Please clear up density altitude for me

[Thomas B]

... I realize that at some future date the FAA will in all likelihood require all UAS pilots to test but I suspect the test for recreational piloting will be somewhat less stringent.

You stated the reason well and also expressed my hopes, not for me alone, but for all recreational pilots. Thanks for the reply, appreciate the thoughts.

 

[FoxhallGH]

In my part 107 studying I’ve been so confused by density altitude. Here’s a question that always confuses me..

What effect does high density altitude have on the efficiency of a UA propeller?

Answer: Performance is decreased.

Does the word high refer to the density of the air or the altitude? I took this to mean the air is more dense which, I think, would increase performance.

This is not a clear question - even when another country's version of English is being taken into account!
For me, an altitude with high air density, is sea level!
High altitudes of course have lower air density ...
Based on the answer, the question should read "What effect does low density air at high altitude have on the efficiency of a UA propeller?"

 

[Cymruflyer]

Understand the facts stated. Perhaps what I should have stated is something like, the test should have a version for recreational use or maybe a separate rest for drones weighing less than a pound.
I still fail to see the relavence or the usefulness of an air density question to pilots of drones like the Mavic series, Spark, etc., especially since by law altitude is limited to 400ft. and, as you note, the battery weighs the same (+ or - a few electrons) throughout any flight.
Not arguing against the law or getting certified, arguing the use of questions completely irrelevant to the recreational drone pilot.

Altitude to a small point is not too important once you have taken off in a real aircraft. The part that density altitude plays for piloting decision making is, do I have the runway length I need to safely take off, given the all up weight of the aircraft, and the prop type and size and the engine HP for this flight, given what the ambient temp will be when I am ready to give full power to take off. All that is relative to a pilot, and of course I am not speaking of a drone pilot here, but mention this to help you understand the importance of density altitude equations.

Once you are off the ground and clear of anything in front, you can slowly gain altitude and go to where you are heading. The altitude makes no difference. However, with that said, let's talk about altitude. Your comment also means you may not be aware of the temp difference at altitude. Since you are only going up to 400ft, there will be no appreciable difference, but then sometimes there is, because you may find there was an inversion layer just above you and you might climb into a temp difference of 10+ degrees. I have taken off in an open cockpit aircraft in Colorado in the winter and it was freezing, yet within about 400 or so feet during climb out, I found the ambient temp increased so much for a little while between altitudes, that I could comfortably take off my gloves and fly. Now as I continued to climb, I suddenly went through that inversion layer and it got extremely cold again, but for those that are not aware, an inversion layer does happen.

However, normally as you climb the temp decreases, thereby slowly increasing your performance as you climb, above what you experienced at take off (density altitude). So it just gets better, but then you also have altitude increase to account for and the drop off in performance, when flying high in the Rockies for example, so it is always a trade off in real flight.

It is never a bad idea to learn more about aviation, despite the fact that you will just be flying a very light toy sort of drone, so knowing about density altitude is a good thing, so that you can understand that it probably will not make any difference to your flight. And that is a plus, right?

 

[Thomas B]

Altitude to a small point is not too important once you have taken off in a real aircraft. The part that density altitude plays for piloting decision making is, do I have the runway length I need to safely take off, given the all up weight of the aircraft, and the prop type and size and the engine HP for this flight, given what the ambient temp will be when I am ready to give full power to take off. All that is relative to a pilot, and of course I am not speaking of a drone pilot here, ...
...so that you can understand that it probably will not make an difference to your flight....

My point is your summary. No need to ask a question requiring a lot of study and math when it won’t make a difference to a given type of pilot taking the test. Why I hope the FAA will devise a test for recreational drone pilots.

Appreciate the excellent summary of the relavence to the pilot of fixed wing aircraft.

 

[Gunteacher]

Exactly! Your question is very valid.
The question should be phrased "What effect does "high altitude have" on the efficiency of a UA propeller?

high density altitude seems to be an oxymoron. At high altitude the air is less dense.

"Aircraft pilots know this.... Lower air density penalizes pilots in three ways: The lifting force on an airplane’s wings or helicopter’s rotor decreases, the power produced by the engine decreases, and the thrust of a propeller, rotor or jet engine decreases. These performance losses more than offset the reduced drag on the aircraft in less dense air. "
We don't have worry about combustion engine power loss, only propeller lift force loss.


Good point.
I guess the only thing comes to mind is that the Mavic will compensate, but the battery life will be shorter.

So it seems that the shorter battery life would impact mission planning such as mapping and other longer missions at greater distances. The cause and effect are the same whether you are flying a small drone or a manned aircraft it seems. Physics does not care. While this may be unimportant to a recreational flier, a commercial operator may need to plan around this performance degradation to accomplish his mission.

