DO winglets have effect on Vref when landing?

[sky-738]

Hello everyone,
I fly many kinds of 737s including 737_800 with and without winglets . When landing at the same weight , the one which have winglets have a small Vref then the one without winglets , just about one or two knots.
Can anyone tell me why ,because what I learned is that winglets is for fuel saving when cruising .

[underfire]

738 without winglets?

[B737900er]

This must be a simmer.:}

Winglets also reduce drag, thats why the Vref will be slightly different between winglet and non winglet aircraft.

[RVF750]

Defo a simmer The rest of us just fly what the box tells us......

[737Jock]

Vref is derived from the stallspeed. Just google on the effect that winglets have on stallspeed and you will find your answer.

Sorry don't feel like writing a thesis on winglets, amongst the just fly brigade. Think induced drag, which also explains the fuel saving....

Reading some of these answers though makes me despair....

[737Jock]

As for 737-800 without winglets:

Photos: Boeing 737-8Q8 Aircraft Pictures | Airliners.net (http://www.airliners.net/photo/Britannia-Airways-(Sterling/Boeing-737-8Q8/0015774/L/)

OMG:eek:

[underfire]

I guess there are far more 800's without winglets than I thought...Ryanair, SAS, Delta, and a few others, seems it was common when the 800 first came out...

Realted/unrelated...The A320 with Sharklets is significantly different...

[J.L.Seagull]

Underfire,

No it's not! An A320, with or without sharklets has the same Vapp speed (equivalent to Vref on a Boeing, I think).

Yes, there are some performance, operational and limitation differences, but Approach Speed is not one of them! :-)

[Wizofoz]

Plus, as far as I an am aware, the major savings made by winglets are in high AofA situations like climb and approach, less so in cruise.

[underfire]

I meant the performance of the wing, not the OP Vref (sorry)..the sharklets make it very, very slick compared to the A320 with the typical winglets....the pilots noted it was far more difficult to slow it down.

EDIT:

Does Vapp differ with sharlet vs non-sharklets? It is calc'd by the FMGCS, (Vls +5), but is it the same?

Plus, as far as I an am aware, the major savings made by winglets are in high AofA situations like climb and approach, less so in cruise. Opposite is true...

Edit: OT..just wondering..the drivers said they were not briefed on the difference, and didnt know they had a sharklet plane until they got there and did a visual. Is this typical? They even mentioned they had a tough time slowing it down, I noted lots of speed brake after config 1, and then almost right to full flaps, pretty far out...(good thing there were 10 pax instead of a full load...)

[J.L.Seagull]

Underfire, Wiz,

This has been discussed before, and I'm too lazy to type it all again. There's a thread "A320 with Sharklets" in which I've made a couple of posts. Good detailed info there.

BTW, sounds like a suspect training & safety team if the pilots weren't briefed on the introduction of sharklet A320s in the fleet. There are fuel imbalance limitations, auto-land crosswind limitations, and a whole bunch of other stuff that the trainers love bringing up every 6 months! LOL!

I've never flown a 737, so I'm not going to pretend I know anything about that bird, but would be happy to talk about the A320 sharklets if you'd like.

[RAT 5]

RYR's first a/c did not have winglets and then were retrofitted as later deliveries came with them as standard.
Curious why the takeoff & landing cross wind limits were different with & without. Was told it was the usual 'demonstrated' figure. Given it was used as a limit I wondered why a demo was necessary for such a small mod. There is no ground clearance issue, and I'd be surprised with there was a roll handling issue. Why not just use the higher value for both variants?

[underfire]

JLS,

Thanks, will look at the thread.

I know the 737 series with winglets is really slick compared to the ones prior. I would guess that the reduced drag near te ends of the wings and improved airflow from the sharklets, that the A320 would have similar features.

In the training, is this something the pilots are made aware of, the increased speeds, and issues with energy management on final?

[J.L.Seagull]

Rat5,

The exact limitation difference is for AUTOLAND WITH AUTOMATIC ROLL-OUT; 15kt vs 20kt for non-sharklets aircraft.

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 guess, Airbus felt that the NWS and flight control systems weren't up to the task once the aircraft was on the ground, hence the limitation reduction.

[tdracer]

The 737-800 did not have winglets at EIS, in fact they were not certified until several years after the -800 entered service. I believe they are now baseline on all new production -800/-900 variations.


