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Turbofan Tips: Newsletter Challenge (September 2017)

Posted August 31, 2017 5:01 PM
Pathfinder Tags: challenge question turbofan

This month's Challenge Question: Specs & Techs from GlobalSpec:

The low-pressure rotor of a PW100G-JM turbofan engine spins at a maximum rpm of 10,047. At what speed are the fan blade tips travelling at this rpm, and what (if any) noise concerns would this generate?

And the answer is:

Turbofan engines take advantage of momentum transfer efficiencies by moving large masses of air. A larger diameter turbofan moves more air, but must spin slower due to prevent the blade tips from reaching very high velocities.

The tip of a fan blade travels a greater distance than the section near the center for each revolution of the fan. For the same fan rotation speed (revolutions per minute), the tip of a fan blade will move at a higher speed (m/s) than the base of the blade. As fan diameter increases, the longer blades can cause blade tips to travel at very high speeds.

To calculate the blade tip speed, the fan diameter and fan rotational speed are needed. The Airbus A320neo is powered by the PW1100G-JM, which has a fan diameter of 81 inches. The maximum rotation speed of its low pressure rotor is 10,047 rpm. Since there is a gear train with a ratio of 1:3.0625 between the low pressure rotor and the fan, the maximum rotational speed of the fan is 10,047 / 3.0625 = 3,281 rpm.

Blade tip speed = π x D x S

Where

D is fan diameter, and

S is fan rotation speed

So the maximum blade tip speed of the PW1100G-JM is 353 m/s. [ π x (81 in) x (3,281 rpm) = 83,4912 x (0.0254 m/in) x (1 min / 60 sec) = 353 m/s ]

The speed of sound is 343 m/s, so the blade tips are travelling at supersonic speeds at maximum rpm. For a simple, straight-bladed propeller operating in free air, supersonic motion would cause shock waves to form, generating a large amount of noise. But in turbofan engines like the PW1100G-JM, the fan blades are shrouded by the casing around the engine. The casing or nacelle helps guide the airflow into the compression stage of the engine, minimizing shock waves. In addition, the casing has noise-reducing liners.

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#1

Re: Turbofan Tips: Newsletter Challenge (September 2017)

08/31/2017 5:15 PM

Ahha! trick question, there is no PW100G-JM turbofan engine....

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#2
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Re: Turbofan Tips: Newsletter Challenge (September 2017)

08/31/2017 9:11 PM

Most likely a typo...probly shuda been PW1000G-JM

https://en.wikipedia.org/wiki/Pratt_%26_Whitney_PW1000G

I'm guessing that the answer is Mach1, which would correspond to about a 1 foot radius at that RPM (sonic boom).

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#3
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Re: Turbofan Tips: Newsletter Challenge (September 2017)

08/31/2017 10:15 PM

..."By putting a 3:1 gearbox between the fan and the low-pressure spool, each spins at its optimal speed: 4,000–5,000 RPM for the fan and 12,000–15,000 RPM for the spool, the high-pressure spool spinning at more than 20,000 RPM"...

Well first of all the fan is the front blade, considerably larger than 1 ft.....the low pressure rotor is turning at 3 times the fan rotor speed, so the 10,047 figure would be /3 or 3349 rpm...depending on which model the engine is it could be anywhere from 56" to 81" in diameter....and I would guess 81"...which would put the tip speed at 1.05 mach...

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#4
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Re: Turbofan Tips: Newsletter Challenge (September 2017)

08/31/2017 10:49 PM

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#5
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Re: Turbofan Tips: Newsletter Challenge (September 2017)

09/01/2017 6:50 PM

Actually, it's a little more complicated than calculating the circumferential speed of the blade tips. The important factor is the blade speed with respect to the air and not to the engine itself. But the blades are designed to achieve supersonic speed relative to the air for efficiency reasons.

A discussion of this can be found at:

https://aviation.stackexchange.com/questions/22316/why-do-turbofan-blade-tips-exceed-the-speed-of-sound-while-prop-tips-should-not

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#6
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Re: Turbofan Tips: Newsletter Challenge (September 2017)

09/01/2017 8:00 PM

Yes I would think air density and relative humidity would have a lot to do with operational characteristics...but probably why the engines are so loud on takeoff and seem so much quieter cruising at altitude ~ 0.08 mach...

