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Old 11-27-2019, 04:24 PM   #1
Theodore42
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Default No Vapor Return Modeled?

From the manual 51-127-5 page 22:

(Concerning the vapor return): "Also, check the tank occasionally in flight to make sure that it isn't completely full. If you don't do this, the recovered gas, which may amount to as much as 10 gallons per hour depending on operating conditions, will be lost through the overflow pipe."

When I read this I tested it (about a year ago). It suggested that there were only specific conditions that would result in 10 gph. I remember GUESSING that conditions conducive to vapor lock would result in a higher vapor return rate, but my tests at that time didn't detect a discernible vapor return.

I tested max heat at SL and at max altitude. Since I couldn't find anything I just assumed that I didn't know what operating conditions caused the max vapor return rate. If I was operating the aircraft such that the vapor return rate was only 2-3 gallons per hour, I wouldn't have been able to detect the return with the tests I did.

DCS has far more subtle engine effects modeled, so I would describe this as a bug rather than pedantic aviation nerd rage.

*UNLESS it is modeled and I just don't know how to cause it.
Does anybody know for sure what "operating conditions" would cause the most vapor return? Is it just the engine setting, or would it be affected by ambient temp and atmosphere?
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Old 11-27-2019, 04:49 PM   #2
grafspee
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I did test it. Notice that tanks gages are very precise in range from empty to 5 gallons, i did burn left tank dry, then i made flight on right tank for a good 30-40 minutes with various power settings. Even 0.5 gallon should move needle, i tried to switch to the left tank but engine refused to work so tank was still empty.
Where is our vapor
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Old 11-28-2019, 06:57 AM   #3
Weta43
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Perhaps just incorrectly sitting in the tank it was drawn from - in which case the end result has no effect on range, weight or performance, only on the proportion of time you have to select the left tank to keep fuel evenly distributed.

A bug, or an oversight, but a very insignificant one.
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Old 11-28-2019, 09:27 AM   #4
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Maybe to increase vapor separation we need to depressurize fuel system so fly high,kill booster pump, low rpm maybe then vapor will liquefy in left tank.
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Old 12-02-2019, 10:57 PM   #5
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Looks like late models had vapor return to fuselage tank
Question is, is -25 and -30 late version or early version.
Looking at booster pump switch i assuming that we have later version because of 2 position switch
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Old 12-03-2019, 06:17 PM   #6
Theodore42
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Quote:
Originally Posted by grafspee View Post
Looks like late models had vapor return to fuselage tank
Question is, is -25 and -30 late version or early version.
Looking at booster pump switch i assuming that we have later version because of 2 position switch
There is a note in that manual on p22:
"In later planes the vapor return goes to the fuselage tank instead of the left wing tank. With these planes, use the fuselage tank first. However, the only way of finding out to which tank the return is connected is to ask the engineering officer." Or, in our case, the DCS manual

Quote:
Originally Posted by Weta43 View Post
Perhaps just incorrectly sitting in the tank it was drawn from - in which case the end result has no effect on range, weight or performance, only on the proportion of time you have to select the left tank to keep fuel evenly distributed.

A bug, or an oversight, but a very insignificant one.
It's important for a few reasons:

Firstly and simply, the startup checklist says to start from whichever tank has the vapor return line. So we are all ritualistically simulating this act but we can't experience the negative effects for failing to follow the checklist (wasting fuel). If it is 10 gallons per hour, that is kind of a big deal.

However, there is also the matter of fuel management:

If you are keeping a schedule of switching between external wing tanks and then internal wing tanks, you may end up with a lot more fuel in the vapor return tank than you have scheduled. Probably not the kind of thing that would get noticed unless you are flying a LONG time, but if 10 gallons per hour is the maximum.... it could make relevant an understanding how much fuel is going back to the vapor return tank.

MOST importantly, however, is that the checklist says simply to start up and take off with the vapor return tank. If you have a full fuselage and external tanks, then you are switching to the fuselage tank asap to prevent reversibility. Then you are switching to the external wing tanks so you can drop them if need be. That is a LOT of time spent returning vapor to the vapor return tank... And if it is full then you are losing fuel.

Anyway, I was going to do some testing last weekend to verify that there is/isn't any vapor return but... you know.... that F-16 module
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