Relative Wind Question

[manlymatt83]

New student pilot here, all.

So I understand the theory of relative wind, and I even get the fact that a plane can stall at any airspeed, and it's really all about angle of attack. However, I also know that the relative wind can change. How long does it take to change?

For example, let's say I'm in a nose down free fall (forget the ground exists for this question) and apply back pressure. I realize that if I apply that back pressure too quickly, I will stall, because my relative wind is 100% vertical and there is downward momentum, and I'm reaching a critical angle of attack even though my nose is still pointed towards the ground. But at what point does that new direction become my relative wind, to where applying that "back pressure" allows things to even out?

 

[Capt. Geoffrey Thorpe]

How long does it take to accelerate your airplane to the new trajectory?

Angle of attack is pretty much a function of how hard you (and the trim) are pulling. Pull and the angle of attack goes up - push and it goes down. It's that simple.

Speed plays into the equation by determining how much G force is generated at any particular angle of attack. At high speeds you will pull a lot of Gs before you stall. At very low speeds the G load will be minimal (even less than one (assuming not level flight)) when you stall.

 

[write-stuff]

Remember that the relative wind is opposite the aircraft's line of flight. So, as quickly as the line of flight changes, so does the relative wind.

 

[manlymatt83]

So the reason the relative wind doesn't change immediately when you pull back on the elevators is because there's momentum in the original direction?

 

[jordane93]

Remember that the relative wind is opposite the aircraft's line of flight. So, as quickly as the line of flight changes, so does the relative wind.

This. As you soon as you change the flight path of the airplane, your relative wind is changing.

 

[manlymatt83]

This. As you soon as you change the flight path of the airplane, your relative wind is changing.

but that leaves me confused. If I apply back pressure to raise the nose the relative wind doesn't immediately change, right? If I did, there would never be a different angle of attack other than 0?

 

[Dan Thomas]

So the reason the relative wind doesn't change immediately when you pull back on the elevators is because there's momentum in the original direction?

Yes.

Too many pilots have crashed because they have never understood the accelerated stall and the conditions where it can happen. They'll do a buzz job on the airport or some friend's place and pull up sharply, and the airplane flicks over and dives into the ground before they know what happened. Or they'll do steep turns at low airspeed and low altitude.

Newton's first law says: [FONT=helvetica,geneva,arial][/FONT][FONT=helvetica,geneva,arial]Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. [/FONT]

So the airplane wants to travel in an established direction and needs an external force to change direction, and that force usually requires a change in lift from the wings. Ask for too much and you have trouble.

See http://www.planeandpilotmag.com/pilot-talk/ntsb-debriefer/the-accelerated-stall.html#.VUT7kZPfpL8

You are wise to want to understand this. Many can't be bothered, often because they don't realize just how inportant it is.

Dan

 

[Capt. Geoffrey Thorpe]

but that leaves me confused. If I apply back pressure to raise the nose the relative wind doesn't immediately change, right? If I did, there would never be a different angle of attack other than 0?

If you pull back, the angle of attack changes just as fast as the aircraft can rotate (limited by the polar moment of inertia).

The trajectory of the aircraft takes longer to change and that rate is a function of the change in lift and the aircraft total mass (A = F/M)

 

[manlymatt83]

If you pull back, the angle of attack changes just as fast as the aircraft can rotate (limited by the polar moment of inertia).

The trajectory of the aircraft takes longer to change and that rate is a function of the change in lift and the aircraft total mass (A = F/M)

And the difference between trajectory (which takes longer) and angle of attack (which is more immediate) is what causes the potential stalls.

 

[Jimmy cooper]

Having done some serious buzz jobs in a mooney years ago I can say that wings level, one can really do a magnificent buzz job on a combine, tractor, etc provided one does things gently and still returning to altitude quickly , not jerking the controls, etc. not go into a climbing turn with some Gs thrown in. That's where trouble lurks. Keeping the speed near red line, being gentle, no problems. Then of course, never never return for an encore. Be gone!

