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mustafagander
I am not an authority on performance but I think you need to examine your statement. If aircraft can be controlled with precision below Vs stall wouldn't be such a bugbear. Below is JAR definition. JAR/FAR 25.149 Minimum control speed (b) VMC[A] is the calibrated airspeed, at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane with that engine still inoperative, and maintain straight flight with an angle of bank of not more than 5 degrees. (c)VMC[A] may not exceed 1.2 VS with • Maximum available take-off power or thrust on the engines; • The most unfavourable centre of gravity; • The aeroplane trimmed for take-off; • The maximum sea-level take-off weight |
Originally Posted by 737Jock
(Post 8426304)
in fact its impossible to be less then stallspeed, might be equal.
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I would rather say since VLs is not affected Vs1g should also be not affected by A320 sharklet.
VLS = 1.23 Vs1g g * The 1.23 factor is applicable to the fly-by-wire aircraft |
Just curious, but wouldnt less drag mean more lift?
If that is so, couldnt you have a lower stall speed? |
After this apparently sincere post -
When you hand fly an approach in gusty crosswinds, you need to 'fight' a little more with the aircraft to keep it on the centerline. This is due to the added vertical surface area of the sharklets blowing the aircraft to one side. I had to reply with the difficulties winglet equipped a/c have maintaining LNAV in crosswinds. ;) |
Re: this whole winglets + crosswind = can't-fly-straight
The only conclusion I can come to is that these guys are having a laugh by making silly statements and I completely missed the sarcasm. (Wouldn't be the first time!) That or I should have my license taken away and go back to ground school. :confused: |
dirty sanchez
Winglets reduce the effect of span wise flow, which increases the strength of wingtip vortices. Increased vortex = increased drag (vortex drag which is part of the induced drag ie drag which is induced by the aircraft's velocity. and there is no such this as wingtip vortices. The discontinuity of viscous flow over the top of the wing is the cause of drag....winglets tend to move the area of discontinuity outward along the wingspan, hence less drag ...it likely has more to do with winglet/wing bending, hence the newer scimitar and active winglet designs...nothing to do with the vortices that the wing creates. In regards to advanced wing design...one should note that even the 787 has vortex tabs down the wing...yet Airbus products do not. |
OK guys,
Now I'm back on the ground, further info to my apparently shocking statement that Vmca can be and in fact is below Vs1g on big Boeings by a good margin. Assuming 380T and close to sea level standard conditions, Mr Boeing assures me, via his certified Performance Limitations Manuals, "observance of which is required by law", that for RR Vs1g clean is 205Kts and Vmca is 117Kts while for GE Vs1g clean is also 205Kts and Vmca is 120Kts. You quote, vilas, that Vmca may not exceed 1.2Vs but that does NOT mean that it cannot be lower, in fact it infers a lower speed. How Vmca is determined when it is below Vs1g is beyond me - I'm just a pilot, not a designer. |
mustafagander
If what you stated is correct that means aircraft will stall at 205 KTS but with critical engine failed and full power on others it will not stall even at 117KTS. There is no question of flying below Vs, this Vmca could not have been demonstrated may be theoretically derived. As far as my reference to Vmca I have already corrected that to VLS which is definitely 1.23 Vs1g. |
vilas,
Boeing says no such thing. All that their graphs prove is that you will never have a Vmca problem, you'll stall first. The graphs say that you'll stall around 205Kt. So, one less thing to worry about with a V1 cut. Remember that Vmcg tends to drive some scheduled speeds and it usually is above Vmca. There's no "if" about it old mate, it comes from Boeing certified data. I imagine that there are several thousand copies around. I, too, have wondered how Vmca could be demonstrated for certification! Just BTW, the notorious PA44 Seminole also has Vmca just a bit below Vs1g in most conditions, hence the AFM warning about intentional single engine speed in big print. |
So you take-off in clean configuration mustafagander?
Maybe you rotate at 205 kts and blow your tires that are most likely rated to 195kts? Even if you could theoretically derive that VMCA is below VS, you still can't fly below VS. As such VMCA only has a practical meaning at or above VS. You can't fly straight with 1 engine inoperative and a max bank of 5 degrees below stallspeed. |
mustafagander, you might wish to consider the configurations which apply to the speeds you quote; are they like-for-like?
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Mustafa
That is what I am saying if the Vmca is lower than Vs then it cannot be a demonstrated speed but theoretically derived off Corse you cannot fly at that speed. |
Nice to see so many clever people on here!!
