Airplane physics question

  • Thread starter Thread starter under18carbon
  • 118 comments
  • 5,187 views
Status
Not open for further replies.
Bram Turismo
A different question then :p

What if you were flying a plane, and the speedometer shows your doing 100 knots, however, you have headwind, and the speed of the headwind is estimated at a 120 knots. Is the plane flying backwards?

Probably a "obvious answer" question, but I heard some mates of me discussing it the other day...
Click to expand...

Is the speedometer an airspeed indicator or a GPS speed readout?
 
Bram Turismo
What if you were flying a plane, and the speedometer shows your doing 100 knots, however, you have headwind, and the speed of the headwind is estimated at a 120 knots. Is the plane flying backwards?
Click to expand...

It depends - If the plane's airspeed indicator shows a speed of 100 knots, and is reading indicated airspeed, then the plane is moving backwards at 20 knots.

If the airspeed indicator is actually a GPS readout then the plane is moving forwards at a speed of 100 knots, relative to the ground. If my memory serves me, it would show as ~220 KIAS on the airspeed indicator.

I hope that makes sense, I can see what I mean in my mind but I have trouble putting it into words. :dunce: I may have my airspeeds mixed up, I can't remember the exact differences between them.
 
That's correct.

And I've flown my R/C plane backwards (relative to the ground) on numerous windy days.
 
I've seen BIRDS fly backwards, and across headwinds. It's why a Kite works: it's not moving in relation to the ground, but the (sometimes rather significant) amount of lift, combined with a (usually) low weight, means the kite stays up...

Unless you're Charlie Brown. In this case, anything can happen, including spontaneous combustion.
 
Simply stated; no, the plane will not lift off as it remains stationary relative to the ground (assuming the treadmill is capable of speed equal to and opposite of the thrust of the engines). Plane wings only provide lift when the air travelling over the wing is faster than the air travelling under the wing. Creating a suction which lifts the plane wing. I believe it's called the Bernoullie effect.

Does the wind blow through your hair as you "run" on a treadmill? No.
 
ERacer
Simply stated; no, the plane will not lift off as it remains stationary relative to the ground (assuming the treadmill is capable of speed equal to and opposite of the thrust of the engines). Plane wings only provide lift when the air travelling over the wing is faster than the air travelling under the wing. Creating a suction which lifts the plane wing. I believe it's called the Bernoullie effect.

Does the wind blow through your hair as you "run" on a treadmill? No.
Click to expand...


Did you read the thread before you posted? Like, actually read the thread?

Please read what I have quoted for your convenience, okay?

Danoff
This is a dynmaics problem and should be treated as such. Let's look at the forces acting on the aircraft. Let's assume that the wheels are frictionless.

Force 1: Thrust (applied by the engine) acting in the forward direction with a force F.

There are no other forces acting on the aircraft (neglecting drag). If the aircraft is translating on the conveyer at a velocity S, the aircraft will speed up with respect to S with acceleration F/M. If the conveyer is turned on at the same moment the thrust is applied, the aircraft will speed up with respect to zero with acceleration F/M.

The conveyer does nothing due to the movement of the wheels.

To see this, all you have to do is remember that the earth rotates and that therefore the runway is moving (much like the conveyer)... this does not stop the aircraft from speeding up.

The problem is poorly posed because it is impossible for the conveyer to keep up with the wheels after the engines are turned on. The more the conveyor speeds up, the more the wheels will speed up. The aircraft will accelerate regardless. The treadmill is simply rolling the wheels - not forcing the aircraft.
Click to expand...

Azuremen
Beat me to it, but I was thinking of maybe doing a free body diagram :dopey:

The only force the wheels exert on the aircraft is the normal force caused by the earth pushing up, assuming the bearings in the wheels are perfect. All we are concerned about is the Sum of the horizontal (x) forces acting on the plane, which are drag and the prop.

Drag is a function of area and velocity. So the plane must be moving relative to the air in order for drag to be an issue.

Thus, drag is not regarded at this point, as we are just debating in the plane will move.

So, the only x force on the plane frame is the force of the prop.

From that, it is clear the net force along the x axis must have a magnitude. So the plane must move.

The wheels themselves would be a completely different system, aside from the y forces as a result of the normal force and the plane's weight.
Click to expand...

