Blackbird
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Sonic Cruiser

Sun Mar 25, 2007 7:56 am

What characteristics gave the Sonic Cruiser such good performance even when right up to the sound-barrier? Also, how extreme was the shift in the center of lift? (Was it controlled aerodynamically, or by fuel-ballast?)

Could any of these characteristics be used to develop a supersonic plane that would experience virtually no drag jump from Mach 0.9 to 1.2 with supersonic performance at least comparable to a modern high tech airfoil?

Andrea Kent
 
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Jetlagged
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RE: Sonic Cruiser

Sun Mar 25, 2007 8:20 am

I'm not sure the Sonic Cruiser had that much better performance, nor would it have cruised right up to Mach 1. Quoted cruise was 0.95. The big change in centre of pressure happens above Mach 1, so any small trim changes would probably have been accomplished with fuel shift (just a guess you understand) and a Mach trimmer.

Again pure speculation, but being a canard configuration with FBW computer control the CG could have been positioned well forward to give neutral longitudinal stability allowing zero or positive canard lift for trim and so minimum trim drag. The main wing was a delta for good transonic performance. The canard would have allowed trailing edge flaps to improve takeoff and landing performance.

Cruise fuel flow was 15-20% higher, but transit times were 20% less, so overall fuel cost was about the same. The alternative offered by Boeing (the 7E7) had 20% less fuel burn at normal cruise speeds. Basically the airlines didn't buy the idea of paying the same to cruise faster. They preferred the idea of paying less to cruise at the same speed (now the 787).

As for the transonic drag rise, the best you can do is get through this region as fast as possible. Even the Sonic Cruiser couldn't change that. Transonic cruising is not a good idea.
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Blackbird
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RE: Sonic Cruiser

Sun Mar 25, 2007 12:34 pm

Are you sure about that statement about the center of pressure cange occuring after supersonic speed? I remember reading that the center of pressure shifts aft when you lose the upwash at the front of the wing.... that would occur as you near and reach the speed of sound.

As I understand it the bulk of the shift in the C/L occurs in the transonic range (high subsonic, low supersonic) range, even though some shifting occurs at higher mach numbers.

Andrea Kent
 
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RE: Sonic Cruiser

Sun Mar 25, 2007 4:41 pm

Quoting Jetlagged (Reply 1):
Quoted cruise was 0.95.

Actually, Boeing's website at the end of the program changed from the initial .95 - .98 to just .98 mach.

It was rumored that Boeing even investigated the possibility of Mach 1.2-1.3 supercruise on transoceanic route segments as range was planned to be between 9,000 and 10,000 nm (meaning every conceivable route would have substantial transoceanic sectors).  spin 
 
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RE: Sonic Cruiser

Sun Mar 25, 2007 10:48 pm

Quoting Blackbird (Reply 2):
Are you sure about that statement about the center of pressure cange occuring after supersonic speed? I remember reading that the center of pressure shifts aft when you lose the upwash at the front of the wing.... that would occur as you near and reach the speed of sound.

There is a shift of the C/P as you say, which is why many subsonic aircraft have a Mach trimmer, but the big change occurs when the flow over the wing is supersonic, i.e. above M=1.0. By definition this is still in the transonic range. Sorry I didn't make that clearer in my first reply.

Put very approximately the C/P will move from around the 25% chord point (subsonic) to about the 50% point (supersonic). At least that's what my hazy memories of aerodynamics lectures tell me.  Smile
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Blackbird
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RE: Sonic Cruiser

Mon Mar 26, 2007 2:49 am

Jetlagged,

So, the flow over the wing is supersonic, but the leading edge is still in the subsonic flow, and you got a shockwave moving from front to back over the wing when the C/P shifts aft a lot?

Additionally the shift from 25% chord to 50% chord is to my knowledge only truly accurate on straight wings. On swept wings, delta wings, or arrow wings, the shift is less.

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Starglider
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RE: Sonic Cruiser

Mon Mar 26, 2007 4:38 am

Quoting Blackbird (Reply 5):
So, the flow over the wing is supersonic, but the leading edge is still in the subsonic flow

Analysis results show that the flow around large portions of the aircraft will be supersonic, even if very thin airfoil sections are used.

Big version: Width: 605 Height: 381 File size: 43kb


The figure above presents a typical Mach number distribution on the surface as well as a section cut through the outboard wing. It is clearly visible, that the flow around the canard and the outboard wing is completely supersonic at Mach 0.98, even with thin airfoils. The Mach number on the fuselage and the inboard wings does not exceed M = 1.1 over most of their surface.

Source: DLR Institute of Aerodynamics and Flow Technologies, Germany.


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RE: Sonic Cruiser

Mon Mar 26, 2007 6:16 am

And during these moments when the plane is subsonic, right up against the sound barrier, and most of the wing (damn near all of it) is supersonic with the exception of a tiny area of the front of the wing, and a tiny amount of the back, the center of pressure has shifted rearwards most of the way?

