Headwinds also affect GPH not just MPG

[SkyHog]

My mind was blown tonight. I was pointed to a few different discussions that say that when flying into a headwind, the increased drag also increased the amount of fuel consumed per hour.

I was always under the impression that sure, it takes longer to fly the same distance, therefore you'll burn more fuel to go the same distance, but 1.5 hours of flight was 1.5 hours of flight as far as fuel burn goes.

I won't reveal the names of the people telling me this, just in case they're wrong (I'd hate to get anyone singled out), but am I the only one that didn't know this? WHY DON'T POHs show this???

 

[SCCutler]

Nick:

You know better than this. Trust your instincts, or soon, you'll be calculating how much extra airspeed you need to take off from a conveyor belt.

 

[Steve]

This sounds like the same myth about turning downwind is more dangerous than turning upwind.

The engine and airframe have no clue which way the wind is blowing. They are operating in a fluid. Only the track and speed relative to the ground is affected by wind, not relative to the fluid. Even gust loads aren't going to affect fuel burn.

Now, if you adjust the power setting in attempt to compensate for the variable ground speed/track your fuel burn will be different.

Just where is this increased drag coming from? The Higgs field?

 

[fgcason]

Where are they saying the increased drag supposed to come from?
As long as the plane is moving through the airmass independent of the ground, 100kts is 100kts regardless of what the airmass is doing over the surface. (Terrain induced turbulence will use some energy but real world it's probably quite trivial until conditions get to be exciting)

Conveyor Belt Runway Part II.

 

[jesse]

They are wrong.

But...then again. I've been known to be wrong too...well only once...and I was pretty tired.

 

[SkyHog]

ok - a source:
http://www.megginson.com/blogs/lahso/archives/2004/12/20/wind/

 

[Steve]

Hey, the guy is changing the throttle to go faster....that ain't headwind induced, that is pilot induced...

It is true that a headwind in one direction is never compensated for when flying in the opposite direction at the same power setting due to the time of exposure to the head (or tail) wind. My pvt instructor pointed this out to me early on. I have been fortunate lately having tail winds in both directions on several flights by changing altitudes or waiting for fronts to pass.

 

[MauleSkinner]

Sounds like an old Director of Ops I had who said you can't fly a Sabreliner 3 hours westbound

 

[fgcason]

ok - a source:

I think I see the source of the confusion here.

When the windspeeds start becoming a significant percentage of the cruise speed (like the 60kt example in a 120kt plane) you have to start balancing the endurance vs velocity (reduced endurance) numbers at some point to get to the desired destination. This is not a set the throttle and leave it situation. A realistic though extreme example: If the plane is cruising at a very fuel efficient 60kts into a 60kt headwind, GPH is obviously lower BUT the fuel consumption to destination goes to infinity. The only way to reduce fuel consumption to below infinity is to speed up thus increasing fuel flow and reducing the total endurance. This obviously results in more airframe drag from the increased airspeed however the total fuel consumption to destination drops considerably when the otherwise normal cruise airspeed number starts to approach windspeed.

That brings us back to:

I was always under the impression that sure, it takes longer to fly the same distance, therefore you'll burn more fuel to go the same distance, but 1.5 hours of flight was 1.5 hours of flight as far as fuel burn goes.

You are correct that 1.5=1.5. I suspect you're not taking into account that the throttle plate must be opened further (thus increased fuel flow) to force the time requirement to remain at or less than 1.5hrs to the desired destination for various headwinds. The windspeed isn't increasing the drag, the pilot shoving the throttle in is increasing the drag.

Do not confuse fixed throttle setting endurance with endurance vs range balancing.

 

[mikea]

Sounds like an old Director of Ops I had who said you can't fly a Sabreliner 3 hours westbound

There was the inDUHvidual airline counter agent who said Los Angeles was closer to New York for those who fly eastbound.

 

[JohnLScott]

There was the inDUHvidual airline counter agent who said Los Angeles was closer to New York for those who fly eastbound.

