Yeah, but that number comes from “effective exhaust velocity”. The wiki states earlier:
“For air-breathing jet engines, particularly turbofans, the actual exhaust velocity and the effective exhaust velocity are different by orders of magnitude. This is because a good deal of additional momentum is obtained by using air as reaction mass. This allows a better match between the airspeed and the exhaust speed, which saves energy/propellant and enormously increases the effective exhaust velocity while reducing the actual exhaust velocity.”
OK. According to that page, effective exhaust velocity is different from actual exhaust velocity:
Specific impulse can include the contribution to impulse provided by external air that has been used for combustion and is exhausted with the spent propellant. Jet engines use outside air, and therefore have a much higher specific impulse than rocket engines. The specific impulse in terms of propellant mass spent has units of distance per time, which is a notional velocity called the effective exhaust velocity . This is higher than the actual exhaust velocity because the mass of the combustion air is not being accounted for. Actual and effective exhaust velocity are the same in rocket engines operating in a vacuum.
The thrust of a (stationary) jet engine is its mass flow rate multiplied by its exit velocity, whereas the kinetic energy of the exhaust stream is its mass flow rate multiplied by the square of its exit velocity. The whole reason for high-bypass turbofans is to lower the exhaust velocity by recruiting a huge mass of air, thus allowing a given level of thrust to be produced with a lower power (fuel) requirement. I can’t find a cite right now, but my recollection is that the biggest turbofan engines have actual exhaust velocities well under 1000 MPH.
Ignorance fought, thanks.
May be old hat on stationary gas turbine engine powerplants, but for road-going big diesels, it’s pretty much the norm now. If you’ve got a diesel-powered vehicle that uses diesel exhaust fluid (DEF), there’s an SCR in the exhaust system.
Apples and Oranges. The real problem with Diesel engines is not NOx but particulate emissions (DPM). There is no effective solution to that problem, as far as I know.
“ DPM has a significant impact on California’s population. It is estimated that about 70% of total known cancer risk related to air toxics in California is attributable to DPM.”
NOx from diesels remains a problem; the EPA is pushing toward tighter regs on this front.
Diesel particulate filters, despite their operational drawbacks, are very effective at controlling tailpipe PM emissions.
As others have already suggested …
For aircraft of that general payload and desired short runway capability they need ~750SHP.
That’s two top-end ICE or one bog standard turboprop. The airframe needed to carry two ICEs would be heavier. AvGas is hard to find in remote areas; jet A is more available and in a pinch it’ll burn Diesel fuel just fine, especially at altitudes below the flight levels where fuel cooling & precipitable water freezing aren’t concerns. And as you suggest reliability is orders of magnitude better.
Fuel efficiency is nice, but probably not the primary concern. In fact the greater power/weight of the turbo and the lighter airframe vs a twin ICE may well mean you can travel farther on however big the tanks can be given a target payload.
Said another way, perhaps the more structurally efficient turbine aircraft can carry enough extra fuel vs the equivalent ICE aircraft that it has more range despite burning more fuel to do so. In sparsely populated areas, or in archipelagos, range has a go/no-go quality all its own.
Well under.
The reason a fighter with afterburner as you might hear at an airshow sounds so hellaciously loud is exactly because pf the Mach 1+ exhaust stream interacting with the surrounding atmosphere. It’s hideously inefficient in every engineering measure of merit except for power per pound of structure and power per cubic volume of structure. Both of which are at a super-premium in a fighter.
The fact modern airliners don’t sound like F-whatevers at takeoff is all about the vastly increased mass flow and vastly reduced flow speed.
On the ‘blades rotating at 1,000 MPH’ that Napier mentioned upthread, my first thought was, “Relative to what?” And my second thought was, “Aren’t turbine blades’ translational velocities deliberately lower than transonic, because it takes a lot more energy (and noise) to move material faster than sound?”
