Rime Grime

[old man emu]

Rime ice is formed when small supercooled water droplets freeze on contact with a surface which is at a sub-zero centigrade (Celcius) temperature. Rime ice can form on aerofoil and engine inlet leading edges and thereby affect aerodynamic characteristics of wings and the airflow into engines. We learn this during our student days, but in Australia, we are not likely to encounter it as we puddle jump in VMC conditions. But ...

 

I'm doing some environmental risk management documentation for a couple of Sydney secondary airports, and one of the things I have been looking at is the environmental risks associated with washing aircraft. I went back to the basic question, "Why do we wash aircraft?"

 

Sure, there are some who like to maintain a flashy appearance of their little toys, but I am looking at aircraft that work to earn their keep - flying school aircraft, air work aircraft and the smaller end of the charter market. These workhorses are out there virtually every day, cutting holes in the sky to make a quid.

 

Then I started to think about why washing aircraft is an environmental risk. Planes are pretty clean. They operate high above the deposits of road grime that soon make the underside of cars and trucks look like the La Brea Tar Pits. Engine bays are usually oil-free, and as soon as an oil leak is found, the aircraft is into the hangar for servicing. Where else can the dirt that has to be washed away come from?

 

I had a look at Sydney's air quality. Liverpool Sydney South-west, Australia Air Pollution: Real-time PM2.5 Air Quality Index (AQI)

 

The two things to look at are PM10 and PM2.5 . PM is the abbreviation for particulate matter. PM10 refers to particles less than 10 micrometers (0.01 mm) in size. The National standard for PM10 concentration is 50 micrograms per cubic meter of air.

 

When aircraft are flying through the air over the Sydney Basin, they are constantly hitting these micro-particles, which stick to the leading edges of wings, tailplanes and propeller blades. Just as rime ice destroys the aerodynamic flow of air over these surfaces, grime does the same. Over time, the accumulation of grime will reduce the efficiency of the lift producing surfaces. Maybe not as catastrophically as rime ice, but it will affect the economics of aircraft operation through increased drag and reduced lift, both of which require more power to produce the "book" performance figures. More power means more fuel burn, which means it costs more per hour to keep the aircraft in the air.

 

And are these pollutants unique to aircraft? No. They are the dust, smoke, pollen and salt that are the components of Sydney's dirty brown haze. So wash your aircraft every couple of months. Do it on a grassed surface. Is this going to cause irreversible harm to the environment? Well, the NSW Environment Protection Authority has not seen fit to issue any documented procedure for washing cars, and by logical extension, aircraft.

 

 

[djpacro]

New Service Letter for exterior washing of Citabria and Decathlon: http://www.americanchampionaircraft.com/assets/sl-450.pdf

 

 

[skippydiesel]

Hi OM Emu - I fly in/out of the Sydney Basin very regularly - the main" contaminants" of my aircraft leading edges are bugs. Not so much, but still present, at this time of the year. From dim memory (it hasn't rained for some considerable time, I get occasional mud spatter of flap & empennage when conditions dictate. As far as micro particles are concerned - I cant say I have registered much but with the exception of the Lane of Entry & the concessional Victor 1 I dont fly over the city.

 

 

[old man emu]

The aircraft that need frequent washing are those based and mainly operating in the Sydney Basin where the air is sometimes chewable.

 

Here's an experiment to carry out from now until New Year:

 

Get a sheet of glass, about the size of a normal window pane.

 

Wash it all over so it is squeaky clean.

 

Mark off four sections on the pane.

 

Working from the right hand side, lay down strips of masking tape to create borders between the sections.

 

Cover three of the four sections with a wide strip of paper, or cling film, taping the edges to the masking tape borders.

 

Place the sheet of glass in an open uncovered area, clean side up.

 

On the first day of the month, remove one strip of covering from the glass, starting with the 2nd strip from the left.

 

After the last strip has been exposed for a month, compare each section to see how much solid matter has been deposited on the glass.

 

This experiment will give you an idea of how dirty Sydney's air is.

 

If it rains, keep going with the experiment.

