skier

Can someone explain what induced drag is? To me it seems like a poor attempt at explaining a lack of lift. I came to this conclusion because induced drag gets higher at slower speeds. If this is true, then there is the most induced drag when you are stopped.

I could be thinking about this completely wrong, and if so can someone explain it to me

BMatthews

Induced drag is the drag that the wing makes when it produces lift all right. But the speed issue is hiding the real cause. Induced drag is related to the lift coefficient of the wing. At high speeds you only require small lift coefficients to produce the lift needed to support the model. But at low speeds you still need the same lift so the wing must operate at a higher lift coefficient and that produces drag. And the extreme point is when we say the wing is stalled. That's where the drag coefficient just to very high levels for small gains in lift.

Look over a lift to drag chart for an airfoil and then go play with an imaginary model wing in FoilSim and see how the lift coefficient changes as speed changes.

e=mc2

Right you are kenny! The induced drag is highest... that is why you are stopped LOL

beepee

skier,

Look at it a different way. Induced drag is the vector component of lift that is parallel to the direction of flight. During cruise flight, at high speed the angle of attack is small so the lift vector is NEAR vertical, only a small component of left is directed rearward. In slow speed flight, your wing's angle of attack is high so the component of lift is well split between lift perpendicular to flight (holding the aircraft in the air), and lift dragging on the aircraft (induced drag).

That provide adequate new confusion?

Bedford

tommy321

Have a look through the thread on ground effect a couple months back. Ground Effect and Induced Drag kinda go hand in hand. There are several ways of looking at induced drag, all of them equally valid ways of explaining it. One of them is as follows:
The wingtips make vortices...The kinetic energy used to create these vortices has to come from somewhere. This energy actually comes from the airplane engine (or gravity in the case of a glider), and since the engine has to pull harder to create these vortices, we think of it as an additional drag. That's why contraptions that reduce wingtip vortices (winglets, high aspect ratio, etc.) are really trying to reduce induced drag. Flying close to the ground, "in ground effect" helps destroy wingtip vortices, and therefore reduced induced drag.

See the link below. A discussion on induced drag always comes to ground effect at some point, and a discussion on ground effect always comes back to induced drag...
[link=http://www.rcuniverse.com/forum/m_1887016/anchors_1887016/mpage_1/key_induced%252Cdrag/anchor/tm.htm#1887016]Ground Effect Thread[/link]

LouW

Induced drag is the cost of producing lift (basically what BeePee said). An actual wing cannot produce lift without also producing induced drag. Induced drag is not directly related to speed. It is related to angle of attack. In level flight, to fly slower requires a steeper angle of attack, which results in higher induced drag. When the angle gets too steep, the wing stalls producing little lift and a lot of drag, but this drag is mostly form drag, not induced drag. Since the airplane cannot maintain level flight slower than the stall speed (except when being sustained by thrust alone) there is no induced drag in this flight condition.

There are only two significant ways to reduce induced drag, high aspect ratio, and light wing loading. These both reduce the angle of attack required for a given amount of lift, thereby reducing induced drag. (wing tip design and planform also affect induced drag, but not nearly as much as AR and wing loading.) An actual wing design is a compromise depending on the desired flight profile and the relative importance of induced vs form drag at the design flight conditions.

Tall Paul

Yesterday I flew a 2:1 aspect ratio delta and a 4.5:1 AR wing in the same wind on the slope. Both were electric powered. The 2:1 plane was stopped in mid-air, with power on, by the increased wind where the slope lip changes to horizontal... the wing had no trouble getting thru.
The delta also had very poor penetration in the wind away from the slope.
Basically the difference was the induced drag of the low AR plane being higher than a better AR configuration.

