# parasitic drag vs induced drag..... could someone please explain the difference to me



## pinsog (Oct 25, 2013)

I thought I knew so much after reading about WW2 airplanes for 30 years and then I get on this sight and feel like a retard (my apologies if I offended any other retards out there by using the term retard)

Could someone please explain the difference between parasitic drag and induced drag? Thank you


----------



## Matt308 (Oct 25, 2013)

Parasitic drag for you retards is just a formula away from induced drag. Recommend not using parameter "retard".

End of lesson.


----------



## Shortround6 (Oct 25, 2013)

See: Parasitic drag - Wikipedia, the free encyclopedia

It seems to be a good explanation for those of us who are mathematically _challenged_ to be PC.


----------



## pinsog (Oct 25, 2013)

Matt308 said:


> Parasitic drag for you retards is just a formula away from induced drag. Recommend not using parameter "retard".
> 
> End of lesson.



I don't feel any smarter than before. Is it like people saying HP is more important than torque when in fact they are interconnected rpm x torque/5252=HP?

The reason I ask is in the 1000 to 1200 hp long range interceptor post, someone said the Spitfire had higher parasitic drag but the Mustang had higher induced drag. I have NOOOOO idea what they mean.


----------



## pinsog (Oct 25, 2013)

Thank you Shortround6, 

I saw your message when I posted mine. That explains it a little better.


----------



## GregP (Oct 25, 2013)

Parasitic drag is composed of form drag, skin friction drag, and interference drag. Form drag is the drag caused by the form of the aircfcraft pushing the air out of the way. Skin friction drag is caused by less than perfect skin smoothness, such as non-flush rivets, scratches, dirt on the surface, etc. Interference drag is caused by the interseciton of shapes (mixing of airflow at intersections), such as a wing sticking out 90° (or at whatever angle) from the side of the fuselage. Parasitic drag starts out small and increases as speed increases.

Induced drag is the dag caused by the production of lift by the lifting surfaces, which can include the fuselage as well as the wings and tail. Induced drag is typically high at low speeds and decreases as the speed increases.

So the drag at low speed is mostly induced drag while the drag at high speed is mostly parasitic drag.


----------



## Shortround6 (Oct 25, 2013)

See the graph in the link, Both change with speed even on the same airplane. 

Not all airplanes follow the same curves. Without test results for the actual speed/s in question it is hard to say which was higher. we do know the Mustang had less total drag but how it was divided up?


----------



## JtD (Oct 26, 2013)

Induced drag is pretty easy to estimate fairly accurately. Again here's a link to wiki: Lift-induced drag - Wikipedia, the free encyclopedia. If you check the formula, the only thing unknown will be the wing efficiency, which doesn't vary by a wide margin in comparable flight states. It's also what the Spitfire has over the P-51, because of the elliptical wing shape, outside of the lower weight. Once you know induced drag, and total drag, you know parasitic drag.


----------



## drgondog (Oct 27, 2013)

Drag has two distinct 'buckets'. 
Drag due to Pressure Distribution and Drag due to Shear Stress. Typically parsed and categorized as noted below:

Drag due to Lift (Induced drag associated with the lifting surfaces - primarily the wing but also canards, horizontal stabilizer, wing twist,etc.)
Form Drag (Drag associated with separation of flow such as boundary layer build up over wing or separation of flow aft of a nacelle, etc) 
Parasite Drag (All the drag components associated with profile/surface of the airplane including nacelles, masts, landing gear, gaps in control surfaces, skin friction, Radiators, etc. Any element of the airframe and wings that are exposed to airflow)
Compressibility Drag (Drag due to transonic and supersonic drag rise, also includes propeller tip drag contributions)

Within Induced Drag, there are several factors which contribute to Induced Drag. They are Coefficient of Lift based on airfoil selection - which in turn is a function of (weight, wing area, velocity and density of air).

There then Aspect Ratio and Plan Form (which accounts for deviation from Minimum Induced Drag elliptical plan form). All airfoil data is expressed as Infinite wing span with zero tip losses. Many plots are adjusted for Aspect Ratio by changing the plot to reflect a finite wing with varying span to Area ratios. So, the wing span/wing area as well as planform ranging from Ellipse to Delta, to Trapezoidal are included in the calculations. 

