Stop Disrespecting the Turboprop

WIB air January 23, 2017 0

A Colombian Air Force EMB-314 Super Tucano. Michael Pietrucha photo Planes with these engines are perfectly fine for modern air forces by MICHAEL PIETRUCHA The...
A Colombian Air Force EMB-314 Super Tucano. Michael Pietrucha photo

Planes with these engines are perfectly fine for modern air forces

by MICHAEL PIETRUCHA

The minds behind the Air Force’s plan for a new light attack aircraft, the OA-X, always envisioned a turboprop engine powering the plane.

If that statement didn’t suddenly cause your blood pressure to spike, you probably aren’t the intended audience for this piece. Surprisingly, this simple and obvious reference to a powerplant is the subject of much angst inside the Air Force and out.

The choice was “emblematic of a service that has lost touch,” according to one particularly egregious article. Somehow, the turboprop has found itself prematurely assigned to the technological dustbin, widely viewed as a “less advanced” form of propulsion and one that is unsuitable for a modern air force.

The emotional response to the turboprop is just that — an unthinking reaction based on a lack of critical thought. If anybody has lost touch, it is those who evaluate their needs within a worldview that assumes that newer is always better and that proven designs have no value.

The turboprop is exactly the powerplant those responsible for the OA-X concept — including the author — wanted for the aircraft outlined in the plan, for the environment we expected it to fight in.

The requirement

When we at the Air Force’s Air Combat Command first conceived of the OA-X in the mid-2000s, we based it conceptually on two Vietnam-Era aircraft. These were the A-1 Skyraider and the OV-10A Bronco — both propeller-engine aircraft.

We started with these aircraft because of their attributes. The A-1 had good loiter time, a heavy weapons load and the ability to take punishment. The engineers who crafted the OV-10 designed it with short, austere runways in mind and it also had good endurance.

Afghan Air Force A-29s. U.S. Air Force photo

What we wanted for our new design was a modern equivalent that wrapped them both together into an aircraft that matched the precision engagement capabilities of modern fighter and attack with the long loiter time and rough field capabilities of the Vietnam War-era aircraft. And we wanted it to be fast, cheap and suitable for the Air Force and foreign partners.

Those desired attributes led us to a powerplant discussion, unwittingly following the same developmental path that led to the A-10 Warthog — a process that had started some 40 years earlier.

The A-1 had a massive Wright R-3350 Duplex-Cyclone radial combustion engine that generated 2,700 horsepower. The OV-10 featured a pair of Garret T76 turboprops, each putting out 715 horsepower.

During their U.S. military service, both aircraft operated extensively in “low and slow” environments and they were damned good at it. Historical references from Vietnam were replete with comments about the utility of propeller-engine aircraft.

When we were building the OA-X concept, the Colombian Air Force had just started to prove that the Embraer EMB-314 Super Tucano was a superlative combat machine in an irregular environment. A single Pratt & Whitney Canada PT6A turboprop — the same motor in the U.S. Navy’s and Air Force’s Beechcraft T-6 Texan II trainers — powered the plane.

Developing up to 1,600 horsepower, the modern, computer-controlled turboprop gave the Super Tucano a better power-to-weight ratio at combat load than either the A-1 or OV-10A. Indeed, crunching the numbers on the modern light attack birds — including Beechcraft’s armed T-6 variant, the AT-6C — revealed that the wing loading and power-to-weight ratio were uncannily close to a P-47D Thunderbolt II of World War II fame.

There was clearly potential there.

Modern turbine engine types. NASA art

The basics

In my dimly remembered time as a cadet, I vaguely recall classes on aerodynamic propulsion — not necessarily the most dynamic educational subject ever, but not useless either. It may have been in an AS100 survey class during my freshman year, or it may have been from a test I had to pass in Civil Air Patrol.

Nevertheless, it covered the basics of air vehicle propulsion — reciprocating engines, turboprops, turbojets, turbofans and rockets. The basics were enough to give a flavor of the propulsion systems — which moved aircraft around and which occupied different propulsion niches.

Which one was better? Well, none of them actually.

It all depended on the aerodynamic environment that you were operating in at the time. Modern propulsion systems are mostly variations on a turbine engine.

