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"Tip Drag"-Induced Or Skin Friction?  
User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 9
Posted (13 years 7 months 1 week 5 days 22 hours ago) and read 3059 times:

Hi guys. My Question is about the "Type" of Drag that is produced at an aircraft's wingtip.

I understand that there are two "Main Catagories" of Drag, "Induced" and "Parasite". I understand the definitions of these two Types of Drag as well as their sub-catagories. I know that one of the sub-catagories of Parasite Drag is "SKIN FRICTION".

I also understand what causes "Wingtip Vortices". They are the result of "High Pressure AIR" under the wing, trying to Wrap itself around the wing tip, in an effort to Fill In the "Low Pressure AIR" over the wing, in order to achieve "Dynamic Equallibrium" of the Air Molecules on both sides.

However, what I DON'T understand, is how the simple "Diffusion of Molecules" from High Pressure to Low Pressure can create DRAG! I have a real "Mental Block" when I try to visualize how this works. How does AIR PRESSURE cause Drag?

***Is it possible that what is "Actually" causing Drag at the wingtip, is a form of "SKIN FRICTION" caused by "High Speed" Air Molecules "Rubbing" against the wingtip skin as they wrap around the tip during Diffusion? If so, would Wing Tip Vortices still fall under the catagory of "Induced Drag?"

I can mentally picture Skin Friction causing Drag at the wingtips more than I can Pressure Differential.

If you can help me, that would be Great!!!

Chris






"Just a minute while I re-invent myself"
16 replies: All unread, jump to last
 
User currently offlineMiller22 From United States of America, joined Nov 2000, 721 posts, RR: 4
Reply 1, posted (13 years 7 months 1 week 5 days 20 hours ago) and read 2969 times:

not sure if this will answer your question or not:

The difference in high pressure below, and low pressure above the wing meet at the wing tip. When they "slide off the edge of the wing" and meet, they create turbulence in the form of a spinning vortex. Just like any turbulence formed by an antennae or surface, it causes drag. To answer your question of whether the drag is induced or parasite, it is induced. Wingtip vorticies are induced by the creation of lift on the aircraft. Since the vortice is a direct result of high and low pressure, it is a direct result of lift, thereby making it induced drag.

Hope this helps.


User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 9
Reply 2, posted (13 years 7 months 1 week 5 days 19 hours ago) and read 2962 times:

Hi Miller22. Thanks for your response. Do you believe that the drag formed at the wingtips could be a "combination" of both Induced and the "Skin Friction" part of Parasite drag?

Could it NOT be that the "Spinning Vortex" is Induced by the phenomenom of lift and the resulting behavior of High and Low Pressure Air at the wingtip, but the Vortex only creates Drag because it causes Friction on the skin of the wingtip as it spins against the surface?

I really can't seem to get a grip on how Air Pressure "alone" can cause drag. It seems to me that the vortices generated along the tip must be causing Friction as they spin off the wingtip.

Who knows, maybe I'm just going Insane!!

Chris



"Just a minute while I re-invent myself"
User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6541 posts, RR: 54
Reply 3, posted (13 years 7 months 1 week 4 days 20 hours ago) and read 2954 times:

Dear Mr. Spaceman, I think that I can help you a little.

The total "induced drag" on a plane is the drag which is a direct consequence of lift production.

But some of the generated lift is "spilled" over the wingtip, and the power, which should have helped to lift the plane, generates a tip vortex instead.

That part of the induced drag, which is a consequence of tip vortex generation, is called "induced tip vortex drag". So it is in fact not an "extra" drag, just the normal drag from lift production when the lift gets lost.

In fact any disturbance of the atmosphere created by a flying plane is the result of some energy being transferred from the plane to the atmosphere. The total energy of the disturbance will always be equal to the total drag (when we exclude the disturbance generated by engines - it's another game).

So in other words we can say that the energy, which is going into creation of the tip vortexes, is what we call "induced tip vortex drag".

