Title text:
Carefully maneuvering the balloon down a mineshaft in an effort to break the OTHER altitude record
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3161/
You can’t measure airspeed in a balloon
Balloons don’t carry such instruments, but they do experience airspeed. Balloons can climb and descend at over 500fpm. We experience vertical “wind” at those speeds.
Balloons are tall enough that the envelope can be above a wind shear, while the basket can be below. I’ve experienced 15kt shears, enough to deform the bottom of the envelope into a “question mark”.
On most flights, pilots will experience 5-7kt shears at certain times.
Sure you can. Just need a pitot tube!
The Pitot tube doesn’t work as the ballon is moving (more or less) with the wind, i.e. both pressure tubes, the one for static pressure and the one for total pressure, in the Pitot tube experience (almost) the same pressure, resulting in measuring (almost) zero dynamic pressure and thus velocity.
It records an air speed of 0. Isn’t that working exactly as intended?
Isn’t one usually interested in the speed relative to the ground?
No. The comic is talking about airspeed: speed relative to the wind.
Ah, ok. I somehow got that wrong.
You can, it’s just usually zero, which is what I think the comic is suggesting
What if they ascended to normal altitude, then lowered a tether back down to the ground? Probably break some records as the wind swung you down to the ground.
I don’t think that would be level-flight
Tether yourself on three sufficiently far apart points with no slack in the rope, and you will be stationary
It would be wind speed wouldn’t it.
Airspeed of hot air balloons would be very similar to wind speed, except for when there’s a quick change in wind speed as the balloon would have some momentum
What’s the world manned uptime record?
Randall isn’t a hot air balloon pilot.
Most balloons are about 100’ tall. The difference in wind speed between the surface and 200’ AGL can be 15kts. (it can be much greater, of course, but shears that strong are below flight minimums).
It is not at all unusual to descend through a shear such that the envelope is in 20kt winds, while the basket is in 5kt winds. It’s rather scary, actually, because our aerostatic aircraft start experiencing aerodynamic effects: “False Lift”. These effects only exist while the balloon is crossing the shear. Once it passes through the shear, all that aerodynamic “false” lift disappears, and the balloon starts sinking like a rock.
Yep, the last two comics are quite fundamentally wrong. It’s as though he didn’t fact check them in the most basic way. Slightly disappointing
The document forgery one? Isn’t that just a joke? What exactly do you fact check about a hypothetical forged document?
Ah sorry, I meant the one before that with the continents and space.
Ok… Plate techtonics were widely accepted in the late 1960s according to my search, which is exactly what the previous comic claims (make sure to read the image text).
What did he get wrong?
I think it was just meant to be a joke although I did learn something about hot air balloons… enough to know that I’m never going in one
Is airspeed relative to wind speed at ground level? Or relative to wind speed the craft is experiencing?
I imagined it would be relative to wind speed at the craft, as measured by some instrument. Which would make the comic at least true in a general sense, as it does state the altitude is constant?
Edit: yep, air speed is relative to the air the vessel is moving through
Edit: yep, air speed is relative to the air the vessel is moving through
Just to complicate it a little bit, “Airspeed” usually refers to “Indicated Airspeed”, which is provided by measuring the ram air pressure into the pitot tube relative to the static air pressure. It’s a measure of dynamic pressure rather than actual speed. It’s how fast your wings think they are moving through the air.
If your wings need 100kts in the thick air at sea level to lift off the runway, they will need to “think” they are moving at 100kts when you get into the thin air at 30,000ft.
Depending on altitude, you might actually be moving at 400kts past a weather balloon, but your “Indicated Airspeed” might only be 100kts.
Putting my flight instructor hat on here, this is mostly correct.
“Indicated airspeed” (IAS) is what number your aircraft’s airspeed indicator is pointing to. As Rivalarrival described, the airspeed indicator is a barometric instrument that compares ram air pressure with static pressure to measure the dynamic pressure, which is a function of airspeed. Indicated airspeed is an indication of how the airplane will “feel,” how much force will act upon the aircraft in maneuvers, which is why force limit speeds such as maximum flap and landing gear extended speeds, stall speeds, max normal operating speed and never exceed speed are marked on the airspeed indicator.
