Wing Loadings: The Idiots Guide?

[Maximowitz]

Salutations,

Could the more technically minded please explain in laymans terms the benefits and deficits of high and low wing loadings in aircraft design? Over the years I've read numerous times that Messerschmitt designs favoured a high wing loading which was then criticised. If it was a bad idea why would he do it?

Thanks in advance.

 

[HoHun]

Hi Maximowitz,

>Could the more technically minded please explain in laymans terms the benefits and deficits of high and low wing loadings in aircraft design? Over the years I've read numerous times that Messerschmitt designs favoured a high wing loading which was then criticised. If it was a bad idea why would he do it?

Wing loading is not that useful a parameter if you neglect the ability of the wing to create lift, which depends on the airfoil - and Messerschmitt tended somewhat towards higher-lift airfoils.

Ignoring these differences for the moment, low wing loading results in low stall speeds and thus in good take-off and landing performance, and in good manoeuvrability. High wing loading (for an aircraft of identical mass) results in a small wing with little drag in high-speed flight and thus in high speed.

The take-off and landing performance can be improved by the use of lift-increasing devices such as slats and flaps, which Messerschmitt used.

Turning manoeuvrability was the aspect Messerschmitt traded off for speed. Since WW2 experience showed speed to be the most important aspect in fighter combat, that probably was a good decision on Messerschmitt's part.

Regards,

Henning (HoHun)

 

[Maximowitz]

All becomes clear. Thank you Henning.

 

[kool kitty89]

The leading edge wing slats spanning the outer ~40% of the Bf 109's wing allong with the choice of airfoil helped improve turning ability though. (and unlike the Spitfire, the Bf 109 had a maneuvering/combat setting for the flaps to improve turning ability as well, at the expense of increased drag)

Additionally, unusualy for the time (but common on modern jets), was the feature of the Bf 109's ailerons to "droop" when the flaps were lowered, effectively act as flaps along th outer portions of the wing.

wikipedia's page on slats: Slats - Wikipedia, the free encyclopedia


The 109's slats operated automatically by means of air pressure. The early models had problems with aileron "snach" or a jolt in the controls when the slats deployed as well as problems with jaming under high G loads and being rather sensitive to dirt. These problems were solved, however, on the 109F.

 

[Soren]

The automatic LE slats where fitted for mainly one purpose, improving maneuverability in the horizontal plane aka turning performance. The slats operated by means of airpressure, starting to extend at around 10 degree's AoA. The slats increase the Clmax (Or plain simple: Lift) over the covered areas by ~25%, lowering the lift-loading and improving the turn performance of the a/c. That they improved take off and landing performance was a beneficial side effect, if you can all it that.

 

[Captain Dunsel]

The problem is, to lower the wing loading you either have to lower the weight or add more wing area. Lower weight usually means a less robust structure, as in the Zero and Oscar. Very maneuverable, but also very vulnerable to gunfire.

Adding wing area isn't always the answer, either, as you now have a bigger structure, with more weight, and more drag. Aspect ratio now becomes more important, too.

Confused? Then add in drag and consider biplanes and triplanes versus monoplanes. It really gets squirrelly!

One point you have to remember when designing a plane, be it a full-scale bird or a model: Every airplane is a series of compromises.

CD

 

[Soren]

The Automatic LE slats is one of those brilliant devices that allow you to keep your wing small for low drag and still have lots of lift available when you start to maneuver. Hence why nearly all fighters since have either LE slats or the improved LE flaps.

 

[claidemore]

Of course LE slats also increase drag when they are in operation. Somewhat negating the initial advantage of a lower drag wing. Something that doesn't get mentioned often in discussions about LE slats.
In any case, kudos to Sir Fredrick Handley-Page for inventing them.

 

[stevie teever]

I am another prime example of the diminishing gene pool!

 

[kool kitty89]

Aarrrgh! SPAM!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[kool kitty89]

The problem is, to lower the wing loading you either have to lower the weight or add more wing area. Lower weight usually means a less robust structure, as in the Zero and Oscar. Very maneuverable, but also very vulnerable to gunfire.

Getting back on topic, lightening the structure is just one of a few main ways of modifying the weight of an aircraft. (fighters in particular)

There's armament, fuel capacity/load, and armor/fuel protection equipment to consider as well. All with trade-offs of course.

 

[drgondog]

The problem is, to lower the wing loading you either have to lower the weight or add more wing area. Lower weight usually means a less robust structure, as in the Zero and Oscar. Very maneuverable, but also very vulnerable to gunfire.

And very tough to accomplish with an existing airframe design - the P-51H ended up being an entirely different BOM from the P-51D and B even though the lines were very similar. They also reduced the design and limit loads about 5% going for 11g instead of 12 for 'ultimate'

Adding wing area isn't always the answer, either, as you now have a bigger structure, with more weight, and more drag. Aspect ratio now becomes more important, too.

