podair

Ram Air

18 posts in this topic

As I tend to fly relatively high, 8,000-10,000ft, pulling the ram air open makes quite a bit of a difference on my 1967 M20F, even with a new supposedly more efficient Challenger/K&N Filter.

I fly WOT with 2500 RPM and get around 1.0-1.5in more manifold pressure at those altitudes. With that, I systematically get 145kt tas with 32 liters per hour (8.5gal), 21inch and 2500 rpm, ram air open and density altitudes between 10k and 11k, about 10-15 lop. Not bad for a stock 1967 Mooney with a 3 blade prop.

However, in certain circumstances, I have become a bit paranoid about using it lately (despite having run the engine flawlessly for 5 years with those settings!), especially when not in gliding distance of land (I cross the English Channel on a regular basis). I have noticed at times some ever so slightly rough running (or is it a case of over water auto rough?!), but I think it is more due to running slightly lean of peak with adequately balanced (but not GAMI) injectors.

What is the downside? Of course I understand it is unfiltered, one should not use it in cloud or dusty environment.

How high is the risk of intake icing in clean air? Have there been cases of engine stoppage with ram air open?

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You also should NOT use Ram-Air if moisture (rain/clouds/mist). Sometimes my plane runs rough when I pull Ram-Air door open while LOP at altitude...Usually a little more fuel eliminates this. I commonly run 50-70 LOP in my M20E. Adverse winds/cloud levels will sometimes result in my staying at lower altitudes (3500-5500 feet). I usually have 9.5-10.0GPH fuel flows down lower.

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"How high is the risk of intake icing in clean air? Have there been cases of engine stoppage with ram air open?"

Pod air,

The cause of intake ice is... moisture and cold.

The pressure drop generated by the nearly closed butterfly valve creates a localized drop in temperature in the air entering the engine.

A drop in temperature and pressure can cause water to fall out of solution. Evaporating fuel in carburetors also cools the air entering the engine. Temperatures below freezing can cause freezing.

Things to avoid:

Avoid high RH, near freezing temps, carburetors, and low power settings.

Clouds at your altitude are a hint at RH.

WOT has no pressure drop related to the butterfly valve.

My thoughts anyways. My only experience with ice was in an M20C with very high RH. It was a hazy day and carb heat was the solution to the challenge.

Best regards,

-a-

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You also should NOT use Ram-Air if moisture (rain/clouds/mist). Sometimes my plane runs rough when I pull Ram-Air door open while LOP at altitude...Usually a little more fuel eliminates this. I commonly run 50-70 LOP in my M20E. Adverse winds/cloud levels will sometimes result in my staying at lower altitudes (3500-5500 feet). I usually have 9.5-10.0GPH fuel flows down lower.

Why would you keep Ram Air closed in moist air ?

Air filter is there to protect from abrasive material not water.

Fuel combution ===> water (and plenty of it) in the engine that evaporates)

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Numerous aircraft, such as the Extra 300 with a 1500hr engine TBO have no air filter and run with the equivalent of a ram air open all the time.

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You also should NOT use Ram-Air if moisture (rain/clouds/mist). Sometimes my plane runs rough when I pull Ram-Air door open while LOP at altitude...Usually a little more fuel eliminates this. I commonly run 50-70 LOP in my M20E. Adverse winds/cloud levels will sometimes result in my staying at lower altitudes (3500-5500 feet). I usually have 9.5-10.0GPH fuel flows down lower.

Not to shift the thread but 50-70 LOP is leaner than optimal at cruise above 5K. . Try 15-25 LOP.

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Why would you keep Ram Air closed in moist air ?

Air filter is there to protect from abrasive material not water.

Fuel combution ===> water (and plenty of it) in the engine that evaporates)

From my POH:

WARNING

Turn Power Boost off when operating in icing conditions. Icing conditions prevail any time the temperature and moisture conditions combine to produce the possibility of impact ice.

Using unfiltered induction air when flying in snow or other IFR conditions can be hazardous. Snow can accumulate in the fuel injector impact tubes or moisture can freeze in the passages under icing conditions and cause loss of power. Therefore it is imperative that the Power Boost NOT be used when flying in sleet, snow, rain or moisture-laden air near freezing temperatures. Under these conditions ice can form in the inlet duct or fuel injector unit even though no visible moisture is apparent on the airframe.

In summary 'cause my POH says so.

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Not to shift the thread but 50-70 LOP is leaner than optimal at cruise above 5K. . Try 15-25 LOP.

My fuel burn is greater than 10GPH at 15-25 LOP Brian (talking below 5500 feet)

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Make sure and check the rubber gasket for service/function on a regular basis. Mine was coming apart when I bought the plane.

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I use the Ram air all the time. It gives me just over 1 inch up at 14,500 ASL. I never use it in bad weather or below 5000' AGL (not including the mountains, as I am sometimes 1500' AGL while at 14,500 ASL.) :blink:

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in icing , i would be more worried to clog the air filter with ice

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M20J has a spring loaded alternate air door. Note it must be checked at annual.

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Alternate air door is right next to filter / ram air. If those are closed or clogged, all bets are off the engine needs air to run so moist air and dust can come in.

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My fuel burn is greater than 10GPH at 15-25 LOP Brian (talking below 5500 feet)

I get it it. So is ours. Its 15-25 LOP OR 10.0 GPH for me too, whichever is less.

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I only use ram air when at full throttle 7.5K or higher. I mean it you are not WOT and want more MP just give it more with the little lever. am I missing something here?

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I only use ram air when at full throttle 7.5K or higher. I mean it you are not WOT and want more MP just give it more with the little lever. am I missing something here?

Yes WOT. I'll open mine duing my climb usually around 3k to 4k feet.

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I only use ram air when at full throttle 7.5K or higher. I mean it you are not WOT and want more MP just give it more with the little lever. am I missing something here?

Yes. If you are NOT WOP you save money buy getting free power with the ram air open in clean air.

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you save money buy getting free power

I am not sure I understand, how you get free power when not WOT. 22" MP is still 22"MP weather the the Ram air is open or not, Perhaps less drag through the filter, which might make sense but not sure this is true or not.

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