 62 deg
- Blade centre: R = 0.3, $V_{rot}$ = 0.3 * 700 = 210 m/s, velocity pitch angle = arctan (130/210) = 32 deg.
- Blade tip: R = 0.5, $V_{rot}$ = 0.5 * 700 = 350 m/s, velocity pitch angle = arctan (130/350) = 20 deg.
Note that these are unloaded velocity triangles, the blades can only generate power with air coming in at a local angle of attack. We're rough estimating, so you could take a pitch of 25 deg at tip, 35 at centre, and linearly twist onto 45 deg at root (most of the power is generated at the outer part of the blade).
