Maybe it is a stupid question (probably is), but I really don't get it why prop engines (single, twin or turbo) are so bloody loud and they have really high fuel consumption. The best example is C172 with 160hp engine (like a car). So, 160hp very noisy engine, and it will drain 7 gallons/hour, or will do 15 miles per gallon, which is 4.5 liters on 25km or 18 liters/100km. Why? Is the loudness connected to exhaust system, or...? Why is consumption way higher than the car with the same hp engine?

 

 Just surfice it to say they are built for performance and reliability rather than comfort and economy.

I'd imagine it'd be because cars have a gear box, so when you gear up into higher gears you're burning less fuel while travelling faster. Props don't have gear boxes, so it'd be like driving around with your car in first all the time... Comparing those two fuel consumptions and you're not too far off...

 

Regards,

Ró.

Hmmmm good start
Now I'm interested about RPM numbers of the engine. Cessna's will "rotate" around 2200-2300rpm (propeler), so how high is  RPM of the engine?

In the vast majority of piston propeller aircraft the propeller is bolted directly to the crankshaft. This means that the propeller is always turning the same RPM as the engine.  The reason why the RPM is so low compared to something like a car engine is because if you spin a propeller much faster the tips will start to go supersonic which greatly increases the drag on the prop and reduces efficiency. 

 

As far as noise goes most aircraft engines have relativity small mufflers or none at all if they're turbocharged.  Also, a lot of the noise you hear, especially at high RPM, is propeller noise as the tips get closer to the speed of sound.  The thing you have to realize with aircraft piston engines is that they haven't really changed all that much since 50s/60s. 

 

 

Props don't have gear boxes

 

Actually, some do. They are called variable pitch props. Also, imagine dismantling your car's exhaust system and having nothing but a header coming out of your car's engine. Your quiet car would become a snarling, police attention getting, monster. Or, imagine your mother-in-law without a gag. :LMAO: 

The thing you have to realize with aircraft piston engines is that they haven't really changed all that much since 50s/60s. 

 

 

Great point.  Now is a new Cirrus more fuel efficient than a 1983 Cessna 182,  are newer prop planes quieter (nevermind turbo props)?

Actually, some do. They are called variable pitch props.

 

True, but I don't think that's what he meant.  Most are still directly coupled to the crankshaft and prop RPM = engine RPM.

 

But there were several notable piston engines designed with gearboxes which did offer a (fixed) geardown - Sorry, don't remember the engine designations off the top of my head, but the Beech Duke (the original, not the turboprop conversion) was an example aircraft.

 

As others have noted, prop noise is generally as big a component as engine noise as the prop tips approach supersonic speeds.

 

Scott

Also C172 when on idle is way louder than a car. That is not becuase of prop speed.

 

 

But there were several notable piston engines designed with gearboxes which did offer a (fixed) geardown

 

That's true too and like you, I can't remember the aircraft they were on. However, back to my point about variable pitch... Even thought they remain directly coupled to the crankshaft or through a reduction gear to the engine, VP's do act as a rudimentary transmission in that they do change performance (or maybe better, efficiency) for a given RPM.

 

 

True, but I don't think that's what he meant.  Most are still directly coupled to the crankshaft and prop RPM = engine RPM.

 

Yes, that's correct. But, even though the engine and prop are still turning at the same RPM, with a constant speed prop the change in pitch allows the engine to operate more efficiently for a given RPM. So because of that a variable pitch prop is like a transmission in a car as Tom alluded to.

 

 

 

But there were several notable piston engines designed with gearboxes which did offer a (fixed) geardown

 

The Continental GTSIO-520 in the Cessna 421 is the most common one I can think of. The prop is geared lower than the engine so the engine can spin faster, which more engine RPM=more horsepower.

 

 

VP's do act as a rudimentary transmission in that they do change performance (or maybe better, efficiency) for a given RPM.

 

Agreed. :-)

 

Scott

 

 

Also C172 when on idle is way louder than a car. That is not becuase of prop speed.

 

Even at idle, a major component of piston airplane noise is the prop.  Really.  Props are by nature noisy things.

 

Note: I'm not saying it's all prop - as noted, engine noise reduction is minimal to non-existent on most piston aircraft.

 

Scott

Also C172 when on idle is way louder than a car. That is not becuase of prop speed.

No Muffler as Tom mentioned...  Think of it more like a car with straight pipes...  Or a Harley with No Muffler!

A Cessna 172S with a 180 HP engine is 360 cu. in. which is approaching 6 liters if I am not mistaken. The cylinders are very large and have a long displacement.

The exhaust pipe is approximately 3 in. wide and follows a very tiny muffler which is located only inches below the engine.

I would say that the prop and engine both produce a lot of noise, with the prop being louder than the engine at higher power settings.

Thanks all for good explanations, now I can sleep well.