Wings

[m37b1]

Question about wing shape and drag.

While I recognise and understand the impact airfoil shape has on lift/drag, I'd like to know more about wing shape as viewed from above or below.

For instance. You have the Spitfire's elliptical wing, the P-40's straight leading edge and tapered trailing edge, DC-3 is the opposite of the P-40, the P-38 has a leading and trailing taper. These all feature somewhat rounded wing tips. Then you have the P-51, with it's rather blocky squared off tips.

What is the influence on drag?

Please forgive my aerodynamic ignorance . . . I'm an engine guy.

 

[pbehn]

Simples....The Spitfire was the best, nothing to do with aerodynamics it looks pretty, everyone wants to fly a pretty plane and no one wants to get shot down by one.

The plan form of a wing isn't as important as its cross section (aerofoil shape) The P40 Spitfire and P 51 represent a sort of progression in aerodynamics. The P 40 was pretty much stock technology for the time it was designed. The Spitfire was designed to have the thinnest wing possible consistent with a retracting undercart and 4 guns in the wings (later increased to 8 ). The Spits wing had a twist in it which warned of stall at the limit but increased drag. The P51 was from the next generation with lamellar flow wings, much less drag compared with a spit but there were a few draw backs. All is compromise, spitfires had their wings cropped to improve low altitude performance and had wings extended to improve high altitude performance. The DC3 was a transport A/C completely different requirement altogether.


There are plenty of guys here who can quote chapter and verse on all the ramifications of wing design, it was a life's work for many.

http://wp1113056.server-he.de/ABL/20-forschung/laminarfluegel/laminarfluegel_en.htm
http://en.wikipedia.org/wiki/Elliptical_wing

 

[drgondog]

When I get some spare cycles I will weigh in.

 

[drgondog]

First of all, Induced Drag is defined as all drag due to lift of the wing. At Zero lift state there is no induced drag. All airfoils possess two dimensional values for lift as a function of attack, drag as a function of attack, and pitching moment of the wing - expressed as non-dimensional Coefficients. Additionally, the airfoil section which contain the dimensions of both upper and lower surfaces, will also have pressure distributions, all expressed as a function of the chord position of the wing section.

The airfoil sections and their properties are for wings of infinite span with NO induced drag. For real life the Aspect Ratio of Span^2/Area is the primary factor which mitigates induced drag, along with Oswald efficiency which is a non-dimensional expression (and empirically derived) to takes into account the non-elliptical lift distribution of lift for wings of general shape.

When diving deeply into drag calculations, Oswald efficiency 'e' should include the variation of Parasite Drag due to Lift.. For example, when an aircraft is at high angles of attack and increasing the AoA, the amount of parasite form drag of engine nacelles as the flow behind them separate. For most WWII aircraft 'e' = .85 is frequently used if no other sounder values are available via wind tunnel

Abbott and von Doenhoff "Theory of Wing Sections" is a good place to start.

The elliptical plan form provides least induced drag from a theoretical perspective. The Spitfire is semi elliptical and has leading edge twist - both of which increase induced drag but given parity in Aspect Ratio and comparable leading edge twist to a trapezoidal wing, it should have slightly lower induced drag.

The tapered/trapezoidal style wing approaches the elliptical plan form and adding a rounded wingtip provides slightly better tip vortex shedding while a winglet is best aerodynamic contributor toward reducing induced drag for a given plan form.

So, summarize:

For the same plan form general shape (but vary with respect to Aspect ratio) the factors that would increase Induced Drag for a/c 1 heavier than a/c 2, at the same velocity, at the same altitude, with exactly the same wing

Induced Drag = (CL)^2/(Pi*AR*e)

CL for heavier ship is >CL for lighter ship. An example would be P-51B vs P51D in which the B is 350 pounds lighter, CL for B is lower ----------------> Induced drag for P-51B is lower.

If you added more leading edge twist to the P-51B you would increase the induced drag and maybe achieve parity with the D.

If you increased the wing span of the P-51B while maintaining the wing area (i.e. like a Ta 152 vs FW 190), you increase the Aspect ratio of the wing with greater span. --------------> even greater reduced Induced drag for P-51B compared to standard wing P-51D

If you put leading edge slats on the P-51 and eliminated the wing twist, you would reduce induced drag until achieving very high AoA.

That is about all that I have at the moment.

 

[N4521U]

Whot?
I'll have to re-read.
But interesting.

 

[vikingBerserker]

Now you have my curiosity up, is there a good book or a resource for a list of the different Planforms like the "Gott 398"? I see the names of these quite often but really have no idea how one is different from another.

 

[drgondog]

Offhand, No. I have not seen any publication which a.) displays many plan forms and b.) then follows up with attributes like AR or Oswald efficiency..

 

[pbehn]

Also, it must always be remembered that the wings do other things, like hold the undercart and weapons. The Spitfires wing had a bit of a drawback in shoe horning MGs and later cannon into them. If the early WW2 planes were designed only with minimum drag in mind they may well have had difficulty landing on a made up grass strip with a pilot having minimum experience.

