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DO winglets have effect on Vref when landing?

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DO winglets have effect on Vref when landing?

Old 9th Apr 2014, 12:54
  #41 (permalink)  
 
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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
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Old 9th Apr 2014, 13:13
  #42 (permalink)  
 
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Originally Posted by 737Jock View Post
in fact its impossible to be less then stallspeed, might be equal.
Incorrect.
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Old 9th Apr 2014, 13:33
  #43 (permalink)  
 
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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
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Old 9th Apr 2014, 15:56
  #44 (permalink)  
 
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Just curious, but wouldnt less drag mean more lift?

If that is so, couldnt you have a lower stall speed?
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Old 10th Apr 2014, 06:14
  #45 (permalink)  
 
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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.
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Old 10th Apr 2014, 12:49
  #46 (permalink)  
 
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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.
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Old 11th Apr 2014, 03:16
  #47 (permalink)  
 
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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.
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Old 11th Apr 2014, 11:09
  #48 (permalink)  
 
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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.
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Old 11th Apr 2014, 12:18
  #49 (permalink)  
 
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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.
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Old 11th Apr 2014, 12:52
  #50 (permalink)  
 
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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.
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Old 11th Apr 2014, 12:56
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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.
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Old 11th Apr 2014, 13:09
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mustafagander, you might wish to consider the configurations which apply to the speeds you quote; are they like-for-like?
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Old 11th Apr 2014, 13:55
  #53 (permalink)  
 
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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.
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Old 11th Apr 2014, 17:19
  #54 (permalink)  
 
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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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Old 12th Apr 2014, 03:38
  #55 (permalink)  
 
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LNIDA
The Boeing 737 Technical Site
You can read it yourself.
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Old 12th Apr 2014, 11:11
  #56 (permalink)  
 
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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

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.
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Old 12th Apr 2014, 12:44
  #57 (permalink)  
 
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Originally Posted by 737Jock View Post
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?
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Old 12th Apr 2014, 23:11
  #58 (permalink)  
 
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Not to mention JAR/FAR 25.149(f) ...:
(f) V MCL , the minimum control speed during approach and landing with all engines operating, ...
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Old 13th Apr 2014, 02:28
  #59 (permalink)  
 
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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.
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Old 13th Apr 2014, 08:45
  #60 (permalink)  
 
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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.

Last edited by 737Jock; 13th Apr 2014 at 09:13.
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