Stability can be simply classified as static stability or dynamic stability.
- Static Stability. If an object is disturbed from a given position and following this disturbance it tends to return to this position of its own accord, it is said to be statically stable. If, following the disturbance, it continues to move further and further away from its original position, it is said to be statically unstable; if it remains in the disturbed position, it is said to be statically neutrally stable.
- Dynamic Stability. If an object is statically stable it will return to its original position, but in doing so, it may initially overshoot. If the amplitude of the oscillations decreases and dies out, it is said to be dynamically stable. If the amplitude of the oscillations increases, then it is said to be dynamically unstable, and if the oscillations continue, but at a constant amplitude, it is said to be dynamically neutrally stable.
Stability in the Hover
Consider a helicopter hovering in still air when a gust of wind affects the rotor disc from the side. The disc will flap away from the wind and, if no corrective action is taken by the pilot, the helicopter will move away from the gust. After a short while the gust of wind dies out but, because the helicopter is moving sideways, it will now experience an airflow coming from the opposite direction. The helicopter will now slow down as the disc begins to flap away from this new airflow; in addition, the fuselage will tend to follow through as an overswing, thereby tilting the disc further than it was tilted before, and the helicopter will move sideways back towards its original position faster than it originally moved away. The movement of the helicopter will
result in it experiencing continual sideways changes in the airflow affecting the disc and, although it will be statically stable, because the amplitude of the oscillations will be continually increasing, it will be dynamically unstable. The effect of a gust of wind from any direction will produce the same effect on the disc, therefore the helicopter is dynamically unstable in the pitching and rolling planes.
A gust of wind will also affect the tail rotor. If for example the helicopter has a starboard mounted rotor and is struck by a gust from the starboard side the tail rotor’s angle of attack will decrease. Assisted by the weathercock action of the fuselage, the helicopter will then yaw into the gust, ie to starboard. The aircraft will also move away from the gust and in so doing it will reduce the effect of the gust on the tail rotor. The aircraft will then experience an airflow from its own sideways movement and the aircraft will yaw to port. Following the movement of the helicopter it can be seen that the fuselage will be alternately yawing to port and starboard with each successive sideways movement of the helicopter. Therefore, when hovering, the helicopter is statically stable but dynamically unstable in the yawing plane.
Stability in Forward Flight
If a gust of wind from the starboard side strikes the fuselage of a helicopter with a starboard mounted tail rotor in forward flight, the immediate effect is for the tail rotor’s angle of attack to decrease and the helicopter to yaw to starboard. But the inertia of the helicopter will continue to keep it on its original flight path; weathercock action will then return the fuselage to its original position. In forward flight, therefore, the helicopter is both statically and dynamically stable in the yawing plane.
If a gust of wind affects the disc from ahead, the disc will flap back, and forward thrust will reduce and the aircraft will decelerate. Because the centre of gravity is below the thrust line, the inertia of the fuselage will cause the aircraft to pitch nose up, taking the disc back further and thus decreasing speed even more. When the speed has stabilised at a lower figure, the fuselage will start to pitch down below its original position (pendulosity): at the same time the disc will flap forward relative to the fuselage (reduced flap back due to lower speed). Now the speed will start to increase with the helicopter descending in a shallow dive and, as the speed increases, the disc will begin to flap back again and the cycle will be repeated, but with increasing amplitude. The helicopter will finally be pitching outside control limits unless cyclic correction is applied early in the cycle. The helicopter is, therefore, statically stable because each oscillation will take it through its original position, but is dynamically unstable because the amplitude of the oscillations progressively increases.
One method of improving stability in forward flight is by fitting a stabiliser at the tail of the fuselage. Its purpose is to help prevent the fuselage from following through when a gust of wind causes the disc to flap back. As the fuselage begins to pitch up, the increasing angle of attack on the stabiliser will damp down the movement and the rearward tilt of the disc will be greatly reduced; the reverse effect takes place when the fuselage pitches down. It should be noted, however, that the stabiliser will produce adverse effects if the helicopter is moving backwards: following a gust of wind which causes the disc to flap forward, the fuselage will pitch nose down and the tail will pitch up; this will increase the lifting force on the stabiliser, thereby increasing the pitch-up movement of the tail to a dangerous degree.
The autostabiliser, is the simplest form of control system. The autostabiliser is a damping device without the ability to hold a given datum, hence a helicopter autopilot often consists of an autostabiliser to which long term datum holding is added. There are two types of autopilot which may be fitted to helicopters:
- Basic Autopilot. A basic autopilot provides long term datum holding of one or more variables but does not permit the pilot to introduce demands through his controls. Trimmers may be used to make limited adjustments.
- Directed Autopilot. A directed autopilot provides long term datum holding of one or more variables and also permits the pilot to introduce demands through his controls. Such an autopilot is also described as an Attitude Manoeuvre Demand System and may also be called Automatic Stabilisation Equipment.
As you can see, helicopter stability is not a simple topic but in general, helicopters are statically stable and dynamically unstable.
The helicopter is statically stable because each oscillation will take it through its original position, but it is dynamically unstable because the amplitude of the oscillations progressively increases.
For really technical information you can try reading this: http://www.aerojockey.com/papers/helicopter/report.html
How Many Aircraft in the World?
