View Full Version : Performance in clouds

Chuffer Chadley
13th Jun 2007, 14:41

Here's a little question that I haven't been able to find a satisfactory answer to so far...

Why does IAS (in my heavy turboprop) increase in cloud?

I've noticed this particularly when in the descent (at Vmo, yes, clacker, capt gets a beer). I understand the following to be true:

1) Humid air is less dense than dry air, and clouds are humid.
2) IAS is a function of density, increasing with increasing density.
3) Turbine thrust also increases with density.
4) Some high-performance jet aircraft (eg Sea Harrier) use(d?) water spray into the intake to increase power at critical phases of flight.

Remembering that I have observed IAS increase when entering a cloud, how do we reconcile the above point 1 with the others? It occurs to me that a cloud is humid (low density) air, with water droplets, so the density of the air+water needs to be considered. But I might be wrong about that.

Can anyone help with my conundrum?


13th Jun 2007, 15:19

As t point 4..the water injection into a jet engine is to lower the EGT, as to the rest waiting for the site's gurus to post their say:hmm:

13th Jun 2007, 21:40
We get the same in the ATR. My understanding is that when entering clouds from above one usually crosses temperature inversion (as this is where clouds usually end). Since the air in the cloud is colder than above, the engine power increases. Also, when entering convective clouds, one usually encounters an updrought, which generally causes an increase in speed... Stand by to be corrected though!

13th Jun 2007, 22:52
What type of clouds are they? If they have significant vertical development, that would indicate a rising airmass. In order to maintain altitude in such clouds you would need to pitch down, which would have the side effect of increasing airspeed due to reduced induced drag. Effectively you are getting free energy from the rising air.

Chuffer Chadley
14th Jun 2007, 13:03
Good stuff!

OK- the bit about cooling EGT makes sense- lower temp, higher density (all other factors being equal). And we've all observed a temperature drop entering a cloud.

As to the updraught, there might well be something in it, however, I think I've noticed the effect on stratoform type clouds as well.

So, in summary- the density of the air in the cloud is relatively high, due to the temperature difference.

Anything else to add, or is that a complete solution? I'm not sure!


16th Jun 2007, 16:43
Agree with above posters.

I think the temp.reduction and updraught in cloud would mainly explain the IAS increase.

I think the humidity/moist air is less dense argument doesn't hold water (sorry). But here's why:-

Cold air is basically dry. And we fly airplanes mainly in cold air.

Even at 30 deg C, air can only be occupied by 4% volume of H2O molecules. This means a density difference of just 1% between 0 and 90% relative humidity. This translates to an IAS difference of 0.5 %.

Air at -40 deg C can only be occupied by 0.2% volume of H2O molecules. This means that even 100% saturated air at this temp. is virtually the same density as dry air, so virtually the same IAS.

What about the water/ice droplets in the cloud then?

Well, they have an insignificant effect on the gas (air) behaviour. Water occupies 1/800th the volume of air, so the presence of suspended droplets hardly changes the air density.

Temp. reduction in cloud though:-
Say you enter a cloud and the temp drops by 10 deg C. Not uncommon, sometimes more. eg temp 20 deg C, =293k
293/283= 1.035. A 3.5 % decrease. thus a 3.5% increase in density.

Water ingestion in jets.
This has been discussed before here.