Sharing what I'm learned
I'm not a QFI, but I do occasionally help teach groundschool on a course for baby test pilots. I've posted below some notes from that course which might be of interest.
All is (c) Genghis, but by all means any instructor who can make use of this feel free to copy it and make appropriate us. One of these days I'll find the time to put all this together and publish a book, but I'm far too damned busy at the moment.
There are some tables that normally go in this text, but they're only really relevant to test pilots, and anyway I don't know how to post a table in Pprunetext.
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The Unaccelerated and turning flight stalls
The unaccelerated stall is very important to the airworthiness of a fixed wing aeroplane in a number of ways. It is essential to the definition of many of the critical operating speeds - the approach speed and Va in particular are functions of the stall speed. It also defines the manoeuvre margins for a turning aeroplane, and for most light aircraft codes, decide whether the code may be applied at all, particularly for microlights.
The conduct of stalling tests on a new or significantly changed aeroplane is a potentially hazardous business, and flight testing practices is a whole separate subject; however, the references cover this in good detail. There are however a number of points pertinent to the testing which are not always well understood, and are worth repeating here: -
· The stall is entered at flight idle, at (by convention) a deceleration rate not exceeding 1 kn/s from a trimmed speed of around 1.4 Vs..
· It is impossible in all but some gliders to conduct a stalling test in level flight, the aircraft will be descending.
· No two aircraft types will give identical stall or warning, it is important that the characteristics for the type and variant is identified, repeatable, and documented in the pilots handbook. Many airworthiness standards give overly simplistic definitions of the stall, which may or may not reflect what actually happens - be guided by these, but not bound by them.
· Stall speed data is meaningless from an airworthiness perspective without the ASI having been calibrated .
· Stalling characteristics will vary with CG and power setting. With CG expect to see higher stall speeds at fwd CG, and more dramatic post-stall behaviour at aft CG. With increased power settings, stalling speeds are likely to be lower with more nose-up stalling attitudes. Increased power again, is likely to generate more dramatic post-stall behaviour; some aircraft which are extremely benign when stalled at idle power will tend to enter immediate incipient spin (sometimes with no stall warning) when stalled at high power settings (two examples of the latter characteristic are the UK air cadet variant of the Grob G109b motor-glider, and the Ultraflight Spectrum microlight).
· The stall is not necessarily marked by the classical pitch-break beloved of flying instructors. It may be marked by a loss of nose-up authority (most microlights), an AoA triggered Klaxon (the Aviasud Mistral), an air horn supplied by air pressure from the wing under-surface (Cessna 150), or other cues. The bottom line is that the stall occurs when the pilot ceases to have absolute control over the aircraft - but learn and understand the precise definition used by the standard in use. Equally, ensure that the test pilot, flight test engineer, and airworthiness engineer are all in complete agreement as to the stall definition in use for a test programme, and subsequent documentation.
· Wing-drop at the stall is normal to most aircraft, it is the magnitude of it that is critical to airworthiness. The wing drop limits of all standards assume that correct recovery action is being taken.
The turning flight stall
For the purposes of civil certification, the turning flight stall is universally carried out at 30° of bank in a co-ordinated turn, although requirements for power and flap settings may vary. Since this is an unaccelerated stall, again the entry rate will be no greater than 1 kn/s.
It is almost inevitable that any aircraft will suffer from undemanded rolling at the point of stall, and this is generally more marked in a turning flight stall. There is no universal value which dictates the acceptable limits of this, and it is important to be aware of what the relevant standard says. The terminology universally used for this undemanded rolling motion is “into the turn” and “out of the turn”. If an aircraft is banked to the right, and at the point of stall it banks more to the right, then it is said to have rolled “into the turn” (this incidentally is a characteristic most commonly associated with low wing aircraft), whereas if it tends to roll towards wings level (a characteristic most often found in high wing aircraft) then it is said to have rolled “out of the turn”.
Regardless of the precise wording of any requirement, it is important that stalling from a co-ordinated turn cannot cause a spin. The author has seen several aircraft which did display this tendency (i.e. never assume that it won’t be there!), the best known of which is the North American Harvard which routinely will enter an incipient spin from a stall at PLF or MCP - a characteristic which is believed to have killed a good many student pilots during WW2. Similarly any marked pitch-up or control force lightening at or near to the stall should be regarded as unacceptable.
