I’m a country-boy physicist, grew up on a farm, doing farm things, working with farm things. Most of the other physicists I know are city-boys.
Coming home on a commuter flight recently, on a Dash-8 (turbo-prop), a discussion about physics between me and a city-boy physicist friend came up. I asked him if hew knew how a big plane “brakes” in order to slow down once it gets planted on the runway?
I went to school with this fellow and he got just as good of marks as I did, and does research just as complex as mine.
So the first answer from this fellow, of course, was that the airplane just uses normal wheel brakes, on the wheels of the plane, and that’s why it makes the “bark” when it lands.
So, I told him that the bark he hears is actually just from the non-rotating wheels making contact with a very fast moving surface, and so this is kind of like doing a “burn-out”, or popping the clutch, in your car and making that sound of rubber skidding on asphalt. The brakes aren’t ON as the plane lands!
So then he said, “Well, it’s after that that the brakes come on, then.”
“Sure”, I said, but followed, “Don’t you hear the change in sound coming from the engines?”
“Oh right”, he said, “sorry I forgot, the engines reverse too.”
I said, “Right, the engine reverses thrust, but do you know how it does that?”
He said: “They reverse the direction they’re spinning.”
And THAT is the difference between a farm-boy physicist, and a city-boy physicist. It actually makes me wonder how city-boys can get by in physics at all if they have such a profound lack of physical intuition. Do they truly understand the mechanics and the forces and the energy behind the concepts they are otherwise able to write physics exams on? It’s kind of strange, isn’t it?
I wonder if it is predominantly city-boys who believe in the greenhouse effect, and country-boys who understand that you need the real-time Sun to actually make physics happen. I’m sure there are exceptions to this, of course…I just thought this was very interesting.
If you are able, please comment below about what was wrong with his answer, and why a farm-boy would get it, but a city-boy might not:
I’m not sure you can make such a broad analogy between city-boys and farm-boys. I think the problem is that your friend based his answer on an assumption (just like the climate debate). He then compounded his error by assuming his assumption was good enough to be able to make an assertion. A better understanding or knowledge of (propeller) aerodynamics and pitch control mechanisms would have avoided the necessity to assume. 🙂
I suppose one could argue that the farm-boy lives in the real world and usually has to try to fix stuff himself but that is probably too uncharitable. Many sensible folk live in the city! This is where an analogy between the ‘city’ and ‘climate science’ ends…
I’m surprised at you both. Everyone knows that the plane slows down after landing because of CO2. The 0.0004 of the atmosphere takes a break from heating the entire global surface by +33K and rushes in front of landing aircraft. I would have thought the increased drag from this local density anomaly would have come to you intuitively.
I’ve built a few computer models, all reviewed by my ‘scientist’ friends, which prove this effect to >95% certainty. Fair enough, they didn’t work initially; however, when I gave the CO2 molecules capes and little masks there was nothing they couldn’t do.
Do you think I might win a Nobel award?
[“Do you think I might win a Nobel award?”]
Well, as your theory is as feasible as some others around, I’d say you’re in with a good chance! 🙂
Thanks for keeping it light…
A physicist who does not know about reversing props? Or the inability to reverse turbines? Does he know that cooler can’t make warmer? Was he a community organizer or something like that with an honorary physics degree from Metropolis U with a job in government saving the world?
The answer is obvious. Your colleague is typical of the problem that the world is being subjected to at an ever increasing and alarming rate – overwhelming numbers of individuals that are educated beyond their intelligent capabilities. They are for the most part living off memory script having lost their way. For many us that have spent decades in the professional world quickly note that anyone can obtain a degree in any profession, smart or dumb. But the real question becomes, is the individual any good and productive or just floating along in some fantasy world? The real discriminators are natural curiosity and pure common sense. Your youth provide you with the environment where common sense drove the day and along the way instilled in you the concept of shame – most, like your colleague lacks this human trait. One might conclude that he, like so many, are true liberals who can only function when in pursuit of some ideology and do so without question. Don’t be discouraged, just accept reality and continue as you do.
Joe, I’m a Train Driver.
I guess working with mass, velocity & momentum I know that something with as much energy as a Jet turbine could never actually ‘reverse’ whilst going forward without it becoming a ‘former Jet turbine’ !
It would be like putting a car traveling at 300 mph in reverse gear.
I don’t know if that answers your ‘city boy’ – ‘country boy’ quiz but the answer to ‘reverse thrust’ is that the jets exhaust gases are redirected.
Exactly, Peter. I thought anyone should get it but I guess it makes sense that only people who’ve worked with machinery before know that if you try to reverse that amount of momentum in that short of time, you simply get destruction!
I mean this conversation went on AS we watched the prop! The fellow was watching it…I told him to watch it because I hoped he would see the pitch angle of the prop change. Instead, somehow he thought the prop reversed the direction it was rotating!
[“I don’t know if that answers your ‘city boy’ – ‘country boy’ quiz but the answer to ‘reverse thrust’ is that the jets exhaust gases are redirected.”]
Just to clarify, there is no alteration of jet exhaust direction. The exhaust gasses power a turbine which in turn rotates the propeller. After landing, the pilot selects reverse thrust on the propellers (and there is normally a safety catch or quadrant to prevent inadvertent selection). This alters the pitch angle of the propeller blades, reducing the angle through the ‘fine’ angle to the ‘braking’ angle. Once in the braking range, the propeller thrust vector acts in the opposite direction to normal and the (now reverse) thrust slows the aircraft instead of driving it forward. After landing you may not only hear the blade pitch change but also the throttle increase because now the engines are driving the reverse propeller and increased throttle increases the braking. Once the aircraft has stopped, it can actually be taxiied backwards by use of this pitch change, unlike a jet.
Arfur, on jet engines they do actually re-direct the jet exhaust forward, and this too produces negative thrust: http://en.wikipedia.org/wiki/Thrust_reversal#Jet_aircraft
Excellent description of how the prop version works!
Does this mean we have a consensus? Perish the thought! 🙂
Some planes with props have reverse pitch, the props turn the same way but the pitch of the props is reversed. Jet a/c with engine braking pave deflectors that push the jet wash forward. In no way would it be possible to reverse direction of rotation.
@Augie – Thanks for that thoughtful reply, it is exactly correct.
A jet aircraft can and has used reverse thrust to back up the aircraft in the terminal area. See this youtube video.
Cool! Clamshell design.
Clamshells to reverse thrust on jets engines have been around for decades. After touchdown, the jet engines ALWAYS rev up big time while the plane sheds speed. After slowing significantly, the engines rev down, the clamshells are stowed, and the low thrust forward used to cruise to the terminal. Brakes indeed are used for stopping at the terminal.
Some aircraft also deploy air brakes on landing – a portion of the upper surface of the wing is raised to a near vertical position for maximum effect. Air brakes may also be used at shallower elevations during descent – partly to reduce velocity, but also to lose height more rapidly by reducing wing lift and counteract the ailerons – it has the advantage that lift can be rapidly restored should a stall threaten. This video shows both functions in operation on a 777:
Brakes on the wheels are only used at rather lower speeds – sometimes to avoid overshooting a taxiway turnoff – when air brakes would have little effect as some basic Newtonian mechanics will confirm.
And here’s a prop plane reversing under its own power: