Aspect Ratio in Aviaiton

Aspect ratio is the ratio of wing span to its aerodynamic breadth or chord. For most wings the length of the chord is not a constant but varies along the wing, so the aspect ratio AR is defined as the square of the wingspan divided by the area of the wing planform. (Ref: Wikipedia)

I can go on with formulas and other technical details. But let's not get caught up in various representation. What exactly is aspect ratio? What does it do?

Aspect ratio tells you how the wings perform aerodynamically. 

By looking at the value of aspect ratio, you can tell if the wings produce more lift or not. 

Simply put, if the value of aspect ratio is high then the wings produce more lift. 

If the value of aspect ratio is low, then those wings produce less lift. 

Why is it like that?

Check out the picture below:

Aspect Ratio of A = 4

Aspect Ratio of B = 25

And both have same wing area. 

Still the shape is different. Why?

Again, look at the picture below:

It's the Solar Impulse 2 and an A330. Both have different aspect ratios. Solar impulse has high aspect ratio while A330 has lower aspect ratio. Why?

It is to get higher lift to drag ratio. 

Simply put, the lift should be higher than drag. Of course all the aircraft have a positive lift to drag ratio otherwise they won't fly. But we can increase the value of lift while keeping drag value at bay, so that at higher lift to drag ratio, the aircraft produces tremendous lift force.

But if we increase lift the drag also increases. After all their parameters are same. 

How do we increase lift but not drag?  

That is where aspect ratio kicks in. 

With keeping the surface area same, they can reduce drag by extending the wing. As the wing is extended, the amount of air spillage(due to pressure difference) is less from bottom of the wing to top of the wing. 

These so called wingtip vortices try to pull back the aircraft because its a drag force (called induced drag) acting opposite to thrust. 

When you have a reduced wing tip area, then the amount of spill is less and thus drag is reduced.

Still why do we need to extend the wing? Can't we just make the wing tips short? 

Well, surface area directly effects lift force. It's one of the parameters. So when you cut short a wing's tip, the surface area decreases. That does not help gaining lift. We need a way to keep the surface area constant and then make the wing tips short. And the solution is the design of "Aspect Ratios". 

If high aspect ratios provide high lift to drag ratio, then why in the world we need low aspect ratio wings?

Because having high lift to drag ratio is sometimes not the priority. 

Speed is. Maneuverability is. Fuel storage space is. 

What do you think is the aspect ratio of this F-102 Starfighter:

It's 2.35. 

It's not efficient. It has a poor lift to drag ratio than a Cessna 172. 

Then why are they designed like this? 

They are designed for the speed. 

You see, velocity is one another parameter of lift. (Read more about lift)

So, to compensate decreased surface area; velocity is increased. 

F-102 was the first aircraft to break Mach 2. 

So is the Bell X-1 which broke the sound barrier in October, 1947  for the first time in history. Did it have a high aspect ratio? 

Nope. Aspect ratio of Bell X-1 was " 4 ". 

SR-71 Blackbird flew at Mach 3 with an aspect ratio of 1.7.

As you know, SR-71 is a legend. It was from the legendary Skunk Works where everything fly fast and high.

There is another aircraft from Skunk Works that fly high (above 70,000ft) with an aspect ratio of 14. 

That's right - 14. 

That is 9 times more than the aspect ratio of SR-71, and it still flies above 70,000ft.

And that aircraft is the private jet of NSA & CIA. :)

The one and only Lockheed U-2. 

U-2 is a subsonic surveillance aircraft. And when operating at high altitudes, the density of air is less and they don't have the luxury of high speed. The requirement to produce tremendous lift is the highest priority. So in order to compensate, Lockheed engineers gave an high aspect ratio wing to U-2; thereby increasing the surface area and reducing drag. 

Most of the surveillance aircraft that operates at high altitude, where air is scarce, uses high aspect ratio wing. 

Commercial airliners need speed as well as lifting capability. So they settle to have an aspect ratio somewhere between 6 and 10.

Boeing 747 has an aspect ratio of 6.96.

The new Boeing 777-9X has an aspect ratio of 9. I have an article about the new Boeing 777 9X. 

Finally, here is a chart plotted between Mach No. and Aspect Ratio. 

You can see from here that aircraft that has a high aspect ratio has speeds below mach 1.

Aspect ratios are great. By looking at just it's value, we can tell lot about the planes that have them. 

Thank You for reading guys.

CK

Why Boeing 777 9X transform its wings?