Recovery from the unaccelerated stall
Most aircraft will pitch down, with varying degrees of severity, at the point of stall. This naturally puts the aircraft into a dive from which the existing trim setting should allow it to recover without pilot input - although for an efficient (and comfortable) recovery correct handling by the pilot - initially to allow the aircraft to recover flying speed, then to pull out of the dive and re-establish straight and level flight (probably with an increase in power) is considered normal.
However, it is important (and some standards, although not all do discuss this) to ensure that the acceleration immediately following the stall cannot lead to an inadvertent exceedence of Vne / Vmo / Mmo. Also the pullout (especially that pull-out naturally caused by the pitch trimmer setting) must not cause the normal acceleration or Angle of Attack limits to be exceeded.
The accelerated or dynamic stall
Whilst for the purposes of determining stalling speeds a 1 kn/s deceleration is universally used, this is not necessarily appropriate to every real-world situation. Indeed, most microlights will have some difficulty in entering a stall at only 1 kn/s. For this reason, most standards also require consideration of a more rapid stall entry, either from a rapid wings-level pitch-up, or from a steep turn. The specific requirements of each standard do however vary considerably; this is unsurprising since in the more coarse manoeuvres, the different weight-classes of aircraft inevitably will be flown in very different ways.
How aircraft behave in response to this is very variable. Most aeroplanes will display a more marked nose-down pitching motion (or a measurable one, if none existed before) at the point of stall. Aeroplanes with laminar flow lifting surfaces are likely to display less wing-drop at the stall (because both wings stall at the same time), whilst aeroplanes with more conventional wing surfaces are likely to display more wing drop. Some high wing aeroplanes, when stalled from a steep turn will tend to roll naturally wings-level before naturally resuming straight and level flight (probably as the wings stall, pendular stability becomes the dominant roll effect).
The following table shows the main requirements of the various standards. The reader should however treat this with caution - all the standards are somewhat vague (and often probably too relaxed) in their requirements, and it is most important to construct tests which reflect the way in which the aircraft will actually be flown. Most organisations dealing routinely with flight testing (particularly those dealing with the lighter end of aircraft certification) will almost certainly have their own type or class specific schedules and guidance which should also be referred to - and may well be much more useful than the main certification standard.
This table doesn’t give much information on test conditions. Unless stated otherwise, test conditions are the same as for 30° turning flight stalls discussed above.
The stall warning margin in a dynamic stall
For all the listed standards, stall warning requirements are the same as for the unaccelerated stall. Whilst at first glance this is entirely sensible, in practice it is not and the sensible airworthiness team will aim to go beyond the minima of the standards.
Consider for an example, JAR-23. JAR -23 has a requirement for a minimum 5 knot stall warning margin, and a test deceleration speed for the dynamic stall of 3-5 knots per second. This potentially would give a pilot as little as 1 seconds warning of the impending stall - only enough for the most alert pilot (in an aircraft high elevator power and a very short SPO) to take appropriate action. For any aircraft that is likely to be routinely flown in violent turning manoeuvres (e.g. aerobatic aircraft, utility aircraft used for short field or agricultural operations, any military aircraft or most training aircraft) it is important that the pilot is given clear and unambiguous warning of the impending dynamic stall. Apart from the obvious artificial stall warning devices, ways in which this might be achieved include high stick forces, very high nose-up pitch attitudes, alarms triggered by AoA sensors.
Going beyond the minima of the airworthiness standard in this way can require some courage, particularly if potentially expensive modifications to the aircraft may be required. However, any recent set of fatal accident reports should furnish one or two cases of aircraft loss due to a pilots failure to recognise the dynamic stall - this and the risk of litigation is generally sufficient to convince the most bloodyminded company accountant.
References
- Flying Qualities and Flight Testing of the Aeroplane, Darrol Stinton
- Flight Testing Homebuilt Aircraft, Vaughan Askew
- Flight Test Guide for Normal, Utility and Acrobatic Category Airplanes, FAA, AC23-8