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Variable Pitch For Jet?

Wed May 29, 2002 7:50 am

I was just brainstorming and thought, could you incorporate the variable pitch technology that is used on props with Turbojet/fan engines? Is it feasible and justifiable?

Ziggy  Smile
Posts: 38
Joined: Mon Aug 27, 2001 9:26 am

RE: Variable Pitch For Jet?

Wed May 29, 2002 8:18 am

They are developing variable pitch technology. Many aircraft have variable stators to help avoid compressor stalls. The variable pitch rotor blade concept will help with efficiency and save weight by not having to have a thrust reverse section
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RE: Variable Pitch For Jet?

Wed May 29, 2002 11:16 am

GE introduced an engine with variable pitch inlet guide vanes and variable pitch first few stages of stators back in about 1954 (the J-79). Convair used a commercial version of it (CJ-805) in 1959 through 1962 in the 880 and 990. But the fan engine version didn't have varying pitch on the fan.

However, an infinitely variable transmission between the power takeoff for the fan and the fan could do as much, if it could gently reverse direction of rotation. But the fan inlet possibly would need to be placed aft of the turbojet inlet.


RE: Variable Pitch For Jet?

Wed May 29, 2002 3:06 pm

The P&W PW306A turbofan engines that power the Gulfstream G200 (Galaxy) have variable inlet guide vanes and first-stage variable stator vanes. Both sets of vanes are hydraulically actuated by fuel pressure from the hydromechanical fuel control.
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RE: Variable Pitch For Jet?

Fri May 31, 2002 8:06 am

Yeah, variable pitch stators and inlet guide vanes are one thing, variable pitch fan (rotor) blades are a whole other story. Does anyone have any information about who might be working on variable pitch fan blades? I can't imagine the extra mechanical complexity is worth any increase in efficiency. Variable stator vanes have a relatively simple actuation system, thanks in part to the lack of centrifugal load on the blades and the fact that the mechanism is mounted on the casing, not within the rotor hub. For variable pitch fans, some robust hub-based mechanism would have to be developed (just like on a variable pitch prop, only a heck of a lot more blades and therefore less space to fit the mechanism into).

For reverse thrust, the effects of the airflow in the fan duct going forward could get a little complicated. The bearings might not appreciate the axial load on the rotor suddenly reversing. Maybe it's just me but a cascade thrust reverser with a translating cowl mechanism seems like it would be lighter and more mechanically simple than reverse pitch fan blades. Besides, who cares how efficient the engine is in reverse thrust?
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RE: Variable Pitch For Jet?

Fri May 31, 2002 9:12 am

I remember back when I was a little kid, before I knew anything about aviation, I thought that they did have a variable pitch, and to use reverse thrust, they would change the pitch and just add power to the throttles!

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The Engine Would Flame Out.

Fri May 31, 2002 10:20 am

In the case of a front fan engine if you changed the pitch of the fan blades for reverse thrust the compressor would be starved for air, and the engine would just shut off, except with power plants whose fans don't have blades right at the shaft (inner diameter).

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Joined: Wed Apr 24, 2002 8:04 pm

RE: Variable Pitch For Jet?

Fri May 31, 2002 8:35 pm

The only company, that I know of, that tried to develop a variable pitch turbofan was Turbomeca of France. That was sometime ago and I don't know many details and I don't think it was ever certified. The engine was intended to be used on corporate aircraft.
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Joined: Fri Mar 01, 2002 9:31 am

RE: Variable Pitch For Jet?

Sat Jun 01, 2002 12:22 am

VIGV's were initially used to prevent compressor stalls. They are still on JT8s' and many other for the same purpose.

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Joined: Wed Mar 08, 2000 6:15 am

RE: Variable Pitch For Jet?

Sat Jun 01, 2002 12:31 am

The JT8Ds have never had variable inlet guide vanes (VIGV). There is the J52-P-408, which is similar to a JT8D minus the fan, that has VIGVs. Incidently, most new engines do not have have inlet guide vanes at all. But they do have variable stator vanes (VSV) in the high pressure compressor.
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Joined: Tue Aug 08, 2000 10:34 am

RE: Variable Pitch For Jet?

Sat Jun 01, 2002 2:06 am

F-4s are still flying. The ones with the GE J-79s have VPIGVs and VPSs.

It looks to me that the unducted fans could probably adjust pitch fairly easily.

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RE: Variable Pitch For Jet?

Mon Jun 03, 2002 12:37 am

The Embraer 145/135 with the Rolls Royce engines have variable pitch fan blades known as Compressor Variable Geometry (CVG). They are hydraulically actuated and automatically controlled by the Full Authority Digital Engine Control (FADEC). Like other engines with this arangement, their purpose is to improve compressor efficiency and prevent compressor stalls.
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Joined: Wed Sep 06, 2000 10:30 pm

RE: Variable Pitch For Jet?

Mon Jun 03, 2002 4:44 am

I think, instead of fan blades, you mean variable geometry STATORS, not blades, in the the 1st - 6th of 14 stages of the COMpressor.


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