Qs about turbofans

jetman

New Member
How fast [RPM] does the fan [N1] and the the turbine [N2]turn in a turbofan engine at full power?
Do the blades go supersonic?
If they do is there a shock wave and sonic boom inside the engine?
Thanks for input jetman
 

Eagle

New Member
The N1, N2 are read in percentages, not in RPMs.

So as for what the actual speed is... who knows?
 

Mr_Creepy

Well-Known Member
Well you're supposed to know!!! It's in your FSI training I hope! Although I suppose it doesn't matter much.

The Beech 1900 turns 104% at 39,000 rpms. 100% is something like 37,842 I believe.

Of course, I would have to ground the airplane if I see it only turning 38,999 at 104 % !!!!!


NOT!

Yeah you're right. Who knows? And who cares?
 

Derg

New Arizona, Il Duce/Warlord
Staff member
Turbofan **ANSWERS** :)

I dug around a 727, 737-200/300 and my MD-88/90 flight manuals and I couldn't find any reference to an actual number.

But alas, a quick telephone call to Juegen Badura who is the MD-88/90 Technical Manager for Delta says that at 100% on the MD-88 engine (Pratt&Whitney JT8D-219), the RPM is about 8,900 or so and the compressor/turbine blades do not go supersonic.

Hey, how's that for customer service!
 

PhotoPilot

New Member
Wow! 39,000 RPM is 650 rotations per second! I'm not yet a pilot and won't even pretend to know anything in-depth about a turbine engine, but MAN that sounds fast.

Doug's numbers seem a bit more possible to my internal-combustion-on-the-ground brain, but I suppose that there could be a large difference in RPMs between two engines. Where did you get the higher numbers from?

Just curious!


(And who knows! Maybe I'll need to know in 5 or 10 years when I might actually be flying a turbine aircraft!
)
 

IrishSheepdog

Sitting in the median
In the Saab, our gas generator is around 44,720 RPM at 100%, and the propeller is around 22,000 RPM at 100%. 1750 SHP.
 

Baronman

Well-Known Member
[ QUOTE ]
22,000 RPM at 100%. 1750 SHP

[/ QUOTE ]

Those some fast moving blades!!

Might wanna check that before your next test!!
 

PhotoPilot

New Member
My internal-combustion-on-the-ground brain is humbled! Even though I look at them every time I fly commercially, I am VERY impressed that someone can build engines that withstand the heat, rotational speed, and structural stresses encountered every day by turbo-fans. 44,000 RPM! Dang!

And I think I'm hot stuff for learning to work on my own car . . .
 

Alchemy

Well-Known Member
39,000 RPM ????

Let's say the compressor blades are 4 feet in diameter. That means a circumference of about 12.6 feet. So they're moving 12.6 feet 39,000 times every minute, that's 490,088 ft per minute, 8,168 ft per second, or 5,569 miles per hour, roughly mach 7.3 at sea level under standard conditions. Wouldn't the engine disintegrate at these speeds?
 

Derg

New Arizona, Il Duce/Warlord
Staff member
[ QUOTE ]
39,000 RPM ????

Let's say the compressor blades are 4 feet in diameter. That means a circumference of about 12.6 feet. So they're moving 12.6 feet 39,000 times every minute, that's 490,088 ft per minute, 8,168 ft per second, or 5,569 miles per hour, roughly mach 7.3 at sea level under standard conditions. Wouldn't the engine disintegrate at these speeds?

[/ QUOTE ]

Yup, I remember those numbers from Beech 1900 training.

I failed to ask Juergen at what point the engine is running at 8,900 -- which seems pretty low if you ask me.
 

Alchemy

Well-Known Member
wow, I didn't realize they moved that fast.....impressive stuff. So is a lot of the noise from jet engines the result of mini-sonic booms from the fan blades or am I way off.

I've read "The Turbine Pilot's Flight Manual" per doug's reccomendation and they didn't delve too deeply into shaft rotation speeds......
 

Derg

New Arizona, Il Duce/Warlord
Staff member
[ QUOTE ]
I've read "The Turbine Pilot's Flight Manual" per doug's reccomendation and they didn't delve too deeply into shaft rotation speeds......

[/ QUOTE ]

Primarily because it really doesn't matter that much.

Even N1 and N2, in a real basic sense, don't really matter as much as ITT or "Inter Turbine Temperature". The engines can almost spin themselves to light speed, but if the temperature gets too high, party over.
 

jetman

New Member
Re: Turbofan **ANSWERS** :)

Thanks guys for replies ,Doug it was very nice of you to dust off the old manuals and make the phone call, the service is EXCELENT
thank you.
About the 8900 RPM i guess that is the speed of the high pressure turbine NOT the speed of the fan[if the fan is 7 or 8 feet in diam. turning about 150 RP second the blades would travel at about 3750 feet per second kind of supersonic ////////
Take care guys jetman
 

IrishSheepdog

Sitting in the median
[ QUOTE ]
[ QUOTE ]
22,000 RPM at 100%. 1750 SHP

[/ QUOTE ]

Those some fast moving blades!!

Might wanna check that before your next test!!

[/ QUOTE ]

Nope, that's correct. Plus, we have overspeed protection at 25,000 RPM (1573 Np, which is read as RPM but is divided by a computer to give us that number... divided by 15.9 I believe). Our normal Flight range of prop RPM is 1150 to 1396, multiply that by 15.9 and you get the actual RPM.
 

Mr_Creepy

Well-Known Member
PT67 (Beech 1900D engine) turbines are much smaller than 4 ft in diameter. I doubt they are even a foot. I used to show them in my ground school. Even the centrifugal compressor turbine fit pretty well in my hand. Probably about 9-10" in diameter.
 

Alchemy

Well-Known Member
ahh, okay, that makes more sense. with 10" blades the rotational velocity at the tips is only about mach 1.46 I think.
 

Buzo

Well-Known Member
The 1900C with PT6-65B's puts out 1100HP. The turbine speed like John said is 39,000 at 104%, and 37,468rpm at 100%. The prop rpm is reduced through a reduction gearbox to a max of 1700rpm. Above 1768 rpm a governor kicks in to prevent it from going faster.
 

A300Capt

Freight Dawg
[ QUOTE ]
In the Saab, our gas generator is around 44,720 RPM at 100%, and the propeller is around 22,000 RPM at 100%. 1750 SHP.



[/ QUOTE ]

I would think that reduction gear system would reduce the engine's high internal rotation speeds to something far lower for the propeller. I find 22,000rpm for the propeller much too high. Seems to me the the tips would go supersonic.
 

Mr_Creepy

Well-Known Member
I am sure he means the rpm before the gear ratio reduction. I know the 1900 has something like 17.6:1 reduction.

Let me do the math here. The Props turn at 1700 rpm when the Power Turbine is doing 29,920 rpm. 29,920 / 1700 = 17.6. Wow I was right!
Notice that the Power section does 29,920 and the compressor section can be going as fast as 39,000.

That is the "free turbine" principal, also known as the "air clutch" of the Pratt & Witney design.

Anyway in what I hope will be a final answer (Regis, is that your final answer?) to this thread:

No the turbine blades do not go supersonic. The shock wave would surely cause compressor stalls and probably serious damage to the engine.
 
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