Time for a PJ?

What's the difference between a very light jet (VLJ) and a personal jet? No specific definition of a personal jet exists, but I have my own. A personal jet (PJ) is a single-engine airplane with no more than six total seats, a maximum operating altitude of 30,000 feet or less, and a top speed of 300 knots, or only a little more.

In contrast, the VLJs that are making significant progress in development all have twin engines, and all that I can think of are aiming for a maximum operating altitude of 41,000 feet. Cruise speeds are expected to be 340 knots or more, and Eclipse is even promising autothrottles, a feature now found only on the most expensive business jets. Certainly owner pilots are attracted to the VLJs, and many will be owner flown, but the VLJ mission is likely to be the same as other business jets only flown in a smaller airplane.

With only one engine the PJ is automatically in a new category of simplicity of operation, and lower costs. A single-engine airplane also forces people to accept the remote possibility that if the worst happens at the worst time a forced landing off airport is inevitable. Thousands of pilots make that decision every day so it's not that big a deal for an owner-flown airplane, but non-pilot passengers may make a different choice. With its lower operating altitudes the PJ will be less challenging to design, build and, most importantly, will be less demanding to fly. With only enough pressurization needed to keep the cabin at 8,000 feet while cruising at, say, 25,000, the PJ can be lighter because less structural strength is required to contain the cabin pressure.

Like any other single-engine airplane, the PJ must have a maximum stall speed of 61 knots in landing configuration at maximum takeoff weight to meet certification rules. That means approach and landing speed will be no greater than 79 knots, a speed that any pilot experienced in high-performance singles will find comfortable, and landing runway length will be minimal.

But there will be a number of challenges in designing a PJ and delivering the runway performance and IFR range that owner pilots want. The difficult issues are inherent in a jet versus a propeller, whether the prop is turned by a piston or turbine engine. The most efficient jet is not as thrifty as a piston or turboprop, and jets just can't match the takeoff performance of a propeller airplane of similar power.

Though no airplane that meets my definition of a PJ has entered service yet, there is one in the past that came close. That first and only sort of PJ was actually built nearly 50 years ago in France, and it was not a big success. The Morane-Saulnier MS 760 Paris Jet doesn't exactly fit my definition of a PJ because it had two jet engines, but they were buried so closely together in the fuselage that the airplane had virtually centerline thrust. It was a small airplane with six or fewer seats. The Paris Jet was conceived as a military trainer and liaison airplane, but was later optimized for what was then called "executive" transportation, in other words, a personal jet. The early models had four seats, but a six-seat version was also built.

The Paris Jet was powered by two Turbomeca Marboré VI turbojets, the same engines used in the Magister V-tailed French military jet trainer. The engines were rated at 1,058 pounds of thrust for takeoff, but the Paris Jet weighed 8,820 pounds maximum for takeoff. That's a thrust-to-weight ratio of 4.2 pounds of airplane for every pound of thrust. Compare that to a Beech Premier that has a thrust-to-weight ratio of 2.7 pounds of airplane and you can begin to imagine how the Paris Jet performed on the runway. With only one engine running at the start of the takeoff roll a Premier, or most other existing business jets, would be only a little more lethargic in acceleration than the Paris Jet with both Marborés blazing away. And the high velocity exhaust of a turbojet such as the Marboré is not nearly as effective at producing acceleration from low speed as the greater mass of lower velocity exhaust from a turbofan jet engine, so the Paris Jet was even more sluggish than the thrust-to-weight numbers indicate.

Once the Paris Jet was up and cruising, it could threaten 310 to 330 knots true airspeed, but only in the lower altitudes where fuel flows were out of sight. A more realistic cruise speed was around 270 or 280 knots in the mid to high twenties. Advertised range was around 1,000 nm, but that was landing with barely enough fuel to taxi in, and with no wind. Realistic IFR range was not much more than half that for the Paris.

Clearly, the Paris Jet had many compromises in performance and range, but it was a jet, and many thought that would be enough to make it a success. But it wasn't. Beech even took on U.S. marketing rights and as far as I know, didn't sell any. Several Paris Jets are still flying in the U.S., but they are an oddity even though they have full standard certification.

