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

[Shortround6]

" 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]

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

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.

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

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").

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.

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.

Yes the Mustang also had twist for the same reasons - maximum aileron authority while the rest of the wing was going to hell in a handbasket..

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.

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

 

[GregP]

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]

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]

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]

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

You can easily over-ride the effects of wash-out by using the ailerons at high AoA.

Why would you wish to 'over-ride the effects of washout' at high AoA, when the purpose of wash out is HAVE an effect with the ailerons when the inboard lift distribution is departing those lovely wings?

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.

The intent of washout/wing twist is to achieve control over rolling moment due to uneven inboard loss of lift.. otherwise you have a distinct and often fatal 'malignant' stall characteristic of a slow or snap roll with no hope of recovery until the wings have resumed flow over the outer wing where the ailerons are.



Would you prefer a different definition of 'aileron authority' to describe the ability to counteract a rolling moment during a stall?

Also, the separated airflow will disrupt the flow over the tailplane, and the buffet will be felt by the pilot, giving plenty of warning.

Which of course does you no good if the aircraft departs due to loss of control of your ailerons and any subsequent rolling moment

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.

 

[swampyankee]

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]

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]

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.

 

[drgondog]

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]

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]

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..

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]

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]

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.