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#8
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Re: Turbofan Tips: Newsletter Challenge (September 2017)

09/02/2017 12:11 AM

I kept expecting the video to give some useful information about altitude, but it never happened, except possibly on the dials and gauges in the cockpit. I'd have to use stop-frame to possibly read any of those...

The speed of sound drops with increasing altitude (decreasing temperature), up to the troposphere, so for a given fan RPM, the fan tip mach number will increase with altitude. Based on that, the fan noise should get louder at altitude.

Of course the engines are spinning faster during takeoff than they are at altitude. so naturally they will be louder during takeoff.

What is the significance of that ≈ 0.08 Mach?

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#9
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Re: Turbofan Tips: Newsletter Challenge (September 2017)

09/02/2017 1:06 AM

That's the cruising speed of the A320...

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#10
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Re: Turbofan Tips: Newsletter Challenge (September 2017)

09/02/2017 1:33 AM

Surely you meant ≈0.8, not 0.08!

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#11
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Re: Turbofan Tips: Newsletter Challenge (September 2017)

09/02/2017 1:46 AM

Haha...

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#7
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Re: Turbofan Tips: Newsletter Challenge (September 2017)

09/01/2017 11:11 PM

Had seen that link before and it pretty much sums up the question but doesn't give an exact answer to the question...because there is no exact answer yet there are many correct answers - n attacks.
In a vacuum one is correct - but meaningless. In high pressure situations there is another. In a massive deluge there is another.
So the best answer is - open another bottle of that excellent 2002 Malbec (2004 Merlot would do at a crunch) and ponder the excellent work from the people at PW

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#12

Re: Turbofan Tips: Newsletter Challenge (September 2017)

09/02/2017 11:53 AM

I've always wondered why it says : The answer to this challenge will be posted later this month, right here on cr4.

The answer better be posted on cr4, because it wouldn't do much good to post the answer on the Martha Stewart cooking school website.

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#13
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Re: Turbofan Tips: Newsletter Challenge (September 2017)

09/02/2017 2:32 PM

Doesn't matter, I could still find it....

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#14
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Re: Turbofan Tips: Newsletter Challenge (September 2017)

09/03/2017 12:11 AM

Perhaps that's why I rarely see the posted answer! I've looked at many of the challenges, and posted on a few, but only remember seeing one or two official answers.

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#16
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Re: Turbofan Tips: Newsletter Challenge (September 2017)

09/07/2017 8:54 AM

FYI: the original thread is edited around the last Monday of the month with the correct answer. Or at least the answer the author of the question provided.

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Re: Turbofan Tips: Newsletter Challenge (September 2017)

09/07/2017 11:36 AM

I think my attention span is probably better than most, at least for scientific/engineering stuff, but a month is way too long! I have always expected the answer to be provided in a week or so.

I think I remember at least one or two cases where a correct answer was confirmed after only a few days. That's as it should be.

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#18
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Re: Turbofan Tips: Newsletter Challenge (September 2017)

09/07/2017 3:14 PM

Indeed, that's an easy change we can make. Voila, your answer!

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#19
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Re: Turbofan Tips: Newsletter Challenge (September 2017)

09/07/2017 5:37 PM

Thank you!

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#15

Re: Turbofan Tips: Newsletter Challenge (September 2017)

09/04/2017 2:28 AM

..."The fan of an IAE V2500 engine (that power the Airbus A320) spins at a maximum speed of 5,650 RPM. The fan diameter is 63.5 inches (1613mm). The distance covered by the blade tip, in one revolution is 1.613m X π = 5 meters. 5 meters X 5650 RPM = 28,630 m/minute = 477m/s, which is about 1.4 times the speed of sound at sea level. If the PW1100G’s 81 inch diameter fan (2057mm) is spun at the same speed, the blade tips travel at 608m/s, which is almost twice the speed of sound! (This is responsible for the characteristic chainsaw noise that can be heard from an Airbus A320’s engine when taking off at close to full take off power). A large amount of energy will be needed to overcome the drag associated with the high speed of the blades (this is different from the drag that the blades pose to the oncoming air stream). In addition, the airflow becomes more complex."...

http://theflyingengineer.com/page/4/?s=ATR

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