flaps -->induced drag

mcfly · Post by **mcfly** » Sat Feb 17, 2007 2:20 am

How come flaps don't increase Induced drag if they increase lift? In my training manuals it says that vortices are worse when flying slow and CLEAN with high power such as t/o's and landings. What's the dealeeyo?

skye · Post by **skye** » Sat Feb 17, 2007 5:58 am

Flaps don't increase Lift, they increase "coefficient of lift" (or CL). In equilibrium (non-acclerated) flight, Lift=Weight; that is, the wing only creates as much Lift as required to support the aeroplane's Weight. The Lift is proportional to airspeed (squared) and CL. The increase in CL allows the required Lift to be achieved at a lower airspeed without stalling the wing (exceeding CLmax), than would otherwise be achieved under no-flap configuration, hence the benefit of flaps for landing. Therefore, if lift is the same, the induced drag is the same. Actually, the deflection of flaps tends to disrupt the normal flow of vortices around the wing, and therefore actually reduces the effects thereof; so induced drag is reduced with flaps deployed. This effect, and the CL increase outlined earlier provide the benefit of flaps during takeoff, but up to a point due to the increase in parasite drag. Hope this helps.

mcrit · Post by **mcrit** » Sat Feb 17, 2007 9:07 am

Flaps also increase the drag coefficient by tilting the lift vector further aft and that results in the increase in induced drag.

mcfly · Post by **mcfly** » Sat Feb 17, 2007 2:15 pm

"Flaps don't increase Lift, they increase "coefficient of lift" (or CL). "

uhh....well if flaps increase CL, then flaps do indeed increase Lift.

Upon closer inspection, i was wrong in thinking flaps dont increase induced drag. Flaps DO increase Induced drag.

What i still dont get is..--> How can flaps increase Induced drag without Increasing Vortices? (in the context of safety when flying behind jets)

Is it because they split the induced drag into 2 weaker vortices per wing rather than 1 stong? 1 vortice @ flap tip and 1 vortice @ wingtip? ???

Tango01 · Post by **Tango01** » Sat Feb 17, 2007 3:48 pm

In clean configuration, more lift would be required for a given speed, due to the fact that the Cl is lower as compared to when the flaps are down. Therefore, the strength of the vortex wil be greater.

As far as flaps producing lift or not, this fact will be argued for ever. It's just like leaning the mixture, everyone has a different opinion.

husky · Post by **husky** » Sat Feb 17, 2007 4:36 pm

Flaps cause a reduction in wingtip vortices because the effective angle of attack of the wing outboard of the flaps is reduced for a given airspeed and descent rate (or level fliight or climb) with the flaps down. A wing with flaps along the WHOLE trailing edge would not see this effect.

TFE731 · Post by **TFE731** » Sat Feb 17, 2007 4:52 pm

mcfly wrote:
What i still dont get is..--> How can flaps increase Induced drag without Increasing Vortices? (in the context of safety when flying behind jets)

Is it because they split the induced drag into 2 weaker vortices per wing rather than 1 stong? 1 vortice @ flap tip and 1 vortice @ wingtip? ???

Because the induced drag you are thinking of is NOT the induced drag that's increasing. I remembering arguing this years back when I was a flight instructor with fellow colleagues as I discovered here in Canada vs the US, this type of induced drag doesn't get covered as thoroughly in training and texts.

I'll put it as simply as possible.

The lift vector created by a wing is ALWAYS perpendicular to the wing chord line and since most wings out there are built with some degree of 'angle of incidence', the lift vector is always tilted slight rearward as opposed to being exactly straight up (or 90 degrees) in level flight. Therefore, this vector of lift has components both horizontally and vertically (X, Y). The horizontal component of the lift (X) is acting in the opposite direction of the flight path of the aircraft, therefore opposing it. This is the type of induced drag we are talking about here and what is altered with flaps.

When flaps are extended you are changing the effective 'camber' of the wing. Easiest way to view this is to draw two identical cross sections of wings on paper. On the second add a little hanging flap. Now draw a straight line between the leading edge and trailing edge of both cross sections. Notice the one with the flap has a steeper angle of 'chord' now. Now draw your lift vector exactly perpendicular to both of those lines. See how the one with flap is more rearward, therefore the horizontal component increases while the vertical component decreases hence more of that lift vector is now opposing the flight path of the aircraft, hence the increase in induced drag in this configuration!

I'm sure now I'll read this back to myself and it'll be clear as mud. I will try to resource some diagrams to supplement this explanation as it is MUCH simpler to understand with drawings.

TFE731 · Post by **TFE731** » Sat Feb 17, 2007 5:05 pm

Couldn't find any so I took the liberty of drawing it myself:

Blue Side Down · Post by **Blue Side Down** » Sat Feb 17, 2007 6:02 pm

mcfly wrote:
"Flaps don't increase Lift, they increase "coefficient of lift" (or CL). "
uhh....well if flaps increase CL, then flaps do indeed increase Lift.

Only momentarily as they are moving from one deflection to another. That's why you get the 'bump' you feel as you hit the lever. After that, Lift is equal once again to weight- if Lift were 'greater' than weigth, you'd accelerate upwards indefinately... F= m*a, yo.

So, during the movement of the control surface, you get a momentary acceleration. After it stops moving, the lift the wing produces is equal to the weight of the aircraft is equal to the lift that the wing was producing before the flap was deployed.

looproll · Post by **looproll** » Sat Feb 17, 2007 6:16 pm

TFE731 wrote:Couldn't find any so I took the liberty of drawing it myself:

nicely done!

mcfly · Post by **mcfly** » Sun Feb 18, 2007 2:17 am

Very well explained and drawn. I'm not gona act like i'm not impressed. Thanks dudes.

fanspeed · Post by **fanspeed** » Sun Feb 18, 2007 9:00 am

TFE731 wrote:Couldn't find any so I took the liberty of drawing it myself:

Try always perpindicular to the relative wind. Average relative wind, that is. Think of flaps as creating the same amount of lift, only at a lower speed. So the CL does change, and for the same lift, you need a lower speed as is evidenced by:
Lift = CL½ρV²S

5400AirportRdSouth · Post by **5400AirportRdSouth** » Mon Feb 19, 2007 1:53 pm

Ok, that was awesome. Total lightbulb moment.

Thanks for the question and the answer,

5400

mellow_pilot · Post by **mellow_pilot** » Mon Feb 19, 2007 9:27 pm

No one has really answered the question about bigger vorticies without flaps (at least not in the cursory glance I had at the thread) so here' goes...

The easiest way I've found to expalin this phenominon (sp?) is to say that vorticies are related to wing loading. The harder the wing is working, the bigger the vortex. (I know, I know, it's not completely accurate, but let's KIS here folks).

So if, given the lift formula above, you have a small V², S is small (no flaps) and th lift required is large (ie max gross @ takeoff) the wing is working like a 15 yr old at Tim Hortons at the lunch rush (your CL is high, since you've increased the angle of attack to produce the lift needed.)

Again, really simplistic way of looking at it, not entirely correct from and advanced theory perspective (much like the FTGU), but it gets the idea across. I hope.

Now, where'd I put that Nomex suit...

flaps -->induced drag

flaps -->induced drag

Ding!