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More Funny Fuel Flow Facts

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yankee-flyer

Well Known Member
Flew 143WM for a short while this morning and made the following observations.

Density Altitude (DA) was a little more than 1000 feet greater than MSL.

During takeoff and WOT climb fuel flow (FF) readings was 5.2 - 5.3 gph UNTIL I climbed through approximately 6000' DA. From that point on FF rose slowly into the mid-7 range and stayed there when I leveled off at 6500 MSL (7700 DA) and established cruise at 5200 rpm. FF remained in the mid 7's (highest I've seen) during cruise.

I pulled the electric pump fuse. FF dropped to 0 (THAT got my attention but engine kept running) and then slowly built back up to 4.3- 4.6 with a fair amount of fluctuation. Note that I don't see much fluctuation with both pumps running.

Re-inserted the fuse and FF slowly climbed into the 7+ range again.

Descended to 4500 MSL (DA 5600) and fuel flow readout returned to "normal" 4.1 to 4.3 gph.

Landed and check fuel tank sight gauge. Used just over 2 gallons in 0.6 hours flying time. Interestingly enough, the fuel-remaining readout said I'd used 2 gallons, too.

Whatever is happening certainly seems to be related to DA.

Scott, it's hard to believe that the fuel pump would not leak below 6000 DA but would lose 3+ gph above 6000 DA. There is NO smell of fuel at any time.
I know our carbs are supposed to be altitude compensating; could some malfunction be occuring there? John and Dick have seen this with both the old and new style pumps; I'm still running the old pump at 120 hours.

Dick, I've followed John's lead in entering a return value into the Dynon-- have you done so? I don't remember seeing this behavior before that but maybe I wasn't watching since I knew the FF was off anyway.

I'm thinking we may need to forward all this to a Dynon rep???

Wayne 120241/143WM
 
Wayne,

That is a very interesting dataset. During future flights, I will try to remember the density altitude when the fuel flow rises. What you observe with fuel flow is VERY close to what I am observing.

YES, forward your data and text to Dynon and request their analysis. Would love to see their response.
 
Wayne and Marty,

Both times I have seen this behavior it has been above 6000' or so DA. I never really thought about it before, but you're right, that is where it occurs. Interesting. Yes, let's get a weigh-in from Dynon on this.

John
 
Response today from Dynon

Got this back today-- I think the answer is yes but I want to check the plans

Wayne

Wayne,

I wonder if this has something to do with where the fuel flow transducer is located. I believe there is a return fuel line.


Is the fuel flow sensor installed in line between the gascolator and the fuel pump? I wonder if the error you are seeing is caused by the amount of flow being returned to the fuel tank by the fuel return line at the fuel splitter.

Mike H
 
Same Thing

Just returned from a Dallas trip where I was cruising at 9500'. My fuel burn was 7 gph and my fuel pressure was indicating 4.1 psig. About an hour and a half into the trip my fuel burn dropped to 5.0 gph and fuel pressure dropped to 3.2 psig. It seemed as though they are related. Question is... have ya'll checked your pressure when your fuel burn changes radically at the same power settings??

Potential variables
1. Dynon "read out" changes with altitude.
2. Fuel pressure changes with altitude influencing the actual flow of fuel through the fuel line / flow meter.
3. Carbs are actually using the fuel causing the high fuel burn rate.

Just wondering.


Jersey
 
The last time I experienced this the flow jumped up into the 9 - 10 range, and the pressure did not change at all. I plotted it out from the Dynon data.
 
Just stumbled onto this thread, but I see some crazy numbers here.

The Rotax at WOT sea level should be pulling more than 5.2GPH, more like almost double that. The carboy is pressure compensating, so you will see changes.

Is there a fuel return line involved on these? and where is the measurement taken?

Just food for thought.
 
Please send your readings

to:

[email protected], addressed to Mike H. I think there are several of us seeing this, probably more than we know about. Flew for 1.4 hours Saturday, all at or below 6000 DA (roughly) and fuel flow was a steady 4.1 - 4.2 gph. I thought about checking pressure, but not while I was seeing the funny readings. I'll do that next time I climb higher. You guys flying to Oshkosh might have a good chance to check this phenomenon out.

