# Tag Info

20

Low density altitudes mean your engine generates more power, and your wings generate more lift. Everything is better with low density altitudes, right? The potential danger in low density altitudes is that your engine generates more power than it can handle. Even without exceeding the maximum rated RPM and manifold pressure, you can exceed the rated power ...

16

With more flaps, you will lift off at lower speed, but will have worse climb performance due to the increased drag. On short field you are limited by the field length, so you need to lift off early and therefore need to use more flaps. But in the mountains you are limited by the climb performance. Therefore you need to use less flaps. Of course, this is ...

13

The optimum altitude depends on your aircraft, the engines and the weather. But most important is to decide what you are trying to achieve. If you would like to optimize your fuel consumption per distance travelled, the altitude you will fly at will be higher than if you try to optimize your fuel consumption per time unit. Fuel per distance travelled is ...

12

There are procedures with temperature restrictions, related to altitude constraints. An example is Innsbruck: The text in the red box says: Procedure N/A below AD temp -7° For effect of temperature on altimeter: How will the altimeter read in air colder than ISA?

11

An oversimplification for sure: Turbochargers essentially compress the air going into the engine (cylinders) to maintain (near) sea level pressure internally, so the external density altitude doesn't affect the performance of the engine. The "liability" at low density altutudes is that it is possible to over-boost the engine, potentially damaging it. See ...

11

The atmosphere approximates an ideal gas, and as such you can relate pressure and density through the ideal gas equation. The form we use in meteorology uses mass density and is given by: $$p={\rho}RT$$ where $P$ is pressure in units of Pa, $\rho$ is density in units of kg m-3, $R$ is the gas constant for dry air (287 J kg-1 K-1) and $T$ is temperature in ...

11

(Source) The basics Height: absolute height above ground, measured using a radar altimeter (like the helicopter on the right) QFE: the barometric setting an airport broadcasts that makes an altimeter on ground read zero. This gives height only around the airport if the area is relatively flat. Also known as Above Field Elevation QNH: the barometric setting ...

10

The air in the lungs is in pressure equilibrium with the cabin, but not temperature equilibrium. It's always at body temperature. So the only variable that can affect how much oxygen is available to the body is the pressure of the outside air - thus pressure altitude, not density altitude, is physiologically relevant. That said, if the density altitude is ...

10

Generally, climbing as high as you can would be close to the optimum strategy. In the end, the altitude is given by the optimum lift coefficient in cruise, which can be approximated as $$c_L = \sqrt{\frac{1-n_v}{n_v+3}\cdot \pi \cdot AR \cdot \epsilon \cdot c_{D0}}$$ Now it will be important to get the thrust over speed dependency right, which is expressed ...

10

Yes, through the Reynolds number The Reynolds Number is a function of airspeed and is used to predict the transition from laminar to turbuluent flow: $${Re} = \frac{\rho v L}{\mu} = \frac{v L}{\nu}$$ The critical angle of attack is affected by the separation of the boundary layer, which, in turn, depends on the laminar or turbulent qualities of said ...

8

The caution is probably the default prefix for a nonstandard DA notice, not anything particular to the fact that it's negative. You're correct that lower DA will improve performance, and an FAAsafety.gov document on density altitude (PDF) doesn't discuss low DA at all.

8

QNH is the atmospheric pressure adjusted to sea level. So, a reported QNH of 29.92 means the pressure is 29.92 inHg at sea level, regardless of the reporting station's elevation. If you're on ground at 3,000' MSL, you can rotate the knob to display 0' altitude, the pressure reading then on the altimeter window will be QFE (Field Elevation). (Source) Real ...

8

If the altimeter says the station pressure is 29.92" sea level, but I am at 3000', then the airplane still thinks it's at sea level when it comes to performance. Is that wrong? Although the station pressure may be reported as 29.92", that does not mean that a barometer there will show that as the current pressure. The reported value is the actual pressure, ...

7

Assuming our lungs are like our engines They are not. Engine (at least spark-ignited reciprocating one) uses most oxygen in each charge and its RPM is limited, so it can't be run faster to compensate for the lower air density. However, our bodies only use small fraction of oxygen from each breath and we normally breathe much slower than we can if need be. ...

6

I have analyzed this information before, and this is what the chart looks like with one line for each weight category if you convert the temperature and pressure altitudes to density altitudes and then chart that vs. the takeoff distance: You can see that I have drawn a sort of best-fit line for this information. I did not add humidity as a factor because ...