To account for such variations, an Oswald Factor is inserted to reflect the overall efficiency of the wing tip geometry to the total Induced Drag of the Wing. 

Spit versus Mustang
The Spit had bulgy 'stuff like radiators, gaps in wheel covers, radiators, cannon barrels , poorly designed windscreen (up through Mk IX), rough camo paint, plus a wing with much higher profile drag as well as parasite drag due to rivets and gaps in sheet metal surfaces combined with greater wing surface area.

Mustang positives - no bulgy stuff, well designed canopy windscreen, flush wheel door covers, optimized radiator cowl design, second degree upper and lower airframe lofting profile, smaller wing surfaces with all gaps and rivets filled, sanded and painted.

Spit 'positives' - Thin wing delaying onset Mach Divergence Drag/Transonic Drag although the Mustang with max wing thickness to wing chord ratio at 45% vs Spit 24% meant the velocity gradient was less on the Mustang suggesting that onset Mach Divergence Drag was delayed when compared to conventional wing designs at same free stream velocity. 

The Spit had a slightly better plan form design re: tip losses/Oswald factor 

Summary - Mustang had 30+% less Zero Lift Drag but slightly higher Induced Drag at top speeds - yielding significantly higher speeds at all altitudes for same engine HP.


----------



## stona (Oct 27, 2013)

Your description of the finish of a Spitfire wing is inaccurate, certainly after the introduction of the smooth (Type S) cellulose paints in 1942. The Air Ministry in conjunction with Supermarine and sub contractors went to great lengths to establish exactly how much of the wing had to be filled sanded and smoothed (in a multi step process which took 50 man hours, one quarter skilled, for each aircraft) to have the minimum detrimental effect on performance. 







This does not reflect your description:

"...._rough camo paint_, plus a wing with much higher profile drag _as well as parasite drag due to rivets and gaps in sheet metal surfaces_..."


The Ministry also spent considerable efforts along with manufacturers in developing and improving smooth paints with a suitable camouflage finish.

The RAF in turn devoted considerable resources to instructing and informing maintenance personnel in how to maintain the finish on so called high speed aeroplanes, of which the Spitfire is obviously one.

Cheers
Steve


----------



## drgondog (Oct 27, 2013)

Steve - I hereby stand corrected on the surface prep for 1942 era Spits. Thanks for the article.


----------



## OldSkeptic (Oct 27, 2013)

A major srouce of induced drag is the formation of wingtip vortices. Wikipedia gives a simple and stright forward summary:

Lift-induced drag - Wikipedia, the free encyclopedia


> "Theoretically a wing of infinite span and constant airfoil section would produce no induced drag. The characteristics of such a wing can be measured on a section of wing spanning the width of a wind tunnel, since the walls block spanwise flow and create what is effectively two-dimensional flow.
> 
> A rectangular wing produces much more severe wingtip vortices than a tapered or elliptical wing, therefore many modern wings are tapered. However, an elliptical planform is more efficient as the induced downwash (and therefore the effective angle of attack) is constant across the whole of the wingspan. Few aircraft have this planform because of manufacturing complications — the most famous examples being the World War II Spitfire and Thunderbolt. Tapered wings with straight leading and trailing edges can approximate to elliptical lift distribution. Typically, straight wings produce between 5–15% more induced drag than an elliptical wing.
> 
> ...



So, taking the Spit vs Mustang again as an example, the Spit's induced drag was certainly lower*, but the Mustang's parasitic (all the other surfaces on the plane) was considerably lower, _especially radiator drag_, which was much higher on a Spit than the Mustang (in fact probably at least half, maybe more, of the total parasitic drag difference could be accounted by the radiator drag alone). 
The one exception was probably that the Spit had a lower tailplane drag, which offset the difference a bit.

* Because of the elliptical shape and airfoil change near the tips, noting that they both had washout.


