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Density altitude, and what it does to the aircraft


Anticept

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This post was in another thread, and it was was after their last backup when the hardware failed, so it ended up lost to the abyss. I'm reconstructing the original the best I can. I'm also making a couple tweaks to be more in line with aviation instruction, rather than international standards.

 

This is a very technical post, for those who might be interested in learning what Density Altitude does to their aircraft.

 

First, some definitions:

 

IAS - Indicated Air Speed - This is what is read from the instrument within the cockpit, and is what is most often used in POH and manuals.

 

CAS - Calibrated Air Speed - This is what is read from the instrument, and adjusted for installation, instrument, and position of sensors error. CAS is generally very close to IAS, but no installation, instrument, or position is perfect.

 

TAS - True Air Speed - This is IAS/CAS (depending on precision you want), but adjusted for temperature and pressure altitude (aka, density altitude).

 

PA - Pressure Altitude - Basically, how much the air above a ground reference station weighs. We use ground stations as a reference for two reasons: the weight changes at different altitudes (and while some fancy math can get us an approximation, the second reason invalidates the use of this approximation), and everybody buzzing around NEEDS to use the same reference. It doesn't matter if the ground reference is slightly off, that just means all aircraft *together* will be flying a little higher or a little lower.

 

DA - Density Altitude - For aviators, density altitude is pressure altitude adjusted for temperature. There's also a more complicated version of DA, and it's more for meteorologists. They use humidity and air viscosity, as well as the mass of a volume of air. It's more for weather prediction, rocketry, and spaceships though, so don't worry about it.

 

L/D ratio - Lift / Drag ratio - This is the deciding ratio which dictates your rate of climb, descent, and airspeeds for power settings. Lift is exponentially proportional to airspeed, but unfortunately so is drag. In fact, drag has TWO curves! Low speed drag, called induced drag, is a curve which is inversely proportional to airspeed, while high speed drag, called parasitic/form drag, is proportional to airspeed. Generally where drag is the most minimal is where "best glide speed" is set, and very close by will be "best rate of climb".

 

Here's an illustrative graph of L/D:

Drag_Curve_2.jpg

 

 

 

High DA Aircraft effects:

 

First and foremost, a high DA will cause the engine to lose volumetric efficiency, resulting in a corresponding loss of power. Non-turbocharged reciprocating engines (otto cycle engines) are constant volume engines, which means that air density greatly affects how much mass of air is drawn in the cylinder (volumetric efficiency). Since mass, and not volume, directly correlates with engine power, a lower density means lower power. In fact, when you close the throttle, you are lowering the mass of material drawn into the cylinder, lowering volumetric efficiency. As a side note, this is in contrast to brayton cycle engines, aka jet engines, which maintain a constant pressure up until they can't draw enough air to maintain the pressure, and they dump excess air overboard by the use of bleed air valves. They adjust their power by fuel mass flow alone.

 

The second effect is less pronounced, but just as important, and affects all aircraft. A high DA means there is a lower air density, which means less air molecules per unit volume. Because there are fewer air molecules per unit volume, this also means there are fewer air molecules participating in the aerodynamic model in any one particular time snapshot. We have two ways of making up for this loss of molecules. We can either increase the force of each air molecule acting on an airfoil, or increase the number of molecules acting on the airfoil in a particular unit of time. What happens in reality, is we move the airfoil faster through the air, which means BOTH per molecule force, and molecules per unit time acting on the airfoil, increase, until the aerodynamic forces are back in balance. An aircraft moving in high DA conditions has a higher TAS (it actually moves faster per unit time) than an aircraft moving in low DA conditions.

 

Thirdly, aircraft instruments are also affected by high DA. Since there are fewer molecules participating in exerting force on diaphragms (older instruments used for illustrative purposes, but solid state sensors are also affected), we need to either increase the force exerted per molecule, or increase the number of them acting on the diaphragm per unit time, in order for an instrument in high DA conditions to read the same as an instrument in low DA conditions. As with airfoils, reality has it that we perform both when the aircraft moves faster through the air. In a way, aircraft instruments and wings "feel" like they are moving at 60 knots, even if the aircraft's true airspeed is significantly higher in the case of low orbit flying :P.

 

 

Results that are seen from high DA:

 

On takeoff, the reduced volumetric efficiency (loss of power), combined with the loss of molecules participating in the aerodynamic model per unit time, means that we need a longer distance, and a faster ground roll, to take off from an airfield on a high DA day. Remember though: even though we are rolling faster for takeoff, the IAS/CAS will still read the same speed as a low DA day! We have fewer molecules in high DA conditions, so we need to move FASTER for the INSTRUMENT and WINGS to "feel" that takeoff speed.

