Principles of Flight...

[south coast]

I have been having a debate with a friend about the most efficient wing...and by that I mean the best lift to drag ratio.

I was under the impression, the most effective and efficient wing is a clean wing.

The lift created by a clean wing generates 'x' amount of drag.

Now, when trailing edge flaps are deployed, I understand that lift is increased, and drag is also increased.

Question: Is the increase in lift and drag giving the wing with flaps down a better lift to drag ratio, better efficiency , than that of a clean wing.

P of F buffs needed on this one....

Thanks

[Tarnished]

The answer is ...... it depends.

Too big a subject to try to do it justice here, the best book I've read that covers this well is called Design for Air Combat, but I leant it to someone and never got it back and I can't recal who I gave it to. Think it is a "Janes" publications.

Depends what you mean by "most efficient, most effective" for what?? Endurance, range, rate of turn etc.

T

[south coast]

Does it matter...

For example...if when the flaps are deployed, and the angle of attack increased, so in turn the lift has increased.

But since the lift has been increased, so therefore has the induced drag which is a by-product of lift. Also, profile drag has been increased because we now have something in the airflow.

So, is the ratio of lift to drag any better than when the wing was clean and not producing as much lift, but less drag?

:uhoh:

[FE Hoppy]

I agree with tarnished there is no straight forward answer. For any wing cl/cd changes with IAS and Alpha. So if you want to define efficient you will have to specify an altitude a speed and a weight and then compare the various flap settings.

There is a mod around for 73s or 72s that deploys the flaps by a couple of degrees in the cruise to reduce drag. I think it floats the ailerons up a bit also. This shows that at the average cruise conditions the aircraft is used the wing as originally designed is not at optimal Cl/Cd but by making use of the flaps it can be improved.

Think the article was in Flight International a month or so ago.

[expedite_climb]

I would answer the question by taking a step back from the technical side.

Do aircraft fly in the cruise with flaps deployed ? No, so it must be more efficient with a clean wing.

Flaps are used on the approach so that a decent descent rate can be maintained while keeping power on.

[Genghis the Engineer]

High lift devices increase CL.max. However, induced drag is a function of CL², so Cd will certainly go up.

Obviously it depends upon the individual characteristics whether CL/Cd will go up or down - although my money would be on down in most circumstances.

G

[dicksynormous]

Its a very straight forward principle.

You pay me the right money i fly efficiently. The only Flaps are the pursers if she runs out of beverages.

Hope this helps

[Fokker-Jock]

From flying gliders in the past the answer is it depends on the wing profile. Some wings get better Lift/Drag ratio with flaps extended, some don't. Some even get the best Lift/Drag ratio with negative flap setting. Assuming you're flying at speed for best Lift/Drag ratio, changing your flap setting without changing your speed will decrease Lift/Drag ratio.

In competition glider flying, some advanced gliders change to a negative flap setting in order to increase the speed for best Lift/Drag ratio, therby being able to fly faster with a lesser loss of altitude.

[dudduddud]

Obviously it depends upon the individual characteristics whether CL/Cd will go up or down - although my money would be on down in most circumstances.

Then you add on the increase in profile drag due: more frontal area and it becomes a given.

[Bre901]

Fokker-Jock



I'd just like to point out that for most flapped gliders, the "absolute best" L/D is acheived with 0 deg flap settings, when the wing is in its "cleanest" state.

Negative flap settings are indeed used to increase the L/D at high speeds, but the max L/D for a given flap setting is generally lower that the max L/D you get at 0 deg flaps (at a lower speed).

more here (http://www.sli-institute.ac.uk/~bob/flaps.htm) (with polar curves which should make things clearer)

[Fokker-Jock]

I believe we agree! This is what I wrote:

Assuming you're flying at speed for best Lift/Drag ratio, changing your flap setting without changing your speed will decrease Lift/Drag ratio

[Bre901]

Fokker-Jock

I agree we agree ;) , especially on the second part of your post

My point was more on the first part of your post :
I haven't seen yet a glider polar for which max L/D is achieved for a non-zero flap setting.

But I haven't seen the polars of all gliders built in the world, of course

[PPRuNeUser0172]

I suspect that expedite climb got his/her aeronautical lessons off the back of a cereal packet.

"Flaps are used on appoach to allow a rate of descent to be maintained at a constant power" or words to that effect.

So reduction in stall speed, increased camber, increased wing area, better low speed handling characteristics are all just unwanted side effects??

Why dont people keep quiet about stuff they clearly know very lilttle about.

As the saying goes, a little knowledge is dangerous......................
;)

[M.85]

the 737 flaps selection 1 through 5 (maybe up to 15..but not sure,woll have to look it up) provide more lift than drag.

M.85

[Keith.Williams.]

If we plot the increase in Cl and the increase in Cd against increasing flap angle we will find that we get most of the Cl increase in the first few degrees of flap deployment and most of the increase in Cd in the last few degrees. This makes it tempting to suggest that increasing flap angle by a few degrees will increase lift:drag ratio, but in most cases it does not.

Under "normal circumstances" increasing flap angle will decrease lift:drag ratio. My reason for saying "under normal circumstances" is that there is at least one special case in which this is not true.

Imagine an aircraft for which the clean stall speed is 100 kts and the stall speed with full flap is 80 kts. What flap angle will give the best lift:drag ratio if we fly at 90 kts? It does not take too much thought to see that under these very special circumstances it is the full flap configuration.

Even under more normal circumstances the conventional wisdom can be incorrect. Most users of the 737 would argue that zero flap gives best lift:drag. But a recent article in Flight magazine reported a modification that reduces drag in the cruise by setting the flaps at a very small downward angle (less than 1 degree I believe). All this really means of course is that BOEING could have done better in designing the wing had they chosen to do so.