----------



## drgondog (Oct 27, 2013)

OldSkeptic said:


> A major srouce of induced drag is the formation of wingtip vortices. Wikipedia gives a simple and stright forward summary:
> 
> Lift-induced drag - Wikipedia, the free encyclopedia
> 
> ...



The tip Vortex is a phenomenon of finite span wings.

'It' creates Drag by altering the flow field about the wing in such a fashion as to alter the surface pressure distribution in the direction of increased drag. Because the tip vortex has a downward component of velocity it has the effect of 'tilting' the lift vector 'backwards' (if you will accept this analogy) so that an incremental force acting parallel to the free stream is created (incremental drag)


----------



## stona (Oct 28, 2013)

drgondog said:


> Steve - I hereby stand corrected on the surface prep for 1942 era Spits. Thanks for the article.



No worries, it's the point of a forum, to exchange information! Glad I could help.
Cheers
Steve


----------



## OldSkeptic (Oct 28, 2013)

drgondog said:


> The tip Vortex is a phenomenon of finite span wings.
> 
> 'It' creates Drag by altering the flow field about the wing in such a fashion as to alter the surface pressure distribution in the direction of increased drag. Because the tip vortex has a downward component of velocity it has the effect of 'tilting' the lift vector 'backwards' (if you will accept this analogy) so that an incremental force acting parallel to the free stream is created (incremental drag)



Easier to think of air as particles. Basically air rushing in, but because the plane is moving it 'seems' to look like a spiral falling behind the plane. Remembering there are no 'sucking' forces involved, only 'pushing ones.
The wing is pushing air out of the way, at the tips it rushes in horizontally as well as vertically.


----------



## stona (Oct 28, 2013)

drgondog said:


> The tip Vortex is a phenomenon of finite span wings.)



What you want for less "draggy" tip vortex patterns is a nice elliptical knife-edge wing tip ........like on ......

Cheers

Steve


----------



## drgondog (Oct 28, 2013)

The Spit had a 'better plan form' closer to elliptical but also diminished its approach to elliptical efficiency by introducing wing twist. 

The trapezoidal plan form of the Mustang with tip chord to root chord ratio was pretty close from perspective of Oswald factor. 

The 109 with no twist was perhaps more efficient than both....


----------



## N4521U (Oct 28, 2013)

I must say............. 
This innocent question has turned into a great thread of information.

Well done exchange of well thought out information, and just proves why this forum is so well attended.


----------



## OldSkeptic (Oct 28, 2013)

Not really, wing twist and profile changes (both of what the Spit had) can actually reduce induced drag, if done right of course.

Remember the twist makes the tips more perpendicular to the air flow, ie a lower angle of attack. This in itself will reduce drag, at the cost of lower lift in level flight.
This also helps stalling, since the tips have a lower stall speed than the inner wing, again because of that varying angle of attack. 
The greater the angle of attack the higher the stall speed (with the usual caveat, or weasel words, "all other things being equal").

The Mustang had wing twist too, but the Spit combined that with a profile change. Those 2 factors gave the Spit its gentle stall, combined with the shape and thinness this maintained low induced drag even with such a low wing loading.

The price paid was the complexity, so you didn't get something for nothing.

Now the 109's slats, were not your modern ones, all hydraulically and computer controlled. They were on or off, often quite abruptly too.They did mean a very simple wing with a good landing stall speed at the price of control issues when one or more slats came into operation in higher speed maneuvers (even in landing with cross winds and so on ...brr). There was also a drag issues due to gaps (etc) in the leading edges and of course maintenance, dust, snow, ice and mud clogging up the slats were not a lot of fun.

You pays your money and you takes your choice.


----------



## bobbysocks (Oct 28, 2013)

drgondog said:


> The Spit had a 'better plan form' closer to elliptical but also diminished its approach to elliptical efficiency by introducing wing twist.
> 
> The trapezoidal plan form of the Mustang with tip chord to root chord ratio was pretty close from perspective of Oswald factor.
> 
> The 109 with no twist was perhaps more efficient than both....



which wing design for the 109...some had a rounded and some were squared off....or didnt it matter in this case?

do you know how much drag is reduced by removing the scoop on the mustang? or what the translation into gain of mph would be? a lot of the later reno racers removed the scoop and had the rad cooled a different way. might not have been ( and probably wasnt ) feasible for a ww2 fighter... its just something i have always been curious about.


