Why do the curtains on lorries get sucked in when the lorry is moving ?

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  • Why do the curtains on lorries get sucked in when the lorry is moving ?
  • If you ever look at a curtainside lorry when it’s moving, the curtains will look like this.

    How does that happen, when a wing works like this.

    The air on the outside of the curtain is moving at the speed of the lorry.
    The air on the inside is near enough stationary.

    So, if air lifts an aeroplane wing, why doesn’t it suck the curtains outwards ?

    Premier Icon somouk
    Subscriber

    Bernoullis principle I believe.

    Premier Icon Drac
    Subscriber

    Lorry sides are flat so you don’t get the lift just suction, hold two strips of paper close to each other and blow they get sucked in together.

    jock-muttley
    Member

    The air inside the lorry is at a lower pressure than the air moving past the side which as it’s moving will be at a higher pressure ergo pushed in…

    I think you are both missing the point here.
    Why do lorry curtains do the opposite of what they should according to the Bernoullis principle ?

    IanMunro
    Member

    Dunno about the lorry but the wing image of lift creation appears to be wrong. The wing’s looks like it’s got a negative angle of attack. Lift isn’t generated by air having to flow faster over a top surface. Otherwise aircraft wouldn’t be able to fly upside down.

    cynic-al
    Member

    MidlandTrailquestsGraham – Member
    I think you are both missing the point here.
    Why do lorry curtains do the opposite of what they should according to the Bernoullis principle ?

    They don’t.

    Pressure outside > pressure inside (presumably air can escape more easily at the back than the front).

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    IanMunro – Member
    Dunno about the lorry but the wing image of lift creation appears to be wrong. The wing’s looks like it’s got a negative angle of attack. Lift isn’t generated by air having to flow faster over a top surface. Otherwise aircraft wouldn’t be able to fly upside down.

    They fly upside down with the use of flaps/ailerons rather than the Bernoulli effect of the wing.

    spooky_b329
    Member

    Turbulence from the cab (wings don’t like turbulence, they stop creating lift) and air being sucked out the rear doors, creating low pressure inside.

    Also his air deflector is too low, I’m not an expert but its probably the turbulence from the exposed headboard that is creating the strong ripples at the top of the curtain.

    Premier Icon Onzadog
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    Anything to do with a turbulent boundary layer?

    Premier Icon epicyclo
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    MidlandTrailquestsGraham – Member
    I think you are both missing the point here.
    Why do lorry curtains do the opposite of what they should according to the Bernoullis principle ?

    Interesting – never thought about it before.

    Must be something to do with turbulence. Maybe the lorry isn’t going fast enough for a smooth flow along its sides.

    …air being sucked out the rear doors…

    This was my guess.
    If there’s any gaps in the bodywork, then the venturi effect of the air passing over them would create low pressure inside the trailer.
    It was just a guess though. I wondered if anyone actually knew the answer.

    stumpy01
    Member

    MidlandTrailquestsGraham – Member

    The air on the outside of the curtain is moving at the speed of the lorry.
    The air on the inside is near enough stationary.

    So, if air lifts an aeroplane wing, why doesn’t it suck the curtains outwards ?

    How is the air on the outside of the curtain moving at the speed of the lorry? This isn’t true, except for a very thin boundary layer close to the body – probably less than a few cms at most.
    The lorry is pushing itself through the stationary air (assuming local wind conditions are zero).

    You are saying that a rectangular object (lorry) should pass through the air in the same manner as a teardrop shaped object…..
    This ‘lift’ principle has caused issues for manufacturers’ in the past. Probably get some details wrong here, but the original Audi TT didn’t initially have a spoiler. The spoiler was quickly added along with EPS, after (I seem to remember) several high speed crashes caused by excessive lift from the body shape. Not hard to see why, if you look at the original TT in profile…

    IanMunro
    Member

    Why do lorry curtains do the opposite of what they should according to the Bernoullis principle ?

    Because Bernoullis principle originally applied to venturi, and a lorry ain’t a venturi. That’s not to say that Bernoullis principle can’t be applied to some part of the lorry’s fluid dynamics, but I dare say there’s a lot more going on.

    So for instance, they may be a much higher area of low pressure on the roof of the lorry than on the sides producing a net low pressure within the trailer causing the sides to suck in.

    IanMunro
    Member

    They fly upside down with the use of flaps/ailerons rather than the Bernoulli effect of the wing.

    Are you saying a different set of physics apply when a wing is upside down? 🙂
    ‘Bernoulli effect’ doesn’t know what shape the wing is, or which way up it is. Wings don’t generate lift due to Bernoulli effect. It’s a lot more complicated, but the ‘Bernoulli effect’ is a oft taught lie to simplicate a far more complicated truth.

    Premier Icon aracer
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    cynic-al wrote:

    They fly upside down with the use of flaps/ailerons rather than the Bernoulli effect of the wing.

    I very much doubt aircraft use flaps when flying upside down (and most aircraft which fly upside down a significant amount probably don’t have any), and ailerons act in opposite directions on each wing, so if one was generating lift the other wouldn’t. In fact aircraft fly upside down by having an angle of attack on the wing which directs air downwards (and hence creates lift by Newton 3). Strangely enough that’s also how they fly the right way up – despite what most people understand, Bernoulli’s principle contributes only a minor proportion of the lift.

    Premier Icon aracer
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    Eeen wrote:

    the ‘Bernoulli effect’ is a oft taught lie to simplicate a far more complicated truth.

