Rohan Holtkamp at Dynamic Flight <<dynamic>>
writes:

This year before the World Hang Gliding Championships in Forbes I
discovered that a flex wing hang glider can achieve L/D of over twenty to one.
My first reaction after reviewing the data was WTF?! And next - where did I make
a mistake? Further testing on six occasions over three separate days confirmed
the numbers. I guess you want to know more so here's the background and method
so in true scientific tradition you can go test it for yourself.

I have spent about 25 years of flying hang gliders in competitions all around
the world, my first, second and third gliders were home-built rigid wings with a
web of wires, they looked like and were nicknamed clothes lines. After this I
flew flex-wing hang gliders and was sponsored by Airborne and Moyes, this flying
was in the pursuit of getting good comp results and improving the hang glider,
more specifically the glide performance without sacrificing handling or circling
sink rate. On some occasions with some gliders both areas of performance can be
improved with one adjustment.

Flying hang gliders in competitions is a subjective method of testing as
conditions encountered and lines taken during each flight can warp ones sense of
how the glider is actually performing, it takes many days of flying against the
best of the best to determine if you have settings or adjustments that appear to
be working either positively or negatively. Further discrepancies can be
introduced if your competitors are making adjustments as well. Recent
improvements in GPS track-log analysis are a significant step in the right
direction but the scientist in me was clamoring for more objective testing so
real improvements could be quantified and locked into production of any given
hang glider.

Outside of a large wind tunnel and sensitive load cells, accurate glide
performance analysis requires the hang glider to be flown in laminar air that's
neither lifting or sinking, these conditions are commonly found above the
convective layer over flat terrain early in the day without postfrontal,
prefrontal or shear influences. For my testing I was aero-towed to about 8000ft
asl early in the morning above flat terrain into these conditions over nine
days. I have some video of these test but still have to download it from the
camera.

Sensitive and calibrated instruments are also required, - we need to accurately
determine forward velocity through the air and downward velocity through the air
to calculate glide performance through the air. This is often expressed as L/D
ratio, a ratio of how many units we travel horizontally to vertically through
the air, like 20:1

Prior to gathering data, calibration of the ASI was done. To calibrate forward
velocity the instrument pod with ASI and GPS was fitted to a vehicle 2meters
above the front bumper and on a still morning we made six passes in different
directions along Dynamic Flight Park runways at GPS indicated speeds of 50, 60,
70 and 80 kph then adjusted the ASI to read with the GPS. I used a Brauniger IQ
Comp/GPS with impeller type ASI mounted at the front of the pod, the pod was
mounted into the airflow longitudinally and horizontally as it would be on the
glider.

Next we had a look at the vario (VSI) and set the averager to 12 seconds, this
updates the screen every second with the previous 12 seconds of averaged sink
rate. This instrument has a temperature compensated altimeter that is considered
to be accurate by experts in the field though it may still prove to be a
variable after other pilots use different instruments in the same way. Time will
tell.

After towing to height I selected the VG setting and established the correct
harness attitude and indicated air speed. The harness used was my Dynamic Flight
RR Mk3 with dual chutes and full standard Airborne pack-up gear. 50kph
plus/minus 1kph was held for three minutes, the average sink rate was recorded
every 5 seconds unless the airspeed varied more than 2kph. The same was done for
60kph, 70kph and 80kph until the convective layer was encountered, usually
around 500' agl on the first glide and 1000' or 1500' on the second and third
glides of each morning. It was too difficult to write down the numbers and hold
airspeed so we videoed them and broadcast them on a one watt UHF radio for
recording on the ground. I cross referenced this sink rate against the altitude
lost over time at that speed to verify accuracy of the averager and that RF
output did not interfere with the readings.

Sink rate was recorded in meters per second, airspeed was calculated to meters
per second; 50kph=13.888m/sec 60kph=16.666m/sec 70kph=19.444m/sec
80kph=22.222m/sec

On three separate days two Airborne Rev 13.5's with factory settings and my RR
Mk3 harness returned sink rate averaged over three minutes at 50kph of 0.63m/sec
=> 22:1, at 60kph at 1m/sec => 16.7:1. Other days we achieved better sink rates at
these speeds but found it unrepeatable so concluded errors or lift must have
been encountered. We did get better L/D at 46 to 47 kph but is was more
difficult to hold for any length of time. L/D like this is fantastic for
recreational flying as you can cross gaps with ease and get very high in ridge
lift, furthermore this low sink rate allows you to get up in the lightest of
thermals. These tests were done with a pilot and harness weight (clip-in) weight
of 90 to 93kg.

Competition flying demands good glide angles at airspeeds up to 90kph so further
work will be done to optimise performance at this range, hopefully without loss
of this low speed excellence. Between flights we adjusted settings on each of
the five gliders tested, this helped determine that small setting changes can
make significant performance changes and surprisingly some negatively - lower
outer sprogs on some topless gliders make glide angle worse!

We also tested another 2012 built topless glider at manufacturer and other
settings but could not return L/D this good.

To make these tests requires considerable investment in time, equipment and
money, none of which I have an abundance of, therefore this is the extent of my
investment I am willing to give away to the general flying community (Airborne
excepted). Please accept that further emails to this discussion will most likely
get a brief or no reply as I will be away from the computer building harnesses,
testing and tuning gliders or teaching new pilots to fly hang gliders, well at
least until winter sets in.