Retrofit of the winglets is pretty simple on the 737-800/-900 due to minimal structural modifications required. 737-700 requires significant structural changes hence retrofit is quite a bit more difficult and expensive.
The -700 BBJ uses the wing from the -800, and hence can take winglets with minimal structural mods.

[737Jock]

It slows down slower, I wouldn't say the differences are dramatic for a a320 Sharklet.
It's a nice excuse though for an approach that would have been pretty much as bad in a non-sharklet a320.
yes I have flown both, and the most annoying thing about the sharklet variant is the wingstrobes that light up the flightdeck at night!

[underfire]

BTW...I posted pics of the wing and flap settings here..

http://www.pprune.org/tech-log/537474-a320-sharklets.html

its a nice looking wing, too bad about the strobe effect...I have a meeting coming up with the design folks at AB, I will ask them if they are going to fix it.

[Intruder]

According to the 744 DDG, one winglet may be missing. There is a performance penalty of 9435 kg, but no change in ref speeds.

Interestingly, you can't takeoff with both winglets missing...

[arn3696]

...Due to lack of static wicks

[J.L.Seagull]

Underfire,

sops say:
"Set the STROBE selector to ON, before entering the runway. The flight crew can switch off the strobe lights if the lights cause any visual trouble during the flight."

This is specific to A320s with sharklets, and is listed as a TDU (temporary documentary unit). I.e. It can be implied that Airbus knows about the issue and are working on a fix.

In the mean time, just pull the sunshades down. It's 90% effective. :)

[underfire]

JL...you have sunshades on the flightdeck? :}

App is but a small portion of flight, so enroute must be a @#$%&

but thanks for the info...I am sure the drivers reading the thread will appreciate!

EDIT: No really JLS, I am not trashing AB in any way..I feel their wing design is first class...I have been dealing with them on many fronts from the initial A380 'winglet' design...the A320 wing is very, very good..and as noted, doesnt have the vortex tabs as standard equipment.

[sky-738]

ok, thanks everyone! Still have few question , why the 737 700 is hard to install winglet compare to 737 800 , both of them have the same wingspan. Do u mean the wingspan is the same but the structure is different? And also, why the 700 and 800 have different app speed when in same weight? Because the length of fuselage? Need an expert!

[tdracer]

Sky - the 737-800 has a significantly higher max gross weight than the -700, and hence the wing is aerodynamically the same but structurally different.
As noted, the 737-700 based BBJ has a -800 wing to give it a higher max gross weight, which is normally used in combination with body fuel tanks to give it longer range relative to the run of the mill 737-700.

[underfire]

td has it correct, the 700 and original 800 wings needed to have reinforcement added for the winglets...later 800 wings had it built in from the start.

Some 800's have the same approach speed as the 700...

[misd-agin]

Planes with winglets struggle to stay in LNAV due to crosswind pushing on winglets.

Previous poster mentioned difficulty in tracking centerline e while hand flying.

[bubbers44]

Do you also trim downwind rudder while enroute so that crosswind component doesn't affect the vertical stabilizer?

[Intruder]

That's self-correcting, because it makes the nose point upwind when the tail is pushed downwind. :)

[slam525i]

trim downwind rudder while enroute so that crosswind component doesn't affect the vertical stabilizer
Wha? :confused: Cross-wind doesn't affect anything with regards to trim. The airplane doesn't know what the wind is...

Edit: Oh I see you got it from this:
Planes with winglets struggle to stay in LNAV due to crosswind pushing on winglets

Again, the airplane doesn't know what the wind is. It just flies in the body of air, whether the body of air itself is moving or not. The idea that an airplane will struggle to stay in LNAV due to a cross wind is just plain silly.

The earlier poster mentioned gusts on approach. Gusts will change the aircraft's flight, due to inertia. That plus when you're on approach, you're flying relative to the ground.

While enroute, in a stable condition, wind doesn't affect how any airplane flies. You (without autopilot/FMS/etc.) just point it a little upwind to compensate for it.

[vilas]

A320 Winglets do not affect Vmca which means it doesn't affect Vs or Vs1g. So it shouldn't affect Vls or Vapp.

[john_tullamarine]

A320 Winglets do not affect Vmca which means it doesn't affect Vs or Vs1g

Might you elaborate a bit ?

[rsiano]

The most fuel efficient plane in the world today is the Boeing 787. It has no winglets at all. Why? Because winglets by themselves contribute nothing.

