HONEYWELL TECHNICAL NEWSLETTER
Copyright November 2, 1998 Honeywell Inc. All Right Reserved.
Business and Commuter Aviation Systems, Glendale, Arizona.
Re-published by Cytonix with permission from
Honeywell Inc.
SUBJECT: RADAR DESENSITIZATION IN RAIN, WATER FILMS ON RADOMES, AND HYDROPHOBIC
COATINGS.
INTRODUCTION
Over the years we have received reports from operators of various aircraft
types who have experienced degraded radar operation while flying in rain.
Many of these reports have sufficient detail and describe events that can
easily be explained as 'normal under the stated conditions' and are caused
by the limitations of any radar or in the way the radar was being operated.
However, a number of reports, some of which are quite detailed, have not
previously had a ready explanation.
To provide a service to our customers and the industry, Honeywell launched
a continuing investigation of these phenomena. We feel that we have identified
the primary cause of radar desensitation when flying in rain and although
we have not yet identified a final solution, we are providing this information
now. Understanding the phenomenon will increase pilot confidence in the
radar and reduce maintenance costs by preventing unneeded radar removals.
BACKGROUND
A typical pilot report is that the radar is working normally, rain in the
area is displayed on the radar to good ranges, as well as being seen visually.
Then, if the aircraft flies into light rain (green level), the sensitivity
is degraded so severely that very little (if anything) is displayed on the
radar. In most cases, the REACT field is not brought into a short range
to indicate that there has been significant intervening signal attenuation
and that the radar may be uncalibrated. Then, shortly after the aircraft
flies out of the rain, the radar works normally again.
This phenomenon is intermittent; it doesn't happen every time the problem
aircraft flies into rain. Tests of the radar system and aircraft after the
event do not find any problem. Discussions with pilots and maintenance personnel
indicate that all brands of radar and airframes may suffer from variations
of this phenomenon.
DISCUSSION OF THEORIES
The leading theories about possible causes of this phenomenon are:
a) a water film on the radome.
b) icing on the radome
c) p-static on the radome
d) some combination of the above
A water film on the radome easily explains the 'can't see anything when
the radome gets wet' phenomenon and we believe it to be the principal cause
of this class of reports.
Figure 1 shows a graph of calculated loss in radar sensitivity caused by
water filming on the radome. A film of about 8 mils (0.008 inch) on the
radome will decrease the radar sensitivity about 1 color level; red targets
become yellow, yellow becomes green, and green becomes black. Our laboratory
experiments agree closely with the calculated values at that film thickness.
A water film thickness of 35 mils would decrease the radar sensitivity by
almost 3 color levels.
The graph in Figure 1 only shows the attenuation effects of a water film
out to 40 mils, but the function is cyclical. After approximately 35 mils
the attenuation decreases to minimum at about 70 mils, and then increases
again to a peak at about 105 mils.
We have asked aircraft OEMs if they have an idea of how much of a water
film could form on the radome, but no one really knows as it is very difficult
to measure in flight. One comment was that they would be surprised if it
would get 100 mils thick.
A water film as the source of the signal loss also explains why the REACT
field remains at its 'no attenuation' position, rather than coming into
a very short range to show the significant attenuation and an uncalibrated
radar. Every radar has a 'blanking range'. The radar receiver is turned
off (blanked) during the transmit pulse and for a short time afterwards,
this typically is at least 1/4 mile, but can be more depending on the transmit
pulsewidth. Because the water film is within the blanking range, the receiver
and REACT circuits are not active and therefore cannot compensate for or
indicate the presence of the attenuation caused by a water film on the radome.
Figure 1. Water film thickness verses radar sensitivity loss.
Mathematical models do not support icing as the cause of this phenomenon.