So why do I think a newly designed PJ can succeed when the first and only PJ (Paris Jet) was a failure with owner pilots? Advances in jet engines will make the difference.

The engines in the Paris Jet were first generation centrifugal flow jets that had horrible fuel efficiency. The fat little Marboré VI engines burned more than one pound of fuel to make every pound of thrust in cruise while a modern turbofan uses about half that amount of fuel. Thanks to efficiency of the turbofan engine a newly designed single-engine PJ will need to carry less than half the fuel the Paris Jet needed. That weight savings in fuel improves runway performance, climb, and range, not to mention saving big bucks at the fuel pump.

But the turbofan engine and its issues will still be a challenge for new PJs. Diamond has announced some basic specifications for its D-Jet, including that it will use the Williams FJ-33 but hasn't determined a takeoff thrust rating. According to Williams the FJ-33 is capable of producing around 1,200 pounds of thrust for takeoff, so with its expected maximum takeoff weight of 5,071 pounds the D-Jet should have a thrust-to-weight ration of around 4.2 pounds of airplane to each pound of thrust, the same as the Paris Jet has with both engines running. That thrust-to-weight ratio does not herald sprightly takeoff performance for the D-Jet, but the Williams engine is a turbofan that should be better at producing takeoff acceleration rates.

Another challenge for the D-Jet and other PJs to come is getting air into the engine. Naturally, the single jet engine must be mounted on the centerline so that means it has to be in the fuselage. It may be possible to mount the single jet engine on the vertical fin above the fuselage a la DC-10, but that would create difficult center of gravity issues with the engine so far aft, along with pitch change whenever power was adjusted, so the fuselage is the only realistic place for a single jet engine. With a passenger cabin ahead of the engine in the fuselage, it's impossible for me to imagine how a central inlet duct like many single-engine jet fighters have, could be used. The people are sitting where a single central inlet duct needs to be.

Instead of a central engine air inlet PJs will need to have twin ducts on either side of the fuselage, or an S-shaped duct that takes in air above the cabin. Diamond is showing wing root air inlets for the D-Jet in its preliminary design renderings, which is the most logical and probably the least complicated to implement. However, the ducts must make fairly sharp bends from the inlets in the wing roots toward the center of the fuselage and then back to the engine itself, and those bends will rob engine efficiency. Forcing the air through the duct adds drag so less ram air pressure reaches the engine, and turbulence can develop in the ducts when the airplane is flown at low speeds or high angles of attack. Too much turbulence in the duct can cause the engine compressor to stall and the engine to quit running. Inlet duct design has been problematic on most jet engines mounted in the fuselage, but modern computer design programs-plus the 50 year's worth of experience in designing single-engine military jets-should help reduce surprises and minimize engine efficiency losses.

Electronic engine controls in modern engines such as the FJ-33 tame the quirks of a jet. The pilot can do whatever he wants with the thrust lever without fear of exceeding engine limits, or of moving the lever so quickly that it disrupts engine operation as can happen in mechanically controlled jet engines. In fact, having only one lever to move and no limits to observe when moving the lever will be one of the nicest pilot workload relievers in the D-Jet and other PJs to come.

Diamond has not finalized any specifications on the D-Jet, including the price, and Cirrus has been talking about a PJ in only the most general terms. Diamond does expect the price of the D-Jet to be under $1 million, and maximum cruise speed to be around 315 knots, but that speed will need to come back to 240 knots for long-range cruise to stretch the 1,740 pounds of fuel the airplane is expected to carry.

As you can see from the very preliminary Diamond performance numbers for the D-Jet, a PJ is not going to be usefully faster, or have as much range, as some turboprops. Diamond hasn't even published an estimate of runway required for takeoff, but you can bet the turboprops, single or multi, will beat it in that category, too. But it's a jet, and I'm betting for many pilots that will be enough to fuel desire, particularly if the D-Jet can come to market for $1 million or less. The VLJs that are in the works are really miniature business jets, but the D-Jet and other PJs that I believe will be along soon, are, well, personal jets, and that is a category that hasn't been addressed in almost 50 years.