Turboprop, turbofan, propfan

[Mach Stall]

why faster aircraft use turbofans compared to slower aircraft using turboprops

The short answer is that, in the domain of most subsonic business and commercial aircraft, there is actually no aerodynamic constraint preventing turboprops from competing in speed and efficiency with turbofans.

A longer answer might benefit from some books or courses, but I'll take a stab at an executive summary, as this is an area I've spent some time researching.

The introductory thing you need to understand is that some of the terminology results solely from historical marketing pressures. The term ''propfan'' was coined to get away from the old-fashioned association of a propeller. A propfan is, in every way, a propeller. The "fan" inside a turbofan is also a propeller. Of course, in each instance, for any airplane, the design of the propeller must be optimized for its range of freestream conditions.

Once you understand this, you can appreciate that the major distinctions with a difference wrt turbine propulsion involve: (i) ducted vs unducted propeller, (ii) bypass ratio, (iii) overall propulsive efficiency.

The first key thing to understand about propellers is that a propeller blade is a wing (and actually wings can be viewed as a propeller disc of equivalent span). Just as what keeps an airplane aloft in straight-level flight is the downward thrust vector from the airflow as it is slightly redirected downward by the wing (think Newton's 3rd law), the propeller blade redirects some of the local airflow across the blade into the axial direction, thus adding thrust.

The second thing to understand is that in subsonic aerodynamics, "jet" is actually a four-letter word. You see, a jet, by definition, is a relatively fast-moving focused stream of fluid, and this is also a definition of inefficient propulsion. To maximize subsonic propulsive efficiency, you want to move a large area of air at a speed that is just slightly higher than the freestream velocity (i.e., the exact opposite of a jet). Incidentally, this is why helicopters have very large diameter blades, so they can maximize the propulsive efficiency at very low speed. And recalling the first point that wings and propellers work the same, this is also the reason why sailplanes have long wings, so that the downward airflow resulting from the wings (aka "lift") is spread over a large area and is thus slow-moving -- and thereby efficient.

A third thing to understand, just to complicate matters, is that the jet engine is a truly bizarre engine in that it has constant thrust instead of constant power (for a given fuel flow across varying freestream velocities). Strange realizations ensue from this bizarre bit of thermodynamics. For example, the faster a jet engine goes, the more efficient it becomes (and the slower it goes, the less efficient) in terms of power per fuel flow. So if you're going to design a jet, it should go fast.

But even when flying fast and subsonic, the inefficient propulsion of the jet exhaust stream begs for some improvement, and this is where the turbofan comes in (and why there hasn't been a true commercial or business jet airplane in 40 years).

Back to nomenclature, a ducted propeller coupled with a jet engine is called a turbofan; however, the bypass ratio -- the ratio of mass flows between propeller and jet engine -- can vary considerably. You can loosely think of the bypass ratio as the ratio between high efficiency propeller thrust (slow moving, large area) and low efficiency jet thrust (fast moving, small area).

So why don't all turbofans have very high bypass ratios? It is for a number of reasons. The size, drag, and weight of the fan and shroud can become considerable at high bypass ratios. Other engineering constraints, such as the optimal vs constrained fan rpm (if an ungeared fan) must also be considered, as must other things such as high altitude considerations.

Now we need to dispel a prevalent myth about propellers. It is absolutely not true that propellers must lose much of their efficiency at supersonic speeds. Actually, if properly designed, propellers can operate with very high efficiency, almost as high as conventional low speed propellers, far in the supersonic regime. The main constraint has much more to do with noise and vibration than efficiency. And note that the ducted propeller of a modern commercial aircraft turbofan is often designed to reach around Mach 1.11 local tip velocity for takeoff.

So could a business or commercial aircraft be designed for Mach 0.80 flight with an unducted propeller (aka propfan)? YES. In fact, it has been demonstrated with many different tests. Would it have greater propulsive efficiency than a turbofan? YES (but moreso at Mach 0.76 than Mach 0.90 because of the jet engine speed-efficiency relationship described above). But the current downsides are noise, vibration, and perception. Where the industry appears to be going, at present, is towards even larger turbofans with higher bypass ratios, geared fans, and variable pitch blades.

