Density Altitude

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ACE
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Density Altitude

Postby ACE » Wed Jul 27, 2005 3:20 pm

The following contains info put together by Jan Coetzee, a well known Bapsfontein flier who also produced the book on Microlight Airfields.

I look forward to comments/corrections

Density Altitude
Density altitude and hot, high, humid conditions work hand in hand to influence the aircraft’s performance. The easiest way to visualize density altitude is to think of a column of air resting on the sea and the sea level temperature is 15 degrees Celsius. The pressure of this column on the sea is 1013.2 hPa. The higher we go up into the air the cooler it becomes. This happens at a rate of 2 degrees Celsius per 1000 feet. This is short the International Standard Atmosphere (ISA). There is also a height correction of 120 feet that must be made for every degree Celsius that the temperature differs from the standard temperature of 15 degrees Celsius.
Take an example of an airfield that is 5000 feet above sea level and it is a nice hot afternoon of 30 degrees Celsius. According to the standard atmosphere, the temperature should be 15°C minus (2°C per 1000 ft) = 15 – (2 x 5) = 15 - 10 = 5°C. This 5°C is the ISA temperature for that airfield. Ambient temperature of 30°C equals ISA (5°C) + 25°C. Correction for this extra temperature of 25°C above the standard temp. is to be made: 120 x 25 = 3000 feet. That airfield is for all practical flying reasons 5000 + 3000 = 8000 feet above sea level.
We all know that the higher we go the thinner the air gets and the less oxygen there is (that is why your car does not perform so well up on the Highveld as down by the sea). As it is with all gasses, air is dense at low temperatures and less dense with high temperatures. This is where the hot high humid condition comes in. If it is a hot day, the air molecules will not be packed tightly together. There will be space for water vapor to take up the space between the air molecules and for flying you need air. Low temperatures will give more air to increase engine performance, to increase lift from the wing and thrust from the propeller. Vise versa happens with high temperatures. Total performance losses in the order of 27% can be experienced at high above sea level areas. In short what all this boils down to is. On a cold winters morning on the Highveld you will with a fully loaded all up weight plane get airborne within a 100 meters and climb out like a home sick angel into the sky, where as on a hot, muggy summers afternoon you will battle to get airborne in 300 meters.
“HOT, HIGH and HUMID” conditions (as explained by Jan Coetzee)
It is a well-known fact that "hot, high and humid" is not good for flying. I have been looking up some figures to put numbers to this, and came up with some very interesting results:
First some background:
The weight of a volume of air is influenced by temperature, pressure (altitude) and the amount of water vapour that it carries. This last can be expressed as the dew point of the air - the higher the dew point, the higher the amount of water vapour in the air. We know that when the dew point is the same as, or slightly below, the ambient temperature, then the vapour can easily - and quickly -- turn into visible mist. When the dew point is very low (for instance minus 10deg C) then there is so little vapour in the air that the vapour will only come out of the air at a temp of -10C, and it will then immediately turn into frost (this is the black frost of very cold and dry winter mornings). What goes against our intuition, is that the fact that the more water vapour that is dissolved in a mass of air, the lighter, that air mass becomes.
Here are some figures giving the weight of 1 liter of air in grams, under different conditions and at a pressure of 750mBar (equivalent to an altitude of approx. 350ft AMSL):

Temp deg. C. Dew Point in deg. C. Grams/liter
20 -10 1.1876
20 0 1.1861
20 10 1.1833
20 20 1.1610

This means that the higher the humidity, the less the mass that is displaced by the propeller, therefore the less thrust. Also, the lighter the air mass, the less the carrying capacity of the wing, and the higher the speed that is necessary to keep the wing aloft. The less the mass of air that is ingested by the engine, the less the amount of oxygen available to burn fuel, therefore the engine develops less power. The difference between the best and the worst case above is not all that bad (only about 2%), but the effects on thrust, engine power and Lift is cumulative.
Some more figures:
Temp deg. C. Dew Point in deg. C. Grams/liter
0 -10 1.2518
10 -10 1.2296
20 -10 1.1876
30 -10 1.1485

10 0 1.2280
20 0 1.1861
30 0 1.1469

10 10 1.2251
20 10 1.1833
30 10 1.1442

20 20 1.1610
30 20 1.1394
30 30 1.1311

From the above figure it follows that the difference in air mass on a very cold dry Gauteng winter morning (0C, Dew point -10) and on a very hot steamy afternoon immediately after a rain shower (30C, dew point 30) can be as much as 10%. This means 10% less thrust due to less air mass, as well as 10% less power, as well as 10% more speed necessary for lift. 0.9 x 0.9 x 0.9 gives 0.73, which means you only have 73% of the performance relative to the cold morning! More: At a pressure of 620mB (giving an equivalent altitude of somewhat more than 5000 ft AMSL) all the above values can be reduced to approximately 82% of the above. The moral of the story is. Take GREAT care on hot muggy days.
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Postby lamercyfly » Wed Jul 27, 2005 9:10 pm

Hi guys and gals.

First of all, the dude that wants to cruise around around at 12 000 is getting into advanced flying.

So, getting advice is good. Well done and thankyou for knowing your limits.

First of all consider the Air Law.

You will most probably be encroaching into controlled airspace, so make sure of this.

Secondly, air- law states that at no time may an un-pressurised flight exceed 10 000feet ASL. You may, however, exceed this for a maximum of 120mins(2 hours), up to a maximum of 12 000'. You must then immediately descend below 10 000' again.

Human Performance Limitations states that a person of average fitness should experience NO Hypoxia problems up to 10 000'ASL. The max., height recommended for unfit or heavy smokers is 8 000'ASL.

O.K. so now you are legal, and you have thought about the human limits here, so what next?

What about the weather?

Read Jan's mail for that.............

And then your engine and prop.

To expect your two stroke motor to function at heights of more than 2000 more or less than the Main Jet you have installed, is unreasonable.

So, organise a buddy(a real 'maar-gat') to sit in the back so that he can change your jets for you on the way up. etc., etc., etc.,

In a nutshell, there are days, most probably only vultures, eagles and hang-gliders will be in the air, that you can expect to get to 12 000 using your ol' Rotax 503 and one tank of fuel.

About Indicated Airspeed, Calibrated Airspeed, True Airspeed etc., and how this relates to stall speed, etc., Go ask your instructor to explain, and if he can't, 'skop' him between the eyes, and then ask FundiFF.

Talk later,

David
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Email: lamercyfly@gmail.com
Mobile: +27 (0)746495744
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ACE
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Postby ACE » Thu Jul 28, 2005 8:31 am

Morning David and Anette,

Touching on the advanced flying aspect, I read your posting on the Yahoo SA Micro site after the Brits accident wherein you mentioned an advanced microlighting course including doing a total strip-down of one's wing and then re-assembling it, preferably with one's own trike.

I for one reckon this is a damn good idea, especially since my own training did not go into this too much, I will definitely contact you when I'm next down in Natal.

Regards.
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Postby Fairy Flycatcher » Thu Jul 28, 2005 8:56 am

Not sure if this was meant to be a PM, but thanks in any case.

We are putting together a "Trike Master" course, and later a general "Flight Master" (for all microlights) or mayby "Sky Master" not sure what name yet, and as soon as we have finalised the sylabus, and where it fits into MISASA's ARO, we will start advertising. I am hoping for mid October
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