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jetmech
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Tail Surface Sweep Angles

Sun Oct 15, 2006 4:32 pm

In this photo, it can be seen that the leading edge of the vertical fin of the 737 (400?) is inclined back somewhat less than the leading edge of the vertical fin of the 747-400 ER (ignoring photographic distortion).

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Photo © Sam Chui


Is this difference in vertical fin leading edge inclination angle analogous to the sweep angle of a wing? IIRC, for a 747, the cruise speed is M 0.85 with a 37.5 degree sweep angle at the 1/4 chord. For a 737, the cruise speed is M 0.745 with a sweep angle of 25 degrees at the 1/4 chord  Confused .

Does the trend of increasing sweep angle with cruise speed also apply to the vertical tail and for similar reasons?
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FredT
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RE: Tail Surface Sweep Angles

Sun Oct 15, 2006 4:53 pm

Stabilizers are wings, doing the same job. The only real difference is that for the vertical stabilizer, it operates at an angle of attack of zero degrees under nominal conditions. Thus, all the same principles apply.
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zeke
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RE: Tail Surface Sweep Angles

Sun Oct 15, 2006 9:13 pm

Quoting JetMech (Thread starter):
Is this difference in vertical fin leading edge inclination angle analogous to the sweep angle of a wing? IIRC, for a 747, the cruise speed is M 0.85 with a 37.5 degree sweep angle at the 1/4 chord. For a 737, the cruise speed is M 0.745 with a sweep angle of 25 degrees at the 1/4 chord .

Does the trend of increasing sweep angle with cruise speed also apply to the vertical tail and for similar reasons?

Lets look at some wing sweep angles...

717 24.5
737 25
a320 25
757 25
a300 28
a310 28
a330 29.7
a340 29.7
767 31.5
777 31.6
727 32
707 35
747 37.5



Cruise speed does influence the sweep angle, a more modern wing can achive a lower wave drag than an older wing with a higher sweep due to the section used.

Vertical tails

707 30
a320 34
737 35
757 39
767 39
a300 40
a310 40
717 45
747 45
a330 45
a340 45
777 46
727 53


Vertical tail can be swept back more to give the rudder a longer effect moment arm for engine out, or in teh case of the 727, for the elevator.

757 27.5
a320 29
717 30
737 30
a330 30
a340 30
747 32
767 32
a300 34
a310 34
777 35
707 36
727 36
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2H4
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RE: Tail Surface Sweep Angles

Sun Oct 15, 2006 10:10 pm




Quoting Zeke (Reply 2):
Vertical tail can be swept back more to give the rudder a longer effect moment arm for engine out

Isn't the primary reason for sweep based on increasing spanwise flow, and Mcrit as a result?



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zeke
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RE: Tail Surface Sweep Angles

Sun Oct 15, 2006 10:42 pm

Quoting 2H4 (Reply 3):
Isn't the primary reason for sweep based on increasing spanwise flow, and Mcrit as a result?

One could have a lower sweep angle for many of the aircraft listed, however you would need a longer fuse or larger rudder to get the same control. E.g. the 747 has a lower vertical tail sweep than the 777, but a higher cruise speed.

Dont see any advantage in increasing spanwise flow, normally designers add devices/features to control spanwise flow.
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Buyantukhaa
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RE: Tail Surface Sweep Angles

Sun Oct 15, 2006 10:57 pm

Zeke, what is the third list of angles you mention? First is wing sweep, then vertical fin sweep, but the third?
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2H4
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RE: Tail Surface Sweep Angles

Sun Oct 15, 2006 11:20 pm




Quoting Zeke (Reply 4):
Dont see any advantage in increasing spanwise flow, normally designers add devices/features to control spanwise flow.

Oh, ok. For some reason, I was under the impression that increasing spanwise flow (within reason) increased Mcrit, delaying the onset of supersonic flow (and resulting drop in efficiency) over the airfoil.

Apparently, it's been too long since my aero classes...  Sad



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vikkyvik
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RE: Tail Surface Sweep Angles

Sun Oct 15, 2006 11:40 pm

Quoting 2H4 (Reply 6):
Oh, ok. For some reason, I was under the impression that increasing spanwise flow (within reason) increased Mcrit, delaying the onset of supersonic flow (and resulting drop in efficiency) over the airfoil.

Apparently, it's been too long since my aero classes...

Haha, no, you're actually correct about that. Swept wings (leaving aside vertical stabs for now) do increase the spanwise flow component, reducing the chordwise component (is that the right term?), and increasing Mcrit. Therefore, you can fly faster with less drop in efficiency. You also need to fly faster to obtain the same lift, keeping everything else constant.

