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.
 

ferranrosello

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Ok, by now is clear that the rotor disc has a positive angle of atack always. However this is not an explanation of why "why does the nose pitch down when the stick is pushed forward? "

This is because the center of gravity wants to be situated always in before the rotor spinning axis. When you move the stick forward the aircraft "has to move" to keep the cg in position.

More technically explained:
The cg is the axis around what the aircraft rotates. Is a little bit forward of the rotor spinning axis because of the tail Horizontal stabilizer, that creates a moment around the cg (a downwards lift) that is compensated by the rotor thrust. If yo move the stick forward, you will move the rotor thrust "behind" the cg and now the moment will be unbalanced. The aircraft "has to rotate" downwards until the cg moment is canceled...
 

Housedr

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If you picture it this way it might help with your visualization.
imagine Your gyro is always flying straight land level.
in this imaginary gyro we are flying at 60 and the stick is neutral. Push the stick forward and the rotor disc moves forward but the hinged body can’t move all the mass that fast, so it lags a bit then assuming you keep that stick position, you speed up but are still flying level to the relative wind So the nose of the gyro body appears to pitch down,Losing altitude in the process.
if you were to add power at this point without introducing a relaxation of forward stick pressure , you would continue to fly very stably into the ground with the gyro at the same attitude to the relative wind.
pull back or relax the forward pressure and instantly the rotor tilts back, but the body will lag just a bit before the nose eventually comes back to straight and level climbing flight with the same rotor carriage angle to the relative wind. (For a given airspeed)
 

Vance

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If you picture it this way it might help with your visualization.
imagine Your gyro is always flying straight land level.
in this imaginary gyro we are flying at 60 and the stick is neutral. Push the stick forward and the rotor disc moves forward but the hinged body can’t move all the mass that fast, so it lags a bit then assuming you keep that stick position, you speed up but are still flying level to the relative wind So the nose of the gyro body appears to pitch down,Losing altitude in the process.
if you were to add power at this point without introducing a relaxation of forward stick pressure , you would continue to fly very stably into the ground with the gyro at the same attitude to the relative wind.
pull back or relax the forward pressure and instantly the rotor tilts back, but the body will lag just a bit before the nose eventually comes back to straight and level climbing flight with the same rotor carriage angle to the relative wind. (For a given airspeed)
It appears to me I am not picturing your visualization well.

In the gyroplanes I have flown when I move the cyclic forward my indicated air speed increases.

When I add power I climb.

What am I missing in your visualization?

Why do you fly into the ground when you add power?
 

Housedr

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If you establish a dive with stick still forward And never neutralizing it just continuing to hold it forward in your dive the carriage and the rotor will reach an equilibrium just as if it’s in level flight. Where am I wrong. Adding power at that point will just make you hit the ground faster
 

WaspAir

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If you establish a dive with stick still forward And never neutralizing it just continuing to hold it forward in your dive the carriage and the rotor will reach an equilibrium just as if it’s in level flight. Where am I wrong. Adding power at that point will just make you hit the ground faster
The aircraft is not like an arrow, merely going in the direction it is pointed.
Adding power in steady state horizontal flight, changing nothing else, doesn't give you a simple speed increase along the same horizontal path. It makes you climb. If you want to go faster but remain in horizontal flight, you pitch forward for that higher speed and add power to maintain the desired altitude (some view the pitch/power relationship in the reverse, but regardless, both pitch and power must change to go faster without climbing).
Why would steady state flight in any other condition (such as descending) create a different behavior?
Adding power in a steady descent, changing nothing else, will not make you go faster along the same descent angle. It will make that angle more shallow, and if you have enough excess power to add, it can level your path or even make you climb, at a speed set by your pitch.
 
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Housedr

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I think you might have not read my original post or I may have not articulated it well.
Would you agree that the gyro flys through relative wind and as long as there is wind going through the rotor the gyro does not know the earth is below it or in front of it.
Would you agree the the rotor is tilted back about 9 degrees typically do that the relative wind can spin the rotors?
So if you were in a dive- not a glide- dive dive dive! Lol
And you are let’s say max power max speed a setting that would be fastest available speed in level flight-
Are you still going to climb out of that dive?
 

WaspAir

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So if you were in a dive- not a glide- dive dive dive! Lol
And you are let’s say max power max speed a setting that would be fastest available speed in level flight-
Are you still going to climb out of that dive?
In any steady-state flight at any acute angle of your path to the horizontal (whether up, down, or level) that you can achieve, adding power, without any other change, will cause a clockwise rotational motion of the needle on your vertical speed indicator. How far it moves depends on how much power you are able to add.
 
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