Fuel consumption question

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josephades

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Happy new year!

I am trying to figure out the following, with a piper archer PA-28-181 (and of course for all similar planes)

Say you have a 300+ nm cross-country trip. In my case, going northwest from KFRG to CYKZ.

- The headwind at 3000 ft averages 25 knots, resulting in flight time of 3 hours 30 min
- The headwind at 8000 ft averages 40 knots, resulting in flight time of 4 hours

In both examples, mixture is leaned down to where the engine gets rough, and then slightly enrichened. Moreover, cruise RPM are both set to about 2550. Lastly, the headwinds are pretty much coming from the same heading you are flying, give or take a couple of degrees.

I know that the higher you go, the less fuel is needed, but in the above example, due to the fact that there is a 30 minute difference in flight time, does it really not make any difference? Or because I am higher, I am still expected to use less fuel at 8000 despite the increase in flight time?

If the fuel consumption would be the same or greater at 8000 ft due to the additional flight time, let's say that the difference between flight times would be significantly less. Rather than being 30 minutes apart, let's call it 15-20 minutes. Would that make a significant difference?

Thanks in advance for all responses
 
think in terms of gallons per hour (GPH) and go from there. Don't bother to be confused with the power output and altitude.

I'd just calculate the time and engine setting that gives a gph....then just multiply them and add taxi and fudge factors and go with it....and leave an hour reserve for good measure. The difference of a few gallons shouldn't matter in your planning.

1. 3.5 hrs * 8.9 (gph) = 31.15 gal

2. 4.0 hrs * 8.5 (gph) = 34 gal
 
You're not flying a turbine, your fuel consumption is not significantly better over a 5000 foot difference. Go for the lowest time enroute. Now, I'll take a 20 minute longer trip if it means cruising above 5k where the bumpies usually subside and the temperature becomes naturally air conditioned. Pax comfort is another consideration to manage in my mission. Solo I'd low level a 420NM trip like a pirate just because I wanna get there quicker lol, but not with pax.
 
josephades said:
Happy new year!
I know that the higher you go, the less fuel is needed,
Click to expand...

In a normally aspirated piston engine airplane, range is independant on altitude. Many of the still standing Max Conrad long distance records where done at low level.

The following is plagiarized from a review of Aerodynamics for Naval Aviators.

"""1) a normally aspirated piston powerplant loses propulsive efficiency at a rate that negates the true airspeed advantage of flying higher; therefore the maximum range of an airplane with this type of powerplant will be obtained when it is flown at the lowest practical altitude, and at max L/D AOA."""
 
josephades said:
... I am trying to figure out the following, with a piper archer PA-28-181 (and of course for all similar planes) ...
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Good on you for this question and for trying to understand fuel burn.

But remember the old military rule: "No plan survives first contact with the enemy." Winds will not be what you expect. You may forget to lean after takeoff. Consumption may not be exactly what the book estimates.

So ... plan your fuel to as many decimal points as you like, but as @Check_my_Six says: Make sure your plan leaves an hour in the tanks at your planned landing time. And stop for fuel along the way if necessary. "Necessary" comes into play if you find yourself with thoughts like: "I'll probably make get there without needing much of my reserve."
 
A surprising number of fuel exhaustion incidents are caused by pilots trying to stretch the fuel in the tanks "just a few more miles." The fuel regulations seem to be forgotten as soon as the written is passed.

Bob
 
I've done this trip many times without issue. Not trying to stretch or anything. Just trying to figure out what works best based off other people's experiences.
 
It just depends on the winds....it could be different each time. Either way it's only a difference of a few gallons. Not worth calculating to the nats rear IMHO.

There are a few websites that will optimize your route for winds and fuel burn....I don't use them, but someone could chime in with those links.
 
Until you have actually proved you can match the fuel burns shown in the table or developed adequate corrections factor the best bet is to pick the highest cruising fuel burn in the chart and assume that is what you will burn. Once you learn what the airplane and you actually achieve you can start using less conservative numbers.

Brian
CFIIG/ASEL
 
bobmrg said:
A surprising number of fuel exhaustion incidents are caused by pilots trying to stretch the fuel in the tanks "just a few more miles." The fuel regulations seem to be forgotten as soon as the written is passed.

Bob
Click to expand...
There's a big difference between 'stretching fuel' and trying to find the most efficient altitude to cruise at.
 
brcase said:
Until you have actually proved you can match the fuel burns shown in the table or developed adequate corrections factor the best bet is to pick the highest cruising fuel burn in the chart and assume that is what you will burn. Once you learn what the airplane and you actually achieve you can start using less conservative numbers.

I would definitely go for the less headwind and lower alititude with 15kt difference.


