Wing Loadings: The Idiots Guide?

[kool kitty89]

Wing Loading vs "Lift-Loading"

The thing wing loading actually tells you is how much lift/area the wing is producing in level flight. (in level flight weight=lift)


But to get an idea of comparing aircraft in extremes of flight (max turns, close to stall) you need to compare the weight of the a/c and the maximum lift the wing can generate. (lift/drag ratio is also very important in these comparisons as well as are stall characteristics -which determines the ease of the pilot to pul tigfht turns without stalling)


Soren often uses a comparison he referrs to as "lift loading" (which I have seen elsewhere, but I'm not sure if it's a true term in areodynamics)

This is basicly: Weight of aircraft/(wing-area x CLmax) CLmax being the coeficient of lift at the critical angle of atack.
This compares the maximum amount of lift able to be created by the wing and the weight (mass really being more aplicable here) of the aircraft.



Other factors for turn performance would be lift-to-drag ratio, power loading or power/mass (the reciprocal of power loading), and stall characteristics (determining the ease of reaching Clmax). With all other factors being equal the aircraft with an advantage in one category will turn better. (lower "lift-loading," higher lift/drag, more power/mass, or being able to get closer to Clmax)


The key factor for lift-to-drag ratio for a wing (at lower airspeeds) is induced drag ("drag due to lift"), which is minimised when lift distribution is elliptical. However ellipitcal lift distribution results in a wing with very violent stall behavior and the shape makws the wing complex and expensive to manufacture. Tapering the wing also reduces induced drag and is a much simpler (albeit less effective) solution. Increasing a wing's aspect ratio also reduces induced drag.


As speed increases, induced drag decreases and parasitic drag rises. (and at very high speeds -in this context- wave drag starts to become a factor)

 

[kool kitty89]

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

I don't think the Bf 109K is really comperable in that sense, it wasn't really "stripped down" as sometimes claimed. (iirc it weighed a bit more than the G-10) The 109K was intended to streamline and standardize production.

 

[Timppa]

My 2 cents:

Generally wing loading is a strong indicator of a relative sustained turn capability. However to claim anything categorically, you should specify the exact conditions and go through the calculations all the way.

The parameters needed are:
Weight, wing area, wing span, air density, e -factor, engine power, propeller efficiency, exhaust thrust, maximum speed and stall speed.
(I once made a spreadsheet to calculate turn rates, I hope I did not miss anything )

The stall speed is needed to calculate the CLmax for the actual airframe. I would not take this from a table of wing sections.

 

[drgondog]

My 2 cents:

Generally wing loading is a strong indicator of a relative sustained turn capability. However to claim anything categorically, you should specify the exact conditions and go through the calculations all the way.

The parameters needed are:
Weight, wing area, wing span, air density, e -factor, engine power, propeller efficiency, exhaust thrust, maximum speed and stall speed.
(I once made a spreadsheet to calculate turn rates, I hope I did not miss anything )

The stall speed is needed to calculate the CLmax for the actual airframe. I would not take this from a table of wing sections.

Agreed but I would add air density (for calcs at different altitudes and thrust conditions), and you still need a Cl Vs AoA plot out of airfoil section data for intermediate calculations for induced drag,

you need flight test data specifying altitude/hp and velocity to attempt to calculate Profile Drag in level flight as a starter.. (and this is increasingly unreliable as velocity reduces and AoA increases). While profile drag in general is much less important at low speeds, trim drag should be looked at nevertheless.

- and you would like to have flight test data for power on/clean stall speeds to give you a check against calculations. Timppa is right about this being a better ClMax than the 'perfect 2-d wing section'..

Additionally, Most single engine fighters have a severe tendency to sidelip or roll at stall velocities in a turn well above level flight power on stall speed. It is in this lower range of velocities where spanwise components of velocity coupled with high AoA turns spreadsheet calcs to crap.

 

[drgondog]

Wing Loading vs "Lift-Loading"

The thing wing loading actually tells you is how much lift/area the wing is producing in level flight. (in level flight weight=lift)


But to get an idea of comparing aircraft in extremes of flight (max turns, close to stall) you need to compare the weight of the a/c and the maximum lift the wing can generate. (lift/drag ratio is also very important in these comparisons as well as are stall characteristics -which determines the ease of the pilot to pul tigfht turns without stalling)


Soren often uses a comparison he referrs to as "lift loading" (which I have seen elsewhere, but I'm not sure if it's a true term in areodynamics)

This is basicly: Weight of aircraft/(wing-area x CLmax) CLmax being the coeficient of lift at the critical angle of atack.
This compares the maximum amount of lift able to be created by the wing and the weight (mass really being more aplicable here) of the aircraft.

Actually I have yet to see what Soren's 'lift loading' calcs do except for finding a very quick way to calculate one point of Stagnation pressure (therfore Velocity squared) of the freestream. Thinking more through this it implies that as Wing Loading and Clmax increase or decrease proportionately, so that the value of WL/CL remain the same, the velocity squared will always be one value?


