Hi guys,

After a long and intense period of modern airliner-only flying, where everything is calculated for me, and all I have to do is follow the magenta line, I've started to get more interested in the older ways of airline flying. Not sure why it appeals to me, but I guess the screaming turbojets and old cockpits, with scattered dials galore, must hold a place in every aviators heart.

 

After reading Al's review of the CS 707, I started looking at the EPR charts included in this link. Now everything makes sense, and for the most part I understand what EPR is, where the gauge is located etc, but I'm confused as to what the blank boxes stand for. (note, I'm referring to the 300JT4-11-12-tothrust.jpg)

So if I have a pressure altitude reading of 750 feet, and the temp is 25C, would I just use the same thrust as i would a pressure alt. of 250 feet, or something different?

Thanks,

You would use 2.32 for derated or 2.47 for a full power takeoff.

On those charts, if there is no other value listed in the other columns, you'd use the one that is listed in the nearest suitable column, i.e. if it only lists -1000 and SL and nothing in the subsequent altitude column, then you'd use the SL value for 250 feet.

 

Although where the engine modeling on the CS 707 is concerned, it is often a moot point, because they just are not simulated to the level of detail where it really matters that much. In fact if I had one complaint about all Captain Sim's older jets with JT8 engines, this would in fact be the one complaint, since it was kind of the art of driving the things beyond the navigational and handling skills they also required. Thus you might find that you follow the EPR charts to the letter with the CS 727 and 707 and it doesn't do what you'd expect, and because that is so, sometimes you just have to firewall the throttles on them.

 

But, all that aside, and back with those charts and deciphering them. You might have noticed that the values sometimes don't seem to make a lot of sense on the face of it, i.e. you would tend to think that you'd want more and more power because of thinner or warmer air, and so you might expect to see a fairly linear increase of EPR settings across the charts as the air gets thinner or warmer; but of course you don't see that on those charts. So why is that?

 

Well, the reason you don't see it looking as simple as you might expect, is to do with how much warm air can be handled by the engine, which goes beyond simply the need to suck sufficient air into the engine for the fuel/air mixture. Essentially, the problem is that if you take too much warm air into the engine, you can swamp it at higher temperatures or thinner density altitudes, and then you'd get less efficient thrust, so you kind of need to 'take it easy and hurry up' with jet engines when operating them in warmer thinner air, or in other words, you can actually accelerate quicker by allowing the engines to operate as best as they can manage, rather than trying to force them to give you all they've got and overwhelming them. There is, obviously, a little bit more to it than that simple explanation, but I hope that helps you make some sense of why those charts are often a bit of a puzzle.

 

You might be interested to know that A2A are at present working on an AccuSim version of the General Electric J79 jet engine found in the McDonnell Douglas Phantom II, which is an aircraft they are intending to simulate when that work is completed. Although being the methodical types that they are, they will first be simulating just one of these engines, placing it in a Lockheed F-104 Starfighter, in order to perfect the process.

 

They'll be on with that very soon now that they've got their P-51 Mustang out of the way, and it is my understanding that the dynamic way in which their Merlin engine found in their Mustang and Spitfire behaves, is also the way they are intending to simulate the way the J79 works, i.e. with its operational parameters working outside of FSX and it being a modular software component that can be slotted into airframes that they model visually and for flight parameters in the FDE, as the Merlin simulation on the Spit was slotted into the P-51. So we may eventually see a much more realistic jet engine simulation in FSX if all goes well. And of course if they pull that off, then it would also be a spur to other developers to match that level of sophistication.

 

In the meantime, I'd like to see Captain Sim make a Boeing 720, which of course had the Kreuger flaps of the kind found on 727, so that we don't need a four mile dry lakebed to get at least one variant of the 707 off the deck at MTOW LOL

 

Al

You would use 2.32 for derated or 2.47 for a full power takeoff.

Thanks, Chris. I see where your getting those numbers, too, as I missed the arrows through the horizontal columns.

 

You might be interested to know that A2A are at present working on an AccuSim version of the General Electric J79 jet engine found in the McDonnell Douglas Phantom II, which is an aircraft they are intending to simulate when that work is completed. Although being the methodical types that they are, they will first be simulating just one of these engines, placing it in a Lockheed F-104 Starfighter, in order to perfect the process.

Oh yes, I remember hearing about the A2A F-4 and F-104. Does anyone have any idea as to how far development has come on them? I'm not much into warbirds, but I love the Spit and Mustang, and would love to see something from that era modeled perfectly