airplane riddle (taken from another board)
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Chris_Ford
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Okay, I'm going to be the idiot that bites (note: I'm also the idiot that doesn't believe in physics... but that's another post)... But the wing isn't going to have any wind over it, is it? The thrust provided by the engines would merely cause the wheels to spin faster and nothing more, and since the tires are independent, there wouldn't be excess lift/thrust to make the plane airborne... And even if it did get airborne, as soon as it took off, it'd lose enough relative wind to make the airplane stall
Ex.: Let's put the plane on a treadmill and support it from one point. With no engines running, the wheels spin up, but there's no wind over the airplane. Now let's replace the support by firing up the engines, and bam, there should be no difference.
Also, since I mentioned how I don't believe in physics, please explain how a spring gets heavier when you stretch it. That's absolute BS.
Ex.: Let's put the plane on a treadmill and support it from one point. With no engines running, the wheels spin up, but there's no wind over the airplane. Now let's replace the support by firing up the engines, and bam, there should be no difference.
Also, since I mentioned how I don't believe in physics, please explain how a spring gets heavier when you stretch it. That's absolute BS.
I believe that you are thinking more about something like an automobile, where the motive force is the car's wheels driving against the surface it is standing on. An airplane is different, because it does not use its wheels to drive it forward. The only purpose of the wheels is to reduce friction between the airplane and the surface supporting it, allowing the power of the propeller pulling (or pushing if you prefer) on the air to make the plane move. It does not matter if the airplane is on solid ground, or ice, or a treadmill moving forward or backward, the wheels will just "free-wheel", and the aircraft will move forward.Chris_Ford said:
I put an analogy together for this same question for a different website, maybe it will be useful here (?). My post was talking about the difference between an automobile and an airplane on a treadmill.
therookie
New Member
I've tried to leave the physics out of this as much as possible. So for those of you who would like to point out that due to Newtons laws the prop actually doesn't "pull" the plane...feel free 
This situation most likely be best explained by looking at what exactly moves the airplane foward (both in the air and one the ground). The propeller (or for those of you with fancier setups, jet) pulls the plane foward. The wheels are free spinning, that is nothing besides the friction with the ground will cause them to move. Compared to the thrust generated by the prop (jet) the friction of the wheels is largely negligible.
Therefore, at the instant the scenario begins (assume the plane and treadmill both begin at a standstill) the prop pulls the plane foward and the plane accelerates exactly the same as on a normal runway. The wheels will spin faster, as the ground is "moving faster" underneath the plane, but because the wheels have no accelerating (or braking unless the brakes are engaged) effect on the plane, the speed of the ground is irrelevant.
More in Depth:
If you want to assume the treadmill begins at Vx or Vy backward then the plane will have to accelerate twice as much as it's initial velocity will be negative V(whichever). However, assuming that a tailwind of that velocity while the plane is stationary on the treadmill doesn't flip it, this will still be well within the capabilites of any plane (including the might 152 I currently captain!)
Hope this helps! I've done my best without drawing pictures...
-John
This situation most likely be best explained by looking at what exactly moves the airplane foward (both in the air and one the ground). The propeller (or for those of you with fancier setups, jet) pulls the plane foward. The wheels are free spinning, that is nothing besides the friction with the ground will cause them to move. Compared to the thrust generated by the prop (jet) the friction of the wheels is largely negligible.
Therefore, at the instant the scenario begins (assume the plane and treadmill both begin at a standstill) the prop pulls the plane foward and the plane accelerates exactly the same as on a normal runway. The wheels will spin faster, as the ground is "moving faster" underneath the plane, but because the wheels have no accelerating (or braking unless the brakes are engaged) effect on the plane, the speed of the ground is irrelevant.
More in Depth:
If you want to assume the treadmill begins at Vx or Vy backward then the plane will have to accelerate twice as much as it's initial velocity will be negative V(whichever). However, assuming that a tailwind of that velocity while the plane is stationary on the treadmill doesn't flip it, this will still be well within the capabilites of any plane (including the might 152 I currently captain!)
Hope this helps! I've done my best without drawing pictures...