----------



## Shortround6 (Oct 28, 2013)

" a lot of the later reno racers removed the scoop and had the rad cooled a different way. might not have been ( and probably wasnt ) feasible for a ww2 fighter... its just something i have always been curious about."

I believe (and could well be wrong) that the Reno racers use sort of a heat exchanger. Coolant from the engine circulates through a tank of water heating the water in the tank until the water turns to steam and boils off. Not too good for long distance flights.


----------



## drgondog (Oct 28, 2013)

OldSkeptic said:


> Not really, wing twist and profile changes (both of what the Spit had) can actually reduce induced drag, if done right of course.
> 
> *Negative. Wing twist adds a small increment to Induced drag - no mater how well or poorly it is done*
> 
> ...



True - but overstated as to complexity as the pilot of the later models became adjusted to the characteristics.


----------



## GregP (Oct 28, 2013)

I know of a few Reno racing Mustangs that eliminated the scoop. One was the ill-fated "The Galloping Ghost." A second was "Stiletto." A third was "Race 45," flown by Anson Johnson in 1946 and 1949. I'm not aware of any others.


----------



## gumbyk (Oct 28, 2013)

drgondog said:


> Actually what 'twist' accomplishes is ONLY to reduce the angle of attack for the outboard span where ailerons are located and provide for stalling Inboard first so that roll control is maintained as the inboard (less twisted) airfoil stalls first
> 
> 
> As the inboard section of the twisted wing stalls, the wings lose the greater part of its Lift and effectively Stalls even when the tips maintain some lift distribution and attached flow for aileron authority.



Its not about maintaining aileron authority. You can easily over-ride the effects of wash-out by using the ailerons at high AoA. Its about creating benign stall characteristics. If the wing stall unevenly (and they almost always do), if the loss of lift is restricted to the inner portion of the wing, the rolling moment is reduced. Also, the separated airflow will disrupt the flow over the tailplane, and the buffet will be felt by the pilot, giving plenty of warning..


----------



## N4521U (Oct 29, 2013)

I was just thinking............. simply put.........
Rivets are like little Parasites sticking to the skin of the plane.
If you stick your head out the window you're Inducing drag!


----------



## drgondog (Oct 29, 2013)

GUMBYK


> Its not about maintaining aileron authority.
> 
> *Well, yes it is. It is about maintaining flow over the outer wing/tips and subsequent aileron control at high angles of attack*
> 
> ...



If the wing is not twisted or has no slats there is an opportunity for a low speed level flying aircraft to stall 'outboard-in'. This is a condition of zero aileron 'authority' and hence condition in which the pilot loses instant roll control.

As you seem to quibble about the definition of 'authority' I hope we can substitute 'authority' with 'control'

Going back to the foundation of finite wing airfoil charachteristics.

1. A finite wing lifting line, and the attendant chordwise circulation vortices reach a discontinuity at the tip. When that happens the higher pressure below the wing 'leaking' to the lower pressure region of the top surface as vortices departing in the direction of the free stream - except that they have a distinctly different departure angle than the free stream Angle of Attack causing the relative AoA at the tip to be higher than the inboard untwisted airfoil sections.

2. The downward component of the air velocity is the foundation of the Induced Drag because it has the effect of operating opposite the Lift of the wing. The effect varies as Angle of Attack increases along the wing until the boundary layer builds up leading to adverse pressure gradients and separation.

3. The purpose of creating a span wise change of the angle of the airfoil chord line with respect to the free stream velocity vector is to reduce the local angle of attack along the outboard section of the wing so that it is the last section to stall as the local AoA increases... in other words the stall theoretically originates from the root region and moves progressively outward.

4. ...thereby (theoretically) enabling functioning ailerons in an un-stalled portion of the wing so that pilot controlled deflections thereof may profile for control of rolling moments that may arise as the lift distribution degrades unevenly.