A very, very high level of p-static may cause the phenomenon. We have had
a report where the Saint Elmo's Fire was very intense and obscured the view
out of the windscreen, rendered nav and com radios useless, and prevented
the radar from displaying targets. However, in the vast majority of reports
regarding radar phenomenon when the radome gets wet, no Saint Elmo's Fire
or radio interference is reported. Therefore, we do not feel that p-static
is involved in these cases.
We believe that water filming on the radome is the primary cause of this
phenomenon.
A POSSIBLE SOLUTION
Hydrophobic coatings prevent water from sheeting; in other words, the coating
will cause water to bead up and run off the treated surface. Hydrophobic
coatings are also called 'rain repellents'. Many aircraft operators, airlines,
and OEMs already use hydrophobic coatings on windscreens.
Hydrophobic coatings have been used for several years on ground based and
marine radomes to reduce sensitivity degradation during rain and on microwave
antennas to reduce 'rain fade'.
In mid 1997 we were working with an operator who regularly experienced the
'can't see anything' phenomenon. While the aircraft was on the ground with
the radar painting weather targets, a hose was used to spray water on the
radome and the targets disappeared as they did in flight and water filming
was observed on the radome. We suggested that they apply a hydrophobic coating
to the aircraft radome. They used a silicon oil based product sold for automotive
windshield applications. This operator feels that it solved their problem
and reported it to us. Based on the apparent success for this aircraft,
we recommended hydrophobic coatings to a small number of other operators,
both fixed-wing and rotary-wing, who regularly experienced the desensitation
phenomenon. In all cases, the operators feel that the application of hydrophobic
material to the radome has cured their problem.
Because of the small size of the experimental sample, we cannot absolutely
state that water filming on the radome is the cause of all reports of this
phenomenon and that hydrophobic coatings are the cure for all operators.
However, because of the theoretical studies, the experiments, and great
success that the sample operators have had, we feel it is time to offer
the idea to all of our operators for consideration.
PRACTICAL HYDROPHOBIC COATING CONSIDERATIONS
There are several formulations for hydrophobic coatings. Some are based
on silicon dioxide, some are based on simple silicone oils, and others on
fluoropolymers.
There are two specifications for the effectiveness of a hydrophobic coating.
The most common specification is 'contact angle'; that is the angle between
the surface and line drawn from the point of contact (between the surface
and the water droplet) that is tangent to the water droplet. The bigger
the contact angle, the more of a sphere the droplet is and the better the
hydrophobic properties. For example a droplet of water on a non-treated
surface may have a contact angle of 15° (a 'fried egg'), some products
have a contact angle of about 95° (a little more round than a hemisphere),
others claim a contact angle of 115° (more of a sphere), and yet others
claim a contact angle of 140° (even more of a sphere). We are told
that for a coating with a smooth surface, about 120 degrees is the theoretical
limit and that for coatings with a texturized surface the angle can go to
about 150 degrees. The other specification is 'surface energy', the molecular
force of attraction between the water droplet and the surface. The smaller
the number, the better it is.
The surface energy of the products range from 6 to 30 dynes/cm, for reference,
teflon is 18 dynes/cm.
We have been evaluating available hydrophobic coatings in an effort to find
one that is very good for radome application. Unfortunately, we have not
yet found a perfect one.
The silicon oil products sold for automotive windscreen use have several
drawbacks. Their hydrophobicity is not the best, typical contact angles
are 90 degrees and contact energies around 25 dynes/cm. Their effectiveness
only lasts one or two flights. They can get into paint and make refinishing
difficult. They can damage some types of plastic windscreens. One aircraft
manufacturer has expressed a concern that they may damage kevlar composite
materials. Some silicone based coatings have been reported to degrade precipitation
static performance in high speed aircraft. Clearly, these type of products
are not the final solution to radome water filming and are not recommended
at this time.
The silicon dioxide coatings have excellent hydrophobicity, typical contact
angles are 140°. However, they generally have a texturized surface
which is quite fragile and they are not clear. Coatings of this type are
sold primarily for ground based microwave antennas. We feel that this class
of hydrophobic coatings will not be satisfactory for aircraft use.
