Flying with a pulse - oximeter

[FastEddieB]

Interesting hypothetical.

 

Let me think on it.

 

And while I do so, ponder this:

 

 

It combines goofy with technical in an odd way, but has a lot of info to digest.

[FlyingMonkey]

The "excess thrust" argument seems to negate the commonly held belief that AoA is the determinant of lift, and that a climb or descent is related to AoA.  If an plane's wing has an AoA such that its lift equals its weight, it flies straight and level.  If the AoA is decreased, lift is reduced and the airplane descends; of AoA is increased, lift is enhanced and the airplane climbs.

 

I think excess thrust beyond what is required for level flight is needed to maintain a climb, but once the airplane is in a steady climb, the airplane is in a steady state at a higher AoA and the excess thrust is balanced by the increased drag of the higher AoA.  The airplane is back to a state of equilibrium, with the thrust equal to drag, but with lift higher at the new AoA and a climb as the result.

[Tom Baker]

Hi Doug,

 

I think you are answering a different question.  The question I am asking is whether an airplane with thrust that is always horizontal and therefore making no contribution to upward forces on the airplane (in opposition to the effects of weight) can still climb as a result of lift from the wings alone.  

 

My impression is that Eddie has stated that it would not and that the forces that differ between level flight and climbing flight are the result of thrust alone, but I am not convinced.  I wrote the above comment as a hypothetical to explore that statement.

 

fg

 

Fred,

 

Having an engine rotate so it would stay parallel to the ground or level would not work out well. While the ground is a good reference point for us when we fly, it is not as important as the direction the airplane is moving through the air. Imagine what your proposed rotating engine would do if you tried to do a loop in the airplane.

 

Now if your engine could rotate to stay parallel with the relative wind it might work out OK.

 

For the most part on a low powered airplane like the CT it is not the excess thrust pointed at a downward angle that makes the airplane climb. It is the fact that the excess thrust allows you to maintain the relative wind at a positive angle in relationship to level. As long as you have enough thrust, regardless of the angle, to keep the relative wind at a positive angle the airplane is going to climb.

 

As for gliders. Once a glider is in the air by what ever means it is descending in the air mass. Because the glider has such a slow descent rate it will go up as long as the air mass is rising at a rate greater than the glider is descending through it. They use higher speeds to move from one area of lift to another. They can use the energy from that speed to climb, but once that energy is gone they are back to relying on the rising air mass.

[FlyingMonkey]

Fred,

 

Having an engine rotate so it would stay parallel to the ground or level would not work out well. While the ground is a good reference point for us when we fly, it is not as important as the direction the airplane is moving through the air. Imagine what your proposed rotating engine would do if you tried to do a loop in the airplane.

 

 

That's the point of a hypothetical, to take something to an extreme or logical conclusion to see where an argument's limits are.  

[Doug G.]

Hi Doug,

 

I think you are answering a different question. The question I am asking is whether an airplane with thrust that is always horizontal and therefore making no contribution to upward forces on the airplane (in opposition to the effects of weight) can still climb as a result of lift from the wings alone.

 

My impression is that Eddie has stated that it would not and that the forces that differ between level flight and climbing flight are the result of thrust alone, but I am not convinced. I wrote the above comment as a hypothetical to explore that statement.

 

fg

Of course it can. Air over an airfoil creates lift. The Wright Brothers proved that in their wind tunnel.

[Doug G.]

Assuming of course that you accept Bernoulli.

[Tom Baker]

The "excess thrust" argument seems to negate the commonly held belief that AoA is the determinant of lift, and that a climb or descent is related to AoA.  If an plane's wing has an AoA such that its lift equals its weight, it flies straight and level.  If the AoA is decreased, lift is reduced and the airplane descends; of AoA is increased, lift is enhanced and the airplane climbs.

 

I think excess thrust beyond what is required for level flight is needed to maintain a climb, but once the airplane is in a steady climb, the airplane is in a steady state at a higher AoA and the excess thrust is balanced by the increased drag of the higher AoA.  The airplane is back to a state of equilibrium, with the thrust equal to drag, but with lift higher at the new AoA and a climb as the result.

 

Andy,

 

It might be a good time to go back and study up on aerodynamics.

I have never heard that AoA is the detriment of lift, but an increase in lift does increase drag.

Climb and descent is not related to AoA. Speed is related to AoA.

If the AoA is reduced the airplane will increase speed. It is true that it will descend, but the reason is because you don't have the reserve power needed to maintain level flight at the new airspeed.

If you increase angle of attack the airplane will slow down because of the increased AoA. Depending on speed and power setting before you increase AoA the airplane may climb or descend. If you are in the region of reverse command (back side of the power curve) the airplane will descend with a increase in AoA.

[FlyingMonkey]

Andy,

 

It might be a good time to go back and study up on aerodynamics.

I have never heard that AoA is the detriment of lift, but an increase in lift does increase drag.

Climb and descent is not related to AoA. Speed is related to AoA.

If the AoA is reduced the airplane will increase speed. It is true that it will descend, but the reason is because you don't have the reserve power needed to maintain level flight at the new airspeed.

