cctsurf

Higher with Headwind?

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Just curious on the thoughts of our mooneyspacers here...

Tomorrow, I have a flight scheduled where if I stay low, I won't have a lot of headwind (approx. 15mph) or if I go high, I'll be into a fairly stiff 30mph headwind.  I don't have my JPI installed yet, soI can't do a true experiment.  Does the efficiency of flying high outdo the headwind or would it be better to stay low and pay for the gas but be going faster?

I've thought about this a couple of times, I have some experimental data that flying low costs a lot more, but I'm really not certain which to choose...

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Check out either skyvector or 1800wxbrief.com. You can put in your performance numbers including fuel burn, then put in your flight plan and see approximately what the flight time is and fuel burn at different altitudes. They both use the winds aloft forecast coupled with your performance numbers taking into account higher fuel burn and slower ground speed in the climb. I've found them to be fairly accurate. 

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The thing is you won't know what the winds are until you actually fly.  I like altitude because it is usually smoother and you have more options in case it turns into a really bad day.  In my experience, you just gotta go check it out by leveling off at proposed altitudes and make a decision. 

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It depends.  The sweet spot in NA Mooney’s seems to be 4-8k, but it depends on headwind vs turbulence.  If it is bumpy down low and I have to pull power to stay out of the yellow arcsthen I will opt to go higher for smooth air.  Depending on wind speed, it may be a wash...but smoother. I find Foreflight useful for planning optimal winds.

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With 15 mph difference it is better to stay lower as long as it is safe.  I use the 1800wxbrief  tool for flight planning and find it very good.  I was climbing out a few weeks ago into a 40 mph headwind,  it hurt to see 80 knots over the ground.

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I tossed it through Skyvector (I'm not signed up on 1800wxbrief), and it was very clear.  The trip will take approximately 9 minutes longer and yet burn 3 fewer gallons at altitude  (for very rough calculations).  I much prefer the altitude, as they say, "altitude is options".  I'll probably even make up some more of that time by the efficiency of the airplane at that altitude.

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1 hour ago, cctsurf said:

Tomorrow, I have a flight scheduled where if I stay low, I won't have a lot of headwind (approx. 15mph) or if I go high, I'll be into a fairly stiff 30mph headwind. 

Define "low" and "high". To me, low would be 10,000 ft and high would be 17,000 ft.

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Just now, KLRDMD said:

Define "low" and "high". To me, low would be 10,000 ft and high would be 17,000 ft.

In the past, when I fly "low," I fly 3,500 or so, when I fly high, I push to the top of where I can fly without oxygen (12,500, 10,500).  I'd really like to go higher, but I don't have oxygen.  I don't have hard numbers, but I burn approximately 8gph at "high" altitude and closer to 10gph at 3,500...

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Is the OP's question a TAS vs GS at identical power settings question?

Does the OP want to maximize speed or efficiency or both?

Notes on The efficiency/speed curve for NA engine powered Mooneys....

Determine your wants and needs for power generation/fuel flow/engine efficiency, air resistance, flight smoothness & glide safety, then adjust for wind...

1) Power drops off with altitude, about 1” MP per 1k’...

2) Fuel Flow drops off with MP...

3) air resistance drops off with altitude...

4) Engine Efficiency is improved with WOT...

5) Red Box theory, has a limit of 65% bhp. Somewhere around 21” of MP.  Check your specific engine details for this... this applies to engine leaning... lean near peak...

6) Engine out options improve with altitude... one minute per 1k’ or so.  Check your glide ratio in your POH for this...

7) Ride smoothness improves with altitude...  East Coast:  getting above 3k’ agl is pretty helpful.  More wind the higher you go...

8) VFR And IFR altitudes with direction apply...

9) Using O2?...  gives additional options for the tail wind returning flight....

10) Altitude guidance I typically use...

  • In state... around 4.5k’ (low)
  • Between states... around 12.5k’ (high)
  • Local flight... 2.5 or 3.5k’
  • no O2 used, but measure ox saturation anyway.

11) Make adjustments for wind after (1-10)

To go the fastest, in an NA plane, it requires running WOT, balancing power generation with air resistance... this altitude magically works out to be around 7-8k’...  as usual...watch your FF, and %bhp For leaning Safety.... are you using 100°F ROP..? Or something closer to peak?

To go the most efficient, in an NA plane, requires leaning towards peak while avoiding the red box @<65%bhp

To really gain an appreciation for thin air and high power settings... go TC or TN'd Mooney.

To get usable wind info while you fly... ADSB or XM seems to work...

 

PP Thinking out loud. Not a CFI... let me know if I missed or mixed something...

Best regards,

-a-

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Range is basically independent of altitude. I flew from Kissimmee FL to Houston on 39 gallons at 2400’ once. Headwinds.  Any higher required a fuel stop and we would arrive later.  So we did it nonstop. It was also the first trip I did a LOP takeoff from brake release and the first trip I ever pulled the throttle off the stop. There’s more than one way to get there but less headwind always makes sense. 

Edited by jetdriven
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As you increase altitude your TAS also increases. You’ll have to fly it and decide if the headwing is beating your TAS or vice versa. By noting what GS is doing. In other words select altitudes that best optimizes GS.

So I think in your scenario I would fly the altitude that gives the best GS (forgetting ff.) Once I saw my GS dropping I’d request a different altitude.

