How is the load factor less than 1g at VSmin?

[IFLY_INDIGO]

Going through the A320 FCOM, I read airbus saying that ' for a conventional aircraft, the reference stall speed, VSmin, is based on a load factor that is less than 1g.' I am confused how is this possible? stalling speed is minimum flight speed and it would ensure sufficient lift for a given weight unless we are manoeuvring the aircraft. After the stall speed, yes, the load factor is less than 1, till the time stall recovery is made.

[vilas]

IFLY_INDIGO
At CLmax is Minimum speed at which load factor is one. Vs is slightly after that at which load factor is minus. That is reference speed for conventional aircraft.

[bubbers44]

The only way I can see stall being computed at less than 1g is in a steep climb or descent. A hammerhead stall maneuver isn't really a stall because the aoa is almost zero when your as drops to almost zero.

[FLEXPWR]

INDIGO,

As I read, stall by definition is a reduction of the lift generated by an airfoil. If you are loosing lift, technically you start going downhill, thus your load factor has to be less than 1G.

From this, can we say that the stall speed is the speed at which the aircraft is no longer able to maintain level flight for a set of fixed parameters?

In this case, there are two separate speeds we can look at: one is the stall speed (less lift, nose drop, less load factor), the other one, slightly higher (maybe just one knot or so), would be the minimum steady speed for level flight , i.e. no loss of lift at his point. That is before the aircraft stalls.

Does this make sense?

[Tourist]

Stall is not speed dependent, it is only AoA dependent.


Since this is the case, you can base a "reference stall speed" on any g you like. They happen to have chosen one that is less than one.


I assume they want a "stall speed" to refer to because speed is the closest/most useful allegory to AoA in the normal Airbus cockpit.


If you really wished, you could fly an Airbus at 10kts without stalling, or conversely, stall it at 200kts.


You could also stall it whilst climbing at 5000ft/min

[FLEXPWR]

Tourist, I am not sure to grasp your explanation, neither on the stall load factor, nor how it applies to Airbus.

Airbus had to redefine the stall speed characteristics as the FBW is based on constant G load (when no input is made on the joystick), thus the FBW is trying to maintain 1G in level flight, right to the point where lift is no longer available. Of course, in practice, the lay pilot may never see this, as Aplha Floor would kick in before that point is reached.

You mention "They happen to have chosen one that is less than one."

Who is they? I suspect "they" did not happen to chose, as you might suggest, arbitrarily, but "they" must have observed that as the lift first decreases (definition of stall), so is the load factor.

A perfect 1G stall is something that a computer can do. I don't think any pilot is able to maintain exactly 1G as the stall is initiated.

I'd be happy to see inputs from fellow pilots and their views on the stall applied to FBW aircraft.

[roulishollandais]

If the wing shape has no sudden nose-drop, Just let the plane descending ! the nose-up pull is limited by what they call "protection" ! They say lift is still enough...

Airbus had the idea to give that defiition of their fake "no-stall" (Re rudderruddererrat link in thread Habsheim post 269", and comments rudderruddererrat, Chris Scott and Haserlnuts39 posts 269-272).)
http://lessonslearned.faa.gov/Indian...0Condition.pdf (http://lessonslearned.faa.gov/IndianAir605/A320%20Final%20Special%20Condition.pdf)

Doing that they sold both the idea that
1. we don't need to use an AoA sensor,
2. no use of inertial HUD,
3. having only the speed on the HSI, and
4. the biggest idea to sell ad libitum their aircrafts : "they do not stall". Now we know they are lyuing.

[de facto]

Hi there,


Reference of VS is for conventional aircraft and represents the speed when lift suddenly collapses(load factor<1).

The yanks use VS,so for example VREF is based on VS which cant be less than 1.3 VS.

Airbus and EASA use VSR (reference stall speed) which may be not less than VS1G (calibrated speed/aoa at which max lift coefficient is just before lifts start decreasing).

For example, vref cant be less than 1.23 VS1G for flaps used (1.23VSRO), VLS on airbus i think.
V2 minimum for example can not be less than 1.13 VSR ...

[Tourist]

"A perfect 1G stall is something that a computer can do."


I don't think the word stall means what you think it means.........

[john_tullamarine]

Comments -

(a) no specific background with Airbus certification

(b) speed reduction during the final approach to stall is quite slow

(c) low thrust = descent = reduced load factor

(d) stall definitions have varied over the years

(e) always one needs to check the frozen certification standard for a particular aircraft before throwing numbers into a discussion.

(f) current heavy rules for FAA can been reviewed here (http://www.ecfr.gov/cgi-bin/text-idx?SID=f6f91fc781336ac9f6699d86a82641aa&node=14:1.0.1.3.11.2.155.9&rgn=div8). One might note the requirement to correct for load factor.

[roulishollandais]

Where is it easy to find the"stall definition" used, the date of certification and modifiations, and additions, in the different ICAO countries?

[BARKINGMAD]

And I thought load factor referred to the ratio of bums on seats to the total of seats on a particular service?