BMatthews

I suppose it depends on what you consider induced and certainly tip votex drag is part of the cost of making lift with any real airplane but I always saw induced drag as specifically being related to the airfoils profile drag only and that is what the coefficient of drag for an airfoil was all about hence the link in the performance charts between the lift and draf coefficients (Cl and Cd). Adding in the tip losses is raising the bar to analyse specific wing shapes along with the airfoil. After all the wing's overall performance depends on many factors. Not just aspect ratio, which IS the biggie, but also it depends on wing tip shape and fuselage interaction and a host of other things that are not related to the airfoil's possible performance.

Ah, maybe I'm just caught up in the semantics thing....

banktoturn

Induced drag is not directly related to speed or to angle of attack. As BMatthews pointed out, it is related to the coefficient of lift. When the wing operates at a high coefficient of lift, induced drag is high. This condition tends to occur along with the related conditions of high angle of attack and low airspeed. Induced drag is not related to the profile drag of the airfoil. The reported values of the coefficient of drag may sometimes have some representative component of induced drag added in, but I'm not sure.

banktoturn

LouW

Since for a given wing, lift coeficient is directly related to angle of attack, it is not an error to say that induced drag is also directly related to angle of attack.

probligo

Diverting just slightly...

One of the things that confused me, and which took a while to get straight in my head is the relationship -

Induced Drag > Velocity > profile drag.

As I play round in the realms of ff glider, and this is right at the bottom of the drag bucket... profile drag is very low due to the low airspeed, and induced drag is high because of the "high" Cl numbers (typically 1.0 - 1.3). Consequently you see no modern F1x models with aspect ratios of less that 12 or even 15.

The ideal solution, one which I am still figuring, would seem to be a polyhedral wing where the tip "fits into" the vortex at glide speed... [sm=idea.gif]

BMatthews

Sorry, I said profile drag above and that's far from right as you guys pointed out. Profile drag is the drag due to the wings frontal area IIRC while the induced drag is related to the coefficient of drag.

Probligo, your analysis is spot on from where I'm watching from. As for your miracle wingtips there's some folks playing with a wingtip that curves back and attaches to the wing itself a few inches in from the tip. Sort of like wingtip Mobius loops... They may indeed be onto something but I feel the design I saw was too small and probably produces more drag due to the small size of the loop insides.

Besides I think they may have beat you to it already.... Remeber the Hobie Hawk?

skier

thanks everyone. i am less confused now

probligo

Every time I see a photo of a Hawk, I drool. I have my dribble sheet draped over the k/b right now...and that is just from thinking about it.

Actually elliptical dihedral is just the first step...now take those beautiful swooping tips and sweep them back, a bit like some of the extreme four taper rc gliders, and then a bit more...

I have it in my eye, but damned if I can draw it on paper.

Take it back and attach it to the wing? Nah!!! Remember the diamond wings and other attempts to avoid induced drag? Dont work for me, anyway.

The most extreme version, and the one that comes from about three different computer models, is the "C Wing". Brilliant concept. but it don't want to turn corners apparently. Not good for ff, perhaps for sloping?

<e> Actually, there is one thing that comes back to mind, and which is often missed in this discussion on induced drag.

The last, and most easily overcome, generator of induced drag is the wing plan.

Briefly, a straight plank generates most induced drag, a pure elliptical planform generates ZERO induced drag. A straight plank to 60% half span, then taper to 80% root chord is about 40%, the multi-taper plan forms probably are close to elliptical - close to zero.

This side of the argument - if I got it right - links local wing loading with the amount of induced drag.

LouW

An elliptical planform does not generate zero induced drag. Induced drag is merely a minimum with elliptical lift distribution, but only slightly less (<10%) than that of a moderately tapered wing. In fact the elliptical form is only more efficient when a constant wing span is considered. For a constant area and root chord, a tapered wing with a little wider span will be more efficient than the elliptical form with a shorter span. With an elliptical planform each section operates at the same angle of attack across the span which is most efficient in producing lift. However it also means that the entire wing will stall at the same time which gives a rather abrupt stall.