The Ta 152 is a very good example where that approach was beneficial

Confused? Then add in drag and consider biplanes and triplanes versus monoplanes. It really gets squirrelly!

One point you have to remember when designing a plane, be it a full-scale bird or a model: Every airplane is a series of compromises.

CD

good summary. always interesting to note that P-51H/Me 109K's are hard examples to find - in which the primary airframe is lower in weight than its progenitor. Just about every notable combat a/c grew in weight to achieve more versatility - and lost some manueverability as a result

 

[Captain Dunsel]

Stevie Teever's just describing one way to increase drag...

Seriously, Koolkitty 89, I agree. You can decrease weight by using some radically new structure, such as carbon fiber. In birds like the Oscar, there were several concessions made to improve maneuverablity. Not only was the structure made lighter, but the armament was kept (at least, in the earlier models) to only twin 7.7mm guns. Oscars could outmaneuver just about anything, but lacked sufficient firepower against most Allied aircraft.

One of the more fascinating (to me) facets of aviation development has been the biplane/triplane versus monoplane in air combat. How many countries stuck with the more maneuverable (but slower) multi-winged planes, instead of going to monoplanes? Maneuverability lets you get on the other guy's tail, but if he is faster, he can not only run away before you get a good shot off, but he has the option of avoiding conflict altogether. Maneuverability may let you win the fight, but speed lets you decide if you want to fight in the first place.
That was a valid concept, up until the age of reliable missiles. Now, you can't go fast enough to outrun a missile, but you may be able to outmaneuver it. So, will maneuverability trump speed, after all?
CD.

 

[drgondog]

That was a valid concept, up until the age of reliable missiles. Now, you can't go fast enough to outrun a missile, but you may be able to outmaneuver it. So, will maneuverability trump speed, after all?
CD.

IMO nope - if you can't detect it you can't outmanuever it. While the threat is missle and radar, or stealth/missle and radar, tomorrow brings x-ray lasers, particle beams, etc (well maybe not tomorrow but weapons technology will continue to improve to point where airframe to carry man is too expensive).

The USAF was ahead of its technology in the 50s and lost the air fighter dominance because the missles didn't work. They do now but the stealth is hard to find before you are dead.. but the chips get larger, the signal processing better and sooner or later stealth won't be dominant either.

 

[syscom3]

..... The USAF was ahead of its technology in the 50s and lost the air fighter dominance because the missles didn't work. ......

The Sidewinder was considered a success.

 

[DerAdlerIstGelandet]

One of the more fascinating (to me) facets of aviation development has been the biplane/triplane versus monoplane in air combat. How many countries stuck with the more maneuverable (but slower) multi-winged planes, instead of going to monoplanes? Maneuverability lets you get on the other guy's tail, but if he is faster, he can not only run away before you get a good shot off, but he has the option of avoiding conflict altogether. Maneuverability may let you win the fight, but speed lets you decide if you want to fight in the first place.
That was a valid concept, up until the age of reliable missiles. Now, you can't go fast enough to outrun a missile, but you may be able to outmaneuver it. So, will maneuverability trump speed, after all?
CD.

I just did a nice term paper for an Embry Riddle class I was taking that covered that in it as well. Got a 100 on it by the way...8)

 

[Soren]

Of course LE slats also increase drag when they are in operation. Somewhat negating the initial advantage of a lower drag wing. Something that doesn't get mentioned often in discussions about LE slats.
In any case, kudos to Sir Fredrick Handley-Page for inventing them.

Nope they don't add any extra drag at all. The higher lift however equals higher drag as drag is a byproduct of lift.

Everything esle being equal, the higher the Clmax the higher the Cdi.

 

[kool kitty89]

Soren, according to that comparison chart of various high-lift devices used with the Clark Y airfoil, the lif to drag ratio did drop somewhat with the higher CLmax with the slot/slat than it had been with the plain airfoil.
Which would seem to imply that more speed would be lost in a turn at CLmax with LE slats, than with a larger wing of similar planform, airfoil, and lift without the use of slats.

The difference, while fairly large (~30% lower lift/drag), was certainly not enought to counter the advantages of the smaller slatted wing. (much better speed due to the reduced drag of the smaller wing, a lighter structure, along with better roll-rate due to the smaller wing and lower drag in maneuvers that didn't require the use of slats)

 

[claidemore]

Nope they don't add any extra drag at all. The higher lift however equals higher drag as drag is a byproduct of lift.

Everything esle being equal, the higher the Clmax the higher the Cdi.

They absolutely increase drag Soren, when they are extended, not when they are flush with the wing, if that is what you are referring to.

 

[kool kitty89]

Yes, but in that context drag is increased compared to what? If they weren't there the wing would be stalled at the same AoA where they're fully extended.

If you mean compared to a (similarly proportioned) larger wing producing the same amount of lift without salats (as per my discussion above), that would have greater drag (not to mention weight) as well, though lift:drag would be higher at CLmax.


Another note is that no washout (to improve stall characteristics) needs to be incorporated into the wing disign if slats are used.