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2H4
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RE: Tail Surface Sweep Angles

Mon Oct 16, 2006 12:28 am




Quoting Vikkyvik (Reply 7):
Swept wings (leaving aside vertical stabs for now) do increase the spanwise flow component, reducing the chordwise component (is that the right term?), and increasing Mcrit.

Ha, cool...it's good to know I'm not losing my mind entirely!

So, obviously, there's a point at which you've got too much spanwise flow. What measures are taken by aerodynamicists to walk the edge of that line, and control spanwise flow as precisely as possible?

I suspect wing fences play a very large part. If I'm not mistaken, certain variable-geometry wings are equipped with wing fences that pivot, and are always aligned with the longitudinal axis of the aircraft.



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FredT
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RE: Tail Surface Sweep Angles

Mon Oct 16, 2006 1:42 am

Quoting 2H4 (Reply 6):
Oh, ok. For some reason, I was under the impression that increasing spanwise flow (within reason) increased Mcrit, delaying the onset of supersonic flow (and resulting drop in efficiency) over the airfoil.

Reducing the chordwise velocity component of the air by sweeping the wing does indeed increase M_crit/M_drag_divergence and is the big reason for sweeping wings, regardless of if they are of the kind which keep you flying or the kind which keeps you pointed in the direction you are flying.

Quoting Zeke (Reply 4):
Dont see any advantage in increasing spanwise flow, normally designers add devices/features to control spanwise flow.

That's not the spanwise flow depending on sweep though, but rather the spanwise flow which result from lift being generated. Wing sweep does have a significant effect on the spanwise flow.

Not saying that getting the pressure center further aft does not help. It does, and means you can get away with a slightly smaller stabilizer. It also comes with a weight penalty of its own though, as the structure becomes more complex than what a straight, unswept aerodynamic surface would require.

I suspect that the main driving force behind many slower aircraft having swept stabilizers is, however, fashion. Building things to look slick and fast does sell.
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zeke
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RE: Tail Surface Sweep Angles

Mon Oct 16, 2006 7:07 am

Quoting BuyantUkhaa (Reply 5):
First is wing sweep, then vertical fin sweep, but the third?

Its the horizontal stab.

Quoting 2H4 (Reply 6):
For some reason, I was under the impression that increasing spanwise flow (within reason) increased Mcrit, delaying the onset of supersonic flow (and resulting drop in efficiency) over the airfoil.

The cos of the flow is reduced, the sin is increased. The cos component is reducing wave drag, however by increasing the sin component I dont recall that being better at all.
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jetmech
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RE: Tail Surface Sweep Angles

Mon Oct 16, 2006 4:13 pm

Thanks for all the informative replies people  Smile!
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AmericanB763ER
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RE: Tail Surface Sweep Angles

Tue Oct 17, 2006 3:45 am

Quoting Zeke (Reply 2):
747 45
a330 45
a340 45
777 46

Quite interesting to see that the 777's tail is actually more angled than the 747's although the 747 appears to the eye to have a stronger swept vertical stab than both the 777 and the A330/340. Big grin

I always wondered what's the reason for the enooormous sweep in the 727's vertical tail.
 
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jetmech
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RE: Tail Surface Sweep Angles

Fri Oct 20, 2006 4:25 pm

I have an additional question, this time about the sweep of wings. From what I know of this design feature, sweeping wings is an attempt to increase the critical Mach number of an airfoil. Sweeping does this by making the airfoil section "thinner" by decreasing the thickness to chord ratio.

Imagine you have a straight wing with constant chord. For this example, imagine the section thickness is 0.15m, and the chord is 1m, thus making the thickness / chord ratio equal to 0.15.

To increase the critical Mach number, this section is swept at an angle of 45 degrees. The thickness is still 0.15m, but from trigonometry, the "effective" chord has increased to 1.41m. The swept wing now has a thickness / chord ratio of 0.1064, which has increased the critical Mach number.

My question is, wouldn't it be easier to design a straight wing with a constant chord of 1.41m in the first place . I assume the oncoming air would "see" exactly the same thickness/ chord ratio, so wouldn't this wing have the same critical Mach number as the swept wing . I imagine a straight wing would be easier to design and manufacture.

The question could be extended to a wing that has a tapering chord length like the 747. This is much harder to picture though. Imagine "slicing" a 747 wing into slices 1cm wide, with the cuts parallel to the effective chord line.

Now imagine arranging these slices such that the trailing edges form a straight, perpendicular line from the side of the fuselage. The oncoming "air" would see exactly the same effective chord lengths as before, thus in theory, shouldn't this new wing have exactly the same critical Mach number as the original 747 wing . It would have the additional benefit of being easier and simpler to design and construct.

Swept wing "slicing". Lame, I know!


Original image; http://www.stanford.edu/~kasidit/images/wing_planform.jpg

[Edited 2006-10-20 09:41:50]
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