Turbojets? Great for fighter aircraft where thrust trumps all other considerations, like in the mighty F-4 Phantom II fighter jet — particularly down low.

Turbofans? They had better efficiency than the turbojets, but tended towards greater diameter. The larger the diameter of the fan, the more efficient.

Rockets were great for a kick in the pants during takeoff and necessary if you wanted to fly where there was no air. Ramjets were for the speed demons who flew the SR-71 spy plane at speeds above Mach 3.

And turboprops? A propeller turned by a small jet turbine and really efficient if you didn’t need to scream through the “bozosphere” at the speed of heat.

A GE90 turbofan. General Electric photo

All jet engines need air, so they can heat it up and expel it, creating thrust. A turbojet gulps air through the intake and all of the air goes right into the hot turbine section.

Turbofans put a fan on the front of a turbojet, which adds thrust via air that is accelerated by the fan, but which does not flow through the combustion chamber. Instead, the system kicks it out the back as thrust.

Turbofans have a bypass ratio that refers to the amount of air that bypasses the turbine’s “hot section” compared to the air that goes through it. The bypass ratio of the powerful low-bypass Pratt & Whitney F100 on an F-15E Strike Eagle fighter bomber is 0.63 to 1 — or for every cubic foot that flows through the turbine, a little more than half a cubic foot bypasses it.

The giant GE90 turbofan on the Boeing 777 airliner is among the most fuel-efficient turbine engines ever and is the world’s largest turbofan. It gets this fuel efficiency via a very high bypass ratio and large size — an option not suitable for a small aircraft.

The GE90 has a bypass ratio of 9 to 1, meaning that for every cubic foot of air that is sucked into the turbine section, nine times more pass through the bypass section behind the massive fan. Putting this in context, the reason for the turboprop’s efficiency becomes clearer.

A turboprop is essentially a high-bypass fan — with a ratio as high as 100 to 1 — that has no duct to channel the airflow. A turboprop typically gets only about 10 percent of its thrust from the jet turbine, with the remainder coming from the propeller. And a high bypass ratio means low fuel consumption per pound of thrust.

Northrop’s 1968 A-X design concept. U.S. Air Force art

The original specifications for the A-X — the Air Force project that produced the A-10 — involved turboprop propulsion, based on a Lycoming T55 turbine. The authors of the 1968 concept formulation package noted that at slow airspeeds — up to 460 miles per hour — the turboprop had a significant thrust advantage over the turbojet and turbofan and this was greatest with slow speeds.

These attributes would enable short takeoffs and good low-speed maneuvering. Furthermore, the study indicated that the turboprop designs were not “volume-limited” in the same way as a streamlined high-speed aircraft and could thus carry a lot of fuel.

However, internal squabbling over the contract design delayed the A-X program. By 1970, suitable turbofan powerplants appeared on the market.

But serendipity rather than capability ultimately drove the final decision to power the YA-10 with turbofan instead of a turboprop. As chance would have it, the Navy paid for the development of General Electric’s TF-34 — to go along with the S-3 Viking anti-submarine plane — at precisely the time contractors were looking for a suitable turbofan for A-X prototypes.

When plane makers submitted proposals for the project’s second round, four of the six submissions featured the TF-34. That late in the A-X’s development, the aircraft designs had grown so large and complex that turbofans looked like the obvious choice for their simplicity and thrust class — not their efficiency.

U.S. Air Force T-6 Texan II trainers. Air Force photo

The boring stuff, propulsion efficiency

Generically, the turboprop is among one of the most efficient forms of aerodynamic propulsion — at least up to a certain airspeed. In effect, the engine takes advantage of the fact that propellers are highly efficient forms of propulsion, but are functionally Mach-limited.

The forward airspeed of a propeller aircraft is inherently linked to the maximum rotational speed of the prop itself. At slower speeds, turboprops beat out modern turbofans handily on propulsion efficiency.

And it is at slower speeds that a combat aircraft enhances its utility in some “counter-land” attack missions, including close air support for troops on the ground and forward air control, guiding other aircraft around the battlefield.

