Model World - Aug '94
by Stan Yeo
A lot of modellers, when
it comes to judging wind strength tend to over-estimate the speed of the wind and
under-estimate their / model's capability to fly in it. On the other hand, some newcomers
to slope soaring are completely unrealistic as to the winds their lightweight models can
fly in. The purpose of this article is to offer advice on flying in strong winds. Often,
once the model has been launched and got away from the slope, out into the 'clean' air,
the flying is very exhilarating and a lot of fun; forgetting of course the occasional
WHAT IS A STRONG
How long is a piece of
string? The definition of a strong wind will depend on the pilot's ability and the model's
capability to fly in it. A novice pilot with a lightweight rudder elevator model would
find 10 mph a handful whereas my definition would be nearer 40 mph with the right model.
Whilst flight testing the Wingbat we were flying in winds gusting up to 60 mph as
measured with a Dwyer wind gauge at the top of the slope. This was without ballast on a
wing loading of 10.5 ozs./sq. ft. I must admit it was not pleasant. My eyes were watering,
we had to remove the frequency pennant to avoid damaging the transmitter aerial and stand,
leaning into wind, with one foot in front of the other, to prevent being blown over.
Others have been certified for less!! It is all down to model selection and experience.
This does not mean the inexperienced should not fly in strong winds, just that they should
seek the help of a suitably experienced flyer when doing so. How else are they going to
Model selection depends
mainly on structural strength and how clean aerodynamically the model is. Obviously a
lightweight open structure thermal soarer is not going to be suitable, neither is a boxy,
draggy, rudder elevator slope trainer. The thermal soarer is out because it is unlikely to
be structurally strong enough whilst the slope trainer is out because it is has too much
built in drag. On the slope trainer the flying speed can be increased by adding ballast
and moving the Balance Point forward but this is at the expense of Glide Angle. There is a
point where the Glide Angle becomes so steep that the lift produced by the slope is
insufficient to keep the model airborne.
Most modern intermediate
models of modest wing loading (greater than 10 ozs./sq.ft) can cope, unballasted, with
winds up to 35 mph. They may require a little nose weight to increase pitch stability and
a small amount of down trim but that is all. A typical wing loading for my models is 10 -
12 ozs/sq.ft and I never ballast to cope with strong winds. On rudder elevator models I
occasionally add a small amount of nose weight but this is only to desensitise the model
for the inexperienced pilot as the increased flying speed increases control response.
THE WIND ON THE HILL
For most slope soaring
hills there is an optimum wind speed. Below this speed and the lift decreases, above it
and the lift gets flattened resulting in a lift 'ceiling'. This is particularly noticeable
on coastal sites overlooking the sea. The reason is the air above the hill has got inertia
and the faster the air is travelling the more inertia it has got. The air that is being
deflected upwards by the hill has got to overcome this inertia so the higher the wind
speed the more difficult it becomes and hence the apparent lift ceiling. On inland sites
this phenomena is less apparent because the terrain in front of the hill disturbs the air
giving it less straight line inertia.
This 'flattening' of the
air as it passes over the hill also causes the wind speed to increase because the
free-stream air and the hill are acting as a venturi. With a venturi, if the
cross-sectional area is decreased, then the speed of the fluid passing through it
increases pro-rata, consequently the wind speed at the top of the hill is at its greatest.
If we were able to measure the wind speed 50 metres out from the hill we would find that
it was considerably less than at the top of the hill. Likewise if the wind speed were
measured halfway down the hill we would get a similar result.
When flying your models,
irrespective of the wind conditions, this venturi effect is not only present on the top of
the hill but on the sides as well. If you are unfortunate enough to allow the model to
drift downwind and off to the side of the hill it will be doubly difficult to penetrate
back to the main part of the slope. Not only will the wind speed be higher but there is
the added complication of no lift. It is often advisable in these circumstances to cut
your losses and land as best you can on the side of the hill. Continuing to attempt to
regain a position in front of the slope could result in a long walk and/or a severely
damaged model. It is wise to formulate a plan for this and other possible eventualities by
exploring the flying site. Construct a mental picture in your mind of the airflow above
and around the hill, identify possible landing areas and think through all the options
should you get caught out.
There is very little
model preparation that can be done except to check that the structure is intact and there
are no loose bits of covering that can be torn off by the wind. Flying in windy conditions
is a rigorous test of the model and any weaknesses in the structure will be cruelly
examined. If you intend flying a basic trainer / intermediate model then move the balance
point forward by up to 10% of the mean chord. The more 'draggy' the model the more forward
the balance point. To make headway in strong winds the model must fly faster than the
speed of the wind. There are two ways this can be achieved. One is to apply down elevator
trim the other is to move the balance point forward. In the first instant a small amount
of down trim can be used to increase the model's flying speed but a point is soon reached
when using more down trim just results in a steeper nose down flying attitude (dive)
without an increase in forward speed. Some of this deterioration in glide angle is due to
the design of the model and the increase in drag that accompanies an increase in speed but
some of it is due to extra drag being generated by the down elevator. If the balance point
is moved forward then some of this down elevator can be removed allowing the model to fly
more efficiently. Moving the balance point forward has a secondary effect, it makes the
model more stable in pitch. Not a bad thing considering the stronger the wind the more
turbulent the air will be.
The first golden rule,
for the inexperienced, when flying in windy weather is never turn downwind, always turn
into wind. Likewise, aerobatics should always be exited with the model pointing into wind
i.e. away from the slope. There are three potential problems when flying downwind:
1. The ground speed of
the model is very high which means any accident is likely to be serious.
2. The flying speed of
the model is invariably too slow because the pilot is alarmed at how fast the model is
travelling relative to the ground. This results in poor control response and the model
flying in a semi-stalled condition waiting to be fully stalled when the controls are used.