If you increase angle of attack the airplane will slow down because of the increased AoA. Depending on speed and power setting before you increase AoA the airplane may climb or descend. If you are in the region of reverse command (back side of the power curve) the airplane will descend with a increase in AoA.

 

Huh...NASA sure thinks AoA relates to lift:

 

https://www.grc.nasa.gov/www/k-12/airplane/incline.html

 

The very first line of the article:

 

"As a wing moves through the air, the wing is inclined to the flight direction at some angle. The angle between the chord lineand the flight direction is called the angle of attack and has a large effect on the lift generated by a wing."

 

Also noted:

 

"For thin airfoils, the lift is directly proportional to the angle of attack for small angles (within +/- 10 degrees). For higher angles, however, the dependence is quite complex. "

 

Since most of our flying is at less than 10° AoA, this seems relevent to this discussion.

 

Their attached image also seems to indicate a relationship:

 

[gbigs]

The "excess thrust" argument seems to negate the commonly held belief that AoA is the determinant of lift, and that a climb or descent is related to AoA.  If an plane's wing has an AoA such that its lift equals its weight, it flies straight and level.  If the AoA is decreased, lift is reduced and the airplane descends; of AoA is increased, lift is enhanced and the airplane climbs.

 

I think excess thrust beyond what is required for level flight is needed to maintain a climb, but once the airplane is in a steady climb, the airplane is in a steady state at a higher AoA and the excess thrust is balanced by the increased drag of the higher AoA.  The airplane is back to a state of equilibrium, with the thrust equal to drag, but with lift higher at the new AoA and a climb as the result.

 

Not thrust, but energy.  And lifting the nose only changes the direction of thrust.  Again, ignoring the energy of the air and airframe ignores the full reason an aircraft (glider or powered) climbs.

[Tom Baker]

Andy, Don't try and put words in my mouth, I never said AoA is not related to lift!!

[Tom Baker]

Andy, what controls the speed of the airplane?

[FastEddieB]

The "excess thrust" argument seems to negate the commonly held belief that AoA is the determinant of lift, and that a climb or descent is related to AoA. If an plane's wing has an AoA such that its lift equals its weight, it flies straight and level. If the AoA is decreased, lift is reduced and the airplane descends; of AoA is increased, lift is enhanced and the airplane climbs.

I don't have time right now, but I think you really need to rethink that.

 

In practical, day to day flying, keeping that mental image will likely do no harm.

 

But if you do go on to advanced licenses and ratings, pretty sure you'll get certain select questions wrong on a regular basis until you rethink this.

 

Best to remember that, simplified, in any steady state unaccredited flight condition - whether it's straight and level, climb or descent - lift will equal weight*. If it doesn't, in a transient state, it will soon balance again.

 

 

*To be concise, in any steady state unaccelerated flight condition, total up forces must equal total down forces, by definition. Any time they're not, the plane will no longer be in an unaccelerated state, until it stabilizes and regains equilibrium.

[Tom Baker]

For a given airplane at a given weight and CG there is only one matching AoA for each speed. A low AoA will result in a higher speed, and as you increase the AoA the airplane will slow down. This goes all the way to the critical AoA where the airplane stalls. Whether the airplane will maintain level flight or climb depends on whether you have enough power to maintain the corresponding speed, or an excess in power to make it climb. Just because you increase or decrease the AoA doesn't mean the airplane will climb or descend, but it will change the speed it is flying through the air. In fact there are times when an increase in AoA will make the airplane descend, and a decrease in AoA will make it climb.

 

The wing is producing lift anytime there is air flowing across the wing in the proper direction regardless of AoA. With a low AoA unless the speed is high enough to support the weight of the airplane it will descend. Even when you reach the critical AoA the wing is still producing lift, just not enough to support the airplanes weight.

[FlyingMonkey]

For a given airplane at a given weight and CG there is only one matching AoA for each speed. A low AoA will result in a higher speed, and as you increase the AoA the airplane will slow down. This goes all the way to the critical AoA where the airplane stalls. Whether the airplane will maintain level flight or climb depends on whether you have enough power to maintain the corresponding speed, or an excess in power to make it climb. Just because you increase or decrease the AoA doesn't mean the airplane will climb or descend, but it will change the speed it is flying through the air. In fact there are times when an increase in AoA will make the airplane descend, and a decrease in AoA will make it climb.

 

The wing is producing lift anytime there is air flowing across the wing in the proper direction regardless of AoA. With a low AoA unless the speed is high enough to support the weight of the airplane it will descend. Even when you reach the critical AoA the wing is still producing lift, just not enough to support the airplanes weight.

 

I agree with this.  I thought we were talking about normal flight speeds though, not stalls or "the back side of the curve" where all of this other stuff happens.

 

In practical terms, if you have sufficient speed and increase AoA, the airplane will climb.  If you don't add excess thrust to maintain the climb, airspeed will decay and the climb will deteriorate, further increasing AoA (assuming the attitude remains constant) potentially to the stall.  One can say excess thrust causes the climb, I think it's more correct to say excess thrust enables higher lift to be generated, which causes the climb.

