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How Come Rain Doesn't Affect Lift?  
User currently offlineKay From France, joined Mar 2002, 1884 posts, RR: 3
Posted (2 years 1 month 3 weeks 6 days 21 hours ago) and read 4987 times:

As per the title

I was flying a light GA aircraft the other day under torrential rain. Wondering how the weight of water slamming against the wings doesn't affect lift, or how the performance of the aircraft in general never seems affected despite the force of the water, when other weather factors have significant implications on the behavior of the aircraft, etc

I always found this interesting, what is the answer?

Thanks!

20 replies: All unread, jump to last
 
User currently offlineflipdewaf From United Kingdom, joined Jul 2006, 1562 posts, RR: 1
Reply 1, posted (2 years 1 month 3 weeks 6 days 20 hours ago) and read 4974 times:
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Lets do the Maths on it,

F=Ma (basically means force is equal to the rate of change of momentum)

Wiki tells me that "violent rain" (blackadder would suggest a hat) is >2"/hr but we'll pput that in sensible units because this is 21st century so 1.38*10^-5m/s.

The projected upper surface of a GA plane I will estimate at ~15m^2.

Rate of accumulation * Area of aircraft
1.38*10^-5 * 15 = 2.08*10^-4m^3/s (this is the volume of rain that falls on the aircraft every second)

Volumeetric rate * Density
2.08*10^-4 * 1000 = 0.208kg/s Mass of water hitting the aircraft per second

The velocity of a rain drop is ~10ms^-1

Mass flow rate * velocity = momentum
If we assume that all the rain stops moving when it hits the airframe and so the momentum becomes effectively 0 then the change in momenum and hence force becomes:

0.208 * 10 = 2.08N

This is less than the difference of whether you remembered to go for a leek before you left or not.

Hope that helps.

Fred


User currently offlinejollo From Italy, joined Aug 2011, 212 posts, RR: 0
Reply 2, posted (2 years 1 month 3 weeks 6 days 18 hours ago) and read 4892 times:

Quoting flipdewaf (Reply 1):
we'll pput that in sensible units because this is 21st century

Am I hallucinating? An Englishman putting down in writing that SI units are the sensible choice for doing "the Maths"?   Hats off...

What about relative velocity? In your analysis, the a/c is parked on the ground under pouring rain; what about flying through at, say, 130 knots (or about 150 mph, or, in "sensible" units, circa 67 ms^-1)?


User currently offlineGST From United Kingdom, joined Jun 2008, 930 posts, RR: 0
Reply 3, posted (2 years 1 month 3 weeks 6 days 18 hours ago) and read 4857 times:

It could also be that many light aircraft are conservative low performance designs, devoid of features like aerofoils highly optimised for laminar flow over as much as the surface as possible. This is in stark contrast to many modern gliders, whose wings are extremely susceptible to water drops causing significant loss of performance.

This is nothing to do with the energy of the rain so much as the aerodynamic effects of "roughness" from thousands of raindrops on the wings. One moment you can be flying a high perfomance efficiency machine with dry wings, and the next have performance more like an unusually aerodynamic brick. But this is a quirk of some very high performance aerofoils, probably unlike what the plane you were flying in had.


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 4, posted (2 years 1 month 3 weeks 6 days 17 hours ago) and read 4771 times:

Quoting jollo (Reply 2):
What about relative velocity? In your analysis, the a/c is parked on the ground under pouring rain; what about flying through at, say, 130 knots (or about 150 mph, or, in "sensible" units, circa 67 ms^-1)

Most of your velocity is forward...the forward projected area of the airfoil exposed to the rain is very small, so the resultant force is even smaller than what you get from above (and shows up as drag, not as weight).

If you have significant vertical velocity relative to the rain then that would show up as an effective weight change but it would have to be absolutely enourmous (bigger than normal forward airspeeds) to make a meaningful difference.

Tom.


