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Single Engine Jets

posted Apr 17, 2011, 2:42 PM by George Finlay   [ updated Oct 8, 2011, 11:04 AM by Nathaniel Cauldwell ]

Tim Slifkin hit a bird at 6000 feet, over the Hudson River. It was night, and he was on his way back to Morristown NJ from Nantucket MA in a Mooney cruising at about 140 knots, The windscreen cracked on the right side but held together. The first-time passenger sitting on the right was startled, but reassured by the pilot’s calm demeanor. Fifteen minutes later, they landed at Morristown. An examination of the airframe revealed signs the bird had been sliced by the prop, glanced off the engine cowling, and then off the windscreen. Feather samples were sent to Rutgers University, where the bird was identified as a Canada Goose (Branta Canadensis). Adults have a typical wing span of about five feet, and weigh about fifteen pounds.

While you currently see single-engine turboprops like the Pilatus PC-12 in passenger charters, you do not see single-engine jets.  One major reason may be FOD, foreign object damage. Air entering the Pratt and Whitney PT6 on the Pilatus must pass through a screen and make a 180 degree turn to enter from the rear. This reverse-flow design protects the delicate turbine blades from birds, ice, and other objects that can be sucked into a conventional jet or turbofan engine, where reverse flow is not an option.

As part of the certification process for aviation jet engines, four pound chickens, or reasonable facsimiles, are fired into the engines from air cannons at 180 knots to simulate bird strikes. Contrary to popular belief, an engine does not need to keep on producing power after such abuse. It simply needs to survive long enough for an orderly shutdown. But that is not good enough if the airframe does not come with a backup engine.

There are two companies working on single-engine personal jets, Cirrus and Diamond, both using the Williams FJ33 engine. There is also the PiperJet, planned around the Williams FJ44-3. But I do not think Piper will survive to build that airplane. (More on that subject in a later post.)

On the Cirrus version the engine is mounted on empennage, with the intake just behind the cabin and the thrust nozzle between two stabilizers in a V-tail. The Diamond uses two intakes mounted to the left and right of the cabin, just above the wing. The engine is integrated in the empennage, with the single thrust nozzle mounted on the bottom.

In 2000, Sam Williams, founder of Williams International, manufacturers of the FJ33, along with well-known aircraft designer Burt Rutan, were granted U.S. patent no. 6089504 on an airframe design that attempts to address single-engine jet FOD. The basic idea is to protect the intake by placing it close to the fuselage and behind a forebody (the cabin) where large objects like birds are less likely to be able to reach. Reached by email, Mike Van Staagen, Vice President for Advanced Development at Cirrus stated categorically that this patent was not used in the design of the Cirrus Jet. However, to the untrained eye, the intake on the Cirrus Jet does appear to be protected to some extent by its position behind the cabin, particularly at the high angle of attack required at takeoff.

The question remains how Cirrus and Diamond plan to address the overall issue of FOD, including bird strikes, and damage or malfunctions caused by ingestion of other foreign matter including hail, rain and ice; also other causes of compressor stalls and flameouts in other turbine engine installations, including crosswinds and the sideslips that pilots are prone to use to deal with them.

Pilots familiar with the Boeing 727 know that engine #2, which has an intake with an S-turn, is more prone to compressor stalls during takeoffs in crosswinds than either #1 or #3, with straight intakes. High crosswind components across the #2 intake disrupt the airflow into the intake, where it is already disturbed by the turn. This tends to lead to variations in the pressure field created by the compressor blades. Those variations can ripple through the engine and in extreme cases cause compressor surge and flameout.

Turbine designs include features designed to help minimize compressor stalls. All utilize bleed valves to relieve excess pressure and some have a system for changing the angle of the stator vanes, which are fixed to the engine body and are not part of the turbine, to help increase air pressure in case of disrupted airflow through the intake.

All turbine operating instructions call for igniters, normally switched to OFF or AUTO for normal operation, to be turned ON for takeoff and for any other situation where engine failure would be particular troublesome or something might possibly interfere with combustion, as when penetrating heavy rain or turbulence. Provided one has enough altitude, restarting a piston-driven propeller airplane is a routine operation. Restarting a turbine in which combustion has ceased is not routine or quick and therefore requires significantly more altitude than what is needed to restart a piston engine. A restart is not an option when an engine fails on takeoff in any airplane. Turbines and turbofans are at higher risk of engine failure on takeoff due their vulnerability to ingested foreign objects and crosswinds. Up until now, except for the injector seats employed in single-engine military jets, the only acceptable solution to the risk has been to install a minimum of two turbine engines, either one of which can supply sufficient thrust for a successful takeoff.

We need to know more about what Cirrus and Diamond are planning to do to deal with this issue.

Links:

Video of bird strike

Sam’s and Burt’s patent