Unbeknownst to much of the public, the propeller actually made a huge comeback in the 1970's after being displaced by the sexy jet with its concealed thrust mechanism, but the propeller had to sneak back onto the stage, hidden inside the duct of every turbofan. In the future, the propeller may again wholly emerge from its cloak as an "unducted rotor," "propfan," or some clever name yet to be trademarked. And in the very long term, calculations show the unducted propeller should ultimately win out for subsonic transportation, at least up to about Mach 0.80-0.85.

[riff_raff]

Mach Stall - Nice post.

I would just add that the reason commercial jet aircraft use high bypass turbofan engines is because that is what works best at the speeds and altitudes they operate at.

[A Squared]

Originally Posted by barit1

Stepping back a couple decades, the R-2800 18-cylinder aircooled piston engine presented a cooling challenge, and the cooling drag was substantial. But it was a reliable and well-respected machine and successful in the DC-6, Martinliners, and Convair twins.

Convair took a different approach to cooling it, and ducted the engine exhaust into augmenter (eductor, if you will) tubes in the aft nacelle, where the cooling air was drawn out at the wing trailing edge.

I doubt this created any thrust in itself, but since it improved cooling with less external drag, it had the same overall effect as a small jet.

For what it's worth, Douglas manuals for the DC-6 used to claim that they got 600 lb (IIRC) of "jet thrust" from the exhaust. It never seemed to me like it was a particularly useful piece of information, so I don't know why Douglas felt compelled to print it in their manuals, but I do recall running into that bit of trivia back when I was flying the DC-6.

[Flash2001]

I've been told that the "radiator", actually a convector, of the P51 aircraft made a net contribution to the force moving the aircraft forward. That is to say that the thrust exceeded the drag.

After an excellent landing etc...

[A Squared]

Originally Posted by Flash2001

I've been told that the "radiator", actually a convector, of the P51 aircraft made a net contribution to the force moving the aircraft forward. That is to say that the thrust exceeded the drag.

I have seen that claim also, although the reading I have done suggests that realizing a net thrust may be a little bit of an overstatement. Yes, the designers did pay a lot of attention to exploiting the Meridith effect, and yes, when the cooling exhaust was considered separately, there was thrust present. The articles I have seen which go into the subject with a lot of technical detail all seem to suggest that the exhaust thrust never exceeded the intake drag and the form drag of the radiator fairing. I have seen some sources claim that the thrust was 90% of the total drag.

In support of that, I would point out that some racers of modifies P-51's have replaced the stock radiator with a boil-off cooling system, in order to reduce the frontal area and intake drag of the stock cooling system. This isn't absolute proof of anything, but it's certainly an indication that the racers in question believed that the cooling system was not a source of net thrust.

[megan]

That is to say that the thrust exceeded the drag.

Not quite, it was estimated that cooling drag had been reduced by 90%.

[barit1]

why faster aircraft use turbofans compared to slower aircraft using turboprops

It's the other way around. When an aircraft is designed specifically for efficient shorthaul work from shorter runways, the turboprop is preferred because of its greater static thrust. This makes the plane cruise slower than a jet (turbofan), but for short stage lengths, that's not much of a disadvantage.

[DaveReidUK]

I've been told that the "radiator", actually a convector, of the P51 aircraft made a net contribution to the force moving the aircraft forward. That is to say that the thrust exceeded the drag.

If only they'd had space to hang a load more radiators under the Mustang, then they could have dispensed with the need for an engine at all.

[A Squared]

Originally Posted by DaveReidUK

If only they'd had space to hang a load more radiators under the Mustang, then they could have dispensed with the need for an engine at all.

Yep, coulda replaced that complicated, expensive Merlin with a boiler.

[twistedenginestarter]

Be careful you don't simply equate aircraft speed with engine speed. Some turboprops are designed to go a lot faster than others, getting on for turbofan speeds. Some operators may be prepared to pay the extra fuel for this (eg A400M) but the ATR shows people tend to prefer slow-but-economical.

[megan]

the exhaust thrust never exceeded the intake drag and the form drag of the radiator fairing

Figures given by Lee Atwood, of North American, gave the figures of cooling drag 400 pounds, and thrust produced by the radiator 350 pounds, net cooling drag thus 50 pounds. Bare in mind full power prop thrust was in the order of 1,000 pounds, so reduction in cooling drag was not inconsequential.