For airspeeds up to 370 miles per hour, the turboprop has a superior propulsive efficiency over turbofans and turbojets. They beat out even the high-bypass fans by as much as 25 percent and turbojets by a staggering 40 percent.

Rolls Royce art

At 370 miles per hour, the turboprop reaches its maximum propulsive efficiency, which then drops off while jets continue to climb. At around 460 miles per hour, the turboprop drops to the efficiency of a high-bypass fan and at 575 miles per hour it is no more efficient than a turbojet.

For the speed and altitude regime where we expected the OA-X to operate, the turboprop was the most efficient propulsion type available. Its attributes contributed to the staying power of the engine as an extremely popular propulsion type for small aircraft in general, including short-haul airliners.

Turboprop-powered small aircraft have demonstrated low fuel consumption, well under 490 pounds per hour on average. For an OA-X class of aircraft, the engine would allow the planes to eke out significant flight times — 2 hours or more — on a paltry internal fuel load.

The OA-X we proposed consumed only 5 percent as much jet fuel as a Strike Eagle for the same amount of flying time. Put another way, an F-15E pilot could taxi on the ground for 6 to 8 minutes and burn the same amount of gas one of our proposed aircraft would use up in an hour .

In addition, the turboprop had major implications for fuel supply, as well as consumption. The smaller fuel needs meant U.S. or friendly forces could supply forward bases via local resources and airlift, avoiding the logistical fratricide common to fuel supply operations in Iraq and Afghanistan.

As for maintenance, the PT6A excelled in particular. The T-6s engine can normally run for 2,250 hours before repair crews have to inspect the “hot section” — which they can do without even removing the engine from the plane.

A full overhaul isn’t due until 4,500 hours and only then must technicians pull the engine out completely. Even at a high utilization of 900 hours per aircraft per year, the PT6A can stay on the plane for five years between overhauls.

A PT6A engine from a T-6 trainer sits on a test stand. Rick Goodfriend/U.S. Air Force photo

More interesting, survivability

The survivability of a turboprop’s core turbine is similar to a jet engine as far as actual combat damage goes. The turbine sections are essentially identical and damage mechanisms are similar.

However, a PT6A has a very small turbine that is often protected by armor plating. The propeller is exposed to a lesser extent than the fan disc inside a jet engine, because it is not continuous — which is why you can shoot guns through a moving prop and not a fan section.

The infrared signature of a turboprop-powered aircraft is completely different from a jet. A jet aircraft stands out in two portions of the infrared spectrum based on the propulsion — typically the nozzle and exhaust plume — and because of aerodynamic heating caused by friction effects on the leading edges of the wings and other surfaces.

Older heat-seeking missiles like the Soviet AA-2 Atoll or early American AIM-9 Sidewinder variants rely on a shot from tail aspect where the exhaust plume dominates the spectrum between 2 and 5 microns. Pilots can shoot newer missiles — newer in this case meaning under 40 years old — from any aspect because the weapons can see into the far infrared, from 8 to 12 microns, where hot metal emissions start to pop out.

For these weapons, a jet presents a pretty visible infrared target in most directions, because of both the hot engine and friction-heated surfaces warming up as the aircraft passes through the air. Though still a turbine, the front-mounted “tractor” PT6A turboprop vents exhaust in a smaller exhaust shroud on the front of the engine rather than through a hot tailpipe.

As a result, this hot exhaust immediately mixes with ambient air,while the wing partly blocks the view from below. The well-mixed exhaust plume then emerges aft of the wing.

The difference in exhaust plume temperature between a fighter’s low-bypass turbofan and a turboprop is staggering. NASA measurements on the F-15A at “military power” — no afterburner — showed a pretty constant exhaust plume temperature of 950 degrees Fahrenheit, regardless of altitude.

A PT6A turboprop produces lesser volumes of 1000-degree exhaust at much lower exhaust velocities, and mixes those exhaust gases at a ratio of around 60 to 1 with ambient air, resulting in an exhaust plume that is shorter, narrower and cooler than a jet. By the time the exhaust from the Super Tucano’s engine reaches the trailing edge of the wing, it isn’t even hot enough to boil water — and that’s at full power.

