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abletofly
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Speed At Fanblade Tip?

Wed Dec 13, 2006 5:11 am

I was just wondering when I saw the picture of the PIA 777 with the massive engines. What's the speed of the fanblade tips at max thrust?? There must be a factor due to the engine diameter??

Regards
 
gh123
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RE: Speed At Fanblade Tip?

Wed Dec 13, 2006 5:20 am

Good question - I wouldn't know the answer.......but I'm interested to find out.

Slightly off topic I was once told that the tip of a helicopter's rotor went round at about 800-900mph.
 
CHRISBA777ER
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RE: Speed At Fanblade Tip?

Wed Dec 13, 2006 5:20 am

The tips actually go supersonic - on higher bypass fans thats what the grinding noise is when the engine is at full power.
What do you mean you dont have any bourbon? Do you know how far it is to Houston? What kind of airline is this???
 
Poitin
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RE: Speed At Fanblade Tip?

Wed Dec 13, 2006 5:30 am

You might be better off moving this to the Tech/Ops forum, where all the airplane geeks hang out. I am sure they can tell you by make and model of engine.
Now so, have ye time fer a pint?
 
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abletofly
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RE: Speed At Fanblade Tip?

Wed Dec 13, 2006 5:35 am

I know that there's an issue with some wings on commercial planes and supersonic speeds, but is that no big deal with fanblades? If and when they go supersonic on the tips..?

Regards.
 
N231YE
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RE: Speed At Fanblade Tip?

Wed Dec 13, 2006 6:00 am

Quoting CHRISBA777ER (Reply 2):
The tips actually go supersonic - on higher bypass fans thats what the grinding noise is when the engine is at full power.

That's correct.

Quoting AbleToFly (Reply 4):
I know that there's an issue with some wings on commercial planes and supersonic speeds, but is that no big deal with fanblades? If and when they go supersonic on the tips..?

I believe that the shockwaves resultant from the fan blade tips moving at supersonic speeds increases efficiency. The only downside, as stated, is noise. Rolls-Royce has addressed this problem, using swept (curved) fan blades as found on the Trent 900s, which power the Airbus A380. I do not know the physics behind how swept blades work by reducing noise and increasing performance, so that is open to someone else...


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DfwRevolution
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RE: Speed At Fanblade Tip?

Wed Dec 13, 2006 6:02 am

Quoting N231YE (Reply 5):
I do not know the physics behind how swept blades work by reducing noise and increasing performance, so that is open to someone else...

It's not unlike the drag reduction accomplished by sweeping the wing of an airplane.
I have a three post per topic limit. You're welcome to have the last word.
 
N231YE
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RE: Speed At Fanblade Tip?

Wed Dec 13, 2006 6:13 am

Quoting DfwRevolution (Reply 6):
It's not unlike the drag reduction accomplished by sweeping the wing of an airplane.

I never thought of it that way. Thank you.
 
oly720man
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RE: Speed At Fanblade Tip?

Wed Dec 13, 2006 6:33 am

From some data on the GE90 I get the tip speed to be 406.5m/s (speed of sound being around 340m/s)

Fan diameter 123" (radius 1.56m) and 2485 rpm (=41.6 revs/sec)

Diameter 123", increased to 128" for GE90-115
http://www.geae.com/aboutgeae/presscenter/ge90/ge90_20000724.html

RPM here for fan blade failure test.
http://www.geae.com/aboutgeae/presscenter/ge90/ge90_19950202c.html
wheat and dairy can screw up your brain
 
SlamClick
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RE: Speed At Fanblade Tip?

Wed Dec 13, 2006 6:45 am

Quoting AbleToFly (Reply 4):
I know that there's an issue with some wings on commercial planes and supersonic speeds

To understand that issue you need to remember that there are two possible "mach" numbers here: One is the actual free-stream mach number that the airplane and its wings are making good through the air. The other is a local flow speed as a result of the shape and interference from other bodies nearby etc.

The speed at which the local aiflow goes supersonic over some parts of the wing of a subsonic airliner is called the critical mach number or MCRIT.

Since many smart engineers and brave test pilots thoroughly explored this subject in the 40s and 50s the modern airliner has wings that accomodate local supersonic flow all the way up as fast as the designers intend the plane to be flown. So on a typical subsonic jetliner of recent years, if MCRIT is M0.75 and the plane is intended to cruise at M0.80 then the flow over parts of the upper wing surfaces will be supersonic with no serious harmful effects.

