Relative fuel consumption

[Ju52]

Hi folks,

I am currently reading a lot on allied bombing during WWII and was wondering how much bombing (especially of the USAAF and RAF) was dependent on weather conditions. Intuitively, I would think that more wind, for instance, should result in further distances flown by the allies because the wind over Germany usually comes from the West, i.e. tail wind for the allies. However, tail wind might be bad for gaining lift...
I already had a closer look myself at the distances flown in 1943/44 and wind speed and found that higher wind speed seems to go along with less distance. This does not make much sense to me, precisely because the wind over Germany usually comes from the West and therefore should be helpful for allied bombers.
Do you have any explanation for this phenomenon?

Cheers,
J

 

[tyrodtom]

It's a 2 way trip, the wind that's behind you going is going to be opposing you flying back to home base.

No matter which way the wind is blowing,or what direction you intend to fly once airborne, you take off as close to into the wind as the runway will permit you.

 

[Ju52]

Yeah, that makes a lot of sense. That's why it seems even more weird that I can find a clear relationship between wind speed and distance (more wind, less distance). I thought there might be some technological reason behind that ...

 

[BiffF15]

The winds actually helped from a fuel perspective for the first half of the flight. The plane burns a lot of fuel in the climb which is exasperated by heavy A/C weights. Tail winds facilitate covering more distance with less fuel. Once bombs are delivered and outbound fuel is burned off the plane is light. Best case scenario for fuel burn except for the head winds during RTB. This is of course with no consideration given for enemy fighters, flak or weather.

Cheers,
Biff

 

[fliger747]

Though it is a two way trip, any steady wind aligned with the direction of flight will result in a increased fuel burn as opposed to a zero wind. That the aircraft will have a varying "cargo" load on each leg and may use a different altitude in reality with differing wind components on each leg, make things messier. Additionally as the aircraft get lighter, the optimum cruising speed decreases, which will modulate the wind effect possibly more.

 

[Ju52]

Many thanks for the answers!

So, just to make sure I understood everything correctly: Essentially what I am taking from this, is that wind is always "bad" in terms of fuel consumption no matter in which direction it blows. Consequently, the aircraft is able to fly longer distances with the same amount of fuel when there is no wind compared to a situation with wind (be it tailwind or headwind). Is this correct?

 

[tyrodtom]

I have limited experience with light aircraft and helicopters.
I have never seen a tailwind increase my fuel consumption. Less flight time for a given distance, less fuel burned.

Though if I had had to fly back under the same wind conditions, my added flight time might have been more than what i'd gained on the initial flight.

In the case of WW2 bombers, once they formed up their formations, they almost never flew directly to their targets.
They had various doglegs, and feints in their course to try to confuse the enemy as to what exact target they were headed for, and their return flights would not be anything like 180 degrees of their inbound course.

 

[BiffF15]

Many thanks for the answers!

So, just to make sure I understood everything correctly: Essentially what I am taking from this, is that wind is always "bad" in terms of fuel consumption no matter in which direction it blows. Consequently, the aircraft is able to fly longer distances with the same amount of fuel when there is no wind compared to a situation with wind (be it tailwind or headwind). Is this correct?

Tailwinds decrease and headwinds increase fuel consumption. Crosswinds depends on direction and severity.

Cheers,
Biff

 

[pbehn]

Many thanks for the answers!

So, just to make sure I understood everything correctly: Essentially what I am taking from this, is that wind is always "bad" in terms of fuel consumption no matter in which direction it blows. Consequently, the aircraft is able to fly longer distances with the same amount of fuel when there is no wind compared to a situation with wind (be it tailwind or headwind). Is this correct?

I used to regularly fly the route taken by RAF bombers from what was RAF Middleton St George to Netherlands Schipol. There isn't a prevailing wind, it changes with the seasons and depends on the time of year and position of weather fronts. There isn't a fixed route especially from Schipol, sometimes you go direct across the N Sea other times up the English coast and other times up the centre of England flying up the Tees valley to land. The route chosen to avoid strong head winds or catch tail winds. On a one hour flight this could add or take off 10 minutes. It was a similar situation Schipol to Hanover sometimes a head wind some times a tail wind, usually nothing of any significance to a jet.

 

[gumbyk]

Tailwinds decrease and headwinds increase fuel consumption. Crosswinds depends on direction and severity.