This is why the effect of turboprop exhaust on the aircraft skin is limited to soot, not bubbled paint and rippled metal. All other things being equal, the turboprop has the lowest infrared signature of any of the turbine propulsion systems.

The low signature extends into other portions of the infrared spectrum. Aerodynamic heating of the wings and tail is a function of Mach number, with the heat generated as a function as the square of that figure. At the low airspeeds one expects from a turboprop, this results in aerodynamic heating intensities of about one fifth of a fighter going 0.9 Mach — right below the speed of sound.

Even the propeller blade tips on a PT6A, the parts of the airplane that move fastest through the air, never exceed 0.82 Mach at their maximum revolutions per minute. They are cool enough to require equipment to prevent ice from forming in moist conditions.

So if you were to design an aircraft with a low infrared signature, you might well go the F-117 route — a subsonic jet aircraft with the exhaust coming out on the dorsal surface, with built-in airflow diffusion to cool those gases down and no hot metal exposed to observation from the ground.

Or, if you plan on operating in an environment that allows for it, you might just install a turboprop.

Iraqi Air Force T-6 Texan IIs. U.S. Air Force photo

FOD resistance

Turboprop engines vary in their resistance to foreign object damage, or FOD. There are additional factors that impact FOD resistance, such as how engineers have installed an engine within their design and its general location on the airframe.

The PT6A turboprop is the most commonly available turboprop in the 500 to 1,500 horsepower range and maintains the lion’s share of the market in this area. The PT6A-68 variant specifically powers the T-6A/B trainer, the AT-6C light attack plane, the U-28 spy plane and the A-29, the U.S. military’s name for the EMB-314.

It has a very high FOD resistance because the basic design is essentially backwards — with the intake in the rear and the exhaust in the front. This arrangement provides exceptional resistance to debris in the turbine section because the airflow upstream of the intake must make a sharp turn into the turbine — a turn which heavier particles cannot hack.

Pratt & Whitney Canada art

Even with the bypass duct closed, a physical screen protects the compressor from dangerous foreign objects. A turbofan has no similar resistance to damage, even in the stream of bypass air — heavy particles that do not enter the turbine section may nevertheless damage the fan section.

The nacelle inlet of many aircraft models includes an inertial separator provided by the aircraft manufacturer to prevent heavy particles from entering the engine inlet. Most installations incorporate two moveable vanes, one upstream of the engine inlet and the other blocking the bypass duct. For bypass operation, the inlet vane comes down and the bypass duct vane opens up, permitting maximum separating efficiency. In some installations, the vanes are fixed in the bypass mode.

As shown in the diagram above, depicting the inlet vane in the lowered position, air entering the engine inlet must turn hard to get past. Particles heavier than air carry straight through, by their own inertia, into the bypass duct and dump overboard.

It’s a feature, not a bug

For a small combat aircraft fighting primarily in a counter-land role, the turboprop is not only a sensible choice, it is the most practical choice. For this class of aircraft this type of motor makes the most sense unless the dominant requirement for the propulsion unit is to sacrifice most other characteristics for speed.

The reverse-flow turboprop is a comparative fuel-sipper that is easy to maintain, has a very long time between overhauls, has fantastic FOD resistance and produces significantly a lower infrared signature, making it less vulnerable to heat-seeking missiles. In this speed regime and threat environment, the turboprop is the right choice and far exceeds compact turbofans in the parameters that most matter for light attack missions.

As always, the requirements for the aircraft and the environment where it will operate should dictate the choice of systems on board — not a perception of technological modernity. So stop disrespecting the turboprop.

U.S. Air Force Col. Mike “Starbaby” Pietrucha was an instructor electronic warfare officer who flew in the F-4G Wild Weasel and the F-15E Strike Eagle aircraft, amassing 156 combat missions and taking part in 2.5 surface-to-air missile kills over 10 combat deployments. As an irregular warfare operations officer, Col. Pietrucha had two additional combat deployments in the company of U.S. Army infantry, combat engineer and military police units in Iraq and Afghanistan. The views expressed are those of the author and do not necessarily reflect the official policy or position of the Department of the Air Force or the U.S. Government.