The greater the aspect ratio, the relatively lower the downwash angle of the tip vortex - leading to lower Induced drag of the wing at same AoA . However there is a price when the twist is introduced as the local lift distribution span wise is reduced as the local AoA is correspondingly reduced as a function of the semi span. The tip vortex strength is essentially the same but the lift vector is reduced - ergo more Induced Drag of the wing.

Wing twist effects are treated as a separate component in the theoretical Drag analysis but is smaller than AR effect.

Reactions: Like Like:
1 | Like List reactions


----------



## swampyankee (Nov 2, 2013)

pinsog said:


> I thought I knew so much after reading about WW2 airplanes for 30 years and then I get on this sight and feel like a retard (my apologies if I offended any other retards out there by using the term retard)
> 
> Could someone please explain the difference between parasitic drag and induced drag? Thank you



Parasitic drag is caused by air rubbing across the skin of an aircraft, and by areas where there is an area of relatively low pressure behind bluff bodies, like landing gear legs, round antennas, and wires. It's caused by a fluid property called viscosity.

Induced drag is caused by lift.

Or it may be LIFT DEMONS.


----------



## GregP (Nov 3, 2013)

Why did you let the "Lift Demons" secret out of the bag?

I believe Thrust Pixies are classified, so the FIB will be after you.

Now everyone will know!


----------



## swampyankee (Nov 3, 2013)

Wings are tapered and twisted to approach an elliptical lift distribution, which can be achieved at one condition. One interesting result of this is that a tapered and twisted planform may have induced drag when it's total lift is zero: the root section may be producing positive lift, while the tips are producing negative amounts of lift. With an elliptical platform, with no twist, and with constant camber distribution, it's possible to get an elliptical lift distribution at all lift coefficients, but washin or washout, either directly by physically twisting the wing or indirectly by changing the airfoil camber along the wing, may ruin the elliptical distribution. Of course, the fact that there's this big cylindrical or prismatic lump right in the middle of the wing has ruined it already (although flying wings don't have the big lump in the middle, it's also very difficult to get one to be stable with an elliptical lift distribution).

The elliptical lift distribution (note that this is not the same as an elliptical planform) provides the least induced drag for a given aspect ratio; a wing can also be designed for the least induced drag with a given bending moment, which results in a different planform, which is narrower towards the tip than an ellipse. There's a lot of literature about optimizing planform to minimize induced drag under various constraints. Ilan Kroo of Stanford has some on-line publications about this, and if you go to the NASA Technical Reports Server, you could probably find a few hundred articles.

Reactions: Like Like:
1 | Like List reactions


----------



## drgondog (Nov 3, 2013)

swampyankee said:


> Wings are tapered and twisted to approach an elliptical lift distribution, which can be achieved at one condition. One interesting result of this is that a tapered and twisted planform may have induced drag when it's total lift is zero: the root section may be producing positive lift, while the tips are producing negative amounts of lift. With an elliptical platform, with no twist, and with constant camber distribution, it's possible to get an elliptical lift distribution at all lift coefficients, but washin or washout, either directly by physically twisting the wing or indirectly by changing the airfoil camber along the wing, may ruin the elliptical distribution. Of course, the fact that there's this big cylindrical or prismatic lump right in the middle of the wing has ruined it already (although flying wings don't have the big lump in the middle, it's also very difficult to get one to be stable with an elliptical lift distribution).
> 
> The elliptical lift distribution (note that this is not the same as an elliptical planform) provides the least induced drag for a given aspect ratio; a wing can also be designed for the least induced drag with a given bending moment, which results in a different planform, which is narrower towards the tip than an ellipse. There's a lot of literature about optimizing planform to minimize induced drag under various constraints. Ilan Kroo of Stanford has some on-line publications about this, and if you go to the NASA Technical Reports Server, you could probably find a few hundred articles.



I would agree entirely with the above except 

_Wings are tapered and twisted to approach an elliptical lift distribution, which can be *achieved* at one condition_.