[exeng]

Planes with winglets struggle to stay in LNAV due to crosswind pushing on winglets.


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.

[vilas]

John
Actually from Airbus document on sharklets the only speed affected by them is Vmcg by 1KT on CFM. There are some other limitations but not on speed. Everything else remains same. I was trying to work backwards. Since Vmca is 1.2 or some multiple of Vs1g so apparently Vs1g is not affected and Vls is also a derivative of Vs1g. Or simply since Vs1g is not affected all speeds remain same. particular reference to Vmca was not necessary.

[vilas]

rsiano
Winglets are not fads. On an aerodynamically efficient wing design winglet do not contribute much. Development of 787 wing design may have taken that into consideration. But all other aircrafts 747, 737 even A320 now, benefit in terms of fuel saving by reduction in drag.

[Lord Spandex Masher]

The most fuel efficient plane in the world today is the Boeing 787. It has no winglets at all. Why?

It has raked wing tips instead. Effectively the same thing but doesn't look as cool.

Is the 787 the most aerodynamically efficient 'plane in the world though?

[PPRuNeUser0172]

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. Not to be confused with form drag etc which is a zero-lift drag function. That said, winglets have a surface area and therefore produce some form drag but I digress....)

They certainly reduce fuel consumption in steady state flight (the cruise) because like for like an aircraft with winglets will have a lower total drag and therefore require a lower thrust setting for a given speed or will be able to cruise faster for less fuel if you like.

With regards to 787, 747-8 etc with so called raked wing tips is more to do with aspect ratio but as you will note have a large degree of sweep relative to the rest of the wing at the tip. This serves to minimize the effect of the dastardly vortex. Aspect ratio being a function of span2/area is enhanced by increasing the span. Aerodynamically perfect wings would be infinitely long (amongst other things) to prevent the wingtip vortex ever forming.

This would of course make it a sod to park...:E

In answer to the post. Winglets will affect vref, but only slightly. A wing with winglets will stall at a slightly different speed to one without but not more than by a couple of knots. Both lift and drag are directly proportional to surface area and winglets obviously increase both. Like anything, hopefully the benefits outweigh the negatives.

[mustafagander]

Vilas,

Vmca has absolutely nothing to do with any brand of Vs. Nothing at all. Vmca is about control surface power and thrust lever arms, meaning how much force the deflected controls, mainly rudder, generate and how far from the centre line are the thrust lines of the engines.

All the big Boeings I know about have a Vmca below Vs by a comfortable margin. Perhaps you were thinking of Vlo or even V2.

[737Jock]

vmca, with a for air, is below vs? please explain

as far as I know vmca is the speed at which the aircraft can be controlled with the most critical engine failed and a maximum bank of 5 degrees into the live engine(s). which means it is in the air, not stalling. in fact its impossible to be less then stallspeed, might be equal.

[matkat]

I was involved in getting winglets installed on the Flyglobespan -800s cost around $500K to get done but more expensive if factory fitted.

[BARKINGMAD]

"Planes with winglets struggle to stay in LNAV due to crosswind pushing on winglets. "

Statements like this give our PROFESSION a bad name!!

I hope you're not on the same planet as me?

[vilas]

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

[Lord Spandex Masher]

in fact its impossible to be less then stallspeed, might be equal.

Incorrect.

[vilas]

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

[underfire]

Just curious, but wouldnt less drag mean more lift?

If that is so, couldnt you have a lower stall speed?

[misd-agin]

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. ;)

[slam525i]

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:

[underfire]

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.

This statement is completely backwards...

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.

[mustafagander]

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.

[vilas]

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.

[mustafagander]

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.

[737Jock]

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.

[safetypee]

mustafagander, you might wish to consider the configurations which apply to the speeds you quote; are they like-for-like?

[vilas]

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.

[LNIDA]

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)

[vilas]

LNIDA
The Boeing 737 Technical Site (http://www.b737.org.uk/flightcontrols.htm#Short-field_Performance_Enhancement_Program)
You can read it yourself.

[mustafagander]

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.

[Lord Spandex Masher]

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.


Yes you can. You will have the speed at which you stall and then, depending on the rate of deceleration, will get to a speed lower than that. That's known as minimum speed in the stall.

If you can still maintain directional control then what have you got?

[HazelNuts39]

Not to mention JAR/FAR 25.149(f) ...:
(f) V MCL , the minimum control speed during approach and landing with all engines operating, ...

