Aerofoils in heavy rain
 

Away from the breathless excitement of R&N and the Bali thread, what do the experts think of this? ('Avoid' if possible is taken as written......)

Looking at a few papers on the topic, I see figures for an 'up to 30% loss of lift at max alpha', a significant reduction in stall angle and a big increase in drag, particularly on a 'dirty' wing, all of which degrade the g/a performance. If you chuck in the simple downforce exerted by a heavy downpour which increases a/c weight...................................

I believe there has been 'advice' in the past on increasing speeds on approach in these conditions, but I am not aware of any specific guidance in Ops Manuals (in my time). Is it in place now? Should it be?

I think there was a brief discussion a few years back on PPrune.

This thread?

http://www.pprune.org/tech-log/10919...in-effect.html

Yep - that's the boy, thanks. Looks like Davies was wrong.

He should have asked a glider pilot then, they knew all along it has an effect, in some cases quite dramatic.

OMG! Those flying airliners with laminar aeroprofiles in torrential rain are doooooomed! :eek:

BOAC's question was answered by a glider pilot. :ok:

I hear you, the aircraft in the UK should be falling off the sky:E
Seriously,why fly through such heavy rain in the first place?
If downdraft were so so strong,wouldnt the PWS warned them of it?did they disregard it?did they usually disregard it?

Interesting point about gliders.

I remember back in the '70s when the first glassfibre sailplanes were coming along that a slope-soaring Kestrel 19 had to land out when a rain shower came along and reduced lift to such an extent that the aircraft could no longer maintain lift on the slope and was too low to complete a circuit to land (at Challock 1976).

The effect of rain on the performance of the laminar flow glassfibre wing was significant.

KR

FOK

- trying to keep this thread focussed. Discussions about Bali on the Bali thread perhaps?

EDIT: Before you all relegate this to a 'gliding' problem, I suggest you do a bit of searching for research work on jet airliner wing sections.

... with my B737 hat on, I don't remember any guidance for adjustment of approach speeds due to precipitation, just the usual allowance for gusts up to a maximum figure.

Nor do I remember any talk during training of any significant adverse effects of rain on jet airliner wings?

Microbursts yes, but the direct effect of any rain associated with them, no.

Maybe 'scale' and speed dictate the likelihood of adverse affects due to rain rather than laminar flow aerofoil sections as such? - Big fast jets against slow small gliders?

KR

FOK :)

FOK - that is my point - there is no guidance. Have a look at this (wind tunnel) test by NASA from 1992 http://www.cs.odu.edu/~mln/ltrs-pdfs/tp3184.pdf

The theory of the effect may be fine, but the time duration of the effect equates to little effect for most high energy flight vehicles.

Seems like there should be lots of DFDR data in past accidents to support this ?

- unless, of course, it should happen at the moment critique with energy a little less than 'high'?

Folks, its not time to shoot down DP Davies yet, especially as his advice on necessity of utilizing perspective and balance when reflecting on aeronautical matters has been once again proven to be useful - by this thread.

Yes, rain will affect aerodynamic performance of any airfoil.

Yes, the effect was measured in aerotunnels.

Yes, it is significant on laminar aerofoils such as used on high performance gliders or Rutan's canards.

No, there are no modern airliners using laminar wings. Certain Tolouse based brand claims their planes do but while this might not be entirely marketing hype, there are no incidents showing they lose performance or their control forces/efficiency alters in precip. Mind you: some of their make and models are almost tragically underpowered.

No, there are no airliner accidents or incidents where high rate of precipitation was proven to be decisive factor. Even when extreme rain caused both engines to flame out, aeroplanes were controllable down to ground/water contact, outcome largely dependent on the quality of the terrain below. Anyway, there is not much point in worrying about loss of performance due to wet wing if you find yourself in microburst.

Paper quoted that states there were some a) doesn't provide any reference for the claim b) is published via Azerbaijan based pay-to-publish operation that organizes conferences used just to pad the CVs, despite the claim its journals seem not to be peer reviewed at all and consequently has pretty bad reputation in scientific circles.

Clandestino -

"its not time to shoot down DP Davies yet," - it's just that he said "This has no significant effect on the aeroplane", you said "Yes, rain will affect aerodynamic performance of any airfoil" - which is it?

Are you saying that Nasa also "has pretty bad reputation in scientific circles."?
The NASA paper tested "a cambered airfoil representative of typical com-
mercial transport wing sections" - NB not laminar flow.

The NASA summary
"The results obtained for various rain intensity levels and tunnel speeds
showed significant losses in maximum lift capability and increases in drag for a given lift as the liquid water content was increased. The results obtained on the landing conguration also indicate a progressive decrease in the angle of attack at which maximum lift occurred and an increase in the slope of the pitching-moment curve as the liquid water content was increased."

I think on balance NASA wins over Mr C.

posted this on the 'other' thread, but may be more appropo here...

The mechanics of the weather radar and wind shear alert system. Doesnt the windshear alert use the weather radar? I am aware that some models use a l@ser system for windshear, I am just not positive about the connection between the 2 systems.
If the settings for the weather radar had been to look up, with the windshear system auto engage at 2500 feet, does this automatically reposition the dish?

