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When will a diesel be available for my aircraft? |
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News, facts, and comments on the coming revolution for piston-engine aircraft.News of October 05, 2007Diesel, or turbine? Propeller, or jet propulsion?We said many times that the aero diesel will slowly take the market of piston-engined aircraft between 100 and 450HP. Why? Discussing this opens a much wider issue: the relative future of jet airplanes versus propeller airplanes. Let me surprise you: the true turbojet engine is gone. The very common fanjet design consists in putting back on the turbine axis a geared propeller, with the difference that this propeller is captive in a fan duct and looks like a low pressure compressor. At low speeds, it increases the engine force pulling forward the plane (in a go around for instance) and reduces specific fuel consumption. The true turbojet, which still exists only on military aircraft, is a tremendous gas guzzler at low speed. A good turboprop airplane such as the TBM 850 or the Pilatus flies at 270 to 300 knots. The new generation of mini-jets goes just a bit faster. A piston engined plane can fly as fast as a turboprop. The speed limiting factor is the propeller itself, not the engine principle. The fastest piston engined planes fly faster than 400 knots top speed (440 knots for some of them) since the late forties. A propeller airplane, whether with turboprop or piston engine, can easily be designed for a cruising speed of 300 knots. Above 450 HP, the turbine rules. It is much lighter and much smaller, and it burns jetfuel. But it is not fuel efficient at low speeds, which is less important on a big and fast airplane with lots of fuel capacity. It is much more expensive per HP than a piston engine (because of the manufacturing complexity and cost of all turbine components and materials) but the owner can afford it: Soon he will be able to afford a business jet anyway… And he will be driven to it by tax avoidance reasons: An airplane is amortized tax-wise much faster than its real market depreciation. Since depreciation is deductible, it means that 40% of the price of an airplane is financed by the taxpayer. Which is a negligible factor on a Cessna 172, but a very big factor on a $15 million business jet. Under 100HP, meaning for very small LSAs and Ultra-Lights, the gasoline engine rules: it is lighter than a diesel, and its fuel cost is a non issue even if Avgas goes up to $10 a gallon (and eventually it will.) And it is a small market. In-between will be the segment where the aero diesel engine will rule. It burns jetfuel. It has all the advantages on Avgas engines which we have discussed many times, which are more important for smaller planes. And the public still doesn’t realize what a twin-diesel with two 450HP engines will look like. (Think of the German TT62 prototype, or an Adam push-pull). It will be much less expensive than a jet of similar capacity and useful load. It will have a much longer range. Flight management will be far more flexible. Its maintenance costs will be much lower. And it will cruise at 300 knots. Now, if the US and other administrations decide that it is time to extend the amortization time span of all aircrafts and if the disguised tax subsidy gradually fades out, it will be more and more difficult to justify moving VIPs around in a 30 million plane. VIPs may consider that, if President Eisenhower was satisfied in the fifties with flying at 170 knots in an Aero Commander 520, its modern, 300 knots diesel equivalent might be good enough for them? posted by Deena at 4:18 AM |
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