 

On landing, we generally exclude engine effects from the equation due to minimal power (therefore, no volumetric efficiency to consider). However, we are still moving faster in those high DA conditions, so this means we are carrying more kinetic energy, and need a longer distance to stop. Usually though, landing distances are not as affected as takeoff distances, because again, the engine is generally at minimal power.

 

In flight, we rely on aircraft flying characteristics using IAS/CAS, because that is a reliable measurement of molecules participating in the aerodynamic model. This is an important to understand, because an aircraft will stall at the same speed, regardless if you are in high DA or low DA conditions. This also means that glide speed does not change! A glide speed of 60 knots IAS/CAS is still 60 IAS/CAS even in low orbit! It's just that the aircraft has to have a higher TAS in order for the instruments and wings to "feel" that 60 knots IAS/CAS.

 

The aerodynamic model also doesn't change based on DA. If your decent rate is 500 feet per minute at sea level with best glide, then it will also be 500 feet per minute in low orbit. Flight is about balancing the aerodynamic forces in the aerodynamic model, and as previously established, if you lower the air density, then you can make up for it by moving faster through the air (raising TAS). What DOES change as a result, is the glide RATIO. Because you have a higher TAS in low orbit, but the decent rate is still the same, you will glide further as your forward movement is higher, but your downward movement is still the same. Therefore, an aircraft in high DA conditions will actually glide a little bit further per unit time, than an aircraft in low DA conditions. Our graphs for glide distance are linear, because until you get into orbit, the differences between a few thousand feet is negligible. Simplicity rules.

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I have noted I see a distinct difference in throttle use dependent on DA.

Again, I personally have never noticed that.

 

Most of my flying career was in FL. Moving to where I am now, N GA, 2,000' field elevation made no apparent difference in my pattern or power settings.

 

The handful of times I've flown out west in real mountains, I also did not find the need to change my patterns or power settings.

 

Maybe I'm adjusting subconsciously, but regardless I've never noticed it.

 

Argumentum ad absurdum, how would a high DA affect one's power-off approach? Notwithstanding the higher TAS and GS on landing, of course.

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Today DA was 9,000', morning air was stable and surface winds were calm. I didn't have to adjust my pattern it was easy to hit the numbers, much like a good winter day.  

 

Later in the day, all bets are off, might take full throttle to adjust for shear in the pattern.

 

DA doesn't change your glide angle but it does increase your true speeds at a given glide.

 

Negative wind shear in the pattern calls for throttle (or a tighter pattern) and is far more likely in summer or lee side turbulence.

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. . "Negative wind shear in the pattern calls for throttle (or a tighter pattern) and is far more likely in summer or lee side turbulence." . . .

By the term, "negative wind shear," do you mean decreasing performance wind shear?
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The engine develops less horsepower and torque due to fewer oxygen molecules entering the combustion process per volume of air [...]

 

This is what volumetric efficiency measures. Those other air molecules are also important in the calculations too, because that's what the engine is designed around. If you took an engine designed for our ~28% 21% oxygen atmosphere, and pumped in 100% pure oxygen, you would slag your engine because the other 72% 79% is used as a heat sink, and helps prevent detonation.

 

 

As for the rest, if your plane has a tendency to sink faster than the standard approach slope (nearly all do), then yes, higher DA will affect thrust performance of your aircraft, and as you said, you need to open the throttle a little bit more (however, idle does contribute some thrust, so you shouldn't really need to make too much of an adjustment). If, however, your aircraft has a glide ratio greater than 19:1, then DA would not matter at all for a landing.

 

As an option, when you come in for an approach for downwind, you can turn base a bit sooner. You'll approach the slope from a steeper angle, but you will use less fuel in the process.

 

 

Why did I say 19:1 glide ratio? The standard approach is 3 degrees for small aircraft. That means for every 100 feet of altitude lost, you need to make a forward movement of ~19.08 feet to maintain that 3 degree glide slope. If your plane does that naturally, then we can remove the engine variables, so throttle is no longer considered.

 

 

MATH: Your vertical distance is side alpha, your horizontal distance is side beta, and the slope in degrees is angle Beta. Using TAN (3 degrees) = X / 100 and solving for X, we come up with ~5.2 feet. Then, divide 100/5.2, and you get ~19:1 glide ratio.

 

346px-Rtriangle.svg.png

 

 

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I mentioned this thread to a fellow CFI and he immediately asked, "If you need more power in the pattern at high DA's, how do glider pilots manage to fly there?"

 

I thought it was interesting how he gravitated towards a similar argument to mine,

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This is what volumetric efficiency measures. Those other air molecules are also important in the calculations too, because that's what the engine is designed around. If you took an engine designed for our ~28% oxygen atmosphere, and pumped in 100% pure oxygen, you would slag your engine because the other 72% is used as a heat sink, and helps prevent detonation.