Cheers,
Biff

Crosswind component always decreases groundspeed, on both outbound and return legs. It may be this that is causing the question from the O.P.

Usually the head/tailwind component will more than counter it, but if you've got a crosswind at 90º to your track, it'll slow you down.

 

[pbehn]

Here two wind speed maps for north sea (surface wind)

and the forecast two days later
View attachment 521413

 

[MIflyer]

Well, the effect of winds can be adjusted to a considerable degree by changing altitude. USAAF bombers needed to be at 20,000 ft plus over the target to lessen the effects of AAA as well as make fighter interception more difficult. The FW-190 tended to run out of steam starting at about 20,000 ft and the Luftwaffe night fighter force would have had trouble getting up there, too.

On the way back from the target they could lose altitude if they avoided flak concentrations.

 

[pbehn]

Well, the effect of winds can be adjusted to a considerable degree by changing altitude. USAAF bombers needed to be at 20,000 ft plus over the target to lessen the effects of AAA as well as make fighter interception more difficult. The FW-190 tended to run out of steam starting at about 20,000 ft and the Luftwaffe night fighter force would have had trouble getting up there, too.

On the way back from the target they could lose altitude if they avoided flak concentrations.

I know MI, I was just posting to show that their isn't really a prevailing wind, the wind at altitude can be different and that isn't just a "binary" difference either. The two charts show winds from East Anglia where most US bombers were based having crosswinds/tailwinds towards central Germany on one chart and tailwinds on the other.

 

[tyrodtom]

The winds at 20,000 will usually be quite a bit stronger than the surface winds.

Right now in the USA winds at 30,000 feet show several areas with wind speeds close 80-90 knots. At 40,000 even stronger in some areas.
That's certainly enough to impact time in flight.

And sometimes winds at higher altitudes show different directions than surface winds.

 

[BiffF15]

The winds at 20,000 will usually be quite a bit stronger than the surface winds.

Right now in the USA winds at 30,000 feet show several areas with wind speeds close 80-90 knots. At 40,000 even stronger in some areas.
That's certainly enough to impact time in flight.

And sometimes winds at higher altitudes show different directions than surface winds.

Tyrodtom,

You are spot on. The software my airline uses will have us fly a longer route for more favorable winds. Going direct can cost you dearly on some flights.

Cheers,
Biff

 

[fliger747]

It's worth remembering that here wasn't much known about winds aloft in WWII compared to today. Even the B-29's were surprised about the "jet Stream. I've hit over 700 its ground speed over Southern Japan with a favorable "Jet". Bucking the Jet heading "West" one can sometimes out climb the core, load permitting.

Any wind in a two way flight under same conditions will result in increased fuel consumption. It's a bit about the story of grandpa who walked five miles to school. uphill both ways.

An extreme example, plane with 100 kt TAS against a 100 Kt headwind, he does get 200 Kts on the return leg, but never gets there in the first place. Less extreme examples somewhere in the middle, but you get the point.

 

[Ju52]

Thanks to everyone for your detailed answers!

The winds at 20,000 will usually be quite a bit stronger than the surface winds.

Right now in the USA winds at 30,000 feet show several areas with wind speeds close 80-90 knots. At 40,000 even stronger in some areas.
That's certainly enough to impact time in flight.

And sometimes winds at higher altitudes show different directions than surface winds.

I heard before that wind speed varies much less in high altitudes.. So, you are essentially saying that wind speed is more extreme in say 20'000ft above ground compared to wind speed in the ground?

 

[fliger747]

I spent some 25 years doodling around in the stratosphere. The strong winds will usually be associated with the Jet Stream, the core of which will depend somewhat on the season and latitude. With the 747 we could (if light) get up into the mid 40's which would usually be above the strongest winds, which might, over Japan in the Winter be in the mid thirties. I have also seen it be dead calm at 35,000'. Depends where you are in the global circulation scheme.

But yes, as a generalization, winds aloft will usually be greater at altitude than those at the surface unless you are in a mountain pass or have some other orographic effects. It is not however a linear function.

 

[gumbyk]

Windy as forecasted

This site shows winds at all altitudes up to FL450 for the globe, great bit of resource.

 

[tyrodtom]

According to that site the wind locally ( Pound, Va) at ground level, about 1200 ft , is coming from almost exactly south at 9 knots.
While at 34,000 feet it's coming from just a few degrees south of west, at 126 knots.
A good illustration of surface winds not having much relation to winds at high altitude.