Why the Cybertruck is in Nevada = SAE J2807 Towing Test

[PilotPete]

But the point is, the weight of the trailer does change. That just means that you could pull the trailer empty or full and it won't change anything besides braking. I can confirm that this is real. Plus how much does a trailer full of live chickens weigh anyways. Can't be more than a few hundred pounds total

Watch the video if you don't believe me. The weight of the trailer does NOT change. Demonstrated with live birds and a model helicopter. I understand how someone might think it would, but it doesn't. The birds are not levitating, they are flying. The vertical vector of lift requires two forces to overcome gravity. A higher pressure below and a lower pressure above. Off the back of an airfoil there is a downwash. Above is a lowered pressure. While that energy become spread out over distance, it does not disappear. eventually, it is felt by the floor of the trailer in a force equal to the amount of lift created. IF the trailer were an open trailer, and IF the trailer were traveling fast enough for the downwash of air to miss the trailer floor and hit the ground outside the trailer, and there was no roof, then yes, the trailer would feel less weight. But within a closed trailer, there is no free lunch.

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[windydrew]

I feel like they should have taken a baseline with the birds not in the trailer and then put them in to land, then start the stir arms. I feel like they shouldn't have used pigeons because I doubt they can even pick up the weight difference between flying or not. Chickens would have much more weight. But I do feel like I'm underestimating the wind force from them flying so maybe it doesn't matter. Unless they were tethered and provided lift force......

 

[PilotPete]

I feel like they should have taken a baseline with the birds not in the trailer and then put them in to land, then start the stir arms. I feel like they shouldn't have used pigeons because I doubt they can even pick up the weight difference between flying or not. Chickens would have much more weight. But I do feel like I'm underestimating the wind force from them flying so maybe it doesn't matter. Unless they were tethered and provided lift force......

When you look at their data, you can see the pulses of wind hitting the floor. They refer to that data as “noise”. When the helicopter flies, the load is constant with little “noise”. However, when the helicopter lands, you can see a huge spike, both positive and negative, showing the vibration of that little helicopter on the trailer. That is how accurate their gear was. The baseline was the birds sitting still. There wasn’t sufficient deviation from that number to need a comparative baseline. As to weight, chickens weigh more, but are much harder to get to fly. Turkeys would have weighed even more. Like I mentioned, it’s one of those stories that gets brought up in every PHY101 class.

Now if you want to think about a PHY 201+ story, I have one about energy exceeding the speed of light by moving from a straight line (shortest distance) into a wave (longer distance) but taking the same amount of time to get from one point to another…

 

[CyberGus]

When you look at their data, you can see the pulses of wind hitting the floor. They refer to that data as “noise”. When the helicopter flies, the load is constant with little “noise”. However, when the helicopter lands, you can see a huge spike, both positive and negative, showing the vibration of that little helicopter on the trailer. That is how accurate their gear was. The baseline was the birds sitting still. There wasn’t sufficient deviation from that number to need a comparative baseline. As to weight, chickens weigh more, but are much harder to get to fly. Turkeys would have weighed even more. Like I mentioned, it’s one of those stories that gets brought up in every PHY101 class.

Now if you want to think about a PHY 201+ story, I have one about energy exceeding the speed of light by moving from a straight line (shortest distance) into a wave (longer distance) but taking the same amount of time to get from one point to another…

Yeah but what if I'm hauling a trailer full of EmDrives

 

[BaddaBing]

Just guessing TRI Motor comes in under the IRA $80K.
A bargain if you ask me!

Thanks for your effort but I am no farther ahead. If I wanted a guess I would have asked the old man sitting on a park bench feeding the pigeons.

 

[rudedawg78]

Thanks for your effort but I am no farther ahead. If I wanted a guess I would have asked the old man sitting on a park bench feeding the pigeons.

You get speculation because the questions you asked have not been officially answered. Hopefully, your questions will be answered in the coming weeks/month.

 

[WHIZZARD OF OZ]

Thanks for your effort but I am no farther ahead. If I wanted a guess I would have asked the old man sitting on a park bench feeding the pigeons.

Elon could tie an accurate message to the pigeon.

 

[Scott Beall]

That’s a fun physics question, but the short answer is, even if they fly, the weight is the same.

Not if you put them on the roof, like Gov. Romney's dog. Then they'd add lift reducing the trailer weight and maybe a little forward momentum.

Who needs an aux solar panel on the truck when you have a cage of chickens?

 

[CyberGus]

Thanks for your effort but I am no farther ahead. If I wanted a guess I would have asked the old man sitting on a park bench feeding the pigeons.

