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Thursday, May 24, 2012

Kill the drag!

A Taliban drag queen being escorted by Afghan forces to face justice!

No, the drag queen is just a joke, I'm talking about "winglets" and why is there such a variety?

In crew talk here, a winglet refer to two things.
1. That wingtip thingy-magic that's supposed to make our flights cheaper by saving fuel.
2. Male F/As that act like drag queens (ever seen one with his arms out to his sides and both hands pointing upwards when greeting you?).

It's no brainer, I'm not talking about killing male F/As that act like drag queens (even though I don't want to be locked up in a hotel room with one). I'm talking about our basic need to save fuel!

I'm not going to talk about the history of the aerospace engineers conjuring up funny equations. I'm also going to talk on the business side. The truth in the history of the winglet that we know today, is that no one cared about drag reduction wingtip devices until Iran decided to get people to think about fuel savings (if there's one pariah we need to thank with today's improved fuel efficiency technology, we reluctantly have to thank the Ayatollah).

Let's start with a little aerodynamics though. We don't put these things up just because they look pretty.

Where does the drag come from?
Lift for an aircraft is created because the lower side of the wing has higher pressure than the top. All sounds nice and dandy, but what about on the wingtip where both the upper and lower sides meet.

Tip Vortex by Citizenhom (Creative Commons)

Air is like a fluid, it always wants to move from high pressure to low pressure. So, the high pressure air near the wingtip, spills over to the lower pressure air, hence air from the lower side moves to the upper side of the wing, and as it moves forward, it creates a vortex behind it.

This creates drag... the vortex sucks its energy from the airplane, causing it to be less efficient.

Photo by NASA

As planes go higher and faster, this drag, increases. Scientists and businessmen in the industry agree to do one thing... 


Thanks to rising fuel prices since Iran fell, the urge to save fuel consumption continue. The old days of flying fast is long gone as it takes a lot of energy to punch an airplane through the air at high speed. Flying slower reduces the energy needed, hence lower fuel consumption. Unfortunately, lower speed means you've reduced the aircraft's total drag, but increasing the percentage of wing-tip vortex drag out of the total drag.

You can fight this drag by various means:
1. Increase the wing span whilst maintaining the aspect ratio. This reduces the lift-induced drag (including the wingtip vortices), but the question is, just how wide do you want the wingspan to be? There's a limit to this, both economic and structural constraints.
2. Optimize wing loading distribution by concentrating the lift nearer the body of the aircraft, so that you can afford minimum pressure differential at the wingtip, but this has manufacturing and maintenance complexities.
3. Put up a barrier!

Wingtip devices, are method number 3.

1. Containment method.
Ever seen a downward facing wingtip? This is made to contain the air from spilling over to the side. This is effective for slow speed aircraft, but for high-speed aircraft such as commercial jet, most of the spillover happens behind the wing, and the vortices are much much larger than the downward wingtip.

2. Wing-tip Fence like you find on the A300-600, A310-300, A32X family (except A320-100) and A380

Photo by: Joe Roland/V2 Photography
  • Method: Putting a small vertical fin at the wingtip, reduces the vortex effect. The small size focuses on the small radius high speed wingtip vortex which is the major component of wingtip vortex drag.
  • Benefits: Drag reduction at low weight penalty therefore still suitable for short flights. Small size does not affect crosswind landing/take off limits. Minimum interference drag too!
  • Problems: Not effective for speeds significantly higher than wing's optimum speed as the vortex size will render the fence useless. However, the Airbus use supercritical wings which makes its optimum speed on the higher end of the operating envelope.

3. The Standard Winglet like you find on the 747-400, A330 and 340

Photo by: Joe Roland/V2 Photography
  • Method: Adds aspect ratio without significantly adding the wingspan. Just add the span and hammer the tips upwards (sarcasm). The cant angle (angle of the wingtip going up) and the toe angle differs from aircraft to aircraft. The rotating vortex going up is stopped by the winglet, the cant angle will capture this to push the wing up, and the toe angle pushes the flow backwards as a form of, flow induced thrust. As long as the "induced thrust" is higher than the remaining induced drag, this method can be very effective.
  • Benefits: Big boost in aerodynamic efficiency and increases high altitude cruise stall margin. Enables the aircraft to fly higher.
  • Problems: Additional weight is something to watch out for. Interference drag exists (nothing stopping the lower airflow spilling over), and the boundary layer drag hitting a sharp angle will create another vortex.

4. Winglet-Fillet Combo of the MD-11

  • Method: Standard winglet added with a lower wingtip fence below the winglet.
  • Benefits: As per standard winglet but with lower interference drag.
  • Problems: I'm never clear on the penalties caused by this, but many suspect it was never really perfected as it's sole major user in the past, the MD-11, didn't really sell as well as we all expected (but that's another story).