 

[tomo pauk]

Spitfire's wing was quite capable to hold 4 cannons, so there is no problem re. that. It's wing, while offering excellent drag charcteristics, it was also of big area. Hence it provided low wing loading, so landing and taking off was 'childishly easy when compared with Bf-109' as said by a German pilot.
The thin/'non-draggy' wing can be designed to use high-lift devices to further improve low speed handling; the thick/draggy wing has as good as no means to become more streamlined.

 

[pbehn]

Spitfire's wing was quite capable to hold 4 cannons, so there is no problem re. that. It's wing, while offering excellent drag charcteristics, it was also of big area. Hence it provided low wing loading, so landing and taking off was 'childishly easy when compared with Bf-109' as said by a German pilot.
The thin/'non-draggy' wing can be designed to use high-lift devices to further improve low speed handling; the thick/draggy wing has as good as no means to become more streamlined.

It was designed to hold 2 MGs per wing later increased to 4. It held 4 cannon later but usually only 2 because the cannon needed to be kept warm and that was difficult with the outer 2, the first cannon armed spits were a disaster the pilots begged to go back to MGs. Of course with all the gizmos to allow control at low speed anything is possible. I was just stating that an eliptical wing purely designed for minimum drag could be dangerous, they stall suddenly. (so I have read here).

 

[GrauGeist]

Now you have my curiosity up, is there a good book or a resource for a list of the different Planforms like the "Gott 398"? I see the names of these quite often but really have no idea how one is different from another.

Take a look at this wiki article regarding all aspects of wing types.

Not only does it cover successful types, but attempted or theoretical types as well.

Also, check out the citations and references at the bottom, you may find a good selection of books to look for, from that list.

Wing configuration - Wikipedia, the free encyclopedia

 

[tomo pauk]

It was designed to hold 2 MGs per wing later increased to 4. It held 4 cannon later but usually only 2 because the cannon needed to be kept warm and that was difficult with the outer 2, the first cannon armed spits were a disaster the pilots begged to go back to MGs. Of course with all the gizmos to allow control at low speed anything is possible. I was just stating that an eliptical wing purely designed for minimum drag could be dangerous, they stall suddenly. (so I have read here).

The reason why the wing was of minimum drag was it's thin profile, perhaps the thinnest of all ww2 combat aircraft. The thickness to chord percentage, at wing root, was only 13%. Spitfire's wing have had also some twist, so the root was 1st to stall, giving still a good deal of control and plenty of stall warning.
There is many reasons why the cannon armed Spit was sometimes a probelm child. One was that, until maybe Griffon or two-stage Merlin was installed, the weight penalty was too great if 4 cannons were installed. Another thing was the insuficient heating. Neither of those issues have had much, if any connection with capability of the wing to lug 4 cannons around.

 

[pbehn]

The cannons in the spit were difficult to install. As I remember they had to be canted at an angle which meant feed was poor and slight flexing of the wing caused jamming. The cannons had to be kept warm, difficult to arrange on the outer two. The 8 MGs were so spread out along the wing they were unpopular with some pilots, like firing a pepper pot.

Of course wing twist flaps slats etc improve low speed control which is what I was saying it is a compromise. The wing along with all the aerodynamics had to house wheels, fuel(sometimes) and weapons in most fighters, keeping cannons warm came later.

 

[oldcrowcv63]

Simples....The Spitfire was the best, nothing to do with aerodynamics it looks pretty, everyone wants to fly a pretty plane and no one wants to get shot down by one.

 

[tomo pauk]

The cannons in the spit were difficult to install. As I remember they had to be canted at an angle which meant feed was poor and slight flexing of the wing caused jamming.

Think you've mixed up the drum- and belt-fed Hispano cannons. Drum-fed ones were indeed having problems with feed, the drums laying on cannons' side in Spitfire's wing. When belt-fed variants were introduced, they were also rotated so the ammo was being fed from the top. The jamming due to miss-feeding was as good as cured.

The cannons had to be kept warm, difficult to arrange on the outer two. The 8 MGs were so spread out along the wing they were unpopular with some pilots, like firing a pepper pot.

The low capacity of gun heating was to blame, it had nothing to do with cannons. The big spread of the MGs was a result of a 'better safe than sorry' attitude, that sought to avoid leading ammo belts above outer MGs to the inner MGs.

 

[drgondog]

Careful about being locked into thin wing as The key to low drag when compared to fatter wing (like a Mustang)

While the thin wing of the Spit gave it an advantage in a Mcr dive, the wing also contributed to a LOT more friction drag (significantly more are x 2 for top and bottom surface) to add to the shape-parasite drag of the 20 mm extended Hispanos vs flush 50 caliber, pair of under-wing radiators, open wheel well as well as the surprisingly draggy windscreen - which was not somewhat improved until the Mk XIV IIRC.

Later, when the 51s arrived in NMF state and the Spit continued camo paint -----> more difference in friction drag.

 

[pbehn]

Think you've mixed up the drum- and belt-fed Hispano cannons. Drum-fed ones were indeed having problems with feed, the drums laying on cannons' side in Spitfire's wing. When belt-fed variants were introduced, they were also rotated so the ammo was being fed from the top. The jamming due to miss-feeding was as good as cured.