In regards to this thread...wet wings and Reynolds numbers...CFD gone mad in regards to Bernoulli vs Newton..

NASA and wind tunnels...interesting concept, but in reality, its all smoke and mirrors.

In working with some testing, I just found out that there has NEVER been a model tested in a crabbing position..NEVER. Makes you wonder...

Let me pass on an actual observation of rain on a wing - from a perspective not available from the front office....

Right in the middle of my primary flight training, I flew as pax on (coincidentally) a 737 from MEM to IND. On approach, IND had numerous summer thunderboomers in the area, with ceilings under the CBs at about 4000, and intermittent strong downpours (we passed through about 5 on the inbound leg to 23R).

Each time we entered the heavy rain, I could see, from my window seat just above the right wing, a "bead" of water pulsating along the leading edge of the deployed slat. Or at least the "edge" that I could see (the actual leading edge was likely so low as to be out of sight from my location about 6 feet back).

That build-up of water pulsated between 1 and 2 inches in thickness (2.5-5cm) in the heaviest parts of the rain; reducing, of course, to nothing when we exited each rain column.

For a beginning pilot, it was an impressive demonstration of just how much water can build up from the forward movement of the wing through a torrential downpour.

What do you mean? Track over the ground is surely irrelevant....the relative wind over the wing is parallel to the the longitudinal axis of the aircraft (in balance flight)...

There is quite a difference between specific airfoils (NACA 64-210 and 0012) tested in wind tunnel with simulated rain spray and real aeroplane flying through real rain. If one's agenda is panic-mongering through distorting the meaning of test results, well then disregard for the not so subtle difference is understandable.

Also, in my level 4 English there exists such a thing as insignificant effect so "it will affect" and "it has no significant effect" are not at all contradictory, but then I'm no native speaker and might be wrong.

No, but I'm saying that NASA's scientists never pretended their results could be applied immediately to real world unlike those who published their findings through WASET that categorically claim:

OMG! :eek: Them scientists found out the way to help pilots control the aeroplane in heavy rain and zee pilots don't care about it! Is the denial of industry there actually is a problem sign of worldwide conspiracy? :E

NACA 64-210? Certainly not laminar but probably more typical of early jets.

It's report on scud running that ended unsurprisingly. Rain effect is nice distraction.

I guess he meant "slip" when he wrote "crab".

From what I've seen, every wing section seems to have a different reaction to contamination and it's not obvious what it might be until it happens.

I have flown many gliders in precipitation of all descriptions and it's not always the highly laminar flow wings that are affected the most. One particular model with a fairly thick, high lift section vibrated and headed earthwards after a few tiny drops. Mine with a modern, thin 90%+ laminar profile seems to be much more resistant: I've seen water cascading in floods off the wing and the handling doesn't really suffer.

With typical airliner sections, there probably is an effect but it's masked by the amount of power available and the scale of the wing, I feel. If it were that pronounced, you'd have a performance decrement to apply taking off in rain or after de-/anti-icing. We know that hard, irregular deposits are bad and take great pains to rid the airframe of them but are less concerned with liquid ones - think how many takeoffs are made in rain every day without issue. It seems that a wet runway is much more relevant than wet wings.

If I could drag folk off their soapboxes and axes to grind, may I remind all that in post #1 I said:

"I believe there has been 'advice' in the past on increasing speeds on approach in these conditions, but I am not aware of any specific guidance in Ops Manuals (in my time). Is it in place now? Should it be?"

Judging by the responses I detect that no-one so far:

a) Thinks it should be in the Ops Manual
b) That it IS in any Ops Manuals
c) Thinks approach speeds need to be considered

Thus case closed.

.. from a report just read .. it may be that the recent LionAir mishap involved some very heavy rain ?

The Predictive windshear is independent of the wx swith on the B737 NG.
It engages during take off when thrust levers are past the vertical position(wx radar on or off) and during descent when below 2300 AGL.

From my noddy perspective, how about it's got nothing to do with a wet wing but rather the increase in effective density/inertia of the air ? When accelerating over the top of the wing the water particles add inertia and hence reduce speed/increase local pressure. Would expect this to be most pronounced when acceleration/shear at maximum ie high alpha so giving reduced Clmax.

If you should fly a hang glider
 

The higher performance hang gliders get a ten knot increase in stalling speed when wet (from 20kt to 30kt), that's 50% :eek:. My intermediate performance hang glider was entirely unaffected because it had a textile leading edge rather than one with a shiny coat. On occasion I got the whole hill to myself. On the other hand it is a miserable experience flying a hang glider in the rain.

I seriously doubt it had any effect, more likely it was the effect of hitting the surf.

course I don't have any data, but I doubt the veracity of most of what has been reported so-far (it's outside historical data and engineering comprehension)

Energy management
 

The total energy of an airliner compared to a glider at approach speeds makes it prudent practice for the glider to increase speed with wet wings. A glider will typically aim to deploy a setting of half-airbrakes for the planned approach to give a plus or minus tolerance on approach slope and speed.
I suspect that the effects of wet wings on a commercial aircraft will be less significant.