Next challenge (if you choose to accept) Explain how the SFP kit on a 738 improves short field take off and landing performance (the improvement is substantial) |
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Jock, of course we configure as per the Boeing performance figures, F20 always. I used the clean case to avoid bogging down in what is a huge number of combinations when you start considering FR, Rtg1 or Rtg2. Throw in overspeed and 2nd segment considerations and we can post for a month. BTW our tyres are rated to 235Kt so no problem :E
It is not possible to fly to Vmca because it is below Vs1g in almost all conceivable situations, that is my point. How Boeing derived those numbers and satisfied the airworthiness authorities has always made me wonder too. Actually Vmcg is the driver for light weight T/O in the B744 because of the factoring of V1. Like for like, Vmca is always less. |
Originally Posted by 737Jock
(Post 8430095)
Even if you could theoretically derive that VMCA is below VS, you still can't fly below VS. As such VMCA only has a practical meaning at or above VS.
If you can still maintain directional control then what have you got? |
Not to mention JAR/FAR 25.149(f) ...:
(f) V MCL , the minimum control speed during approach and landing with all engines operating, ... |
Exeng - And misd-agin says he is an airline pilot!
I think I've heard it all on this site now. Still you have to try and maintain a sense of humour I suppose. Exeng - I updated my profile for your benefit. |
I refer you to post 42 lord spandex smasher. Or indeed the following:
http://www.skybrary.aero/bookshelf/books/2263.pdf 1.3.2. Minimum Control Speed in the Air: VMCA JAR 25.149 Subpart B FAR 25.149 Subpart B “JAR/FAR 25.149 Minimum control speed (b) VMC[A] is the calibrated airspeed, at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane with that engine still inoperative, and maintain straight flight with an angle of bank of not more than 5 degrees. (c)VMC[A] may not exceed 1.2 VS with • Maximum available take-off power or thrust on the engines; • The most unfavourable centre of gravity; • The aeroplane trimmed for take-off; • The maximum sea-level take-off weight The aeroplane in the most critical take-off configuration existing along the flight path after the aeroplane becomes airborne, except with the landing gear retracted; and • The aeroplane airborne and the ground effect negligible (d) During recovery, the aeroplane may not assume any dangerous attitude or require exceptional piloting skill, alertness, or strength to prevent a heading change of more than 20 degrees.” I guess that if the tail is infinitely long or the rudder is infinitely big theoretically vmca is zero. Doesn't get us a lot further though if you can't get off the ground at that speed. Since the rudder and vertical fin cause drag, I reckon that manufacturers like to keep it as small as possible. |
Regarding VMCL: (which is really just another form of VMCA but in a differrent phase of flight)
JAR/FAR 25.149 Minimum control speed (f) VMCL, the minimum control speed during approach and landing with all engines operating, is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane with that engine still inoperative, and maintain straight flight with an angle of bank of not more than 5o. VMCL must be established with: The aeroplane in the most critical configuration (or, at the option of the applicant, each configuration) for approach and landing with all engines operating; The most unfavourable centre of gravity; The aeroplane trimmed for approach with all engines operating; The most unfavourable weight, or, at the option of the applicant, as a function of weight. Go-around thrust setting on the operating engines (g) For aeroplanes with three or more engines, VMCL-2, the minimum control speed during approach and landing with one critical engine inoperative, is the calibrated airspeed at which, when a second critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane with both engines still inoperative, and maintain straight flight with an angle of bank of not more than 5 degrees. VMCL-2 must be established with [the same conditions as VMCL, except that]: The aeroplane trimmed for approach with one critical engine inoperative The thrust on the operating engine(s) necessary to maintain an approach path angle of 3 degrees when one critical engine is inoperative The thrust on the operating engine(s) rapidly changed, immediately after the second critical engine is made inoperative, from the [previous] thrust to: - the minimum thrust [and then to] - the go-around thrust setting h) In demonstrations of VMCL and VMCL-2, ... lateral control must be sufficient to roll the aeroplane from an initial condition of steady straight flight, through an angle of 20 degrees in the direction necessary to initiate a turn away from the inoperative engine(s) in not more than 5 seconds. |
Minimum control speed and stall speed
Whereas the stall speed varies approximately as the square root of weight, the change of minimum control speed with weight is very small. The reason that it changes at all is the bank angle of 5 degrees allowed in the determination. So if someone says that at a particular weight the minimum control speed is less than the stall speed, he is comparing the stall speed at that weight to the minimum control speed established at a lesser weight and extrapolated to the actual weight.
Another aspect to consider is that the stall speed is determined with power off, whereas Vmc is determined with TOGA thrust on the operating engine(s). Since a component of the thrust contributes to the lift, it is in fact possible to demonstrate Vmc below the power-off stall speed. |
As you see from the requirements vmca is determined at the maximum sea-level take-off weight.
