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Question on Boeing 737 AEW&C "Wedgetail"

VTOLicious

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I was always wondering why the radar antenna is not parallel to the fuselage (viewed from the side). Can someone elaborate the reason?

BR Michael
 

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TomcatViP

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IMOHO, the antenna is with no doubts parallel to the speed vector at cruise. Since the radar system add weight and alter balance from that of a passenger plane, the tilt of the antenna is also aggravated from that of the fuselage (chorwise)...
 

VTOLicious

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IMOHO, the antenna is with no doubts parallel to the speed vector at cruise. Since the radar system add weight and alter balance from that of a passenger plane, the tilt of the antenna is also aggravated from that of the fuselage (chorwise)...
That would make sense and would be my first guess as well. But looking a the pictures I have the gut feeling that's too much angle of incidence for cruise conditions @ 460 kts (853 km/h).

...hey, I just found that pic:
 

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_Del_

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Well, it might not be if the intended station altitude was near the service ceiling, but I agree, it looks like too much alpha. Maybe they were getting adverse airflow on the rudder -- and/or buffeting. Leaves the air less disturbed at the lower portions of the tail, where the chord of the rudder is thickest-- might mean cleaner air for the greater part of the rudder and/or less abuse on the rudder?
 

GTX

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trying to get some good airborne shots of the E-7A showing the attitude it cruises at is challenging. There are some but typically these include it formatting behind a tanker which might give an incorrect impression. These are the best I could quickly find.


 

TomcatViP

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@VTOLicious : I do remember now hearing about a problem with the lift generated by the antenna (aft of CG aggravating the need for trim? ). Hence the exaggerated negative incidence.
 
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Richard N

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It would be great to see wind tunnel video of this configuration being tested. The angle chosen is probably the one for lowest drag and also to not generate lift at cruise.

On a conventional airplane configuration with the wing forward and horizontal tail behind, when the wing generates lift it also wants to rotate nose down about the CG. The horizontal tail is angled to create a downward force to balance that nose down rotation of the wing. Anything behind the CG has to generate a balancing negative lift to keep the aircraft flying level.
 

AeroFranz

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I would think the setting of the antenna was chosen to minimize the generation of any kind of moments. You wouldn't want to have to change elevator size, and risk having to re-stress the fuselage. So they probably picked the mid-mission speed/altitude/weight and changed the incidence of the antenna until the thing was satisfactorily trimmed.
 

DWG

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The other possibility is that rather than cruise or high-altitude loiter, the angle of incidence of the antenna is optimised for take-off or landing, both of which will see the aircraft in a particularly nose-high attitude and where disturbing the original design airflow (of the base 737 NG) would be particularly non-optimal.
 

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Would it be possible/desirable to allow the pod to pivot along the direction of travel? Perhaps allow for the unit to be 'trimmed' while in different flight regimes.
 

TomS

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Would it be possible/desirable to allow the pod to pivot along the direction of travel? Perhaps allow for the unit to be 'trimmed' while in different flight regimes.
No.

OK, you could maybe squeeze out a slight aero improvement, at the cost of significant mechanical complexity and risk (what happens when it jams in the wrong position?) And it complicates the actual radar employment, which now has to compensate for changing geometry between the antenna and various potential reflectors like the wings and tail.
 

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The same “feature” is apparent on those aircraft equipped with the Ericsson PS890 Erieye radar: Saab 340 AEW&C / S100B Argus, Saab 2000 AEW&C and Embraer 145 AEW&C. In “Beyond the Horizon” from Harpia Publishing the reason given is that when in operation the cruising speed is kept as low as possible to save fuel and extend sortie duration leading to the aircraft flying nose up. The exact angle seems to vary according to the aircraft it is mounted on. In the case of the 340 that is nose up 3 degrees.

The Chinese Shaanxi KJ-200 has the same “affliction”.
 

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