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RE: Sonic Cruiser

Mon Mar 26, 2007 8:58 am

Quoting Blackbird (Reply 7):
And during these moments when the plane is subsonic, right up against the sound barrier, and most of the wing (damn near all of it) is supersonic with the exception of a tiny area of the front of the wing, and a tiny amount of the back, the center of pressure has shifted rearwards most of the way?

My guess is, yes. And there the canard served its purpose as a trimming device, to compensate for the shift.
Now where have we seen this before . . . . . i think we know the answer to that question. Boeing bought Rockwell International which used to be named North American Aviation. At least some of the basic design of the Sonic Cruiser had that heritage.


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RE: Sonic Cruiser

Mon Mar 26, 2007 9:03 am

Quoting Blackbird (Reply 5):
So, the flow over the wing is supersonic, but the leading edge is still in the subsonic flow, and you got a shockwave moving from front to back over the wing when the C/P shifts aft a lot?

I meant when the whole flow is supersonic.
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Blackbird
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RE: Sonic Cruiser

Tue Mar 27, 2007 10:08 am

According to that diagram, the flow is still slightly subsonic at the front of the wing.

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RE: Sonic Cruiser

Mon Apr 02, 2007 1:06 pm

Quoting Blackbird (Reply 10):
According to that diagram, the flow is still slightly subsonic at the front of the wing.

A blunt, or round, leading edge will always have flow moving somewhat subsonic until it accelerates as it goes around.
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Blackbird
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RE: Sonic Cruiser

Tue Apr 03, 2007 5:57 am

So, which is it. C/L shifts aft when wing is totally supersonic? Or C/L shifts when wing is almost supersonic?

Because what I read on an aviation site, the shift occurs just before going supersonic. Is the page wrong?

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RE: Sonic Cruiser

Tue Apr 03, 2007 7:12 am

Quoting Blackbird (Reply 12):
So, which is it. C/L shifts aft when wing is totally supersonic? Or C/L shifts when wing is almost supersonic?

Because what I read on an aviation site, the shift occurs just before going supersonic. Is the page wrong?

First thing that matters is a wing's critical mach number. Critical Mach number of an airfoil is when the speed of the airflow over some of the area of the airfoil reaches Mach 1. If the air flowing over an airfoil reaches Mach 1 at some point when the airfoil/aircraft is actually moving at Mach 0.8, the airfoil's critical Mach number is 0.8.

At speeds below critical Mach number, the center of lift is at approx. 25 percent of the wing chord. When speed increases above the critical Mach number and reaches the transonic range with supersonic flow over the wing, generating shock-waves, the center of lift starts to move aft. This will require trimming to counter the nose down effect and is the main reason the all flying tail with irreversible control system was introduced to counter increasing forces as the center of lift continues to move aft with increasing supersonic speed. This phenomena is also known as the "Mach Tuck". If no corrective action would be taken by trimming the aircraft, it would continue to nose over more and more with increasing supersonic speed. Aircraft such as the XB-70 took advantage of its folding wing tips which changed the delta wing geometry in such a way that the center of lift moved relatively forward again when the tips were folded fully down, reducing the amount of trim required to keep the aircraft longitudinally stable.

Starglider
 
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RE: Sonic Cruiser

Wed Apr 04, 2007 5:25 pm

Quoting Starglider (Reply 13):
the center of lift continues to move aft with increasing supersonic speed.

Are you sure about that? I was taught that supersonic air pressure becomess equally distributed along a surface past Mach 1, so the center of lift locks to 50% chord and stays there.

But this should only apply as long as the wingroot shock wave doesn't cover the wing, i.e. airflow normal to the cross section is still subsonic, mathematically.

With the latter I'm refering somewhat to Robert T. Jones' Oblique Wing theory, that a wing sweept of 45* will render no shockwaves across the wing, negating MCR. If BSC had 45-degree wings maybe it could have had less of a wave drag impact, apart fromt he dutch-roll problem?

Unless I paraphrased wrong, do correct, its been a while.
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RE: Sonic Cruiser

Thu Apr 05, 2007 6:34 am

Quoting Lehpron (Reply 14):
Are you sure about that? I was taught that supersonic air pressure becomess equally distributed along a surface past Mach 1, so the center of lift locks to 50% chord and stays there.

By moving aft of the aerodynamic center of lift at increasing supersonic speed, i mean gradually moving aft between 25% to approx. 50% chord. You are correct, at supersonic speeds the whole of the surface behind the crest of an airfoil causes expansion of the relative airflow. The accompanying suction over the upper surface contributes far more to the total lift than at lower speeds, and thus the resultant lift acts much further aft, e.g. limited to around the 50% chord.


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