User currently offlineDavid L From United Kingdom, joined May 1999, 9523 posts, RR: 42
Reply 5, posted (2 years 1 month 3 weeks 6 days 17 hours ago) and read 4771 times:

Quoting flipdewaf (Reply 1):
Wiki tells me that "violent rain" (blackadder would suggest a hat) is >2"/hr but we'll pput that in sensible units

Ah, you went on to clarify. I was chomping at the bit to point out that "inches per hour" is not the velocity of the rainfall.   

Quoting jollo (Reply 2):
Am I hallucinating? An Englishman putting down in writing that SI units are the sensible choice for doing "the Maths"? Hats off...

I hate to break it to you but we've preferred SI units in science and most engineering for a loong time now. The old imperial stuff had almost disappeared from the curriculum by the time I went to school, and that wasn't exactly yesterday, and was completely gone by the time I moved up to senior school.  


User currently offlinejollo From Italy, joined Aug 2011, 212 posts, RR: 0
Reply 6, posted (2 years 1 month 3 weeks 6 days 15 hours ago) and read 4692 times:

Quoting David L (Reply 5):
The old imperial stuff had almost disappeared from the curriculum by the time I went to school, and that wasn't exactly yesterday, and was completely gone by the time I moved up to senior school.

You would be surprised at what some of your fellow countrymen say on the subject in 2012! Along with a lot of Americans, of course...

Quoting tdscanuck (Reply 4):
Most of your velocity is forward...the forward projected area of the airfoil exposed to the rain is very small, so the resultant force is even smaller than what you get from above (and shows up as drag, not as weight).

Makes sense. Does the incleased drag show, for example, in increased engine settings to maintain speed and level flight on an airliner (with larger frontal section and at higher speeds)?


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 7, posted (2 years 1 month 3 weeks 6 days 14 hours ago) and read 4648 times:

Quoting jollo (Reply 6):
Does the incleased drag show, for example, in increased engine settings to maintain speed and level flight on an airliner (with larger frontal section and at higher speeds)?

Theoretically yes, you would need more thrust to maintain speed. However, compared to the overall aircraft drag it's so small that I suspect you're in the noise of the measurements. NAMS (Nautical Air Mileage) testing might be sensitive enough to pick it up but they always do that in clear air (maybe because of rain?) so I don't really now.

Tom.


User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 8, posted (2 years 1 month 3 weeks 6 days 12 hours ago) and read 4539 times:

Gut feeling: The cooling effect rain has on the air it is falling through, plus the de facto water injection in the engine more than offset any negative effects from rain.

Back around the late 70s a very well educated bunch of people argued seriously in public forums that the roughing of the wing surface by the splashes would kill ALL lift and that an airplane could not fly in the rain. They used ultra high speed photographs of the splashes then assumed that the air had to flow AROUND that shape instead of that shape being borne along in the air, as it is. Of course empirical evidence was initially disregarded. (anyone here ever flown in the rain?)



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently onlineStarlionblue From Greenland, joined Feb 2004, 16998 posts, RR: 67
Reply 9, posted (2 years 1 month 3 weeks 6 days 11 hours ago) and read 4509 times:

Quoting SlamClick (Reply 8):
Back around the late 70s a very well educated bunch of people argued seriously in public forums that the roughing of the wing surface by the splashes would kill ALL lift and that an airplane could not fly in the rain. They used ultra high speed photographs of the splashes then assumed that the air had to flow AROUND that shape instead of that shape being borne along in the air, as it is. Of course empirical evidence was initially disregarded. (anyone here ever flown in the rain?)

Whoa. You mean there were public forums before the web? 

Empirical evidence being the fact that planes flew routinely in rain I suppose. Some people...



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinevikkyvik From United States of America, joined Jul 2003, 9776 posts, RR: 27
Reply 10, posted (2 years 1 month 3 weeks 6 days 8 hours ago) and read 4401 times:
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Quoting David L (Reply 5):

Ah, you went on to clarify. I was chomping at the bit to point out that "inches per hour" is not the velocity of the rainfall.

Ha, so was I.

Quoting jollo (Reply 6):
Along with a lot of Americans, of course...

