VIEWS: 6 PAGES: 7 POSTED ON: 3/6/2012
RC HELICOPTER Now that rc helicopters are battery operated, the hobby is now easier to get started. RC electric helicopters are simpler and easier to fly than gas powered kits. The advantage of an electric rc helicopter is that they are simpler in design, quiet, and some can even be flown indoors. The disadvantage is the relatively short flying time, usually between 5 - 15 minutes, depending on the model and battery pack. However, the simple solution is to buy several battery packs. Mini RC Helicopters How does the thrill of flying indoors sound to you? Imagine the fun of flying a mini rc helicopter in you home or even your office cubicle. A great way to relax and relieve stress. Mini rc helicopters are usually powered by small electric motors. Some models are tethered, which means there is a thin wire connected to the helicopter as it hovers. This wire is connected to an external battery providing the power. Since the battery is external, you can use high capacity batteries and fly for a very long time. As I have said before, single rotor RC helicopters are, at first, not easy to fly. They have the exact same controls and follow the same laws as full size helicopters. The first step is to understand these forces and laws. It will then be an easy matter to understand how a helicopter is controlled. To make this as simple and basic as possible, I have broken helicopter flight down into three very basic areas: The Air Foil All things that fly, from birds to supersonic aircraft and yes helicopters too - rely on air foils to create lift. An air foil (the shape of all wings if looked at from the edge) creates lift by producing lower air pressure on the top of the wing than the bottom. I could talk about how this happens, but to understand RC helicopter control all you need to know at this point is that all wings create lift by producing lower air pressure on the top of the wing than on the bottom. In effect, there is a vacuum created above the wing sucking it upwards. You can try a simple experiment right now to demonstrate how an air foil works. Take a normal 8.5”x11” piece of paper and hold it between both hands length wise at the leading edge so the back edge of the paper flops down. You are now holding a very simple air foil shape (rounded leading edge followed by a long trailing edge). Now blow down and across the piece of paper, at about a 45 degree angle. What happens? The back edge of the paper lifts up, even though you are blowing it down wards. You just created lift with your simple but effective paper wing. Helicopters don’t have wings of course – well actually they do... we call them rotor blades. You will see me interchange “wing” and “rotor” throughout this discussion. Just as long as we all realize that rotor blades behave just like wings, it will all make sense – I hope. As long as the wing (or rotor) is moving through the air – low pressure is produced on the top of the wing and pulls the wing upwards. The faster the air flows over the wing the more lift. This is a very basic explanation – there are other forces involved such as drag and turbulence that limit the speed of air over a wing and ultimately limit the amount of lift produced. Angle Of Attack The other component of lift is angle of attack or in helicopter control terminology “PITCH”. If you angle the rotor blade or wing up (leading edge higher than the trailing edge) you get even more lift. We call this positive or high angle of attack. What do you think we call it in the heli world? “Positive Pitch”. This picture shows a typical symmetrical rotor blade with a special RC helicopter tool called a pitch gauge measuring the angle of attack or pitch of the rotor blade. As you can see, this blade is showing a positive pitch angle of about +13 degrees. It might seem you could just use a flat board for a wing and just angle it upwards to get lift, but there would be so much drag andturbulence produced, the lift would be minimal for the amount of power that is being used. In fact you could keep adding more and more power and the only result would be more and more turbulence and drag acting against the lift produced. Air foils are very efficient at creating lift with limited drag and turbulence - that is why they are used. Air Foil Shapes So now let’s take the air foil and pitch idea one step further. Let’s take our basic air foil example from above and create a mirror image of the air foil on the bottom of the wing. If we look at the wing edge on, the top and bottom both have the exact same air foil shape – they aresymmetrical. Thus the name symmetrical wing or rotor blade – same on the top and bottom. Having an air foil shape on both the top and bottom of a wing now means airplanes and helicopters can fly upside down. By changing the angle of attack or pitch to negative when upside down, the rotor now produces lift in the opposite direction – pretty neat. Here is our RC helicopter pitch gauge now showing a negative pitch angle of about -9 degrees. Because this is a symmetrical rotor blade, if the helicopter was upside down, the rotor would actually be creating lift allowing the heli to fly or hover upside down. Certainly not all rotors have symmetrical air foils – some have semi- symmetrical or flat bottomed air foils depending on how efficient the rotor has to be. Symmetrical air foils are not nearly as efficient at producing lift as a flat bottomed air foil shape using equal amounts of power. This is why birds don’t have symmetrical shaped air foil wings. The amount of energy to fly would be too great. I guess that is also why I have never seen a bird flying upside down. A quick note – when I am talking about flat bottomed air foils I am also referring to a hollowed out air foil. This is like our piece of paper wing experiment above, a kite wing, or a bird’s wing. There is no flat bottom on the wing, just the curved underside of the upper wing section. This design is ideal for fixed pitch applications and produces lots of lift - they are sometimes called high lift rotors. I wanted to point this out because many micro and coaxial RC helicopters use this type of air foil rotor design. The obvious reason is because it produces the most amount of lift and has the least amount of weight (high lift to weight ratio). In our helicopter hobby, all fixed pitched helicopters will use flat bottom or hollow air foils to create the most amount of lift for the amount of power available. As a result, smaller engines or motors/batteries can be used for longer flight times and lighter helicopters. This is a very good compromise seeing that fixed pitch helis can’t fly upside down anyways. Most of our single rotor collective pitch helicopters will use symmetrical shaped rotors coupled with powerful engines or motors. This is how we can produce lift when the helicopter is inverted. We simply use our helicopter controls to change the pitch angle from positive to negative. There of course are RC helicopters with collective pitch that use flat bottomed or semi-symmetrical rotor blades to benefit specific areas of flight, such as auto rotations . However, most out of the box collective pitch, single rotor helicopter kits will come with symmetrical rotor blades because they are the best compromise for all areas of RC helicopter control. To Sum Up: Air foils are required to produce lift. By increasing the speed of the wing or rotor we increase lift – up to a point. By increasing the angle of attack or positive pitch of a wing/rotor – we also increase lift. Flat bottomed air foils are more efficient at creating lift than symmetrical air foils. To fly upside down, we have to use symmetrical wings or rotor blades. Fixed pitch helicopters use flat air foils and don’t require as much power to fly. Collective pitch helicopters generally use symmetrical rotor blades but need more power to fly. OK – we now understand all that, so how do we effect lift with our rc helicopter controls? We already know that we can achieve lift by either speeding up the rotor blades or changing the pitch angle more positive or a combination of both. For our fixed pitched heli, we only have one option – increase the speed of the rotor. On our collective pitch heli, we can speed up the rotor, increase the angle of attack, or do both. From the discussion in the fixed or collective pitchsection – we know there are several advantages to using collective pitch and why it is preferred method of helicopter control over fixed pitch. Now we can understand what is exactly going on with our helicopter controls in relation to lift. To lift off the ground or gain altitude, the helicopter must produce more lift force than the force of gravity pulling it down. To sustain a hover, the lift force must equal the force of gravity - the force of lift and the force of gravity are balanced. In this case the helicopter remains at a fixed altitude. To descend, the lift force has to be less than the force of gravity so gravity can pull the heli down. These are very basic examples of lift theory and how it applies to helicopter controls. As we examine more areas of flight theory in the Direction And Cyclic Control section, you will see there are many other forces acting upon the helicopter that cause the amount of lift being produced by the rotors to be changing constantly. This is why there is no such thing as a simple “up-hover-down” helicopter control button on your radio or in a real helicopter – the dynamics are always changing and you have to keep adjusting to stay in control.
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