Compact turbojet engines rather than high-bypass turbofans like modern airliners. Blended into the wing for less drag compared to underslung wing pods like the contemporary rival 707.
Here's some of the story on bypass for jet engines:
Yes, high bypass: With a jet engine, what you pay is kinetic energy, and what you get and want is momentum. For mass m and velocity v, the kinetic energy, that costs you (1/2)mv^2, and for the momentum you get mv.
So, you want to maximize the resulting momentum mv for the paid kinetic energy (1/2)mv^2. So, you want to select m and v to maximize momentum for a given kinetic energy.
Since for a given velocity v, in kinetic energy are paying for v^2 but in momentem getting only v, clearly want to make mass m large and velocity v small.
So use the kinetic energy of the pressure of the burning fuel to turn a big turbine that moves a large mass m by a small velocity v.
That works so well that GE took one of their early jet engines and put on an aft-fan, that is, put a fan, a turbine, free wheeling, not driven by any shafts, on the back of the engine. So the blades of the fan played two roles: (1) Close to the axis of rotation, the blades got hit with the fast, hot gas from the engine, and (2) the rest of the blades, that extended past the engine, pushed on cold air.
The Dassault FanJet 20 used
these GE fanjet engines, and
those were the planes, modified for carrying cargo, used at the start of FedEx.
BUT! My understanding is that this whole story only works for subsonic flight! So, jet engines developed for military fighter jet planes and supersonic flight didn't have fans.
But when the US military wanted a really big cargo airplane, the C5A, they paid for the development of high bypass turbofan engines and used them. With the engines developed (took big bucks), commercial aviation took advantage.
High-bypass turbofans, or just turbofans in general.
In the early days of jet airliners, you had high-velocity exhaust providing all of the thrust. With turbofans, you have much more thrust coming from low-velocity bypass. This is quieter in itself, but also 'shrouds' the high-velocity exhaust.
As bypass ratios get higher, engines generally get more efficient and quieter. The challenge in doing this is mostly engineering the large fans.
That's because it's mostly wing. It dates from the Dash-80 era of slim, pure jets and low-bypass engines. Today's high-bypass engines are big shrouded fans with a jet engine at the center to spin them. Fuel economy is much better, but the engines look bloated.
Future engine economics would have been interesting, as well as digital design models once there was a market for Concorde B+, C and so on. Current engines for commercial jets are bypass fan designs. If the same efficiency gains had been achieved for the type of engine in a concorde... (who knows? maybe military jets are now significantly more efficient)
What’s the deal with high bypass turbofans vs turboprops?
Turbofans look a lot like the jet engine in this video but, as you point out, only the core is the turbine and most of what’s inside the cowling is a fan for pulling air past the engine, which pulls the plane forwards.
Turboprops do this but, to a lay person, in a different way. Four propeller blades instead of hundreds, and no cowling. Is it just cheaper to build a turboprop, and turbofans are the ultimate in terms of performance?
Hmm, I'm not familiar enough with jet engines to say whether that is expected or not. 15% doesn't sound like a huge improvement, I agree.
But I'll note the F130 is a medium bypass business jet engine producing 'only' around 75kN. 8 of those is about 600kN, which could be achieved with two of those newest generation civilian high bypass turbofans used e.g. in the B787 or A350. I suspect the motivation was to choose a new engine that is as similar to the existing one in order to minimize work needed to install them on the plane. There being AFAIU less than 100 B52's in service, I guess minimizing engineering cost has better payoff than minimizing fuel consumption with a high bypass engine.
It seems easier to make an engine that gives very high power at the expense of efficiency than an extremely efficient engine with reasonable power which is what airliners want.
The primary concern was fuel efficiency, which is proportional to the diameter of turbofan engine. Two smaller engines would be less efficient. The same with turbofan airflow hitting the wing.
As far as I know, and according to Wikipedia, most commercial airliners already use turbofans [1], which are high-bypass turbojets [2]. Unless you were referring to turboprops?
So basically this seems to be a extra-high-bypass turbofan achieved by gearing down the fan. But delayed hugely because giant megacorps move very very slowly.
Modern high bypass turbines provide a lot of power for the weight and extreme efficiency. Essentially, they are a gas turbine which then gets to extract extra energy from their exhaust gasses without adding a lot of weight or mechanical complexity.
There are a lot of trade offs involved, but for large 500+ MPH aircraft high bypass turbofans are simply the most cost effective option.
And all GA jet engines now are turbofans with high bypass ratios. Not something you can do at small scale. Would have guessed the relative speed they operate at also plays a role. And wouldn't this require quick changes in thrust? Not sure that heavy big diameter fans could do that.
They're talking about this prototype doing mach 0.7, so it's a safe bet they intend it to have a turbofan. That's much faster than a MQ-1/MQ-9 and also faster than a (turbofan) MQ-20.
Dismissing this design because it requires a jet engine just seems bizarre. Like okay, it doesn't work with piston props... so what?
Turbofan engines have two exhaust components: high pressure/temp exhaust coming from the turbine, and unburnt low pressure air coming from the bypass, which is forced through by the first turbine but not actually burned.
Military craft have low bypass engines, while large civilian craft have high bypass. These different designs trade off engine size and efficiency; large jets are willing to have large engines if they’re more fuel efficient, while military craft have high needs and very little room to spare.
My suspicion is that these techniques will need significant tweaks to work on the very different large engines. Already we’re seeing external nacelle changes to wide body aircraft for noise reasons, rather than the internal design shown here.
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