Pitch Question

JJ Campbell

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When the stick is all the way forward, the rotor is level (or close to it) and there is no elevator. So, why does the nose pitch down when the stick is pushed forward?
 

Vance

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In my opinion the rotor thrust vector (rotor thrust line) is moving aft when the cyclic is moved forward and that lowers the nose.

I feel it is a useful concept because it is why the pitch of the gyroplane is very much related to speed and a more immediate indicator of air speed trends than the airspeed indicator.

I use the sight picture of pitch for speed trends and the air speed indicator to calibrate my sight picture.

The airspeed indicator is always behind the aircraft and using the airspeed indicator to manage airspeed has me behind the aircraft trends.
 

bryancobb

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The cyclic controls the tip-path-plane of the rotor. Visualize the spinning rotor as a colored disc over your head. When cyclic is moved in any direction, aerodynamic forces cause that disc to tilt in that same direction. The body of a rotorcraft is attached to that disc at the center, by a thing sort of like a universal joint called the rotor HUB.

If you are flying along at say 60 MPH and move the stick forward, that tilts the disc or tip path plane in that direction and the rotor dives or descends and the body of the aircraft must follow. The top aircraft is not climbing or descending. The bottom aircraft is a fraction of a second after the pilot has pushed the stick forward. The tip path plane has dipped at the front and risen at the back but the body of the aircraft has not yet reacted, but it will. The arrow shows the body motion. Its nose will go down and tail will rise.
 

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JJ Campbell

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But if the rotor is pitched down, how is air coming up through the bottom to keep it in autorotation? I am probably being really dense but I am struggling to visualize how it works. And, none of the two gyro books I have really explain this part when they discuss the aerodynamics of the rotor.
 

WaspAir

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The rotor is pitched more in the downward direction, when compared to where it was, but not down below the flight path so that airflow would be suddenly coming from above the disc. Remember also that the airflow is opposite the direction of travel, so if you are ptiching nose down, the relative wind will be more from below.. You're not diving into a a steady horizontal current, but always facing relative wind opposite your motion.
 

Vance

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Two pictures of the Predator,

The first one is at 85kts straight and level; you can see the rotor is still well back.

The second is nose down with the 120kt airspeed indicator pegged.

It is easy to see the air is still coming from below the rotor.
 

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Doug Riley

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One might (at first blush) think that the gyro's frame would stay level when the rotor disk is level. That's the way it is in helicopters.

In our gyros, however, we intentionally place the CG far enough forward so that, if the rotor is pulling straight up relative to the horizon (rotor level) and there's no prop thrust, the frame hangs 10-11 degree nose-down. We conduct a "hang test" before we first fly to be sure this is the case.

Why? Because, in level cruising flight, a gyro's rotor is inclined aft around ten degrees. This aft tilt means that the rotor is pulling up-and-back on the gyro at an angle of ten degrees. The gyro's CG must lie on the rotor's up-and-back thrust line, and the frame then flies level in cruise.
 

WaspAir

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That's because you're looking through the disc at the aircraft "below" (with respect to the viewer). Notice how the gyro is rolled toward the viewer and how much of the upper surface of the horizontal stabilizer you can see. You can't really appreciate the depth between the disc and the tail at that angle.
 
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Vance

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Vance - Rotor looks so close to the tail in the second photo.
This is The Predator at the same air show with the cyclic full back at 120 rotor rpm.

If the blade were to sail the blade would hit the rudder about four inches below the top.
 

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Vance

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That's because you're looking through the disc at the aircraft "below" (with respect to the viewer). Notice how the gyro is rolled toward the viewer and how much of the upper surface of the horizontal stabilizer you can see. You can't really appreciate the depth between the disc and the tail at that angle.
The picture was taken from the ground and I was around 100 feet above the ground and 1,000 feet away. I am diving out of a steep turn and you can see my knee board that is about five inches below the edge of the body when looking from the side. I looked for a better picture. Rotor orientation in a picture is just luck. You understand the perspective perfectly J.R. I used the picture because I remembered I had the airspeed indicator pegged.
 
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