Date: 16 Jun 98 02:15:16 From: "P. Wezeman" <firstname.lastname@example.org> Organization: The University of Iowa Followups: 1
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The May issue of Popular Mechanics has a short feature about a new aircraft concept that is being promoted by a company called 3X Jet Inc. The idea is to take a rear-engined jet like the DC-9 or most business jets, and, instead of mounting identical engines on either side of the tail, to install two dissimilar engines, both on the aircraft centerline. One engine is of the same size as normally used on an aircraft of that weight, and the other engine is about twice that big, sized to give optimal efficiency when providing all the thrust at cruise. The plane would start the smaller engine first, use it to taxi to the run-up area, start the big engine there, takeoff and climb to cruising altitude on both engines, and cruise with the big engine only. Before landing, the smaller engine would be started, the plane would land with both engines running, then shut down the big engine after leaving the active runway and taxi with the small engine. The small engine is shown mounted in the back of the fuselage where it has minimal drag at cruise, with some type of low-drag inlet: a retractable inlet or NACA scoop. The disadvantage of this concept is that the weight and cost are higher than a conventional twin-jet because of the larger engine. The advantages claimed are: 1. Lower fuel consumption during taxi by using one engine at greater thrust level; the big engine would be available for steep grades. 2. With 50 percent greater thrust on takeoff, higher useful load, ability to use shorter runways, greater safety from the same length runways, or some combination. Faster climb to cruise with less fuel burned while climbing; greater excess thrust in case of wind shear. Runway length requirement would have to assume loss of the big engine, but with the greater acceleration, there would be more runway remaining in front of the aircraft at any given speed. 3. Lower fuel consumption in cruise with the big engine optimized for that flight regime. 4. Lower maintenance cost. The overhaul cost per pound of thrust is less for one big engine than for two smaller ones. By not using the big engine for taxi, life is extended 10 percent over normal. Small engine is used such a small fraction of the time that it would normally not need overhaul in the life of the airframe. 5. Option of cruise with both engines running at somewhat higher speed. There are several proposals around for supersonic business jets that would need to fly subsonic over land; if these had one big and one small engine instead of three identical engines (as presently proposed) they could use one engines for subsonic cruise. Overhaul cost would be reduced as well, although in this case they would need to overhaul both at about the same intervals. Overall, the 3X company says that total operating cost would be reduced 10-15 percent in typical use. There was a more detailed article about this in Aviation Week about three years ago. The AWST writer seemed generally supportive of the idea; one possible problem they mentioned was that it can be difficult to start engines at cruising altitude. They had comments from several people in the industry who thought the idea was unorthodox but were unwilling to dismiss it out of hand. The unbuilt original idea of the Hustler business turboprop was somewhat similar in concept; it had a single turboprop with a back-up turbofan in the rear of the fuselage. Comments? It seems to me that if 3X Jet can get a STC to convert one of the popular business jets used by Federal Express and its competitors, one of them might well be willing to try it. They already fly single-engine turboprops on many routes. If it proves out in express service, then people might be willing to try it for business and regional jets. How big of an aircraft would this system be practical for? The AWST article said maybe DC-9 size. The obvious extreme would be half the size of the largest existing twin jet, i. e. the Boeing 777. The largest common aircraft of rear-engine configuration is the B-727; might it be practical to retrofit them? I think the cruise engine would need to be in the RR RB-211 class, with the smaller engine in the 25,000 pound thrust range (any alternatives to the CFM-56?) There are a lot of 727's in package service. Peter Wezeman, anti-social Darwinist "Carpe Cyprinidae"