Temperature, density altitude, and performance

[John Myers]

If the mixture is properly leaned a naturally aspirated engine will lose around 3% power per 1000' in elevation above asl. If you don't correct the mixture it can be substantially more than that.

I live in a very mountainous region, and have snowmobiled for a long time. With fuel injection its not as bad now as the old days, but the power loss is still drastic as we climb while mountain riding.

I was referring to most NA airplanes being able to maintain 75% power up until around 7,000ft DA or so.

 

[Southpaw]

https://www.youtube.com/results?search_query=bowling+ball+mortar

The Bowling Bal mortar Video by Doug Dickens was taken in my front yard. I packed the steel home from California for this project. I helped him build this mortar and had it in my shop for at least 3 years in pieces and parts. Its not near as efficient as mine in that it's powder chamber will hold 3 lbs of powder. Its range far exceeds practical use . Last test shot was estimated to be in the 3 mile range. He never did find the ball.
We test fired one a number of years back that made 3 miles . The ball was found by a Border patrolman . We denied any knowledge of its origin. The fellow who found it was with us the day we fired it.
Good memories . Doug has since moved to Oklahoma.

 

[Southpaw]

DD also posted one of my mortar , not a good video but also on this same page . I have a 13 star flag painted on my tube.

 

[Dave S.]

Two other factors to add to this. Colder air will descend especially with a high/low system as will a low rise. This descending air adds an additional kenetic energy force to the denser air as will the ascending warmer air subtract the upward kenetic energy from an already less dense air. This kenetic energy is reflected in the addition or subtraction of weight or pressure recorded for the air column.

The second issue is adding water vapor to the atmosphere which will reduce the density of a parcel of atmosphere because the water vapor is less dense than the nitrogen/oxygen mixture of that parcel.

WarmER air is usually lower in pressure and coldER air is usually more dense with higher pressure. But you could have a warm airmass with little moisture next to a cold airmass with much moisture resulting in two air masses with nearly the same pressure but different temperatures where you would otherwise expect a lowER pressure with the former and a highER pressure with the latter. Normally a coldER airmass would produce a highER pressure than a warmER airmass with a lessER pressure resulting in a pressure gradient force generating a wind generally from the higER area to the lowER area. The addition of the lighter water vapor can change these system routines greatly.

But generally coldER air compresses the air column and the pressure reduction rate as you go higher. Instead of approximately 1in of Mercury per thousand feet it might go to 1 inch per 850 or 900 feet. This effect is the reason for cold compensation procedures in exceedingly cold temperatures; and the axiom “warm to cold look out below!.” Actually it’s “cold or low, look out below.”

Notice I emphasize warmER and coldER and highER and lowER because the characteristics of an airmass is only meaningful when relatively compared to the characteristics of another adjoining airmass. This includes dryER and moistER (dry lines). This is because all weather is the result of the process of nature trying to equalize some imbalance in either differing temperatures, pressures, or water vapor content of adjacent air masses.

This means I owe thanks for my wonderful weather and garden to the law of entropy.

Tex

 

[birdus]

Still working on this. Would it be fair to say:

• Pressure Altitude is the altitude the plane (altimeter) "thinks" it's at based only on pressure. This varies at any given location as the weather changes. It can be determined precisely by setting the altimeter to 29.92. However, it may not correspond to actual altitude, again, because weather can throw it off.
• The performance of an airplane (wing, prop, engine) is slightly different than only what pressure dictates because heat has an impact, but can't be accounted for only by pressure (measured by the altimeter). Therefore, to know how the plane will actually perform, we take what the altimeter tells us and make an adjustment based on heat. Humidity will have an impact, too, but not as much, so we don't bother accounting for it, at least if doing our own computation before takeoff, as it's easy to measure temperature, but not humidity (i.e., we may have a thermometer, but probably not an instrument to measure humidity).

 

[John Myers]

Still working on this. Would it be fair to say:

• Pressure Altitude is the altitude the plane (altimeter) "thinks" it's at based only on pressure. This varies at any given location as the weather changes. It can be determined precisely by setting the altimeter to 29.92. However, it may not correspond to actual altitude, again, because weather can throw it off.
• The performance of an airplane (wing, prop, engine) is slightly different than only what pressure dictates because heat has an impact, but can't be accounted for only by pressure (measured by the altimeter). Therefore, to know how the plane will actually perform, we take what the altimeter tells us and make an adjustment based on heat. Humidity will have an impact, too, but not as much, so we don't bother accounting for it, at least if doing our own computation before takeoff, as it's easy to measure temperature, but not humidity (i.e., we may have a thermometer, but probably not an instrument to measure humidity).

