Turboprop Lack of Noise

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bigblockz8

Pre-takeoff checklist
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For a jet+ a prop I am always surprised by DHC8's lack of noise. This Pilatus PC-12 took off of a 2400 ft runway at a decent power setting and was very quiet! I was disappointed. Out of W00, only see one or two every year or two!

http://www.youtube.com/watch?v=XoEILuqrNPc

I'm new and green with these but why are they quieter? A 205-210 would be LOUD leaving. Even the 172's are.
 
A lot of it has to do with prop speed. Also the angle of the exhaust with respect to the listener, turbines CAN be noisy.
 
Gears. As soon as you add a gear set you can run the prop at an efficient quiet speed and the engine up at a speed where it can make power without getting into detonation. Same reason a 421 is so much quieter than all the rest of the straight shaft-prop coupling fleet.

Noise control is all about prop tip speed as is propellor efficiency.
 
bigblockz8 said:
For a jet+ a prop I am always surprised by DHC8's lack of noise. This Pilatus PC-12 took off of a 2400 ft runway at a decent power setting and was very quiet! I was disappointed. Out of W00, only see one or two every year or two!

http://www.youtube.com/watch?v=XoEILuqrNPc

I'm new and green with these but why are they quieter? A 205-210 would be LOUD leaving. Even the 172's are.
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Are you kidding me! I always miss the cool stuff going on!:mad2: First Mariah Carey and now this:sad:
 
Henning said:
Gears. As soon as you add a gear set you can run the prop at an efficient quiet speed and the engine up at a speed where it can make power without getting into detonation. Same reason a 421 is so much quieter than all the rest of the straight shaft-prop coupling fleet.

Noise control is all about prop tip speed as is propellor efficiency.
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You can get detonation out of a turboprop?
 
Henning said:
Gears. As soon as you add a gear set you can run the prop at an efficient quiet speed and the engine up at a speed where it can make power without getting into detonation.
Click to expand...

Technically the engine on a DHC-8 (the the Q400 I believe) is a free turbine, so there's gears, but they aren't directly connected to the main turbine inside the engine. :)

So yeah, gears... but not what most people think when they think of gears connected to an engine via a transmission.
 
Sam D said:
You can get detonation out of a turboprop?
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No, but you can overheat the turbine under similar pressure/back pressure type situation. Where as a a piston engine runs pulses of pressure, a turbine runs continuous pressure, but regardless, too much pressure leads to too much heat.
 
But they serve the same purpose by reducing turbine speed (N1) to prop speed (N2). Prop speed is further refined by governor system, but the primary change from ~30k/min to ~2k/min is achieved by the gearbox.

denverpilot said:
Technically the engine on a DHC-8 (the the Q400 I believe) is a free turbine, so there's gears, but they aren't directly connected to the main turbine inside the engine. :)

So yeah, gears... but not what most people think when they think of gears connected to an engine via a transmission.
Click to expand...
 
Henning said:
No, but you can overheat the turbine under similar pressure/back pressure type situation. Where as a a piston engine runs pulses of pressure, a turbine runs continuous pressure, but regardless, too much pressure leads to too much heat.
Click to expand...

Got it. Thanks.
 
wabower said:
But they serve the same purpose by reducing turbine speed (N1) to prop speed (N2). Prop speed is further refined by governor system, but the primary change from ~30k/min to ~2k/min is achieved by the gearbox.
Click to expand...

Yup. Just lots of folks reading the original "gears" comment would think there's a big ol' gearbox hooked up to the main turbine. There's still a gearbox, it's just a miracle of modern engineering that air is driving it. :)
 
By the way, the non-direct-geared turbines are loads quieter (to me, anyway -- not measured with a dB sound meter) than the fully geared ones that ARE connected to the main turbine. That's a significant portion of the answer as well.
 
denverpilot said:
Yup. Just lots of folks reading the original "gears" comment would think there's a big ol' gearbox hooked up to the main turbine. There's still a gearbox, it's just a miracle of modern engineering that air is driving it. :)
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Actually a turbine wheel is driving the gear box whether it is a free turbine or direct turbine. the only difference is that the same shaft and turbine that drives the compressor is not the same turbine and shaft that drives the prop gear set. Either set of turbines though is driven by air, but then, so is the piston in a recip.
 