For what its worth, in Formula 1 we did a lot of studies into the effects of rain on the cars wings and bodies.

the basic rule now as a result is that when its dry we use a hydrophobic coating to resist water and dirt as much as possible.

however when its wet, we use a Hydrophillic coating. The reason being that the Hydrophobic coating causes the beading together of droplets which cause greater drag with them having to be pushed off the surface as beads. The beads also cause more surface disruption and them moving about under vibration disrupts the optimum wing tuning frequencies.

BUT with the hydrophillic coating we found that the water forms a nice thin (wetted ) slippery film over the surface with lower drag and better boundary conditioning.

Not flying i know but thought someone might find it interesting.

GB

Interesting post from Bye. The slippery film he describes makes me think of the effect we get from de-icing fluid.

The most I have noticed the effect of rain in over thirty years of gliding was in the Libelle. A beautiful early glass glider which really did not like wet wings. The rate of sink increased markedly.

On the Boeing I fly there is no noticeable difference. However a basic understanding of practical aerodynamics suggests any contamination must effect performance. However the difference between a glider where you may at times be only few knots over the stall speed and a jet where at the slowest point you should be some twenty to thirty knots over the stall speed mean you do not notice it.

The only time it is an issue is with flaps 15 vref ice. I understand Boeing discovered in testing that the margin was reduced in this configuration which is the reason for the add on. Presumably one could expect a similar but lesser effect from rain.

“… a similar but lesser effect from rain”, re de-icing fluid.
IIRC some old studies of the effects of de-icing fluid on lift and drag concluded that any deterioration was minor. However more recently, several manufacturers evaluated the new higher-viscosity de-icing fluids, which depending on aircraft type showed some deterioration or the need to adjust configuration / speeds to maintain controllability standards.
Rain, in comparison with de-icing fluid, has relatively low viscosity and thus might not be such a problem.

I remember....
 

I remember years ago....like 30 or 35 years ago....NASA was doing
a study about the effect of bugs on the leading edges of wings....
how the bugs affect lift. No kidding. Our tax dollars at work.

Never heard what conclusions they came to, if any. Maybe someone
can Google this.

Anyway, I was a flight engineer on the 727...doing my walk-arounds...
along with some NASA guy with a ladder counting bug splats on
the leading edges.

Fly safe,

PantLoad

That was well spent money. Understanding bug contamination on wings leads to a better understanding of airfoils with improved stall characteristics and reduced drag in the cruise. Let's face it, aerodynamically there's not much difference between a well spattered aircraft and one with ice on the leading edges. One contaminant we get rid of, the other we don't.

PM

I seriously doubt it had any effect, more likely it was the effect of hitting the surf.

A second report suggests that there may have been a crew interaction problem/deficit which had very predictable results. Premature to discuss the details I suspect.

that fits about 75-80% of accidents, now to find the other causal factors because all we can do is minimize the 75-80% by a few percentage points

to err is human

JT,
According to Flightglobal's Special Safety Bulletin regarding this accident,
the co-pilot was in control of the landing. The aircraft hit a pocket of heavy rain at a fairly low altitude (flaps and slats out for the landing) and lost sight of the runway, at which point the Captain took over the landing. The Captain then elected a TOGA missed approach but the aircraft did not respond to the climb quick enough or as expected and settled into the water. If true, it might suggest that heavy rain does indeed affect lift capability of the wings with slats and flaps out at slow speeds.

TD

On highly laminar wings this has a major effect, actually.

The Piaggio p180 POH contains this text and reduced figures are available for performance calculations:

"This airplane is characterized by extensive natural laminar flow over the forward and main wings. Insect debris, dirt in general, or rain may force the boundary layer to become turbulent prematurely and the performances are affected by the loss of laminar flow.

The extension of laminar flow as a function of surface contamination is very difficult to determine. However, loss of performance, substantiated by flight test data, ... are indicated if significant in each performance graph..."

Emphasis mine.

So 20 kts in the overrun for your pax?:E

Some glider pilots like to wipe the bugs off the leading edge while flying, typically in a competition.

Don't believe me? Have a look at this (or any of the other vids on that page):

Where do they get the power to operate it from?

From some sort of RAT or battery?

I wonder what the trades in performance vs weight are?

http://www.dg-flugzeugbau.de/Data/mueckenputzer-ein.jpg

"One distinctive feature that is incorporated in the LS10 fuselage is the bug wiper garage. Even though the wing of the LS10 has been designed to be bug tolerant, fitment of bug wipers does not result in a drag penalty as they fit perfectly into the fuselage garage. The distinctive aluminum-carbon design of the bug wiper goes one step further to make the LS10 a distinctive high tech glider.

In addition to the integral bug wiper garages, solar panels are also integrated into the fuselage turtle deck to provide continual battery charging. Two normal batteries are sufficient to provide the required electrical demand for flight inclusive of (optional) bug wiper and turbo operation."

The full article is at www.dg-flugzeugbau.de.