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HeliTorque :: View topic - Tandem Rotors in Autorotation Situations
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HeliTorque Forum Index » Flight Dynamics

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TripleDelta
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PostPosted: Sun Oct 09, 2005 7:32 am    Post subject: Reply with quote

SkidsUp wrote:
I seem to remember from early FW days that AC such as the Twin Otter, when the props went into what was called the "Beta" range, which was a netural pitch, neither pulling or pushing, but effectively creating a huge "disk" that was difficult to push through the air, thus slowing you down, and allowing a much steeper angle of desent without and increase in speed.


That is mostly correct, though using Beta in flight is a rather hazardous operation for aircraft and engines not deisgned for that (the only a/c I know of that can withstand this treatment is the Hercules). Beta in flight can lead to blade overstressing and engine overspeeding, as opposed to its primary use on the ground at taxi speeds to reduce the amount of thrust produced by turboprop engines at minimum working pitch.

However, a propeller on a fixed wing a/c is a different issue than the rotor of a heli. While both work on the same principles in normal, powered flight (even windmilling is "used" in the same way on rotors and constant-speed propellers), the only force it produces is that of thrust in the horizontal plane - that is, its direct contribution to lift, in terms of force produced, is virtually zero. Setting a prop into neutral pitch would not cause the aircraft itself to loose lift, save for the loss of forward speed - but the wing would still be doing its job.

The rotor of a heli is the primary lift generator. The force it creates is in the vertical plane and directly opposes weight. If you were to set the blades into neutral pitch, that is into zero-lift pitch, you'd loose the force counteracting the weight - and frankly you'd be going down like a rock. True, you'd get an element of drag from the blades in the manner you described, but its contribution to slowing the heli down would be negligible compared to the lift produced by letting the rotor spin at some pre-determined pitch - plus, by letting the rotor spin, you'd get forward speed as well, enough to reach a safe landing location.

Think of it this way: do you think that a rotating paper disc would fall slower than a rotating sycamore seed? You can try it out in the park anyway...

EDIT: typos...
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Ascend_Charlie
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PostPosted: Wed Oct 12, 2005 10:11 am    Post subject: Reply with quote

Skids, you seem fixated on the idea of the big disc or plate creating lots of drag.

Have a look at rotor blades when they are stationary - how much of the circle do they take up? Less than 10%, usually more like 5 or 6% ( circumference of a B206 blade disc is around 94', blades around 1' wide. They take up more of the circle at the hub, but you get my idea.) This is called the 'solidity" of the disc. Spinning it around creates the illusion of a solid disc, but so does the fishing line on the end of a whipper-snipper, and it has stuff-all drag in the airflow.
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flyer43
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PostPosted: Tue Dec 05, 2006 10:16 pm    Post subject: Lift and Drag in autorotation Reply with quote

To assist with the understanding of what forces are acting on a rotor system during autorotation, you might want to visit dynamicflight.com The following link should take you specifically to the page on Autorotation. http://www.dynamicflight.com/aerodynamics/autos/

Cheers

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