Parasite drag:

If we define all non-induced drag as parasite drag, then you are right that skin friction drag is a subtype. The other major subtype is a product of the shape of the plane, or the energy needed to reposition the air molecules as then plane is advancing through the atmosphere.

Understanding skin friction drag we must acknowledge that air has viscosity. Butter has a high viscosity, engine oil much lower viscosity, water much lower again, and air much, much lower again. But it is there. And with the great surfaces and high speed it adds up to a substantial total drag.

But then there are two ways to generate friction drag: Laminar boundary layer airflow and turbulent boundary layer airflow.

Laminar flow is, as the name says, when the air molecules close to the surface stick to the metal, and molecules slightly further away from the skin flow at ever increasing speed. You could talk about imaginary layers of air which interact like flat plates sliding on top of each other. This is the favourable type of boundary layer flow, but unfortunately it is only realistic to a major degree on glider aeroplane. On airliners you will find laminar flow only on the leading edge of the wing and some inches back. And only when the leading edge is smooth and clean. Plus on the engine intakes.

Soon the boundary layer will transition into turbulent flow where the air molecules near the surface tumbles up and down in the boundary layer. Since it transfers the effect of the air viscosity further away from the surface, then it also creates more drag than laminar flow.

You will notice that mainly the leading edge of the wings is produced very accurrately, while at other places a small gap between surface panels is less important. On most airliners it simply means the leading edge slats, since laminar flow is in any case completely impossible behind the gap between the slat and the main part of the wing. On the leading edge you will also notice that for instance rivet heads are polished perfectly to shape. It has a noticeable effect on the fuel economy of the plane. Further back, or on the fuselage, the boundary layer will in any case be rather thick and turbulent, and a perfect mirror like surface becomes less important. But still all surfaces of airliners are normally immensely beautifully made (at least when we talk about western production, old Russian planes are generally very noticeably different).

Hope that it helped a little.
Best regards, Preben Norholm



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 9
Reply 4, posted (13 years 7 months 1 week 4 days 19 hours ago) and read 2946 times:

Hi Prebennorholm. Thanks for your reply. I actually didn't expect to many responses because of the nature of the question. This question involves "Aerodynamics", and I think that this forum is aimed more toward "Mechanical Equipment" questions.

I do understand quite a bit about how an aircraft wing flys, however I'm by no means a "Wizard" at aerodynamics, and probably wouldn't want to be....I'ts just to Mind Boggleing!

You obviously know quite a bit [especially if your flying gliders], and I really do appreciate your information.

I understand how beautiful a Laminar Flow wing is. I also understand what you were saying about the Boundary Layer. I am aware that the boundary layer has a "Seperation Point" where it starts to break away, and become turbulent. I am aware that a wing has a "Center of Pressure" as well. I know that these areas on a wings upper surface move forward and aft, as the Angle of Attack changes.

I understand that you were talking about another sub-type of Parasite Drag, which is known as Form Drag [the over all general form], of the aircraft. You can surely imagine the difference in form drag between the Concord and a 747.

Unfortunately though, I'm not completely satisfied with my understanding of exactly how wingtip vortices create "DRAG". I understand the High and Low pressure and Energy that is involved. However, I believe that the spinning vortices MUST be causing some SKIN FRICTION along the tip. I also feel that this happens extremely fast!

Do you believe that there is any "Skin Friction" involved with Tip Vortices Drag at all?

Chris







"Just a minute while I re-invent myself"
User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6541 posts, RR: 54
Reply 5, posted (13 years 7 months 1 week 4 days 17 hours ago) and read 2941 times:

Hi again Chris,
You are absolutely correct about form drag, separation point, centre of pressure etc. But I would probably prefer to use the name transition point instead of separation point here.