-John
Chris_Ford
Well-Known Member
SteveC said:
In your first example, I'm going to disagree, however. Your foot lifts off the treadmill and moves forward, this covers the distance that the belt moves your foot back...
And in your rollerblade example... let's insert some concrete numbers...
You start at 0 m/s (in true physics notation)... You accelerate to 10 m/s as the belt also accelerates at the same rate (and at the same time) to the same speed. Net result: 0 m/s. Basically, this is elementary relativity... If we look at the system as a whole, however, there's no net velocity. Which means there's no wind, thus no lift on our hypothetical wing, yes?
Not saying you're wrong or anything, just trying to get an explanation... I might have to find some rollerblades and go on the treadmill tomorrow
I have no idea what you are talking about here. Well, in the first part ("I don't believe in physics") I think I understand what you mean (I have some issues with Thermodynamics, myself). It's the "spring gets heavier" part I've never heard of before.Chris_Ford said:
Oh and by the way, those are your words, not mine. I certainly wouldn't be so callous as to call you an idiot on an open internet forum.
:bandit:
Chris_Ford
Well-Known Member
SteveC said:
Spring gets heavier: relativity. E=mc^2 E=1/2 kx^2, thus mc^2 = 1/2kx^2. From a physics professor, not me. On that question, my answer was, "No, that's ridiculous. If the answer of (delta)kg isn't 0, I don't believe in physics.
As for the treadmill. The treadmill picks up speed as the airplane is rolling on the ground. This is where the logic hole lies. As thrust from the jet (I'm assuming it's a jet, daggommit) increases, it has to "push" something. The wheels have to push against the belt of the treadmill (because there *is* friction, and I'm assuming there's normal friction, thus the ice example is a slight bit different) So once the airplane starts rolling, the tires *have* to spin, because the treadmill is resisting... Thus, as the tires speed up, as does the treadmill and so on and so on... Someone needs to make a video
Ah, but your body does not move in relation to the ground around you. Yes, you expend energy and you move in relation to the treadmill surface, but you won't feel any wind in your face from moving relative to the earth's surface.Chris_Ford said:
But your numbers are wrong. The treadmill does not cancel you from moving forward.
In my roller blade example you are pulling on the handrails, moving yourself forward. The treadmill moving underneath you does nothing except make your wheels spin. If the treadmill moves backwards at the same speed (-10 m/s) that you are pulling yourself forward (10 m/s), your body will continue to move 10 m/s forward relative to the earth around you. Your wheels will be spinning at 20 m/s because of the treadmill moving the opposite direction that you are.
The act of pulling yourself forward by the handrails is the same thing that an aircraft does by pulling itself forward through the air.
Incorrect. If you look at the system as a whole you will find a mass of air being accelerated *backward* and an airplane accelerating *forward*. It will move in relation to the ground and in relation to the airmass, there will be a relative wind, and the airplane will fly.
Good idea! It would then become very apparent to you... I might have to find some rollerblades and go on the treadmill tomorrow
Here's the way to do it. Put your rollerblades on, and stand on the stationary treadmill. Hold your feet still, facing straight forward. Trial #1 is to pull yourself forward on the treadmill using your hands on the handrail. Notice how much (little, acutally) force is needed to move you forward. Now, remember that your arms holding and pulling on the handrails is the same as the aircraft's propeller pulling against the air. Turn the treadmill on, and hold yourself still by hanging on to the treadmill. Turn the treadmill speed as high as it will go, then start pulling yourself forward by pulling on the handrails. Notice how the force required to pull yourself forward is basically the same as when the treadmill was shut off? The reason is twofold: 1. you are not pushing against the treadmill surface to propell yourself, and 2. the wheels isolate you from the movement of the treadmill because they can turn freely.
Those same two factors are what allow the aircraft to freely pull itself through the air, completely independent of what the treadmill is doing underneath it.
Chris_Ford
Well-Known Member
SteveC said:
Yes, but in our hypothetical example, the treadmill would speed up as I pulled forward, because the wheels would be going faster.
What I think you guys are assuming is that there is excess thrust.... but there isn't any to be found...
