Chowder, if you were in Northwest Indiana and I had the money to rent an airplane I’d be more than willing to take you up and show you how this works in the real world. Unfortunately, I’ve been out of work since November so I’m a bit short on cash, and in any case, you’re across an ocean.
But really, once you experience this in real life it makes a lot more sense.
Heh 
Part of the problem is that we passengers/non-pilots usually do tend to think of groundspeed. That’s just what it sounds like - it can be thought of as the speed that a ground-based observer would measure for a plane passing overhead. It’s useful for us because it answers the question “How long will it take to get from A to B?” That, of course, is the pressing issue for most of us, not considerations of aerodynamics and the like - we just buy our tickets and get herded on a plane. Making the thing fly is someone else’s problem.
Airspeed is the speed at which the wings and surrounding air are moving relative to each other. That’s what concerns pilots, really, because it determines whether or not the plane stays aloft, and how it behaves in the air.
Well dagnabbit 
My intended 3 weeks holiday this year was back to the good 'ol US of A but as I reported in a thread some while ago I won a holiday, courtesy of my car insurance, so on August 23rd I’ll be going to Guernsey.
Across an ocean…pshaw…means nuffink mate so maybe next year I’ll take you up on the offer, I’ll pay.
Hope you find yourself a job before then, keep your pecker up buddy, nil desperandum
Unfortunately, (for me) due to the restrictions on my license, you can NOT pay - you’re not allowed to! Well, OK, technically you can pay half the hourly cost of rental, but I’d be happy if you just buy me lunch (much cheaper).
To be honest - I’d be just as happy to visit the UK and take you flying there, as my license is valid there as well as here, but the last I heard renting a Cessna was about three times as costly on your side of the pond than on mine.
I’m working on it, thanks for the encouragement.
I’ll have a go, though I don’t know if I can help anymore than the others.
Once an aeroplane is airborne its movement is entirely independant of the ground. Take the ground away and absolutely nothing changes with the aircraft*, the only thing of concern to the aircraft is the air mass it’s moving through. That air mass may be doing all sorts of things relative to another object such as the ground, it could be moving over the ground at a thousand mph but as far as the aircraft is concerned, its engine is chugging away pulling itself through the air at 100 mph, and that’s all that matters. So airspeed, the speed of the aircraft through the surrounding air, is the speed that makes the aircraft fly.
Movement of the aircraft over the ground is simply the resultant vector of its own speed through the air and the movement of the air over the ground (wind.)
So an aircraft with an airspeed of 100mph through air that happens to be moving at 100mph in the opposite direction over the ground, your hypothetical 100mph headwind, will be stationary relative to the ground but will still be moving at 100mpg through the air. The same aircraft with a 100mph tailwind will still be moving at 100mph through the air but will have a 200mph speed relative to the ground.
The ground speed becomes important to pilots and passengers because ultimately we’re navigating from one ground feature, such as an airport, to another and so navigation must take into account the effect of the current wind relative to the ground but navigation is not what makes the aeroplane fly.
*This is of course completely false, take the ground away and you have no Earth which means no atmosphere for us to fly through. Ground can also cause turbulence and otherwise disturb the airflow, but mainly only at low altitudes. Also, it is thermal activity on the ground that creates the weather patterns in the first place, but if you can ignore all that for the moment, the point still stands.
That’s not really true. Or at least it’s not as true as you think it is. You may be used to the wind you experience on the ground which is generally influenced heavily by the ground itself and objects on it such as buildings. These things break up the flow of the air and cause turbulence. Once you get high enough to be in air not influenced by the ground you’ll find that it is a solid block of atmosphere moving at a given speed. There are certainly changes in the way it behaves over distance but those distances are normally too great to have any effect on our aircraft. Generally speaking, whenever you are flying and the conditions are smooth, you are flying in a large block of atmosphere moving at a given speed, sometimes that speed can be well over 150 mph in the jetstreams.
So it is quite reasonable to consider the OP’s question to involve a steady wind with no turbulence.
Just to clarify this a bit: The typical speed of any given molecule in the air depends on the temperature, and is always faster than the speed of sound. This is true even in what appears to us to be “still” air. When you’re talking about windspeeds, you have to look at parcels of air much, much bigger than individual molecules, at which point most of this high-speed motion (which is in random directions) cancels out.