Still learning after all these years........................:confused:

[FLEXPWR]

Let me rephrase:

A computer (FBW in this case) will try to maintain exactly 1G all the way to the collapse of lift. A pilot cannot possibly do such a thing with any precision with conventional flight controls flying manually.

Better? :8

[bubbers44]

If you can hold altitude plus or minus 10 ft like everybody I know you will hold 1 g to a stall.

[Tourist]

"A pilot cannot possibly do such a thing with any precision with conventional flight controls flying manually"






I would disagree. (where can I find a smiley that shows how appalled I am at such a statement...)


In fact, it borders on a definition of a pilot!


If you can't do that then in my opinion you should not call yourself a pilot.


When we used to carry out stall check test flights, we were required to fly maintaining level flight while reducing speed at 1kt/second and check that the buzzer, stickshake and stick push all operated at the correct speeds and time intervals. These would not work if we didn't hold altitude.




john


"(c) low thrust = descent = reduced load factor"


Am I misunderstanding you, or are you saying that a descent means less than one g?!?

[bubbers44]

Yes, a descent is less than 1g and as you approach a vertical descent which is common flying aerobatics you are at zero g loads. The hammerhead or wing over maneuver is in zero g during the top as you rudder over into a vertical dive and the aoa is also zero so no stall.

[Machinbird]

Let me really get people going.:E

If you define Nz as your g measurement, then when your aircraft is pitched up in level flight near stall speed, your z axis is inclined to the vertical, perhaps 15 degrees.

In that case, your Nz will be less than 1 in level flight (before you start falling out of the sky), probably somewhere around .96 g.:8
That force you feel on your back accounts for the rest of the gravitational field.

[HazelNuts39]

If you define Nz as your g measurement, then when your aircraft is pitched up in level flight near stall speed, your z axis is inclined to the vertical, perhaps 15 degrees.The applicable regulation defines Nzw as the acceleration measured normal to the flight path, i.e. vertical in level flight, whatever the pitch attitude. In certification flight test the airplane is decelerated 1 kt/second with idle thrust, i.e. on a slightly descending flight path at slightly less than 1 g. As the airplane approaches the stall speed, the Drag-to-Lift ratio increases so the flight path steepens slightly, curving downwards. For most swept-wing airplanes the minimum speed occurs after passing the point of maximum lift, because the airplane is still losing airspeed due to high drag while the flight path steepens further downwards.

[Meikleour]

Surely, with a lot of swept wing aircraft, the wing does not give up 100% of its lift at one exact speed but rather it progressively loses lift. This is different to the straight wing case when there is usually an abrupt stall "break" and pitch down.

Hence the aircraft is going downhill and progressively stalling the lower the speed? That is my understanding of why the g used is less than 1g.

Where is John Farley when you need him? !!

[Tourist]

I was misreading:O

I was reading load factor but thinking g experienced by the pilot rather than load factor ref the aircraft axis.

[john_tullamarine]

In certification flight test the airplane is decelerated 1 kt/second with idle thrust

Worth keeping in mind that, for the formal test programs, the TP folk run a series of deceleration rates so that the book 1 kt/sec (1 mph/sec for even older aircraft) is determined by sums done in the backroom.

No doubt JF will come into the fray at some stage but, in the interim, several of the recent posters know more than quite a bit about this stuff ... so their counsel is worth attending.

[E_S_P]

Meikleour

Surely, with a lot of swept wing aircraft, the wing does not give up 100% of its lift at one exact speed but rather it progressively loses lift. This is different to the straight wing case when there is usually an abrupt stall "break" and pitch down.

Hence the aircraft is going downhill and progressively stalling the lower the speed? That is my understanding of why the g used is less than 1g.

Where is John Farley when you need him? !!

Dont forget there is always the very knowledgeable CliveL who was a Concorde areodynamicist on here http://www.pprune.org/tech-log/423988-concorde-question.html :ok:

[zzuf]

As a one time certification test pilot, as stated by John Tullamarine to achieve a one knot per second approach rate it was usual to bracket about 0.7 and 1.2 knots per second. The boffins did the correction to 1 knot per second.
The minimum speed after all corrections was the certification stall speed.
The problem with this stall speed determination was that high drag aircraft could have a significantly lower g loading at the stall than low drag aircraft - this could result in a stall speed advantage ie a lower stall speed.
Around the time of A320 certification decisions were made by various authorities to change to Vs1g rather than Minimum Speed in the Stall.
Arguments then arose about the safety factors to be applied. It being obvious that (say) 1.3 Vs was being applied to stall speeds (minimum speed in the stall) which really were not equivalent

[Mansfield]

John Tullamarine: "always one needs to check the frozen certification standard for a particular aircraft before throwing numbers into a discussion."

Most important point in this and numerous other discussions!

Zzuf has the best explanation, at least insofar as it corresponds to my memory of what went on in the Flight Test Harmonization Working Group back in the 90's. Earlier certifications were not required to demonstrate stall at a 1G loading; later certs were and will continue to be.

[john_tullamarine]

No doubt JF will come into the fray at some stage

.. or, indeed, zzuf - greetings, good sir, and one trusts that the nautical life still treats you well.

