The downwind turn.

[Armitage]

An aircraft with a stall speed of 50 mph is flying into a 30 mph headwind at a ground speed of 60 mph. The pilot does a 180 degree turn at constant altitude in 30 seconds. Assume the engine on this aircraft cannot accelerate the aircraft an additional 20 mph in 30 seconds.

Does the aircraft stall?

 

[FoBoT]

i wish i was a pilot

i'll go with no, the airspeed after the turn will be 60 mph and that is higher than the 50 mph stall speed

 

[zanieladie]

Well if it does, I hope that *I* am not on it! :Q

 

[Cattlegod]

Only if there is a conveyor belt.

 

[spidey07]

Of course not.

Gravity.

 

[mordantmonkey]

considering the plane had enough power to maintain a ground speed of 60 while flying into a head wind, wouldn't its ground speed increase with a tailwind?

 

[Armitage]

Originally posted by: spidey07
Of course not.

Gravity.

Oops - I assume you mean trading altitude for airspeed. That's not allowed either, and I've edited the OP as such.

 

[FoBoT]

Originally posted by: mordantmonkey
considering the plane had enough power to maintain a ground speed of 60 while flying into a head wind, wouldn't its ground speed increase with a tailwind?

ground speed doesn't matter, stall speed is based on air speed , not ground speed

 

[spidey07]

Originally posted by: Armitage
Oops - I assume you mean trading altitude for airspeed. That's not allowed either, and I've edited the OP as such.

NO FAIR!!!!

 

[CaptnKirk]

60 MPH ground speed (True Airspeed) into a 30 MPH headwind is a 90 MPH Relative Wind.
Turning and flying at a true airspeed of 60 MPH with a 30 MPH tailwind gives you a 30 MPH Relative Wind, and you hit the dirt ball.
Your vehicle is 20 MPH below the stall speed, so you have a pant-load to contend with.

 

[mordantmonkey]

Originally posted by: FoBoT

Originally posted by: mordantmonkey
considering the plane had enough power to maintain a ground speed of 60 while flying into a head wind, wouldn't its ground speed increase with a tailwind?

ground speed doesn't matter, stall speed is based on air speed , not ground speed

yes but isn't airspeed a function of wind direction and ground speed?
what i was trying to say was that if the engine was maintaining the same thrust. the plane would increase speed (perhaps netting zero airspeed change) since it was no longer flying into a head wind. I know you tryed to rule that out in the op but it just seems illogical all around.

 

[FoBoT]

oh
so the throttle setting to maintain 60 mph ground speed with a 30 mph headwind, is relatively high

geez, i dunno

at least this doesn't involve stupid tread mills. i need to find a web site that has some info on this

 

[Yossarian]

I voted no at first glance but I think it's yes after a little thought. Indicated airspeed is 90 at first, so no problem. Indicated airspeed after the turn is 30. Without reducing the angle of attack by descending, the airplane will stall.

*I stand corrected based on skyking's explanation. stable airmass is the difference between the downwind turn and windshear on final.*

This is why windshear on short final is a big problem. 20 kts of headwind can suddenly become 20 kts of tailwind; your indicated airspeed drops by 40 and you have a problem.

 

[skyking]

OK, this is my area of expertise.
It is a common misconception that headwinds, tailwinds, have anything to do with an aircraft that is airborne in a stable airmass.
The airplane is like a boat in a sea of air. It matters not what direction that sea of air is moving, the plane is moving within it.
It will not stall or do anything unexpected when turning from downwind to upwind, crosswind, whatever.
The ground has nothing to do with flight, and what direction the wind is flowing over it is irrelavant.

Now let's talk about what does affect an aircraft in flight. Microbursts, huge downdrafts, windshear. Those are all conditions in an unstable or changing airmass. If a plane flies into a microburst on an approach, it will initially experience a headwind, then a downdraft, and then a tailwind. the aircraft cannot accelerate it's mass relative to these changing relative winds, and can get into trouble.
large jets are in the most danger here, because the engines are reduced to very low power settings. It takes precious seconds to "spool up" or spin the engines back up to full power. A propeller driven aircraft can respond much faster in those conditions, but is not immune to the danger.

 

[LordMorpheus]

I think probably no.

As soon as the pilot starts the turn, the wind will begin accelerating him in the direction in which it is blowin - this combined with the engine should allow him to find 20mph of groundspeed before he needs it.

