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Radio Control Model World - Aug '94

by Stan Yeo

INTRODUCTION

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 missed heartbeat.

WHAT IS A STRONG WIND?

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 get experienced?

MODEL SELECTION

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.

MODEL PREPARATION

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.

FLYING

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.

LANDING

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 problems.

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.

LAUNCHING

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 avoided.

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.

SUMMARY

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 The Article

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')

4. Cartoon

5. Diagram showing effect of presenting underside of model to wind.

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