On some wing designs you can actually see the shockwaves in various places out there in flight.

On propellers, rotor blades and compressor blades getting the tip supersonic must result in super-sub boundaries somewhere along the span of the blade. Must make for interesting aerodynamics for the technologically inclined. Especially for helicopter rotors which have a retreating and advancing side of the blade disc.
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SlamClick
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RE: Speed At Fanblade Tip?

Wed Dec 13, 2006 7:03 am

Quoting Gh123 (Reply 1):
Slightly off topic I was once told that the tip of a helicopter's rotor went round at about 800-900mph.

I've been away from the good ol' Huey for a long time but I think I remember an actual rotor RPM of 344. (With an engine speed of about 6200RPM)

It had a 48 foot rotor diameter.
So I make that hovering tip speed of 516 knots.

Sea level, standard day the speed of sound is 662 knots so the tip speed is about .78M

Redline speed for the same Huey was 120KIAS so the advancing blade tip was doing 516 + 120 or 676 knots or Mach .96 This makes my remembered numbers pretty credible for me because that Mach number would argue for both redline speed and rotor RPM limits being about where they are.

The retreating blade would be doing 516 - 120 or 396 knots or Mach .6 or so.

At the point on the blade span where rotational speed is equal to forward speed or 120 knots - from that point inboard toward the blade root, on the retreating blade the airflow was actually reversed. It flowed from trailing edge to leading edge at that point.

The memory-dependant variables here are actual rotor RPM and rotor diameter. The rest is just arithmetic. If anyone has an old Huey manual lying about I'd welcome correction.
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ex52tech
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RE: Speed At Fanblade Tip?

Wed Dec 13, 2006 7:16 am

Quoting CHRISBA777ER (Reply 2):
The tips actually go supersonic - on higher bypass fans thats what the grinding noise is when the engine is at full power.

Grinding noise?
I was thinking more like a manly Growling noise............grinding..........sounds like somethings wrong.
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jetlife2
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RE: Speed At Fanblade Tip?

Fri Dec 15, 2006 1:37 am

The duplicate thread got deleted, so I'm reposting my response to this thread. The above calculation of the GE90 fan blade tip velocity is correct and results in about M 1.4. Regards
My views are not necessarily the views of the GE Company
 
aeroguy
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RE: Speed At Fanblade Tip?

Fri Dec 15, 2006 4:58 am

Some additional info in this previous thread on engine noise:
https://www.airliners.net/discussions/tech_ops/read.main/86105/
 
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jetmech
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RE: Speed At Fanblade Tip?

Fri Dec 15, 2006 6:22 pm

Quoting Poitin (Reply 3):
You might be better off moving this to the Tech/Ops forum, where all the airplane geeks hang out.

I resemble that remark  Smile !

Quoting Poitin (Reply 3):
Tech/Ops

Easily the best forum on A'net  yes !

Quoting AbleToFly (Reply 4):
know that there's an issue with some wings on commercial planes and supersonic speeds, but is that no big deal with fanblades?

I think that it may have something to do with the fact that a fan assembly is rotating inside a closely fitting duct, which a wing lacks. I don't know how, but I think that this allows a fan blade to be less effected by exceeding the speed of sound  Confused.

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
Poitin
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RE: Speed At Fanblade Tip?

Sat Dec 16, 2006 1:30 am

Quoting JetMech (Reply 14):
Quoting Poitin (Reply 3):
Tech/Ops

Easily the best forum on A'net yes !

Well, maybe best informed. I perfer the Irish Aviation Thread  yes 
Now so, have ye time fer a pint?
 
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Starlionblue
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RE: Speed At Fanblade Tip?

Sat Dec 16, 2006 2:45 am

Quoting JetMech (Reply 14):
I think that it may have something to do with the fact that a fan assembly is rotating inside a closely fitting duct, which a wing lacks. I don't know how, but I think that this allows a fan blade to be less effected by exceeding the speed of sound

To add to the uninformed opinions, maybe the shockwaves can be more closely managed. Also note that the airflow is presumably not supersonic, just the blade tips. So maybe that makes a difference too.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

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