On the GA ramp I think you're hearing the shriek of the inlet rather than the engine itself.
denverpilot said:
By the way, the non-direct-geared turbines are loads quieter (to me, anyway -- not measured with a dB sound meter) than the fully geared ones that ARE connected to the main turbine. That's a significant portion of the answer as well.
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The Pilatus has a PT-6. Here it is:

pt6aengine2.jpg


The compressor turbine is a single-stage that drives the compressor. There's a gap you can see between that stage and the power turbines (two stages); the shaft is discontinuous so that the gas generator section (compressor and compressor turbine) are a lot easier to start. The starter doesn't have to turn the whole affair. Fixed turbines take more power to start.

The PT-6 is relatively quiet compared to the Garret (Honeywell) fixed-turbine competition, the TPE-331. That one makes a terrible racket. Think Metroliner or Twin Commander 690 or Jetstream.

Dan
 
Dan Thomas said:
The PT-6 is relatively quiet compared to the Garret (Honeywell) fixed-turbine competition, the TPE-331. That one makes a terrible racket. Think Metroliner or Twin Commander 690 or Jetstream.
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Yup, the Garret was the one I was thinkin' of... noisy bastards.
 
It's easy to tell which engine is installed by looking at blade angle of the props at rest. Garrett's will be in flat pitch, Pratts will be feathered.

denverpilot said:
Yup, the Garret was the one I was thinkin' of... noisy bastards.
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Jaybird180 said:
Are you kidding me! I always miss the cool stuff going on!:mad2: First Mariah Carey and now this:sad:
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Hang around more! I saw the landing but had no camera at the time! Such a beautiful and slow one too! The most exciting that I've seen was this, two 369E's, PGPD's 520N, and mini mustang.
 
Dave Theisen said:
Prop speed is a constant 1700 RPM
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That's true for the 1900, and probably for the KAs, but the Dash actually turns MUCH slower (at least the 400 series). 1020 rpm for TOGA, 900 rpm for climb, 850 rpm for cruise. It is a very quiet airplane on the outside (as far as TPs go).
 
Teller1900 said:
That's true for the 1900, and probably for the KAs, but the Dash actually turns MUCH slower (at least the 400 series). 1020 rpm for TOGA, 900 rpm for climb, 850 rpm for cruise. It is a very quiet airplane on the outside (as far as TPs go).
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The torque that must be available to shove that much airplane through the sky on slow paddlewheels like that, is impressive.

Side-note: Here's hoping you've had some good news in your day life away from PoA... Sigh. Too many nice folk hurt by that particular aviation-world silliness.
 
Teller1900 said:
That's true for the 1900, and probably for the KAs, but the Dash actually turns MUCH slower (at least the 400 series). 1020 rpm for TOGA, 900 rpm for climb, 850 rpm for cruise. It is a very quiet airplane on the outside (as far as TPs go).
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My comment was specific to the Pilatus. There is no prop control lever. Once you bring the power up high enough to be on the governer, the prop is turning at 1700 rpm. The only thing the pilot can control is torque.
 
denverpilot said:
The torque that must be available to shove that much airplane through the sky on slow paddlewheels like that, is impressive.
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I work on Lockheed Electras converted to fire bombers. They have four 3750 hp Allisons 501-D13s, and the propeller turns at 1050 RPM. The prop shaft imparts almost 19,000 foot-pounds of torque to that big prop.

Dan
 
denverpilot said:
The torque that must be available to shove that much airplane through the sky on slow paddlewheels like that, is impressive.

Side-note: Here's hoping you've had some good news in your day life away from PoA... Sigh. Too many nice folk hurt by that particular aviation-world silliness.
Click to expand...

Actually, very little torque is provided, but a lot of rpm are.;) Gotta love a little mechanical advantage.:yesnod:
 
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