g38
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Douglas A2D Skyshark - Could It Be Made Reliable?

Wed Aug 01, 2012 12:19 am

The A2D Skyshark has always intrigued me, and so at the risk asking a stupid question, I want to talk a bit about its reliability issues.

The mechanical reasons for the Skyshark's failure were that is gearbox was fragile, and it had a vacuum tube prop control system.

My question is regarding the vacuum tube prop control system. I understand in general what a vacuum tube is, but I don't really have any idea what a vacuum tube control system would look like... how would it function. With today's technology would it be possible to replace it with something more reliable, and what would that be? I hope that makes sense, thanks in advance.



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RE: Douglas A2D Skyshark - Could It Be Made Reliable?

Wed Aug 01, 2012 4:45 am

Quoting g38 (Thread starter):
With today's technology would it be possible to replace it with something more reliable, and what would that be? I hope that makes sense, thanks in advance.

A computer.
I don't know what the vacuum tube control system was either - but a vacuum tube is basically a transistor on steroids. Today you would use a computer.
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RE: Douglas A2D Skyshark - Could It Be Made Reliable?

Wed Aug 01, 2012 1:32 pm

Here is an interesting presentation that includes the engine in the A2D.

http://www.enginehistory.org/Convent...on/2009/Presentations/AP_Turbo.pdf

Doesn't really talk about the control system in detail - but there is a slide that shows/mentions a frequency sensitive airframe mounted electric governor. This sounds like a system that would monitor rotational frequency information and then control the props to manage speeds. The tubes would have be used like modern op-amps in a feedback control loop. Probably comparing a standard signal to the prop signals as well as two props to each other (maybe, maybe not. The props are coupled mechanically, so they may not be independently speed controllable.)

Again - today - this would all be done digitally with a computer I'd think.
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prebennorholm
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RE: Douglas A2D Skyshark - Could It Be Made Reliable?

Thu Aug 02, 2012 1:03 am

Vacuum tubes were nothing special for the A2D. All military planes (and ships, tanks etc.) of the A2D era carried hundreds or thousands of vacuum tubes for electronics such as communication, radar, bomb aming computer, jamming equipment and all such things.

The A2D just also had a vacuum tube based computer to optimize the propeller pitch angle according to airspeed, altitude, power setting, and maybe a few other inputs, that way reducing pilot workload.

Complicated military vaccum tube based electronics of that era were generally rather unreliable and a maintenance nightmare.

[Edited 2012-08-01 18:06:25]

[Edited 2012-08-01 18:07:06]
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g38
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RE: Douglas A2D Skyshark - Could It Be Made Reliable?

Thu Aug 02, 2012 2:59 am

Thank you for the information. I haven't had a chance to read the article yet, but I will later tonight. It looks very interesting.
 
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RE: Douglas A2D Skyshark - Could It Be Made Reliable?

Sun Aug 12, 2012 10:19 pm

Yeah vacuum tube was another way to say electronic I guess, as compared to mechanic or manual. So it was an early FADEC.
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RE: Douglas A2D Skyshark - Could It Be Made Reliable?

Mon Aug 13, 2012 11:06 pm

Quoting prebennorholm (Reply 3):
Vacuum tubes were nothing special for the A2D. All military planes (and ships, tanks etc.) of the A2D era carried hundreds or thousands of vacuum tubes for electronics such as communication, radar, bomb aming computer, jamming equipment and all such things.

Thousands? Certainly not. The power needs of such an assembly would be huge, it'd be throwing off heat like mad so that cooling would also be a huge issue, and it'd be unreliable as hell in anything that moved.

Hundreds would be quite improbable as well. Perhaps maybe on the B-29, which was the most advanced aircraft of its time in terms of systems, and cost more to develop than did the atom bomb!

Keep in mind that most tubes were dual-function, they had 8 pin sockets, so one could implement two triodes inside one envelope, or a pentode and a diode, and various other combinations/permutations of interest.

For instance, here's a description of a common WWII aviation receiver and transmitter, with a total of 10 tubes for both Rcvr and Tmtr:

Quote:

LF/MF/HF receivers all use a similar same 6-tube superhet design: r.f. amplifier (12SK7), converter (12K8), two i.f. stages (two 12SK7's, or 12SK7/12SF7), diode detector/BFO (12SR7), and one audio stage (12A6). Transmitters use four tubes: 1626 oscillator, two 1625 finals, and 1629 magic-eye tuning.

Many of the tubes are pentodes, like 12SR7:



Ref: http://www.radiomuseum.org/tubes/tube_12sk7.html

But the 12K8 was a diode/pentode setup:



Ref: http://www.radiomuseum.org/tubes/tube_12k8.html

Also keep in mind these folks were very clever (and they didn't have an Internet to waste time on!) and knew how to minimize their design, which made it more rugged and less heavy. The main amount of weight came from the transformers needed to provide the high voltages needed for the tubes.

Ref: http://en.wikipedia.org/wiki/AN/ARC-5

Quoting Aesma (Reply 5):
So it was an early FADEC.

The above says all it did was control prop pitch, which is far less than anything remotely resembling a FADEC.

