Hypothetical physics puzzle

[Softus]

If an aeroplane was standing still on a normal runway, and a headwind developed and reached take-off velocity, and the plane wasn't tethered to the ground, would it levitate and hover over the ground without moving either backwards or forwards?

If so, what if a giant hand (the type of hand that you regularly see pushing cars along giant conveyor belts) pushed it backwards? Would the 'plane crash to the ground?

 

[holmslaw]

It would rise up be pushed backwards and crash, because don't fly backwards - end of.

Its quite a common occurance>

 

[WDIK]

vector in a wind tunnel, the plane with its brakes on would present little forward resistance to the wind,
so when the air reaches a certain velocity over the wing surfaces lift will be created due to the negative pressure above the wings.
so the plane will rise vertically, but because the plane has no forward propulsion the plane will move backwards, eventually because planes arnt really designed to fly backwards the plane will become unstable and flip.

 

[JohnD]

I don't understand the problem with planes "flying backwards".

You mean, relative to the ground?

What interest is groundspeed to a craft that obtains its lift from airspeed?

There was an early RN Torpedo bomber called a Swordfish (very slow biplane) and if it was launched into a strong headwind with carrier making good progress, the story goes that it would be seen to fly "backwards" relative to the carrier until it gained speed. Of course the plane is flying in the air so the position and motion of the ship is irrelevant to its flight.

edited: oops, my mistake - I gather in this question the plane isn't tethered or propelled so once it loses contact with the ground it will just be carried along by the wind like a leaf.

 

[blondini]

...would it levitate and hover over the ground without moving either backwards or forwards?

No.

 

[holmslaw]

A pretty stupid question really.

 

[Softus]

It would rise up be pushed backwards and crash, because don't fly backwards - end of.

Its quite a common occurance>

A pretty stupid question really.

And yet you rushed to provide the first stupid answer.

 

[Trazor]

A pretty stupid question really.

Not stupid at all........

Scenario 1. No pilot aboard.
If the wind only reached "normal take off velocity", then the plane would not lift off the ground, but would just become unstable. A much greater wind speed would be required to lift the aircraft, which would then quickly crash.

Scenario 2. Pilot on board.
For the plane to take off, its angle of attack must be correct, which is achieved by raising the flaps, which in turn, lifts the nose off the ground, allowing the correct angle of attack.The pilot would have to be pretty stupid to do this, best to lower the flaps, and keep the nose down.

For any one who thinks airflow over a wing, is required to lift a plane so it can fly. Please explain how all aircraft can fly upside down.

 

[blondini]

That will raise a flap alright Trazor.

 

[Softus]

For any one who thinks airflow over a wing, is required to lift a plane so it can fly. Please explain how all aircraft can fly upside down.

By virtue of airflow over the wings.

 

[megawatt]

Trazor Wrote:

Scenario 1. No pilot aboard.
If the wind only reached "normal take off velocity", then the plane would not lift off the ground, but would just become unstable. A much greater wind speed would be required to lift the aircraft, which would then quickly crash.

Whilst on the landing gear aircraft are rigged with negative angles of attack and, therefore, without any elevator input the aircraft would not lift off as you rightly said. The windspeed to make it do so would need to be extreme and, in this case, the likely outcome is that the aircraft would be blown backwards down the runway once the lift was sufficient to overcome its mass.

Scenario 2. Pilot on board.
For the plane to take off, its angle of attack must be correct, which is achieved by raising the flaps, which in turn, lifts the nose off the ground, allowing the correct angle of attack.The pilot would have to be pretty stupid to do this, best to lower the flaps, and keep the nose down.

Oh dear, where DO I start with this one ... I'll assume, in your defence, that you were pi**ed when you wrote it

Firstly Trazor me old mucker ... flaps play no part in raising the nose of the aircraft (rotation about its lateral axis) ... That's the job of the elevators.

The sole purpose of flaps on an aircraft is to increase the effective angle of attack during takeoff (mid flap) to provide more lift at lower airspeeds and to increase drag (full flap) to facilitate lower groundspeeds during landing.

The effective angle of attack of an aerofoil is the angle between the horizontal and a line drawn through the centreline of the aerofoil (a line between the centre of the leading edge through to the tip of the trailing edge ... Or, in this case, the trailing edge of the flap).

If you draw this you'll easily see that, by lowering the flap, you increase the effective angle of attack of the aerofoil.

Once you've grasped this you'll see that what you've said is total boll*cks to be brutally honest

The only thing you've said which makes any sense is

The pilot would have to be pretty stupid to do this

MW

 

[megawatt]

For any one who thinks airflow over a wing, is required to lift a plane so it can fly. Please explain how all aircraft can fly upside down.

Whilst most aircraft are able to momentarily invert as part of an aerobatic manoeuvre, ALL aircraft cannot maintain level flight (which is the broader definition of flight) upside down Trazor.

And those which can are able to do so because their aerofoils are designed pretty much symmetrical to provide lift on both sides dependant upon angle of attack.

I flew a Pitts in my younger days which had such an aerofoil and, as such, was equally capable of maintaining level flight either way up.

The Piper Cherokee I learnt in though and most of the other aircraft I've flown since are totally incapable of maintaing level flight whilst inverted nor are their airframes designed to keep the aircraft structurally intact in this condition i.e. it will come apart at the seams

MW

 

[blondini]

The answer is no, but what would happen?

Scenario 1. The wheel brakes are off.
The aircraft will be blown backwards by the wind. It will be unstable.

Scenario 2. The wheel brakes are on. The forces acting on the airframe will be trying to do two things. There will be lift which will reduce the grip of the tyres on the runway, and there will be a rotating couple which will try to tip the aircraft backwards. I think the aircraft will be blown backwards and it will be extremely unstable.

 

[Softus]

I like blondini's reply.

So, to follow that line of reasoning and augment the original question, what would happen if the 'plane's engines were developing just enough thrust to counter the tendency to move backwards (i.e. an equivalent force, in magnitude, to the wind, but opposite in direction)?

Would it then just hover?

If so, what effect would the big hand have?

 

[blondini]

I'm going to be really really boring...

The aircraft would fly. If the forces were balanced it could be made to maintain position and appear to hover over the runway.

The big hand would push the aircraft backwards, reducing airspeed. Pushed backwards fast enough the aircraft would stall.