“Calibrated airspeed” (CAS) is indicated airspeed corrected for instrument error. The airspeed indicator and the plumbing it is hooked to aren’t perfect, so they’ll be off by a few knots especially near the lower edge of its range. You find a chart in the POH that says “IAS 45, 50, 55, 60 etc” on one line and “CAS 43, 49, 54, 60 etc” on another. Pilots use this for, if we’re being honest with ourselves, nothing.
“True airspeed” (TAS) is indicated airspeed corrected for air density. The airspeed indicator is flawed in concept: It’s a pressure gauge calibrated in units of speed. To actually determine the relative velocity of the aircraft through the air, we have to do a bit of math comparing the outside air temperature with our pressure altitude, this will give us our density altitude. You then do a bit more math to correct calibrated airspeed for density altitude and get true airspeed. E6B flight computers have little windows for this.
Here is my old cardboard E6B from when I was a student. I’ve set an air temperature of -40C over 30,000 feet in the right-hand window, the center window is showing…pretty much exactly 30k feet of density altitude, and we can read true airspeed over calibrated airspeed on the A and B scales. So for 100 knots, we can look at the 10 on the B scale, and read about 164, maybe 165 knots on the A scale. At 30,000 feet and +50C, which literally never happens, your density altitude is ~38,000 feet and 100 KCAS will get you 194 KTAS. Not quite 400 😜
“Ground speed” is true airspeed corrected for wind. To calculate your ground speed, you need your true airspeed as we just calculated, and winds aloft forecasts from one of the government agencies the Republicans are desperate to destroy, and then we do some trigonometry. You can whip out your Ti-83 Plus Silver Edition from high school and SohCahToa this bitch, or you can flip the E6B over to find a handy dandy vector plotter, which does ground speed and wind correction angle calculations by accurately drawing and measuring the triangle. My high school physics teacher called using this thing “cheating,” I call it “a required aeronautical skill.”
That was very interesting! Thank you for taking the time to write this up
Fun fact: In several episodes of the original Star Trek series, Spock can be seen holding and using an E6B flight computer, which is a slide rule invented for the US Navy in World War 2.
At 30,000 feet and +50C, which literally never happens, your density altitude is ~38,000 feet and 100 KCAS will get you 194 KTAS. Not quite 400 😜
This is why I’m a balloon pilot and not a fixed wing pilot. 30,000 is 12,001ft higher than I’ll ever see, and 30kias will probably collapse my envelope and splatter me in a corn field. 😵💫
and winds aloft forecasts from one of the government agencies the Republicans are desperate to destroy,
Can confirm: the accuracy of forecasts in general (and vertical wind profile data in particular) plummeted for this year’s flying season. The GOP is needlessly endangering aviation safety on multiple fronts.
30 knots indicated would take a plane out of the sky too, that’s well below most airplane’s stall speeds. We can probably fix that before doing the Monsanto Slam though.
I imagined it would be relative to wind speed at the craft, as measured by some instrument. Which would make the comic at least true in a general sense, as it does state the altitude is constant?
That is the misconception I am trying to address. Check out this wind report:
1kt of wind on the surface, 5kts at 100ft. My balloon is 120ft tall. My balloon is experiencing those 5kt winds while I’m floating inches off the ground. If I have an airspeed indicator in the basket, it’s going to be reading 4kts. (Actually closer to 5kts due to the change in direction as well as speed)
How about if I’m at 150ft in the basket, in 5kt winds. The top of my balloon is 270ft, in 12kt winds. My airspeed indicator is going to be reading 7kts. I’m going to have 7kts of wind in my face.
I’m trying to point out that the height of a balloon is very often larger than the gradient between two different air masses.
I have experienced 15kt shears: my basket is hanging in an air current 15kts slower than the air current that my envelope is riding within. I have felt 15kt winds in my face while riding in a balloon that is carried by the wind.
I see, that makes a lot of sense. Though perhaps could be “corrected” by placing the instrument at the level on the balloon that receives the average wind speed? For the sake of the comic of course