Other factors for turn performance would be lift-to-drag ratio, power loading or power/mass (the reciprocal of power loading), and stall characteristics (determining the ease of reaching Clmax). With all other factors being equal the aircraft with an advantage in one category will turn better. (lower "lift-loading," higher lift/drag, more power/mass, or being able to get closer to Clmax)

Aspect Ratio is important - if a model of the F-104 had a wing twice the span and half the chord (assuming it was suitable structurally in bending and torsion), it would have the same wing loading and wing area but the induced drag would be far less, the spanwise flow component in high AoA and bank angle would be less.. the profile drag would be higher, acceleration would be less - but it probably would turn 360 degrees with a shorter radius and time


The key factor for lift-to-drag ratio for a wing (at lower airspeeds) is induced drag ("drag due to lift"), which is minimised when lift distribution is elliptical. However ellipitcal lift distribution results in a wing with very violent stall behavior and the shape makws the wing complex and expensive to manufacture. Tapering the wing also reduces induced drag and is a much simpler (albeit less effective) solution. Increasing a wing's aspect ratio also reduces induced drag.

KK - the only fact one could state regarding the aerodynamics of an elliptical wing is that such plan forms represent the lowest theoretical induced drag when compared with other planforms of equal span and chord - with zero twist


As speed increases, induced drag decreases and parasitic drag rises. (and at very high speeds starting near or at Transonic regime -in this context- wave drag starts to become a factor)

For most prop fighters in WWII, compressibility effects started in the .5-.55 Mach range.

 

[drgondog]

The Sidewinder was considered a success.

Depends on which pilot you talked to and whether his sidewinders hit the target or went into the boonies. Ditto Sparrow. Olds and Titus and Kidd were particularly unenthusiastic about the reliablity - hence the campaign for internal guns in all future fighters.

Any sidewinder that hit the target was a success, the argument would be what was the reliability and statistical level to declare 'success'. I would hold post Viet Nam by any reasonable criteria.

 

[kool kitty89]

Thanks for the corrections.

 

[drgondog]

Thanks for the corrections.

KK - you pretty much had it right - think of my comments as extensions of what you said.

 

[kool kitty89]

I completely read through that thread I mentioned on the P-51 vs Spit XIV thread.
On pg.4: A Complete Waste of Space Forums-viewtopic-Lift Loading?

Gene (Crumpp) makes some coments on "lift loading" so maybe this is another thing to to ask him about. (I was a bit surprised he didn't correct the coment on the Fw 190's "low lift" wing though)

 

[drgondog]

I completely read through that thread I mentioned on the P-51 vs Spit XIV thread.
On pg.4: A Complete Waste of Space Forums-viewtopic-Lift Loading?

Gene (Crumpp) makes some coments on "lift loading" so maybe this is another thing to to ask him about. (I was a bit surprised he didn't correct the coment on the Fw 190's "low lift" wing though)

From Gene - "This may help some of you to understand.

Lift loading takes into consideration velocity and dynamic pressure effects while wing loading does not.

In otherwords lift loading is good for a condition of flight and not for comparision outside that condition.

KK- Lift Loading in this context is exactly what it sounds like. A wing in flight produces Lift, and a specific amount of lift based on airspeed, wing characteristics, and AoA. The 'loading' due to lift is expressed as the pressure distribution on the wing. Every different planform and airfoil property and twist will exhibit slightly different distributions span wise and chord wise. The Laminar flow wing of the 51 for example delayed separation CHORD wise and therefore (theoretically) had a greater chord distribution of pressure (lift) than say a Clark Y.

For structural analysis purposes the aeros try to give the bridgebuilers a load distribution on the wing so that the structures guys can look at a three dimensional beam with varying but continuous load on that beam.

It also has very little to do with comparitive performance. A wing will only produce the amount of lift required. Generally speaking the lighter aircraft will have a lower CLmax. It simply does not need to produce as much lift!

All the best,

Crumpp

I'm not as comfortable with Gene's last statement about "low CLmax type" for lighter aircraft but he the generality is probably more right than exceptions. As I think about it there are a wide class of a/c whose wing design may not only be built around high lift/low drag for range but also for short field take off and landing with heavy loads. The trade offs dictate the airfoil, the wing/wing config (flaps, slots, tiplets, landing gear location), position of wing .

 

[Timppa]

..I would add air density..you need flight test data specifying altitude/hp and velocity to attempt to calculate Profile Drag in level flight as a starter.. and you would like to have flight test data for power on/clean stall speeds to give you a check against calculations.

I thought I said all that, or at least was trying to.

and you still need a Cl Vs AoA plot out of airfoil section data for intermediate calculations for induced drag.

Why would you need that ? I just use:
CL=n*W/(0.5*rho*V^2*S) , n is load factor
and
CDi=CL^2/(pi*AR*e)

 

[drgondog]

I thought I said all that, or at least was trying to.