Now do you think it doesn't take off? The wheels are irrelevant...
 
jammyozzy
It depends - If the plane's airspeed indicator shows a speed of 100 knots, and is reading indicated airspeed, then the plane is moving backwards at 20 knots.

If the airspeed indicator is actually a GPS readout then the plane is moving forwards at a speed of 100 knots, relative to the ground. If my memory serves me, it would show as ~220 KIAS on the airspeed indicator.

I hope that makes sense, I can see what I mean in my mind but I have trouble putting it into words. :dunce: I may have my airspeeds mixed up, I can't remember the exact differences between them.
Click to expand...

👍

My friend kept saying it was impossible for a plane to fly backwards, looks like my dad was right after all...

He has both indicated airspeed and a GPS in his 177 RG-II Cardinal 👍
 
Bram Turismo
👍

My friend kept saying it was impossible for a plane to fly backwards, looks like my dad was right after all...

He has both indicated airspeed and a GPS in his 177 RG-II Cardinal 👍
Click to expand...

Your friend is correct. With the exception of planes which can can use devices other than wings to generate lift (i.e. Harrier, Osprey), it is impossible for a plane to fly backwards. It must always be travelling forwards in relation to the medium it moves in, which is air. Otherwise it will stall.

However, it's progress over ground will never match its progress through air unless there is no wind at all.

So, if aeroplane is facing north, and its velocity is 100mph:

If there is no wind:
Plane will cover ground at a rate of 100 miles northwards per hour

If there is a 30mph wind blowing to the west:
Plane will cover ground at a rate of 100 miles northwards, and 30 miles westward, in 1 hour. It's ground velocity will be 104mph, bearing slightly west of north. The plane will still be pointing north, but travelling slightly north west.

If there is a 30mph wind blowing to the north:
Plane will cover ground at a rate of 130 miles northwards per hour.

If there is a 30mph wind blowing south:
Plane will cover ground at a rate of 70 miles northwards per hour.

If there is a 120mph wind blowing south:
Plane will cover ground at a rate of 20 miles southwards per hour (travelling in other words, travelling backwards relative to an observer standing on the ground).

In all these cases, the plane is moving northwards, through air, at 100mph. It's just that the air is moving around too.

Think of it like this. Put a bathtub on a trolley. Fill it with water. Put a toy motorised boat in the bathtub, at the south end, and make it travel northwards through the water. While it is crossing the tub, pull the trolley in the opposite direction, faster than the boat is moving fowards. The boat, as far as it's concerned, covers the length of the bath in exactly the time it expected to take if the bath was standing still. It's progress in the bath was forwards. Relative to the ground, however, it moved backwards.
 
That last example really makes sense Alfaholic. No matter what the plane, or motorized boat, is moving forward, but relative to the ground it moves backwards.👍
 
ERacer
Simply stated; no, the plane will not lift off as it remains stationary relative to the ground (assuming the treadmill is capable of speed equal to and opposite of the thrust of the engines).
Click to expand...
Simply stated, dead wrong. It's an airplane we're talking about here, not a car with wings.
 
I'm simply answering the question as posed in the first post. The plane will not move forward as the conveyor belt is designed to offset the velocity of the wheels, or thrust of the engines. So, the plane is spinning its wheels, going nowhere, and is stationary. The original post also went as far to explain that there is no wind - no wind = no lift over the wings of the airplane. The exception may be that if the plane's jets, or props, were positioned forward of the wings and generated enough airflow over the wings - then it may lift.

And yes, Azuremen, I read your summation - thanks - also noticed you are at Washington State University. Best of luck to you.
 
ERacer
I'm simply answering the question as posed in the first post. The plane will not move forward as the conveyor belt is designed to offset the velocity of the wheels, or thrust of the engines. So, the plane is spinning its wheels, going nowhere, and is stationary. The original post also went as far to explain that there is no wind - no wind = no lift over the wings of the airplane. The exception may be that if the plane's jets, or props, were positioned forward of the wings and generated enough airflow over the wings - then it may lift.

And yes, Azuremen, I read your summation - thanks - also noticed you are at Washington State University. Best of luck to you.
Click to expand...

Like some have said in here, the wheels could spin at a billion miles an hour... they have nothing to do with the airspeed of the plane. If the prop is facing forward, it will pull the plane forward, regardless of wheel speed.
 
