Foreign Turboweenie Porn

[BACONATOR]

My spitball guess would be that in earlier turboprops the reduction gearbox was connected to the compressor turbine. The propellor may need more freedom than the compressor could give it, so maybe holding the turbine constant and changing everything else was the way to go.

Could be talking completely out of my ass. I much prefer the "I Believe" Method of world understanding.

It's been a LOOONG time since engines, but IIRC, technically the PT-6A engine the T-34 uses is actually most accurately described as a turboshaft engine (the same type of engine setup Helos use), which explains the free-power turbine and the gas-coupling, as compared to a traditional turbo-prop. The free-power turbine of the turboshaft engine obviously then powers the output shaft (which is how this type of engine is defined) and that shaft then goes to the gearbox, where it connects to the prop. However, unlike the traditional turbo-prop, the turbine isn't mechanically connected to the gearbox via its own independent shaft.

 

[usmarinemike]

I like the part where they throw each other into a mud pit. I think that would have made Corpus more fun.

I'll bet you do.

 

[HokiePilot]

The best part of the article:

As an interim airplane, the T-34C is going to do a fine job. What remains to be seen is exactly how long that interim is ...

 

[FlyingOnFumes]

The best part of the article:

As an interim airplane, the T-34C is going to do a fine job. What remains to be seen is exactly how long that interim is ...

About as "interim" as the Pratt & "Quitney" TF-30 ended up being as an "interim" engine for the F-14A until the F401 DFE & F101 DFE (Derivative Fighter Engines) were supposed to come online... :icon_roll

 

[Beans]

I had never heard a T34C until I came to FL, and it was really weird to me that it was all propellor noise and no turbine. Now I know why. Good link.

The reason there's no turbine noise is because the exhaust velocity is not high relative to the airspeed. The idea is that a PT6A-type turboprop isn't trying to get any practical thrust out of the exhaust - it's all coming from the prop. For a turbofan/turbojet aircraft, the noise produced by the turbulence between the exhaust jet and the air adjacent to the jet scales by the (exhaust velocity)^8 when the exhaust jet is subsonic... supersonic (like when you see shock diamonds forming on an afterburning low-bypass turbofan) and it scales to the third power.

Turboprop noise mostly comes from the blade tips, as their speed approaches M=1, but that doesn't change over the range of throttle settings with a constant-speed prop.

Measly rep from me for the T-34 stud who works out the blade tip speed at 100% Np. See how close it gets to ~345m/s.

 

[Mumbles]

^ proof that the jet community really is populated by dorks.

 

[porw0004]

I'll play your game.

The prop redlines at just over 263m/s. Even fuel-topping kicks in at 287m/s.

 

[FlyingOnFumes]

^ proof that the jet community really is populated by dorks.

Looks like Beans is a helo bubba....

+1 to Beans, good insight.

 

[Raptor2216]

That was a good exercise in using my degree...I got 263m/s.

 

[nittany03]

519 knots . . . I was too lazy to convert m/s.

 

[C420sailor]

I got 263m/s at 2200rpm.

I think the speed of sound is 343m/s or so on a standard day at sea level?

 

[FlyingOnFumes]

Are you guys using prop tip linear velocity = prop radius x angular velocity (calculated from RPM) in radians?

 

[BACONATOR]

The reason there's no turbine noise is because the exhaust velocity is not high relative to the airspeed. The idea is that a PT6A-type turboprop isn't trying to get any practical thrust out of the exhaust - it's all coming from the prop. For a turbofan/turbojet aircraft, the noise produced by the turbulence between the exhaust jet and the air adjacent to the jet scales by the (exhaust velocity)^8 when the exhaust jet is subsonic... supersonic (like when you see shock diamonds forming on an afterburning low-bypass turbofan) and it scales to the third power.

Turboprop noise mostly comes from the blade tips, as their speed approaches M=1, but that doesn't change over the range of throttle settings with a constant-speed prop.

Measly rep from me for the T-34 stud who works out the blade tip speed at 100% Np. See how close it gets to ~345m/s.

Just to tack onto this post and corroborate my last analysis:

This is true, and is basically the design characteristics of a turboprop and turboshaft engine; All the useful thrust/torque is provided by the prop (or output shaft), and the exhaust velocity/thrust is negligible. Hence why the T-34's and many helo exhausts are routed in odd ways. The exhaust is simply to allow the air to escape and offers negligible force in and of itself.

The turbojet and turbofan engines are the designs which use exhaust gases as a significant portion of their thrust.

 

[C420sailor]

Are you guys using prop tip linear velocity = prop radius x angular velocity (calculated from RPM) in radians?

Angular velocity? Radians? Jesus, you're giving me flashbacks.

Prop circumference (7.5ft x 3.14)

x

Prop RPM (2200rpm)

*

1 min / 60 sec (Conversion factor for fpm to fps)


Then my lazy ass used Google to convert it to m/s.

 

[HokiePilot]

Remember that the tip is also moving forward through the air. I don't feel like doing the math.