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Thermal Updrafts

  • PeterG
  • Topic Author

PeterG created the topic: Thermal Updrafts

Hi all, I was flying solo circuits around Parafield yesterday with interesting weather. A thunderstorm (mild) past through about an hour before and there was still plenty of rain. During the circuits, the plane had a very slow climb rate and a rapid descent rate compared with dryer conditions. Though having a low QNH at 1009, temp at 22 deg and high humidity at 71%, the performance of the engine and aerodynamics can be accouned for during the climb, yet the descent isolated these factors out. Question: Is it fair to say that during dry, warm conditions, thermal updrafts assist the aircraft in the climb and retard the descent on average? With this suggesting the opposite also being true that during wet conditions, the lack of thermal updrafts, the aircraft is less efficient? If this is true, are there calculations available for pre-determining aircraft performance and fuel consumption rate in different weather conditions, in particular wet or dry? I am probably more interested in knowing that i wasn't imagining the effect.
As expected, I am noticing many strange things with flying. This is just one of them.

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  • Richard

Richard replied the topic: Re: Thermal Updrafts

Hi Peter,

Air movement is a complex business and in the situation you describe there'll be multiple factors at play: thermals, as well as temperature / pressure effects on your performance, plus residual turbulence and shear caused by the recent passage of the thunderstorm. In high humidity conditions you can even pick up a bit of carbie ice if you're not careful and that will definitely reduce your performance!
Yes, thermal updraughts will help your climb performance and retard your descent. In an updraught the air you're flying though is ascending. So climbing in an updraught is like walking up a moving up-escalator. To an observer you will ascend very quickly.

If you descend in an updraught, it's like walking down the up-escalator: you can step like mad but to an observer, you'll be descending much more slowly.
No, not really. Wet conditions do not necessarily mean no thermal updraughts. Thermals are triggered by thermal energy: surfaces get heated by the sun and cause uneven heating of air (e.g. the air over a concrete parking lot). The increasing temperature decreases the density of the air and this bubble of warmer air will start to "float" upwards through the relatively cooler, denser air until it reaches a height where it is the same temperature (and density) as the air around it. That's your updraught.

The opposite happens with cooler air. Cool air is denser than warm air and will tend to sink. Now, rain will have an indirect effect in that it tends to cool the air around it (which in turn causes regions of air to sink i.e. downdraughts). However, you can't rule out updraughts just because there is wet weather. You can still get significant thermal formation in wet and humid conditions.

The deciding factor is temperature. Thermal energy is what drives vertical air movement.

As far as aircraft performance is concerned, the main factors affecting performance are temperature and local pressure (e.g. the QNH). Of these two, temperature has the greater effect. They will significantly influence air density and it is air density that will fundamentally affect your performance.

So, the opposite to what you may be surmising is actually true: a warm day will retard overall performance rather than improve it - even if thermals give you a bit of a lift now and then.
Yes, but not for wet/dry conditions since humidity really doesn't have an appreciable effect.

Since it isn't possible to produce figures for all combinations of temperature and pressure, manufacturers calibrate their performance figures against theoretical performance in a standard atmosphere: the International Standard Atmosphere (ISA).

You can use these charts to predict performance but in order to use them you also need to somehow recalibrate the local conditions of the day into equivalent in ISA. This adjustment involves calculating a "density altitude" and there's more on this in the textbook.

Density altitude is basically the altitude in ISA where the air density is the same as the density of the air where you are now. You then use this density altitude to read off the expected performance from the performance charts.

On your example day, 22oC and 1009 hPa parked up at Parafield, your elevation is 57 ft but your density altitude is actually 1017ft! The air you're going to be flying in is actually the same density as the air 1000ft higher in the theoretical standard atmosphere that the manufacturer used in the chart.

If you want to predict your performance for that day, you wouldn't use the figures on the charts for your actual altitude; you would have to use the figures for 1000ft higher than that. So, if you're in the circuit at 1000ft your aircraft will behave as if it's actually at 2000ft!

To summarise then, getting back to your questions:

* On a warm day, thermal updraughts will indeed affect your climb and descent but they are only transient local conditions and you can't plan on them.

* High humidity will theoretically reduce your performance but really nothing worth writing home about. Temperature is the main factor.

* Yes you can use charts to predict your expected performance on the day but you need to calculate a density altitude for your local conditions and use that in the manufacturers charts. These figures will ignore the effects of humidity.

Hope that helps :)



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bobtait replied the topic: Re: Thermal Updrafts

More storms like that today will save the Australian cricket team from almost certain defeat in Adelaide. It seems the weather certainly has an effect on the performance of a cricket team!

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  • thomasha

thomasha replied the topic: Re: Thermal Updrafts

Thanks for the great article.

I take it from the article that air density and wind speed (relative to the ground) are the most critical influencers of total fuel consumption.

If this is the case, do commercial airlines actively chase tailwinds or low density air?

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  • Richard

Richard replied the topic: Re: Thermal Updrafts

Hi Thomasha, welcome to the forums!

Short answer: it depends.

Now for the long answer ;) Favourable wind conditions are always a bonus when flight planning. Getting the best possible ground speed (your actual speed over the ground) will minimise your time in cruise and thereby minimise the fuel you need for the cruise.

In fact, choosing appropriate flight levels was a skill tested in the earlier commercial exams. There you were expected to flight plan a run choosing the most efficient altitude for cruise taking into consideration winds, fuel burn to get to altitude etc etc etc.

As for modern airliners, they are usually either jet or turboprop nowadays. These engines, (especially the jets) are most efficient at high altitude and have ghastly fuel burns at the altitudes we normally fly around in.

Also, on a hot day, it may not be possible to efficiently reach some flight levels which could be used on a colder day.

Flight levels will be chosen for favourable wind conditions but they still have to abide by the flight level requirement for the direction they're flying (see AIP ENR 1.7-10 ).

To complicate things further, the aircraft load will also have a limiting effect on how high the aircraft can cruise. So it also has to be taken into account when calculating the most appropriate flight level.

By the way, since an aircraft burns fuel during cruise, especially during a long flight, you'll find them doing "step climbs". As fuel burns off, they get lighter and eventually reach a weight that lets them climb to the next legal flight level.

So, to finally get around to answering your question, the airlines need high altitude for engine efficiency. They will certainly take wind conditions into account when calculating the most efficient flight level but avoiding "low density air" caused by hot temperatures or low pressure systems is not something they have much control over. They just have to factor in the performance hit when calculating take-off, climb, max flight level and landing performance.

What the airlines are chasing is the most efficient balance between all these factors.



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  • thomasha

thomasha replied the topic: Re: Thermal Updrafts

Thanks Richard,

I appreciate the thorough answer. Thanks for taking the time to write it up.


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  • Richard

Richard replied the topic: Re: Thermal Updrafts

No worries. Have fun :)

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