The 777 winglets and what happens to EPR on the TO roll?

[edawg]

Can't seem to find the answers.



777, supercritical wing, laminar flow, etc but why wouldn't winglets help?


On the EPR, I'm thinking it would fall slightly. I currently fly the CRJ which does not have an EPR readout; any help here?

[629bus]

The 777 200/300 models did not come with winglets because the wing was so efficient is simply did not need them for the aircrafts intended use.

The 777 300ER however does have winglets, you just cant see them from the side (or the untrained eye). The 777ER is a very long range aircraft, so ANY aerodynamic increase in efficiency will go a long way! The last 6ft of the 777ER wing is actually racked, and works just like a traditional wing let, working the same way but in a horizontal manner. Boeing is now of the mind set that it makes not difference to have a vertical or horizontal winglet (look at airbus they go for about 45 degrees).

[SkySurfin]

Correct the EPR drops on the Takeoff roll. Through 80kts you get THR HOLD by which stage EPR should have reached its traget as instructed from the FMC and Autothrottle. After about 100kts you start to notice the EPR roll back. It seems to be fairly linear in relation to acceleration- not sure of exact numbers though sorry.

[FIRESYSOK]

Why is it the winglet causes confusion? Ever fly a glider? A wing with a high aspect ratio? Think in those terms and the answer will come to you.

[edawg]

It's because everything written about winglets does such a great job of 'selling' it's benefits, the only real negatives I can think of would be more initial $$$ in designing the aircraft.


So at risk of sounding ignorant, why DOESN'T a glider have winglets? :ugh:

[capt_einz111]

Gliders are just like airplanes, some have winglets, others don't ;)

[edawg]

I guess I'm a little perplexed, especially given that the 777ER does have winglets... they obviously do provide a benefit even to the superior 777 wing design.

So why would boeing elect not to put them on the 777?

[holdmetight]

why DOESN'T a glider have winglets?

i would say the high aspect ratio of the glider wing already contributes a lot into reducing the effect of induced drag at low speed, which is the usual speed envelope in which the glider flies. another reason would probably be weight... you want to reduce as much weight as you can in the glider, and perhaps the designers thought that an elongated wing would serve the purpose better than having an additional small wing curled up at the wingtip!

i guess it is for similar reasons that Boeing elected not to apply winglets to the majority of its fleet - weight issues. why use extra fuel to carry the extra weight of a winglet when a longer but lighter wing would do the same job? but it was probably different with the 773ER, as mentioned above, due to its longer range, any small aerodynamic benefit would be magnified, therefore the "raked wing" design was added.

another reason would probably be due to the moment caused to the wingtip of the 777. the 777-300 has a wingspan of 61m, and with that length, the moment at the wingtip would be increased if the weight of a winglet acted on it. this is obviously not the case with the 773ER as i understand it has a strengthened wing structure.

my two cents!

[pilot_kenny]

I think so~ B777 series dont have a winglet because of the high aspect ratio, large wing span, and it's said that it's completely design by computers in a way that it's already very aerodynamic even without winglets.

while for the B777-300ER sweepback wing, it does help reduce the wingtip vortex and hence the induced drag. So why does B777-300ER have it but not others? My lecturers have the same two cents - it's for extended range, so even small aerodynamic improvement is very beneficial, say, in terms of fuel consumption.

My two cents

[cx252]

777 was the first commercial aircraft to be designed entirely on computer, Everything was created on a 3D CAD software system which means the current design without winglet was the best and most efficient one for its current dynamics. 777 was offered for floding wingtip, but no airline would like to get one. Floding wingtip and winglet are different animals. the design without winglet maybe was the trend in the airplane industry during the time. Boeing picked the design to increase the windspan rather than add windlet because enginners in Boeing believed that the span increase could also bring up the wing aspect ratio which is another measure of efficiency, by increasing the span of the wing 4/5 of the height of the winglet will have a similar effect on induced drag as a winglet and they can not find the solution for the extra weight penalty by carrying winglets.

cheers

[Happydays]

This is what im thinking...

Winglets reduces wingtip vortex, less energy loss, less induced drag etc.

You can take a weak wing and add a winglet that will help you to reduce induce drag, or you can make it longer, bigger aspect ratio also giving the same result.

But making it longer you needs to make the wing stronger. Now the 777 use a critical wing and it produces more lift aft. That makes the lift/weight couple stronger and as a result of that you now need a bigger tail to increase down force. Boeing then moves the wing forward and needs to make it stronger anyway therefore they just made it longer. The option of longer wing worked out better otherwise the extra weight of adding a winglet would not help improving the fuel burn.

this is what I think ... if im wrong help me out please

[cheez]

I guess I'm a little perplexed, especially given that the 777ER does have winglets... they obviously do provide a benefit even to the superior 777 wing design

So why would boeing elect not to put them on the 777?


Correct me if I'm wrong, but as far as I know, the 777-300 ER doesn't have winglets either.

As I understand it, the ER has raked wingtips which are essentially tapered tips with an increase in incidence towards the tip - kind of like a winglet (in terms of effect) but definitely not the same thing.

In addition, the ER has a longer wing section than previous versions, which serves to increase aspect ratio thereby reducing induced drag.

If I'm wrong, let me know........

[holdmetight]

777-300 ER doesn't have winglets either

you are correct. raked wingtips and winglets bring about a similar effect, but they are essentially not the same thing.

[WaldoPepper]

If you've ever watched an eagle soaring or sailed on a yacht, one of the most important factors in wing design is twist (reducing the AoA toward the tip). By doing this, you will reduce induced drag and therefore have a more efficient wing. Watch the eagle and it will move it's tip feathers constantly, adjusting it's wing design accordingly.

Putting wing twist into a conventional wing is difficult as it is a rigid structure so winglets are used to try and combat the large amount of induced drag at the tip.

If you look at the raked wingtip of the 777, the the Centre of Pressure of the raked section is further aft than the rest of the wing and so a moment arm is created which results in a twist in the wing tip. The twist reduces the lift which in turn reduces induced drag and a more efficient wing overall.

Both methods do the same thing, Boeing and Airbus will say their method is better.

In my opinion, using winglets is more of a "brutal" way of combating the problem, they let the induced drag form and then try and stop it using a physical barrier. By raking the tip and introducing twist in the wing, the induced drag at the tip doesn't form in the first place.

Just my 2 cents worth.

[willnotcomply]

Winglets provide fuel savings in a marked way on ULH ops. On short haul, the benefits are negligible. Some would argue that the wt penalty is decreasing economy on short haul routes. I know that ANA used to operate -400's without winglets exclusively on domestic sectors. Not sure if they still do that?

[cx252]

Both JAL and ANA still operating 747-400D (D stand for Domestic model) But both company are planing to replace them by 777. ANA is considering A380 but not firm yet.

[willnotcomply]

After about 100kts you start to notice the EPR roll back

At pre flt, the FMC will generate a calculated target EPR based upon t/o data entered. On the t/o roll, the EEC's will slightly decrease the target epr. This dynamicaly accounts for the ram air increase. It is a very minor decrease in target EPR. Warning. If actual EPR rollback occurs, as suggested above, INTERVENE!

[SkySurfin]

Sorry for the confusion, wrong wording. After 100kts the target EPR decreases slightly......happy now :ok: Watched it yesterday out of Narita and we had a target EPR of 1.362 and by V1 EPR was 1.342. So just less than a 2% decreases due to Ram rise