 

 

As for the rest, if your plane has a tendency to sink faster than the standard approach slope (nearly all do), then yes, higher DA will affect thrust performance of your aircraft, and as you said, you need to open the throttle a little bit more (however, idle does contribute some thrust, so you shouldn't really need to make too much of an adjustment). If, however, your aircraft has a glide ratio greater than 19:1, then DA would not matter at all for a landing.

 

As an option, when you come in for an approach for downwind, you can turn base a bit sooner. You'll approach the slope from a steeper angle, but you will use less fuel in the process.

 

 

Why did I say 19:1 glide ratio? The standard approach is 3 degrees for small aircraft. That means for every 100 feet of altitude lost, you need to make a forward movement of ~19.08 feet to maintain that 3 degree glide slope. If your plane does that naturally, then we can remove the engine variables, so throttle is no longer considered.

 

 

MATH: Your vertical distance is side alpha, your horizontal distance is side beta, and the slope in degrees is angle Beta. Using TAN (3 degrees) = X / 100 and solving for X, we come up with ~5.2 feet. Then, divide 100/5.2, and you get ~19:1 glide ratio.

 

346px-Rtriangle.svg.png

 

 

 

28% oxygen in the atmosphere? Not sure where in the world that would be. It actually is slightly below 21%

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28% oxygen in the atmosphere? Not sure where in the world that would be. It actually is slightly below 21%

 

 

Oh, you!

 

Corrected my post, thanks :)

 

 

I mentioned this thread to a fellow CFI and he immediately asked, "If you need more power in the pattern at high DA's, how do glider pilots manage to fly there?"

 

I thought it was interesting how he gravitated towards a similar argument to mine,

 

 

They go find an updraft. Gliders are so insanely light, that they can ride air currents into 20,000 feet +. There are actual rules requiring such gliders to use transponders if they go that high.

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So, is there a relationship between DA and descending on base and final to land?  I have noted I see a distinct difference in throttle use dependent on DA.  Why do I think it's DA? Because at the same airport in winter when temps are in the 50s, versus during the summer when temps get into the 80s (and DA doubles), the amount of throttle to maintain the same altitude and base/final sink rates vary on calm wind days.

 

The amount of throttle needed on base and final is traditionally zero.  If you are slow you can lower your nose, if you are short you can tighten your pattern.  Demonstrating your point on base and final doesn't work well.  You say you need more I say you need zero and its hard to get past that.

 

If we back up its a different story.  Lets say you enter at pattern altitude on a 45 and fly the 45 and downwind until abeam at a given speed while maintaining pattern altitude.  In this test your point is easy to demonstrate.  When a CT pilot from the east posts his RPM that he uses in the pattern I always realize that I could not maintain altitude at that speed and RPM.  A 12 minute flight down the hill to Bishop demonstrates your point, there at 5,000' TPA I need about 400 RPM less than I need in Mammoth at 8,000' TPA to maintain the same speed and level flight.

 

The difference between Bishop and Mammoth on any given day well simulates the difference in Southern Nevada between summer and winter.

 

 

I mentioned this thread to a fellow CFI and he immediately asked, "If you need more power in the pattern at high DA's, how do glider pilots manage to fly there?"

 

I thought it was interesting how he gravitated towards a similar argument to mine,

 

 

How a glider lands at DA isn't a good question to use to analyze how additional throttle is needed at high DA.  The glider uses a higher TAS just like we do and otherwise has the same glide ratio as he has at lower altitudes to work with.  At high DA airports he very likely has more lift and sink to deal with and work with.

 

The better demonstration of the need for extra throttle at high DA is level flight at 14,000' DA.  Here you will be close to needing full throttle to maintain altitude at pattern speeds.

 

I have landed at 11,000' but on a road and didn't fly a pattern.  Before the military removed Coyote Flats field at 10,000' I would fly the pattern there.  The glacier in the background goes to 14,000'.

 

FullGlacier00-1.jpg

 

I had a friend that made money from pilots and their family's that landed at Coyote and couldn't take off due to their gross weights and the high DA.  Once they relayed their predicament to Bishop airport my old friend would drive his truck up the 6,000' climb and load the pilot, his family and their luggage into his truck and meet them in Bishop with their airplane and a bill for his trouble.

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The better demonstration of the need for extra throttle at high DA is level flight at 14,000' DA.

I fully understand that if you go high enough, even full throttle will not be enough to sustain level flight.

 

And I can imagine someone getting too low on approach at a high DA, and finding even full throttle is not enough to level off and arrest the sink, especially if inadvertently behind the power curve.

 

But I'm talking about a normal pattern with little or no power in play.

 

I think back to all the diagrams I've seen, and drawn, about where to throttle back, where to put down flaps, where to turn base, and what power settings, if any, were appropriate. That sort of thing. I have never seen the need to tell students that any of those things change at DA's, because I was never taught that nor have I observed that.

