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Old 4th Jan 2011, 06:22   #1 (permalink)

 
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Wing-Loading

I know this might sound like a silly question, but how do airliners manage to fly reasonably well with such horrendously high wing-loadings?
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Old 4th Jan 2011, 10:35   #2 (permalink)

Do a Hover - it avoids G
 
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It is not a silly question but it is not easy to answer your post in a meaningful way without asking you for some clarification:

What is your aviation background?

What do you mean by 'fly reasonably well' (for example do you mean they have high top speeds,are controllable in turbulence, have good takeoff and landing handling characteristics etc etc)

What do you mean by 'horrendously high' (do you just mean many times that of a GA light aircraft? or something else?)

However I will offer that in general aircraft with a high wing loading will need greater distances to takeoff and land (because they will have a higher stall speed), will be more comfortable in turbulence and will not turn so tightly at low speeds as aircraft with a low wing loading.
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Old 4th Jan 2011, 10:47   #3 (permalink)
 
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And for an aircraft that is hover-capable - when in the hover there is little or no aerodynamic wing-loading?

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Old 4th Jan 2011, 10:54   #4 (permalink)
 
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Depends if you call a helicopter an aircraft i guess
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Old 4th Jan 2011, 11:36   #5 (permalink)
 
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Okay, I meant (and should have specified!) hover-capable fixed-wing aircraft! I have a feeling John F will know an awful lot about that.
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Old 4th Jan 2011, 11:56   #6 (permalink)
 
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I bet he does
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Old 4th Jan 2011, 13:35   #7 (permalink)
 
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One can always ask the opposite question: Why do most small aircraft have such low wing loading? Their slow approach speed means that crosswind or tailwind landings are very dicey, and the large profile drag of a big wing limits their top speed. How profitable can a light transport aircraft be with its weather limitations and slow cruise speed?
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Old 4th Jan 2011, 20:03   #8 (permalink)

 
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John Farley,

Quote:
It is not a silly question but it is not easy to answer your post in a meaningful way without asking you for some clarification:

What is your aviation background?
Aviation buff. I'm not a pilot or an aerospace engineer

Quote:
What do you mean by 'fly reasonably well' (for example do you mean they have high top speeds,are controllable in turbulence, have good takeoff and landing handling characteristics etc etc)
Mostly good takeoff performance, but not just that. The other issue has to do with turning performance and such, you'd expect a plane with the wing-loading figures typically seen on a commercial-airliner (727, DC-10, etc) to lose speed much quicker than it does for it's wing-loading in turns.

I have heard about numerous cases in which relatively high g-loads were pulled (not intentionally, by accident) and while the plane lost speed in the turns, you would think that for the thrust to weight ratios an airliner has, and the heavy wing loadings, the plane would have quickly lost all it's speed and just dropped out of the sky like a brick.

Quote:
What do you mean by 'horrendously high' (do you just mean many times that of a GA light aircraft? or something else?)
Well, as I understand it wing-loadings over 85 lbs/ft[sup]2[/sup].

The original 727-100 had a MTOW around 160,000 pounds and a wing-area of 1,650 square-feet, yielding a wing-loading of 96.97 lbs/ft[sup]2[/sup].

The DC-10-10 had a MTOW of 430,000 pounds and a wing-area of 3,550 square-feet, yielding a wing-loading of 121.13 lbs/ft[sup]2[/sup].
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Old 4th Jan 2011, 21:19   #9 (permalink)
 
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There are many more factors in aerodynamic design than just wing loading-- span loading (the flip side of aspect ratio), airfoil, finess ratios, thrust loading, drag, in all its components and installed high lift devices. A number of fighters have greater wing loadings than the DC-10 and perform fine, for their designed mission. Yes, under G loads they lose speed, but FAR 25 design is for 2.5 positive G and all of them possess the power to overcome the resultant induced drag.