 

 

[pmccarthy]

How will this tell me how dirty Sydney's air is? I live in country Victoria. :)

 

 

[APenNameAndThatA]

How do you know that the particles will stick to the leading edge of the wing? Do you see the leading edge become grime coloured? How do you know that such particles will degrade the aircraft performance? Do you have data? Why are you concerned about the leading edge? (The air at the leading edge is static.) Surely bugs would be a much bigger problem than grime, as grime is so much finer. The Foxbat POH says that bugs do not affect performance.

 

From memory, the British painted their Mosquitos a mat colour and lost a knot or two so they changed back. Microscopic grime is going to have a much lesser effect than mat paint, which has visible texture. Plus, the paint was over the entire plane, not just on the leading edges.

 

Surely dirt would have a much greater effect than particles that are literally microscopic. I suppose that you are saying that if you polish your aircraft it will go faster. Has anybody ever found that their plane goes faster after they clean it?

 

The idea that grime would have the same effect as ice is just bizarre. One is literally microscopic and one is literally very macroscopic. On is a surface drag issue (actually, not a surface drag issue, IMHO) and one is an issue because it affects the form of the propeller and wings.

 

How many knots would you have to pick up to compensate for the time taken washing your aircraft every month?

 

If you want to maximise the speed, surely making the surface shiny would be more important than removing grime. Faded paint will be a million times rougher than the effect of grime. Do aircraft go faster when they are repainted?

 

 

[skippydiesel]

The aircraft that need frequent washing are those based and mainly operating in the Sydney Basin where the air is sometimes chewable........................................................................................................................................................................................................................................................................................................

This experiment will give you an idea of how dirty Sydney's air is.

 

If it rains, keep going with the experiment.

Problemo ! Where does the Sydney Basin end/begin ???? If you listen to the Sydney centrist weather reports, it would appear to end somewhere in the arc described by Richmond - Penrith and Liverpool ( I accept the sea as the eastern boundary)l.

 

I would suggest - you are in the Sydney Basin if you can (potentially) see the CBD buildings on a smog/fog free day. If you agree with this definition, then there are large areas of the outer "rim" that are little effected by Sydney pollution. Then again perhaps it's the pollution that defines Sydney.

 

 

[old man emu]

How will this tell me how dirty Sydney's air is? I live in country Victoria. :)

So. Your missus doesn't have to dust as part of house cleaning?

 

"Problemo ! Where does the Sydney Basin end/begin ???? "

 

For the sake of this discussion, the Sydney Basin is bounded by the coast to the east, and the Nepean/Hawkesbury River from the southwest to the northeast at Broken Bay, and a line from Picton to Stanwell Park

 

 

 

[facthunter]

A dimpled golf ball will go further than a smooth one. Boundary layer behavior is not so simple as to presume anything.. Flush riveting is not done on the rearwards parts of big Jets and larger planes can have foot radius leading edges with out penalty (Reynolds Numbers). Bigger planes just get slipperier because they do.. Nev

 

 

[Nobody]

Have you ever noticed that the glider pilots clean the dead bugs of their wings after every day....

 

The dead bug guts give a noticeable decrease in performance on a sleek high performance glider.

 

 

[APenNameAndThatA]

A dimpled golf ball will go further than a smooth one. Boundary layer behaviour is not so simple as to presume anything.. Flush riveting is not done on the rearwards parts of big Jets and larger planes can have foot radius leading edges with out penalty (Reynolds Numbers). Bigger planes just get slipperier because they do.. Nev

My understanding is that a dimpled golf ball goes further than a smooth one because the dimples make the boundary layer turbulent. If the ball is smooth, the flow is laminar. The laminar flow separates from the ball sooner than turbulent flow, giving the laminar-flow ball greater form drag. That is why a dimpled ball is more aerodynamic than a smooth one. (I imagine that a rough ball would be more accurate because the separation of flow from the surface of the ball would be more predictable. But that is just my hypothesis.) Maybe someone on here knows if laminar flow wings (like ? Piper) are more susceptible to bugs than less laminar wings (like Cessna). (I imagine that the difference in the amount of laminar flow is a matter of degree, between Cessna and Piper.)