I am sure you have all wondered at some time or another how many helicopters are in the world today. Well, I did a little research and this is what I came up with.
|Type of Aircraft||Number|
|Active General Aviation Aircraft||360,000|
|Passenger (Commercial) Aircraft||17,770|
General aviation (GA) is defined as all aviation other than scheduled commercial airlines and military aviation.
The 360,000 General Aviation Aircraft includes fixed wing, rotary and private business jets. General Aviation Statistical Databook & Industry Outlook. 209,000 of these aircraft are based in the USA (2012 data).
In 2012 there were 10,055 General Aviation helicopters in the USA and approximately 15,000 rotary pilots.
According to http://www.helis.com/faq/, in 2001; of the 26,500 Civil helicopters, distribution is as follows:
- 46.1% in North America
- 18.2% in Europe
- 12.7% in former Soviet Union
- 12.3% in Asia/Pacific region
- 7.2% in South America
- 3.4% in Middle East
- 0.1% in Africa
Of the 29,700 Military Helicopters, distribution is as follows:
- 33.5% in North America
- 21.1% in Europe
- 16.6% in former Soviet Union
- 13.6% in Asia/Pacific region
- 6.3% in the Middle East
- 4.5% in Africa
- 4.4% in South America?
Boeing forecasts there will be demand for more than 35,000 new planes worth $4.8tn (£3.1tn) over the next 20 years, with airlines keen to replace fuel-hungry older models to cope with high oil prices. http://www.theguardian.com/business/2013/jun/11/boeing-commercial-planes-double-asia-pacific.
According to Flight International’s 2003 Military Aircraft Census, there are an estimated 89,129 military aircraft worldwide.
These figures were very difficult to find and it must be remembered that the numbers are changing constantly.
For those of us who have the opportunity to fly helicopters, it is obvious that we are the privileged few. There are over 7 billion people on our planet today but there are only 56,200 helicopters.
Please let me know if you have any further information and as usual, feel free to leave any comments – good or bad.
Merry Christmas and Happy New Year to everyone.
When a PPL student finishes their training the instructor has to prepare them for the EASA PPL skill test. The skill test involves a flight test and an oral test. As a student, you must pass both of these.
The oral test is normally completed before the flight test and is part of the flight test process. Any students that fail the oral test will not be permitted to take the flight test and the test will be deemed as a ‘FAIL.’
The oral test can take anywhere from one to two hours to complete. The examiner is not going to try and trick you with nasty, tricky questions. He/she is only trying to determine if you have the minimum amount of knowledge to fly safely. This knowledge will have been taught as part of the training syllabus.
Under exam pressure, students often forget things that they have learned in the past. An examiner will appreciate that you are under a lot of stress and will take this into account. You are not expected to know all the answers but the examiner will try to help you to recall the information or at the very least, expect you to know where to find the information relating to any questions asked.
On satisfactory completion of the oral test, the flight test can commence. You will be given time to plan a route provided by the examiner. You will be expected to fly the route and also perform an en route diversion. Also during the flight, the examiner will ask you to perform standard and emergency flight manoeuvres.
On completion of the flight test, the examiner will normally not inform you if you have passed until after the debrief. It is possible to have a partial pass, in which case a retest of the failed flight test items is required. We will discuss this in more detail later.
With all of the above in mind, I have written a book that is aimed at making the EASA skill test easier to pass. The book goes through the psychology of the test and things that you can do to relieve the stress of the test. It also gives a detailed description of what you should have covered in the PPL syllabus and what exactly you will be required to do during the flight test.
The book is aimed at PPL students for helicopters, aeroplanes and airships. There are over 350 sample oral questions for each of these categories but the air law is slightly different and is tailored to the Irish pilot so a few of the air law questions may not apply in other European countries. With 170 pages of information relating to the skill test, this book is the ideal preparation for the skill test.
The book can be purchased on Amazon (UK) by: click here
The book can be purchased on Amazon (USA) by: click here
The book can be purchased on CreateSpace by: click here
EASA Licence Conversion
Now that the EASA regulations have been fully implemented, I am getting enquiries from worried pilots who have let both their helicopter type rating and their licence expire. The old JAR licence needed to be renewed every 5 years. The new EASA Licence (more correctly referred to as a PART-FCL licence) has no expiry date.
The literature relating to renewing an expired licence is difficult to find in the Part-FCL document but after trawling through the relevant documents and with some help from Simon White in the IAA, I was able to get the correct information.
First of all, there is no need to panic or worry if your old JAR licence has expired. It is a relatively simple process to renew it. Anyone wishing to renew an expired JAR license to EASA license should ensure that they do the following (this applies to PPL(H) only – I have not researched the CPL(H) yet):
- Have a current Class 1 or Class 2 medical certificate
- Have passed the English Language Proficiency (ELP) test and reached a level of at least level 4.
- Complete the requirements to renew any helicopter type rating (a licence cannot be issued without a type rating). Refer to my previous post “Renewing an Expired EASA Type Rating“
- In Ireland – complete the application form www.iaa.ie/media/ApplicationfortheRe-Issue1.pdf
- Pass the oral and flight skill test
On successful completion of the above, the Authority (IAA in Ireland) will issue an EASA Licence.
Note that there is no longer a requirement for a Type Rating written exam. The examiner will test applicants verbally before the flight test. You will still be required to have the necessary technical knowledge before the examiner will issue a pass.