 

[GuitarDaddy]

I vote no

60mph ground speed + 30mph headwind = 90mph airspeed

When you make the 180 turn, the throttle doesn't change so the airspeed doesn't change
the ground speed would change to 120mph (90+30)

*edit*
Unless you throw in a conveyor belt, and some constants that defy the laws of physics. Then all bets are off

 

[FoBoT]

i think the example is too simplistic

"It should be reemphasized here that a stall can occur at any airspeed, in any attitude, or at any power setting, depending on the total number of factors affecting the particular airplane."

http://avstop.com/AC/FlightTraingHandbook/StallCharacteristics.html
http://avstop.com/AC/FlightTraingHandbook/

 

[ShockwaveVT]

A: Plane is flying into the wind (30mph) @ 60 mph groundspeed. Thrust provided by its motors gives a net airspeed of 90mph.

B: Plane has turned 90 degrees and is perpendicular to the wind. Thrust provided by its motors still gives a net airspeed of 90mph. Since there is no effective wind, groundspeed = airspeed = 90mph.

C: Plan has completed 180 degree turn and is flying with the wind (30mph). Thrust provided by its motors still gives a net airspeed of 90mph. Wind (30mph) combined with airspeed gives 120 mph groundspeed.

Remember that the plane's motors act on the air, and not on the ground. The wind could be blowing 2,000mph relative to the ground, and the plane could still maintain 90mph airspeed. (above stall speed). As stated above, wind shear is an issue because the change in wind direction overcomes the motor's ability to accellerate the plane. a 30 second turn doesn't seem fast enough to cause a similar effect.

 

[GuitarDaddy]

Originally posted by: FoBoT
i think the example is too simplistic

"It should be reemphasized here that a stall can occur at any airspeed, in any attitude, or at any power setting, depending on the total number of factors affecting the particular airplane."

http://avstop.com/AC/FlightTraingHandbook/StallCharacteristics.html
http://avstop.com/AC/FlightTraingHandbook/

Always have to complicate things

 

[CaptnKirk]

Originally posted by: Yossarian
I voted no at first glance but I think it's yes after a little thought. Indicated airspeed is 90 at first, so no problem. Indicated airspeed after the turn is 30. Without reducing the angle of attack by descending, the airplane will stall.

This is why windshear on short final is a big problem. 20 kts of headwind can suddenly become 20 kts of tailwind; your indicated airspeed drops by 40 and you have a problem.

Or like several plane crash investigations have found:

A) Landing approach speed of 140 K (161MPH) encounters a downburst of air that pans out in front of the plane at 60 MPH.

This has reduced the true airspeed of the craft to 101 MPH almost instantly.

B) The downburst itself is pushing the wings down, according to the volume area of the wingspan as well,
including the weight of the water mass in the form of rain which makes the craft appear to weigh several thousand more Lbs than it really does.

Result

 

[Injury]

I'm probably a moron with dynamics, but if the plane is moving at 60, and the wind coming from behind it is moving at 30... wouldn't you be moving faster than the wind and thus, it would have no effect in lowering the pressure under the wings caused by thrust?

 

[CaptnKirk]

Originally posted by: Injury
I'm probably a moron with dynamics, but if the plane is moving at 60, and the wind coming from behind it is moving at 30... wouldn't you be moving faster than the wind and thus, it would have no effect in lowering the pressure under the wings caused by thrust?

Relative Wind

Airflow UNDER a wing has little to do with how a wing develops lift.
The lift is the result of a pressure difference developed by the airflow over and under the wind,
with the low pressure above the top of the wing causing the wing to move upward into the area of lower pressue - it sucks up.

 

[skyking]

That definition of relative wind is all wrong, I tell you.





It's all about Uncle Frank after the chili cookoff

 

[Demon-Xanth]

The aircraft has a wind speed of 90MPH, if it does a normal turn it doesn't lose airspeed. Thus it'll be going 120MPH after the turn. At this point, the FAA will assume that a terroist has control of the plane and an F15 will be dispatched to shoot it down. So you're boned anyways.

 

[KillerCharlie]

So let me get this straight:

Airplane's ground speed is initially 60mph and a 30 mph headwind (it's flying into a wind that is 30mph wrt to the ground). That makes the IAS (plane speed wrt to the freestream - the "relative wind" as its known by pilots) 90 mph.

The aircraft then atempts a 180 degree turn while holding the altitude constant, but the aircraft cannot accelerate fast enough to gain 20mph. 20mph what - ground speed or IAS? That makes a difference.

You're saying that the engine can't provide enough acceleration, so you're pretty much saying it's going to stall. In real life the engine (held at a fixed setting) would probably accelerate you fast enough so that your IAS is above stall speed once you've turned.

Also, a quick note: stall depends only on angle of attack, not airspeed. When a stall speed is given, that's the speed the aircraft would stall at trying to fly straight and level at a certain weight. If you increase your weight above this reference value, your aircraft will stall at a higher airspeed.