Physics question.. can you figure it out?

[Pablo]

seriously, this is ridiculously easy


the plane will take off at the same air speed, but twice the wheel speed


put the skateboard and fan on a treadmill and it will do the same thing (assuming the bearings in the wheels dont have a crazy amount of drag increase)


put the plane on some dyno rollers that are powered by an engine and measure the force required to hold the plane on the rollers from the nose with a strain gauge mounted to a pole

Youll find that the amount of force measured by the gauge required to hold the plane in place is only a measure of the friction of the wheel bearings, and, is far far far far less than what the engines can generate. If you were to turn the engines on in this experiment you would easily move forward off the rollers once the thrust was greater than the wheel drag which is probably something close to idle.

Aircraft fly regardless of their ground speed, you can hold a plane motionless in the air relative to the ground (fly an rc sail plane into a head wind see for yourself) so whether or not the ground is moving super fast under it or super slow there is no difference in it flying with the wheels touching or the wheels not touching with the exception of the added drag of the wheel bearings.

 

[insane6]

The riddle really does not say anything about jet engine it states PLANE so, now I think if it was a prop driven plane then YES the air from the props would cause lift under and over the wings and take off
Now if it was a jet then I still keep my first answer NO, keep in mind this is coming from someone with a 6th grade education.

 

[TurboJunky]

The riddle really does not say anything about jet engine it states PLANE so, now I think if it was a prop driven plane then YES the air from the props would cause lift under and over the wings and take off
Now if it was a jet then I still keep my first answer NO, keep in mind this is coming from someone with a 6th grade education.

Whether it's a prop driven aircraft or jet it makes no difference. Forward momentum (AKA thrust) on either aircraft is derived from AIR not the runway. On a prop plane the blades of the propeller "bite" the air causing force. That force propels the aircraft forward. The same thing occurs with a jet engine. A turbofan spins and fuel is injected and ignited creating a large amount of energy. That energy is forced out the back of the engine AGAINST THE ATMOSPHERE (AIR)...not the stationary or moving ground. That's why this scenario would have a different outcome if it were a car. The car's forward momentum is derived from spinning the wheels....an airplane's forward thrust is from AND AGAINST the atmosphere (air), not the ground which it's resting (runway or conveyor).

 

[Pablo]

prop planes work the same way as jet aircraft, if you attempted to hold a prop plane still (im assuming you mean the lift generated from airflow passing over the wings on prop driven aircraft driven from a prop on the nose) you would not produce any lift whatsoever, the plane has to have movement through the air not just air generated by a prop, the prop diameter is nowhere near the wingspan and you need most of the entire wing to fly.

If you cut off the wings to match the span of the props (which is probably just a little smaler than the curcumference of the area of airflow generated by the time it hit the wing) youd have like a 4 foot wing section (on a plane with mid mounted wings, a high wing like a piper cub would basically have no wings)
on each side and no flight under any circumstances

 

[Pablo]

Whether it's a prop driven aircraft or jet it makes no difference. Forward momentum (AKA thrust) on either aircraft is derived from AIR not the runway. On a prop plane the blades of the propeller "bite" the air causing force. That force propells the aircraft forward. The same thing occurs with a jet engine. A turbofan spins and fuel is injected and ignighted creating a large amount of energy. That energy is forced out the back of the engine AGAINST THE ATMOSPHERE (AIR)...not the stationary or moving ground. That's why this scenario would have a different outcome if it were a car. The car's forward momentum is derived from spinning the wheels....an airplane's forward thrust is from AND AGAINST the atmosphere (air), not the ground which it's resting (runway or conveyor).

exactly

 

[83hurstguy]

I screwed up earlier when I said it wouldnt fly...

Thought about it and it makes perfect sense that the plane will take off. The wheels do nothing other than support the weight of the aircraft, they provide negligible force along the planes direction of motion on a runway (friction from bearing drag unless brakes are applied).

Prop or jet, the only thing stopping it from flying is a blown out tire.

 

[TurboJunky]

I screwed up earlier when I said it wouldnt fly...

Thought about it and it makes perfect sense that the plane will take off. The wheels do nothing other than support the weight of the aircraft, they provide negligible force along the planes direction of motion on a runway (friction from bearing drag unless brakes are applied).

Prop or jet, the only thing stopping it from flying is a blown out tire.

BINGO!
What caused you to change your mind?

 

[GNSCOTT]

How do you get the required groundspeed for takeoff of any aircraft when it isn't moving on the ground?

Assuming the conveyer belt can reverse the forward plane movement as was the premise of this example.