On slower moving planes such as gliders, and especially model gliders as I have flown during the last 30+ years, it happens so that often the transition from laminar to turbulent flow happens in two steps. First the flow separates somewhat from typically the wing upper surface, and then further down the cord it (hopefully) reattaches with turbulent flow. Those two points are called separation point and reattachment point respectively, and the area between them is called the separation bouble. When such separation boubles get big, then they affect performance very badly.

It gets even worse when the reattachment point suddenly slips behind the trailing edge - the separation bouble gets punctured. It may typically happen on a narrow cord tailplane on a radio controlled model glider airplane at near stalling speed, and it may of course pulse in and out of the trailing edge as the elevator is controlled on the radio transmitter. That can create some very interesting landings, I can tell you. >-:> Many of my friends, who have experiences this, believe to this day that their radio control system had failed.

Separation boubles do not happen on large and/or fast moving aircrafts. Or at least they are so small that they can be completely ignored. They have a transition point where the flow changes from laminar to turbulent. And for airliners that's very close to the leading edge, and certainly never behind the trailing edge of the leading edge slats.

But back to the tip vortex drag:

It is a little difficult to explain by words only, but I'll try.

There is in fact no such thing as tip vortex drag.

But if you compare the performance of a wing section stretching from wall to wall in a wind tunnel with a normal wing with two wingtips in the same wind tunnel, then of course the wing with two wing tips will be inferior since it generates tip vortices. It will produce less lift. Or it will produce the same lift either using a bigger angle of attack or at a higher speed. Both will create more drag. How much more drag is dependant upon the shape of the wing and especially the wingtips, and the extra drag may be reduced by winglets.

But it is that extra drag of any giving wing compared to a teoritical similar wing of infinite span which is called tip vortex drag.

So tip vortex drag is some teoretical animal, since a wing of infinite span cannot exist in reality.

Anyway a lot can be done to minimize tip vortex drag. Or maybe more correctly, make the wing perform closer to the figures of a teoretical wing with infinite span. I have already mentioned winglets. The best method is a very long wingspan - you see that carried out to the extreme on glider aeroplanes. A highly tapered cord will give a wing where a smaller proporsion of the lift is generated at the tips, therefore also less tip vortex drag. You see that on all large airliners. You will also notice that all airliners have a slightly twisted wing - less angle of attack at the tips than at the root. It has the same effect, plus it gives more responsive ailerons at low speed, plus it improves stall characteristics greatly.

Tip vortex drag also varies greatly with angle of attack. If we compare a wing with a symmetrical airfoil section with a similar teoretical wing of infinite span at zero angle of attack, then of course they would perform equally (and produce no tip vortices). But unfortunately none of them will produce any lift either. But as soon any lift is generated, then the difference - or the tip vortex drag - will show up.

But to cut it short. There is no such special drag as tip vortex drag.

Tip vortex drag is the performance difference between a teoretical wing, which generates no tip vortices, and a the performance of a real wing.

Since airplane design involves enormous amounts of numbers, then tip vortex drag is one rather artificial number which designers use in their calculations. But in reality is doesn't exist as a drag which can be explained in one single term.

Puh, did I make the words fairly clear this time?

Best regards, Preben Norholm



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 9
Reply 6, posted (13 years 7 months 1 week 2 days 23 hours ago) and read 2923 times:

Dear Prebennorholm. Thank you so much for the time and effort that you have put into responding to my question.

Obviously I had quite the "mental block" over just how and what caused Drag at an aircrafts wingtip. To see you express your thoughts about Vortex Drag as being a non-existing "Phantom" is very interesting. I can live with that, however, something must be going on at the wingtip that engineers don't like, otherwise they would not have designed winglets.

Whatever these invisible forces are [at the wingtip], that are behaving "against" the thrust of an aircraft, I still believe that they involve "SKIN FRICTION".

