Letchworth Model Aeronautical Society


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Gliders

Hours of endless fun are to be had with any reasonable model glider, out on the hillside enjoying the excellent scenic views, the natural quiet surroundings and the free exercise walking to and from the sites. Slope flying is a very relaxing activity with no fuel or power packs to lug around, no motor starting problems and no messy model at the end of the session, unless you are unfortunate to land it in a fresh cow pat! Slope flying can be quite challenging and just as rewarding as flying a powered model. Careful use must be made of the prevailing wind conditions and any thermal activity to optimise the model performance. Aerobatics present a particular challenge as opening the throttle in an attempt to regain the height lost in manoeuvres will be found to be very ineffective!

The slight drawback of slope soaring is that it not only depends on the wind being of suitable velocity as in all model flying, but in addition it can only be indulged in if the wind direction is up the slope of one of the available hills. Electric flyers find that slope soaring fits in well with our electric activities because when the wind is too strong for our lighter electric models it is sometimes ideal for slope flying. It also helps if you are retired or can get off work easily as suitable days for slope flying do not always occur on weekends! To give an idea of how often slope flying opportunities arise, a few years back when flying slope soaring gliders was our only model flying activity, we were averaging some 50 to 60 slope flying sessions per year. Now when the wind is light we fly electric models even though conditions might be right for slope flying, so that over recent years we have only averaged around 20 to 30 slope flying sessions per year.

Slope soaring

Slope soaring uses the upwards component of the wind velocity as the air blows up the slope of the hill to counter gravity and keep the glider airborne.The hills in this area are fairly modest affairs and so the model must be reasonably efficient aerodynamically, so forget the draggy airframes with high wing loadings. Usable wind speeds for soaring on our usual hills are generally in the range 7 to 20mph, although in the right conditions and with a suitable model you can fly in winds as low as 3 or 4mph, or as high as 25mph. Even higher wind speeds can be flown but it becomes increasingly difficult and the enjoyment factor falls off rapidly. All the sites we fly from are on farmed land and livestock can sometimes be present, so do not forget to shut all gates that you open and leave no litter etc. Also no powered models of any kind, ( i.e. IC or electric ) can be flown on these hills and proof of insurance should be carried although we have never been asked to produce it.

Suitable models

Because gliders are devoid of propulsion systems they provide the simplest and cheapest form of RC model flying. When there is sufficient lift the flight can last as long as the Rx and Tx batteries provide power. The slope gliders we normally fly have wing spans between 1.5 and 2m, and are of the sport aerobatic type. It is also possible to fly scale or semi-scale models of powered aircraft (without power of course), which are referred to as PSS models. Although very low wing loadings of say 6 or 7ozs/sq.ft. can be flown in light wind conditions, as the wind velocity increases the wing loading must also increase in order for the model to penetrate. For general flying a wing loading in the region of say 10 to 12ozs/sq.ft. is recommended, but a loading as high as 16oz/sq.ft. can be quite satisfactory on the right model even in the lighter wind strengths. Low aerodynamic drag also aides penetration and improves the efficiency of the model. A highly cambered wing section may produce good soaring ability in lighter winds but it restricts the speed range of the model and does not provide versatility in a slope soarer. As with powered models the aerobatic ability of the model is improved with a semi-symmetrical wing section or even a fully symmetrical section, although the latter may make retaining altitude difficult in lighter wind strengths Slope models generally should be robustly constructed. There is no point in light construction as ballast will have to be added to obtain a wing loading that will provide a useful penetrating ability. The additional weight might as well be employed in the structure to produce a stronger model. Hill sites are often rugged in nature and landings can sometimes be rough, particularly if the model lands on the sloping face of the hill. As in all types of model flying it is useful if the model comes apart in the event of a heavy landing to reduce damage. Construction using nylon wing bolts, frangible ply plates or rubber banded flying surfaces all help in this regard.

Thermal gliders can of course be flown from the slopes and provide good soaring performance. However they are not particularly robust, and not normally very aerobatic. For those who are not too sure of their slope flying skills, Phoenix make a range of EPP models that are hard to break. The Synergy Duet Sport and the Ban-SHE are typical slope aerobatic trainers. Many other manufacturers produce some very good ARTF models, although the fully moulded models become very expensive. If any of you still build your own models then spruce spars with sheeted wings and plywood sided fuselages are the order of the day. It is useful if your model is able to accommodate extra ballast in order to help penetration on windier days. Some of the more modern transmitters provide for air brakes which are very useful when landing a glider in a tight space.

Dress code

No you do not have to wear a tie when slope soaring but you do have to wrap up warm. Standing on the top of a hill in a 15 to 20 mph wind in the cooler months of the year is not for the faint hearted, but the discomfort can be largely offset by suitable clothing. Even in the summer it is usually necessary to wear extra clothing on the hills. Think in terms of wearing an extra couple of layers on top of what you would need at our site in Letchworth. As the outer layer in the cooler weather we usually wear water proof trousers and jacket which keeps the wind out and keeps you dry if you do get caught out by the rain. Remember there is no shelter and no handy car to dive into! A warm hat, scarf and gloves are also required in the colder weather. It is beneficial to wear stout shoes or walking boots at all times of the year as the ground can be rough. At least the amount of equipment you need for hill flying is only a fraction of what you take to the Letchworth field to fly powered models, which is just as well as you have to carry it up the hill! We carry a small haversack containing our transmitter, a few odds and ends, and a flask of coffee. A mobile phone can also be useful, not only for possible emergencies, but also for ringing the wife to tell her that you will be an hour longer than you told her because you are having such a good time.



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