What do you mean with: he is comparing the stallspeed at that weight to the minimum control speed established at a lesser weight extrapolated to the actual weight? An aircraft will also stall at full-power, so why do you want to compare vmca at full power with a stallspeed without power? The entire point of VMCA is to demonstrate the ability to maintain direction (even though you heading can still change up to 20 degrees) after becoming airborne with the most critical engine failed, the other engine(s) at max to power and a bank maximum bank of 5 degrees. And thats about as far as VMCA goes. Sure you can demonstrate that in the clean configuration at FL200 you can put TOGA on one engine and then stall it, but thats not the definition of VMCA as required by FAR/JAR. It would be another variant of VMC, like for example VMCL. just my 2 cents I realise how you can theoretically, and even practically if it wasn't for the ground being so damn close, get a lower VMC then stallspeed. For me it just has very little to do with the VMCA requirements. |
Just been looking through minibus fcom.
Stallspeed vs1g for A319 at 60T 1+F is about 112kts at 0 pressure altitude The minimum V2 speed for take-off limited by VMU/VMCA is 128kts. Minimum VR is a conservative 113kts. Stallspeed vs1g for A319 at 60T conf 3 is about 106kts at 0 pressure altitude The minimum V2 speed for take-off limited by VMU/VMCA is 119kts. Minimum VR is a conservative 113kts Can't find any exact VMU values unfortunately. But obviously VR is limited by VMU, so I guess the higher V2 limit must come from VMCA. Anyway I have the idea these values are all not that far apart. Certainly not as comfortable as mustafagander claims: For a normal conf 1+f takeoff, a 112kts Vs1g would dictate a V2 of at least 112 x 1.13= 126kts Yet the minimum V2 for VMU/VMCA is 128kts. (but VR is ok at 113kts) For conf 3 both are 119kts |
Originally Posted by 737jock
As you see from the requirements vmca is determined at the maximum sea-level take-off weight.
JAR/FAR 25.149(c)(4) states an additional requirement, i.e. that Vmca may not exceed 1.13 Vsr at the Vsr for maximum sea-level take-off weight (or any lesser weight necessary to show Vmc). |
The entire point of VMCA is to demonstrate the ability to maintain direction
If I may disagree .. maintenance of controlled flight and heading is fine .. but the underlying reason for establishing Vmc is to draw in another boundary for V2. For routine operations, we have no business playing with Vmc ... one should leave it to the TPs to frighten themselves from time to time .. |
If I may disagree, first and foremost it's good to know at what airspeed the aircraft can safely get airborne and control it's direction. If you can't do that then there is not much point in having a V2 speed.
If we then use this information together with other data to be able to determine a safe V2 speed that provides a safety margin on various elements while providing us with a positive climb gradient. Anyway it's a bit of a moot point, chicken or egg discussion. |
The airplane doesn't know what the wind is... I gave him the right answer but he corrected me saying that when flying into the wind, the wind blows against the blunt leading edge of the wing having a significant effect and when flying in a tail wind the wind blows on the sharp trailing edge and therefore has a lesser effect. So you don't gain back what you lost in the headwind. :confused: He also explained why one leg of a V formation of migrating geese is invariably longer than the other. ;) |
He also explained why one leg of a V formation of migrating geese is invariably longer than the other. ;) Maybe he was just being silly to see if you'd buy it, like misd-agin was about the cross-wind LNAV. (I admit, I thought he was serious at first...) |
It's because like cyclists they change the lead. But instead of dropping back straight to the back, they dropout to the side (creating the v-formation) and then crossover to the other side. Which means that one of the legs will be longer.
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A crusty old captain once asked me why on an out-and-back two leg flight with strong winds, it takes more total time than the same flight in calm winds. As an experiment, I plotted direct from airport to airport, and I got 16 knots headwind on the way out, 10 knots tail wind on the way back. I'm so confused. :ugh: |
whats foreflight?
