Wide open at low altitude?

[Swingin]

I will be taking a trip soon and the prevailing weather along the last 100nm or so this time of year may require me down between 1,500' and 2,500'MSL.  The lowest I've ever cruised at was 8,500', and my home field elevation is 4,500', so I'm actually not sure where to run the engine if I'm cruising that low.  Would wide-open, full rich, and 2500RPM get the job done?  Any way to run more efficiently down low?  I'm in an M20E and normally run LOP at altitude...

Thanks in advance for answers to what's probably a pretty simple question.

[danb35]

I guess WOT/2500/full rich would work, but 25"/2500/20-30 LOP would burn a lot less gas.  Might not be quite as fast, I guess...

[Z W]

Highly recommend you not fly 100nm at 1000-1500 agl under a cloud deck, if that is what you are describing. That kind of scud running is a great way to get a close-up view of a cell tower or hillside. Also, if the ceiling drops 500ft you may find yourself trapped with nowhere to go, or in IMC without the rating. If that's the weather, best to wait for another day.

Your engine will do fine. Probably will have to reduce MP to stay out of the yellow arc of your airspeed indicator.

[rbridges]

agree with Zane about scud running, especially in an unfamiliar area.  I read the incident reports, and a lot result from the ceilings getting lower and lower.

anyway, when I goof around the airport (we're at MSL 420) and take people sight seeing over the city,  I will usually do 25, 2500 and pull the mixture out about an inch.  Keep in mind I have a carbureted C model, and I use an EGT to keep an eye on engine temps.

[M20F-1968]

I do not know your time and experience, but if you have to ask this question, you certainly should not be doing it. 

Even if you were, you certaily should not be doing it at anything near full power.

I learned to fly in Portland Oregon where on winter days the ceiling was 3000, drizzle and haze.  Even in a 152 or 172 where the speeds are slowerI never felt comfortable for even short distances, let alone 100nm, 1500 above the ground and 1500 below the clouds.  Conditions can change quickly leaving you no out.  The terrain elevation certainly will change over 100nm, and there is too many risks of cell twos, hills, being in IMC when not rated (which has a 20 sec average time before disorientation).  I assume you are doing this because you are not IFR rated.  Additionally, if there is this kind of weather, you may be in a place like the Pacific NW where there can be risks of icing in the clouds as well.  At Mooney speeds, turn radius is larger leading me to think you do not have a good enough sense of maneuverability.  Think of mountain flying in a canyon.  You can just as easily get unexpectedly cornered in the scenario you describe.  My 2 cents is - DON'T DO IT.

[gregwatts]

Flying at 2500' MSL to land at an airport that is 4500" MSL................that won't work!

[Parker_Woodruff]

Nothing wrong with a low level cruise. Just consult section charts for your minimum altitudes to clear obstructions and plan carefully. I did a lot of WOT cruising at 1000 AGL and lower in my M20J. Typically 2400-2500 RPM and 9.5 - 10 GPH

[Shadrach]

I cruise at 2500-3000MSL all of the time for 60NM hops in an around the MD tristate area (I have family in Winchester VA and Cumberland MD). It's no bigee (provided there's not terrain or towers at the same altitude). As for scud running, it's not the scud running that typically get's pilots into trouble, it's when they run into degrading condiitions and keep pressing on into lower and lower ceilings. Feel free to flame away.

As to your engine. I would run 2500RPM WOT@ 30-40 LOP and see what your CHTs look like. For me it's difficult to keep all CHTs above 300 when low and LOP this time of year.

I actually run 2500 WOTRAO and 30LOP for most lower level ops (1500-3000ftMSL), which usually yields between 150 and 155KIAS. Runs like a top at that setting...

[takair]

Quote: Parker_Woodruff

Nothing wrong with a low level cruise. Just consult section charts for your minimum altitudes to clear obstructions and plan carefully. I did a lot of WOT cruising at 1000 AGL and lower in my M20J. Typically 2400-2500 RPM and 9.5 - 10 GPH

[Swingin]

Thanks for the replies.  Is 10GPH like Parker sees pretty common?  I get right at 8.9 at altitude, I'd be happy with 10 down low.

As for the concern, I appreciate it but I'll be fine.  Commercial, multi, instrument, and I fly something else for a living.  I'm just in the unique position of having never flown a Mooney at or near sea level based on where I live.

[jetdriven]

lean it until you see 10 GPH or less at 2500 RPM.  If it runs smooth, there you go.  It really is that easy.

