Forces on the Autogyro

In the picture you see a (climbing) autogyro with the forces that act upon it. The picture also shows that the airspeed is coming towards the autogyro. That is because we like to say that the autogyro is hanging still and the air is flowing past it. It is quite common to regard an aircraft as a "fixed" object and move the world around it. So the airspeed is the speed of the oncoming air, while the autogyro remains in the middle of our picture.

Let's start with the most obvious force: Weight (Symbol: W). Weight is the force that keeps us on the surface of the earth, and is therefore the reason why we build flying machines like our little autogyro. Weight is also a dirty word. At least, in the aerospace industry. And it should be a dirty word in any amateur aircraft workshop too. Weight is dirty because it causes more weight. More weight means that you need more lift to stay in the air, a more solid airframe, more engine power and more fuel. And each of those things mean still more weight! It may sound contradictory, but the best method of improving an aircraft is to remove everything you don't need. Sometimes you can even make an aircraft "stronger" by removing material (in fact this reduces weight and therefore airloads, making your aircraft more resistent to them).

To counteract the weight of the aircraft, you need some force to keep it in the air: Lift (symbol: L). Lift is a component of the force that is generated by deflecting air downwards. I say a component, because lift is by defenition perpendicular to the free airstream direction (the arrow marked "AIRSPEED"). The component in the same direction as the free airstream is called drag.
Lift can be generated by a rotor, the fuselage and wings. Wings are seldom used anymore, except for an interesting project to build an economical jump-start airplane (see the web page of Cartercopters)
As you see in the picture, the lift is not in line with the weight, and for a climbing autogyro the combination of lift and weight is slowing down the aircraft. If the aircraft was descending, this combination would be speeding up the aircraft (as you all know from gliders).

Where there is lift, there is Drag (symbol: D). Drag is not something bad, it is just something you should live with. Drag keeps you rotor rpm at a nice value and in general it puts reasonable limits to everything. Drag has by defenition the same orientation as the airspeed and is generally slowing you down. There are all sorts of drag:

deflection of air

• induced drag is the drag that is caused by the deflection of the air, and therefore by lift. As you can see in the picture, deflecting an airstream downward does not only give you a vertical reaction (lift), but also a horizontal one. This is the induced drag. You would have induced drag even if the air would have no viscosity at all.
  Induced drag is sometimes associated with tip vortices. This is mainly because of the fact that tip vortices are a side-effect of lift as well. However, there are (theoretical) situations where there are no tips and therefore no tip vortices. In this case you would still have induced drag!
• profile drag. This is the viscous drag of you rotor blades. It comes with the viscotity of the air. Viscosity does not play any part of importance in the airflow, except for a very thin layer around anything in it. This layer is called the boundary layer. You can compare this to a waterflow: if you take something out of a waterflow, it is not dry. A very thin layer of water sticks to the object, even though water does not feel sticky. It is the same with air. A very thin layer of air sticks to the aircraft and causes viscous drag.
• parasite drag. Parasite drag is the drag gerenated by any part of the airplane that is not designed to generate lift, such as the undercarriage, the fuselage, the pilot, etc. As all these components have very complicated shapes, the parasite drag is very hard to predict or calculate.
• aesthetic drag is caused by really ugly parts of an aircraft. As aerodynamically well shaped things are in general very beautiful, it is evident that ugly aircraft parts are not aerodynamically well-shaped and cause an extra lot of drag. This extra lot is called aesthetic drag.
  (There is a theory, however, that awfully ugly front parts of an aircraft could have a negative aesthetic drag. This can be explained as follows: If one would make the front part of an aircraft so ugly that no air molecule with good taste wants to collide with it, a vacuum would be created in front of the aircraft, thus sucking it forward. Experiments have shown that in such cases the vacuum extends over the whole aircraft (thus spoiling the effect) and that nobody wants to fly it.)
  Aestatic drag makes the difference between a Wallis WA-116 and a Bensen B-8. This remark is not entirely fair. Igor Bensen did not optimize his autogyros for breaking records, but for simplicity in construction. Ken Wallis, however, wanted to have more performance. Both designers built the autogyro they had in mind. Still, improving the aerodynamics of an aircraft will improve the looks as well.
• historic drag is the drag that is caused by any part of your aircraft that is not needed anymore and is still there for historical reasons. You should try to avoid historic drag at all times, except for when you are building a replica of a historic aircraft.

finally, thrust (Symbol: T) is the main source of energy.Thrust enables you to overcome the aircraft's drag and thrust enables you to climb. Remember that - in flight - the engine and the rotor are not coupled in any mechanical way. In contrast to fixed-wing aircraft, it is not always wise to give full power in an emergency situation. Often decreasing your engine power will increase the rotor speed. This has everything to do with the rotor angle-of-attack. If you decrease the engine power while holding the stick in the same position, the aircraft will divert downward, thus increasing the rotor angle-of-attack. This also means that you should give more back stick if you give more throttle. (see also the Myths chapter)

Little Nellie parked safely

The actual forces that the autogyro experiences on the ground are dependent of the construction of the autogyro itself. Most single-seaters have a fixed-pitch rotor, and will develop rotor thrust whenever the rotor is turning. Heavier autogyros generally have an articulated rotor, so that the blades can be set to zero-thrust or even negative thrust pitch during take-off. In the very early days, autogyros could spin up the rotor only by making a take-off run. Nowadays, most autogyros are equipped with either a pre-rotator or a jumpstarter

pre-rotators and jumpstarters are devices that can connect the rotor to the engine mechanically. With such a device, the rotor is driven during take-off. As autogyros do not have tail rotors, the undercarriage must be used to counteract the rotor torque.
A pre-rotator is simply used to help the rotor spin up during a normal take-off run. The use of a pre-rotator reduces the take-off distance, but a take-off run is still necessary. The pre-rotator itself will not give the rotor enough speed for take-off. Pre-rotators are generally found in autogyros with fixed-pitch rotors.
A jumpstarter can only be used when the rotor blades can be set to zero-lift pitch. A jumpstarter will drive the rotor to about 1 ¹/₂ times the normal fight rpm. A take-off run is not necessary.

towed gyro-glider

In some countries, instruction in a gyro-glider is very popular. This form of instruction came from the united states and was originally launched as a legal instruction and training method. Because the towing car is always touching the ground and the combination was legally seen as one vehicle, this vehicle was not flying. And because it was not flying, it didn't need to follow aviation rules.