Thanks for the clip Interceptor. 👍

For some reason I was thinking of a jet engined plane when I posted my first reply. Probably because I have been playing Ace Combat :D Jet engines, of course, are typically positioned in such a way that the resulting air blast does not flow over the wings, but rather is pushed out underneath or behind the airfoil.

My second post mentioned that if the jets or props of the plane were positioned forward of the wing - then the plane may lift, as was the case on Myth Busters. I would venture to say that this experiment would not work for a conventional jet plane due to the air thrust from the engines not flowing over the wings.

:cheers:
ERacer
 
ERacer
I would venture to say that this experiment would not work for a conventional jet plane due to the air thrust from the engines not flowing over the wings.
Click to expand...
It wouldn't matter. The jet engines are designed to pull the airplane forward in the exact same way as the prop does. The plane in the Mythbusters episode didn't take off because the engine was pushing air over the wings. It took off because the engine was pulling the plane forward so air would go over the wings.
You are implying that planes fly because the engines act as a wind tunnel, and would therefore lift the plane even if it was stationary. Planes fly because the engines pull the plane forward through the wind.
 
ERacer
Thanks for the clip Interceptor. 👍

For some reason I was thinking of a jet engined plane when I posted my first reply. Probably because I have been playing Ace Combat :D Jet engines, of course, are typically positioned in such a way that the resulting air blast does not flow over the wings, but rather is pushed out underneath or behind the airfoil.

My second post mentioned that if the jets or props of the plane were positioned forward of the wing - then the plane may lift, as was the case on Myth Busters. I would venture to say that this experiment would not work for a conventional jet plane due to the air thrust from the engines not flowing over the wings.

:cheers:
ERacer
Click to expand...

...
You have no clue how a plane works, at all. The front prop on a plane creates so much violence in the air the flow from it is meaningless to the wings. The front prop, or rear, or the jet engines on a plane, or anything that provides propulsion for a plane does NOT provide the lift.

The planes forward motion to the air provides this. Mythbusters plane would have taken off if it was a jet, a plane with a rear mounted prop. Airflow from the engines going over the wings is irrelevant to take off.

You can basically think of the wheels on the plane as completely irrelevant. They are not going to affect the prop's force of the plane, which pulls if forward through the air, and the resulting airflow over the wings generates lift.
 
Indeed, the wheels are irrelevant... which is why early planes had skis... :lol:

That said... I'm surprised it took this long for the thread to re-erupt after Sage resurrected it... unfortunately, I'm not surprised by some of the responses... :D

Like I've said before. If you believe the airplane won't take off, take your plane to a speed shop and have it dyno'd (the automotive dynamometer is probably the most common example of an infinitely adjustable treadmill). I want to know exactly how many wheel horsepower an airplane makes... come back after the dyno guys stop laughing... :D
 
Ah, folks. The light just came on 💡 and I can see now how the whole thing is all about the plane's motion relative to the air, not the ground. The wheel speed is irrelevant. It's all about air speed. I apologise for not thinking this through prior to posting and for my apparent ignorance. Interesting topic.

:cheers:
ERacer
 
Bingo! Another convert to the church of science! :D :D :D
 
No this can not happen because wven though the wheels are moving, the plane is not so there is nothing to provide the lift needed for flight.
 
Sage
*Sigh*

Did you actually read the thread?
Click to expand...

Shouldn't this thread be locked? I mean, we know the answer, we're 100% sure what the answer is, and if you're unclear on it, all you have to do is read a few of the most recent pages to get your answer.

Unless of course we use this thread as a general airplane physics question thread.
 
Loon
Shouldn't this thread be locked? I mean, we know the answer, we're 100% sure what the answer is, and if you're unclear on it, all you have to do is read a few of the most recent pages to get your answer.
Click to expand...
Agreed, but maybe it's the better choice to leave this one open for the ones that haven't been convinced yet. Otherwise, they may open a new one saying "I read the other thread, and you were all wrong".
 
Gods... who's going to say that? They're going to see this locked thread... see the end of it saying: "We locked this so you dumbclucks would stop bumping it. Check out this cool mythbusters video here." and will avoid making fools of themselves.

Which is better, but less fun than having some random newbie wander in here and making a fool of himself every few months. :lol:
 
Seriously, this thread's done. I'm closing it to stop idiots who post in threads without reading them from showing themselves as such. In this thread at least.
 
Status
Not open for further replies.

Latest Posts

Back