Back to the original question, it might (or might not) help to look at it this way: Do everything in the air’s frame of reference. In this frame of reference, the air is, by definition, at rest, but the ground is moving at 100 MPH. But the airplane doesn’t care that the ground is moving at 100 MPH; it just cares about the air, which is stationary.
Leans back on chair with a big smile
My job is done then. 
That is not exactly right, while the aerodynamics of an airplane are dependant on the air volume it is moving through, the whole of the flight variables are not; most importantly gravity and the planes momentum are independent of the air motion.
As an example, if you have a plane doing 100 mph IAS (Indicated air speed, what the speed gauge on the plane says) into a 100mph wind (so it’s stationary relative to the ground). In this situation while the plane is flying it doesn’t have any momentum, so if you suddenly remove the head wind, the plane drops like a rock until it can pick up speed.
Awww *uck it 
, here’s a video of my lightest (23 grams) plane strugling with a brisky breeze: Youtube link You can see how it barely makes forward progress at the beggining, any time it faces the wind headon it hovers up, trading momentum for height. When it steers downwind it looses some height as it begins to pick up speed. I hope that helps to visualize what happens with a plane on a, relatively, strong wind.
Wouldn’t the same thing happen if you were traveling in still air, and were suddenly subjected to a 100mph tail wind?
P.S. That plane weighs less than an ounce?! :o
In terms of lift, yes, but since in your case the airplane actually has a momentum it will go into a ballistic trajectory until the speed is enough to resume normal flight.
Also, since in this case the tail wind would push the airplane forward it will reach flying speed faster than in the case the head wind suddenly dissapears, in that case you only have the engine thrust (and the height/speed trade) to build up airspeed.
Yes, that’s right, less than an ounce and 14 in. wingspan.
Sure, it’ll go into a ballistic trajectory -relative to the ground-, but it will still be falling straight down -relative to the air-, making the momentum insignificant.
I fail to see how it will reach flying speed faster, either, since in both cases you’re having the identical wind speed differential.
OK, someone check my math. Forty smph times 5,280 feet in a statute mile is is 211,200 feet per hour. 211,200 feet divided by 60 minutes is 3,250 feet per minute. Right? I’ve never flown a helicopter that would climb that fast.
Re: airspeed and ground speed. That’s been pretty well covered. In a helicopter the ‘wings’ (rotors) obviously travel faster at the tips than at the hub. So let’s just arbitrarily say that the rotors are going around at 100 mph and forget the distance from the hub. In a hover in still air the ‘wings’ have an airspeed of 100 mph. But the aircraft is stationary above the ground. Its airspeed is zero. Same thing with an airplane, except the wings have to move forward in order to generate lift and the rest of the airframe has to go with it. So in flight, your Bugsmasher 150 has an airspeed of 100 mph and, in still air, a ground speed of 100 mph. Add a 100 mph headwind and the airspeed will still be 100 mph. But the ground speed will be airspeed minus wind speed, so the ground speed will be zero.
The helicopter in hover will still have a 100 mph rotor speed, but it will be moving backwards at 100 mph. (Airframe airspeed 100 mph minus wind speed 100 mph = minus 100 mph airframe speed.) This is not optimal. What if you increase the airframe’s speed to 100 mph in the face of a 100 mph headwind? The helicopter (airframe) will have an airspeed of 100 mph. The ground speed will be zero. The advancing blade of the rotor system will be 200 mph (rotational speed plus wind speed) and the retreating blade will be zero (rotational speed aft -100 mph plus wind speed). And therein lies a problem. The retreating blade will not be generating lift. Bad Things™ ensue.
FWIW a Robinson R22 has a V[sub]NE[/sub] of 104 kts. I don’t recall the actual tip speed, since you’re concerned about rotor RPM and not the actual speed. Could be about 300 mph. A faster airspeed than 102 kts (117.3 smph) risks retreating blade stall, as the rotor is not moving fast enough to generate enough lift.