 

[drotto]

Just to throw a wrench in things. My DPE has a favorite question. An acrobatic or very high performance plane is flying straight up, is the plane stalled?

 

[jordane93]

Just to throw a wrench in things. My DPE has a favorite question. An acrobatic or very high performance plane is flying straight up, is the plane stalled?

If it has exceeded its critical angle of attack then it is stalled.

 

[drotto]

If it has exceeded its critical angle of attack then it is stalled.

Think about what angle of attack means. A plane is FLYING straight up, it is actually not stalled, but why? I actually got this question on my oral.

 

[airdale]

Just to throw a wrench in things. My DPE has a favorite question. An acrobatic or very high performance plane is flying straight up, is the plane stalled?

Think about it this way: Just as the airplane doesn't know that it is flying at night, it doesn't know that it is flying straight up (or straight down, for that matter). All that it knows is the relative wind. So the question does not provide the relevant information needed to determine the answer.

Langewiesche probably discussed this but I am too lazy to look it up.

 

[RotorDude]

Just to throw a wrench in things. My DPE has a favorite question. An acrobatic or very high performance plane is flying straight up, is the plane stalled?

Some military jets have more thrust than weight, so they can accelerate going straight up. In that case they are clearly flying and there is no stall. But even if the thrust to weight ratio is less than one, as long as the angle of attack is below the stall angle, you are not stalled. If you are flying straight up, your angle of attack would be near zero, well below the critical angle.

 

[drotto]

Both airdale and RotorDude are correct!!! By definition the angle of attack is the angle of the relative wind to the cord line of the wing. So in this case the relative wind is heading straight down in relation to the cord line of the wing. The critical angle of attack is therefor not reached, so no stall. Yes, you must have a high enough thrust to weight ratio to make this possible, but that gets more into the balanced unaccelerated flight potion of aerodynamics.

 

[fiveoboy01]

Think about what angle of attack means. A plane is FLYING straight up, it is actually not stalled, but why? I actually got this question on my oral.

I think he answered that already by stating that if the aircraft hasn't exceeded the critical AOA it's not stalled.

Attitude doesn't matter, AOA does.

 

[jordane93]

Think about what angle of attack means. A plane is FLYING straight up, it is actually not stalled, but why? I actually got this question on my oral.

Because it hasn't reached its critical angle of attack.

 

[Edgefly]

Great, if you actually got this question on your oral, you must have gotten the DPEs' answer to it. Share it with us.

 

[drotto]

The DPE answer was that a stall is all about exceeding the angle of attack, and the angle of attack is all about the angle between the cord line and the relative wind. In this situation, the angle of the relative wind to cord line of the wing does not exceed the critical angle of attack so the plane is not stalled.

 

[manlymatt83]

This is all starting to make sense. However, here's another question. Stalling has nothing to do with airspeed, right? Just angle of attack. So why is there a stall speed on a plane? If I'm full throttle and have the trim set for straight and level flight, and apply back pressure, won't I stall at the same critical angle of attack as I would if I was in slow flight and applied that same back pressure? After all, it's just about critical angle of attack as compared to the relative wind, no? What am I missing?

 

[kgruber]

Airspeed indicator is the simplest, cheapest, method of "almost" indicating a stall in normal flight conditions.

 

[drotto]

This is all starting to make sense. However, here's another question. Stalling has nothing to do with airspeed, right? Just angle of attack. So why is there a stall speed on a plane? If I'm full throttle and have the trim set for straight and level flight, and apply back pressure, won't I stall at the same critical angle of attack as I would if I was in slow flight and applied that same back pressure? After all, it's just about critical angle of attack as compared to the relative wind, no? What am I missing?

The critical angle of attack is important because it relates to the laminar flow of air over the wing and Bernoulli's principle. Bernoulli states that lift is created because air moving over top curved portion of the wing is moving faster and is therefor exerting less pressure on the wing then the slower moving air under the wing. Thus lift, but lift also requires laminar flow or smooth movement of air over the wing. At the critical angle the laminar flow separates from the wing and becomes turbulent and lift is destroyed. The plane stalls.

