Tres_Crows... Airplane question!

   / Tres_Crows... Airplane question! #1  

RobertN

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I had a question about air craft. The Reno Air Races were suppossed to be this weekend. I do not know if they ran, since there has been rain, and even a bit of snow in the local area.

So here is my question... If you take a high performance supercharged engine, wouldn't you get better performance in a denser, cooler, moister air? My turbo diesel has an intercooler to cool off that air after it goes through the hot turbo...

Now, part two... If I am right, and you get better performance from the cooler denser air, would that performance gain be offset by by the aerodynamic effects of the cooler denser air on the wings ect?

The same question applies to another event; they had a big Drag Race in Sacramento yesterday. As this cool moist front worked in, wouldn't the engine in a funny car or dragster run better(although maybe marginally as a percentage) than on a warm low humidity day? As long as the track was not too cool as to affect traction, seems they might run a slightly better ET.

Is this on the right track, or totally off base?
 
   / Tres_Crows... Airplane question! #2  
The Reno air races were run this past weekend. Dago Red and Rare Bear battled for first place in the unlimited Gold race.

Yes normally aspirated aircraft engines perform better in cool dense air. The drier the air the better. Since water doesn't burn very well, the less water sucked into the engine the better. On turbocharged engines, the cooler, denser, and drier the air that starts the compression cycle, the better. If you start with cold air, the air temperature after compression will be cooler and therefore more dense. This is the very reason for intercoolers. To try and cool the air and get it more dense before packing it into a cylinder. The intercooler usually sits between the turbo charger (heats the air) and the cyclinders (need as cold air as possbile). While turbo charges make the engine think that it is at sea level by compressing the air before mixing with the fuel, there is a limit as ti how much compressing can be done. Above a certain altitude, turbo charged engines start to lose power as the air gets less and less dense just like normally aspirated engines. There just isn't enough air to compress so the air going into the cylinders becomes less and less dense.

Same thing with the wings. The closer together that the air molecules are (i.e., the colder the temperature), the more lift is produced for a given speed. Since the prop is just an airfoil also, it also is more efficient in cold dry air.

So you get a double bonus by flying in cold dry air. The engine produces more power and the wings and prop produce more lift/thrust. The only disadvantage is that fuel usage increases (vs. flying in warm air) with the cold air as fuel is mixed based in air density. More than a few pilots have neglected to take this into account when flying in the mountains.

For a normally aspirated airplane, a 70 degree temperature at Leadville, Co (elevation 10,000 msl) is equivalent to flying at around 14,000 ft msl (density altitude).

Nobody wants humidity going into the engines.

Probably more than you ever wanted to know.
 
   / Tres_Crows... Airplane question! #3  
I forgot some stuff...
While Dago Red crossed the finish line first, a 16 second penalty for cutting a pylon caused Rare Bear (highly modified F6F) to get the trophy. Sounds like an SEC football game doesn't it??

To answer your specific question about the denser air canceling out the benefits of the improved performance of the engine and wings, I am no engineer, but from experience, the airfoil/engine improvement is more than enough to overcome the increased drag imposed by the denser air. The increased lift provided by the denser air allows a lower angle of attack of the wing to be used which goes a long way to reducing the parasitic drag. I can't give you an engineer's detail of the situation, I'll leave that to the more educated folks here.
 
   / Tres_Crows... Airplane question!
  • Thread Starter
#4  
Since this was a storm front coming in, which brought rain and even a dust of snow over the passes, would that moister air have, even though it was cool and dense, have been detrimental then to performance?
 
   / Tres_Crows... Airplane question! #5  
Yes. Humidity does degrade engine performance because the water molecules (which hinder combustion) displace oxygen molecules (which aid combustion) when compressed and pushed into the cylinder by the turbo charger.

No numbers to back it up, but the amount of degradation from humidity seems slight based on experience. Air temperature is a much bigger factor. The density altitude tables don't even factor humidity into the equation.

By the way, jets (turbines) suffer from exactly the same issues of density altitude as recipricating engines do.
 
   / Tres_Crows... Airplane question! #6  
Rare Bear and most of the strung out unlimiteds the one running high boost and manifold pressure are supercharged have water injection and NOX. Rare bear went from a 2800 cu/in engine to the R3350. Original engine 2400 max hp modified bigger engine 4000+ and it tunes over 2900rpm.
Fuel flow is 600gph at race hp and the oil tank holds 40gal and burns way more than 5gal/hr.

F8F Bearcat
F6F Hellcat

Cheers
Tony

If you want to see what unmodified ones cost check out
barnstormers
or
[url="http://www.courtesyaircraft.com/[/url]"]http://www.courtesyaircraft.com/
[/url]

Tradeaplane.com needs a subsciption
 
   / Tres_Crows... Airplane question! #7  
</font><font color="blue" class="small">( The density altitude tables don't even factor humidity into the equation. )</font>

RockyMtn1, I think you have it very close to exactly right. /forums/images/graemlins/smile.gif

The performance tables for turbojet aircraft use ISA (international standard atmosphere) as the median. Very cold temperatures and moisture cause an icing problem, and that is a definite performance degradation to engine performance and creates severe drag and destroys lift on the fuselage, wings, and stabilizer. The big fan in modern jets works like a centrifuge to move the heavy moisture and ice particles outward from the axis. This air mostly bypasses the core of the engine and does not get as highly compressed as that which will be mixed with fuel for combustion.

High & hot conditions are severe limiters to aircraft performance. Most takeoff performance decisions are made based on weight, altitude, temperature, runway length & slope, and brake energy. Whichever of those is the most limiting factor will determine the maximum payload and fuel (weight) for a safe takeoff. For twin-engine jets, that amount is also based on failure of an engine after V1 speed when the takeoff must be continued with a single engine.

Okay, now I'm getting too technical.... /forums/images/graemlins/smile.gif
 
   / Tres_Crows... Airplane question! #8  
I read somewhere that they suspend flight operations at Phoenix International ( and probably anywhere in Arizona and elsewhere) once the temperature reaches 110 or 115,IIRC, degrees. I guess the air density gets so low at that temperature that they worry about the planes getting enough lift to take off. I didn't think about the affect it would have on the engines, I figured the turbines would take care of that.
 
   / Tres_Crows... Airplane question! #9  
It is true that the temperature at PHX have imposed ground stops for aircraft there, there are several reasons for this situation.

1) There are runway length limtations/restrictions. These are for safety purposes. Aircraft require a certain distance to attain certain speeds (v1,vr,v2). More importantly, there must be a certain amount of runway remaining and available for use in order to stop from a certain speed (Balenced field). This distance is affected by aircraft weight, runway type/slope, runway length, airport elevation, wind direction/speed, etc.

2) Weight restrictions in order to adhere to Item #1. Planes require enough fuel to fly to the destination airport, fly to the alternate airport, and fly for 45 minutes after that. With the extreme air temps in PHX, the possibility is good that aircraft could not carry enough fuel to comply with this restriction and item #1.

3) There are minimum performance requirements for aircraft after takeoff. This performance is also affercted by aircraft weight, aircraft type, and high density altitude. If aircraft cannot meet the minimum performace criteria (usually a minimum climb rate with a single engine), they are required to stay on the ground.

These restrictions are imposed by the FAA, aircraft manufactorers, and company rules. Violating any of these restrictions could result in a loss of flying certificates by the pilot/fine for the company at the least or an aircraft accident at the worst.

Hope this helps.
 
   / Tres_Crows... Airplane question! #10  
Definitely a little more info than the usual newspaper article, thanks!
 
 
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