Meanwhile, the more people that Dynon hear from the more likely thay are to do something.

Wayne 120241/143WM
 
Great info John

Can you forward your plots to Dynon? Physically, there is no way (is there??) that the fuel flow rate can increase significantly in a constant diameter line if the pressure doesn't change. At least, that's what I think Bernoulli said.

Wayne 120241/143WM;)
 
Scanned RPM/Pressure/Flow graph sent....

.... to Mike just now. Wayne, keep in mind that the fuel system measures flow in between the two pumps, and measures pressure after the two pumps. It is concievable that for some unknown reason the return flow to the tank increases dramatically during this event. Doesn't seem likely, but could explain the high rate of flow.

If you want to see the graph, send me an email at [email protected].

John
 
Is it possible the return line in the fuel tank fell down into the tank. Mounted correctly in the filler neck the return line has no back pressure. If it were to come loose from the top of the tank and fall into the tank and get submerged you might get some back pressure that could give you varying flows when the tanks fuel level changed from full tank to half or quarter tank.
 
My experience today - -

It was a toastie 33 C at the airport ( about 980' ). I climber to 7500', and it was a nice 18 C ( about 66 degrees F ). I noticed my fuel flow also rose higher than normal. For the RPM I was at, I would normally see about 4.8 GPH. It was reading 5.8 GPH. The DA was at 9750' when I was at 7500' ALT. I stayed at 7500' for about 1/2 hour. Everything else was pretty normal, but fuel flow was higher. Seemed ok once back on the ground.

John Bender
397.8 hours
 
I know our carbs are supposed to be altitude compensating[/QUOTE]

Rotax carbs are not altitude compensating. They are sometimes called "constant velocity carburetors"

don
 
Semantics?

"The fuel/air mixture the carburetor provides to the engine varies with air density. Air density fluctuates with ambient temperature and altitude above sea level." Quote from Rotax literature

This may not meet the definition of "altitude compensating" exactly but it must be pretty close ;)

Tony
 
Not Semantics, just facts

"The fuel/air mixture the carburetor provides to the engine varies with air density. Air density fluctuates with ambient temperature and altitude above sea level." Quote from Rotax literature


To me this means that the fuel air ratio that the carburetor delivers to the engine varies with air density. This means that the carburetor is not altitude compensating. If the carburetor was altitude compensating then the fuel air ratio would be constant with altitude or air density. The key word here is fuel air ratio (fuel/air mixture) not fuel flow.

By the design of the Bing carb, it does not have any component that will adjust the fuel flow to keep the fuel air ratio the same, at different air density or altitudes.

Don
 
Strange thing about it, although the fuel flow indicates high 7 GPH/hr at higher altitudes, fill-ups indicate an average of 4.5 - 5.0 GPH. I think something may be goofy with the Dynon at 7000' + at DA.

Gary
 
Same Problem

I have experienced the same problem.
Constant altitude, RPM, fuel pressure, OAT, and the fuel flow changed from 5.3 to over 7 in less than 40 seconds.
Fuel flow actually went as high a 10 with no other changes.
rpm 5300
altitude 7500 ft
fuel pressure 4.3
OAT 57

All with no change in actual fuel consumption.

Gary
 
BigJohn said:
.... to Mike just now. Wayne, keep in mind that the fuel system measures flow in between the two pumps, and measures pressure after the two pumps. It is concievable that for some unknown reason the return flow to the tank increases dramatically during this event. Doesn't seem likely, but could explain the high rate of flow.

If you want to see the graph, send me an email at [email protected].

John
Click to expand...

The email to [email protected] just came beack as undeliverable, after 4 days of trying. I re-sent it but don't have any confidence it is going through.
 
Sent your address

directly to Mike at Dynon, John. I'm sure he'd want to see your plots. The only thing I can think of is that the message is too big for their mailbox.

Sure hope you can get the info to him.

Wayne
 
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