I have always thought that the aerodynamic feature you mentioned, requarding the downward "twisting" of the leading edge [less angle of attack], as you approach the wing tip, was quite brilliant. I understand that it's main purpose is to keep the "whole wing" from Stalling at the same time. This helps ensure that the wing will stall at the root first, then progress toward the wing tip, thus allowing the pilot to still have smooth airflow over the ailerons [giving him/her longitudinal control], while recovering from the stall. I believe this feature is called "Wash Out", but I might be wrong.

Chris



"Just a minute while I re-invent myself"
User currently offlineAaron atp From United States of America, joined Mar 2000, 533 posts, RR: 2
Reply 7, posted (13 years 7 months 1 week 2 days 21 hours ago) and read 2920 times:

Spaceman, Remember back to a graph of drag versus airspeed where it shows curves for induced drag and parasite drag. This is the one that most people use to show total drag and L/DMAX (which would of course be the lowest point on the DI curve. I don't have any reference books nearby at the moment, but this graph should be in every private pilot manual ever produced.

Remember that the parasite drag curve starts low and increases with airspeed, while the induced drag starts high and decreases with airspeed. If the Canadians don't teach this on the PPL, you may have to look this up, but it is commonly held knowledge that wingtip vortices are the greatest when the aircraft is heavy, clean, and slow. That is because wingtip vortices are part of induced drag and become weaker as airspeed increases (other factors being held constant).

It can be explained by other methods, but many people can grasp the non-technical explanation more readily.


On another note take a look at this:


Notice that looking at the rear view, the upwash fields extend well beyond the span. "Wingtip vortices" as we call them is a bit of a misnomer. What most people teach regarding the flow leads the student to believe that it is only the flow in close proximity to the wingtip that causes wingtip vortices. The downwash created by the wings is part of this same complex flow pattern. We break it down into many parts to make things simple to understand, but it all fits together.

For this reason, I understand why you believe parasite drag plays a factor. To change your mind, you may have to lead yourself to realize that the flow patterns which induce "wingtip vortices" are merely centered about the wingtip, each flow pattern inducing the vortex have a radius of at least one half-span. I suppose you just have to look at things on a broader scale with an open mind.


I wish I had graphics to show everything a little more clearly, maybe I will try to post others at a later time.

aaron


User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 9
Reply 8, posted (13 years 7 months 1 week 2 days 20 hours ago) and read 2913 times:

Hi Aaron atp. Thanks for your help with reguards to my question. I do apreciate it. I am, however, still thinking that Skin Friction is a present factor when it comes to Drag along the wing tip. You are one of just a few who have dared to touch this question. No one, however, has addressed my reason for wondering about Skin Friction. Therefore, no one has "eliminated" skin friction as being a part of wing tip vortices Drag.

I've been thinking. Maybe my belief in Skin Friction at the wing tip, is based on "VISUAL" cues. When I can actually SEE the Vapour Trails flowing off of the wing tips, I automatically think of Skin Friction.

Let me ask you a question, please. In the following photos, are the vapour trails flowing off the DC-10 and MD-11 being caused by Skin Friction along the edge of the flaps, or by High and Low Air Pressure differential? I guess it's possible that the outer edge of the outboard flaps are acting like a wing tip because once they are extended, there appears to be enough room for High pressure air underneath to wrap around to the top surface of the flap.

I'm trying to keep an open mind here. I'm also trying just to keep my mind!

Also, just to let you know, YES, the Private Pilot Traing course up here in Canada, does cover Wing Tip Vortices. The info in the Flight Training Manual on that subject is quite good, I feel. It does state that Tip Vortices fall under "Induced Drag" as well, but I'm hunting for a more detailed, deeper explaination because I suspect that Skin Friction could also be involved.

Chris


Click for large version
Click here for full size photo!

Photo © Paul Dopson



Click for large version
Click here for full size photo!

