Hi Again,

Here is another one that has been bothering me having done the written practice exams from Bob and WA Aviation College:

Flaps are a lift producing (or augmentation) device - some more than others (eg Fowler and Double slotted Fowler product more lift than ordinary plain flaps.

If you turn to page 7.7 of Issue 5 Bob Tait CPL Aerodynamics you will see the various types of flaps discussed: Plain, Split, Flower and double slotted Fowler.

In discussion in particular of plain flaps, the comment on page 7.7 says "SMALL EXTENSIONS are useful for lift increases, but it generates high drag when "FULLY EXTENDED". (capitals mine)

If I consider most of the flap related questions (which by default would appear to discuss "plain flaps") then I get confused, as to me "lift producing" would suggest that my VSI indicates a POSITIVE rate of climb, however I know when I deploy flaps I am normally reducing power so that the rate of climb is in fact negative.

And yet for most of the questions they ask such as that asked in the back of the BT Aero book on Final Test No 2 which in Question 18 asks: (note there is nothing about "how much" the flap is extended either partially or fully)

Q18: When flap is extended during a glide at constant indicated air speed:
Correct Answer a) Both rate and angle of descent will increase

So my question is (and I know the angle of descent will increase), if the rate of decent is also to increase, it would assume that any lift actually produced is overcome by drag at ANY flap setting, and that in fact the net result for ANY flap setting is actually a decrease in TOTAL lift. (so much for LIFT AUGMENTATION).

Would I be correct in saying this and can anyone shed further light on this, as it seems one of those strange paradoxes, and it frustrates me

Regards

Brook.

Gday brook,

Mkay, I know this isn't DIRECTLY related to your question, but I'll try to get my point across practically..

You lose your engine in a single. You adopt the best glide speed for your type, let's use 65kt (C152). You trim and find you have about 500fpm ROD. You then pick out an aiming point and go for it. Will you take flap? Why? Why not?

You take the first stage, without any control input your nose pitches up (here is your increase in lift). As your speed decreases the nose pitches down (it's trimmed for 65kt right?). What's happened to your ROD?.. It increased. What about your AOD? It increased, as you correctly stated. What's happened to your aiming point? It's moved forward, you've REDUCED your gliding distance because you used lift 'creating' flaps.

So yes I suppose that with any stage of flap, especially full flap, with enough airspeed, drag will overcome any lift created by them. If you're doing a short field take off and use 10deg, do you leave that flap down till you land?

I'm sure you've heard and it's probably in Bob's book that ANY flap will reduce your gliding range and when you do a forced landing or glide approach your instructor will stress that you are not to take that last stage until you're 110% sure you'll make your aiming point.

Hope that helps somewhat!

Cheers

The distance an aircaft can glide in no wind depends not only on the lift being generated. It depends on the ratio between lift and drag - the lift/drag ratio. If lift is ten times drag, then you can glide ten times your height in no wind. This can be easily proved by simple maths.

You will never fully understand lift and drag until you realise that they don't exist! They are simply components of the total reaction on the wing - two sides of the same coin. You can never change one without changing the other.

Imagine a model aeroplane making 10 kg of lift and 1 kg of drag. Its lift/drag ratio would be ten to one and it would glide ten times its height in no wind.

With flaps down it makes more lift [say 12 kg], but it would also make more drag [say 2 kg]. The lift/drag ratio would now be six to one and it would glide only six times its height. The extra lift did not make it glide further because the extra drag spoilt the lift/drag ratio.

Some [not all] aircraft manufacturers recommend the application of a small amount of flap on take-off. This produces an increase in both lift and drag [as always]. The small increase in drag would decrease the acceleration during the take-off run, but we happily accept that for the benifit of the extra lift which allows lift-off at a lower speed. The take-off run is actually reduced.

Thanks Bob and Captainellzy!

Captainellzy just one thing though, you make the statement that is true in normal operations - "as your speed decreases", but if you notice though that in that question (Q18) it mentioned below:

When flap is extended during a glide at constant indicated air speed (note the CONSTANT indicated air speed). I guess this might have been the point more than any that confused me.

So Bob, (thanks for your great explanation it makes a lot of sense in relation to L/D) in relation to your small amount of flap during takeoff as in the last part of the post, would that mean in that specific case at the same CONSTANT IAS that the aircrafts flap was producting a higher positive rate of climb (compared to if there were no flap)? (I am fairly certain it does having flow this quite a few times in the C172 and A22).

So the question relates to that Constant IAS and the application of flap under those specific circumstances.

Please bear with me I can take a while to get things but when I do I really get them if you know what I mean...