Substitute *Approach* for 'achieved' as taper is introduced for a non-elliptical planform to attempt to yield an elliptical lift distribution - but never quite match that elliptical lift distribution and minimum induced drag of the true elliptical planform wing. Wash in-wash out via wing twist is introduced for two reasons;
1.) Provide wing tip liftand resultant aileron authority when the inboard lift distribution is falling apart
2.) Alter the Lift distribution profile along the wing

The FW 190 for example was a strange beast with leading edge twist running out to 80% semi chord - then zero for last 20%. The implication was to squeeze the last possible amount of inner wing lift distribution as the /ac was approaching CL max - but at the risk of a sudden departure at high speed stall due to tip approaching CL max first.. saying 'implication' is about right as I have not found an English translation of the reasoning by Tank 

Having said that, and you implied it, the leading edge design is a factor of lift related drag and treated as a delta to induced drag.

Although not stated in your last paragraph, my assump tion of the predesign approach you mention is to play with calculus of variations to set as boundary conditions Desired pressure (shear and normal) distributions as a function of RN and AoA - until the analysis yields the wing/body Geometry that is optimized for the desired Pressure Distribution results?

I got through both Calculus of Variations and Chaos Theory as applied to boundary layer behavior but never applied either in a practical manner in the Industry.. tools weren't good enough in the 60's and early 70's.

Good to chat..


----------



## swampyankee (Nov 3, 2013)

GregP said:


> Why did you let the "Lift Demons" secret out of the bag?
> 
> I believe Thrust Pixies are classified, so the FIB will be after you.
> 
> Now everyone will know!



Darn. Now I'll have to use the memory eraser.

If I can remember where I left it and my dark glasses.


----------



## swampyankee (Nov 3, 2013)

drgondog said:


> I would agree entirely with the above except
> 
> _Wings are tapered and twisted to approach an elliptical lift distribution, which can be *achieved* at one condition_.
> 
> ...



If I remember (I've just entered my 7th decade; my memory is increasing resembling a sieve used for target practice), Kroo (Design and Analysis of Optimally-Loaded Lifting Systems) used, among other methods, Weissinger's discrete vortex model and performed the induced drag calculations in the Trefftz plane (hey, I can remember my buzzwords....). It wasn't so much calculus of variations (which is, of course, fun for the whole family) as trial and error with reasonable constraints (CONMIN can be rather stupid: it's been known to do things like optimize an airfoil to resemble the symbol for infinity).


----------



## drgondog (Nov 4, 2013)

swampyankee said:


> If I remember (I've just entered my 7th decade; my memory is increasing resembling a sieve used for target practice), Kroo (Design and Analysis of Optimally-Loaded Lifting Systems) used, among other methods, Weissinger's discrete vortex model and performed the induced drag calculations in the Trefftz plane (hey, I can remember my buzzwords....). It wasn't so much calculus of variations (which is, of course, fun for the whole family) as trial and error with reasonable constraints (CONMIN can be rather stupid: it's been known to do things like optimize an airfoil to resemble the symbol for infinity).



This was the paragraph I was referring to.

"_The elliptical lift distribution (note that this is not the same as an elliptical planform) provides the least induced drag for a given aspect ratio; a wing can also be designed for the least induced drag with a given bending moment, which results in a different planform, which is narrower towards the tip than an ellipse. There's a lot of literature about optimizing planform to minimize induced drag under various constraints. Ilan Kroo of Stanford has some on-line publications about this, and if you go to the NASA Technical Reports Server, you could probably find a few hundred articles._

And you answered it. I'll go look. As a reference I am rapidly closing on 70 so we're not far apart. Having said that I spent a year in real Aero/computer modeling then transformed into airframe structures and design geek at Bell when the Industry skidded in late 60's.


----------



## drgondog (Nov 4, 2013)

Interesting paper - I need to spend more time on the section of Viscous Drag and Weights. He states as he should that drag at high lift coefficient is excluded which seems reasonable as the method will not calculate or yield boundary layer growth due to adverse pressure gradient.

He also specifies that the weight is a integrated span wise function of M/I to yield constant stress due to the applied bending loads of the pressure distribution. That would have to mean all the wing weight investigated is a factor of the main spar.

There is 'stuff' I am not familiar with like Aerodynamic Influence Coefficients which I need to ponder 

But thanks for the lead on Kroo.


----------