[misd-agin]

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.

[737Jock]

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.”


If you call stalling flying then I think we have to agree to disagree, certainly in the context of transport category aircraft taking off. I also doubt you would be trimmed for take-off at that speed.

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.

[737Jock]

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.”

[HazelNuts39]

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.

[737Jock]

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.

[737Jock]

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

[HazelNuts39]

As you see from the requirements vmca is determined at the maximum sea-level take-off weight.You are misreading the requirements. Even though Vmca varies little with weight, it is usually determined at all weights where it may be limiting V2, which is usually at the low end of the take-off weight range. It may optionally be determined at the minimum take-of weight because Vmca decreases with increasing weight if a 5 degree bank is used.

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).

[john_tullamarine]

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 ..

[737Jock]

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.

[Mozella]

The airplane doesn't know what the wind is...

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.

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. ;)

[slam525i]

He also explained why one leg of a V formation of migrating geese is invariably longer than the other. ;)

Is it because geese, like pilots, like to keep traffic on their left? :} I wonder if the ponds they land in are more likely to be left-hand circuits and if they have appropriate noise abatement & "dump" procedures. (I'm actually curious about his answer to the V formation leg thing.)

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...)

[737Jock]

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.

[slam525i]

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 aside, I was playing with Foreflight and I was looking at flying from Toronto to Buffalo following the lake shore (basically a C shape with 1 leg longer than the other). Outbound wind was calculated to average 12 knots headwind. Return was average 2 knots tail wind. Same altitude, same route.

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:

[737Jock]

whats foreflight?

[slam525i]

whats foreflight?

Foreflight is an iPad/iiPhone flight planning (and enroute) software. It's really popular in North America, and especially in Canada as it's the only one that has charts for Canada. For us Canadians, it's paper or Foreflight, no other choices.

[underfire]

He also explained why one leg of a V formation of migrating geese is invariably longer than the other.

It is simply because geese are in pairs, so with a leader, one leg will invariably be longer....:}

[vilas]

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?

[mustafagander]

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.

[737Jock]

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.

[737Jock]

http://www.avioconsult.com/downloads/Controlling%20multi-engine%20airplanes%20after%20engine%20failure.pdf

When banking, a component of the weight, leads to a side force due to bank angle (Wsin 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 Wsin 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-

Sorry but I cannot find any evidence at all that VMCA would massively be below Vs in the configuration its actually tested in. And the only reason for that would be the lack of pilots banking into the live engine. Its a crazy notion that basicly would mean the tailfin is way too big, thus weight materials can be saved by making it smaller. While also reducing fuel burn.

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.

[Oktas8]

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!

[737Jock]

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.

[Lord Spandex Masher]

Vmca, min control speed in the air... Below Vs you are not in control...

If you think you can prove opposite you compare apples with pears, but please be my guest try to rotate below Vs and keep it there next time.


Yes, you are. Or can be.

If you're rotating you're not in the air;)

[Oktas8]

maybe if you consider that vmcA is only valid in the take-off configuration you would understand my post.

My aircraft (a turboprop) has a VmcL equal to stalling speed with landing flap at MLW.

It should come as no surprise at all then that Vmc is very much lower than Vs in the clean configuration. Even the classic Vmca is much less than Vs in the specific circumstances pertaining to an engine failure during take off.

Presumably, the size and design of the rudder/fin have been specified for the most limiting case (in my aircraft, a rated power go around at final approach speed & configuration). Inevitably, Vmc is less than Vs in some other configurations. It may be more than Vs in some configurations, in some aircraft.

This doesn't mean that pilots will or should attempt to fly below Vs! It's just a result of compromise in the design criteria, and the different aerodynamic laws pertaining to max CL versus directional controllability.

[vilas]

My question is if Vmca is below VS then how is it demonstrated? It cannot be flown.

[Oktas8]

If the test pilot can't do it, it will be a job for the wind tunnel &/or mathematicians.

These days, more maths and less wind tunnel.

There are people like John Tullamarine or djpil who are more knowledgeable than I about the "how" of airframe development.

[737Jock]

So please explain me then why a manufacturer would put on a fin that is so big that VMCA is below Vs?
Clearly indicating that they can save weight, materials and thus reduce fuel burn.

[HazelNuts39]

My question is if Vmca is below VS then how is it demonstrated? It cannot be flown.

