The car vs the airplane...

[UnAnswerd]

If you told anyone that your car has 140HP, nobody's going to be impressed. I don't have exact figures, but I'd estimate that your typical 4-cylinder passenger car has around 140HP. The top speed in some of these gutless econoboxes is MAYBE 90mph.

What's interesting here, is that if you take a 140HP airplane, it can have a top speed of 135MPH. Why does the airplane go so much faster than the car with the same amount of power???

It has to be because the airplane is more aerodynamic, right????

 

[jknight8907]

Among other things, it's because you don't have the drag of the wheels on the ground, and a 140HP airplane is a lot smaller than a 140HP car.

 

[I.P. Freley]

Some thoughts on the topic...

For starters, that 140hp car is not making 140hp at the wheels because of friction losses in the drivetrain. Figure about 20% lost right there, so 140hp becomes 112hp. How this compares to the loss from crank to thrust in an ungeared airplane powerplant I don't know, though I did at one time.

The 140hp car weighs more than even the MTOW of a typical light airplane, too (at least with modern cars, as even a total lightweight like the Toyota Echo rings in at 2200lbs, a Focus Hatchback weighs over 2500lbs)... Though vehicle weight is more detrimental to acceleration and fuel economy, not top speed, at least not within normal weights (in cars. airplanes are obviously different).

The gearing of that 140hp car is probably not ideal for top speed runs, either. The engineers likely geared it for economy at normal highway speeds.

I'm no engineer, but I have to imagine that the induced drag on a typical light aeroplane is pretty significant, though compared to the drag of the tires for a car, I can't really compare without descending into uninformed speculation. As for parasite drag, I'd like to see how a light single compares to a slippery street car. I'd think it's fair to say that the total frontal area of a Corolla is greater than that of a 172, but that's a guess.

I'll stick with the assumption that the TOTAL drag of a car is greater than that of an airplane, and that's where the big difference is.

A better question, and one easily answered but not as easily understood in a modern context, is why that 140hp airplane needs 320cid to make that power (at approx 8gph at normal cruise), while the car can do it with 90cid (at 2gph at normal cruise).

 

[VNugget]

I'm no engineer, but I have to imagine that the induced drag on a typical light aeroplane is pretty significant,

Only at low speeds. At high speeds, most of the drag is parasite.

A better question, and one easily answered but not as easily understood in a modern context, is why that 140hp airplane needs 320cid to make that power (at approx 8gph at normal cruise), while the car can do it with 90cid (at 2gph at normal cruise).

For starters, the car on the speed run is probably running at at least twice the RPM of the plane. Can't answer about the fuel consumption, though.

 

[TrafficInSight]

For starters, the car on the speed run is probably running at at least twice the RPM of the plane. Can't answer about the fuel consumption, though.

Higher compression means more efficient combustion. Higher RPM means higher output. However, higher RPM and higher compression means higher stress and lower reliability.

 

[I.P. Freley]

For starters, the car on the speed run is probably running at at least twice the RPM of the plane. Can't answer about the fuel consumption, though.

I said "at normal cruise", not a "speed run".

I used the 8gph number as my best recollection of your typical 2-4 seat light airplane at cruise power, the number you'd use on a cross-country flight plan (I've not flown a light airplane or used a POH's fuel burn numbers in over five years, so this is an approximation).

The cruise RPM for that assumed airplane is probably around 2300rpm. An economy car is probably turning slightly more than this at 70mph, maybe 2750-3000, but that is not the horsepower peak for that engine. In any case, that engine is probably doing about 2gph at that rpm... Far less than the airplane engine at cruise rpm, about 1/4 as much in fact. I don't know what road speed/engine RPM produces the most MPG in a "typical" economy car, but I reckon if you ran one at the same 2300rpm typical for a light airplane cruise setting, the car's gph might be even lower, even if the mpg didn't change much.

And yes, I do understand that the auto engine in the airplane would have to run at a much higher RPM to maintain the same HP at the crankshaft and thus the same airspeed, with a commensurate increase in fuel burn for that engine in this application.

Of course modern cars have engines and engine management systems that are far removed from the Lycomings found in most light airplanes, which I remember describing to my students as "the finest technology the 1930's had to offer". I understand the reasons why, to some extent, but even 80's car technology could and should be applied to light airplane engines by now... Even if Porsche tried and failed with their Mooney engine program 20 years ago.

As you point out, the parasite drag is more significant at high speeds (my recollection of 'Aerodynamics for Naval Aviators' has faded somewhat in the last decade!). Even with the wheels, door handles and antennae sticking out, the parasite drag for even a well-used 30 year-old 172 is probably pretty low compared to a street car.

 

[TrafficInSight]

The cruise RPM for that assumed airplane is probably around 2300rpm. An economy car is probably turning slightly more than this at 70mph, maybe 2750-3000, but that is not the horsepower peak for that engine. In any case, that engine is probably doing about 2gph at that rpm... Far less than the airplane engine at cruise rpm, about 1/4 as much in fact. I don't know what road speed/engine RPM produces the most MPG in a "typical" economy car, but I reckon if you ran one at the same 2300rpm typical for a light airplane cruise setting, the car's gph might be even lower, even if the mpg didn't change much.

Compression ratio, as I failed to say adequately above , a higher compression ratio gives you more efficient combustion. The typical engine in your 172 has a CR of 6.75:1 or thereabouts. The typical car today is around 9:1 or 9.5:1. This is fine for cars with computer controlled timing that can advance and retard as necessary to control detonation and liquid cooling to control temperature. But in your air cooled 20/25 degree BTDC no matter what 172 engine a compression ratio that high would be a death sentence.

 

[VNugget]

I said "at normal cruise", not a "speed run".

Whoops, I actually read that in your original post, but I guess I just kinda mentally ignored it.

However, if we wanna compare apples to apples, we need to look at the horsepower for either vehicle when it's at its max.

 

[Gatorman]

Civic -vs- Cessna

My little 1.8 liter Honda Civic will do 120 mph, then I get affraid and let off the gas.

I think 90mph is the top speed of those euro box cars, I think they call them mini's over here.

All kidding aside, I can agree with the geared down ratio of the cars over the straight, bolt-on prop for the horse power rating.

 

[I.P. Freley]

However, if we wanna compare apples to apples, we need to look at the horsepower for either vehicle when it's at its max.

Egads, how dare you make this into a serious topic with actual facts and ramifications!

There's the rub, max HP for just about any street car engine you could name is achieved well beyond 3000-4000rpm, especially in engines that have relatively equal hp to light airplane engines (140-160hp is typical in 2.0L engines, or around 120cid). Therefore, we're now talking transmissions. That torpedoes the whole idea, dunnit?

*sigh*

As an aside, I do wonder why we haven't moved on to liquid cooling and computer controls for airplane engines, aside from the fact that noone would know how to work on them... Liquid cooling, at least, worked fine in the Mustang, even with those wacky Germans shooting at the radiators and cooling lines. I figure even the most hamfisted owner/mechanic would be less dangerous to the continued health of the engine than FLAK.

Or am I just being naiive? I did once have to stop a student from visually checking the fuel level in a Cherokee, at night, with his Zippo...