This is a topic I
have been avoiding for some time, but it's the Western Australian
Summer now and wind is unavoidable. Never been to WA? Well
its home to the biggest Kangaroo ticks I have ever seen, and
supposedly Perth is the third windiest city in the world (after
Auckland and Chicago).
These ticks are no joke. Unlike the lethal and lethargic
blue-bottle ticks of New South Wales, these ones won't kill
you or your pets. But in the months of October and November
they are very active and aggressive, and can run quite fast.
Up your leg in a flash, they can latch on anywhere: I do mean
'latch'. Their mouthparts include a pair of massive hooks
that remain in the flesh even after the body is torn away.
This leaves an itchy spot; which is annoying for months afterward.
I tried killing them insitu with kerosene. They died alright,
but didn't release their grip. Now I cut them out with a modelling
knife. I poke the knife in about 4mm below the head and slice
up toward the head. Usually this cut releases the hooks, but
if not a second incision will get them free. The wound heals
quickly, there being no itchy spot afterward. No pain either,
as the tick injects its own anaesthetic so you won't notice
Unfortunately, wind is not so easily dealt with; Perth has
3 periods of strong winds. There are the August/September
gales in Winter/ Spring, usually occurring on F/F days at
Meckering. The Summer wind pattern comprises strong, hot,
morning Easterly's, followed by a lull of about one hour at
noon, then a strong South-Easterly rips in. This latter is
affectionately known as the 'Fremantle Doctor'; affectionately,
as it is a cool wind which drops the noon temperature from
38 down to about 29 in a few minutes. If the Doctor doesn't
come in, then we here in Perth are in for a hot, sweltering
night. The 3rd period of wind is everything else.
As it happens, most of our F3D flying is coincidental with
the Fremantle Doctor. At our local field (KAMS), the sport
fliers and glider guys pack it in at noon, leaving the field
free for some noise and speed. By the time Mark has his Phelan
F3D gear ready to roll, the wind is in at 20 to 30 kph (at
There are 2 phases for testing F3D props. The first is to
see if the prop lets the motor get up on pipe; the second
is to see if the prop allows the engine to develop full power
in the air. This latter implies that one must know the in-flight
RPM, and, hopefully, also the airspeed. The Doppler method
permits RPM and airspeed to be known, but one must be careful
to apply the correct correction for height and wind. I have
covered this method previously, so you may like to divert
to www.supercoolprops.com at this point for a refresher course
on Doppler measurement.
Briefly, one places a tape recorder outside the course, well
up past number one pylon. In this position, there is a point
between pylons 3 and 1 when the model is flying directly at
the tape recorder, a second point lying between 1 and 2 when
the model is flying directly away from the tape recorder.
Using a computer program called 'Spectrogram', one can readily
determine the frequencies of the sound emitted by the model
at these 2 points; the frequencies are designated 'Fcoming'
and 'Fgoing'. They are readily noted, as the frequency 'Fcoming'
is the highest value of frequency visible on the Spectrogram,
while 'Fgoing' is the lowest (for a given harmonic).
Click to enlarge
The magic part of all this is that these 2 frequencies tell
us the RPM and airspeed in the straights! No in-flight telemetry,
no extra weight: indeed, something for nothing! How often
does that happen!
Unfortunately, sound is carried by the wind. This affects
the perceived values of 'Fcoming' and 'Fgoing'. In the case
of F3D, we locate pylon number 1 upwind, so that we may launch
the model into the wind.
When flying into the wind, from pylon 3 to 1, 'Fcoming' is
reduced below the value for no wind. Similarly, flying from
1 to 3, 'Fgoing' is also reduced. Remember, this is for the
tape recorder placed upwind past #1 pylon .
First consider the Doppler equations for still air (no wind).
With F the number of revs per second, V the speed of sound
in m/s and Vs the speed of the model, also in m/s, the still
air equations are:
Fcoming = V * F /(V-Vs)
Fgoing = V * F /(V+Vs)
Before we can use these Doppler equations to yield RPM and
airspeed, the wind-distorted values of 'Fcoming' and 'Fgoing'
which we observe must be fitted in to these equations.
Click to enlarge
If the speed of the airplane is Vs in still air, then downwind
its speed is Vs + w, and upwind its speed is Vs � w, where
w is the wind-speed in m/s. This is an approximation, since
the model may not be flying exactly upwind or downwind, but
its good enough.
Further, the speed of sound is reduced to the value V-w,
since it has to travel upwind to the tape recorder.
Substituting these values in the Doppler equations and solving
for Vs, the still air speed of the model, we get:
Vs = V * (Fcoming � Fgoing) / (Fcoming + Fgoing)
This is a surprising result, as the wind-speed 'w' is not
there! So the result holds, irrespective of the wind speed!
I guess we don't need the anemometer after all!
But we still want the RPM. Substituting Vs � w for Vs and
W � w for the speed of sound in our equation for Fcoming yields:
F = Fcoming * (V � Vs) / (V � w)
Oops, 'w' is back in our equation; we still need the anemometer!
So the RPM determination depends on wind speed. That is not
real nice. The problem is that the anemometer reading at ground
level can be quite a lot less than the wind-speed at 30m height,
but I guess we are stuck with that.
Now to do a worked example. Let us take the following values
measured in windy conditions:
Fcoming = 685 Hz
Fgoing = 410 Hz
W = 340 m/s � velocity of sound
'w' = 9 m/s � wind speed
then Vs = 340 * (685 � 410) / ( 685 + 410) = 85.4 m/s = 307.5
and F = 685 * (340 � 85.4) / (340 � 9) = 526.9 m/s = 31600
The still air airspeed of the model is 307.5 kph, while the
engine RPM is 31600.
How does this compare with World Class F3D models? After
the last World Champs at Melnik (2003) in the Czech Republic,
the duel between Chris Callow and Robert Bosch continued the
following week at Tours, or more correctly, St Martin Le Beau,
a village some 20 minutes train journey out of Tours. It's
a beautiful spot for camping, right on the river Ther. La
Grappe D'Or is a caravan park only a few hundred meters from
the flying field, right on the river, and an easy walk via
the Gypsy camp. In the morning, we were surprised to find
mounds of dirt outside our tent, like little volcanoes, had
appeared during the night. The culprits are Moles!!
Come the final, conditions were perfect for Doppler. I could
position myself 200m past #1 pylon, there being no wind. These
guys fly low, so there was no height correction either. Robert
was doing 325 kph at 33000, while Chris ran an identical 325
kph at 31600. Robert sprays around a bit, while Chris might
as well be running on steel tracks, so it was easy to identify
the pilots on the Spectrogram trace.
Those speeds are good for sub-60 times, so that is the target.
Mark's speed above is not height corrected. He was up around
30m, so a correction of 5% would see him at 323 kph, not a
Lets hope I have my algebra right on this one. A.E.E McKenzie's
book, 'Sound', is a little hard to follow on this topic.