Just to illustrate a point I made earlier:

http://i.imgur.com/bgKI4J1.jpg

Vmc is normally determined by flight test at minimum weight. At 5 degrees of bank Vmc reduces with increasing weight. That change is small so for practical purposes it may me ignored, which is conservative. So then Vmc is constant with weight.

Stall speed increases with increasing weight. At some weight Vs will be equal to Vmc, and at higher weights it is greater than Vmc.

If the cross-over weight is below the lowest weight that can be achieved with the test airplane, then Vmc is obviously not limiting and the regulator will permit a value determined by analysis to be used instead.

So please explain me then why a manufacturer would put on a fin that is so big that VMCA is below Vs?Firstly, you have to consider all weights and flap settings. Secondly, there are other considerations for sizing the fin, for example the decrab maneuver when landing with crosswind.

[737Jock]

Ever flew on a jet that needs more rudder input during a crosswind landing then in an engine failure at V1 at TOGA ?
No thought not....

I completely understand the theory, the practicality is a bit far fetched imho.

Maybe check this link out again: http://www.avioconsult.com/downloads/Controlling%20multi-engine%20airplanes%20after%20engine%20failure.pdf

Seems that VMCA does change, at least on a 707/dc8. Banking into the dead engine makes it all quite exciting.

Your graph is interesting, but unfortunately its not based on actual data. You could put the blue line higher by reducing the size of the fin, to a maximum of 1.2 x Vs. Thus saving weight and fuel.

[HazelNuts39]

737Jock,

Please explain me why single-engine airplanes have a fin and rudder.

[737Jock]

Fin is for stability.

Rudder is to compensate for crosswind, engine power etc. It controls the longitudinal axis around the vertical axis.

Your point? Its the same reason why multi-engined aircraft have a fin and rudder.
Although there are aircraft that don't have a fin and rudder.

My question was if you ever needed more rudder on a crosswind landing then on a TOGA V1cut with minimum V speeds in a jet (not talking about fancy improved climb speeds or flex takeoffs)?

[HazelNuts39]

Your graph is interesting, but unfortunately its not based on actual data. You could put the blue line higher by reducing the size of the fin, to a maximum of 1.2 x Vs. Thus saving weight and fuel.My graph is generic and schematic, to assist in understanding what I'm writing. Of course you may put the blue line anywhere you like, but you would create a very unusual airplane if you put it at 1.2 x Vs at maximum takeoff weight. Since V2 must be at least 1.1 x Vmca you would severely compromise takeoff field length requirements at all takeoff weights.

[737Jock]

Nor would you put it far below, as it would create an unnecessarily large tailfin.

After all V2 is also a minimum of 1.2Vs or 1.13VS1g. (Vs= 0.94 x Vs1g)

If you would aim for VMCA=Vsx1.09 V2min will meet both requirements. The tailfin would be minimum size and VMCA is larger then Vs at MTOW.

Now it all depends on how much VMCA changes with the decreasing weight of the aircraft. If its very little (almost constant) as you say then this is not a problem. If the change is massive then it needs to be changed in order to keep VMCA at a maximum of 1.2vs...
(Actually that is not true as the restriction is that it may not exceed 1.2VS at Mtow. It doesn't have this limitation at lower weights.)

Anyway I understand how VMCA can be below VS, I just don't think that when it is below the difference is massive. And it probably has more to do with the tailfin being designed for multiple variants of the same aircraft.

[mustafagander]

Numbers Jock.

For B744ER with GE CF6-80C2B1F engines close to standard conditions close to sea level.

Using Full rating thrust, Vs1g @ F20 varies from 160kt @ 412T to 120kt @ 230T whilst Vmca is 120kt and Vmcg is 126kt.

Before you say "gotcha" at the 230T mark, nobody would be at FR at this weight, rather Rtg 1 with Vmca of 117kt or even Rtg 2 with a commensurate reduction in Vmca.

I reiterate, I have no idea how this could be demonstrated but since it is certified data that's what we use.

There's a bit more to tail fin sizing than 1EO ops on the ground but that's a whole new thread drift from the one we are currently involved in.

[Oktas8]

737jock,

Hazelnuts' graph is only generic, as you said. But the point remains true that Vs changes more than Vmc with different configurations or weights.

Here are real numbers from my turboprop. I've included only changes in configuration, keeping weights the same.

