Kay
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How Come Rain Doesn't Affect Lift?

Thu May 31, 2012 2:45 pm

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!
 
flipdewaf
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RE: How Come Rain Doesn't Affect Lift?

Thu May 31, 2012 3:12 pm

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
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jollo
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RE: How Come Rain Doesn't Affect Lift?

Thu May 31, 2012 4:56 pm

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)?
 
GST
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RE: How Come Rain Doesn't Affect Lift?

Thu May 31, 2012 5:20 pm

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.
 
tdscanuck
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RE: How Come Rain Doesn't Affect Lift?

Thu May 31, 2012 6:46 pm

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.
 
David L
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RE: How Come Rain Doesn't Affect Lift?

Thu May 31, 2012 6:46 pm

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.  
 
jollo
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RE: How Come Rain Doesn't Affect Lift?

Thu May 31, 2012 8:37 pm

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)?
 
tdscanuck
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RE: How Come Rain Doesn't Affect Lift?

Thu May 31, 2012 9:30 pm

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.
 
SlamClick
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RE: How Come Rain Doesn't Affect Lift?

Thu May 31, 2012 11:44 pm

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?)
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Starlionblue
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RE: How Come Rain Doesn't Affect Lift?

Fri Jun 01, 2012 12:40 am

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." - John Ringo
 
vikkyvik
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RE: How Come Rain Doesn't Affect Lift?

Fri Jun 01, 2012 3:51 am

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?   
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
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glen
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RE: How Come Rain Doesn't Affect Lift?

Fri Jun 01, 2012 8:58 am

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]
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Kay
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RE: How Come Rain Doesn't Affect Lift?

Fri Jun 01, 2012 12:40 pm

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..?
 
GST
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RE: How Come Rain Doesn't Affect Lift?

Fri Jun 01, 2012 2:12 pm

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]
 
Kay
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RE: How Come Rain Doesn't Affect Lift?

Fri Jun 01, 2012 2:22 pm

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  
 
tdscanuck
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RE: How Come Rain Doesn't Affect Lift?

Fri Jun 01, 2012 4:33 pm

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.
 
SlamClick
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RE: How Come Rain Doesn't Affect Lift?

Fri Jun 01, 2012 5:18 pm

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.
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vikkyvik
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RE: How Come Rain Doesn't Affect Lift?

Fri Jun 01, 2012 5:26 pm

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.
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soon7x7
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RE: How Come Rain Doesn't Affect Lift?

Sun Jun 03, 2012 11:25 am

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!   
 
Kay
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RE: How Come Rain Doesn't Affect Lift?

Sun Jun 03, 2012 4:41 pm

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!
 
vikkyvik
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RE: How Come Rain Doesn't Affect Lift?

Sun Jun 03, 2012 11:59 pm

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.
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