It's true for westbound, isn't it? The Earth is rotating east underneath the airplane.

 

[TMetzinger]

only if the airplane is out in space completely free of the Earth's influence. Picture a helicopter hovering. It's moving at the same speed as the earth.

 

[alaskaflyer]

And birds flying inside an aircraft aren't part of the gross weight calculation until they land...


I'd say that the blogger is correct, as you approach maximum airspeed you begin experiencing diminishing levels of returns in efficiency. He reaches that conclusion through demonstration, while there is a theoretical formula demonstrating the same...let's see, I had it here somewhere...it was for boating not aircraft but it is essentially the same thing (both propellered.)

If I were fighting a headwind and my ETE/ETA was not a factor I would tend to adjust my speed to maximum endurance for safety...unless I began experiencing a negative value groundspeed

 

[flyersfan31]

There was the inDUHvidual airline counter agent who said Los Angeles was closer to New York for those who fly eastbound.

Did you ask for a discounted ticket eastbound, seeing how it's closer?

 

[MauleSkinner]

If I were fighting a headwind and my ETE/ETA was not a factor I would tend to adjust my speed to maximum endurance for safety...unless I began experiencing a negative value groundspeed

Actually, if by "for safety" you mean "maximum fuel in the tanks when you land", you'd want max range speed, increased an appropriate amount for the headwind.

How much increase, you ask? well...it depends upon the airplane, and is generally not published...glider pilots apply a correction based on the L/D graph for the particular ship, although "rules of thumb" can be pretty accurate too.

Unfortunately, I haven't worked it out for sure, but I think if you plot TAS vs fuel flow across the range of speeds for the airplane (obviously, since it's TAS, it would require a LOT of charts to have a complete set), the tangent from the origin will be over the long range cruise speed for the airplane in still wind...if you draw the tangent from the amount of headwind component along the TAS side of the graph, the tangent point will then fall over the TAS for long range cruise accounting for the headwind.

Can anybody verify that, or have a better way?

Fly safe!

David

 

[gismo]

only if the airplane is out in space completely free of the Earth's influence. Picture a helicopter hovering. It's moving at the same speed as the earth.

I'm confused as to your point?

 

[gismo]

Hey, the guy is changing the throttle to go faster....that ain't headwind induced, that is pilot induced...

It is true that a headwind in one direction is never compensated for when flying in the opposite direction at the same power setting due to the time of exposure to the head (or tail) wind. My pvt instructor pointed this out to me early on. I have been fortunate lately having tail winds in both directions on several flights by changing altitudes or waiting for fronts to pass.

I didn't see any mention of GPH decreasing with headwinds in that blog (and that's definitely untrue at a constant power & TAS), but there were some other statements I'd disagree with such as these two:

but averaged over many flights, the best wind is still no wind at all.

Seems to me, from an efficiency and or groundspeed perspective the "best winds would be tailwinds in both direction.

When people want to challenge the true airspeed figures put out by the airplane manufacturers, they often pull out their average groundspeeds, which are inevitably 10 knots slower or more, for which they usually blame the manufacturer’s marketing department.

I'm having a hard time believing that a significant number of airplane owners complain that their average groundspeed from takeoff to touchdown is less than the manufacturers published maximum cruise speed. Sure the manufacturers tend to make claims based on optimal circumstances and realized TAS is likely lower on a real plane, but does anyone really think they should average something close to the max cruise speed when including the effects of wind, climbs, deviations, pattern work etc?

 

[gismo]

Actually, if by "for safety" you mean "maximum fuel in the tanks when you land", you'd want max range speed, increased an appropriate amount for the headwind.

How much increase, you ask? well...it depends upon the airplane, and is generally not published...glider pilots apply a correction based on the L/D graph for the particular ship, although "rules of thumb" can be pretty accurate too.