5

We all know the saying "From High to Low, Look out below" Some were taught that this only applied to pressure differences and thus were instructed to get a new altimeter setting every 100NM or so. This is a good recommendation. Temperature differences from standard also causes errors with the altimeter. The same saying above holds true with temperature ...

5

Check your temperature conversion: 25F is -3.9C... Should be (25-32)*(5/9).

5

You should look in the POH for your specific aircraft (because it may have had modifications to it which would make it different from a "standard" aircraft) and see what it says. The 172P POH that I found online contains no maximum airport elevation for takeoff, and the Type Certificate Data Sheet (TCDS) doesn't either. Therefore, you can only go by the ...

5

The International Standard Atmosphere (ISA) gives “average” values of air pressure and density by altitude. It starts by defining standard pressure 1013.25 hPa (29.92 inHg) and standard temperature 15°C at sea level, and derives the values for higher altitude from physical laws. A “density altitude” is altitude in ISA at which air has the density you are ...

5

There is a variety of reasons for a maximum altitude: Most common is insufficient engine performance to climb any higher. Then your question never arises. The aircraft simply cannot go higher. For more powerful aircraft the partial pressure of the oxygen supply in an unpressurized cabin would drop below the value to keep the pilot(s) alert. For GA aircraft ...

5

You may be able to get a rough estimate of if your engine is producing sufficient thrust utilizing brake pressure. But, how will you determine how much thrust is being produced? Your brakes are not calibrated so carefully as to adequately determine that. You would only be guessing. You might get a better indication of engine performance by utilizing the ...

4

I don't know of a maximum airfield elevation, though I suppose the airplane's service ceiling would qualify as an answer to this question. In regards to the second question, that is largely going to depend on the airfield elevation, runway length, climb gradients needed for obstacle clearance, and ambient atmospheric conditions. These questions would have ...

4

Pressure, density and temperature are related (approximately) through the ideal gas equation. In the general form it is $$PV=nRT$$ Where $P$ is pressure, $V$ is volume, $n$ is amount, $T$ is temperature and $R$ is ideal gas constant. If you have an enclosed container filled with air, volume ($V$) and amount ($n$) are the same, so pressure increases ...

4

A big thing to remember is that $Density=\frac{Mass}{Volume}$. It is unrelated to pressure, and pressure is unrelated to density. Pressure generally increases with temperature only in a gas with a constant volume. This is because you are adding more energy to the system, causing the molecules to become more exited. To put simply, they bounce around harder ...

4

The mistake is subtracting the ISA temperature at sea level from the ISA+18°F temperature at altitude in step two. If your atmosphere is heated up, it needs to be hotter at sea level as well, or your lapse rate in the first step and the denominator is wrong. First calculate density for ISA conditions. This gives 0.7708 kg/m³. Then used the ideal gas law to ...

4

The formula can be derived equaling the weight ($W$) of an airplane with the maximum lift ($L$) generated by its wings. I'm not aware of any special name for it. $$W = L \\ m\cdot g= \frac{\rho_{min}}{2}\cdot V^2\cdot S\cdot c_{L,max}$$ Substituting the aircraft's speed with an expression containing the Mach number ($a$ is the local speed of sound), V=...

4

Add 10% to your computed takeoff distance if it is too humid. Humidity. Humidity is not generally considered a major factor in density altitude computations because the effect of humidity is related to engine power rather than aerodynamic efficiency. At high ambient temperatures, the atmosphere can retain a high water vapor content. For example, at 96°F, ...

4

You would have to have a brake pressure indicator that shows the pressure being applied to the calipers, where you could observe the minimum pressure required to hold the plane when WOT, and some charts or calculations that showed the static thrust being produced with a given amount of brake pressure. But then the amount of pressure varies with a lot of ...

3

Another factor in the decision to fly low or high: turbulence. How important is it to you and your passengers to have a smooth ride? Higher is generally less turbulent but check those Airmets and Pireps!

3

Turbochargers increase available engine performance by increasing the amount of fuel/air mixture that can be burned in the cylinders. This is accomplished by compressing the air before fuel is mixed in. The performance of a reciprocating aircraft engine is directly dependent on how much fuel can be burned and converted into heat energy, which in turn is ...

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