Although most of my work experience has been in inches and pounds, etc., I much prefer doing calculations in metric. I wish we were more of a metric society.

Quoting SlamClick (Reply 8):
Of course empirical evidence was initially disregarded.

I was going to say, did no one ever fly in the rain prior to the late 70s?   



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineglen From Switzerland, joined Jun 2005, 221 posts, RR: 2
Reply 11, posted (2 years 1 month 3 weeks 6 days 2 hours ago) and read 4260 times:

More critical in rain however are laminar profiles as used on gliders. Especially the first generation laminar profiles developed in the 70's are very sensitive to any soiling, because it disturbs the laminar flow. While bugs on the profile are the bigger concern (they stay there for the whole day..., and usually you don't fly in rain with gliders), but flying through a rain shower degrades the performance drastically until the profile has dried up again.
We had a DG200 in our club. It was a very nice plane, but flying through a rain shower with it could ruin your day. And taking off with a wet wing (for example for landing training on a day with some light drizzle) was a nearly suicidal idea.

[Edited 2012-06-01 02:01:03]


"The horizon of many people is a circle with zero radius which they call their point of view." - Albert Einstein
User currently offlineKay From France, joined Mar 2002, 1884 posts, RR: 3
Reply 12, posted (2 years 1 month 3 weeks 5 days 23 hours ago) and read 4162 times:

Thanks for all the replies. Refering to the points made above, am I right that what is being said is:


1- water doesn't make it through the airfoil and breaks up/condensates without coming between the airfoil and the wing?
2- downforce of all the water is negligeable in any case
3- the fact that the airplane is travelling at speed reduces all effects of water anyway because most real contact takes place in an area as wide as the leading edge only, rather than the wing surface (like when you drive a convertible car at speed in the rain, not much water enters the cockpit)

But then:

1- given that the lightest chop of turbulence makes the aircraft wobble in the air and you you feel every movement directly in the yoke, how come the heaviest rain, or no rain whatsoever, makes no difference on the yoke and on the feel of the aircraft? even if we are only talking about the water slamming against the windscreen (forget the flying surfaces).

2- Very interesting point regardling laminar airfoils. But equally interesting is that the effect of rain is reduced to zero in real life if the airfoil isn't laminar..?


User currently offlineGST From United Kingdom, joined Jun 2008, 930 posts, RR: 0
Reply 13, posted (2 years 1 month 3 weeks 5 days 21 hours ago) and read 4116 times:

Quoting Kay (Reply 12):

1- water doesn't make it through the airfoil and breaks up/condensates without coming between the airfoil and the wing?

Perhaps I'm not understanding you properly, but just to confirm you understand that the aerofoil is simply the cross sectional shape of the wing., There is no "between" the aerofoil and the wing, aerofoils are merely used to describe and define the shape of a wing. There are hundreds designs used for different things, and probably thousands that have been superseded by newer versions. Often aircraft do not have the same aerofoil right across the wing, the root and tip for example may have different aerofoils defined with a smooth blend between them.

Just a small portion of the aerofoils of the world, and details of what they are used for http://www.ae.illinois.edu/m-selig/ads/coord_database.html

Quoting Kay (Reply 12):

2- Very interesting point regardling laminar airfoils. But equally interesting is that the effect of rain is reduced to zero in real life if the airfoil isn't laminar..?

It isn't that the effect is reduced to zero, just that it is reduced. I have little power flying experience so I wouldn't know how the feel of an aircraft changes with rain if you have an engine, but I know from flying older gliders (pre-laminar flow wings) out of light rain that performance was degraded noticably. Once I had to land in a serious downpour that suddenly started and barely needed airbrakes on finals such was the performance depreciation.