Brian
CFIIG/ASEL
Click to expand...
 
josephades said:
I've done this trip many times without issue
Click to expand...
Be careful with those words.

If the book says 8.5 GPH budget for 9.5 GPH etc. be conservative with fuel.

As mentioned above, don't worry about consumption changes at different altitudes. You have your GPH figure and as long as you lean accordingly you should meet it.
 
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It's all about winds...I just flight planned this route for my Sierra, and here's what I got.

Screenshot from 2016-12-31 19-59-35.png

Here's the reverse trip, showing the optimal fuel and optimal speed altitudes being different:
Screenshot from 2016-12-31 20-32-01.png
 
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Ryanb said:
If the book says 8.5 GPH budget for 9.5 GPH etc. be conservative with fuel.
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I totally agree. My fuel burn is 8gph and I use 10gph for fuel calculations. I also bump the reserve fuel to 1hour.

By the way I may have the first post of 2017, that is the first post on the Alaska time zone...:)
 
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mscard88 said:
E-6B :D
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If you're looking for punishment, lot more fun ways to go about it

image.jpg



Math and E6Bing it, bah





FltPlan.com FTW, once you plug in your info, tell it where you wan to go and just pick the happiest numbers, ether for your pocket book (fuel) for you azz (time)
As far as altitude not being important in a piston, dude... Winds tend to be a wee bit stronger up top, sometimes I'll climb to get a monster tailwind, but on the return trip tuck down under the major winds, so altitude is a consideration for speed/econ, but it's also a factor for terrain down low and ice up top (the blue shaded altitudes in FltPlan).
image.png
 
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bobmrg said:
A surprising number of fuel exhaustion incidents are caused by pilots trying to stretch the fuel in the tanks "just a few more miles." The fuel regulations seem to be forgotten as soon as the written is passed.

Bob
Click to expand...
True, that is just sad that many of us (yes, "us") don't have enough common sense and self-preservation.
Just looking at my fuel burns and gas gauges, both my needles would point to E should I decide to land somewhere with the required 30-min reserve. Having only 2 gallons in each tank makes me REALLY uncomfortable. In case of a go-around on a turbulent day, I'd hate for a tank to go unported. *eek*

Y'all fly safe in the new year (and if flying today, then "in the New Year" :) ).
 
Fuel consumption is a neat thing. On Friday I flew my 140 from F05 to JZP. with fuel stop at PBF. Cruise altitude 7500.
From F05-PBF, Power was set at 2450, and leaned aggressively. took 3 hours, and burned 26 gallons. From PBF to JZP took 3.9 hours,
power at 2550, and leaned aggressively. Burned 25 gallons.
 
Best way to figure this out is bust out the time/fuel/distance to climb chart from the POH, adjust the distance for the wind over the climb then take the New distance from the TOC to the destination, bust out the POH again for the cruise performance tables, (which you will notice already take into account different speeds and burns for altitude) figure out how much fuel you will burn over the cruise portion from that table

Add the two together and voila your flight total fuel burn.

Do it for the different altitudes to find the exact answer to your question!

Fly and lean it then way the book says and you should come pretty close. I just flew a 277nm flight and came to within 0.5 gallons of what was planned (based on the book) to what I actually I got. As some said until you know the plane and have seen how the burns compare to the book be more conservative
 
Ya'll doin' too much work. I just take the transit time multiplied by GPH at max power, then carry twice that amount of fuel. Lazy, yes. But I'm not going to run out of gas.
 
Without a JPI 830 my fuel burn could be 2-3 gallons off (what I thought) with very subtle adjustments to the mixture control. I'm quickly paying for my 830 (with fuel savings) and have substantially increased my safety every flight since the install.
 
bobmrg said:
A surprising number of fuel exhaustion incidents are caused by pilots trying to stretch the fuel in the tanks "just a few more miles." The fuel regulations seem to be forgotten as soon as the written is passed.

Bob
Click to expand...

I'm not nearly as impressed by these pilots forgetting a reg as I am then forgetting about saving their own butts with an extra hour to land and take on some more fuel.

The fun factor drops considerably when the engine quits because the tanks are dry. If the whole idea is to have fun...

The "penalty" is that I have to fly some more? Sign me up! Haha.

That's just an extension of a joke from a friend who says the same thing to take the edge off for people who are stressing about a checkride. So you fail? So what? You'll just have to fly some more. Horrible fate. :)

But it does seem to apply to a number of things in aviation. If you get all in a hurry, it'll often bite you square in the gluteus maximus.

"Maybe we'll make it" is the Resignation attitude that starts that accident chain... lots of airports down there and most of them have 100LL.
 
And when the winds change,you may find a pucker factor ,your not ready for . Take GPH times hours to fly add the reserve,then there should be no worry.
 
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