The low capacity of gun heating was to blame, it had nothing to do with cannons. The big spread of the MGs was a result of a 'better safe than sorry' attitude, that sought to avoid leading ammo belts above outer MGs to the inner MGs.

I havnt mixed up anything, the original point was that as well as all the aerodynamic issues the engineer must also ensure for example that a fighter can fight. We are making the same points. The introduction of cannon into spitfires was not part of the design and proved problematic with both drum and belt feeds all the problems were eventually solved. As I said heating guns may seem trivial in the context of all the advanced maths and aerodynamic theory posted here but it is still important, experience showed cold cannons didnt fire and so a fighter became just a target.

There was no room in the spitfire wing to feed the ammo over the other MGs so they wernt. In the Hurricane there was so the MGs were close together. There wernt many reasons to prefer a hurricane to a spit but that was one, pilots who could and did get up close and personal preferred the hurricanes concentrated fire to the pepperpot spit. It was a Polish pilot who as I remember made a particular point of it.

 

[tomo pauk]

Careful about being locked into thin wing as The key to low drag when compared to fatter wing (like a Mustang)

While the thin wing of the Spit gave it an advantage in a Mcr dive, the wing also contributed to a LOT more friction drag (significantly more are x 2 for top and bottom surface) to add to the shape-parasite drag of the 20 mm extended Hispanos vs flush 50 caliber, pair of under-wing radiators, open wheel well as well as the surprisingly draggy windscreen - which was not somewhat improved until the Mk XIV IIRC.

Later, when the 51s arrived in NMF state and the Spit continued camo paint -----> more difference in friction drag.

Indeed, the P-51 was at 16.5% (thickness to chord ratio, at root), but also of, as much as practically possible, laminar flow wing profile. The Spit did not have that benefit, it was more of a 'classic' wing? I was trying to compare it with those ones.
Bill, do we have some quantification how much the details mentioned were adding to the Spitfire's drag?

I havnt mixed up anything, the original point was that as well as all the aerodynamic issues the engineer must also ensure for example that a fighter can fight. We are making the same points. The introduction of cannon into spitfires was not part of the design and proved problematic with both drum and belt feeds all the problems were eventually solved. As I said heating guns may seem trivial in the context of all the advanced maths and aerodynamic theory posted here but it is still important, experience showed cold cannons didnt fire and so a fighter became just a target.

There was no room in the spitfire wing to feed the ammo over the other MGs so they wernt. In the Hurricane there was so the MGs were close together. There wernt many reasons to prefer a hurricane to a spit but that was one, pilots who could and did get up close and personal preferred the hurricanes concentrated fire to the pepperpot spit. It was a Polish pilot who as I remember made a particular point of it.

Sorry if I sound too much persistent, but:
Since the cannon and 0.50 ammo has been able to be fed above a neighboring cannon, the LMG ammo would be even easier to feed above a neighboring LMG. The belt fed cannons were far less problematic than drum-fed ones; the angle, or the tilt of the drum-fed ones being a significant contributor. Gun heating, or its low capacity, have had nothing to do with the airfoil chosen for Spitfire.

 

[pbehn]

Careful about being locked into thin wing as The key to low drag when compared to fatter wing (like a Mustang)

While the thin wing of the Spit gave it an advantage in a Mcr dive, the wing also contributed to a LOT more friction drag (significantly more are x 2 for top and bottom surface) to add to the shape-parasite drag of the 20 mm extended Hispanos vs flush 50 caliber, pair of under-wing radiators, open wheel well as well as the surprisingly draggy windscreen - which was not somewhat improved until the Mk XIV IIRC.

Later, when the 51s arrived in NMF state and the Spit continued camo paint -----> more difference in friction drag.

Cheers Drgondog but as I understand it the spit was designed with state of the art aero and so was the Mustang just showing how much things advanced. The Mustang was less draggy and had more room, I bet that would have surprised Mitchell. I think many things with the spit were "needs must" they could have changed the cooling if they were prepared to do without a lot of production. The windscreen wheel well and camo paint I have never really understood. In 1944 did anyone really think Spitfires needed camo paint?
?

 

[pbehn]

Indeed, the P-51 was at 16.5% (thickness to chord ratio, at root), but also of, as much as practically possible, laminar flow wing profile. The Spit did not have that benefit, it was more of a 'classic' wing? I was trying to compare it with those ones.
Bill, do we have some quantification how much the details mentioned were adding to the Spitfire's drag?



Sorry if I sound too much persistent, but:
Since the cannon and 0.50 ammo has been able to be fed above a neighboring cannon, the LMG ammo would be even easier to feed above a neighboring LMG. The belt fed cannons were far less problematic than drum-fed ones; the angle, or the tilt of the drum-fed ones being a significant contributor. Gun heating, or its low capacity, have had nothing to do with the airfoil chosen for Spitfire.

That isnt what they did. In a hurricane the MGs are mounted upright and all close together side by side in a Spit they are all spread out with the ammo boxes between the two outer guns it was two mgs per wing with two added on at the extremes. The Hurricane easily fitted two cannons per wing because its wing was thicker.