Unfortunately, I haven't worked it out for sure, but I think if you plot TAS vs fuel flow across the range of speeds for the airplane (obviously, since it's TAS, it would require a LOT of charts to have a complete set), the tangent from the origin will be over the long range cruise speed for the airplane in still wind...if you draw the tangent from the amount of headwind component along the TAS side of the graph, the tangent point will then fall over the TAS for long range cruise accounting for the headwind.

Can anybody verify that, or have a better way?

Fly safe!

David

Your method is exactly correct IIRC. There are "rule of thumb" estimates for calculating the airspeed adjustment required for best range in a headwind or tailwind but as long as you are already flying faster than the best range airspeed (including the effects of wind) you will lose, not gain efficiency by speeding up in a headwind and you will always gain by slowing down in any wind.

I think the ROT was something like add 50% of the headwind and subtract 25% of the tailwind from the max range speed but never fly below max endurance airspeed no matter what the tailwind. Based on that, I wouldn't gain range by speeding up from my normal cruising speed of 180 KTAS unless I was flying into a wind greater than 60 kt at sea level since my best range IAS is 120 kt. At my typical crusing altitude of 9000 MSL, it would take more than about 40 Kt of headwind component (not 40 Kt of wind BTW), and chances are I'd be flying lower in such a wind unless I was forced up there by terrain and/or turbulence.

 

[MauleSkinner]

Your method is exactly correct IIRC. There are "rule of thumb" estimates for calculating the airspeed adjustment required for best range in a headwind or tailwind but as long as you are already flying faster than the best range airspeed (including the effects of wind) you will lose, not gain efficiency by speeding up in a headwind and you will always gain by slowing down in any wind.

I think the ROT was something like add 50% of the headwind and subtract 25% of the tailwind from the max range speed but never fly below max endurance airspeed no matter what the tailwind. Based on that, I wouldn't gain range by speeding up from my normal cruising speed of 180 KTAS unless I was flying into a wind greater than 60 kt at sea level since my best range IAS is 120 kt. At my typical crusing altitude of 9000 MSL, it would take more than about 40 Kt of headwind component (not 40 Kt of wind BTW), and chances are I'd be flying lower in such a wind unless I was forced up there by terrain and/or turbulence.

Thanks for the confirmation, Lance, and good point on the relationship to "normal cruise".

As far as the ROT, I haven't ever dealt with it in light planes...in gliders, we used either 1/3 or 1/2 the headwind component for the airspeed increase. The LRC numbers in the Hawker are "99%" numbers--they're not exactly LRC, but they'll get you 99% as far--and they're good to 80kt headwind without further adjustment. Obviously rules of thumb will vary.

Fly safe!

David

 

[bbchien]

Thanks for the confirmation, Lance, and good point on the relationship to "normal cruise".

As far as the ROT, I haven't ever dealt with it in light planes...in gliders, we used either 1/3 or 1/2 the headwind component for the airspeed increase. The LRC numbers in the Hawker are "99%" numbers--they're not exactly LRC, but they'll get you 99% as far--and they're good to 80kt headwind without further adjustment. Obviously rules of thumb will vary.

Fly safe!

David

http://www.db.erau.edu/research/cruise/piston.frame.html

 

[poadeleted3]

snip
WHY DON'T POHs show this???

Because it's not true.

The engine in the Skyhawk SPs I fly, for example, will consume 10 gph and produce an airspeed of about 115 kts. The engine doesn't care if it's one whit if those 115 kts are into a 115 kts headwind, or a 115 kts tailwind, or no wind at all it's going to burn 10 gph and give me an airspeed of roughly 115 kts.

 

[SkyHog]

I concede. Y'all's thought was my original thought. 1.5 hours = 1.5 hours makes more sense to me anyways, I was having a hard time wrapping my head around why headwinds would affect fuel consumption.

 

[Anthony]

I think that happened in an old Star Trek episode when they depleted the dilithium crystals faster going towards the Earth. No...wait....that was due to the hole in the space/time continuim or condendrum, can't remember. Never mind.