I haven’t thought about this in a bit, someone more knowledgeable than me can check my work here:

Pressure altitude could be viewed as the ambient pressure “broken” in a specifically agreed upon way, which may be a more helpful view than saying it is “corrected”. As with density altitude, it is based on ISA, which is a political agreement as much as a scientific one. Your plane sees the ambient pressure, including temperature and humidity effects. Performance charts can be stated in altitudes based on the density of this standard atmosphere which are more intuitive units for pilots to work with. We need ISA because when you say “the pressure at 15,000ft” we need to know on what day, since temperature and humidity will change the pressure from day to day, so the powers that be made up a standard day and mechanisms to correct to and from that standard day. Since humidity is not a major factor, for simplicity we usually just use the temp.

Altimeters convert the ambient pressure into units of feet based on this political agreement, assuming this standard day. Since they don’t know the temperature or humidity, and it’s rarely a standard day, we must correct for this by using a setting that accounts for both.

I found learning the practical implications and coming back to the theory later makes things easier. Some DPEs seem to be confused between aeronautical engineers and pilots- information relevant to one is usually not to the other.

 

[bflynn]

This is something I'm confused about (almost ready to begin Private Pilot training).

I believe that on a cold day, my plane will perform better—engine, prop, and wing. I thought that was because cold air is more dense. However, now I'm reading that there is higher pressure with warmer air and lower pressure with colder air. Isn't there a direct correlation between density and pressure?

Apparently, the altimeter will read higher than my actual altitude when the air gets colder (Figure 5.15 in the FAA's Weather Circular). Again, I thought I would need to fly higher in cold air to get a given reading because the cold air is denser and so if I flew higher, then I would have less air above me pressing down in the "column" of air.

Lots of people have given complex examples. Maybe this one is complex too, but here's how I think about it.

With higher pressure, the gas has expanded and is pushing outward harder - think tea kettle. There are fewer molecules per cubic ft of air because it's pushing apart. The local height of the atmosphere actually goes up a little during summer because of the higher pressures!

Lift is all about the molecules of air moving downward. When the molecules are further apart, your wing affects fewer of them as it goes through the air, so less lift is produced.

 

[Rgbeard]

Short answer. YES.

Setting the altimeter to 29.92 will give you your pressure altitude, which will seldom be the same as your actual altitude. (unless the correct altimeter reading is 29.92 at that moment. it happens).

We can easily calculate the BIG things in density altitude and performance, and negate a lot of the minors, such as humidity. It's not worth the time/effort when the value moves the number so slightly.

Practical Use:

You find yourself with a Piper Warrior (150hp) in Flagstaff Arizona on a warm day. (80 deg is warm for Flagstaff). It's you, your wife, and your 40# dog. Your airplane is now a machine chanting: "I think I can... I think I can..." as you roll down the runway.

Do some D.A. calculations, and then go to town for a sandwich, until the temperature drops another 10 degrees. Humidity shouldn't have any effect on this decision.

Still working on this. Would it be fair to say:

• Pressure Altitude is the altitude the plane (altimeter) "thinks" it's at based only on pressure. This varies at any given location as the weather changes. It can be determined precisely by setting the altimeter to 29.92. However, it may not correspond to actual altitude, again, because weather can throw it off.
• The performance of an airplane (wing, prop, engine) is slightly different than only what pressure dictates because heat has an impact, but can't be accounted for only by pressure (measured by the altimeter). Therefore, to know how the plane will actually perform, we take what the altimeter tells us and make an adjustment based on heat. Humidity will have an impact, too, but not as much, so we don't bother accounting for it, at least if doing our own computation before takeoff, as it's easy to measure temperature, but not humidity (i.e., we may have a thermometer, but probably not an instrument to measure humidity).

 

[dtuuri]

Still working on this. Would it be fair to say:

• Pressure Altitude is the altitude the plane (altimeter) "thinks" it's at based only on pressure. This varies at any given location as the weather changes. It can be determined precisely by setting the altimeter to 29.92. However, it may not correspond to actual altitude, again, because weather can throw it off.
• The performance of an airplane (wing, prop, engine) is slightly different than only what pressure dictates because heat has an impact, but can't be accounted for only by pressure (measured by the altimeter). Therefore, to know how the plane will actually perform, we take what the altimeter tells us and make an adjustment based on heat. Humidity will have an impact, too, but not as much, so we don't bother accounting for it, at least if doing our own computation before takeoff, as it's easy to measure temperature, but not humidity (i.e., we may have a thermometer, but probably not an instrument to measure humidity).

Fair enough. Shorter is sweeter, though. Pressure altitude is what you read when you set the altimeter to 29.92. Density altitude is pressure altitude corrected for non-standard temperature. Be able to say it in your sleep.

 

[smv]

...and if you do not happen to have an altimeter handy, the math is pretty easy.

 

[birdus]

Pressure altitude is what you read when you set the altimeter to 29.92. Density altitude is pressure altitude corrected for non-standard temperature. Be able to say it in your sleep.

Yeah, I just like to actually be able to understand things rather than simply regurgitate mnemonics or textbook definitions. That's what I'm trying to do here.