Photo © Scott Leazenby




"Just a minute while I re-invent myself"
User currently offlineAaron atp From United States of America, joined Mar 2000, 533 posts, RR: 2
Reply 9, posted (13 years 7 months 1 week 2 days 14 hours ago) and read 2910 times:

What you are seeing in those pictures are not really a great example of wingtip vortices. Those only show the core of the vortex created from ambient water vapor condensation as a result of the low pressure area behind the tip (look up the Prandtl-Glauert Singularity for more info).

a larger view of the vortex from smoke trails looks more like this:



The drag isn't parasitic from the flow over the wing, because very little of the air in the vortex ever touches the wing surface (or boundary layer flow).


The drag is from the energy that is required to impart this rotational flow (whereas, it could instead be used to accelerate the airplane until a new equilibrium is reached). Instead of creating useful lift, the wing is converting thrust from the engine into rotational flow.


aaron




User currently offlinePanman From Trinidad and Tobago, joined Aug 1999, 790 posts, RR: 0
Reply 10, posted (13 years 7 months 1 week 2 days 7 hours ago) and read 2903 times:

Hey Preb jsut a quick question.

You mentioned in your explanation that one way to reduce induced drag is to have long wings "The best method is a very long wingspan - you see that carried out to the extreme on glider aeroplanes".

I was under the impression that since long wings produced more lift that this increases induced drag. You have me confused now. Could you expand?

Panman


User currently offlineAaron atp From United States of America, joined Mar 2000, 533 posts, RR: 2
Reply 11, posted (13 years 7 months 1 week 2 days 2 hours ago) and read 2897 times:

I think he was inferring a high aspect ratio, not just long wings....


aaron


User currently offlinePanman From Trinidad and Tobago, joined Aug 1999, 790 posts, RR: 0
Reply 12, posted (13 years 7 months 1 week 2 days 1 hour ago) and read 2893 times:

"I think he was inferring a high aspect ratio, not just long wings.... "


That's what I was referring to also Aaron.

Panman


User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6541 posts, RR: 54
Reply 13, posted (13 years 7 months 1 week 17 hours ago) and read 2887 times:

Exactly, what I meant was "high aspect ratio".
And Mr. Spaceman, yes, you are right, the "wing twisting" which I mentioned is called "wash-out".
And Aaron ATP, thanks for the beautiful photo from the Dryden vortex study. It tells better than a thousand words what forces are put to action on the air masses when a heavy plane demands to be kept airborne.

Best regards, Preben Norholm



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineJT-8D From United States of America, joined Dec 2000, 423 posts, RR: 3
Reply 14, posted (13 years 7 months 1 week 17 hours ago) and read 2889 times:

Sweet lord, its induced drag. Talked about this the first week of A+P school. Let it die--lol..JT

User currently offlinePanman From Trinidad and Tobago, joined Aug 1999, 790 posts, RR: 0
Reply 15, posted (13 years 7 months 6 days 19 hours ago) and read 2883 times:

I was under the impression that you get more lift out of high aspect ratio wings (hence the long thin wings on gliders). Thus more induced drag. That's why I asked for Preb to expand a bit.

pAnmAn


User currently offlineA330 From Belgium, joined May 1999, 649 posts, RR: 7
Reply 16, posted (13 years 7 months 4 days 16 hours ago) and read 2873 times:

Parasite drag is a Zero lift drag. Planes in the air however produce an amount of lift using the wings. This lift creates a lift induced drag, the induced drag.
when lift is produced, air is accelerated downwards, the pressure on the upper wing surface being less than on the lower surface. As air flows rearwards, some air will flow around the wingtip from the H pressure area to the L pressure area above the wing, and thus the surrounding air will fill the L pressure in the area above and behind the wing. this will create vortices.
Airflow under the wing (so within the H pressure) will avoid that higher pressure in that area and will try to flow from the fuselage away.
On the contrary, airflow above the wing (so within the L pressure) will try to fill the lower pressure and will try to flow to the fuselage.
these 2 components will meet at the wingtip and will form a twisting flow, called Vortex.
The net result of this vortex is a downwash.
there is more to it, but need to get some sleep...



Shiek!
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