[takair]

Quote: Swingin

I will be taking a trip soon and the prevailing weather along the last 100nm or so this time of year may require me down between 1,500' and 2,500'MSL.  The lowest I've ever cruised at was 8,500', and my home field elevation is 4,500', so I'm actually not sure where to run the engine if I'm cruising that low.  Would wide-open, full rich, and 2500RPM get the job done?  Any way to run more efficiently down low?  I'm in an M20E and normally run LOP at altitude...

Thanks in advance for answers to what's probably a pretty simple question.

[jetdriven]

The yellow arc is not a limitation for low level bumps.  It is a limitation if flying into a 3,000 FPM updraft. 

[takair]

Quote: jetdriven

lean it until you see 10 GPH or less at 2500 RPM.  If it runs smooth, there you go.  It really is that easy.

[jetdriven]

except for the BSFC curve is lower LOP than ROP.   Cylinder pressure depends on fuel flow and RPM.

[takair]

Quote: jetdriven

The yellow arc is not a limitation for low level bumps.  It is a limitation if flying into a 3,000 FPM updraft. 

[Shadrach]

Quote: takair

 OK, I have to ask, how do you manage WOT and 10 GPH at essentially sea level?

[Shadrach]

Quote: takair

 Before you go off in unfamiliar terrain, try it at home.  With low ceiling it is amazing how fast things come up at you at 150kt.  It is also harder to pick out landmarks and airports this low.  Keep in mind antennas are getting higher and you will not have time to look at your sectional to pick out the antennas.  TAWS has made things a little easier, but not all antennas are plotted and not all are accurate.  You are also limited by your yellow arc.  Typically you will get low level bumps and will end up pulling power just for those.  Things happen fast.  Also, remember others may also be squished in that airspace to maintain VFR, you don't have 500 ft seperation so people can pop out of the haze very fast.  If you have to do this be very careful.  See the recent accident in Phoenix for proof of this.

[jetdriven]

Ross thats what we do.  Below ~4000' (probably higher), leaning to 10.0 GPH or less guarantees the lean side of peak and limits LOP power to 75%. Our CHT confirms that with 330s all the way to key West and back from Houston.  It really is that easy.

Quote: Shadrach

 OK, I have to ask, how do you manage WOT and 10 GPH at essentially sea level?

[Shadrach]

Quote: takair

 Then why not simply reduce your manifold pressure?  I am still waiting for somebody to post fuel burn for same speed and running LOP vs ROP.  I would guess they are roughly the same and cylinder pressures are lower (at sea level) running lower manifold pressure. 

[Shadrach]

Quote: jetdriven

[takair]

Quote: Shadrach

 Before you go off in unfamiliar terrain, try it at home.  With low ceiling it is amazing how fast things come up at you at 150kt.  It is also harder to pick out landmarks and airports this low.  Keep in mind antennas are getting higher and you will not have time to look at your sectional to pick out the antennas.  TAWS has made things a little easier, but not all antennas are plotted and not all are accurate.  You are also limited by your yellow arc.  Typically you will get low level bumps and will end up pulling power just for those.  Things happen fast.  Also, remember others may also be squished in that airspace to maintain VFR, you don't have 500 ft seperation so people can pop out of the haze very fast.  If you have to do this be very careful.  See the recent accident in Phoenix for proof of this.

[takair]

Quote: Shadrach

 Then why not simply reduce your manifold pressure?  I am still waiting for somebody to post fuel burn for same speed and running LOP vs ROP.  I would guess they are roughly the same and cylinder pressures are lower (at sea level) running lower manifold pressure. 

[Shadrach]

Quote: takair

I've done it a number of times... My POH shows a 75% fuel burn of 10.9GPH 24X2500 at 2500ft. Calculating it out precisely LOP works out to 9.9gph; so about a gallon an hour difference between ROP and LOP for the same power/speed. I can tell you that running 75% LOP in my bird is much, much cooler than 75% ROP...

[xftrplt]

Last spring, on a flight from MD to the Florida panhandle, headwinds were strong above 2000 ft AGL.  I flew the first 300 NM at 1000 ft AGL (+|-) with a tailwind, skimming ridges in VA and NC.  No FAR's broken, time and fuel saved.  And nice scenery.  Buzzards, hawks and geese are the biggest threat, so it's not the time to have your head in the cockpit.

YMMV