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whats foreflight? |
He also explained why one leg of a V formation of migrating geese is invariably longer than the other. |
mustafagander
I have produced below an extract from Airbus magazine on VMC tests conducted for the A380. It does not give the impression that VMCA is below Vs. I am not saying the two have direct connection but you cannot fly below Vs. VMCA, VMCL, When engines and systems are configured, we start about 20 kt above the predicted value, then, we decelerate slowly keeping heading constant. Necessary rudder increases as the speed decreases, eventually up to the stop. Further deceleration will need some bank to still keep the heading constant. The true VMCA is obtained when the bank angle reaches 5° in the opposite sense to the failed engine. This bank angle is very important as it allows a further speed reduction of about 5 to 10 kt, compared to the same test performed with wings levelled. Where is this strange rule coming from? It is a mystery The tests to obtain VMCL and VMCL-2 are similar. But there is more to do. A demonstration that the roll manoeuvrability at VMC is sufficient must be performed. How do you perform this demonstration when below stall speed? |
vilas,
Were your attachment to become visible to me, I would not be too interested in what it shows. I have been on about BOEINGs, in particular B744. I have quoted from their certified Performance Limitations Manual figures extracted from the graphs therein. Airbus may have an entirely different certification regime, I know not. As I have said, I wonder how such speeds below Vs1g can ever be demonstrated, but there we have it - part of the certified data package. I also mentioned the PA44 which has Vmca below Vs1g under certain conditions and a prohibition on intentionally operating the aircraft below the "intentional single engine speed" as extracted from the graphs in the POH. The bottom line for B744 is that Vmca is below Vs1g for all usable T/O weights. Vmca is also below Vmcg which can be a comfort with a V1 cut. |
what you quoted are irrelevant numbers that do not correlate in aircraft configuration, the only correlation is that they come from the same manual. But the qouted stallspeed is in clean configuration, not the required take-off configuration.
Please provide some correlating numbers. Instead talking about how they are too complicated. I showed numbers from the performance section of the minibus. Although they are certainly not exact, it does not show the massively comfortable margins you claim. |
http://www.avioconsult.com/downloads...%20failure.pdf
When banking, a component of the weight, leads to a side force due to bank angle (Wsin φ in Figure 2), that can re- place the side force Yβ due to sideslip that was required for balance with the wings level. The small bank angle decreases the sideslip angle to a minimum, decreasing the total drag and, hence, increasing climb performance. Side force Wsin φ acts in the centre of gravity and therefore does not cause any ad- verse yawing moments. Because the side force Yδr, generated by the vertical tail with rudder, no longer has to act against the side force due to side- slip Yβ, but only against the thrust yawing moment NT, the rudder deflection need not be maximal, the airspeed can be reduced until the deflection is again maximal, or the vertical tail can be dimensioned smaller to save manufacturing cost and weight and weight. FAR/CS 23.149 and 25.149 allow the engineer designing the vertical tail to use a bank angle of maximum 5 degrees. Reducing the size of the verti- cal tail increases VMCA (for a high enough side force Yδr). FAR/CS 23.149, however, does not allow the vertical tail to be made so small that VMCA exceeds 1.2 times the stall speed (VS). Hence, the verti- cal tail is made just big enough to maintain straight flight while the thrust of the opposite engine is at the maximum takeoff setting, the rudder is maximal deflected and while maintaining a small bank angle as opted by the designer of the vertical tail, usually between 3 and 5 de- Now since they build entire airplanes made from carbon, to save a bit of fuel, do you seriously consider that they would stick on a tailfin that is too big? I don't really care if its theoretically possible. It would not be economical and adjusted as soon as they notice it. Sure if you design a longer aircraft based on a basemodel the tailfin could be made smaller (usually they won't), as the arm becomes longer. But the entire idea of super big margins is just ludicrous. |
737jock, if I understand your post, I think you haven't considered all the variables of configuration. In particular, flap changes Vs a lot but doesn't change Vmc much.
The tailfin may be perfectly sized for controlling the aircraft for an asymmetric go around with landing flap at minimum weight - that is, in a combination of circumstances giving a very low stall speed with a moderate to high Vmc. But in that case, Vmc will be very much lower than required (for certification) in clean or take-off configurations. It could well be below Vs. If I've misunderstood your post, my apologies! |
maybe if you consider that vmcA is only valid in the take-off configuration you would understand my post.
vmc is a term that is way too generic, every phase and configuration has its own vmc. For takeoff its called vmcA, for approach/landing/go-around its vmcL, on the ground vmcG. And you could make up a whole bunch more for other regimes. You have to compare the applicable vmc, to the stallspeed that applies to the configuration and flightphase the aircraft is in. The most likely explanation for a VMCA below vs is that the aircraft has been lengthened and the tailfin kept the same size for commonality. For example the a318 has a latger tailfin then the other minibuses to compensate for the shorter fuselage (arm). But still it won't be more then a few knots. In any case I doubt it because nowhere in flighttesting do I see statements where testpilots put the aircraft into a stall in order to determine VMCA. But as I have said theoratically you could have a vmca of almost zero if your tailfin and rudder is infinitely big or the tail infinitely long. BTW stallspeed doesn't change that much between configurations, from clean to the first configuration the change is massive, thereafter its effect is much smaller. |
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