Wheels do more than hold the plane up they allow it to attain the proper groundspeed required for each plane to take off.

 

[83hurstguy]

My first response this morning was when I was still half asleep... kinda just responded quickly without thinking. After actually thinking my way through the problem, it makes sense that the jet will fly. I am embarrassed to admit I am an engineering student and typed up that first post this morning

The jet pushes on air, NOT the ground. If this isn't understood, watch the link to that skateboard video a few times. I didn't watch it until after I had "re-figured" it out... and this is a perfect example.

http://videos.streetfire.net/player.aspx?fileid=35E964D9-38DB-4EFD-BE8D-D6BA1A43A06B

As a really abstract concept, you would have to move the atmosphere backwards to neutralize the thrust of the plane in order to keep it stationary. But then, the plane would probably fly on its own anyways without engine thrust b/c the moving air would provide lift on the wings... sorry, I'm done...

 

[TurboJunky]

How do you get the required ground speed for takeoff of any aircraft when it isn't moving on the ground?

Assuming the conveyer belt can reverse the forward plane movement as was the premise of this example.

Wheels do more than hold the plane up they allow it to attain the proper groundspeed required for each plane to take off.

Easy. AIRspeed is obtained via thrust provided by the engines AGAINST THE ATMOSPHERE (air) and not against the ground like a car. You need to get away from the thinking that an aircraft's movement is increased by creating force against the ground in which it's sitting, it's not.

The wheels spin anyway when the plane attains speed on a conventional runway. What's it matter if they spin at 2x or 3x their normal speed....nothing.

The wheels for all intents and purposes only support the weight of the plane. As already stated some planes do not have wheels and are outfitted with skis or floats. Those aircraft may require more thrust to overcome the drag created by the surface the aircraft is resting. Thrust is forced against the AIR, not the water or snow, causing the aircraft to move.

The ONLY thing that causes a plane to fly is airflow over the wings. Engine thrust provides this by moving the plane through the air, not how fast a free-spinning wheel/tire rotates. This is where everyone gets tripped up. An airplane can fly as long as it meets the required AIRSPEED. That means it can be airborne and completely motionless relative to the ground. That's why aircraft carriers turn into the wind to launch their aircraft. The more air over the wings the lower speed the aircraft needs relative to the ground to get airborne.

The jet pushes on air, NOT the ground. If this isn't understood, watch the link to that skateboard video a few times.

As a really abstract concept, you would have to move the atmosphere backwards to neutralize the thrust of the plane in order to keep it stationary. But then, the plane would probably fly on its own anyways without engine thrust b/c the moving air would provide lift on the wings... sorry, I'm done...

BINGO AGAIN!

 

[83hurstguy]

Easy. AIRspeed is obtained via thrust provided by the engines AGAINST THE ATMOSPHERE (air) and not against the ground like a car. You need to get away from the thinking that an aircraft's movement is increased by creating force against the ground in which it's sitting, it's not.

An airplane can fly as long as it meets the required AIRSPEED. That means it can be airborne and completely motionless relative to the ground. That's why aircraft carriers turn into the wind to launch their aircraft. The more air over the wings the lower speed the aircraft needs relative to the ground to get airborne.

Agreed. Was just talkin to my girlfriends dad about airspeed... he's been an aircraft mechanic/fleet manager for years. Airspeed is measured by taking the difference between the static and dynamic pressures of the air flowing over the aircraft. He used the aircraft carrier example too, lol.

 

[finallygot1]

I've been reading the replys to this post,so I decided to call my brother.He is an apache helicopter pilot in the Army reserves(just got back from a year tour in iraq),he also flies jets for a major corporation.Depending on the size and design of the plane determines the amount of ground speed needed to produce enough lift for takeoff.I watched a video of a Cessna Citation CJ2 it has to reach a ground speed of almost 110 mph(going straight down the stationary runway)to produce enough lift to transfer the weight of the plane from the wheels to the wings to take off.Makes sense and my brother knows what he is talking abut,as did the 35yr pilot/engineer.It would not take off.

 

[TurboJunky]

I've been reading the replys to this post,so I decided to call my brother.He is an apache helicopter pilot in the Army reserves(just got back from a year tour in iraq),he also flies jets for a major corporation.Depending on the size and design of the plane determines the amount of ground speed needed to produce enough lift for takeoff.I watched a video of a Cessna Citation CJ2 it has to reach a ground speed of almost 110 mph(going straight down the stationary runway)to produce enough lift to transfer the weight of the plane from the wheels to the wings to take off.Makes sense and my brother knows what he is talking abut,as did the 35yr pilot/engineer.It would not take off.