Again, the Part 107 Certificate covers all commercial drone operations for drones under 55 pounds regardless. If you are only interested in recreational flying, don't test for the Part 1107 Certificate. If however, you are using the drone in furtherance of a business, then this certificate is required and the penalties are steep. That is the current reality of our regulation. There is good news however on that front as it seems the FAA is looking at the waiver process to allow an easier application and approval process for Commercial operators to allow night flight and flight over people. A very common request. JMHO

 

[Pontoon-pilot]

No sounds like you don't understand the real world. I suppose you will also say a solution to being poor is to simply come up with lots of money!!!

Denver airport, the old one, could not have a Europe bound 747 take off after about 11:30am in summer because it would never get off the runway. Therefore, if the flight encounter delays in the morning, it simply had to be cancelled and moved to the following morning for departure. What is your simple solution for that scenario please? I know, relocate Denver to New Orleans, right?

Cymruflyer,
You may have misunderstood my response, it was intended as a joke, my suggestions to fix the problem (related to short runway at high altitude) was not easily attenable.
You used your experience flying airplanes at high altitude to explain why it is detrimental to piston-engine powered aircraft on a short runway. I respect that, but we are droning in here... again I am just humoring myself, forgive me.

 

[Former Member]

Altitude to a small point is not too important once you have taken off in a real aircraft. The part that density altitude plays for piloting decision making is, do I have the runway length I need to safely take off, given the all up weight of the aircraft, and the prop type and size and the engine HP for this flight, given what the ambient temp will be when I am ready to give full power to take off. All that is relative to a pilot, and of course I am not speaking of a drone pilot here, but mention this to help you understand the importance of density altitude equations.

Once you are off the ground and clear of anything in front, you can slowly gain altitude and go to where you are heading. The altitude makes no difference. However, with that said, let's talk about altitude. Your comment also means you may not be aware of the temp difference at altitude. Since you are only going up to 400ft, there will be no appreciable difference, but then sometimes there is, because you may find there was an inversion layer just above you and you might climb into a temp difference of 10+ degrees. I have taken off in an open cockpit aircraft in Colorado in the winter and it was freezing, yet within about 400 or so feet during climb out, I found the ambient temp increased so much for a little while between altitudes, that I could comfortably take off my gloves and fly. Now as I continued to climb, I suddenly went through that inversion layer and it got extremely cold again, but for those that are not aware, an inversion layer does happen.

However, normally as you climb the temp decreases, thereby slowly increasing your performance as you climb, above what you experienced at take off (density altitude). So it just gets better, but then you also have altitude increase to account for and the drop off in performance, when flying high in the Rockies for example, so it is always a trade off in real flight.

It is never a bad idea to learn more about aviation, despite the fact that you will just be flying a very light toy sort of drone, so knowing about density altitude is a good thing, so that you can understand that it probably will not make any difference to your flight. And that is a plus, right?

Aside from the issue you mention, what are some other hazards of high density altitude operations?

 

[AnzacJack]

No major hazards for multirotors, just reduced efficiency

 

[FoxhallGH]

Aside from the issue you mention, what are some other hazards of high density altitude operations?

At high operating altitudes where the air is thinner, you can have:
- Slower rate of climb
- Faster rate of descent
- Slower roll rate
- Slower yaw rate
- Possibility of icing on the prop's if you fly through cloud ...
And really cold hands - unless you have a good set of touch-sensitive gloves!

 

[FoxhallGH]

OK - after doing a bit of research, it appears that this phrase 'Density Altitude' has a definite meaning in aviation circles (apologies to pilots out there that already know this ...).

There is a very good article about what the phrase actually means here ... Density Altitude - AOPA

In summary ... "Density altitude is pressure altitude corrected for nonstandard temperature. As temperature and altitude increase, air density decreases. In a sense, it's the altitude at which the airplane "feels" its flying."

In other words, if it is a hot day, and air pressure is low - then although you may be flying from a point that's 1,000 ft ASL, it may be like flying from a point that's at 2,000 ft in 'normal' air.

 

[Cymruflyer]

OK - after doing a bit of research, it appears that this phrase 'Density Altitude' has a definite meaning in aviation circles (apologies to pilots out there that already know this ...).

There is a very good article about what the phrase actually means here ... Density Altitude - AOPA

In summary ... "Density altitude is pressure altitude corrected for nonstandard temperature. As temperature and altitude increase, air density decreases. In a sense, it's the altitude at which the airplane "feels" its flying."

In other words, if it is a hot day, and air pressure is low - then although you may be flying from a point that's 1,000 ft ASL, it may be like flying from a point that's at 2,000 ft in 'normal' air.