 

I might be completely wrong here, but that's how I visualize it.

[Ed Cesnalis]

The hypothetical wings need a little more than to remain parallel to the ground.  They need to be able to adjust the AOA, as soon as you get a positive AOA they are pointed at the sky but if they can't there won't be any climb

[FastEddieB]

One can say excess thrust causes the climb, I think it's more correct to say excess thrust enables higher lift to be generated, which causes the climb.

 

I might be completely wrong here, but that's how I visualize it.

 

 

As I said, visualizing it this way is likely to do you no harm in real life.

 

Your statement is more correct with one change:  "I think it's more correct to say excess thrust enables higher lift to be generated, which initiates the climb".

 

Once in the climb no "higher lift" is in evidence. If you seriously contemplate some of the links and YouTube videos I linked, you will find that in a steady state climb the wings actually generate a little bit less lift than in straight and level flight.

[FlyingMonkey]

As I said, visualizing it this way is likely to do you no harm in real life.

 

Your statement is more correct with one change:  "I think it's more correct to say excess thrust enables higher lift to be generated, which initiates the climb".

 

Once in the climb no "higher lift" is in evidence. If you seriously contemplate some of the links and YouTube videos I linked, you will find that in a steady state climb the wings actually generate a little bit less lift than in straight and level flight.

 

Okay, I understand that.  In the end in a 1g configuration the weight of the airplane is the same.  However, it still seems it takes more lift to *raise* (climb) the airplane than to just keep it level.  If it were not making more lift, it would not climb.  This takes additional thrust because you are fighting gravity.  Which I guess is where your altered lift vector comes in.  

 

I have to admit though the line "if you seriously contemplate" is condescending, and sounds like "if you'd just being such a fool and just understand what I'm saying, all would be well."   I don't think I have not taken this discussion "seriously".

[FastEddieB]

If it were not making more lift, it would not climb.

Yes. It would. To repeat, the wings of an aircraft in a steady state climb actually produce slightly less lift than in straight-and level flight. It's counterintuitive, but true - as shown in diagrams and videos already posted.

 

I have to admit though the line "if you seriously contemplate" is condescending, and sounds like "if you'd just being such a fool and just understand what I'm saying, all would be well." I don't think I have not taken this discussion "seriously".

Sorry. Did not mean it that way. But you keep repeating statements like the above, indicating you still haven't quite gotten it, and each of those links do try to explain why that is so.

 

For some here, "An airplane climbs because of lift greater than weight" seems so firmly entrenched it's hard to overcome it. It's just not true. There may be an "aha" moment, where it becomes clear. In my role as instructor, I have to work pretty hard for that sometimes. You may feel it's not my job to "instruct" here, but I still get some satisfaction when I sometimes get to impart my knowledge to others. Or to be corrected when I do, in fact, have something wrong.

[FlyingMonkey]

Okay, I will keep reading.

[FlyingMonkey]

Immediately ran into stuff saying what I'm saying...like this, again from NASA Glenn Research Center:

 

https://www.grc.nasa.gov/www/k-12/airplane/smotion.html

 

So now I'm really confused.  According to the NASA slides, excess thrust only becomes a significant factor in moderate climbs, not in shallow climbs:

 

"For more moderate angles, high excess thrust can provide an important contribution to the vertical acceleration. The next time you visit an airport, notice the high climb angles used by modern airliners. This flight path is possible because modernturbine engines develop high excess thrust at takeoff. "

[FastEddieB]

The key is "Simplified". Though that is confusing, I admit.

 

Lift>weight = plane accelerates upwards.

 

Weight>lift = plane accelerates downward.

 

Both transient states. Once stabilized, everything balances again, the very definition of "steady state".

[FlyingMonkey]

The key is "Simplified". Though that is confusing, I admit.

 

Lift>weight = plane accelerates upwards.

 

Weight>lift = plane accelerates downward.

 

Both transient states. Once stabilized, everything balances again, the very definition of "steady state".

 

Sure, and still *exactly* what I was talking about!

 

Still reading.

[FredG]

Tom, I was not proposing the design as useful for anything other than a thought experiment.  My point was that Eddie seemed to be saying that an airplane that is climbing does so solely because of the fact that the thrust from the engine is no longer horizontal and has some upward component.  So, I asked him to envision the hypothetical airplane that has horizontal thrust regardless of whether it is in level flight, a climb or a descent.  In such an airplane, what would happen when, during level flight, the pilot pulls back on the stick to increase the AOA of the wing?  Since the thrust vector is still horizontal, will that airplane climb due to an increase in AOA alone?  I believe it will and if it does, my conclusion is that the vertical component of thrust is not the sole reason why an airplane will maintain a sustained climb.

 

fg

[FastEddieB]

I'm still pondering that thought experiment.

 

Similar would be a glider launched by a cable pulling horizontally, with zero upward component. It still gets the glider aloft.

 

We're leaving on a camping trip in the morning, but I promise to think about it while gone.

[FastEddieB]

Check out this thread, covering similar ground:

 

http://www.rcgroups.com/forums/showthread.php?t=1387072

 

Special attention to post #12, which at first glance is a nice summary.