ObOldManRef: I was an electrical engineering student in the early 80s when one of our profs, who had been trained in the 50's, gave us an hour lecture on vacuum tube circuits! Funny thing is I already knew a lot about them, since I had been using them for amateur radio for years by then!
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RE: Douglas A2D Skyshark - Could It Be Made Reliable?

Thu Aug 16, 2012 2:31 pm

Quoting prebennorholm (Reply 3):
Complicated military vaccum tube based electronics of that era were generally rather unreliable and a maintenance nightmare.

Actually that is not true. vacuum tubes were very reliable, but the did have several draw backs we don't generally face today. They consumed a lot of electrical power and produced a lot of heat. What ever they were installed in, radar, radios, televisions, electronics, etc. had to 'warm-up', they were not 'instant on' systems. They also took up a lot of space, compared to todays microchips. If power was inturpted, they had to warm up again. They also can 'burn-out', something they share with today's technology. Also if the glass breaks or they develope a leak, they are done for.

But they do have some major advantages over todays micro processors. They can put a lot more power, and are almost immune to the effects of EMP because a surge has less effect on them as they can also handle more power.

Tubes are making a comeback, of sorts because many believe they refine the power needed much better than a microchip or transister can. Some believe they produce a much higher quality in sound equipment such as a turn table/amplifier for record based recordings of music, etc. Also there are many new military systems that now use tubes.
 
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RE: Douglas A2D Skyshark - Could It Be Made Reliable?

Fri Aug 17, 2012 11:10 am

Quoting KC135TopBoom (Reply 7):
vacuum tubes were very reliable

Maybe when you had a dozen or so in a well designed enclosure, but when you start talking thousands like the first electronic computers had, they were not reliable enough.

Quoting KC135TopBoom (Reply 7):
They consumed a lot of electrical power and produced a lot of heat

And added a lot of weight, due to the need for step-up transformers to get the plate voltage.

Quoting KC135TopBoom (Reply 7):
Some believe they produce a much higher quality in sound equipment such as a turn table/amplifier for record based recordings of music, etc.

Yes, some do, but in some applications, the same effects that certain audiophiles find pleasing are extremely detrimental.

There definitely are roles for vacuum tubes, mostly in high power applications, but there are so many negatives that they only get used in very specific roles.
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RE: Douglas A2D Skyshark - Could It Be Made Reliable?

Sun Aug 19, 2012 9:00 pm

Quoting Revelation (Reply 8):
Quoting KC135TopBoom (Reply 7):Some believe they produce a much higher quality in sound equipment such as a turn table/amplifier for record based recordings of music, etc.
Yes, some do, but in some applications, the same effects that certain audiophiles find pleasing are extremely detrimental.

I'm not in that group, I'm tone deaf (years of listening to J-57s quartets singing, sometimes with water).

Quoting Revelation (Reply 8):
There definitely are roles for vacuum tubes, mostly in high power applications, but there are so many negatives that they only get used in very specific roles.

Correct, and this is correct, too.

Quoting KC135TopBoom (Reply 7):
But they do have some major advantages over todays micro processors. They can put a lot more power, and are almost immune to the effects of EMP because a surge has less effect on them as they can also handle more power.
 
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RE: Douglas A2D Skyshark - Could It Be Made Reliable?

Mon Aug 20, 2012 2:38 pm

Quoting Revelation (Reply 8):
Yes, some do, but in some applications, the same effects that certain audiophiles find pleasing are extremely detrimental.

Musicians, too. I have half a dozen tube powered guitar amps.
 
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RE: Douglas A2D Skyshark - Could It Be Made Reliable?

Wed Aug 29, 2012 2:47 am

Quoting Revelation (Reply 6):
Thousands? Certainly not. The power needs of such an assembly would be huge, it'd be throwing off heat like mad so that cooling would also be a huge issue, and it'd be unreliable as hell in anything that moved.

Dear Relevation, you certainly never disassembled a Hughes MA-1 fire control system on for instance a Convair F-106 fighter. That's 2,520 lbs of mostly tiny little vacuum tubes mounted in 200 individually replaceable blocks. And, yes, it wasn't very reliable, but reliability was improved by 60 updates during its service life. I do not know the exact number of tubes in the MA-1, but the SAGE system on the ground, with which the MA-1 communicated, was built on aproximately 58,000 vacuum tubes.

Quoting Revelation (Reply 6):
Hundreds would be quite improbable as well. Perhaps maybe on the B-29, which was the most advanced aircraft of its time in terms of systems...

Nope, the B-29 was a electronically a very simple or primitive thing compared to for instance an F-106.

Quoting Revelation (Reply 6):
...and cost more to develop than did the atom bomb!

I believe you are right here. SAGE and MA-1 R&D is believed to have cost $61 billion 1960-dollars. That's several hundred billion present day money. I'm pretty sure that no atom bomb cost that much R&D money, but that data is likely still classified.

Quoting Revelation (Reply 6):
Keep in mind that most tubes were dual-function, they had 8 pin sockets, so one could implement two triodes inside one envelope, or a pentode and a diode...

You are talking about grandpa's radio or B&W TV, not billion dollars 1950'es, 60'es, 70'es tube based state of the art military electronics. Tubes were not mounted on sockets, and most were tiny little things not much larger than the transistors that replaced them. A Douglas A2D propeller control computer would be based on such technology, not on grandpa's TV.
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