stickmaker | JOHT 57: Spinning

The Joy of High Tech

Rodford Edmiston

Being the occasionally interesting ramblings of a major-league technophile.

Please note that while I am an engineer (BSCE) and do my research, I am not a professional in this field. Do not take anything here as gospel; check the facts I give. If you find a mistake, please let me know about it.

Spinning

Autogyros have never been properly appreciated; yet the concept refuses to die. It actually seems to have enough merit to survive changes in aviation fashion.

An autogyro - or gyrocopter - is very much not a helicopter... yet there are some strong similarities between them. Both use a rotating wing above the body of the vehicle to provide lift. The rotor for an autogyro is close to what you would find on a helicopter, both mechanically and aerodynamically, though the rotor head is much simpler. Functionally - and this leads to the mechanical and aerodynamic differences – one of the main distinctions is that the rotor does not propel the gyrocopter. Actually, there have been helicopters where most of the propulsion did not come from the rotor, but from another source (such as a propellor in the nose or tail); however, those are rare.

The main difference between the two types of aircraft is important and more significant. In flight, the rotor of the gyrocopter is not powered, but spins like a maple seed pod due to its passage through the air. This spinning through the air produces lift, though not in the same way as a helicopter's main rotor. Actually, since gyrocopters came first - at least as a practical, commercial product - saying that the helicopter produces lift in a different way than an autogyro does is more proper. A helicopter forces air down through the powered rotor disk. A gyrocopter reacts to air moving across and upwards through the (usually) unpowered, free-spinning rotor disk. The details of these different methods of lift production are actually a bit complicated and won't be explored further in this brief study.

Propulsion for a gyrocopter comes from a propellor, either tractor or pusher. As mentioned above, most helicopters get propulsion from the rotor. Helicopters can provide forward thrust by tipping the entire rotor, usually by tilting the helicopter. However, they also can produce thrust by changing the angle of attack of the blades during a portion of their sweep, even with the body remaining completely upright. This second method is much safer close to the ground, and it what makes a helicopter so maneuverable. It can hover, including in a wind, or move in any direction. This added complexity - the ability to change the blades' angle of attack in only a portion of the circuit - is what makes helicopter rotor heads mechanically more complex - and much more expensive - than those of a gyrocopter. If a helicopter's engine fails at altitude, it can autorotate, which involves dropping fast to spin the blade up in flat pitch, then using the collective to increase blade pitch just before landing to turn that kinetic energy into lift, to soften the impact. (This tends to be less a landing than a mild crash.)

A gentler version of that is how a gyrocopter lands routinely. It just does that in a slower and more controlled manner than a helicopter can. Though it doesn't have to go through the switch from powered rotation to autorotation, because it is already autorotating. That is how it flies normally.

A gyrocopter can make vertical takeoffs under the right conditions. Those usually involve a strong headwind. However, many gyrocopters have a power takeoff which can use the engine to spin up the blades in flat pitch on the ground. They then disengage the PTO and hit the collective. They still need a headwind to actually take off vertically, but even in still air the pre-spin greatly reduces the takeoff distance. Without the pre-spin their takeoff distance is still shorter than that of most fixed wing aircraft. Some smaller autogyros don't even have collective pitch control of the rotor, but just use a lever to change the angle of the entire rotor disc, by pivoting the head. These don't use a PTO for spinning the rotor on the ground, either. Naturally, their rotor heads are even simpler and cheaper.

Autogyros are inherently stable and stall proof. Due to this and the autorotation method of generating lift they are safer than helicopters. They are even safer than many types of fixed-wing aircraft, thanks to that resistance to stalling. Haul back on the stick and a gyrocopter climbs until it runs out of speed... then begins settling. At a fairly slow, steady rate, which can be held all the way to landing. This, of course, assumes the propellor is still producing thrust. Without that things proceed a bit faster, but still slower and safer than with a helicopter.

Autogyros were, in fact, designed by a man - Juan de la Cierva - who wanted to create an aircraft which could not stall. The first successful flight was made in 1923.

Gyrocopters are easy to learn to fly. Depending on who you ask - and which specific models you are comparing - they are easier than fixed-wing aircraft, and vastly easier than helicopters.

Most gyrocopters made today are single- or dual-seat. The smallest ones usually lack even a skin, so pilot and any passenger sit out in the slipstream. In the past there were models large enough to carry passengers and cargo. Most of these larger ones were hybrids, with a lower, fixed wing as well as a rotor.

Because of their simpler construction, autogyros are much cheaper than even the smallest, simplest helicopter. This, combined with their safety, makes them a popular aircraft for people who want to fly on a budget. Not only are autogyros inexpensive to own and operate, you can safely learn to fly one with fewer lessons than almost any fixed-wing aircraft. Far fewer than for a helicopter.

People want helicopters because they want true VTOL capability, and perhaps maneuverability. Which ignores the fact that many helicopters need a certain amount of forward speed before they can fly out of ground effect (fly high enough to get away from the extra lift provided from being close to the ground) at least with a full load. Still, helicopters can generally lift straight up off the ground in still air, as well as hover and move sideways and even backwards. Advantage there goes to the helicopters. Which is pretty much all they get when compared to autogyros.

Gyrocopters are inherently more efficient than helicopters. The latter use a substantial part of their power to counter the rotational torque imparted to the body by the air drag on the spinning-the-other-way main rotor. Most use a vertical tail rotor for this. Some use two main rotors. A few use more than two main rotors. Since there is no drive to the rotor of an autogyro there is no torque to counter. (Well, there is a tiny amount from the friction of the rotor bearing, but that's easily countered in flight by the aerodynamic controls.) Because of this efficiency an autogyro can carry more cargo faster further than a helicopter of the same size and engine output.

There is more behind the difference in speed between the two types of rotorcraft than the helicopter's power drain to counter rotor torque. Because of the complexities involved in producing propulsion from the main rotor the helicopter is usually limited to under two hundred knots. Partly this is because the blade moving forward (the advancing blade) gains lift from the forward motion of the helicopter, while the blade moving rearward (the retreating blade) loses lift. At speed, assuming forward motion, this produces a significant roll force. A gyrocopter still encounters this effect, but is less affected by it due to the details of the way the rotor interacts with the air.

Note that - as mentioned above - some autogyro designs have a fixed lower wing. This raises the top speed since the rotor can be put in a low drag configuration at high airspeeds and most of the lift generated by the fixed wing. This arrangement is still slower and less fuel efficient than a pure fixed wing plane, though. Advantage to the fixed wing, though for STOL aircraft which can match the autogyro in rolling takeoff and landing distance the difference is pretty minor.

So why aren't gyrocopters taken more seriously? I suspect a large part of the reason is that they came first, at least commercially when compared to helicopters. Helicopters - with their better VTOL capability and maneuverability - were seen as an improvement, and autogyros fell out of favor. They are seen as old fashioned, even quaint. The stuff of pulp fiction. (As fans of many pulp series know.)

Not for everyone, though. There are still several companies which make autogyros, mostly for sport use.

So, if you need a vehicle easy to maintain and repair, which can take off and land on a small field and is faster with a better range than a helicopter, the autogyro is your best bet.