Why wing-loading isn't always the most important characteristic

[FLS]

Thanks PJ. Equations don't speak to me but I appreciate your optimism. I'll try to figure out what you wrote.  

I suspect the answer will show the relative importance of AOA. I don't usually fly the German planes so I don't have a feel for the performance differences but what I've read here seems to suggest that the Ta152 will have a higher sustained turn rate despite the higher stall speed because of the lower induced drag.

Stony I wondered that too. If I get a chance I'll see if I can get some stall numbers to compare.

[Stoney]

Actually, I do suspect that my 190A8 and D9 speeds are low because they work out to have a Clmax of 1.59, and that is a bit too high from what I think they should be.  They probably should both be a little less than 1.5. 

[PJ_Godzilla]

Well, I've got a way to approximate e.  If we get some no-kidding Cd0 numbers, we're in there.

Stoney, lookeehere... http://www.hq.nasa.gov/pao/History/SP-468/app-a2.htm
B-17G at tthe bottom of this one: http://www.hq.nasa.gov/pao/History/SP-468/app-a.htm

It looks like they link to source as well. I submit this to you and hope it helps (don't know if it will). It's only 5 types we care about but, it's a start.

[FLS]

That's the link I posted for the WW1 glide ratio thread. Lots of good stuff there. Did you notice the DC-3 (C-47) glide ratio is a little better than the P-51?

[PJ_Godzilla]

That's the link I posted for the WW1 glide ratio thread. Lots of good stuff there. Did you notice the DC-3 (C-47) glide ratio is a little better than the P-51?

I steer clear of the WW1 stuff - not that I'm not interested in general. I just think the WW1 topic has a ways to go yet with AH. It needs more purpose and variety.

I'm surprised at that slope thing. It seems like the CDo wpuld be higher for the C-47 - though I haven't checked their aspect ratios. C-47 definitely has a lot of wingspan.

[FLS]

Even if you don't enjoy the WW1 arena I hope you've looped the Dr1.   

[Kenne]

The glide angle at a given speed is a function of the efficiency - that is the lift/drag ratio. Low aspect wings can still have very good L/D and therefore a shallow glide angle, but it just means that they have to glide at a higher speed. The drag part is very important and it includes the parasitic drag as well and this is why optimum glide speed is close to the minimum drag (best climb) speed.

For real planes it is a little difficult to compare between two wings, because in addition to the aspect ratios other factors (shape, profile, twist, total area) will differ.

low aspect='fast' wing, laminar wing, Davis wing?
is L/D direct proportional or inversely?

[Stoney]

low aspect='fast' wing, laminar wing, Davis wing?
is L/D direct proportional or inversely?

The Davis Wings were all very high aspect designs.  You see them on the B-24 and other Consolidated designs.  They were typified by short chords and smaller wing area.  Aspect ratio goes up, given fixed span, with decreases in wing area.  Low aspect wings are sometimes associated with "fast" wings, because its easier to chop down a Reno P-51's wing span than it is to shorten the chord of the entire wing.  Basically, they get rid of "excess" wing area that way, cutting down on parasitic drag.  Usually, this is accompanied by higher wing loading.

[Kenne]

The Davis Wings were all very high aspect designs.  You see them on the B-24 and other Consolidated designs.  They were typified by short chords and smaller wing area.  Aspect ratio goes up, given fixed span, with decreases in wing area.  Low aspect wings are sometimes associated with "fast" wings, because its easier to chop down a Reno P-51's wing span than it is to shorten the chord of the entire wing.  Basically, they get rid of "excess" wing area that way, cutting down on parasitic drag.  Usually, this is accompanied by higher wing loading.

forgive me for asking such 'novice' questions.
so short cord, short span = low aspect...yes?

in the case of the 152, the wing got lengthened and so its aspect increased?

[Stoney]

forgive me for asking such 'novice' questions.
so short cord, short span = low aspect...yes?

in the case of the 152, the wing got lengthened and so its aspect increased?

Aspect Ratio = the wingspan squared / wing area.

So, for an aircraft with a 20 foot wingspan, and 70 sq ft of wing area, the aspect ratio would be:

20^2 / 70 = 400/70 =5.7

Or, for an aircraft with 30 foot wingspan and 157 sq ft of wing area would also have an aspect ratio of 5.7.

You could have a broad chord, short span wing that had a low aspect ratio.  The Cassut III racer is a good example of a short span/broad chord aircraft.

And yes, the Ta-152 had a much longer wingspan than its FW-190 brothers, but it also got a bit more wing area as well.  Overall, it had a higher aspect ratio than just about any other combat aircraft in the war.

[bozon]

is L/D direct proportional or inversely?

Inversely - more lift for less drag means you glide better (smaller angle).
Intuitively, think about it this way: Since you do not have an engine, the energy lost to drag must come at the expense of potential energy. To maintain speed the glider must keep descending, but this need is only due to energy lost on drag. Loose less = descent less.

Alternatively, you can think in terms of forces:
The drag is slowing you down, so you need some forward force to replace the thrust. By tilting the lift forward (flying at an angle downward) the lift has a component that pulls you forward on the horizontal axis.
A simple drawing of a velocity vector at an angle "a" under the horizontal, a lift perpendicular to it and drag opposite to it will show you that in order to balance the forces in the horizontal axis:
L*sin(a) = D*cos(a)
So:
L/D = 1/tan(a)    or:    L/D~1/a  for small angles in radians. Inversely proportional to the glide angle.

[Kenne]

so in the case of the 152 with the least wing loading but the highest stall speed.
might that be due to the increase of leading edge which negated any benefit in increase in span?

so then, would an increase in cord allow the same wing loading but a lower stall?

[Stoney]

so in the case of the 152 with the least wing loading but the highest stall speed.
might that be due to the increase of leading edge which negated any benefit in increase in span?

so then, would an increase in cord allow the same wing loading but a lower stall?

For your first question, no.  Leading edge length (by itself) has nothing to do with with stall speed.  Theoretically, a longer leading edge, if wing area is held constant, means higher span efficiency.   

For your second question, if wing area is fixed, increasing chord length reduces aspect ratio which will (all other things being equal) reduce stall speed.  This has to do with span efficiency.