There is no free lunch. Induced drag is the cost of producing lift in the real world and in spite of his grace and beauty, the hawk has to deal with it also. Tip design and planform do influence induced drag a little, but the cause of induced drag is the fact that the wingspan is not infinite. Higher aspect ratio (wider span) and lower wing loading are the only two factors that have a great effect on induced drag. Aspect ratio is usually limited by structural concerns and as wing loading (and induced drag) goes down, form drag goes up.

As a practical matter, induced drag only becomes significant at higher angles of attack. In normal flight profiles this is when landing where the higher drag allows a steeper approach without picking up excessive speed, which is a plus. One exception to this is pylon racing where minimum drag during steep turns is a major factor in overall speed.

Wing design depends very much on the desired mission and is a compromise between several competing factors. There is no one magic configuration.

Rotaryphile

While increasing aspect ratio is commonly used to reduce induced drag, it must be remembered that induced drag is fundamentally not a function of aspect ratio, but of wingspan. Induced drag is inversely proportional the the square of the wingspan. Increasing wingspan by 10% will reduce induced drag by 19%, regardless of what you do the the wing chord at the same time. It took me a while to get my head around this too, but I found that the concept of span loading considerably simplified drag calculations.

For the same span and lift, a wing of aspect ratio 4 will have the same induced drag as one with aspect ratio 20 as long as the wing is unstalled, but its total drag will be far higher, simply because it has much more form drag and skin friction. Total wing drag is minimized by using the maximum practical span, along with the minimum chord required to obtain enough wing area to keep the wing just a bit below the lift coefficient at which stall burbles begin to increase drag.

rmh

I just cut from foam and go fly it to see if any of the theory is relevant to small models .
The textbook answer to most efficient wing always ends up as a great glider wing --and a lousy low speed aerobatic wing.
Lift is drag and drag is lift . call it whatever you like .
for some planes , the most efficient wing is one where you can trade off high lift for high drag - and still retain high maneuverability and transition from one to another.
For those who don't fly --I mean a setup that has lift at very low angles of attack and instant braking power (all drag )- if suddenly angled sharply
No good for full scale DC3 but GREAT for a high powered low wing loading model aerobat.
This may be a textbook somewhere - probably in Russian --

banktoturn

Dick,

Yes, the theory is relevant. The textbooks don't define efficiency as narrowly as you seem to think they do, and you are using the word "efficient" when you should be using the word "optimal", or "effective". Drag is not lift, and lift is not drag. The textbooks, the English ones, German ones, Russian ones, and all the others, teach theory that predicts and explains the behavior you see with your precious foamies. Sorry, you haven't discovered any new aerodynamic realities, you simply like to fly planes that have some unusual characteristics (efficiency not being among them). Evidently, you also hold some profound disdain for the discipline of aerodynamics. You can save the rant about closed-mindedness, I can go read it 10 other places if I want to see it again.

banktoturn

LouW

You are absolutely correct. Span defines the mass of air being deflected by the passage of the wing, and for the same mass, the angle through which it is deflected must be the same to provide the same lift. Hence, the induced drag will be the same. Aspect ratio becomes quite significant when the total wing area is fixed (for instance contest rules). In that case, the highest aspect ratio gives the lowest induced drag.

Again, minimum drag may not be the optimum design for a particular wing. So very much depends on the desired flight profile. This is especially so with typical R/C models that are grossly overpowered, and are seldom in level flight for more than a few seconds. Additional drag can even be a plus in aerobatic craft to prevent excessive speed on the down line.

I suspect this is some of what Dick Hanson is alluding to. Effective design is what makes an airplane fly like you want it to, and with model aircraft the simplest way to do this is often cut and try.

rmh

Distain? -hardly-
Drag is not lift?
Read back a few comments -"lift drag is the vector component of lift etc..
You apparantly think I have no idea how these things operate -
Try this (another of my corn ball examples )
"lift is drag which works to your advantage
"Drag is lift going the wrong direction."
Too obtuse?
Simply a matter of semantics -
My precious foamies as you call them are simply samples of fun airframes which -due to very light efficient powerplants , allow anyone to try parameters broader than that, which is commonly used.
Have you tried them?
It really is fun and informative.
Perhaps you would enjoy conversing with George Hicks.
He knows EngineerSpeak very well and can interpret these silly ideas I have.

banktoturn

Dick,

It seems like disdain when I read your posts, but I have been wrong before.

