For example many airlines what fly the 757 now have a relitavely low CI therefore the climb and crusing speed will be slightly lower most likely in the mach 0.78 range. So if a 757 had winglets would an airline change the Cost index for the aircraft and could the aircraft fly at a high cruising speed such as mach 0.80-.0.82 in the 757 still being more efficeint and economical. Also i'm talking on the subject of a short haul flight as i know CI's differ from long range cruises to short range.

 

Thanks,

Wowk

I can't answer your question, but I'd like to pose my own and add it to your thread. Hope you don't mind.

 

What I'd like to know is if winglets are so great why did Boeing stop using them? I have a vague understanding of what they are supposed to do aerodynamically. Boeing first put them on the 747-400 as standard, then offered an option for the 767 and 757 as well as retrofit kits. Then they offered them as options on next gen 737s. The 777 and 787 got no winglets at all, not even the option. Why not?

I can't answer your question, but I'd like to pose my own and add it to your thread. Hope you don't mind.

 

What I'd like to know is if winglets are so great why did Boeing stop using them? I have a vague understanding of what they are supposed to do aerodynamically. Boeing first put them on the 747-400 as standard, then offered an option for the 767 and 757 as well as retrofit kits. Then they offered them as options on next gen 737s. The 777 and 787 got no winglets at all, not even the option. Why not?

 

The 747-400 was done in house and is more of a marketing thing then much of an improvement. The 747 winglet is also more of a wing fence because it isn't blended into the wing like it is on the 737/757/767 and because of this there isn't much of an improvement. That is why on the 747-400D models they don't have winglets because they are just extra weight and don't add any benefit on those shorter flights.

 

The winglets are actually not a Boeing design but one from former Boeing engineers that were not listened to. They all retired and started their own company called Aviation Partners and developed their own winglet. The airlines saw their improvement and now Boeing retrofits them on the aircraft in the factory. They also make winglets now for Hawkers and Falcons.

 

The 767-400/777/787 have what is called raked wingtips which have a higher sweep angle nearer the tip this is similar to the effects of the winglets that minimize wingtip vortices and washout. Another reason was that the 777 had such a large wing that if they added winglets it would actually make them to wide for some parking spots.

 

For example many airlines what fly the 757 now have a relitavely low CI therefore the climb and crusing speed will be slightly lower most likely in the mach 0.78 range. So if a 757 had winglets would an airline change the Cost index for the aircraft and could the aircraft fly at a high cruising speed such as mach 0.80-.0.82 in the 757 still being more efficeint and economical. Also i'm talking on the subject of a short haul flight as i know CI's differ from long range cruises to short range.

 

Thanks,

Wowk

 

I was told they fly at the same index but the fuel burn is less because the engines are at a lower power setting due to the decrease in drag.

If i remember correctly, i heard that the other downside to winglets is that it pushes up the landing speed a little and more runway is needed to take off, dont know if there is any truth in it mind you.

Hmm, maybe the following is too technical, but I'll give it a go anyway, let me know if there's anything you don't understand:

 

First off, lift:

The amount of lift a plane produces is equal to 1/2 times the lift coefficient (C_L), times the density of the air (rho), times the surface area of the wing (S), times the (air)speed of the plane squared (V²).

In straight and level flight, the lift must equal the weight of the plane.

This means the faster you fly, the lower your lift coefficient must be to keep lift constant. (The pilot controls C_L by changing the angle of attack of the plane).

 

To sum up: higher speed, lower lift coefficient.

 

Next up, drag:

Drag consists of two parts, zero-lift drag, and induced drag.

The amount of induced drag depends on the lift coefficient. The higher the lift coefficient the higher the induced drag.

 

Finally, winglets:

Winglets reduce the induced drag by a certain percentage. They have little effect on the zero-lift drag (might increase it a bit)

This means that winglets have more effect if the induced drag is larger.

The induced drag is larger when the lift coefficient is higher; i.e. when your cruise speed is lower.

 

Or turning it around: If you cruise at a higher speed, your lift coefficient will be lower. This means your induced drag will be lower and thus the drag reducing effect of your winglets will also be lower. They will still help of course, but the faster you fly the less useful (relatively speaking) your winglets become.

 

As for why winglets went in and out of fashion. A winglet reduces your induced drag, but it also has a certain weight. You probably also have to reinforce your wing structure to carry the winglet, which means adding even more weight. More weight means more drag. Thus when deciding on whether or not to have a winglet you have to trade off the reduction in drag due to the aerodynamic effect of the winglet on the one hand, and the increase in weight on the other hand. Whether the reduced drag is worth the extra weight will vary from plane to plane and even from mission to mission.