Fltplan.com does all this for you btw. 

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9 hours ago, cctsurf said:

In the past, when I fly "low," I fly 3,500 or so, when I fly high, I push to the top of where I can fly without oxygen (12,500, 10,500).  I'd really like to go higher, but I don't have oxygen.  I don't have hard numbers, but I burn approximately 8gph at "high" altitude and closer to 10gph at 3,500...

I hope you’re using a pulse oxcimiter at 12;500 10,500. Even though legal, most people’s O2 levels are sub-optimal there.

The medical speakers at the PPPs strongly recommend using O2 above 8,000 (5,000 at night)

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10 hours ago, cctsurf said:

.....  Does the efficiency of flying high outdo the headwind or would it be better to stay low and pay for the gas but be going faster...

Age old aviation question.  Lots of good observations above.  Don't forget to factor in the length of the trip.  An altitude that makes sense for a four hour leg may be a bad altitude for a 45 minute leg.

Fltplan.com is a great resource for altitude selection.

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I'm leaving for New Orleans in about 3 hours.  Currently DUATS has an 8 minute.s difference between 100 and 40 on a approximately 3:45 flight and 493 miles.  I normally will fly this route at 100 but will go 80 today and see how it works out.  I may try lower at some point for better GS.  I have tried lower sometimes for better GS and it was bumpy so I went back up for a slower smother ride.:o

 

Generally for a flight like this less than 15 minute difference I will fly the altitude I want for flight comfort rather that looking for the best GS.  When is gets to be greater than 15 minute difference then I will seek an altitude twat will give me the best GS and shortest flight time while keeping comfort in mind.

 

Currently departure airport is 7sm, OVC 006 and improving and the tops are reported to be 008.

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9 hours ago, jetdriven said:

There’s more than one way to get there but less headwind always makes sense. 

I agree with the first part, disagree with the second part. Staying down low to avoid headwinds up higher and getting the crap beat out of you in moderate or worse turbulence does not make sense to me. I'll gladly give up 10 or 20 knots ground speed or more under some circumstances for a smooth ride.

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42 minutes ago, KLRDMD said:

 ....I'll gladly give up 10 or 20 knots ground speed or more under some circumstances for a smooth ride.

Nah....!  A true Mooniac sacrifices all for speed. :D

A bent wing spar is a small price to pay for 2, or 3 kts better ground speed. Especially if there is a Bonanza in the area!

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It is just a math problem.  How long to climb, GS at various altitudes, how to descend.  

In general it is hard for NA to develop enough TAS to beat a headwind.  It happens on occasion with a turbo but usually when I look at it the numbers show maybe a 15 minutes savings on a 4hr flight.  It isn’t worth the 15mins to suck O2. 

Foreflight does a reasonable job of figuring out optimum altitude now as well. 

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In practice, you learn to worry less about shaving minutes and a couple of gallons of fuel and more about ride and weather avoidance.  In the first place, the winds aloft will be what they are, not what they are predicted to be, and if your trip covers any significant difference, they will change as you travel.  The winds aloft calculation we run for flight planning purposes is just an estimate, it is not going to be what happens once you get up in the air.  Ride and weather avoidance are more important.  If you planned for 8,000 but that puts you right in the cloud tops and 10,000 or 12,000 puts you in the clear, fly in the clear.  The ride will be nicer and this time of year you avoid possible icing in the tops.  On a few occasions when I have been alone and the adverse winds were so strong that I needed the help of ground friction to get a decent GS, I have flown down around 1,500-2500 AGL for distances.  It is not fun.  It is always bumpy, and because it is bumpy it is tiring, and I would not do that with someone on board who is not experienced in light aircraft.  I usually look for the good ride, so what if I could shave 7 minutes off the flight if I used a different altitude.  I like being in my Mooney, why cut it short without a reason.

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1 hour ago, jlunseth said:

In practice, you learn to worry less about shaving minutes and a couple of gallons of fuel and more about ride and weather avoidance.  In the first place, the winds aloft will be what they are, not what they are predicted to be, and if your trip covers any significant difference, they will change as you travel.  The winds aloft calculation we run for flight planning purposes is just an estimate, it is not going to be what happens once you get up in the air.  

Absolutely true.

Because however improbable it may be, it seems I am able to find a headwind at any altitude in both directions!

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That gliding ring looks nicer to me the higher I go.  

In summer I like to be at 8 or 10k to get a better view of the clouds that need avoiding. Need to “beat” out the weather moving in?  Pour the coals on and fly at the best wind altitude  minus the TAS gain (but most of us in the NA club are probably thinking about differences of a few not tens of thousands of feet). 

This conversation can up yesterday - 153 ktas from ENO to PXT at 8500 last night.  Talked about going lower with my wife - a little less altitude but significantly better winds and it’s easier to fly at night sans oxygen below 8000.  A couple of short water crossings within gliding distance of land at either.  Made the decision to start down when I knew I’d have sufficient gliding distance at 6.5 k to make the same airports... a night time VFR. So in a difference of 2000 ft I went from a GS of 136 to 150.  Not too shabby.  But the slow speed was worth it to me.  

So the short answer is higher is better for me.  The long answer is it depends on the circumstances.  

About to fly with my old and semi epileptic dog this afternoon ... he does fine at 4500, or 5500 but doesn’t do so well above 6500 so we’ll stay low for the flat, short ride home. 

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