Ground speed is relative. An airplane can fly with 0 ground speed. I believe he means "airspeed" which means the speed of the air flowing over the wings. Ground speed is completely different when you're taking off into the wind as opposed to with the wind, which is why aircraft being launched from an aircraft carrier head into the wind. It reduces "ground speed" in order for the plane to become airborne.

Please explain why the aircraft wouldn't move forward.

I'll try this a different way. In order to move an object force must be applied to it. As I've stated a car would remain stationary and not move forward because it applies force TO IT'S WHEELS and that's being neutralized by the conveyor belt. An airplane applies force to the atmosphere (air) and the conveyor belt has absolutely no affect on this resulting in the aircraft being forced forward. All the conveyor belt would do is spin the wheels faster than normal.

 

[Sleeper-6]

I say the thing flies!! Since all the wheels do is freewheel, it doesen't matter what the d**m runway does. Engine produces thrust, plane moves forward creating airspeed, wings produce lift, plane leaves ground.

And that's all I have to say about that.

Unless..............of course due to the runway spinning the wheels twice as fast is they are designed to go, the wheel bearings over heat and lock up before sufficent airspeed is gained for liftoff. Then the tires explode, landing gear rips off. Then the entire thing slams into the runway, explodes into a spectacular fireball and we all get a he!! of a show. :biggrin:

 

[dgreen1069]

Geez, I think I responded way too early in this thread. I realized a short while later that the plane would take off. Think about walking on a conveyor belt. If it was matching your walking speed, you would stay in one place. If you put on a pair of roller skates and attached a fan to your back, you would move forward regardless of the conveyors speed. I retract my original and flawed response and join the rest of those that say the plane will take off just fine.

 

[HighPSI]

It would not take off.

Go fly a kite!

 

[TurboJunky]

Nice!

Geez, I think I responded way too early in this thread. I realized a short while later that the plane would take off. Think about walking on a conveyor belt. If it was matching your walking speed, you would stay in one place. If you put on a pair of roller skates and attached a fan to your back, you would move forward regardless of the conveyors speed. I retract my original and flawed response and join the rest of those that say the plane will take off just fine.

Great analogy! I too was just about to post something very similar.

Think of yourself on a moving sidewalk in an airport. Put yourself on the moving sidewalk with roller skates on and the belt moving "backward". Place a high power fan on your back (or better yet a jet engine) pushing you forward OR with a rope in your hand and 10 guys pulling you forward. Would you move? It's the same thing folks!

 

[finallygot1]

I used the term ground speed only as a reference.yes air speed is what matters,with no wind 100 knots is just that on the ground,ad a 50 knot head wind it reduces the ground speed in half.I called my uncle who flew fighter jets in vietnam,to get his opinion.He said that with enough thrust to compensate it might be possible.I never thought it wouldn't move foward,just didn't think it would produce the air speed to take off.Oh and HighPSI I quit flying kites a long time ago :biggrin:

 

[TurboJunky]

I used the term ground speed only as a reference.yes air speed is what matters,with no wind 100 knots is just that on the ground,ad a 50 knot head wind it reduces the ground speed in half.I called my uncle who flew fighter jets in vietnam,to get his opinion.He said that with enough thrust to compensate it might be possible.I never thought it wouldn't move foward,just didn't think it would produce the air speed to take off.Oh and HighPSI I quit flying kites a long time ago :biggrin:

This is a great discussion....

Well, in my opinion the backward motion of the conveyor belt's friction on the landing gear isn't even close to overcoming the forward thrust of any airplane on rolling wheels. Especially since most if not all high-wing aircraft can be outfitted with floats to operate from waterways. That's a lot of drag.

I'm not a pilot, but only a few hours short of my Private Pilot's License.

 

[finallygot1]

This is a great discussion....

Well, in my opinion the backward motion of the conveyor belt's friction on the landing gear isn't even close to overcoming the forward thrust of any airplane on rolling wheels. Especially since most if not all high-wing aircraft can be outfitted with floats to operate from waterways. That's a lot of drag.

I'm not a pilot but just a few hours short of my Private Pilot's License.

I agree this is a good discussion

I'm not a pilot either,so I consulted 2 very expirenced pilots.This is a hypothetical situation anyway,TT/A1233 saw your crash footage you are one lucky man.Glad to be having this discussion with you.Good luck on getting your PPL,I have now moved to undecided