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Post by aerofoto - HJG Admin on Jul 15, 2006 9:49:55 GMT
Since discussion here has focussed on early turbojets and water injection in particular, I thought 'd share the following transcript from an article written by a retired QANTAS Airways Captain (Geoff BROWN) who flew the first B707-138's from 1959. I quote directly from his memoirs as follows as follows ....
The JT3C turbojet engines were fitted with water injection to increase the density of air passing through the engine and so increase the thrust. This it certainly did but it also enveloped the airfield in clouds of black smoke. Water consumption was greater than fuel consumption on takeoff - 570 gallons. Should all water not be consumed on takeoff, provision was made for dumping the excess, resulting in an interesting white plume behind the aircraft. The water was stored in a keel tank behind the wheel wells. Mr Boeing in his wisdom had fitted the water pumps in line ahead, one for number 1 & 2 engines and another for # 3 & 4 engines, so that on initial climbout the forward pump ran out of water first causing a very distinct swing with the loss of 7% power on one side. Not dangerous but disconcerting at first. Why on earth they weren't connected to 1/4 and 2/3 is anybodys guess"
Captain Geoff BROWN retired during 1977 having accumulated some 14,000 hours .... some 3000 of which were accumulated on QANTAS Airways B707's.
Mark C
AKL/NZ
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Post by christrott on Jul 15, 2006 15:01:25 GMT
If he was only loosing 7% of his thrust on one side when the water ran out, then they were using reduced flow injection systems. 2200LB of 10,800LB is 20%, not 7%.
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Post by garryrussell on Jul 15, 2006 15:49:32 GMT
Now why didn't I think of that  You know chris couldn't be more true about the 707 and 135 being two different aircraft. Sure they share the same roots but the one thirty five is a bit fatter than her sexy sister. Then again most of you already knew that  The 135 is the thinner one  the 707 is four inches wider Garry |
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Post by garryrussell on Jul 15, 2006 16:23:35 GMT
If the dry thrust is 10800 and water puts it up by 2200 to 13000 in total, the increase is 23.37%
If the engines are running at 13000 and the water cuts off the 2200 it added to the total, the reduction in thrust is 17 percent.
It is not a 20% increase and it is not a 20% decrease and no way can the increase match the decrease as a percentage.
I wonder if the 7 is a misread of 17.
Garry
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Post by Jordan Claus on Jul 15, 2006 18:23:06 GMT
Now why didn't I think of that You know chris couldn't be more true about the 707 and 135 being two different aircraft. Sure they share the same roots but the one thirty five is a bit fatter than her sexy sister. Then again most of you already knew that The 135 is the thinner one the 707 is four inches wider Garry Wow I didn't know that. But then why does the 707 look sleeker and thinner than the 135? It always looks like the "35" has a bit more junk in the trunk. ;D Is the 135 shorter also? |
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Post by garryrussell on Jul 15, 2006 18:24:49 GMT
I think it just the effect of length and general cleaness of the fuse.
And of course the 707 320/420 has a reduced width to length ratio due the the longer fuse making if in effect thinner in proprtion.
The 707 had to be wider because Pan Am wanted six abreast seating.
Garry
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Post by Jordan Claus on Jul 15, 2006 18:36:23 GMT
That 'splains a lot. Thanks Garry. You learn somethin' new every day.
Jay
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Post by christrott on Jul 15, 2006 21:16:36 GMT
If the dry thrust is 10800 and water puts it up by 2200 to 13000 in total, the increase is 23.37% If the engines are running at 13000 and the water cuts off the 2200 it added to the total, the reduction in thrust is 17 percent. It is not a 20% increase and it is not a 20% decrease and no way can the increase match the decrease as a percentage. I wonder if the 7 is a misread of 17. Garry Only problem Gary is that Water Injection is a modifier, not a baseline performance adjustment. For you to make your calculations based on the thrust with water injection is incorrect since you're making your calculations on a modifier. Just like with afterburners, water injection produces unreliable performance. One time the increase in identical conditions might be 2250#, the next time it might be only 1500# depending on the quality of water. But the point is, the performance is measured from the baseline, not from the modified rating (which is just an educated guess anyway. |
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Post by garryrussell on Jul 15, 2006 21:18:34 GMT
Im am just stating the true percentage relationships of those figures thats all and pointing out the percentage up is not the same as percentage down.that's all
Garry
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Post by christrott on Jul 16, 2006 1:05:54 GMT
He's not loosing 17% of 13,000#. He's loosing 20% of 10,800#. He's going from having a modifier (the water injection) to not having water injection, so he's loosing 20% of his baseline thrust, not 17% of his total thrust since the injection is an after-the-fact issue. That's what I'm stressing here is that the thought process being used is incorrect. If your afterburners fail, you didn't loose X% of the thrust with the afterburners, you just loose the afterburners and lost however much the modifier was for the baseline thrust.