Yes, airliners have undergone proof levels G-loadings, usually they have been associated with a deep nose-low attitude and no amount of G prior to breaking the airplane would result in a stall and "falling out of the sky".
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Old 5th Jan 2011, 01:32   #10 (permalink)

 
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galaxy flyer,

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There are many more factors in aerodynamic design than just wing loading-- span loading (the flip side of aspect ratio), airfoil, finess ratios, thrust loading, drag, in all its components and installed high lift devices.
I assume by fineness-ratios you mean T/C ratio; by span-loading you mean the ratio of span to chord; and thrust loading you mean T/W ratio?

Quote:
A number of fighters have greater wing loadings than the DC-10 and perform fine, for their designed mission.
Like the F-105, it performed decently well in regards to sustained agility when flying faster than a bat out of hell at low altitudes when lightly to moderately loaded; at lower speeds it bled off speed way too easily, and at full-loads, even it's high-speed agility fell off.

Quote:
Yes, under G loads they lose speed, but FAR 25 design is for 2.5 positive G and all of them possess the power to overcome the resultant induced drag.
They can all hold 2.5g without loss of airspeed?

Quote:
Yes, airliners have undergone proof levels G-loadings, usually they have been associated with a deep nose-low attitude and no amount of G prior to breaking the airplane would result in a stall and "falling out of the sky".
I was largely talking about while in level flight, not in a steep-dive.
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Old 5th Jan 2011, 01:49   #11 (permalink)
 
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No, span loading is weight divided by span, related to aspect ratio which is what is what you are speaking of.

Yes, fineness is T/C and thrust loading is T/W

The performance under G loads is related to specific excess power, that is, wing loading causes loss of IAS, specific excess power is how the plane can overcome the increase in induced drag. The Thud didn't have a lot of Ps at high loadings or low speed.

I cannot speak for all transport category aircraft but mostl of them at reasonable altitudes can sustain 2.5 Gs for at least 180 degrees and need no more. At cruise levels, 1.2G or 1.3 G is about standard stall margins and sufficient.

Not many transport category planes have any need to exceed 2.5 G at any time and would absolutely need to only in an out of control dive recovery.

Wing loading is not a factor in any conceivable flight regime for the types of planes we are talking about. Transports don't engage in turning fights for survival or "scissor" with another plane to get landing priority.


Were it not for these "horrendously" high wing loadings they could not do their jobs as jet transports. Try designing a jet transport with a wing loading of, say, 50 lb/ sq ft.
GF
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Old 5th Jan 2011, 01:55   #12 (permalink)
 
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Quote:
I assume by fineness-ratios you mean T/C ratio; by span-loading you mean the ratio of span to chord; and thrust loading you mean T/W ratio?
Jane-Doh,

not exactly,
Fineness ratio, is more about the physical cleanness of the wing it concerns boundary layer T/C is the thickness ratio

Span loading, is the weight divided by the span-a measure of how much load is distributed along the span the

T/C is the thickness ratio...important when chord changes as in flap deflection

The maximum wing load or the maximum weight to ever be carried by the wing is carried at the stall...and is dependent on how far above the stall speed you are if you are at Vs then only one g can be pulled at 2Vs then four times the load will be imparted at the stall at 3Vs 9g...etc...

I hope that helps...

Quote:
One can always ask the opposite question: Why do most small aircraft have such low wing loading? Their slow approach speed means that crosswind or tailwind landings are very dicey, and the large profile drag of a big wing limits their top speed. How profitable can a light transport aircraft be with its weather limitations and slow cruise speed? 4th Jan 2011 07:56
Great Question
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Old 5th Jan 2011, 01:58   #13 (permalink)
 
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I stand, actually sit, corrected by my learned colleague, PA

GF
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Old 5th Jan 2011, 02:02   #14 (permalink)
 
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PA and Jane DoH

Actually small planes have such low wing loadings due to FAR 23 limiting stall speeds to 61 knots due to crash survival considerations. Cost consideration rule out sophisticated high lift devices, hence large wing areas. Th Helio was an exception but long gone.

