Is my old Rover more reliable than small aeroplanes?
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95% (my guestimate from professional experience) of car failures nowadays are electronic failures of some kind; electronics which are not fitted to aero engines primarily because (AIUI) of the certification cost/unit sales ratio and the lack of a requirement to meet specific emmision & economy targets.
That said, I wonder how long car engines would last if you were made to change the oil & filter every 50hrs?
In addition, modern cars are, by law, required to be made of mostly recyclable materials. Materials which unfortunately are not durable.
We can, therefore, quite legitimately blame it on the tree huggers next time our (modern) car breaks down!
An old Mercedes on the other hand, will go on and on and on with proper maintenace.
That said, I wonder how long car engines would last if you were made to change the oil & filter every 50hrs?
In addition, modern cars are, by law, required to be made of mostly recyclable materials. Materials which unfortunately are not durable.
We can, therefore, quite legitimately blame it on the tree huggers next time our (modern) car breaks down!
An old Mercedes on the other hand, will go on and on and on with proper maintenace.
Pompey till I die
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Maybe
There are many more planes around which are 15+ years old than cars. Cars from the 70s are rare on the roads, aircraft are still flying strong.
QED planes are more reliable and last longer. Reasons are debatable but I think that sort of answers it.
QED planes are more reliable and last longer. Reasons are debatable but I think that sort of answers it.
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Well, 130,000 miles at an average speed of, say, 60mph (yes Rovers will do that) is more than 2000 hours.
Secondly, my car, a VW Passat, develops peak power at 5500 rpm. It cruises at about 2500 rpm at 70 mph in 6th gear, and that, at part throttle, i.e, the manifold pressure is probably quite low. So it isn't developing anywhere near the typical 65-75% cruise power of your typical Lycoming or Continental. It's more like around 25% power. Car engines are therefore quite under-stressed for most normal folks. A racing car engine would probably be a better comparison to an aircraft engine...
My wife has a diesel Passat with 110,000 miles on it. At 40 mph, that represents 2750 hours. While Lycomings are nominally rated at 2000 hrs TBO, many stretch them to nearly 3000 hours (not something I would recommend, but it is legal at least in Canada). But that's 2750 low-stress hours in the car, and 2750 high-stress hours in the plane.
I would say that our old air-cooled plane engines are very reliable. You rarely hear of one fail in flight due to a catastrophic mechanical failure. It does happen, but the most usual reasons are fuel starvation/contamination/mismanagement or carb ice.
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One of my old Helicopter engineering mentors had a theory that nobody would put up with a car whose gearbox sounded like a helicopters Main Gearbox, had doors that closed like those on a helicopter, or required the maintenance of a chopper!!
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I seem to recollect that the bodywork of the Rovers that I owned rusted away long before the engines gave up the ghost. The engines were still running but that's no good if the condition of the bodywork means an MoT failure!
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Lot of talk about high stresses on piston aero engines running at 75-100% of rated output for long periods, but let’s look at this from another point of view.
My 4 cyl O360 is rated at 180 bhp. 360 cu in = approx 5904 cc, or 1476 cc per cyl. Now how many modern cars engines of 6 liters are rated at a maximum output of only 180 bhp?
Most are producing better than 400, and go up from there.
All of them would have no difficulty producing 180 bhp at 2700 rpm almost indefinitely!
In short we must accept that even running at max rated output, most N/A aero engines are not mechanically highly stressed, but there are a couple of caveats, driven by the KISS principle:.
Modern car engines make their max rated outputs at 5-7000 rpm, much too fast for a propeller where tip speed is the limiting factor. In order to reduce the prop speed a gearbox or other type of reduction mechanism would be needed, and here we run into the KISS thing. In GA most propellers are bolted directly to the crankshaft thus avoiding all that extra complication and the added weight of a reduction mechanism. The disadvantage is we are now restricted (normally) to a maximum of approx 2700 rpm to make all the power we need for takeoff and flight.
Cooling is the major hurdle. It is much more efficient to liquid cool a piston engine. It provides even temperatures throughout the engine, effectively controls hot spots, and because it is more efficient at carrying away unwanted heat, it allows the engine to make higher horse power for a given size vs. air cooling. It’s disadvantages are high weight, added complexity, leaks can be catastrophic, and radiators are prone to FOD.
Air cooling fits the KISS principle again, nothing to leak, no pumps, no radiator, and very little added weight. The problem is air cooling is not as efficient; it has difficulty dealing with hot spots, is difficult to control, and allows vast temperature differences to exist throughout the engine. Careful monitoring of engine cylinder head and oil temperatures is therefore needed to ensure long engine life. Because engine temperatures are difficult to control most GA aero engines are not rated for very high outputs given thier capacty vis a vis automotive engines.
(One should also mention detonation, which is a real issue for aircraft fitted with C/S props. Detonation cannot be heard by the pilot in an air cooled aircraft engine, therefore careful adherence to the POH or the use of a sophisticated engine monitor is needed to avoid it.)
So there you have the two main differences between aero engines and car engines, both dictated by the KISS principle: The need to make reliable power and torque at a relatively low rpm, coupled with the simplicity and low weight of air cooling.
Horses for courses, one might say.
Regards,
White Bear.
My 4 cyl O360 is rated at 180 bhp. 360 cu in = approx 5904 cc, or 1476 cc per cyl. Now how many modern cars engines of 6 liters are rated at a maximum output of only 180 bhp?
Most are producing better than 400, and go up from there.
All of them would have no difficulty producing 180 bhp at 2700 rpm almost indefinitely!
In short we must accept that even running at max rated output, most N/A aero engines are not mechanically highly stressed, but there are a couple of caveats, driven by the KISS principle:.
Modern car engines make their max rated outputs at 5-7000 rpm, much too fast for a propeller where tip speed is the limiting factor. In order to reduce the prop speed a gearbox or other type of reduction mechanism would be needed, and here we run into the KISS thing. In GA most propellers are bolted directly to the crankshaft thus avoiding all that extra complication and the added weight of a reduction mechanism. The disadvantage is we are now restricted (normally) to a maximum of approx 2700 rpm to make all the power we need for takeoff and flight.
Cooling is the major hurdle. It is much more efficient to liquid cool a piston engine. It provides even temperatures throughout the engine, effectively controls hot spots, and because it is more efficient at carrying away unwanted heat, it allows the engine to make higher horse power for a given size vs. air cooling. It’s disadvantages are high weight, added complexity, leaks can be catastrophic, and radiators are prone to FOD.
Air cooling fits the KISS principle again, nothing to leak, no pumps, no radiator, and very little added weight. The problem is air cooling is not as efficient; it has difficulty dealing with hot spots, is difficult to control, and allows vast temperature differences to exist throughout the engine. Careful monitoring of engine cylinder head and oil temperatures is therefore needed to ensure long engine life. Because engine temperatures are difficult to control most GA aero engines are not rated for very high outputs given thier capacty vis a vis automotive engines.
(One should also mention detonation, which is a real issue for aircraft fitted with C/S props. Detonation cannot be heard by the pilot in an air cooled aircraft engine, therefore careful adherence to the POH or the use of a sophisticated engine monitor is needed to avoid it.)
So there you have the two main differences between aero engines and car engines, both dictated by the KISS principle: The need to make reliable power and torque at a relatively low rpm, coupled with the simplicity and low weight of air cooling.
Horses for courses, one might say.
Regards,
White Bear.
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