Biplanes for ww2: where could've these still mattered?

[special ed]

Biplanes very much did matter in a number of roles: the Hs 123 in the daylight ground attack role; the Gladiator and Cr.42 as fighters; the CR.42, Ar 66 and Go 146 with the Luftwaffe's Nachtschlachtgruppen … Håkan Gustavsson has a whole website devoted to this subject: surfcity.kund.dalnet.se/

Way back in the Windows 95 & 98 days I found that site and figured it was gone today. With little computer memory and even less computer ability than today, I printed much of his data and made notebooks. Thanks for the site update.

 

[Howard Gibson]

Thank you for the excerpt.
The aircraft of the 1940s will hopefully be using more modern wing profiles than what is shown there

I am not sure that is a bad airfoil. A wing section must do three things...

1. Generate lift, generally equal to the weight of the aircraft
2. Generate minimum drag
3. Work as an efficient structural section.

That third item became particularly important when cantilever wings were developed during WWI. A thin wing section is a functional structure if the wing is a set of short cantilevers supported at both ends. If all the attachment and support is at one end, the wing must be way stiffer and stronger. Early cantilever monoplanes needed to be very light, so the wing sections needed extra structural efficiency. They needed to be thick. A thin cantilever section must be heavily constructed. On a 400mph aircraft with 1000+HP, this is manageable.

Thick airfoils were not well understood back in the day. The figure is what I call a Bernoulli airfoil I analysed it at zero angle of attack, using Bernoulli's equation. It works. For an aerodynamically efficient shape, you need to round off the leading edge, and move the top of the curve to about 1/3 chord. You wind up with something that looks a lot like a Clark_Y airfoil. By WWII, newer, more symmetric airfoils that could not possibly work by Bernoulli's principal, were developed empirically in wind tunnels.

 

[tomo pauk]

I am not sure that is a bad airfoil. A wing section must do three things...

1. Generate lift, generally equal to the weight of the aircraft
2. Generate minimum drag
3. Work as an efficient structural section.

That third item became particularly important when cantilever wings were developed during WWI. A thin wing section is a functional structure if the wing is a set of short cantilevers supported at both ends. If all the attachment and support is at one end, the wing must be way stiffer and stronger. Early cantilever monoplanes needed to be very light, so the wing sections needed extra structural efficiency. They needed to be thick. A thin cantilever section must be heavily constructed. On a 400mph aircraft with 1000+HP, this is manageable.
Thick airfoils were not well understood back in the day. The figure is what I call a Bernoulli airfoil I analysed it at zero angle of attack, using Bernoulli's equation. It works. For an aerodynamically efficient shape, you need to round off the leading edge, and move the top of the curve to about 1/3 chord. You wind up with something that looks a lot like a Clark_Y airfoil. By WWII, newer, more symmetric airfoils that could not possibly work by Bernoulli's principal, were developed empirically in wind tunnels.

As above - the designer of a biplane even for the second half of the 1930s will not choose the wing profile from ww1.

 

[Shortround6]

It is not the wing profile itself.
The drag of a biplane was considerable. The two wings interfere with each other.
Consider the Gladiator and the Hurricane. The Gladiator used the RAF 28 airfoil (wiki?) and that airfoil has a max thickness of 9.8% at 30% of cord. Practically a razor blade compared to the Hurricane. However the Gladiator has 323 sq ft of wing area for a 4600lb airplane. The Hurricane has 257 sq ft of wing for a 6400lb plane (MK I ?)
The Gladiator has got about 25% more wing area for surface drag.
Gladiator had a best landing speed of 57mph (not stall speed)
Hurricane had a best landing speed of 67mph.
Gladiator is not getting a lot of lift from that wing area.
But if you throw in all the struts and wires on the biplane the drag really starts to build up.
If you try to use a 250 sq ft biplane wing you need a thicker wing = more drag. Maybe not a lot thicker, Soviet I-15 used a 11.7% air foil (?) and 236sq ft.
At the speeds most biplanes flew they were not running into compressibility problems.
The wakes from the wing interfere with each other creating induced drag.

 

[tomo pauk]

The drag of a biplane was considerable. The two wings interfere with each other.

The wakes from the wing interfere with each other creating induced drag.

Is there some easily-available resource that puts some numbers on that?

 

[Shortround6]

Is there some easily-available resource that puts some numbers on that?

No, because the interference can depend on the size of the wings, the distance between the wings and the amount of stagger the wings have.
On some planes the wings didn't have the same angle of attack.