[Edited 2012-06-01 07:12:54]

User currently offlineKay From France, joined Mar 2002, 1884 posts, RR: 3
Reply 14, posted (2 years 1 month 3 weeks 5 days 21 hours ago) and read 4097 times:

Quoting GST (Reply 13):
Perhaps I'm not understanding you properly, but just to confirm you understand that the aerofoil is simply the cross sectional shape of the wing., There is no "between" the aerofoil and the wing, aerofoils are merely used to describe and define the shape of a wing. There are hundreds designs used for different things, and probably thousands that have been superseded by newer versions. Often aircraft do not have the same aerofoil right across the wing, the root and tip for example may have different aerofoils defined with a smooth blend between them.

I see, my mistake, I actually meant the air flow (not airfoil) that flows over the wing  


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 15, posted (2 years 1 month 3 weeks 5 days 19 hours ago) and read 4050 times:

Quoting Kay (Reply 12):
1- given that the lightest chop of turbulence makes the aircraft wobble in the air and you you feel every movement directly in the yoke, how come the heaviest rain, or no rain whatsoever, makes no difference on the yoke and on the feel of the aircraft?

Turbulence alters the airspeed over the entire wing. Since lift is proportional to airspeed squared, even small changes in airspeed make very significant changes in lift. The impact of rain is extremely small relative to the size of the forces involved in turbulence.

Tom.


User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 16, posted (2 years 1 month 3 weeks 5 days 18 hours ago) and read 4018 times:

Light chop and turbulence should not affect airspeed in any meaningful way. Wind shear from various causes will. Light turbulence can be thought of as a micro version of the winds you are flying in. Just as a plane flying a heading and speed through a mass of air that has its own velocity vector over the surface, the course and distance made good will be the sum of these vectors. So it is in light atmospheric turbulence. The plane is flying along smoothly through a parcel of air when that air itself suddenly gains a vertical component. The mass of the airplane just cannot respond to the sudden change of direction. You are out the other side of that perturbation almost as soon as you feel it.


Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlinevikkyvik From United States of America, joined Jul 2003, 9776 posts, RR: 27
Reply 17, posted (2 years 1 month 3 weeks 5 days 18 hours ago) and read 4010 times:
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Quoting Kay (Reply 12):
1- water doesn't make it through the airfoil and breaks up/condensates without coming between the airfoil and the wing?

Not sure what you mean - rain certainly hits the wing. It's path would be altered by the airflow around the wing, but I doubt it would completely miss the wing.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlinesoon7x7 From , joined Dec 1969, posts, RR:
Reply 18, posted (2 years 1 month 3 weeks 4 days ago) and read 3535 times:

Quoting glen (Reply 11):
and usually you don't fly in rain with gliders


I flew for a commercial glider operation and we did fly in rain, of course you can have rain along with VFR conditions however from time to time we would encounter local down pours which when entered, I would notice the variometer dip negative. I don't feel it was the weight of the water as much as the rain itself forced the ambient air down with it. I have noticed the same while flying single engines. As far as the "big bang theories math on top," you guys gotta get outside more often. I'll never look at rain drops the same again!   


User currently offlineKay From France, joined Mar 2002, 1884 posts, RR: 3
Reply 19, posted (2 years 1 month 3 weeks 3 days 19 hours ago) and read 3428 times:

Quoting vikkyvik (Reply 17):
Not sure what you mean - rain certainly hits the wing. It's path would be altered by the airflow around the wing, but I doubt it would completely miss the wing.

But then, that is exactly what I wonder. How much percent of the surface of the wing is disrupted/covered in uneven rain drops that disrupt air flow, at any given time, it must be quite a percentage (say 30% as a rough average?).So, regardless of the very weak force of the water, physically we don't have the required shape for a large portion of the wing!


User currently offlinevikkyvik From United States of America, joined Jul 2003, 9776 posts, RR: 27
Reply 20, posted (2 years 1 month 3 weeks 3 days 11 hours ago) and read 3309 times:
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Quoting Kay (Reply 19):

I see what you're saying. For laminar flow gliders, this may be an issue as others have stated. For airliners and such, most of the boundary layer is turbulent anyway, and I doubt whatever drops of water will have much effect. They'll likely get flattened into thin sheets/streams anyway. The overall effect on the airfoil shape would be really small.



"Two and a Half Men" was filmed in front of a live ostrich.
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