Vs @ MLW: F0 104kts, F35 79kts
Vmca @ MLW: F0 94kts, F35 78kts

Why have a wastefully large fin & rudder, you asked. The fin & rudder is designed for the most limiting case. At other weights or configurations, Vs and Vmc won't match up.

My turboprop lacks the high lift devices of a jet, so there is "only" a maximum 10kt split between the two. I would expect the difference between F0 Vs and F0 Vmc to be even bigger in a jet.

[kip]

Ok, this made me read up stuff that is too theoretically ;-)

Yes, Vmca can be below Vs - but this is NEVER a speed we can fly, so why bother!?

This paper http://www.avioconsult.com/downloads/Staying%20Alive%20with%20a%20Dead%20Engine.pdf
Explains very well the correlation...

...and then back to flying the speeds in the box :ok:

[john_tullamarine]

Harry pushes a particular wheelbarrow .. and that's fine .. his papers are worth reading for general pilot education.

One needs to keep in mind for civil line operations -

(a) Vmc is a problem at very low weights which are not the routine.

(b) beware of empty positioning flights and flights off very short runways where (a) becomes much more pertinent

(c) beware of the routine approach to airline simulator training where the low weight region rarely is investigated. Indeed, I can recall doing so with an operator which routinely used high V2 overspeed on the 732 .. the immediate results for crews so exposed was, shall we say, eyebrow-raising. However, we left them with a much better understanding and competence after a bit of explanation and practice ... I initiated this work specifically because, while using significant overspeed schedules routinely (and in the sim), they had one aerodrome on the network for which the aircraft routinely did a low weight positioning flight .. with min speed schedules.

The military, for reasons beyond me but, I guess, related to the perceived need to be able to play at the boundaries, tends, in my view, to have an unhealthy preoccupation with Vmc. I've frightened myself enough times to leave that region to the QTPs these days ..

[Mozella]

Is it because geese, like pilots, like to keep traffic on their left? :} I wonder if the ponds they land in are more likely to be left-hand circuits and if they have appropriate noise abatement & "dump" procedures. (I'm actually curious about his answer to the V formation leg thing.)

The question about one leg of a V formation of birds being longer than the other is usually asked by a Captain to a wet-behind-the-ears copilot after saying, "You know something about aerodynamics, right?"

The idea is to trigger a long reply about up-wash, down-wash, wing tip vortex shedding, flapping wing flight theory, complaint wing shapes, and other extended vocalization consisting mostly of BS. When the eager co-pilot finally runs out of wind, the Captain puts on his serious face and gives the answer: "It's because there are more geese on that side".

Captains invariably find this hilarious ................ copilots not so much.

[vilas]

Kip
The link you posted certainly helped. What Mustafa stated Vs has nothing to do with Vmca and Vmca is lower by margin raised the question how did they demonstrate the speed as is required by the authorities? It is obvious that it is derived speed if below stalling speed. Actually publishing Vmca below Vs is of no value and is rather misleading that it can be flown.

[PPRuNeUser0172]

Underfire

Why do you claim there is no such thing as wingtip vortices? Would "vortices formed at the wingtip caused by the combination of spanwise and chord wise airflow mixing" be a better description?

I am not saying vortices only form a the wingtip but it is widely accepted this is where they are most prevalent. Any part of the wing trailing edge will have this issue where span and chord wise flow meet, clearly exacerbated by sweep. This in turn increases the downwash, reducing the effective relative airflow which turn affects the total reaction in favour of drag, hence lift induced drag.

Either way, this induced drag is reduced by winglets. The fact that poor winglet design in terms of wing blending can increase interference drag was not part of my discussion or that of the original poster.

[underfire]

Would "vortices formed at the wingtip caused by the combination of spanwise and chord wise airflow mixing" be a better description?

Simple response is NO, vortices do not form at the wingtip.

One may observe a compression discontinuity at the wingtip, engine cowling, or at a flap edge, but this is not the wake vortex rollup created by the aircraft.

In the next year or so, there will be a radical shift in the understanding of the mechanics of wake generation. I noted opposing arguments from Boeing and Airbus on the issue of wing design, airflow, and vortex generation, which was a bit surprising at this point. Then again, even the 787 has vortex tabs on the wings....

[PPRuNeUser0172]

Under fire,

Well that is certainly radical. What technological breakthrough has led to a better understanding of this situation? I wait with baited breath...