Yes, drag is not lift. Your new definition is not correct either, and it is inconsistent with your comments in a previous post about drag being beneficial. Drag can certainly be beneficial, but that does not make it lift. Lift and drag have unambiguous definitions, and they just aren't the same thing.

No, not too obtuse, just incorrect. You should look up the definition of the word 'semantics'. Semantics, unlike, for instance, a particular choice of words, always matters.

No, I haven't tried foamies. I think they sound really cool, and I think your description of them is accurate, except when you start to say that the "book theory" doesn't apply to them. I'm sure they are fun, and I know that flying them would be informative.

banktoturn

rmh

OK --there is no lift from any drag generated
and there is no drag from lift generation
Now I have it.
( I look at lift and drag as being sorta like pitch black night and bright sunlight.
they are at the ends of a theoretical "light" line but actually irrevocably linked. )
like a bright night or a dark day

banktoturn

Dick,

No, you don't have it, and you aren't interpreting my statement correctly. One component of drag, induced drag, is indeed caused by lift, but that does not make it equivalent to lift. Lift is not caused by drag. Lift and drag are not two extremes on a spectrum, as light and dark are. That model is fundamentally wrong.

banktoturn

rmh

I see your point and I do understand why drag is NOT generally intrepreted as lift - It simply confuses things.
How about this:
Let's say a panel comes loose on the side of a 747 .
What does it do?
does drag hold it in position if the plane yaws slightly?

I would bet money that it "lifts" off the fuselage.
Whe a rudder is moved Do we call that side lift?
I know -- goofy
BUT as long as the rudder makes the craft yaw - be it in a 90 degree position or a 10 degree position- it still works
When does side lift change to drag and as long as it works -do we still call it lift?
On my newest aerobat - (a pretty large gas powered one) -- I depend on the entire aft section of the fuselage to act as a yaw damper - so what is doing this ?
Equalized drag ?
By using a low aspect ration wing (3.3) I can eliminate a lot of rudder fin area and simply flatten the fuselage and balance fore/aft areas enough to control yaw stability .
The short wing can be yawed easiyl and drastically without generating a snap roll.
Net effect is that the aerobat can switch from using the wings for lift --to using the fuselage
It DOES work -in fact the test models will simply loop smoothly in this knife edge position.
No I don't expect to se this setup on a passenger plane -

onewasp

I've been quietly following this thread and your criticisms to Dick's points regarding induced drag, lift etc.

I can no longer stay quiet.

You are heavily involved in a 'jot and tittle' clarification through some well meant (perhaps) criticism of Dick's plainly stated 'aerodynamic principles/conclusions'---{the I've done it and this is what happens school}-- while you appear to be placing yourself in a 'this doesn't pass muster' and incidentally, I'm far above it all elitist position. Your personal profile would seem to preclude your having any real authority to adopt this rather vaulted position. Perhaps there is much you have left out of your profile, but at this point, those are the facts.

I know of a number of Dick's very successful designs in Pattern, TOC, IMAC etc. Perhaps you should enlighten us all by posting some of your successful designs so that all may see and evaluate your background, which you feel entitles you to 'correct and/or pass judgment', with some inherent disdain incidentally, on the comments made by someone who is well known and quite successful in the very field in which you wish to occupy a self appointed position of 'learned critic'.

From where I sit, and I've had a few thousand hours of flight experience in this hobby we've got a proven designer/experimenter (Dick) politely responding to condescending comments from someone who has close to no credentials (you) . My 'jot and tittle' comment stands.

Since I am no match for Dick in the 'what makes them fly' category, I have nothing further to offer. My 45 years plus in building, operating and personal designs/modifications thoroughly aligns me with Dick's comments. You Sir are simply splitting hairs!