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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.
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News of February 06, 2010"I am at TBO, worried about Avgas future. Why can't I find a diesel for my airplane?"Subscribers send me this question more and more often, about all kinds of airplanes from Cessna 150 to Piper Meridian and Cessna 340, showing that aerodiesel is now credible, and this is not surprising. But a purpose of this newsletter is to explain that things are not at all that simple. You cannot just expose your engine mount, remove your propeller, disconnect hoses, tubes and wires, remove your old engine, install the new diesel on the engine mount, reconnect and reassemble, put propeller back, and here it is. Your model of airplane, with a diesel, will be a totally different plane from the firewall upfront and for major instruments and controls on the dashboard. (It will also perform different but this is not the issue here.) A diesel being high compression generates a lot of heat that needs to be evacuated; and at same time it is a smaller engine, meaning its capacity to evacuate heat directly from the engine metal core is less than on a gasoline engine of same power. So you need much more heat exchanging capacity and you need to modify the cowl and air intakes to make sure that air does get in and out to carry all that heat out of the exchangers. And some diesels are liquid-cooled, replacing an air cooled engine. So far, this redesign has been effected and STCd by FAA on the most popular models in the Cessna 172 and 182 families. If you fly experimental, you can consider the Thorpe 211 (still waiting its certificate as an LSA) and one or two models of RVs. The propeller is completely different for two reasons: The rpm is lower; and the vibration pattern generated by the engine is different. You can get rid of everything related to spark-plug ignition (magnetos-related, wiring…) since diesel is compression ignition. There may be some weight & balance adjustment to implement because both the weight in your nose and your tanks (Jetfuel is more dense than Avgas) have slightly changed. Some fittings of your old plane may need to be checked and refitted because of the new vibration pattern which, at certain rpms, may gradually induce leaks for rain through the windshield or other. While checking your fuselage and cockpit for such, your mechanics may discover corrosion damage and suggest a complete inspection, and some refitting, which will conclude with a new paint job… Your engine controls are reduced to one only: the throttle. Propeller pitch/constant speed is monitored automatically. Mixture control disappears. All instruments monitoring engine (EGT, CHT, oil pressure, rpms, etc.) are replaced or disappear. And since so much will change, for the cost you might as well replace avionics: A diesel plane, even small, is capable of full IFR flying because of its extended range, so why not take advantage of it? Fuel tanks remain, but all fuel flows need to be checked and sometimes altered to handle Jetfuel; also it may happen that a conversion will specify smaller fuel tanks because you won’t need the same capacity and will save weight that way, which you may need elsewhere: Engine cooling for instance. Example: A 182 SMA with regular 92 gallons tanks can fly almost 1,500NM at best economy. Are you sure you may need to fly such legs? And then keep in mind that the FAA is somewhat worried at envisioning that some 100,000 singles, averaging ages of more than 35 years, be modified in such ways and be on their way for a new career. We notice therefore that, while more and more convinced that diesel is indeed the future, its policy is very conservative about granting STCs to dieselized airplanes. In many cases it does make sense to favor new airplanes, including new airplane designs, to make it to the market being dieselized at OEM level. Think about it. This being said, will there be other models to be STCd for diesels than the 172 and 182? 5 years ago I was forecasting a large wave of conversions, including conversions of twins of 300HP and plus per engine, for professional and commercial uses, where diesel will justify the most. Now I see that the scope of this forecast should be limited to singles and twins which are still in production and on the catalogue of a manufacturer now. That’s not very many: The production of new piston-engined planes has again gone dramatically down, and it looks like reviving production volumes and catalogues will accompany diesel introduction. At same time, we wrote about the probable scenario for shifting from leaded to no-lead Avgas and to diesel, and why it will be the occasion of across-the-board dieselization. But all that, as everything in aviation ever since Orville Wright was complaining to Wilbur that their engine wasn’t ready yet, will take time. Andre Teissier-duCros posted at 11:34 AM |
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