I had thought that was a reason for the unGodly noise a giant, fast propeller aircraft like a Tu-95 makes, because the blade tips on the 18-20 foot wide propellers were supersonic? And then there was the Thunderscreech…
The speed of sound increases as the pressure does. The highest tip speeds of compressor blades in the inner compression stages are faster than the speed of sound in the surrounding atmosphere. In addition, modern compressor blades take advantage of supersonic speeds to increase compression through small shock waves.
A couple of relevant threads elsewhere:
TL,DR: The tips of the main fan are indeed supersonic at max RPM, but the ducting makes the efficiency hit quite a bit lower than for an unducted prop. The remaining efficiency hit near the tip is considered an acceptable design compromise for the increased performance on the inner portion of the fan.
Building on this, the latest innovation is geared fans. Which turn significantly slower than their ungeared predecessors. 3:1 being the current ratio, but the general belief is that ratio will go up = fan turn even more slowly as the industry gains experience with these things.
This helps match the loading of the fan that consumes shaft power versus the turbine wheel(s) that deliver power into the same shaft. Giving the turbine more leverage over the fan also assists with engine acceleration which is a surprisingly difficult part of GT design.
The other big thing gearing does is slow the tip speed for any given fan diameter. So fans could be 3x the diameter before they get into the supersonic regime compared to current practice.
See for more:
Yeah, loud.
I was in Georgia back in the late 90’s, right over by Robins AFB. Met up with an old friend that worked over there. I got a bite to eat across the street, then hung out in the parking lot watching C-5’s practice touch and go’s. That’s an interesting sight.
Then an F-22 came in and did a touch and go, and from about 3/4’s mile away, I was just about knocked over.
Was at an airshow back in the '80s when a B-1 bomber did a slow fly-by. After they passed the crowd, they pitched up and away and lit all four burners. THAT was loud.
To be ridiculously pedantic, a turbine is still an ICE.
I wonder why you dismiss the complexities of piston manufacturing.
You understand pistons are not round at room temperature, but are at operational temperature. Much solid model thermal mapping must be done.
They are not made from any old chuck of aluminum, but rather a forging that has a ton of r and d, or a cast Hyper-eutectic aluminum alloy.
Quite right. As is a diesel, a wankel, and a few other weirdballs. “Otto cycle engine” (“OCE”?) seems a bit much for casual use though.
In official FAA parlance what I’ve been calling ICEs are “reciprocating engines”.
Fair enough. You’re right, manufacturers have advanced the details of piston design and fabrication to get the best performance possible out of them. But I stand by my main point: pistons are relatively easier to make than turbine/compressor blades, and most engines have six or fewer of them (compared to dozens of blades in a GTE).
AIUI, different elevations of the piston are turned to different diameters, with the expectation that the crown will run hotter than the skirt (and should therefore be a smaller diameter at room temp). Are you saying they also machine pistons to be out-of-round at room temperature?
It’s actually more complex than that.
The cross-section of a piston is not circular; it’s elliptical. And the eccentricity varies in a complex way as you work down from the crown towards the base. This link contains a 2-page approachable intro that gives the gist:
I had a relative who ran the maintenance shop for a huge civil government motor pool during and immediately after WWII. They often had to make their own pistons from billet since the factories were producing war materiel only. He talked about the tradeoffs of the degree of elipticality on wear and on ease of production.
What he said. I have seen posts by people using wire edm, electrical discharge machining, to make the Christmas tree receivers for the roots of blades on a blisk, blade disk. Tight tolerances for form and dimensions. Very expensive.
Well, and the turbine engines are so carefully balanced that if you see a passenger jet parked outside, you might see the intake/bypass fan turning in a light breeze.
I’ve been curious as to the differences between a turbofan and a turboprop engine.
There are obvious differences just looking at them but in the end they seem to be doing the same thing…using a propeller/fan blade to push air for thrust. IIRC turbofans get something like 90% of their power from the bypass air. Only 10% of thrust is the air exiting the turbine. So, in both engines, the main goal is using the turbine to move big fan blades to push air.
The bit that goes through the turbine…not a big deal except to turn those blades.
In the end they seem variations on the same theme despite them look so different.