3. The model always gets
blown back further than intended leaving the pilot with the difficult, often impossible
task, of regaining lost ground.
As soon as the model is
not pointing directly into wind it will drift back towards the slope. This has to be taken
into account when carrying out aerobatic manoeuvres by starting them well away from the
slope, both horizontally and vertically. There is safety in height and distance!!
Obviously if you keep the model pointing into wind and it's flying speed is greater than
the wind speed then eventually it will disappear to the horizon. To prevent this happening
fly the model cross wind and allow it to drift back towards the slope making sure that the
turn at the end of the cross wind leg is into wind. The model will be describing an
elongated figure '8' if it's flight path is viewed from above.
For the more experienced
and those with adequate supervision try a full 360 degree turn. The turn must be started
well out and be well co-ordinated. Up elevator must be used to prevent the speed building
up in the turn. Any excess speed will result in a 'zoom' climb when the model is returned
to a level attitude, into wind. This leads onto the second golden rule of flying in windy
weather, never let the wind see the underside of the model. Presenting the
underside of the model to the wind will result in a deep stall and the model being blown
back towards the slope out of control.
Spins are a manoeuvre
that require plenty of horizontal and vertical space between the model and the hill.
During a spin the model loses height rapidly but gets blown back towards the hill.
Normally the airspace in front of and below hill provides an additional safety margin
during aerobatics should the recovery not go as intended but in the case of the spin this
area is not always available due to the model's drift into the slope. When recovering from
a spin ensure that the model recovers facing into wind. This is achieved by anticipating
how long the spin continues after the control sticks have been neutralised i.e. if the
models takes a further half turn to recover from a spin then neutralise the control sticks
when you can see the top of the model.
Yes I know the phases of
flight are launch, bomb around and land but in preparing for a flight I assess the
conditions first, think about the landing and then launch the model. Launching the model
without a landing plan is a recipe for disaster, particularly in marginal conditions.
Landing and landing techniques is a complete article in its own right. There is not space
in this article to discuss anything but the basics and highlight some of the potential
The object is to land
the model safely with no damage. To do this the model must land in a level attitude, into
wind with the minimum of forward ground speed. Some sites will permit you to adopt a
conventional rectangular landing approach whilst others may require that the model is
'belly flopped' onto the side of the hill. Whatever the method do not attempt to land
unless you are confident in your ability to succeed. If you are not confident get a more
experienced modeller to land the model for you. This does not guarantee success but it
does reduce the odds!
As advised earlier in
this article, unless you are familiar with the site take a stroll around it. Identify
suitable landing areas and try to imagine the prevailing conditions. Locate any possible
obstructions and decide how they are going to affect your approach. Before attempting to
land carry out several practice runs to assess the conditions. It may be that the wind is
so strong that the normal circuit has to be replaced with a crosswind drift back towards
the hill until the model is in suitable position where upon it is pointed into wind and
'driven' onto (not into!) the ground. These are all options that must be considered.
Finally, when the model
has touched down do not relax but be prepared for the wind to try and re-launch it. Hold
in down elevator and keep yours eyes locked on the model. Over the years I have seen a
number of models become airborne again after the pilot has put the transmitter down
thinking the model has landed safely. Rudder elevator models are particularly prone to
this so get someone to hold the transmitter for you whilst you retrieve the model.
Having decided how you
are going or not going to land as the case may be you can now think about launching the
model. As mentioned previously the wind is at it's strongest on top of the hill and it's
lightest at the bottom. The objective, on launching the model, is to get it out and away
from the hill as quickly as possible and into the good soaring air in front of the hill.
To do this the model must be 'driven' forward, away from the slope, against the wind. The
method I use is to 'contour fly' the hill by flying the model down the hill, a few feet
off the ground, then pushing out, away from the hill, slowly converting the excess speed
to height without losing ground by allowing the wind to get underneath the model. Any
tendency for the nose to rise must be countered by re-applying the down elevator. Do not
worry about the model being low as the model will soon gain height and assume a more
normal position in the sky.
When launching the model
go a quarter way down the hill where the wind is not so strong. If possible solicit the
help of an experienced modeller to release the model. Carry out final radio / trim checks
and apply a small amount of extra down elevator trim. Get your assistant to gently
launch the model in a slight nose down attitude. Be prepared for anything to happen! With
any luck the model will sail away from the slope without any problems but if it does not
the model is more likely to leap into the air and get blown backwards than dive into the
ground. Diving into the ground from the launch is not usually too much of a problem but
leaping into the air requires a cool nerve and a steady hand if an accident is to be
On cliff sites it is not
possible to go down the hill to launch the model or advisable to contour fly the hill!
Instead dive away from the cliff face at an angle of 20 to 30 degrees and recover as
before. Flying at cliff sites can be quite disconcerting, as the frequency pennant ribbon
often points out to sea due to the 'curl-over' at the cliff edge. This curl-over must be
taken into account when landing. Where possible, it is advisable to go a long way back to
avoid the turbulence.
In reading through this
article I am conscious of three things:
1. A cautious approach.
2. There is a lot of
material to absorb.
3. The experience needed
for this type of flying.
The cautious approach is
advocated because models are expensive, take a long time to build and are easily trashed.
I would hate to think how many modellers are lost to the hobby each year because their
models are needlessly wrecked. The only way to avoid this is by modellers gaining the
necessary knowledge and experience under the supervision of suitably experienced flyers.
This means plenty of background reading and flying whenever possible. See you in the next
storm force 9!
Required To Support
1. Photographs of
Wingbat and typical intermediate slope models.
2. Section of hill
showing windgradients / venturi effect.
3. Plan view of
crosswind flying (figure '8')
5. Diagram showing
effect of presenting underside of model to wind