 

 

[facthunter]

You golf ball summation is correct. I don't think we would really know whether a slightly roughened surface us better than a smooth one, on planes this size.. Perhaps on models much more so. VG's make a lot of difference on some places. Like the "S" duct on the middle engine of a B 727. A couple missing and the engine will stall. Nev

 

 

[pmccarthy]

You could ask the guys from the naked skydiving championship.

 

 

[skippydiesel]

You could ask the guys from the naked skydiving championship.

The mind boggles !

 

 

[facthunter]

Goosebumps make them fall faster plus once your extremities shrink you are more streamlined. Nev

 

 

[Marty_d]

The mind boggles !

That's not all that boggles.

 

 

[old man emu]

Here's something to consider about the weight of pollutants a plane could impact during a flight.

 

1. Air Quality Index. (AQI)

 

The AQI is a ratio of the observed pollutant concentration to the maximum concentration standard set under Section 14 of the National Environment Protection Council Act 1994 , expressed as a percentage.

 

Two grades of suspended partidulate matter (grime) are specified in the Act - PM10 and PM2.5. The 10 and 2.5 refer to the diameters of the particles (< 10 micrometers, or < 2.5 micrometers). The standard concentration value for PM10 particles is 50 ugm/m^3. A ugm is one millionth of a gram.

 

The equation for the calculation is:

 

AQI = (Observed pollutant concentration)/(Pollutant Standard Level) x 100.

 

This equation can be re-arranged to determine the pollutant concentration from the published hourly AQI value:

 

AQI = {Observed/50} x100

 

AQI/100 = Observed/50

 

AQI/100 x 50 = Observed

 

AQI x (50/100) = Observed

 

1/2 x AQI = Observed pollutant concentration ugms/m^3

 

In other words, the weight of suspended pollutants 10 micrometers or less in diameter in one cubic metre of air is equal to half the AQI value. Today (1/8/18) the AQI recording for PM10 at Camden was 46, meaning that the weight of particles in a cubic metre of air was 21 ugms. Doesn't sound like much, does it?

 

2. What weight of grime impacts with a plane?

 

a) For this calculation:

 

The wing of a Jabiru is close to 10 cm at the leading edge, measured vertically from upper to lower surface. Therefore the area surface of a metre of leading edge is 0.1 m^2. This is taking the leading edge to be flat. Wrong in practice, but acceptable for this calculation.

 

b) Distance flown:

 

One nautical mile = 1852 metres. 54 nautical miles is equal to 100,008 metres. For this calculation forget about the extra 8 metres.

 

If a Jabiru flew 54 nautical miles through air with an AQI ( PM10) of 46, then the leading edge would impact:

 

0.1 m^2 x 100,000 m x 23 ugms of grime

 

= 10,000 x 23 ugms of grime

 

= 230,000 ugms of grime

 

= 230,000/1,000,000 gms of grime

 

= 0.23 gms of grime

 

 

[APenNameAndThatA]

0.23 g of grime. Okay. I’m listening.

 

 

[facthunter]

You are trying to paint a GRIMe Picture.. Nev

 

 

[old man emu]

The wingspan of a Jabiru is 9.5 metres. Let's call the total length of leading edges 10 metres (don't forget the tailplane)

 

Therefore, in flying 54 nautical miles, the leading edges would impact 10 x 0.23 gms of grime = 2.3 gms.

 

That is equal to a tad over 0.04 gms per nautical mile.

 

 

[APenNameAndThatA]

The wingspan of a Jabiru is 9.5 metres. Let's call the total length of leading edges 10 metres (don't forget the tailplane)Therefore, in flying 54 nautical miles, the leading edges would impact 10 x 0.23 gms of grime = 2.3 gms.

 

That is equal to a tad over 0.04 gms per nautical mile.

Yep. I'm all ears.

 

 

[Nobody]

The density of air is 1.2kg/m3. A jabiru doesn't seem too upset by the impact...

 

 

[Marty_d]

Enough of this filthy talk....