Hacker News Reader

Comment by fransje26 | Hacker News Reader

Slightly pedantic, but this is not completely correct.

It had a lower Cd with the extended hump, but it still had a higher drag, due to the larger wetted area.

Could you expand upon that for us interested laymen?

Ah. It was based on a series of performance graphs from Boeing, dating back to my aircraft-design days where that very question was discussed and I was baffled enough by the contradiction to ask for detailed clarifications to the presenting professor.

Unfortunately, I don't have access to those old notes, and couldn't quickly find what I was looking for online.

So for now, due to lack of proper supporting arguments, I would say: scratch that.

The Cd is normally normalized by the wing area, and the wing planform was the same between the 747-200 and 747-300. So if the Cd was lower for the -300, the total drag should be assumed to be smaller.

I remember hearing that the -300 cruised at a slightly higher Mach number, I assumed that was enabled by the center of pressure being moved back a few metres. But I have no idea what kind of drag the hump transition imparted on the upper fuselage. I kind of assumed that the low pressure zone above the rest of the aircraft would make up for most of the drag penalty of the transition (in general, not -300 specific). I'd love to hear your thoughts on the matter.

Assuming that the boundary layer is laminar for the entire length of the extended part of the fuselage during cruise - which is probably a good assumption, correct me if I'm wrong - then I wouldn't expect that the increased wetted area to add any significant drag. There is no additional frontal area, and no additional low pressure transition (just one moved back, obviously). But I'm not an aerospace engineer, just an interested laymen, so I'm just guessing. For all I know the straight staircase reduced drag compared to the classic spiral!

At transonic speeds, wave drag dominates both induced and parasitic drag. The hump reducing drag is about shape (area rule).

Thank you. I'll read about the different drag types.

> [..] cruised at a slightly higher Mach number, I assumed that was enabled by the center of pressure being moved back a few metres.

In high-subsonic cases, it normally points to slightly better aerodynamics in the areas flirting with transonic regimes. The name of the game at those speeds is to try to delay the onset of drag-inducing shocks. These shocks will typically first appear on the wing, or in an area heavily influenced by the wing.

In the case of the -300, with the extended bubble coming down at about half the wing root chord, it is possible that the bubble downflow / low-pressure area positively influenced the flow over the wing to slightly delay the transonic onset and further effects.

Pulling the bubble further aft made things worse again.

> Assuming that the boundary layer is laminar for the entire length of the extended part of the fuselage during cruise

Mm. No, I would expect the flow to be turbulent well before that.

> and no additional low pressure transition (just one moved back, obviously).

Careful where you put your low pressure zones, the wing aerodynamics are critical and sensitive! :-) (see explanation above)

But to make things even more interesting, the 747- 300/400/8 cargo all use the short bubble. :-)

Thank you.

  > But to make things even more interesting, the 747- 300/400/8 cargo all use the short bubble. :-)

I only recently noticed this on 3 minutes of aviation. Considering the 4 7 was originally designed for the passenger jets to be easily converted to cargo, I found it quite peculiar that the two -400 variants left the factory with different fuselages.

Reply on news.ycombinator.com