Alternate air related load drop

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ememic99
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Re: Alternate air related load drop

Post by ememic99 »

chili4way wrote:
Tue Apr 21, 2020 3:26 pm
My guess is that there is a difference in ALT AIR intake air availability at high altitudes & higher power settings between the left and right engines. This is when air availability becomes the limiting factor to power production. Since the engines are oriented the same way in both nacelles, the ALT AIR intakes are asymmetric relative to the airframe. Airflow differences between the inboard and outboard side of the nacelles may create differences in interior air pressure that limits available air at the ALT AIR box. This happens at high altitudes and higher power settings because the turbocharger is ‘maxed-out' and cannot make up this difference.
Exactly! I just got a phone call from my mechanic few minutes ago and he explained me the issue in the same way.
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Re: Alternate air related load drop

Post by Boatguy »

My understanding is that the actual power being produced is not changing, as indicated by the lack of yaw and no change in IAS. So is this just a difference in how the FADEC is measuring/reporting power?
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Re: Alternate air related load drop

Post by ememic99 »

I guess so.
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Re: Alternate air related load drop

Post by chili4way »

I think there is an actual difference in power being produced and reported. However, at high altitudes, the same low air density that improves TAS relative to IAS likely also changes the relationship between engine power and propeller thrust, particularly in steady-state cruise with a constant speed propeller at the high RPM setting.

I wouldn't be surprised if there was some minor amount of yaw due to the asymmetric power. The amount depends on how asymmetric the thrust is.
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Re: Alternate air related load drop

Post by ememic99 »

I checked once again few things yesterday flying at 14.000 and 17.000 feet. Critical altitude for CD-155 engines is 8.000 feet, maximum they can deliver at 14.000 feet is 89% and at 17.000 is 85%. The flight was in smooth air and I flew at least 5 min in each condition to let the things settle.

Flying at 14.000 feet:
- at 70% load opening alternate air didn't affect either engine
- at 75% load opening alternate air didn't affect either engine
- at 80% load opening alternate air resulted in right engine load drop to 76%

Flying at 17.000 feet:
- at 70% load opening alternate air resulted in right engine load drop to 66%
- at 75% load opening alternate air resulted in right engine load drop to 70%
- at 80% load opening alternate air resulted in right engine load drop to 74%

RPM fluctuated briefly when opening/closing alternate air but settled at previous value in two-three seconds. IAS and TAS were non affected with opening/closing alternate air (maybe one knot slower but it's hard to say). I noticed small right yaw.

Right yaw at 17.000 ft, left engine 75%, right engine 70%, both engines 2120 RPM:
jaw.jpg
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Re: Alternate air related load drop

Post by Colin »

Totally fascinating and I'm so glad you did that work. If I were in IMC and opened the alternate air it would have freaked me out to lose that much power in ONE of the engines and I am not sure I would have made the connection.
Colin Summers, PP Multi-Engine IFR, ~2,600hrs
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Re: Alternate air related load drop

Post by ememic99 »

I confirmed above findings on Monday with one additional notice and one conclusion. I noticed the load on left engine rises 1%. And the conclusion is that it's not just a presentation of drop in load on right engine - there is a drop in power but it's not reflected in IAS. The reasons can be various, probably related to thin air at these altitudes.

Here are few pictures showing flight data.

Flying at 15.000 feet:
- at 85% load opening alternate air resulted in right engine load drop to 81% and left engine to jump to 86%
- visible right yaw
- IAS/TAS not affected
PFD150.jpg
MFD150.jpg
After closing alternate air, everything returned to normal.
PFD150_2.jpg
MFD150_2.jpg
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Re: Alternate air related load drop

Post by Boatguy »

Now I feel obligated to try the alternate air on my DA40NG!
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Re: Alternate air related load drop

Post by ememic99 »

Boatguy wrote:
Fri May 08, 2020 4:21 pm
Now I feel obligated to try the alternate air on my DA40NG!
I guess you won't notice anything below 14.000 feet (or even higher). Austro has critical altitude 10.000 feet, so alternate air will start to influence load above 13.000-14.000 feet.
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