Four forces

[HF17]

This might be a stupid question, again. But I’m trying to think about what the four forces of flight would look like for an airplane behind the power curve while in a descent. I understand that ahead of the power curve, there is a horizontal component of lift and weight acting forward making the airplane speed up. Is this what the forces would look like while being behind the power curve in a descent?

 

[EdFred]

Four forces are the four forces. All the happens is some of the arrows (vectors of the forces) get bigger or smaller depending on what you are doing.

 

[HF17]

Four forces are the four forces. All the happens is some of the arrows (vectors of the forces) get bigger or smaller depending on what you are doing.

I guess my question is that if an airplane is descending behind the power curve, the lift vector is pointing forward because it has to be perpendicular to the relative wind. Shouldn’t be pointed aft because of a higher induced drag?


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[murphey]

You forgot the 5th force of flight.


$$$

 

[nauga]

I guess my question is that if an airplane is descending behind the power curve, the lift vector is pointing forward because it has to be perpendicular to the relative wind. Shouldn’t be pointed aft because of a higher induced drag?

Lift is by definition perpendicular to the relative wind, drag is parallel to it. You can resolve these into any number of vectors with any number of magnitudes and relative directions if you choose, but those choices wouldn't follow convention.

Nauga,
all thrust, no vector

 

[murphey]

I guess my question is that if an airplane is descending behind the power curve, the lift vector is pointing forward because it has to be perpendicular to the relative wind. Shouldn’t be pointed aft because of a higher induced drag?


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No…the drag vector gets bigger.

 

[HF17]

No…the drag vector gets bigger.

Drag would increase due to the lift vector pointing rearward


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[EdFred]

Drag would increase due to the lift vector pointing rearward


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No. Lift doesn't go rearward. Drag increases because....drag increases.

 

[rhkennerly]

What you’re missing in your diagram is angle of attack. The natural forces don’t change direction. Lift us up. Gravity is down. Behind the power curve, hanging on the prop (as opposed to gliding power off) the direction of thrust rotates around the vertical axis. So, you rotate the plane to be nose high. You do not change the direction of lift.

 

[HF17]

No. Lift doesn't go rearward. Drag increases because....drag increases.

I think this picture in the PHAK made me think that

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[nauga]

The tilted vector is not pure lift, since it's not perpendicular to the relative wind. Downwash causes to the total aerodynamic force vector to tilt aft, but when that vector is resolved into lift and drag the additional aft component (parallel to the relative wind) comprises induced drag.

Nauga,
and curved air

 

[HF17]

It seems like some sources say that lift acts perpendicular to the relative wind and others say that lift acts perpendicular to the downwash.

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[MauleSkinner]

It seems like some sources say that lift acts perpendicular to the relative wind and others say that lift acts perpendicular to the downwash.

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Those weren’t necessarily written by people who knew what they were talking about.

 

[dmspilot]

Induced drag that "has the same effect as" an aftward lift component does not mean that's what it is. It's a simile. Drag is drag, lift is lift. They are different components.

The back side of the power curve is shown on an airspeed vs. power graph, you aren't going to demonstrate it with force vectors. Power is work done over time, the four force diagram doesn't have a time component on it.

 

[HF17]

Induced drag that "has the same effect as" an aftward lift component does not mean that's what it is. It's a simile. Drag is drag, lift is lift. They are different components.

The back side of the power curve is shown on an airspeed vs. power graph, you aren't going to demonstrate it with force vectors. Power is work done over time, the four force diagram doesn't have a time component on it.

Instead of saying “the back side of the power curve,” I should’ve said a slow speed steep descent. Like this picture:

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[MauleSkinner]

Instead of saying “the back side of the power curve,” I should’ve said a slow speed steep descent. Like this picture:

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Yes, that’s what the four forces look like. If you split them into vertical and horizontal components, the horizontal ones will be equal (but opposite) as will the vertical ones.

 

[HF17]

Yes, that’s what the four forces look like. If you split them into vertical and horizontal components, the horizontal ones will be equal (but opposite) as will the vertical ones.

Ok. I guess I was confused about was why the lift vector is pointing forward and not aft. I understand that lift is perpendicular to the relative wind, but the PHAK (and some other sources) says that lift is perpendicular to the downwash.


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[nauga]

It seems like some sources say that lift acts perpendicular to the relative wind and others say that lift acts perpendicular to the downwash.

I don't think you really have a definitive source that says "lift acts perpendicular to the downwash." You might have a few that talk about downwash "tilting the lift vector," but they are really talking about downwash tilting the total aerodynamic force vector (the vector sum of lift and drag). If you're basing your position (I hesitate to say 'argument') on pilot training literature you'd be well-served to look at an aero engineering textbook or two.

Nauga,
and his vector detector

 

[HF17]

I don't think you really have a definitive source that says "lift acts perpendicular to the downwash." You might have a few that talk about downwash "tilting the lift vector," but they are really talking about downwash tilting the total aerodynamic force vector (the vector sum of lift and drag). If you're basing your position (I hesitate to say 'argument') on pilot training literature you'd be well-served to look at an aero engineering textbook or two.

Nauga,
and his vector detector

Ok, that makes much more sense. I thought they were just talking about the lift vector. Thank you


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[Dan Thomas]

That diagram is incomplete. You need this one:



See that "resultant force?" The combination of lift and drag at high AoA looks like that. As AOA increases, the drag vector gets longer and the resultant tilts back even more.

 

[nauga]

See that "resultant force?" The combination of lift and drag at high AoA looks like that. As AOA increases, the drag vector gets longer and the resultant tilts back even more.

At the risk of confusing things even more, I think you've got cause and effect reversed. The "resultant force" is not a result of lift and drag, the conventions of lift and drag are a result of breaking the aerodynamic force vector (labeled as "resultant force") into components perpendicular and parallel to the relative wind. Downwash tilts that total force vector aft, which resolves to an increase in drag, not vice versa.

It's also unfortunate that lift isn't perpendicular and drag parallel to the relative wind in your drawing

Nauga,
master of confusion

 

[Dana]

Instead of saying “the back side of the power curve,” I should’ve said a slow speed steep descent. Like this picture:

Yes, that's correct. But an aeronautical engineer might look it from a different frame of reference, like this (makes the math simpler):



In fact, because lift is so much greater than drag, the resultant of lift and drag vectors mean that at high AOA the wing is actually being pulled forward (relative to the aircraft centerline, not to the direction of flight).

 

[squincher]

This might be a stupid question, again. But I’m trying to think about what the four forces of flight would look like for an airplane behind the power curve while in a descent. I understand that ahead of the power curve, there is a horizontal component of lift and weight acting forward making the airplane speed up. Is this what the forces would look like while being behind the power curve in a descent?

You lack artistic ability.

 

[HF17]

You lack artistic ability.

I wasn’t trying to make it look artistic


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[MauleSkinner]

You lack artistic ability.

Disclaimer I generally use…
“My niece has her Master’s degree in Studio Art. She did not get that talent from my side of the family.”

 

[nauga]

You lack artistic ability.

Yet several of us were able to interpret it and have a reasonable discussion about the physics behind it. Seems like it served its purpose.

Nauga,
restrained

 

[steingar]

 

[dtuuri]

Those weren’t necessarily written by people who knew what they were talking about.

I think it falls under the concept of an "easily understood workable falsehood that explains a complex incomprehensible truth."