There’s another thing to watch out for, re: hovering. If your airspeed is under about 15 kts (or zero, in a hover) and you’re descending around 300 fpm, you’re in a condition called ‘settling with power’ or ‘vortex ring state’. You’re descending through your own downwash. Pulling collective (adding power to the system – normally used to climb) increases the speed of the downwash so you go down even faster. There are two ways to get out of the vortex ring state: The easiest is to simply increase your airspeed (in any direction, but forward is the most obvious) to greater than 15 kts and fly out of it. The other way is to lower the collective to increase your rate of descent and fly out of the bottom of the downwash. Since helicopters tend to stay low, this might not be an option. One of the helicopters I rented crashed when the pilot failed to recover from settling with power. (No injuries to the pilot or his dad; helicopter totaled.)
Ah, uhmmm… yeah, sorry I got a little confused there, scratch that second sentence you quoted.
Slinks away in shame
Oh please, we all know an airplane would take off from a treadmill.
Don’t forget that a plane’s wing is usually set at a positive angle of incidence on the fuselage. Airfoil and controlability issues aside, If you were flying backwards the fuselage would have to be tilted pretty high in the air for the wing to maintain a positive angle of attack. Flying backwards would have significantly higher drag. But that would be the least of your problems.
Of course, the airplane isn’t flying backwards. It’s flying forwards at 100 miles per hour through an air mass that itself is going 100 mph in the opposite direction. The airplane doesn’t care. A good analogy would be running on a treadmill. Your body thinks it’s running forward, and it is - relative to the treadmill band. But because the band is going at the same speed in the opposite direction, you run in place.
My personal best landing distance in a Cessna 150 is about 30 feet. I was ferrying one in from our flying club for maintenance, and the wind speed was high enough that my ground speed at touchdown was about 10 mph. I manged to land and turn off right at the threshold taxiway. The controller was suitably impressed.
Sure, but I was limiting my post to a steady wind scenario rather than trying to describe what happens in severe windshear.
You see this is why I love this board.
One asks a simple question and immediately there is a wealth of info on the subject matter.
Wiki may be good but the SDMB beats the shit out of it when positive answers and insight info is concerned
I wasn’t clear enough with my response, I admit. I tried to word it in such a way that, yes, it is possible to get an airplane airborne if you strap a big enough engine to it. It won’t be flying in the traditional sense, though; that is, the wings wouldn’t be providing the bulk of the lift, it’d have to be the actual engine (which means it’d pretty much have to be a jet). The example I used, the stealth, was designed in this way; aerodynamics were a secondary design criteria, which is why it’s unstable on all three axis. Without fly-by-wire, they would be impossible for a human to control.
The best part is, some of these guys actually know what they’re talking about. :smack: I just have an intro level fluid dynamic class and a semester of fluid and thermal design (mostly pipes and pumps) under my belt (well, and a teensy weenie little bit of nuclear design experience). I’m thinking about taking compressible fluid flow next semester though, we’ll see.
Dude, if Chowder comes over next year for a brief air tour by all means look me up, I know I can’t pay you or pay for the flight but I am more than willing to… ahem… purchase that stick of Jucey Fruit for the approximate value of a good portion of the flight, or kick in for the appetizers and cover the tip, or whatever.
That’s cause we write the Wiki. Here’s my article, which has nothing to do with airplanes (and desperately needs updated as soon as I can find the time).
Even pilots some times forget this basic rule of flight when maneuvering near the ground or in reference to the ground. Bad things result. Many thoughtless pilots have crashed making the turn from base to final when the wind is behind them on base leg.
We are taught ground reference maneuvers and they are required to get your license. But lack of practice and lack of forethought happens to many. The maneuvers are taught at a relatively safe altitude usually where you will find a road that is 90 degrees to the wind at you altitude and you will attempt to fly a nice uniform S curve along the road while the air mass is trying to move you off the road.
As for a pipeline patrol pilot to survive, a major brush up and retraining of your thinking is necessary. The things needed for safe flying are not there. Altitude, airspeed and time to think.
You are low, slow and if the engine quits, you are in an instant world of hurt. No day dreaming, relaxing, or any of that foolishment is allowed.
On top of all the other stuff, you need to plan turns very carefully so that the actual track of the aircraft is such that you can do your job of looking at the pipeline. Coordinated turns are seldom used and great care needs to be taken to ensure that the airflow over the wings is sufficiently maintained … I was lucky, I had a great teacher. I survived over 2500 hours of that kind of flying… Hot, or freezing, rough, bugs on the windshield work. Definitely not the glamor side of making a living in aviation…