    That always seems strange, given that the truth is actually a lot simpler. I think it’s just a repeated myth which has gained credence due to the amount it’s been repeated (as above, it’s not quite a lie, as the effect does contribute to a minor extent).

    stumpy01
    Member

    If you look at most aircraft flying upside down, they do so with a pronounce tail down attitude.
    This maintains the ‘angle of attack’ to the airflow, which will still generate lift but not as much as is created when the aerofoil is the ‘right way up’.
    A flat board will generate lift if presented to the airflow at a slight angle – it just doesn’t do it as well as an aerofoil form.

    Most aircraft can still generate lift with a slightly nose down attitude as the wing is set at an angle to the fuselage.

    I should remember more about this, given that my dissertation was about wings in ground effect and how ground effect affected lift.
    Haven’t used any of it since graduating 14 years ago though, so it’s very woolly!

    cynic-al
    Member

    *removes “armchair-engineer” pin-badge and commits hari kiri with it*

    Premier Icon aracer
    Subscriber

    stumpy01 wrote:

    This maintains the ‘angle of attack’ to the airflow, which will still generate lift but not as much as is created when the aerofoil is the ‘right way up’.

    As an expert on this, is that because the airfoil is more efficient the right way up, or is that actually Bernoulli? Anyway, don’t aerobatic aircraft have symmetrical section wings (and presumably little to no inbuilt angle of attack), which work just as well both ways up? I’m fairly sure that is the case for many models at least.

    Premier Icon tomhoward
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    What if it was on a conveyor belt?

    Premier Icon Rusty Spanner
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    I’ve checked on the Daily Mail website:

    Apparantly, it’s due to all the illegal immigrants in the trailer breathing in at the same time.

    project
    Member

    Curtain sides are just that curtains, to keep the stock secure and dry, by aaplying more tension to the side straps the curtain could be made more taught,but then you would most probably buckle the roof.

    There is also the effect of the wind being blown round the front of the truck and swirling down the sides, also cross winds have an effect.

    izakimak
    Member

    If the curtains are tensioned properly they don’t get sucked in and if the roof support pillars are in the correct place the roof won’t flex either.

    khani
    Member

    If the lorry was on a conveyer belt would the sides still suck in?…

    mrben100
    Member

    I struggle with long sentences i would imagine the cab is causing turbulence meaning the air isn’t flowing “smoothly” over the trailer. An aeroplane wing would not have this problem.

    Apologies if any else has already written this.

    Edit: therefore there would be a relative increase of pressure to the outside

    Premier Icon sparksmcguff
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    Don’t know if this adds much but my brother was talking about his time in the OTC recently and about how if you are sitting in the back of one of those army trucks the low pressure inside the covered area of the truck sucks air into the back of the truck. The resulting mix of exhaust smoke and cigarette smoke is apparently very unpleasent.

    stumpy01
    Member

    aracer – Member

    stumpy01 » This maintains the ‘angle of attack’ to the airflow, which will still generate lift but not as much as is created when the aerofoil is the ‘right way up’.

    As an expert on this, is that because the airfoil is more efficient the right way up, or is that actually Bernoulli? Anyway, don’t aerobatic aircraft have symmetrical section wings (and presumably little to no inbuilt angle of attack), which work just as well both ways up? I’m fairly sure that is the case for many models at least.

    Well, a flat board with a positive angle of attack will generate lift – just not much.
    An aerofoil is designed for it’s particular use; so for example, heavy lift transport will have quite a thick wing section giving it a pronounced aerofoil shape to provide plenty of lift at low speed (at the cost of increased drag), while something like a fighter jet will have a much shallower aerofoil profile as it will be able to generate plenty of lift at the higher speeds it will be travelling at.

    So, an aerofoil will work best in it’s design condition (which isn’t generally upside down), but it will still generate lift when upside down if presented to the air at enough of an +ve angle.

    Yeah, I think you are right – aerobatic aircraft do seem to have symmetrical aerofoil patters (or at least very close to), so when flying with the wing at a level attitude (probably slightly nose down) the wind wouldn’t generate any lift.

    To keep things simple for the CFD modelling I did on my dissertation, I chose to use a symmetrical wing as it made putting it all into the CFD software a lot quicker. I found an aerofoil generator online that was meant for model aricraft. You put in the NACA code of the wing you wanted and how many points you wanted and it gave you the co-ordinates as a tabulated file.

    This isn’t the one I used, but is basically the same thing:

    http://airfoiltools.com/search/index?m%5Bgrp%5D=naca4d&m%5Bsort%5D=1

    andyl
    Member

    My thinking: Get a plastic bag stretched out on an oven shelf and blow at it and you will see the bag going into the gaps.

    When the truck drives through the air it is displacing it. The air is being pushed back at the truck by the atmospheric pressure. As the sides of the truck are all saggy then the air is basically blowing back into the side of the truck. Add that to all the turbulence which means there is no nice turbulent boundary layer to follow all the rules. If there were serious air leaks like an open back then things will be different again.

    now if your truck was travelling at Mach 1 then things would be different as the air doesnt have time to respond.

    globalti
    Member

    Spooky was the first to get it – massive area of low pressure behind the lorry (drafting, anyone?) is sucking air out of the doors and creating a light vacuum inside, enough to pull the curtains in.

    The air will be flowing fairly smoothly along the sides – a big truck or bus has a much lower coefficient of drag than a motorbike.

    CountZero
    Member

    A flat board will generate lift if presented to the airflow at a slight angle – it just doesn’t do it as well as an aerofoil form.

    An F-104 Starfighter being a case in point; the wing’s not much more than a razorblade, with anhedral, it’s pretty much its forward velocity keeping it in the air, doesn’t much matter which way up it is.
    Until the fire goes out…
    ‘How do you get hold of a Starfighter? Buy a patch of ground, and wait…’ 😉
    In reality, I think the 104’s wing is a flat diamond in section.

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