Electronic Ignition - seeing through the smoke screen

[Jean]

My last flight report is #5671 (not #5566). I had a closer look at the data.
We flew at FL 070 and OAT was 0°c. Maximum CHT was 353 during climb and under 340 for the rest of the flight. I checked data between 32:25 and 36:20 (GAMI test) and noticed that EGT 1,2 and 4 peak about at the same time. It's not so clear for EGT 3. Anyway, the engine remains quite smooth LOP (Tempest Fine Wire plugs).
When I look at Fuel flow and TAS curves, I see that TAS decreases faster than FF. So the miles/gallon ratio is deteriorating LOP.
So, I would say that I am happy with the engine's performance and consumption as it is. I will keep flying close to peak EGT when outside the "red box" when CHT's and EGT's look good. Our engine has 920 hours and good compression rates.
I don't know the cost of Electroair but I doubt that it would pay itself fuel wise.
BTW, the picture was taken through side window with a Panasonic FZ200 which has a really bright lens for night shooting.

[Antoine]

Back from Egypt )

Thank you Jean. I have studied your report and could not quite understand the GAMI tests (it seems your fuel flow increases before the EGT peaks whereas my peaks occurred on the descending fuel flow slope).
However it is very obvious that you have a very well balanced fuel injection system.

Did you have GAMI fix it or is it stock?

[Jean]

Antoine,
We have the stock injection system, no GAMI fix, but runs much smoother LOP since we switched to Tempest Fine Wire Plugs.
Here are two graphs of the "GAMI test", one showing EGT/FF and the second FF/TAS.
As you can see our TAS decreases faster than fuel flow when leaning.

[Tommy]

Jean, you're lean of peak around 8.3 gph and your speed at that point is around 138.0.
That is pretty much where I end up and cruise at. Under 6,000 msl 8.3 gph is pretty much the sweat spot
for cruise. EGT's decrease along with cylinder head temps and settle nicely at 8.3 gph.

Anything leaner and the engine does not sound good.

[Antoine]

OK got it now. Thank you.

It seems that one of your cylinders shows a rather broad EGT "plateau" instead of a peak. But overall they are very nicely matched and I suspect this is one reason why you get such excellent efficiency.

On the subject of TAS dropping fast on the LOP side, this is I think normal as the power curve drops very sharply too. It seems that if you do 10-15 degrees F LOP, you are only losing 1 KTAS and you get about 138 KTAS on 7.5 GPH which is very very good.

I also noticed from your flight reports that you like to use low RPMs, unlike most of us here.
Do you also find certain RPM spots that "feel" better?

My plane for example prefers 2280 to 2300, somehow I never use the band between 2280 and 2400. In faster cruise it really likes the 2450-2500 area.
It can go down to 2150 but somehow does not sound "happy" there so I hardly ever do it. Maybe it will become more harmonious once the fuel flows are better balanced.

(I have in the meantime sent my GAMI test data to GAMI and waiting for their feedback.)

[Antoine]

Jean, you're lean of peak around 8.3 gph and your speed at that point is around 138.0.
.

Huh? You're getting me really confused now, Tommy. I though that I had finally decoded the chart and found the LOP side more around 7.5 GPH.

I think I need a LOP course!

[Chris B]

As you can see our TAS decreases faster than fuel flow when leaning.

Hi Jean -

I think this is an artifact of the scales on the graph.

In percentage terms:

• TAS decreases 6% (139 to 131), but
• FF decreases 16% (8.3 to 7.0)

That looks like a great trade-off! Or at least an option to consider if fuel cost is a priority.

Chris

[Jean]

As you can see our TAS decreases faster than fuel flow when leaning.

Hi Jean -

I think this is an artifact of the scales on the graph.

In percentage terms:

• TAS decreases 6% (139 to 131), but
• FF decreases 16% (8.3 to 7.0)

That looks like a great trade-off! Or at least an option to consider if fuel cost is a priority.

Chris

I guess you're right, Chris

My plane for example prefers 2280 to 2300, somehow I never use the band between 2280 and 2400. In faster cruise it really likes the 2450-2500 area.
It can go down to 2150 but somehow does not sound "happy" there so I hardly ever do it. Maybe it will become more harmonious once the fuel flows are better balanced.

Is it the plane or you who doesn't sound happy? When we started reducing RPM (we generally used 2400 RPM) in our cruise settings, we also found the sound felt strange. in fact, it is just different. After a few minutes, it sounds "normal" again.
BTW, Lycoming's manual shows 2400 RPM as the best fuel efficiency setting. But, fuel flow is proportional to rotation speed. So reducing RPM dramatically reduces FF (assuming full open throttle).
On the other hand, rotation speed is regulated by propeler blade pitch control. I guess that, for a given cruise speed, there must be an ideal blade pitch (angle of attack) corresponding to a certain rotation speed. I never could find any documentation about this. Pushing the engine to 2500 sure provides more power but, thrust/drag wise, I am not sure it is so efficent (even if it is effective).
Anyway, given the price of AVGAS in our countries (2,52 €/liter), we decided to stick to 130-135 kts for our long rides. This also improves our range and reserve. Flying LOP will make it even a little better

[Tommy]

Jean, you're lean of peak around 8.3 gph and your speed at that point is around 138.0.
.

Huh? You're getting me really confused now, Tommy. I though that I had finally decoded the chart and found the LOP side more around 7.5 GPH.

I think I need a LOP course!

You are correct Antoine. I read it wrong.

[Antoine]

IO360_Powerflow_settings.pdf

(17.5 KiB) Downloaded 106 times

Jean here some recommended settings from the POH of the DA42-L360 (I use this one because it does not have the 2400 RPM limitation).
Diamond seems to suggest that we run the lowest RPM that gives the desired power output.
We can also see the impact of increased friction when going from say 2000 to 2400 RPM. Fuel flow goes up for the same power.

But as you said, there are other factors such as prop efficiency variations.

As to optimum cruise, I think that from a fuel economy perspective, you are absolutely right to cruise at 130-135 - the gain in block time from higher speeds is less than 10%.
When I did my trip to Lebanon, on certain legs I had no choice but to exploit the very best MPG this plane would give. I recall sitting between 7 and 7.5 GPH all the time, with speeds similar to the ones you mentioned.

This said, these days I am treating myself to 140+ speeds, just to enjoy the results of the work on the plane's aerodynamics. It is silly, but a couple "homemade" knots put a big smile on my face!