I believe that I covered that a while ago, because I thought that is what we were actually talking about. Density altitude was something we had to take seriously every day in the summer flying up in the Rockies. That is also why I was explaining about a 747 needing to take off before the ambient temp got up in later morning, from Denver airport. Because it would not be getting off the runway if it waited too much longer if there was a delay, and the flight would be cancelled until the following morning. Density altitude is the "Effective" altitude at which you will be taking off, when ambient temp and altitude is calculated in, along with aircraft performance, weight being carried and runway length available.

If you recall reading, I also wrote about flatlander pilots with low wing aircraft, who would land at high altitude, 8,000ft plus and then when taking off on a hot morning, set their flaps to lift off and think they were really flying, but were actually still in ground effect, then clear the runway at the end, where the ground dropped away a bit and dump their flaps to clean up the airplane, still thinking they were actually flying, only to suddenly find they were now out of lift and just drop away, to sometimes a crash.

Yes, density altitude is something to be taken seriously and properly calculated for prior to a take off, when flying off high altitude runways. That is why we used to put the aircraft away after 9:30am when I lived up in the Rockies, because our runway was not going to be long enough to get off the ground and or clear the fence and the rising ground after the runway ended. It was also why we had a scales to weight a passenger prior to take off on summer mornings, because none of them ever gave their real weight. We had to have the husbands leave the room so the wives could get on the scales without them seeing what the numbers were.

I remember once in Nevada getting on a little 10 seat tourist plane for sight seeing, where they would ask and make a note of what weight you were when boarding. The woman in front said she was 145lbs, and me at 165lbs, right behind her thought, ohhhh hellp no. Now I am going to have to say I weigh 225lbs just to even out the missing pounds between us, to keep it a safe flight.

 

[Pontoon-pilot]

For a lame folk, "high density altitude" could mean an altitude where the air has high density.
What is low density altitude then? Lower elevation where the air has higher density, right.

Edit:
high density altitude =high altitude, where air density is low
low density altitude = low altitude where air density is high

what is it called when it is low altitude with low density -hot temperature, with high humidity (Huston tX)

 

[AnzacJack]

Yep, welcome to aviation.

Wait until you get people explains theory of lift. Still many people get this wrong. I have text books that are wrong

 

[FoxhallGH]

My point is your summary. No need to ask a question requiring a lot of study and math when it won’t make a difference to a given type of pilot taking the test. Why I hope the FAA will devise a test for recreational drone pilots.

One thing that this does give us though, is that if there is no 'dumbing down' for Drone Pilots getting part 107 / PfCO - then there will be a bit more respect from regular aviators. The last thing we want is to give an excuse to the Aviation industry to label us as amateurs ...

 

[AnzacJack]

what is it called when it is low altitude with low density -hot temperature, with high humidity (Huston tX)

You would have a higher density altitude. The heat plus low pressure would give you performance equivalent to a higher altitude than what you are at. Basically add 30ft for every hPa under ISA (international standard atmosphere) which is 1013hpa and add 120 ft for every degree Celsius over what it should be at your altitude. (ISA temp is 15 c at sea level and minus 2 c for every 1000ft)
No calculation for humidity, but all humidity over 0% will decrease performance

Remember it’s
Higher “density altitude”
Not
“Higher density” altitude

 

[Cymruflyer]

You would have a higher density altitude. The heat plus low pressure would give you performance equivalent to a higher altitude than what you are at. Basically add 30ft for every hPa under ISA (international standard atmosphere) which is 1013hpa and add 120 ft for every degree Celsius over what it should be at your altitude. (ISA temp is 15 c at sea level and minus 2 c for every 1000ft)
No calculation for humidity, but all humidity over 0% will decrease performance

Remember it’s
Higher “density altitude”
Not
“Higher density” altitude

And that standard pressure of 1013 in the USA would be 29.92 in case anyone was wondering.

 

[AnzacJack]

Yes sorry about that. You guys use different units to us. Your measuring your pressure in inches of Mercury
Or about 14psi

I can’t remember what the lapse rate for inHg is.

 

[Cymruflyer]

Yes sorry about that. You guys use different units to us. Your measuring your pressure in inches of Mercury
Or about 14psi

I can’t remember what the lapse rate for inHg is.

It's about 1 Hg per 1,000 ft.

 

[AnzacJack]

Of course. Much easier for running numbers in head

 

[CAG]

and reduced efficiency of propellers

Not just props but all airfoils. Fixed wing aircraft have less lift and longer take off distances. More than one private pilot who flew in to Grand Canyon airport on a hot day has found that after taking on a full load of fuel that added to the full passenger and baggage load, the pine trees at the end of the runway now present a major hazard because the plane will not get out of ground effect.