GF
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Old 5th Jan 2011, 02:33   #15 (permalink)
 
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Quote:
Actually small planes have such low wing loadings due to FAR 23 limiting stall speeds to 61 knots due to crash survival considerations
GF
One learns to think differently about aviation everyday, thanks
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Old 5th Jan 2011, 02:46   #16 (permalink)

 
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galaxy flyer,

Quote:
No, span loading is weight divided by span, related to aspect ratio which is what is what you are speaking of.
I'm sorry, I got that backwards...

Quote:
thrust loading is T/W
Understood

Quote:
The performance under G loads is related to specific excess power, that is, wing loading causes loss of IAS, specific excess power is how the plane can overcome the increase in induced drag. The Thud didn't have a lot of Ps at high loadings or low speed.
That's the energy-maneuverability formula right?

Quote:
Were it not for these "horrendously" high wing loadings they could not do their jobs as jet transports. Try designing a jet transport with a wing loading of, say, 50 lb/ sq ft.
It would need to have very thin wings that were very large for it to even remotely work, though I can think of at least one proposed commercial aircraft design that had a wing-loading of 62.6 pounds/square-foot. It was called the L-2000; it was built by Lockheed as a competitor in the SST program.

In a more serious note, I assume that planes designed to sustain high g-loads (fighter-planes) are built with wings that are excessively large for what is needed for optimum efficiency in level flight?


Pugilistic Animus,

Quote:
Fineness ratio, is more about the physical cleanness of the wing it concerns boundary layer T/C is the thickness ratio
Fascinating.

Quote:
Span loading, is the weight divided by the span-a measure of how much load is distributed along the span
Yeah, I got span-loading and aspect-ratio backwards

Quote:
T/C is the thickness ratio...important when chord changes as in flap deflection
You mean like fowler-flaps?
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Old 5th Jan 2011, 11:07   #17 (permalink)

Do a Hover - it avoids G
 
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Jane

Thanks for responding to my request.

I will not answer it directly now as subsequent posts by yourself and others have moved things on a lot further.

You clearly are familiar with a lot of aspects of aircraft design and performance but I feel have got some of the basic building blocks a bit jumbled up in your head.

I will PM you with some of the basics of lift and drag which I think might help you straighten out some of your concepts.

However all other things being equal if you literally double the wing loading of an aircraft (by filling an airliner with fuel or covering a military aircraft with bombs) then you really won’t change much beyond
  • The stall speed which will be 1.414 times what it was before (this will up your takeoff and landing speeds and distances)
  • The aircraft will have much more inertia (this will make the aircraft appear ‘sluggish’ in response to you trying to change its flight path – especially in pitch compared to the light case)
When it comes to the effects of wing loading on manoeuvre there are two aspects called the manoeuvre boundary and the thrust boundary.

The manoeuvre boundary is a measure of how much g you can momentarily pull at a particular speed (this will reduce as wing loading goes up)

The thrust boundary is a measure of how much g you can sustain without loss of airspeed and is primarily affected by the thrust available. It will suffer as you put up the wing loading but likely less than the manoeuvre boundary.

JF
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Old 5th Jan 2011, 16:16   #18 (permalink)
 
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Low wing loading

My hang glider has 188 square feet and a wing loading less than 1.5 pound per square foot even carrying a porker like me.

It will take off at 18 mph, and will fly in a dive at 44mph. In all but the calmest conditions it requires active control inputs to avoid being thrown around like a leaf.

For a sport it's fine. As a means of transport it leaves a lot to be desired.

Reasonably????? Well?????

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Old 5th Jan 2011, 17:44   #19 (permalink)
bearfoil
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Mine was a rogallo, and had a glide ratio of about 6:1. Its wing loading was ~ 1.75lbs/foot2, and yeah, it flew like a potato chip in a gale.
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Old 5th Jan 2011, 22:03   #20 (permalink)
 
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Two thoughts...

1) Air pressure at sea level is around 2000 Lbs/sqft so is 85 Lbs/sqft really "high" in percentage terms (just 4%)?

2) Reynolds number. Big fast wings are more efficient than small slow ones.

Low Reynolds Number Airfoil Design
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