posted on Jul, 25 2005 @ 04:40 PM
You said that the helicopter was above the overcast, right? It was probably the overcast diverting and/or absorbing the noise from the rotor. Clouds
and fog will reduce the distance that high frequency noise will travel. That is why a fog horn puts out a low frequency noise.
True except that the primary noise component, the 'booming whack' of the blade breaking shocks off it's tips and collapsing the wake of preceding
blades, in it's BVI/HSI states is actually a _low_ frequency.
The best way to stay quiet in a helicopter is to make careful use of terrain features so that something like a ditch or building/treeline in front of
you and any 'reverb' (an ampitheater like effect) is deflected away from likely observer angles as a function of total dbA LOS transmission
distance. Of course there will often be a hover or NOE power penalty associated with this but you would be amazed how little noise you can put out
the front of the disk, simply by sitting on the cushion.
In forward flight, you want to unload the rotor disk, preferrably by using lift wings and an alternative (mixed) propulsor mechanism to the
conventional tail rotor. Something like the VFDP will work very well though there is still a 'sound channel' after it's passage. In a
conventional helo, simply putting the helo in a 'coast' mode with relaxed collective and a /slight/ descent rate (which also helps keep the rotor
speed low and out of vortex ring on the leading side while maintaining directional control at lower tail rotor speeds) will reduce sound levels by
20-30%. Alternatively, you can go exotic and stop the rotor altogether which is what the X-Wing/CRW systems do.
The third 'technical' route is to start messing with blade design as a function of lift efficiencies at a given blade chord and spacing variation
designed to disperse dbA over a wider range of infra, low and mid frequency principle peaks with rise/dwell times that actually 'cross over' to
cancel out secondary harmonics. In terms of total 'felt' amplitute of sound the effects can be extreme. For a trained soldier will note a subtle
'overpressure' sense on his inner ear when unseen helos are operating in his area as an expression of resonant infrasound buildup just shy of
conscious detection in the inner canals.
Similarly, the 'buzz' of registerable (low/mid range) noise intermix that is overlapping peaks and secondaries upon the ears dipole (fractional
wavelength) can be made to rise and fall out of phase with each other so that direction finding becomes seemingly multi-compass point impossible to
Typically this is done by altering blade spacing in either vertical (sectional) or horizontal (planform) passage lines by a few degrees of cant or
rotation so that each effectively 'cuts it's own air' rather than adding to the wake and tipspeed vortice interaction of a (fixed angle=fixed
passage time) blade ahead of it.
DbA reductions peak reductions on the order of 11-20 are possible but there are matching efficiency penalties on the order of 15-20% total lift and
forward cruise/Vne airspeeds as well.
Given helicopter's lack of propulsion independent lift vector (you pull the cyclic back and there is an instantaneous IAS deficit applied as the disk
tilts and the fuselage follows to further decrease the rearwards propulsive angle, providing all kinds of no-horizon stall and vertigo cumulative
psychology factors). Coupled to potential hazards of blade and inlet icing. All while in general operation nearer to flight hazards like power lines
than a fixed wing would encounter, it would be highly unlikely for a helicopter to be operating inside the cloudbase in civillian controlled
Instead, visual invisibility is more a function of isolating vehicle relative motion against a given brightness index of background. Usually by
minimizing total silouhette presentation and adjusting color scheme relative to a horizon-line 'glare' of transitional separation (the army goes
under, the marines go over).
The human eye is very good at perceiving differences in target spatial motion against a fixed set of gross object shapes (not against the trees, but
maybe against the road or over the power line on the near side etc. etc.).
Yet if you remove these cues to near-far rangefinding and 'process of elimination' set-association matching, it becomes easy to get the eye to
defocus to a '4ft blur' level of recognition, looking at a bright terminator just above the local horizon.
This glareline occupies an angle of sky that varies between 10 and 30` over the horizon and you can hide a veritable army inside it, provided they
don't throw enough of a silouhette to literally 'black hole in sky' decrease the ambient light level in that particular sector of observation.
The key to covert route nav then being to minimize the distance at which _lateral_ threats can see the full vehicle shape moving past their position
(usually as a function of navigation away from occupation zones or obvious transit lanes/chokes) while also holding down the range at which threats
immediately along the flightpath can hear the approach before themselves being within counterdetection/engagement limits of say 1-3,000ft
Treelines and other elevated terrain/obstacle LOS blockers can again help here as there is a definite 2D 'cage effect' (don't look over the bars)
upon human visual psychomotor responses, even in experienced soldiers relative to first sight from a higher platform looking down.
Which is why most vehicle column will have an air cav screening element.
More exotic approaches are being investigated. An A-10 was, in about 1992-93, painted with an electrochromic polymer that altered the positioning of
ferrous pigment balls to change coloration and brightness when charged with varying voltage patterns of electricity. Eventually this system was
automated relative to light sensors placed around the airframe.
Results were mixed as it was found that putting small, fixed, geometries (a planview silouhette of a smaller airframe or a false shadow) inside the
bigger outline of the Devil's Cross caused more lead errors in range:rate tracking. Than did the 'amorphous/rolling' shape change of photosensor
driven paint reduce initial detection while yanking and banking above observers.
Yehuti lights and what not are also possible if you don't plan to close the range but the most interesting/advanced ideas are those resulting from
VCSEL technologies which seek to turn conventional AMLCD pixel generation 90` to the vertical to current techniques so that you get the effect of a
football stadium billboard display in a flexible sheet that is roughly the thickness of a heavy sweatshirt.
If you can combine the A-10 electrochromic efforts with the brighter and more diffuse (some say holographic) intensity/feature geometry detail of the
VCSEL. And apply it to MUCH smaller platforms (which generally also require less air disturbance from a smaller disk with fewer, wider, blades
operating at lower tipspeeds, meaning a smaller acoustic footprint directly under the vehicle), like the X-50 Dragonfly and the A-160 Hummingbird.
We could easily see a limited ability to mimic skybackgrounds on fixed limited sectors at altitudes which challenge human vision to acquire a 10-20ft
long silouhette against a much brighter saturated 'blue' (or overcast grey) backdrop.
i.e. The smaller the chopper, the smaller the assumed silouhette:range factor is for base visual or coupled visual:auditory acquisition threshold
distances. And the more you can (acceptable weight penalty) work with advanced masking systems to remove the remaining visual planform or emitted
noise factors from at least the human range of detection norms.
A helo which is never seen from above can double the weight of a look-up defensive screen on it's belly. Or employ an A-Sound generator to
compensate for short 'burst' flight intervals in which it climbs after gliding over a target area. Or simply increase the grazing angle of any
sensor system to the point where standoff yields effective invisibility anyway.