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

[pinsog]

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]

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

End of lesson.

 

[Shortround6]

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]

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]

Thank you Shortround6,

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

 

[GregP]

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]

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]

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]

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]

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]

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

 

[OldSkeptic]

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.

Similarly, a high aspect ratio wing will produce less induced drag than a wing of low aspect ratio because the size of the wing vortices will be much reduced on a longer, thinner wing. Induced drag can therefore be said to be inversely proportional to aspect ratio. The lift distribution may also be modified by the use of washout, a spanwise twist of the wing to reduce the incidence towards the wingtips, and by changing the airfoil section near the wingtips. This allows more lift to be generated at the wing root and less towards the wingtip, which causes a reduction in the strength of the wingtip vortices.

Some early aircraft had fins mounted on the tips of the tailplane which served as endplates. More recent aircraft have wingtip mounted winglets or wing fences to oppose the formation of vortices. Wingtip mounted fuel tanks may also provide some benefit, by preventing the spanwise flow of air around the wingtip."

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]

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


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.

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]

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]

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]

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]

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]

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]

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]

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.