Hi Folks,

Following on from this, one thing has always got me thinking, when we deploy flaps people say drag increases, I can understand that. But is it induced drag or parasite drag that increases? I would think its both. Is there one that has more weight than the other?


Cheers.

Hi Basketball,

My understanding of the drag effect of flaps it would primarily at low speed be induced drag, due to the speed range at which flaps are deployed, and by definition, since that drag is induced by the airframe, and serving a USEFUL purpose in reducing the stall and changing the nose attitude, I would suggest that it was induced drag. However if there was no useful pupose in having the flaps deployed, since the flaps can be retracted, you could consider the drag as excess induced drag within the speed range below VFE, but above speed range for VFE such drag would OBVIOUSLY parasitic drag, since forward motion is now the primary aim of the aircraft and you are now above the recommded spreed that you would normally induce such drag, thereby making it parasitic.

Brook

Hi Brook,

Thanks for the response. I had a similar thought, but when I thought about it more, perhaps I am confusing myself, i read that induced drag is greater for a clean wing than one with flaps deployed, due to the greater angle of attack that the clean wing needs to make in order to create the same lift as one with flaps deployed, making me think that it would be parasite drag when flaps are deployed. Furthermore, and I am not sure about this, when flaps are deployed the load of the lift is produced towards the wing root, with less loading on the outboard section, when compared to a clean wing. As a result less spillage occurs at the tips I.e. less induced drag.

But then I also read that Induced drag depends on 2 factors, the amount of pressure differential and the time allowed for the air to flow over the wing. With flaps deployed you are flying slower, so I would assume more time is allowed for the higher pressure beneath to migrate around the tips, even though the tips experience less loading as mentioned above, thereby increasing induced drag.

Hoping for some pointers to help me in my justifications and where I am going wrong.

Cheers,

Hi Guys,

A good topic for discussion this one - excellent question and some good logic being applied so far.

Induced drag is the drag by-product of lift production. It comes from the surfaces contributing to lift (e.g. wing, tailplane). Parasite drag on the other hand is caused by surfaces not contributing to lift (e.g. undercarriage, aerials etc).

Deploying flap increases lift and drag but, at higher flap settings, drag is increased more significantly than lift. More than half of the lift increase occurs within the first 50% of the flap deployment range and more than 50% of the drag increase occurs within the last 50%.

With small flap settings, the flaps are contributing more lift than drag and I would wager the majority of the drag is therefore induced drag. With higher flap settings, the flap increases drag more than it increases lift so you'd have an increase in parasite drag.

Deploying flaps also increases the effective angle of attack which will increase induced drag.

Flaps change the effective camber of the aerofoil essentially producing a whole new aerofoil shape with altered lift/drag characteristics.

Flaps may also significantly increase the effective chord length of the aerofoil thereby decreasing the aspect ratio of the wing which will also increase induced drag.

So, the actual breakdown of drag types being produced would have to be dependant on the type of aircraft, the type of flap (plain, split, fowler), the airspeed as well as the flap setting selected.

To put it simply, for a constant airspeed though I would say a low flap settings cause more induced drag, higher flap settings create more parasite drag.

Cheers,

Rich

Brook,

OK I read the question again, but same same, as you extend flap you gradually ease forward to maintain a constant airspeed, angle and rate of descent still increase.

I would have immediately said flaps create parasite drag, but a few good points have been made! And having Rich break it down so cleanly also makes me sway.

Good topic!

Hi Basketball, Rich, and Captain Ellzy!

Been a great source of Interest too me so thanks for raising it Basketball, and reviewing it Richard and Captain Ellzy!

Richard I get your angle, it makes sense too - in my mind I have the picture of the induced drag / parasitic drag curve, and see the parasitic drag kicking in higher up the speed curve, but by definition you are spot on. Thanks for your input too Captain Ellzy!

I found a this to be useful as well www.centennialofflight.gov/essay/Theorie..._Flight/drag/TH4.htm
Induced drag is the drag created by the vortices at the tip of an aircraft's wing. Induced drag is the drag due to lift. The high pressure underneath the wing causes the airflow at the tips of the wings to curl around from bottom to top in a circular motion. This results in a trailing vortex. Induced drag increases in direct proportion to increases in the angle of attack. The circular motion creates a change in the angle of attack near the wing tip which causes an increase in drag. The greater the angle of attack up to the critical angle (where a stall takes place), the greater the amount of lift developed and the greater the induced drag.

All of these types of drag must be accounted for when determining drag for subsonic flight. The total drag is the sum of parasite and induced drag.

There is a great little Iphone Ipad app I like called Wtunnel Pro which gives visual / kinesthetic learners like me a helping hand. You can change the AOA of an object and see visually the resultant effects.

You could always retract those flaps at a safe speed and altitude

Brook