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Question on Flaps

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Question on Flaps

Old 26th Jul 2016, 07:26
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Question on Flaps

Hi,

I am reading an aerodynamics book and feeling confused about two paragraphs which is about flaps.

"Think of the trailing-edge flaps at their early extension as lift flaps (when the lifting capability of the wing is increased significantly for an acceptable penalty in drag - say for take off)"

"When flaps are lowered, lift increases but so does drag. The drag increase is proportionately greater than the lift increase, even at small angles of flap extension, i.e. the L/D ratio is less with flap extended."

If the L/D ratio was less when any flap extension, why could a take-off flaps shorten take off distance (for some aircraft such as the C172)?

Thanks,
Issac
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Old 26th Jul 2016, 10:43
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Hi Isaac,

You need to accelerate to achieve sufficient airspeed to create enough lift to fly. The acceleration is opposed mostly by the aircraft inertia (mass). The 2 components of Drag (friction and lift-induced) only become relevant with speed and lift (itself a function of speed).

Hence, as you accelerate along the runway for take off, Drag is not the main force you are overcoming. A small increase in drag - or reduction in L/D - is acceptable at that stage if the gain in Lift Coefficient is sufficient; since you can lift off at a lower speed your ground roll is shorter.

But you will climb slower with flaps extended than 'clean'.

On some aircraft the gain in lift coefficient is so small it doesn't merit using flaps for takeoff. This is the case with some (most? all?) C172 models.

Using flaps for landing also is not necessarily due to the extra lift, but because (a) the change in aircraft attitude means you can see better out the front and (b) you may want to slow down.

B.
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Old 26th Jul 2016, 14:38
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If the L/D ratio was less when any flap extension, why could a take-off flaps shorten take off distance (for some aircraft such as the C172)?

looks like you don't have any real piloting experience. never mind, lets make it simple.
First thing: stall speed. in simple words, a speed when there is no longer lift, just a drag. Plane will stop flying. Stall speed is changing with flaps configuration. Stall speed with clean plane (no flaps) is higher than stall speed with full retracted flaps. Which means you can fly slower with flaps extended. And you will be airborne at lower speed (which means shorter take off) with some flaps. (and this is why are flaps 10 recommended for 172 and short/soft field takeoff when no flaps are needed under normal condition) As was already written, you don't care about drag so much during takeoff and you have an engine which is helping you with this. Consider regular GA plane like 172, you will always do a takeoff on full power but you will need something like 60% (or even less) of power for leveled flight (depends on many condition,weight, altitude, temperature,etc) but power requirement will increase when you extend flaps, so you have to add power to keep leveled on same speed. At some point there could be no additional power and you have to choose to stay leveled on lower speed or start descending on same speed.
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Old 26th Jul 2016, 14:53
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If you kept power setting the same then you'd be right. But at the back end of the drag curve you have to increase power to maintain the same IAS. Couple this with the lift effects of the flaps and you get a shorter T/O run. Gets more complex when you want to consider angle of climb vs speed to altitude. Different aircraft have got different speed/flap setting depending on whether you want maximum rate of climb (ft/sec) or maximum angle of climb (avoid large building/hill ahead). All depends on playing off the vectors between forward speed vs vertical speed. In most circumstances going faster (up to a point) will get you higher, quicker (in terms of lapsed time) but will also use up more forward distance (hence meaning you might not clear an obstacle ahead).
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Old 27th Jul 2016, 18:27
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If the L/D ratio was less when any flap extension, why could a take-off flaps shorten take off distance (for some aircraft such as the C172)?
Well I don't know how much flying experience you have Issac, but this question seems perfectly reasonable to me. In fact I would say that it shows that you are devoting some serious thought to the problems of flight.

An efficient L/D ratio is important in flight if one is considering climb or descent gradient or cruise performance.

For take-off and landing, one is more concerned with the time and distance required either to get airborne or to come to a halt so the airborne performance is not so much of an issue.

As others have stated, especially baikonur whose explanation I like, the use of flaps lowers the stall speed and therefore enables the aircraft to lift off at a lower speed, with all the benefits that accrue (mainly the shorter ground run). The effect of the flaps on the subsequent climb may not be as beneficial but that comes under the heading of 'compromise', of which aviation is full!

Keep thinking about these things and keep asking questions. That is a good way to learn!
eckhard is offline  
Old 10th Mar 2019, 15:01
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Originally Posted by issaccph View Post
"When flaps are lowered, lift increases but so does drag. The drag increase is proportionately greater than the lift increase, even at small angles of flap extension, i.e. the L/D ratio is less with flap extended."
I don't get why CD-increase is greater then CL-increase, even with small angles of flap. I have a graph in my book showing that CL increases a lot with 10 flaps and CD increases just a bit with 10 flaps. How come "CD increases proportionately greater than CL"?

Thx!

P.S. I tried to add a picture of the graph but I get an error that I can't add an URL because I don't have 10 posts yet. The admin is not very helpful either...
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Old 10th Mar 2019, 19:27
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This diagram from 'Handling Light Aircraft' by Julien Evans might be helpful in explaining use of flaps for take-off:


Discorde is offline  
Old 11th Mar 2019, 12:29
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Gnoe, you've used an old thread to ask your question in. It would have been better to start a new one.

In short: Cd is very much influenced by frontal surface area. Have a look at a Cessna wing from straight ahead (looking at the leading edge, use a stepladder if needed to get your eyes level with the wings) with flaps up and with flaps 10 degrees down and try to see how much the frontal area has increased with just those 10 degrees. Now go to 20 degrees, 30 and 40 (if possible on that Cessna). It's the equivalent of sticking your hand out of the window of your car while driving, palm facing front.
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Old 12th Mar 2019, 10:25
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Many gliders as well as having positive flap settings for slow speed flight and landing can also put their flaps negative to -5 or -10 degrees settings to reduce drag and improve the L/D at higher speeds - but in doing so increase the frontal area... It's more a case of having an appropriate camber for the speed at which the aircraft is being flown for small flap settings either plus or minus. Larger positive settings do add drag disproportionately to deliberately worsen the L/D.
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