But Whizzard IS an old man sitting on a park bench feeding pigeons

 

[JBee]

When you look at their data, you can see the pulses of wind hitting the floor. They refer to that data as “noise”. When the helicopter flies, the load is constant with little “noise”. However, when the helicopter lands, you can see a huge spike, both positive and negative, showing the vibration of that little helicopter on the trailer. That is how accurate their gear was. The baseline was the birds sitting still. There wasn’t sufficient deviation from that number to need a comparative baseline. As to weight, chickens weigh more, but are much harder to get to fly. Turkeys would have weighed even more. Like I mentioned, it’s one of those stories that gets brought up in every PHY101 class.

Now if you want to think about a PHY 201+ story, I have one about energy exceeding the speed of light by moving from a straight line (shortest distance) into a wave (longer distance) but taking the same amount of time to get from one point to another…

Do tell.

Speed of light is only restricted, "if" you limit the wavelength like they like doing in textbooks. You could argue that time doesn't exist as a constant, therefore rendering any misconception about light speed being a constant, as also void. In reality time is just a sequence of events, but not determined by more than the fluidity of the fields we operate in.

Electricity travels at the speed of light (300,000,000m/s) but it takes a second for the same electron to travel a meter through a cable. It's a long way between particles with lots of detours when travelling through a solid. Hence Starlink using a a "wireless" medium having lower latency.

I feel like they should have taken a baseline with the birds not in the trailer and then put them in to land, then start the stir arms. I feel like they shouldn't have used pigeons because I doubt they can even pick up the weight difference between flying or not. Chickens would have much more weight. But I do feel like I'm underestimating the wind force from them flying so maybe it doesn't matter. Unless they were tethered and provided lift force......

Arguably, the Myth-busters test results are, like all good science, subject of the test procedure used.

In German there is a good saying for this: "Wer mist, mist mist". Which translated means "who measures, measures rubbish". A very common phrase in electronics.

The easiest way to understand what's happening is to consider that the birds and copter are acting inside of a closed system, aerodynamically speaking. The walls of the trailer contain all of the resulting airflow that lifts the birds or helicopter, meaning their flight can only impact on the air inside the container. Had they done a test in a driving vehicle with a caged container that would allow air flow through the container where the birds were flying, the results would have been different.

First up the helicopter does not have a typical wake profile and downwash is in a helical form, centralised around the axis of the main rotor. Further the helicopter never leaves the wing in ground effect altitude in the test, and effectively exerts all of the downward pressure from the lift it is creating directly on the floor. This means that without outside airflow the trailer is a closed system and the force contained within it stay the same.

The same can also be said about the birds, however their stationary flight relies heavily on manipulating turbulent airflow under the wings to create lift. The helicopter does not do this once it's out of ground effect. Most larger birds can only "hover" for a short period, and typically transition to forward flight as fast as they can as lift is less energy consuming at speed for them. (which btw is a good trick to avoid birds attacking your drone, you just simply increase altitude, and they give up pretty quick because they can't sustain the same climb rates)

The same can be said for the birds net "air deflection force" however, in that it is contained within the closed system of the container.

Now in good Mythbuster form we should also consider how to achieve the result. So if the bird where in a cage, and you drove them down the road and they took flight, then depending on their altitude over the bed, and the resulting airflow from behind the cab, it might well be that the aerodynamics are such that the birds, say flying on the side of the cages, only produce a small amount of downforce on the truck bed, which is less than if they were all sitting on the bed floor. This is of course due to the fact that it is now not a closed system, and the outside aerodynamics interact with the birds flight.

It's probably good to point out that lift isn't generated, as commonly thought by pressure differential, rather by the force generated by deflecting air mass into a different path. The pressure differential is only the result of the deflection of air, but not the cause. Imagine using a large spoon as a paddle in the water, you moving a "mass" of water in the spoon creates a force that acts in the equal and opposite direction. The same applies for lift, in that the motion of a airfoil through the air, displaces air mass downwards and produces a force that counteracts the force of gravity, by pushing up against it.

So accordingly, the airflow being deflected from the birds flight in a cage version truck would at least partially miss the truck bed. You could also imagine a cage that is wider that the truck bed, or a truck bed that is clear to the ground in between the wheels, that would produce similar results. In fact the deflected air off the top of the cab might also help reduce the bird mass of the truck.

Now in saying all that there is a difference between mass and downforce. They are not the same, and a reverse argument might make it clearer as to how it works.

For example a F1 car can produce more than it's own body weight in downforce on the track, to get better traction. But that doesn't mean that it has to budget that extra downforce in fuel reserves, in order to lift it whilst going up a hill. The downforce will create extra drag on the drivetrain and wheels etc just like an actual mass, and also through induced aerodynamic drag that also creates the negative direction of lift. But it will not require energy to create a force to lift the vehicle to a different altitude, because there is no extra mass it needs to lift that is created by the downforce.

Hope this helps.

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