5. Blended Winglet a la 737NG and soon the 320NEO

Photo by: Joe Roland/V2 Photography
  • Method: Same as the standard winglet but is not "stabbed into" the wing, instead, it is curved to eliminate the interference and boundary layer drag. This leaves literally no drag penalties for high speed cruise that dogged the standard winglet and winglet-fillet combo. And oh, yes, it looks good (to the certain extent that it tempted Airbus)
  • Benefits: Self-described in the method.
  • Problems: Weight is an obvious one, especially when deployed on "small" aircraft such as the 737NG. Crosswind limits are affected too, because you're basically adding two big sails on the end of the wings. The curve also require wingspan increases before you even go to the winglet. This eats gate space.

6. Raked Wingtips a la 767-400, 777-200LR/-300ER, 787-8

There's a pair of raked wingtip somewhere in this photo
(photo by: Joe Roland/V2 Photography)
Designers have long thought on how to reduce drag for more than the 3.5 % (wingtip fence and standard winglet), 3.75% (the winglet fillet combo) and 4.5% of the blended winglet. (Note. these numbers are just rough and I forgot where the numbers came from!) They broke the 5% barrier with the raked wingtips.
  • Method: Higher sweep angle at the wingtip than the rest of the wing.
  • Benefits: Increases aspect ratio & reduce lift induced drag, by spreading the wingtip longer with the higher sweep angle. This has the same effecft as the winglet and blended winglet without reducing the crosswind landing and take off limits. It is also surprisingly, not heavy!
  • Problems: Gobbles down gatespace like no other wingtip device because it really does increase the wingspan, this is why the 787-3 was designed with a winglet and not a raked wingtip.

But that's not all! Don't forget the other ideas:

7. Non-planar wingtip
This is a collection of various ideas. The A350XWB will use this by combining the blended winglet and raked wingtip idea. Another non-planar wingtip is the spiroid winglet. Aviation Partners (who designed the blended winglet for the 737NG) is researching this on some bizjets, and have reported reduction in fuel consumption by over 10%, however, it does look strange!

8. The MAX's hybrid wingtip

Lucky that looks don't always match aerodynamics
(image from Boeing)
How do you expand a wingtip fence, to become some form of a blended winglet without the weight, but also reduce the interference and boundary layer drag common to the winglet and winglet-fillet combo? Simple, you get a winglet-fillet combo, and hammer it to pieces till it's all bent with no sharp corners! The improved performance over the blended winglet, is around 1.5%, and you don't get the gate space growth associated with the raked wingtip. The only problem is, it looks ugly!

ENOUGH WITH AERODYNAMICS! What's in it for the passengers?
Let's start with what you don't want... 
  • Additional advertising space on the winglets (not available on raked wingtip). 
  • Lower costs. Let's admit it, fuel is just too damn expensive now, so reducing fuel burn by drag reduction, is making flights cheaper for us.

BUYER BEWARE! You may not get the results you expected!
I want to focus on the battle of the millenium, the A320 vs 737 war, which the upcoming battle is going to be the NEO and the MAX. I'm well aware of the touted benefits of the Blended Winglet. However the marketing material is so effective that we often forget the reality. We're now seeing 737NGs with those winglets flying short routes.

Here's the bad news from the realists. Short routes with winglets cost more than without! On short routes, climb becomes a greater portion of the total flying time, so does flight time spent at low levels, and it is in these conditions that the winglet's weight can end up as a penalty (climb), or its parasitic drag due to increased aircraft surface area (low level).

I looked at the trip performance numbers of the 737NG with and without winglet. Anything 300NM or less, you'd be stupid using the winglet. Above 600NM, the benefits start to come in. The Airbus A320 however, isn't a loser against the 737NG with the blended winglet, but the comparison between the two, does accentuate one thing: WEIGHT RULES AT SHORT DISTANCES! The A320's heavier airframe lose out on flights below 300NM, but beyond 600NM, it's all up to the aerodynamics and engines, and even with a mere wingtip fence and it's amazing supercritical wings, the 320 (with the IAE V2500-A5s or CFM56-5Bs) beats the 737NG on a per ton payload basis, consistently.

Before adding winglets to your 737NG or A320, consider
the composition of the mission profile
(photo by: Joe Roland/V2 Photography)
Airlines must therefore, when faced with the option to have wingtip devices, take a careful look. How long is their average mission profile? Are the rotation segregable or not between the ultra short-haul and the rest of the aircraft type operations? Does the added weight of the wingtip devices affect the landing weight limitations based on the mission profile to the extent of penalizing payload or not?

The questions to answer goes on and on as fuel prices go up and up.

However, operations department may be churning operating cost numbers behind the scenes, but the airline as a whole must not forget that the profit consists of two components: Revenue and Cost. Costs can be as low as possible, but revenue is also important. That is why, depending on the mission profile, getting a bigger and more aerodynamically efficient wingtip device for your aircraft, may not always be the answer. Operations department must be more commercially aware.

But why am I talking about this when the new aircraft with options to have a wingtip device or not is only the 737NG? Simple, the A320 and its upcoming NEO will have the option of a wingtip fence or a "Sharklet"...

(image from Airbus)

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