Airplanes and sailboats

[Brian D]

Floating around the internet is this theory: if a plane is on a tarmac that is moving at the same speed the wheels are turning, will the plane take off?So I present to you: a boats speed is 10 knots heading into a 10 knot current. Will the boat make headway?

 

[Rob Hessenius]

Apples to Oranges

Brian- Its not a comparable situation. All that matters to a plane is that there is enough wind/moving air to create lift. So if they are moving at a sufficient speed the wing will create lift. The airplane has to be headed in the same direction that the tarmac is moving to have the proper aerodynamics that allow it to fly. As far as the boat goes; If I read this correct, if he has/she applied enough power that would allow him to go 10 mph and was against a 10 mph current, he would have a 0 vog. But if he was going 10 mph in a 10 mpg current, he has applied the equvalant power that would allow him to go 20 mph with 0 mph water movement. Rob Hessenius

 

[Robert Gainer]

It’s not the same thing at all.

It’s not the same thing. In a current of 10 knots on the nose the boat making 10 knots in the water will not advance over the bottom.An airplane when it reaches a suitable speed in the air will take off no mater what the surface she is sitting on is doing. A small plane can go up while standing over the same ground spot if there is enough headwinds. All the best,Robert Gainer

 

[Dan Johnson]

Brian...

Several years ago in a light air July race on the Chesapeake we rounded a channel marker that was a race mark on a broad reach, trimmed in to close hauled run to the next mark and went about 100 yards until we hit the channel's outgoing tidal stream. We sat there with perfectly trimmed sails, tell-tales streaming just right, knot meter registering 3 knots, and the GPS indicating 0 VMG. We just sat there for a while not moving, then we slowly began to move, sternfirst, towards Baltimore!Hmmm...tarmac moving at the same speed the wheels are turning...seems to me that the plane wouldn't "take off" until it's moving fast enough over the tarmac for the wings to develop "lift". Can they "take off" tail first if you reverse the prop?

 

[tom]

What is the Speed of the Wings

An airplane flies with it's wings not it's wheels. When the wind speed that the wing experiences reaches a certain speed the plane can fly. I've actually seen an article where they landed an ultralight airplane on top of a houseboat. Some ultralights will fly at only 20 mph or so, so if you have a houseboat going 15mph into a 10 mph wind it's possible. Aircraft carriers go into the wind to help aircraft take off and land. Even small changes in wind can effect take offs and landings. A cessna 150 will fly at about 50 mph(depends upon air temperature and load) If you take off into a 10mph wind the plane will fly at 40 mph relative to the ground. If you take off with the wind you have to go 60 mph relative to the ground. Relative to the ground becomes real important on a short runway as it takes a certain distance to accelerate or brake. High ,hot and heavy has caused many a plane to run off the end of the runway because they couldn't get airborne. High and hot air gives less lift. Heavy requires more lift which means more speed, it's a double whammy because a propeller and engine are also less effective when it's high and hot .  If a boat is going 10 knots into a 10 knot current it will be standing still relative to the ocean floor. But it will be going 10 knots through the water. The ocean floor becomes real imporant when you are trying to go in through a pass on an outgoing tide and the waves are standing up tall. But 10 knots is a dream speed for my sailboat that has a hull speed of 7 knots.

 

[Ross]

During one of the spring floods

the river current was about 3 or 4 knots and my boat sat in the slip with all of her lines slack looking for all the world like she was going up stream. Then she would yield to the current and stretch the springs and bounce back to slack lines again but she just stayed in her slip the whole time and didn't go anywhere. Now there is another thing to consider. With wheels the point at which the rubber meets the road the velosity is zero. the velosity of the hub is the same as the velosity of the vehicle and the velosity of the top of the wheel is twice the velosity of the vehicle.

 

[Brian D]

It was met to be a comparison...

It was a preface to the question about the current and boat. The plane thing has been argued to death. I didn't intend it to be argued more.

 

[Mike]

I don't think it can fly

If the plane needs 100 mph to take off, and the tarmac was moving at 100 mph in the opposite direction that the plane was travelling, there would be no lift. The only air speed would be from the wind. If the wind was at 10 mph into the nose of the plane, and the tarmac was at 100 mph into the now of the plane, and if the plane was accelerating at 100 mph with its wheels on the tarmac, the plane would be stationary to the tarmac. The wind at 10 mph would register as only 10 mph air speed. The plane would have to increase it's speed another 90 mph to get enough lif to get the wheels to break contact with the tarmac.