I don't recognize the second equation. My college education ended over 25 years ago.Chris_Ford said:
Sorry, that is an incorrect assumption. The tires do not push against the belt. If an aircraft required it's wheels to drive against the ground to get it to move, how could an amphibian aircraft ever take off without something to *push against* the water? How would a plane on skis ever get moving?
The jet engine, or propeller (doesn't matter which) reacts against the airmass to move itself. It does not push against the ground. Shoot, if the jet had to push against the ground to get it to move, how in the world does it ever stay in the air?
Chris_Ford
Well-Known Member
SteveC said:
Right, I agree 100%. But what I'm saying is that as a byproduct of this thrust being generated, since the airplane is on the treadmill and there is friction, the wheels will turn, just like they turn when you takeoff on a regular runway. As these wheels speed up, so will the treadmill. This keeps the airplane in the same location as it is in the start. If we had an icy treadmill, I'd agree
There is plenty of excess thrust.Chris_Ford said:
Here is a different way to look at the same problem.
You will agree that a person can move a Cessna 170 over flat ground by pulling it by hand? The force to do so is probably something like 40 or 50 pounds. That force is required to overcome the rolling friction in the wheel bearings. (It takes a higher force to get it moving initially which is overcoming the static friction which is higher, but no longer is a factor once the wheels start turning.) That being true, it would also be possible for a person to be standing out front, holding a rope tied to our airplane-on-a-treadmill and hold it in place while the treadmill moves backward at some slow speed (walking speed for example), yes?
The thing to remember is that rolling friction (such as in a wheel bearing) does not go up as the speed increases. This is the key! Our person standing in front of the airplane, holding a rope with about 40 or 50 pounds of pull against it, can hold the airplane from moving backwards whether the treadmill is moving at 1 mile per hour, or 100 miles per hour!
Given that a C-170 generates some hundreds of pound of thrust, there is plenty of excess thrust and the plane will move forward (in relation to a bystander) no matter how fast the treadmill runs in reverse. The treadmill cannot exert enough force on the airplane to keep it from moving forward. There is not enough friction in the wheels to overcome the power of the engine!
Think about it. If there were enough friction in the wheel bearings to keep the plane from moving forward, a plane could not take off from a non-moving runway.
Exactly. The wheel bearing's whole purpose is to reduce the friction between the ground and the airframe. And it does so whether the ground is stationary or moving, with almost no change in the amount of drag transmitted to the aircraft.Quote:
Originally Posted by Chris_Ford
If we had an icy treadmill, I'd agree
Instead of an icy treadmill assume the frictionless point (or nearly frictionless) is the wheel bearing. Same result.
Chris_Ford
Well-Known Member
SteveC said:
Agree that mu does not change, the thing is that as the airplane goes faster and faster, so does the treadmill. Regardless of how much thrust is actually generated. This keeps the plane in one location no matter how hard the engines are pushing! If it was able to take off of a treadmill, why don't Japanese airports have big treadmills to launch planes? (Japan is my example because I'd imagine it'd be cheaper to build a large scale treadmill than a whole new island for an airport...
Honestly, I can see how you can argue both ways and as far as I'm concerned, this is why physics is pure and utter crap. It's the religion of sciences
Sorry. The treadmill can go as fast as it wants and it cannot stop the airplane from moving forward. It can go 100 times as fast as the airplane and the plane will still move forward in relationship to the air (and the ground and our intrepid Mr. Taylor sitting alongside the treadrunmillway in a lounge chair, drinking Margaritas).Chris_Ford said:
The airplane is pulling itself through the air, the wheels spin freely underneath it, and the treadmill can go forward, backwards, or stop and the airplane will still take off. The treadmill needs to be 2000 feet long, or whatever length is needed for a regular take-off, but it cannot stop the airplane from moving.
Because a 10,000 foot runway on an island is cheaper than a 10,000 foot long treadmill on an island!
Just as the treadmill cannot stop a plane from taking off if it is running backwards, it does not help the plane in taking off if running forward.
But only one way is correct. The other is wrong (your way, in case you weren't paying attention).
I don't work on correcting mental illness. You're on your own with that problem.... and as far as I'm concerned, this is why physics is pure and utter crap. It's the religion of sciences
Can't help you with that delusion.
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