Well we survived the 700 mile round trip in the Colt to Otterburn on the other side of Newcastle at the weekend, but something got me scratching my head.
On the trip up on Friday (almost all motorway*), the trip computer mpg went up to a stable 50mpg. Bit higher than I was expecting, but not unreasonable.
But then coming back on Sunday, virtually the same route in reverse apart from bypassing Newcastle itself, it never went above 42. Stable again, just 8mpg lower.
Now I know that trip computer mpg readings aren't gospel, but I would expect them to be consistent given the same conditions. I certainly wasn't going any faster on Sunday, and we didn't experience any hold ups at all compared to the one 10 or 20 min jam on Friday. Roads were dry both days.
The only significant difference I can think of is that it was pretty windy on Sunday, and I could feel the car been blown around. If we had a strong tailwind on Friday, and a strong headwind Sunday, could that explain the big drop in recorded mpg? If not that, any other ideas?
* route: A329M M4 A404M M40 M42 M1 M18 A1(M)
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My first thought was head/tail wind and I see you suggest that as a cause.
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>> My first thought was head/tail wind and I see you suggest that as a cause.
You think it would make that much difference? And I don't remember it being that windy on Friday. It would explain it though.
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Yes, I too think the answer is blowin' in the wind.
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My experience on driving from Somerset to Sunderland and back on a regular basis, thus sharing quite a lot of your route, is similar.
In windy conditions there can be a significant difference between mpg figures comparing Northward to Southward bound figures. The prevailing wind is almost always behind you heading North.
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Rain also makes a difference to tyre drag.
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Ok, thanks guys - you live and learn. I'll check the weather forecast before travelling long distances next time :)
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The wind will certainly make a difference, and possibly a small variation in the speed. When I had a Defender 90 which is about as aerodynamic as the typical outhouse, the mpg dropped a lot the faster than 65 one went. Even 70 was noticeably worse and 85 which it would happily maintain was in "good greif" territory.
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Just try cycling on a windy day and you'll soon appreciate the effect of head and tail winds!
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I can never understand why flying in an easterly direction from the US you can arrive ahead of schedule if there is a tail wind. Say you fly at 500mph, the tailwind is 70, you are flying faster than the wind.
Pity me. I've always been hard of learning.
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Air speed.
The speed of an aeroplane is relative to the speed it is moving though the air around it. The engines are pushing against air, not against the land. If that air is also moving in the same direction, then the plane is traveling across the ground that much faster.
Land speed = air speed + wind speed (if they're in the same direction).
Its similar to the reason that gliders can go up. In reality the glider is still falling through the air, but the air it is falling through is going up (in a thermal, for example) - potentially producing a net upward movement.
Last edited by: No FM2R on Thu 23 Oct 14 at 11:08
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>> I can never understand why flying in an easterly direction from the US you can
>> arrive ahead of schedule if there is a tail wind. Say you fly at 500mph,
>> the tailwind is 70, you are flying faster than the wind.
>> Pity me. I've always been hard of learning.
I guess the effect of the tailwind is to increase speed over the ground at a given airpseed or Mach number (EDIT - wot Mark said.).
AIUI the effects of airspeed and aerodynamics at cruise altitudes though are quite complex. The difference between stall speed and the max airspeed allowed for structure get closer the higher you go. The phrase 'coffin corner' was coined by those discovering the effect in early days of jets.
Last edited by: Bromptonaut on Thu 23 Oct 14 at 11:11
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>> I can never understand why flying in an easterly direction from the US you can arrive
>> ahead of schedule if there is a tail wind.
Think of the airport when someone is on a travelater and someone is on the walkway alongside. They can both be walking at the same speed but the one on the travelator will cover the distance quicker.
Last edited by: gmac on Thu 23 Oct 14 at 11:30
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>> I can never understand why flying in an easterly direction from the US you can
>> arrive ahead of schedule if there is a tail wind. Say you fly at 500mph,
>> the tailwind is 70, you are flying faster than the wind.
>> Pity me. I've always been hard of learning.
If 500mph is your airspeed, then your groundspeed (what determines your travel time) is going to be 570 isn't it?
They will presumably compromise and save a bit of fuel rather than arrive hours early. But as the prevailing winds are westerly, there's probably a bit of that built in to the schedules.
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I would second Martin Astons post - on a bike you can be putting in an awful lot of effort to pedal in a slight breeze whilst all the cosy drivers are completely unaware of the effect the wind is having on their cars.
There's probably a formula somewhere (where's NumberCruncher?).
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>> Just try cycling on a windy day and you'll soon appreciate the effect of head
>> and tail winds!
Seconded. I've an enduring memory, circa 1988, of pedalling DOWN Glencoe against a wind funnelled up the valley.
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>> >> My first thought was head/tail wind and I see you suggest that as a
>> cause.
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>> You think it would make that much difference?
Easily.
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There's a lot of power in the wind.
Just think of all those wind turbines....
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Bet this guy wishes he had a tail wind!
www.youtube.com/watch?v=7q7K1bBhjkY
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trip computer mpg - Bromptonaut