You've seen the videos. The new Boeing 777 9X has wings that transform. I'm not exaggerating, but it looks damn cool. Period.

But why did Boeing make something like this? Why does the wing transforms? Why does it fold?

The answer lies in just one word : "Efficiency". 

Let me tell you, wings are marvelous piece of engineering. And we don't see wings like this everyday. 

The world today need designs that are effective and efficient. Effective is doing right things while efficiency is doing things right. As you know, fuel is not a luxury anymore. The effective thing to do is design something that uses less fuel to fly. And do it with a particular design tactic that does its job brilliantly. 

Boeing 777 is designed for long haul flight. They are designed to fly between international cities. From Dubai to London. Or from Paris to New York. 

With long haul flights, cruising in high altitudes for a long time requires wings to work more. If they can produce more lift, the engines can relax a bit. In fact with Boeing 777 9X, the airliners save up to 12% fuel per seat. 

If wings are to provide more lift - one of the thing they should have is high aspect ratios. (I will write an article about aspect ratios tomorrow). 

And who do we look to when you want high aspect ratio designs? 

Gliders. 

Gliders have long wings. And they fly high without an engine. 

Long Wings provides less drag and high lift.Boeing took this concept and brilliantly integrated into its wing design. And so, final wing span is so big that it is almost same as that of an A380. 

Still, that explains only the length of the wing. Why does it need to "fold"?

Its for ground handling. 

Wings only are used when flying. But they also have to compatible in ground. 

I once worked with Honeywell's Advanced Virtual Docking Guidance system. This system had all commercial aircraft types specification in its database. It has the specification of Boeing 777 also.The old series have wing span 7 Meters shorter than new one.

Now with introduction of 777 9X, do you think that all these ground specs will change? Do they have to update the wing span length so as to dock the aircraft properly and connect the air bridge? Do they have to update this info in all international airports around the globe?

Of course not. 

The new wing folds the extra 7 meters. They fold it so that the new Boeing 777 9X is welcomed to all airports without having to go through any painful system updates. This is how Boeing want its aircraft to be. Easy, effective and efficient. 

So there you go. The reason behind transforming wings. 

Thank You for reading guys. 

Let me know if you have any doubts.

CK

MH370 : Year One, What happened?

MH370 has turned from a mere flight crash to a legend. The flight which disappeared over Indian ocean on March 8th 2014 will complete its one year coming Sunday. At the time of this writing, no solid clue or evidence have been found. When we think back about this worldwide-propaganda, all everyone wants to know is "What happened?". How can a $300 million Boeing 777 ,with no crash record since its first flight in 1994, disappear with no trace? How did this happen in the first place?

As an aeronautical engineer and an ATC professional myself, I cannot fathom the reasons for this covert enigma. For the public outside aviation industry, it's just another mystery. Something to tangle with crazy conspiracy theories. Even though modern world communication have reduced great distance between people, there are plenty of places on our planet where a commercial aircraft with 239 people can disappear.

Radar technology which was operational in 1940s, are designed to cover the land area. Radar was developed for world war to track enemy aircraft. Soon the technology was commercialized for passenger aircraft. With this technology it is possible to track flights 200 miles off the land. After that, no body cares to track it.

When the Air France Flight 447 crashed in the Atlantic ocean in 2009, talks for a space based tracking was proposed, but none came into effect. Flight 447 crashed while still in its intended flight route, and still it took two years to find the black box.

Promising improvement in the technology was made with ADS-B (automatic surveillance dependent broadcast). But it is not mandatory in all the countries. Since MH370, ICAO has decided to put all commercial aircraft to track every 15 minutes. This will be in effect from December 2015.

The search still continues. It has become the aviation's most expensive search in the history. By now 40% search area has been completed. And this is underwater. Flight has many things that can float. Still the mystery remains on why nothing has been found yet. Not even a single piece.

The transponders went offline two minutes after the radio contact. How? Deliberate? Fire?

And it was headed to the vast south Indian Ocean. Pilots have the option to switch off the transponders for safety reasons. It's ironic though.

The real loss is for the families and loved ones of the 239 passengers/crew who are fated to mourn over bodiless presumed deaths. It's really sad. Our prayers are with them. Also the team searching for the ill-fated flight, they have a job like no-other- we express our gratitude. We put our faith on them, that in time we will know what happened.

Our industry is aviation and we are the safest. We learn from our mistakes, unexpected mishaps. It's not an easy job, its not a perfect system - but it is only one we got. This is what we do. We write the rules with blood.

With high hopes - Chan CK