The coatings containing silanes are designed to adhere to glass and we believe
that they will not adhere well to polyurethane paint. Although these coatings
are already used in aircraft for windscreens, it appears that they will
not be suitable for radome use.
The fluoropolymer (teflon like) coatings have many attractive properties,
but also have drawbacks. These do have very good hydrophobicity, typical
contact angles are 120° and surface energy of 6 to 10 dynes/cm. They
bond well to polyurethane paint and are quite impact resistant so they should
wear well. However, these coatings are very thin and may show 'newtonian
fringing', shimmering colors like those in a soap bubble when the light
angle is right. They are clear, but may dull the gloss of the finish a bit.
There is not a solvent that will remove it, however one vendor advises us
that it will reflow if additional coating is applied to one that is deteriating.
If you elect to try this type of coating, practice your application procedure
on painted scrap metal first!
One vendor has incorporated their hydrophobic material into polyurethane
paints. These coatings are very hydrophobic, the contact angle is about
120 degrees. As they are polyurethane, the impact and erosion resistance
should be very good and they are compatible with polyurethane aircraft paint.
In the author's opinion, these are the most promising products yet, although
normal paint build-up considerations apply. The single part polyurethane
may be easier to apply, but it has a slight amber tint which might not be
satisfactory for some applications. The two part polyurethane based coating
is clear. The manufacturer is investigating the possibility that their proprietary
hydrophobic material could be used as an additive to the standard aircraft
paint, this could be the ultimate solution.
Some concern has been expressed about hydrophobic coatings reducing the
effectiveness of the diverter strips on the radome. Because the coatings
are insulators, it is reasonable to assume they would increase the ionization
voltage between diverter segments, which is bad. However, a water film may
increase the ionization voltage as well. We have asked diverter strip vendors
for definitive answer and a laboratory experiment is planned to answer the
question. Until more is known, we recommend 'masking' the diverter strips
if you elect to apply a hydrophobic coating.
Concern has also been expressed about hydrophobic coatings reducing the
effectiveness of anti-static paint on radomes. There are two classes of
anti-static paint. One is an anti-static primer which is intended to prevent
a static discharge that has caused a pinhole through the top paint coat
from penetrating the radome material. As the top paint coat is an insulator,
it is hard to imagine how a thin hydrophobic coating could degrade the performance
of the anti-static primer. The other class of anti-static paint has a top
coat that is slightly conductive. The most common of these conforms to MIL-C
83231 type 2, this is the black finish seen on some radomes. We have discussed
the issue with a leading vendor of this paint and they feel that there would
be very little degradation of the anti-static properties of their product.
A laboratory experiment is planned to confirm this.
At this time, neither the coating vendors nor Honeywell are ready to make
any absolute claims about the effectiveness or safety of using hydrophobic
products on aircraft radomes, only that it holds considerable promise. The
possible negative effects (if any) to aircraft finishes or construction
materials are not fully known and may vary with the product. If you have
any questions about these issues, contact your aircraft OEM.
We will continue to work with vendors of hydrophobic products, aircraft
O.E.M.s, aircraft operators, and radome vendors, in an effort to better
understand the phenomena and find the best solutions.
As the experience base grows, we will revise this TNL to keep everyone up
to date. Please contact: John Jackson at (tele) 602-436-4794, (fax) 602-436-4100,
or (e-mail) john.jackson@cas.honeywell.com with questions or comments.
Prepared by: Signature on File in Customer Support Engineering, Customer
Support Engineering
Approved by: Signature on File in Customer Support Engineering, Manager,
Customer Support Engineering
Approved by: Signature on File in Customer Support Engineering, Engineering
Technical Manager/Lead
Cytonix Corporation
8000 Virginia Manor Road
Beltsville, MD 20705
phone: (888) CYTONIX or (301) 470-6267
fax: (301) 470-6269 or email: emailbox@cytonix.com