Why would you need that ? I just use:
CL=n*W/(0.5*rho*V^2*S) , n is load factor
and
CDi=CL^2/(pi*AR*e)

Actually you are right..brainfreeze on my part for all values of Cl until approaching CLmax..

I have seen too many spreadsheets that are effectively at one altitude and hp setting and usually at SL which is the only reason I mentioned density. Thes a/c all had different power conditions in middle and high altitudes and rho/rhoSL is significant for calculating Thrust at higher altitudes.

Having said this, for comparisons between two a/c at that altitude, ignoring density should only result in an equivalent margin of error between the two, rather than favor one over the other.

The biggest single issue in getting a good fundamental dynamic model is getting a reliable Parasite Drag component at the velocities of interest, then relying on those values throughout the turn dynamics including high angle of attack.

 

[Soren]

So are we done talking about lift-loading ?

 

[Soren]

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.

Again NO they do not add any extra drag at all what'so'ever! The slats increase lift by accelerating air over the top of the wing when it would've otherwise stalled, greatly increasing the Clmax critical AoA. But like I said with any increase in lift comes an increase in drag as-well.

In short all the slats do is increase the Clmax critical AoA of the wing, they don't add any drag themselves at all, if anything they decrease drag at the original critical AoA by actually accelerating air over the wing. That's just how they work.

Here's how you calculate induced drag:
(Cl^2) / (pi * AR * e) = Cdi

 

[drgondog]

So are we done talking about lift-loading ?

Depends on whether you think 'lift loading' is Wing Loading divided by Cl. If so you don't quite have the concept down yet.

 

[drgondog]

Again NO they do not add any extra drag at all what'so'ever! The slats increase lift by accelerating air over the top of the wing when it would've otherwise stalled, greatly increasing the Clmax critical AoA. But like I said with any increase in lift comes an increase in drag as-well.

In short all the slats do is increase the Clmax critical AoA of the wing, they don't add any drag themselves at all, if anything they decrease drag at the original critical AoA by actually accelerating air over the wing. That's just how they work.

Here's how you calculate induced drag:
(Cl^2) / (pi * AR * e) = Cdi

Deploying slats increase lift And drag. That is anothe reason why pilots reported a 'tug' when say the high wing deployed first.

 

[kool kitty89]

Soren, what doesn't make sense about claidemore's question is, the extended slats increase drag compared to what? I already discussed this on the previous page (post #18 ), but if the slats weren't there the wing would have stalled at the AoA where the slats are fully extended, which would certainly result in greater drag than if the stats had been there to delay the stall.

However, somthing important to note (also mentioned previously) is that at the new Clmax acheived with the slats fully deployed, the lift-drag ratio has dropped significantly. (according to the drag estimation chat with the Clark Y airfoil)

 

[Soren]

Yes KK the L/D ratio will fall ofcourse but not by a whole lot: 7.5 to 5.35.

You should be looking at the Fixed slot to get an idea of the L/D ratio of the wing with slats extended as they go straight out like the slats used on a/c from the 40's and throughout the 50's. Later on they introduced the drop down type which increased lift even more by increasing the camber of the wing as-well. but the L/D suffered.

 

[claidemore]

Doesn't greater 'wetted' surface area equal more drag?

When the slats are open, isn't the surface area of the slat tripled? (top of slat, underside of slat, area of wing the slat was sitting in? not to mention the mechanism of the slat itself. I believe even adding something as small as a mirror or external antenaes has an effect on performance.)

Doesn't air movement across the wing create friction, ie drag? I believe the term is parasitic drag?

Wouldn't accelerating air across the wing create more friction, ergo more drag? (that being said, see bottom of this post.)

k.k. They increase drag when extended, compared to the wing with slats 'un-extended'.

Some sources:

Suppression of Dynamic Stall with a Leading-Edge Slat on a VR-7 Airfoil

The slat caused a slight drag penalty at low angles of attack

Wikiversity

One of the pioneer of the leading edge device was Handley Page who first tried this on his aircraft. First of all he used a fixed leading edge slat which provided him with high lift coefficient but the associated drag was detrimental to maximum speed.

The same article says that leading edge devices increase drag, but the lift/drag ratio is improved.

From Absolute Astronomy:

They are usually used while landing or performing manoeuvres which take the aircraft close to the stall, but are usually retracted in normal flight to minimise drag.

LOL, it ain't rocket science, sic, if they didn't increase drag, they wouldn't have to be automatic opening, they could be open all the time!

Also, according to "Sythesis of Subsonic Airplane Design", the slat does not increase air flow over the leading edge, but causes 'decreased' flow velocities.

 

[Soren]

Sorry but you're wrong Claidemore,

The leading edge slats allow the aircraft to fly at a high angle of attack (lower speed) by accelerating the air between the slat and the wing (venturi effect).

Also where would the extra wetted area come from exactly ?? The slats don't just magically appear, when the extend they leave behind the space where they once sat remember