The stall speed is the slowest speed of the relative wind over the wing where laminar flow is maintained. You must remember this is only in straight and level flight. As you bank or do other maneuvers the stall speed will increase, so at a 30 degree bank the stall speed will no longer be the one indicated on the AI.

 

[Dan Thomas]

This is all starting to make sense. However, here's another question. Stalling has nothing to do with airspeed, right? Just angle of attack. So why is there a stall speed on a plane? If I'm full throttle and have the trim set for straight and level flight, and apply back pressure, won't I stall at the same critical angle of attack as I would if I was in slow flight and applied that same back pressure? After all, it's just about critical angle of attack as compared to the relative wind, no? What am I missing?

To stall the wing in cruise flight you'd have to pull hard and feel some very noticeable G loading. If the cruise speed is above maneuvering speed, stalling the airplane could easily damage it, perhaps fatally. That's what maneuvering speed is all about: the wing will stall before the airframe's certified load limit will be exceeded.

Most flying is at normal load factors, or 1 G. the "stall speed" makes sense there. The mistake some people make is thinking that the stall speed never changes and that they have a good margin above it when they start fooling around, pulling G's in pull-ups or steep turns or whatever.

Some pilots want an angle-of-attack indicator, but all they really need is to know that stall speed rises with load factor, and that if one is feeling heavier in the seat, the stall speed is rising. Got to keep thinking, keep aware. To some of us older guys, an AoA indicator is too much like ABS brakes in the car: they can dumb-down a driver by automatically keeping him out of trouble until the conditions finally get bad enough that no fancy doodads can save him. There have been some air transport accidents simply because the pilots trusted the doodads to do their thinking for them.

Dan

 

[fiveoboy01]

Stalling has nothing to do with airspeed, right? Just angle of attack.

I'm no aero-dynamicist, but I'm not entirely sure that's correct. While you can certainly stall the aircraft in any attitude at any airspeed, the airspeed affects the AOA along with the weight of the aircraft.. Ergo CFIs telling students to watch the airspeed on final.

If you had an AOA indicator you could likely ignore the airspeed indicator but most aircraft aren't equipped. So as said in a post above, mind your weight, maneuvering speed, and how hard you're twisting the yoke.

 

[Capt. Geoffrey Thorpe]

The critical angle of attack is important because it relates to the laminar flow of air over the wing and Bernoulli's principle. Bernoulli states that lift is created because air moving over top curved portion of the wing is moving faster and is therefor exerting less pressure on the wing then the slower moving air under the wing. Thus lift, but lift also requires laminar flow or smooth movement of air over the wing. At the critical angle the laminar flow separates from the wing and becomes turbulent and lift is destroyed. The plane stalls.

The stall speed is the slowest speed of the relative wind over the wing where laminar flow is maintained. You must remember this is only in straight and level flight. As you bank or do other maneuvers the stall speed will increase, so at a 30 degree bank the stall speed will no longer be the one indicated on the AI.

Picking a little bit of a nit: It has very little to do with "laminar" air flow. A turbulent boundary layer can actually remain attached at higher lift coefficients (which give higher adverse pressure ratio - going from the minimum pressure in the middle of the wing to the higher pressure at the trailing edge.) and delay the onset of a stall. That's why some aircraft have vortex generators.

But, at stall, the flow does separate from the wing upper surface.

(Lift also has very little relation to differences in the length between the top and bottom of the wing as some others may suggest.)

Given the curve above, the airfoil will be stalled at angles of attack above about 17 degrees. The load (aircraft mass * G loading) at which that happens is a function of the speed since maximum lift = Cl maximum * 1/2 * air density * Velocity squared. If you set the lift equal to the weight of the aircraft, then you can calculate the 1G stall speed from V = square root(2*weight/(Cl maximum *density) ).

