I will agree the ‘theoretical’ question has holes in its wording, but, the answer seems to raise more questions than give answers.
-A fixed wing airplane, with a NET airspeed of ZERO, creates enough lift for take off?
-Without airspeed there is no lift. This would require a natural and constant head wind equal to or greater than the airplanes STALL speed. Otherwise the airplane is earth-bound isn’t it?
If the airplane and conveyor have a ‘theoretical’ speed of 100 mph and 100 mph (equal but opposite directions), why do the wheels have to suddenly travel at 200 mph?
-This A=A+5 equation (and accompanying text) to me would suggest the airplane does not “appear stationary”, but is in fact travelling forward on the converyor at 5 “units”?
-Finally, the conclusion is that the airplane takes off? Wouldn’t this suggest that all we need to launch airplanes is a free-wheeling conveyor and a quick release tailhook? The only reason we have runways is to land them?
I realize you are all very busy, and probably already explained this in threads, but the original question is still in the archives with no links to explain it better.
As John W. Kennedy said, the answer is not that the aircraft flies with 0 airspeed, but that the treadmill as described is not capable of keeping the aircraft from moving forwards through the air.
Before you post again on the issue, be aware that anything you could possibly bring to the discussion has already been raised, rebutted, and re-argued adnauseum here.
To keep a long thread short, the answer depends on what you decide the question really is (it can interpreted a number of ways), how deep you want to get in to physics, how many of the factors in the question you arbitrarily decide don’t fail at real world values, etc.
In other words, any answer is right, as long as change the question accordingly.
The confusion arises because there are two possible ways to interpret they way the conveyor belt is supposed to work. It’s simpler to first think about having a car on the conveyor belt, and then compare what changes with an airplane.
Configuration A: The conveyor belt matches the speed of the car’s wheels, so that the car ends up staying in the same place. If the car’s speedometer reads 50mph, then the belt moves at 50mph, and the car stays put. Even if the driver steps hard on the gas, as long as the conveyor belt can keep up the car goes nowhere.
Configuration B: The conveyor belt matches the speed of the car itself, relative to the ground. In this case, if the car’s speedometer reads 50 mph, then you would have the car’s actual speed at 25 mph, with the conveyor belt moving in the opposite direction at 25 mph.
With an airplane, the problem with configuration A is that it simply doesn’t work. Since an airplane propels itself by pushing against the air (unlike the car which pushes only against the ground) simply moving the conveyor belt in the opposite direction would not keep the plane from moving; it just makes the wheels spin faster.
Cecil’s answer to this problem assumes that the conveyor belt is in configuration B. Let’s say the plane can take off at an airspeed of 100mph. When the plane reaches 100 mph, the conveyor belt is moving 100mph the other way, and its wheels are spinning at 200mph. No problem.
So the question in Cecil’s column boils down to: will the treadmill described hold the plane to zero airspeed, or not? The answer to which is either “yes” or “no”, depending on your assumptions.