[zzuf]

Hi JT residence now just S of PH. Nautical pursuits seem to involve vehicles with multiple redundancies, minimum displacement circa 80,000 tonnes, and effective anti slop devices for the champagne.
I should also have previously added that certification stall testing involves two very different test procedures.
1. Stall speed determination, 1 knot per second in the configuration which will give the highest stall speed.
2. Stall handling demonstrations, up to 4 knots per second, power settings of (IIRC) that for around 1.6 Vs, configuration to give the most unsatisfactory handling characteristics. These are highly dynamic stall approaches in both straight and turning flight. The pitch attitude for a straight stall, fairly high power and 4 knot per second can be disconcerting. The stall speeds during such manoeuvres can be very low due to the low g loading in a reducing angle climb and the high approach rate.
For those who may not be aware, high approach rates give a lower stall speed because of alpha over swing beyond the normal stall and airflow separation is delayed.
The low stall speeds contribute to reduced damping of any departure and less response to aerodynamic control inputs.
In some ways it is a pity that line pilots never see how well modern aircraft handle even when grossly abused in this manner.

[awblain]

Using the awareness of this 1 knot per second deceleration to find the stall during testing - about delta-g=0.5/9.8=0.05 - illuminates the answer brilliantly. Added on to the downward acceleration as the lift drops associated with the stall.

It's obvious when presented so plainly, yet the picture conjured up by "a stall" is of something happening with almost no changes leading to it, which is also strange when you think about it, since it's obviously something caused in response to changes in circumstances.

[John Farley]

Hi guys

A few comments if I may:

Clearly the business of certification in general (not any specific test) is about getting good data about how the aircraft flies. The best (ie repeatable) data comes from measurements taken in as near steady state circumstances as possible. When steady state is not possible then one lets the least important term change – a good example being if you want to plot angle of attack against g at a particular speed. In this case inevitably when tightening the turn you will eventually not have enough thrust available to hold speed and height so you descend as required to hold the speed – in this case letting height change.

When it comes to this topic – involving stalling in some particular configuration of flaps gear etc - then just selecting idle power and holding height (flying club style) the aircraft will decelerate quite rapidly (speed not constant, AoA not constant) and so when the thing eventually ‘stalls’ you will likely overshoot the actual stall AoA and finish up at an even lower speed, an even higher AoA and really in some quite unknown post stall condition where nothing is steady. This can be unpleasant for the inexperienced student and could even become dangerous should a spin result. But the main thing is such an exercise is useless when it comes to getting good (ie repeatable) flight test data.

Hence the slow 1 kt per sec approach to the speed reduction so that the aircraft is in quasi steady state so far as the aerodynamics are concerned with nothing changing rapidly. Of course you have to accept a loss of height to do this. However such a change of height does not really affect the plot you are able to get of IAS vs AoA which is the object of the exercise. Even more importantly (especially with a new type) you are only creeping the AoA up and so if some aerodynamic event occurs (buffet, wing drop, aileron snatch, tendency for the nose to pitch up etc) then you only have to relax the back pressure a tad and you are instantly back to where you were before the event. All nice and safe and leaves you able to record the data as appropriate (well in my day you had to write it down – although today everything will be recorded for you of course).

So far as the 1 kt per sec goes I was always taught that this was a maximum acceptable rate of reduction and the slower the better. So if you took 5 sec to knock another 1 kt off all to the good.

Off topic and speaking personally if ab initio flying instructors taught stalling this way then the student would not get alarmed and actually perhaps enjoy flying slowly rather than being frightened whenever the ASI needle starts to get lower than usual.

[roulishollandais]

Off topic and speaking personally if ab initio flying instructors taught stalling this way then the student would not get alarmed and actually perhaps enjoy flying slowly rather than being frightened whenever the ASI needle starts to get lower than usual. :DAnd refresh regularly these basics by airline instructors.

[DozyWannabe]

A computer (FBW in this case) will try to maintain exactly 1G all the way to the collapse of lift. A pilot cannot possibly do such a thing with any precision with conventional flight controls flying manually.

Sorry to dig up this older post after the useful info above, but I figured I'd better be the tech guy here.

What computers can do (if given precise sensors), is detect deviations from the required flightpath, sometimes before they would become perceptible to the Mark 1 homo sapiens, and react more quickly to correct those deviations. This is not due to inherent superiority to human pilots, it's simply because that is all the computers are designed to do. As Tourist points out, some pilots can fly incredibly accurately - enough to give the computers a run for their money in any case.

However, regardless of whether controlled by a human or a computer, an aircraft will always be subject to the laws of physics.

What this means is that if the forces acting on the aircraft exceed the abilities of the aircraft's control authority to hold flightpath and orientation precisely, the computers are programmed to do what pilots are trained to do, namely trade off a degree of precision to maintain stability.

Also on occasion, despite the design, testing and certification processes being among the most thorough and rigorous in history, real world scenarios - usually weather-related - will demonstrate areas that were not taken into account during the design.

Computers are there to assist pilots, not replace them - and I suspect that will remain the case for many years to come.