 

[tom]

Ross I think that you are wrong.

For the wheel to roll the speed of the bottom of the wheel must be the same as the speed of car. Just in opposite direction. Since the top and bottom of the wheel are connected they must travel at the same speed. I guess that the confusion is the changing point of reference. The car is traveling linearly at 50 mph and the wheel is rotating with it's outer circumference at 50 mph. Using 2 phi r you can convert 50 mph into revolutions per hour. Interesting logic problem. The motion of the outer circumference of the wheel relative to the car would constantly be changing between +50mph and -50mph and reaching zero when it was going perfectly vertical. I guess that since the car would always have a +50 mph relative to the ground you could say that the bottom was going 100mph....but then you'd be mixing references. Very simular to saying that a boat is going through the water at 10 knots then saying that since it is going with the current it is going 20 knots. Two refernces yes it is going 20 knots over the ground but it is still only going 10 knots through the water. A floating boat or a flying plane behave as if the ground doesn't exist..well at least until they bump into it.

 

[Steve O.]

Who cares about the speed of the tarmac?

What is the airspeed? If the airspeed (or wind) is such that it will create lift on the wing, it will fly. The wheels and the tarmac have bearing on how or why the aircraft flies.A bigger question is, what about the man walking 2 mph north on the train traveling 50 mph south? Obviously he's walking to the bar car.

 

[Bob]

How interesting

Thats like Lindbergh when he flew across the Atlantic asking himself if that small fly roaming around the cockpit that came with him made his plane any heavier? Watch newsreels aircraft taking off aircraft carrier steaming into an intense headwind back in WWII and you will see aerodynamics in place. The wheels have to catch up with the speed of the plane, rather then the plane limited by the speed of the wheels. Overcoming drag I guess.Bob

 

[mortyd]

answers

two answers. the boat will make no headway, and not understanding that the fly will lighten the airplace as soon as it takes off means not understanding the aerodynamic concept of lift. what if the cockpit were open? how far above the airplane would the fly have to go to lighten the airplane? why don't scales measure airplanes flying over them?

 

[Ross]

Tom, If there were any relative motion between

the wheel and the road at the point of contact then the tire would be slipping. Otherwise known a spinning your wheels.

 

[Persy]

the fly...

hmmmmmmm.....if the fly is flying inside with Charlie, then the plane is carrying his weight, or providing refuge from wind resistance or something (along with the air it is moving in which the fly flies) and moving him forward at the plane's speed, so some of the energy is consumed. If the fly is out of the cockpit, the fly is on his own and is left behind...some energy somewhere had to be consumed to move the fly at the plane's speed (I think)My conclusion: the fly has to be using some of Charlie's energy.fire away....

 

[mortyd]

answers

any object, anywhere, in steady level flight due to its onw lift due to its own velocity is weightless. period.

 

[Ross]

MortyD, The fly is displacing his volume of air

since he is heavier than air and the aircraft is limited in the volume of air it can contain then the presence of the fly will have increased the take off weight of the aircraft.

 

[tom]

Ross I believe you are wrong

Speed is distance divided by time. At any instant nothing is moving. So at the very instant that a point on the tire is in contact with the road and you stop time then yes there is no relative motion. But if you take this theoretical point that is in contact with the road at point x and then measure the distance at another time other than zero then that point on the tire will have an average speed the same as the car if the time interval is a whole number of tire revolutions. To generalize any point on the tire will have the same average speed as the car. It's speed will never be negative relative to the ground (it will always move forward just more slowly as it is going vertical). You just have to remember that speed is distance over time and any moving object's distance approaches zero as the time interval approaches zero. Even a bullet can be stopped if you have fast enough film.

 

[tom]

Ross you are right about the fly

Think of a fish bowl. If you have the bowl full to the brim with water and add a fish the bowl will weigh more it will weigh more by the weight of the fish minus the weight of the water displaced. The key thing is the fish changes the average density of a fixed volume. It doesn't matter if the fish is swimming or lying on the bottom. Now if the fish is floating with neutral bouyancy the fishes average weight would equal the weight of the water displaced and there would be no net change in weight..

 

[Steve Kamp]

Moveable Tarmac?

nm

 

[NautiDuck]

The boat will sink

The boat will not make headway. The boat will not go backwards. The boat will not take off in flight. If there is a fly on the boat, and the fly opens a seacock, the boat will sink right where it is.