Post by Allen Peterson on Oct 10, 2009 21:06:59 GMT -5
Tom, I've been pondering some things about the placement of engines on the light jets. It seems that two engines at the tail are now a standard placement for engines. But where is the best place for one engine? The November issue of flying has articles on the Piper Jet and Diamond's D-Jet, and then there is Cirrus' The Jet. I think I would be more comfortable with the placement if the engine on the D-Jet, even if there some intake losses. Having the engine close to the wing just seems better to me. Back in the 70s I knew a guy who flew 727s. He said that when cruising at high altitude you had to watch the RPMs on the center engine since at the angle of attack at cruise the nose could blank the air flow to it. It seems to me that the Cirrus jet would have the same problem, and maybe to some extent, the Piper jet. Since a lot of accidents with private pilots happen on approach when they are low and slow it doesn't seem like a good idea to have all of that mass way back there. What do you think? As a side issue, I've always wondered about the MD-80. The engines have a noticeable up angle. If that is required to maintain flight attitude, then what happens if both engines fail? Would an equivalently loaded MD-80 been able to make it to the Hudson as the A-320 of Flight 1549?
Post by Tom Goodrick on Oct 11, 2009 10:53:53 GMT -5
You bring up a good point. But there are a few issues you have not mentioned. To see my attempt to solve the problem, look at my "Honey Jet" named for the fact it looks a lot like a honey bee. (I'll send you a copy since it was long-ago taken off my web site.) I put the air inlet over the cabin and directed the air as smoothly to the engine as possible.
The twin jet configuration with the engines mounted on the rear of the fuselage is the most efficient design by far and has many advantages. One is that with two engines, the FAA relaxes the limit for stall speed because a single engine remains to fly to an airport when one engine fails. A single engine aircraft must stall at 60 KIAS with flaps unless they get a waiver because they install strong seats[/i] which does not please me when thinking of riding a small jet skiding into the woods after an emergency landing.
Any pilot of a single engine aircraft knows that emergency power-off glides and landings are a real part of their operating experience.
Two engines on the tail have very little yaw effect when one engine is lost on takeoff. Mounting the engines on the wing as in the 737 requires the addition of a big vertical stabilizer and strong rudder. Those add weight and drag. But a big problem with jets on the back of the fuselage or even over the fuselage is ice injestion. Ice forms on the fuselage and on the wings and is shed periodically when heaters are turned on. If this ice flies into an engine you have a problem sooner or later.
Local flow above, behind and under a wing is complex. The speed changes considerably compared to "freestream" speed. The direction also changes. There is strong downwash back by the tail. The jet engine inlets are very sensitive to angle of attack of the incoming flow so they must be pointed up to account for the downwash angle which can be of the order of 8 degrees.
The twin engine configuration also makes use of the extremely efficient packaged engine with an inlet going straight back to the engine going straight back to the exhaust. The engine can operate at peak efficiency only in this configuration. Any jet design needs good efficiency in terms of lbs of thrust per lbs of weight. Also, since T/W = CD/CL for the aircraft, drag enters the relation in a big way. Big, oddly-placed inlet scoops tend to add a lot of drag. The early fighter jets like the Sabre Jet addressed these problems by using powerful engines which limited their weight and payload but got the job done at that time.
An engine operating behind and above a fuselage can be in very complex flow with moderate turbulence as vortices form and shed from the fuselage at even a slight angle of attack.
Modern computaional capability has improved engine inlet design considerably. But attempts to make variable inlet geometries have not been entirely successful. This is the only way to cope with some aircraft flow problems. It is heavy and very expensive to build, control and maintain such a system. That is why you won't see it on a VLJ.
Any of the light, "personal" jets are made possible only by improvements in jet engine efficiency in thrust per weight. Doing anything in the design that reduces that efficiency is bad.
As shown recently in my FS model of the Embraer Phenom 100, efficient climb is the most important part of any jet's operation, more important than cruise in limiting the range. Climb depends very strongly on L/D in a condition where induced drag (due to lift) is raising the drag well above what it is in cruise. My model may be off some but it sure appears to me the Phenom will not make its range spec and i have not seen any pblished evidence it does. The articles recently published give no evidence of meeting such a capability as 1178 nm IFR range. The FLYING article says "To achieve the maximum range capability we would have needed to climb to FL 410 where fuel flow would have been about 200 pph less." I think the range may be calculated for conditions at FL 410 with only an optimistic estimate of fuel consummed in climb.
My view is that people should stop wasting effort trying to make VLJ's and just utilize the many fine small jets we have like the Beech Premier, Cessna CitationJet, and Learjet 31A.