 

[airdale]

... So why is there a stall speed on a plane? ...

There isn't a stall speed on an airplane.

What you are probably thinking of is Vso, which is stall speed "in the landing configuration." Level or slightly descending flight, full flaps, gear, etc. It's just one of the airplane's many stall speeds. The Vso number is particularly useful because 1.3 x Vso is a good target approach speed near the end of the runway. (Remember there's probably a difference between calibrated and indicated airspeed, though that's another thread.)

More than you wanted: http://www.ecfr.gov/cgi-bin/text-id...=text&node=14:1.0.1.1.1.0.1.2&idno=14;cc=ecfr

 

[AKBill]

The critical angle of attack is important because it relates to the laminar flow of air over the wing and Bernoulli's principle. Bernoulli states that lift is created because air moving over top curved portion of the wing is moving faster and is therefor exerting less pressure on the wing then the slower moving air under the wing. Thus lift, but lift also requires laminar flow or smooth movement of air over the wing. At the critical angle the laminar flow separates from the wing and becomes turbulent and lift is destroyed. The plane stalls.

The stall speed is the slowest speed of the relative wind over the wing where laminar flow is maintained. You must remember this is only in straight and level flight. As you bank or do other maneuvers the stall speed will increase, so at a 30 degree bank the stall speed will no longer be the one indicated on the AI.

Very well stated.

This thread reminds me of an accident that killed a friend of mine. He was flying a Beech 18 loaded with cedar shingles. The plane was loaded close to maximum load limits. The load shifted on takeoff. The change in CG stalled the plane.

Very sad, loosing a friend. He was a very experienced high time pilot. RIP Tom.

 

[drotto]

OK as Light and Sporty said, laminar flow is a little bit of a misstatement. The better term would be the boundary layer must remain very close to the wing. When the boundary layer separates, the plane stalls. It is a little bit of a nit-pick because the FAA does use the word laminar flow and boundary layer in a very similar manner. The FAA in fact makes a big deal about the laminar flow over the wind in relation to frost, and that is a question that appears on the written test.

 

[Jimmy cooper]

Some military jets have more thrust than weight, so they can accelerate going straight up. In that case they are clearly flying and there is no stall. But even if the thrust to weight ratio is less than one, as long as the angle of attack is below the stall angle, you are not stalled. If you are flying straight up, your angle of attack would be near zero, well below the critical angle.

An F15 pilot explained to me that it was fun to go straight up at full bore until it ran out of air at about fifty thousand, stalled and fell over.

 

[FastEddieB]

New student pilot here, all.

So I understand the theory of relative wind, and I even get the fact that a plane can stall at any airspeed, and it's really all about angle of attack. However, I also know that the relative wind can change. How long does it take to change?

Some good answers so far.

My advice would be to beg, borrow or steal a copy of "Stick and Rudder".

And read it, of course.

It was my very first aviation read and I think it pointed me in the right direction early on.

 

[Jimmy cooper]

Some good answers so far.

My advice would be to beg, borrow or steal a copy of "Stick and Rudder".

And read it, of course.

It was my very first aviation read and I think it pointed me in the right direction early on.

It should be required reading with a test before a ppl is issued.

 

[Xscaliber]

Some good answers so far.

My advice would be to beg, borrow or steal a copy of "Stick and Rudder".

And read it, of course.

It was my very first aviation read and I think it pointed me in the right direction early on.

Back in my training days, Go out and buy Stick & Rudder was one of the first things my instructor told me to do, it was very good advice the book is ageless

 

[Matthew]

Back in my training days, Go out and buy Stick & Rudder was one of the first things my instructor told me to do, it was very good advice the book is ageless

Even before I took my first lesson a crusty old CFI told me to get and read that book first.

 

[FastEddieB]

I mentioned it before, but here's the page of the Whole Earth Catalog entry that led me to 'Stick and Rudder", and ultimately to a 40 year flying adventure:

Might be a little more than $8.95 now!