As with thrust reversers, the availability of water injection cannot be used as a part of most of the calculations for flight planning, thus if you should loose WI prior to V1, you can still safely stop or if after V1 you can still take off. However, the problem is that doesn't account for partial loss of the WI system, so the aircraft isn't designed to necessarily handle that at extremely high weights.
Anyways, the point is that you're looking at the numbers from the wrong direction. Since it's a modifier, you make all your calculations based on the baseline.
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Post by aerofoto - HJG Admin on Jul 16, 2006 2:07:36 GMT
The figure of "7%" is what this particular QANTAS captain quotes in his memoirs. Other versions of his same article make reference to the same figure too .... so .... that's what I'm going with. This particular QANTAS captain ought to know .... after all he flew the darned things  Mark C AKL/NZ |
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Post by christrott on Jul 16, 2006 3:06:21 GMT
As I said before, it's possible that the 707-120s used smaller pumps than the KC-135As.
Sadly, however, I've also found many pilots really don't know, even ones that have flown the airplane for years. I've run into a couple pilots who flew the 727 for nearly 20 years each and neither knew how to setup the fuel panel to transfer fuel to another tank. Ran into a 10+ year 767 Captain who didn't know where the crossfeed and fuel pump buttons on the overhead were. Had a full DC-10 crew and FedEx maintenance office stumped when I asked them what the overspeed limit for the CF6 was. In fact, none of the manuals even stated it for them to be able to know.
While I have a lot of respect for pilots (hell, I am one myself), I know that many of us sadly don't have nearly the knowledge that we probably should. I know I'm still learning things about the little Cessnas I fly that I never knew and are quite good to know now that I know them.
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Post by aerofoto - HJG Admin on Jul 16, 2006 6:53:20 GMT
YEAH .... quite likely !
What tends to happen too sometimes Chris is that when pilots transition from one aircraft to the next they go through whats referred to as "AUTOMATIC DUMP".
In other words however well they knew their last aircraft, 9 times out of 10 that knowledge is of little or no siginificance at all to their next aircraft.
Unless memoirs are recorded at the time (which I happen to know was in fact the case with regard to this old retired QF captain whom I've quoted) past knowledge is often quickly forgotten/traded through that basic necessity of familiarlarisation with the new.
Mark C
AKL/NZ
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Post by christrott on Jul 16, 2006 19:09:02 GMT
Yeah, I've heard of that. What I'd be interested in seeing though is whether the 707-120s really had 4 water switches like the panel shows. I ask because on the KC-135, there were only 2 and they were on the FO's (or as the USAF called them at the time - Co-Pilot) side. One turned on the water, one dumped it. Every so often, the CP would accidentally hit the wrong switch and instead of getting that extra power, there'd be a sudden lightening of the aircraft and a lot of people commending about how odd it was to see a KC-135 taking a piss....  |
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Post by aerofoto - HJG Admin on Jul 16, 2006 21:13:32 GMT
In reality I suspect they most probably didn't, but, even among my extensive resources I've got no imagery relating to the B707-120 or -138 panels to be able to verify. I'm inclined to think water injection switches fascillities would probably have been located on somewhere around the center pedestall (throttles) or even the F/E panel .... but .... that's just a wild guess on my part When George designed these panels the biggest problem that confronted him was where to accomodate everything .... and in a logical manner which would still promote its being easy to use/access rather than having to employ a sub panel. Smaking it on the main 2D panel (even though this's most probably not authentic) was the best solution .... both for ease of access and 2D viewing whilst galloping down the runway. It's just another one of those little compromises which we're forced to have to sometimes accept .... and that's about as real as we could get it Anway .... the water injection we have for the -120/138 pane is neat little feature which works fine as is nice to have .... regardless of any infedelity. Mark C AKL/NZ |
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