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...

 

[VNugget]

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).

That was my point exactly. The original question was why do airplane engines have horsepower ratings of about half the displacement in cubic inches, while car engines have about twice that (meaning roughly equal HP and displacement).

Power = displacement * BMEP * RPM, right?

Well displacement and manifold pressure are the factors we're keeping constant (for naturallly aspirated engines), and the compressions ratios are about the same, so the only thing left is RPM. Car engines are rated at at least twice the RPM of aero engines, so they develop twice the power per dislpacement. Makes perfect sense, right?

 

[I.P. Freley]

I'm agreeing with most of what you say, but it's been a while since my recips class and I don't remember how BMEP relates to displacement and RPM... Since I don't remember what BMEP is aside from what it stands for! I'm guessing it has something to do with compression ratio, but my brain is farting reaaaal good right now.

The problem I see with the above is that modern auto engines are far more powerful than their more ancient brethren in cars, to the extent that a 3-liter auto engine in something as mundane as a Honda or Mazda sedan makes as much power as an engine of twice the displacement from thirty years ago, with lower-octane fuel and no commensurate bump in compression ratio. Of course they achieve it at a higher RPM... But not something as crazy as TWICE the RPM.

Obviously I've forgotten a lot about the basics of engines, at least as far as the math is concerned.

 

[VNugget]

It's not just modern engines, either. I watched a history channel show about muscle cars from the 50s-70s, and all of the engines they showed as examples had the same numbers for HP and displacement.

 

[Some guy]

The top speed in a car is limited by the gearing in the drive-train. That engine can only turn so fast before it becomes a pile of parts.

 

[Gatorman]

lol...who wants to pull out a 350, bolt on a prop and see if it will fly?

 

[coolyokeluke]

The problem I see with the above is that modern auto engines are far more powerful than their more ancient brethren in cars, to the extent that a 3-liter auto engine in something as mundane as a Honda or Mazda sedan makes as much power as an engine of twice the displacement from thirty years ago, with lower-octane fuel and no commensurate bump in compression ratio. Of course they achieve it at a higher RPM... But not something as crazy as TWICE the RPM.

Obviously I've forgotten a lot about the basics of engines, at least as far as the math is concerned.

Those smaller engines with higher operating RPM's would require reduction gears if affixed to a propeller. They don't make much torque at low RPM like the older, larger displacement car and airplane engines. So not having a gearbox is a weight saving issue as well as something else that could break in an airplane. I'm speculating here, but I think the idea of air/vs liquid cooling comes down to the same issues; weight and complexity. As far as sticking a 350 V8 on an airplane, lots of homebuilders have done it. One of the issues I always hear about with car engines in aircraft is that our large displacement low horsepower aircraft engines are designed to run flat-out, where car engines rarely and only briefly make maximum power. I'm all for advancing the state of aircraft engine technology, but there are reasons (one of which has sadly been liablity issues) that our engines have changed little in the last 50 years: relaiblity and weight.

 

[avbug]

As far as sticking a 350 V8 on an airplane,

ORENDA!

Making a comparison between automotive applications and aircraft applications is nonsensical.

Aircraft powerplants are limited in RPM by propeller constraints.

I've run piston engines in aircraft at much higher RPM's, especially automotive engines such as the Subary Stratus) in aircraft installations...but with very little gain in performance at higher revs.

 

[UnAnswerd]

lol...who wants to pull out a 350, bolt on a prop and see if it will fly?

I have a welding book that shows a finished engine mount used for the installation of a 4.3L V6 engine into an airplane. The 4.3 is essentially a 350 minus 2 cylinders.

I also remember seeing some kitplane with a Chevy 502 installed........

 

[TrafficInSight]

There was an article in a mid 80's issue of AOPA Pilot about a Globe Swift with a Buick 255 V-8 in it. It never caught on.

 

[I.P. Freley]

The top speed in a car is limited by the gearing in the drive-train. That engine can only turn so fast before it becomes a pile of parts.

Top speed on most cars is drag-limited, not gearing-limited, as in topping out at redline in top gear...

Many cars don't achieve top speed in their top gear, though. In Corvettes, at least the more modern ones with a 6spd, top speed can't be achieved in 6th because the gearing is too tall, but it can redline in 5th gear, which would be an example of what you said.