 

Maybe if someone was flying a very wide and very flat pattern that required a lot of power, the need for more power at high DA might become apparent. I don't fly that way, nor do my students, so it has really never come into play.

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 I'm talking about a normal pattern with little or no power in play.

 

I think back to all the diagrams I've seen, and drawn, about where to throttle back, where to put down flaps, where to turn base, and what power settings, if any, were appropriate. That sort of thing. I have never seen the need to tell students that any of those things change at DA's, because I was never taught that nor have I observed that.

 

Maybe if someone was flying a very wide and very flat pattern that required a lot of power, the need for more power at high DA might become apparent. I don't fly that way, nor do my students, so it has really never come into play.

 

If you are talking no power at play then how do you maintain TPA until you begin your descent?  Surely you don't fly the whole 45 entry plus the entire downwind descending?  If not you need a throttle setting sufficient to maintain TPA and it is that setting that demonstrates the difference in power requirements at high DA airports.  Granted you could maintain TPA without power if you entered at a high speed and then remained level in the pattern bleeding off speed with the throttle closed but I'm assuming we are not talking about that.

 

Charts and diagrams with power settings ( in RPM ) have never made any sense to me going back to the 1980s and they still don't today including posts here and FD's website.  I could never achieve the RPM in my skyhawk and in my CT with an optimized prop pitch I can achieve the RPM but I need an extra 500 or so to maintain TPA.

 

You claim that you and your students 'don't fly that way' but its not a matter of choice.  If you enter the pattern here at 11,500' DA and need to maintain TPA and a given IAS you will need a lot more power than if you did the same at Copperhill.

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If you enter the pattern here at 11,500' DA and need to maintain TPA and a given IAS you will need a lot more power than if you did the same at Copperhill.

Of course.

 

I've so stipulated.

 

But I think we're talking past each other.

 

Discounting thermals and shear, once abeam the numbers and making your initial power reduction, do you find you need more power in your pattern to maintain a given IAS on base? I thought you already answered in the negative, but it's been a long thread.

 

It may, in fact, be required, and I just never noticed it. Probably because I'm at a very low power setting or idle anyway by that point.

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I don't have a need to add power after my reduction but it has nothing to do with DA.  It is because I adjust my pattern, tight enough to make the runway, slipping if needed.  Unless a controller is calling our turns we should all be adjusting our patterns for winds with the same objective, making the runway and our target (the numbers).

 

100burgers finds a need but he is obviously flying a  bigger pattern.  You too find one when you have to extend downwind for traffic.

 

Talking about the need for power when descending to the runway where there is no need for power is almost pointless.  That's why I keep taking it to level at TPA or service ceiling where the increased need for throttle at increased DA can be easily demonstrated.

 

We aren't quite talking past each other because the charts that you bring up would support your point if they actually worked in very high DA environments.

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Oh, you!

 

Corrected my post, thanks :)

 

 
 

 

They go find an updraft. Gliders are so insanely light, that they can ride air currents into 20,000 feet +. There are actual rules requiring such gliders to use transponders if they go that high.

The world record glider altitude is over 50,000 feet.

The DG 800, a popular high end self-launch glider, gross weight is 1320 lbs.  Many racing glider are ballasted in many gallons of water to increase speed.  The Ventus 2B holds 440 lbs of water and has a gross weight of 1115 lbs.  My old LP-49 has a gross weight of 700 lbs.  I'm not sure these are "insanely light".

Gliders are no different than any other aircraft that has to comply with 91.215 on use of transponders in class C, B and A.  A starts at 18,000 feet.  That notwithstanding, there are many glider Class A windows which are agreements between the local ATC and a glider organization which permits operations in class A airspace.

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Jim:

 

Yes, those are insanely light :P.

 

As for the transponder requirements: I've ran into people who thought gliders were exempt from them. When I said "there are actual rules", it was to drive the point home about it.

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Gliders have to comply with 91.215. That is the easy way to put it. Various aircraft have various requirements in 91.215. Gliders are not like a 2014 Cessna Skyhawk are not like a 1946 Champ. None are EXEMPT from 91.215. Looking at it from the point of view of other aircraft and not the aircraft in question is ego-centric and confuses the matter.

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Gliders have to comply with 91.215. That is the easy way to put it. Various aircraft have various requirements in 91.215. Gliders are not like a 2014 Cessna Skyhawk are not like a 1946 Champ. None are EXEMPT from 91.215. Looking at it from the point of view of other aircraft and not the aircraft in question is ego-centric and confuses the matter.

 

Hi Jim!

 

They aren't exempt, no, but there are quite a few exemptions granted to gliders by that section itself. Which is why it is said, gliders are odd. Rightfully so though, since they don't have generators on board and transponders are extremely power hungry little buggers.

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