TIGHAR
Amelia Earhart Search Forum => General discussion => Topic started by: Jay Burkett on July 14, 2014, 11:52:04 AM
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I have a question regarding the Bendix Loop Antenna.
All of the photos I have seen where the loop antenna antenna is installed the loop is "facing" fore/aft. This would appear to be a high drag orientation. It would seem like a lower drag orientaion would have been to rotate it by 90°.
My guess is that Kelly Johnson's range/performance calculations were for an aircraft in the factory "clean" condition. That loop antenna is always seen in what appears to be the draggiest orientation. It is also necessarily installed way forward on the fuselage (as opposed to further back where it would be somewhat submerged in the boundry layer where it would have casued less drag).
In my experience accidents are always caused by multiple, seemingly related, details. Take an aircraft that may have been "bent", not flying straight, with a draggy repair (the patch), throw in some winds aloft that were stronger and in directions that were unanticipated and some poor visibilty and/or a few clouds and things suddenly don't turn out as they would have liked.
So, does anyone know if the orientation we see is the recommended orientation for the antenna to be positioned in when not in use?
This does not help us find the aircraft, but, it may help explain the loss in the first place.
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There is a large knob inside, corresponding to the Loop (http://en.wikipedia.org/wiki/Loop_antenna) which allows rotation and has a 360° azmuth to show relative bearings to the radio signal's null points. It seems critical in their failure to navigate to Itaska that they did not understand its proper operation (http://en.wikipedia.org/wiki/Radio_direction_finder). I doubt they even thought about how to orient it for minimum drag...
You would need to orient the loop "facing forward" for clearance to open that hatch they used to enter/exit the cockpit.
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You would need to orient the loop "facing forward" for clearance to open that hatch they used to enter/exit the cockpit.
Which is why all of the photos of the airplane on the ground show the loop facing forward.
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Beyond the photos on the ground, the photos of the Electra in flight seem to have the loop facing forward, so to speak.
The March 1937 photo of her over the Bay Bridge in SF the loop is facing forward, and the photo in flight over Java the loop is mostly facing forward. Seems they just left it in the ground configuration when in flight.
Doesn't seem like that would be the preferred in-flight configuration, but ...
amck
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That makes sense. This brings me to another thought. How easy would it be to damage that antenna? It looks like it is in the perfect location to be misused as a hand-hold when climbing into, or out of, the aircraft. In the various photos there seems to be an abnormally large number of individuals standing ON the aircraft. In one photo there were five (5) guys standing on the inboard wing on either side of the cockpit. It is not too much of a stretch for someone to loose their footing and grab onto anything they could to prevent falling. To the uninitiated the loop antenna is metal and substantial looking -- substantial enough to appear to be able to withstance such use without damage.
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The USAAF concur with that thought Jay...'DANGER NO HANDLE'
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The USAAF concur with that thought Jay...'DANGER NO HANDLE'
It's interesting to note that the loop also has "FRONT" stenciled on it and when that is turned so that it is toward the front of the aircraft, the loop would be perpendicular to the center line of the aircraft just as AE usually had the one on her ship turned.
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Well spotted Woody. They look quite solid as well, until you discover what they are made from. You wouldn't want to be pulling yourself up by grabbing hold of a circular and hollow aluminium tube. I guess that the USAAF might have found out the hard way hence the stencilled yellow paint warning.
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I guess that the USAAF might have found out the hard way hence the stencilled yellow paint warning.
It would sure seem to be.
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I’ve been fooling around with radios and antennas since I was in high school and I have been a licensed radio amateur since the mid -1980s. I have also designed installations for more than a few antenna installations on aircraft (some of those I can actually talk about ;) ). So, you can understand that I have a passing interest on this topic. I wonder if the loop antenna could be twisted out of orientation with the calibration shown in the cockpit. I’m sure that there was probably some sort of indicator and a tuning knob of some sort which would show the center position and degrees left and right (probably at least ±90° and maybe even ±180°). I wonder if the registration could have been inadvertently changed so that the orientation was no longer “zeroed”? Likewise, I wonder how much force it would take to damage the antenna so that it still received signals, but, would not indicate a null direction when operated. Does anybody know how delicate these things were? Just curious …
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If that indicator calibration (Extremely Important for Navigation) had changed, they would probably have noticed after trying to open that exit hatch.
She had reported a lack of ability to get a 'Null' on a flight test at Lae but attributed it to her close proximity to the transmitter, never being told that you can not ever get a Null from that high frequency signal she used. A later ground test confirmed that it was working properly (on a usable frequency, I'm sure).
A lot of published Airport Instrument Approach Procedures were based on (low frequency) Transmitters located on the Airport, would expect they would know that. You do encounter a brief "Cone-of-Confusion" when passing directly above a ground transmitter and a lot of Airport Beacon Transmitters can't be received beyond ten miles, others (especially Marine Radio-Beacons) can be used for many hundreds of miles.
Her only confirmed Itaska Radio Reception was receiving the continuous code (A's or dit-dah's) she requested them to transmit. She reported that she could not get a Null (direction), but of course she couldn't, since it was a 7,000 Kilo-Cycle (Khz) code-only High Frequency rather than a 190-1750 Khz Low to Medium Frequency that her Loop could have gotten a Null from.
The much improved 'Hooven' Automatic Radio Direction Finder (which she had removed before her second try) eliminated manual turning of the loop and was housed inside a Fairing behind the cockpit. It would still have been restricted to the lower radio frequencies.
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Useful link re: antenna, radio wave propagation etc...
http://www.apparentlyapparel.com/uploads/5/3/5/6/5356442/_____practical_antenna_handbook_fourth_edition_carr.pdf (http://www.apparentlyapparel.com/uploads/5/3/5/6/5356442/_____practical_antenna_handbook_fourth_edition_carr.pdf)
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The Eric Chater letter says that Earhart's radio was tested on a ground station transmitting on 600 meters (500kHz) and it worked fine. The Itasca could transmit on 425kHz and 500 kHz and at these frequencies Earhart's loop antenna would have worked good. Why she told them to transmit on 7500 kHz is one of the many enigmas that surround her disappearance.
A bit of trivia: The Itasca was using CGR-32-1 receivers. These were 10 tube, dual conversion radio's that were made by RCA special for the Coast Guard.
Bruce W Badgrow
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I did some double checking and I found that the info I gave that the CGR-32-1 receiver was a dual conversion set was in error, it was single conversion. Sorry about that
Bruce W Badgrow