Load factor in stabilized climb/descent < 1G

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MrManH

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MrManH
Hi everyone,

I originally learned that in a stabilized climb or descent, the load factor is 1G which for all practical intents and purposes is close enough to correct.

Following the example in this thread:
http://www.pprune.org/professional-...d-factor-during-stabilized-climb-descend.html

The load factor when the airplane is in a stabilized climb or descent is slightly less than 1.

While I understand the math, I don't understand why the load factor is less than 1 in a stabilized climb. A climb requires down-force on the tail to increase the AoA of the wings, this down-force increases wing loading, doesn't that mean a slightly increased load factor?
 
MrManH said:
While I understand the math, I don't understand why the load factor is less than 1 in a stabilized climb. A climb requires down-force on the tail to increase the AoA of the wings, this down-force increases wing loading, doesn't that mean a slightly increased load factor?
Click to expand...
Setting aside the math (yawn, math bores me), in a near vertical dive or climb you'd have near zero lift. That would be a very low load factor wouldn't it?

dtuuri
 
So you would still have 1g on your body but the wings are not loaded. Is that why?
 
SixPapaCharlie said:
So you would still have 1g on your body but the wings are not loaded. Is that why?
Click to expand...
Yes.

Furthermore, by my reasoning, the definition of load factor is =lift/weight, so if thrust provides the entire balancing force in a vertical climb there is no need for lift or tail down force either for that matter. Same for a vertical terminal velocity dive. More horizontal flight paths cause more load factor as the wings assume more and more of the load. If the thrust vector in a climb results in a nose-up moment, the tail down force would be lessened versus a descent resulting in the unintuitive fact that the wing produces less lift in a climb than in a descent. The net effect then would depend on the placement of the engine with respect to the CG and the thrust available.

dtuuri
 
dtuuri said:
Setting aside the math (yawn, math bores me), in a near vertical dive or climb you'd have near zero lift. That would be a very low load factor wouldn't it?

dtuuri
Click to expand...

In a perfectly vertical climb, thrust is the only source of "lift". In a vertical descent, the airplane is being pulled by gravity. In both cases the wings produce zero lift. However in a non vertical climb, lift must be increased (higher AoA) to be greater than weight, wouldn't that increase in lift result in a higher load factor? It makes sense to me that the load factor would be decreased in a descent, but not in a climb.
 
MrManH said:
In a perfectly vertical climb, thrust is the only source of "lift". In a vertical descent, the airplane is being pulled by gravity. In both cases the wings produce zero lift. However in a non vertical climb, lift must be increased (higher AoA) to be greater than weight, wouldn't that increase in lift result in a higher load factor? It makes sense to me that the load factor would be decreased in a descent, but not in a climb.
Click to expand...

There is no difference between a climb or descent in this respect.
The load factor is the ratio of lift to weight, with the informal units of "g".
In either climb or descent, the lift is reduced, since thrust or gravity (respectively) are carrying part of the load, hence the wing must reduce its lift to maintain a steady state. The lift reduction is controlled by angle of attack, which is set by the pilot. As noted by others, in the extreme case of a pure vertical climb or descent, the lift would be reduced to zero, but in a typical climb or descent angle it would only be reduced slightly.
 
MrManH said:
The load factor when the airplane is in a stabilized climb or descent is slightly less than 1.

While I understand the math, I don't understand why the load factor is less than 1 in a stabilized climb. A climb requires down-force on the tail to increase the AoA of the wings, this down-force increases wing loading, doesn't that mean a slightly increased load factor?
Click to expand...

This makes me realize that load factor and G-load are not the same thing.

Assuming this is correct, which on first thought it seems to be, then in a stabilized climb, even a vertical one, some or all of the weight is being offset by thrust. This means lift produced by the wings is less than weight, meaning the load factor is less than one. However, unless the aircraft is accelerating, the aircraft and pilot still experience 1G. The pilot doesn't feel heavier and most certainly does not feel lighter.

On a side note, it is a common mistake that even the FAA makes to measure load factor in "G" units. Since load factor is ratio of lift to weight, and both are measured in the same unit (e.g. pounds), the units divide out and make load factor a unitless number.
 
RotorDude said:
There is no difference between a climb or descent in this respect.
The load factor is the ratio of lift to weight, with the informal units of "g".
In either climb or descent, the lift is reduced, since thrust or gravity (respectively) are carrying part of the load, hence the wing must reduce its lift to maintain a steady state. The lift reduction is controlled by angle of attack, which is set by the pilot. As noted by others, in the extreme case of a pure vertical climb or descent, the lift would be reduced to zero, but in a typical climb or descent angle it would only be reduced slightly.
Click to expand...

No debate on 0 load factor in a pure vertical climb or descent. However I could be wrong but I'm not sure I agree with the statement in bold. If you were flying at cruise power and entered a shallow climb, you wouldn't have any extra thrust and yet you'd be climbing and at some point end up in a stabilized climb where your airspeed has stopped decreasing. Increasing the AoA does increase lift.
 
RotorDude said:
In either climb or descent, the lift is reduced, since thrust or gravity (respectively) are carrying part of the load
Click to expand...

I believe gravity doesn't carry part of the load during a descent; drag does.
 
dmspilot said:
I believe gravity doesn't carry part of the load during a descent; drag does.
Click to expand...

Gravity partially replaces thrust in a descent, while drag counterbalances thrust, as always. When the forces are balanced (steady state), the wing feels less of the "down" component of the weight, produces less lift and hence less has a lower "load factor".
 
RotorDude said:
Gravity partially replaces thrust in a descent, while drag counterbalances thrust, as always. When the forces are balanced (steady state), the wing feels less of the "down" component of the weight, produces less lift and hence less has a lower "load factor".
Click to expand...

That's not what I disagree with. You said earlier that gravity "carries the load". The load of what? Gravity doesn't carry a load, that doesn't make sense, and gravity is not one of the four forces. In a vertical dive, the upward acting force is drag and the downward acting force is weight. Therefore, drag "carries the load" of the aircraft's weight.
 
MrManH said:
No debate on 0 load factor in a pure vertical climb or descent. However I could be wrong but I'm not sure I agree with the statement in bold. If you were flying at cruise power and entered a shallow climb, you wouldn't have any extra thrust and yet you'd be climbing and at some point end up in a stabilized climb where your airspeed has stopped decreasing. Increasing the AoA does increase lift.
Click to expand...

Correct. But what you are ignoring is that when you climb, your flight path becomes inclined upwards, i.e. the relative wind is no longer parallel to the ground. So you increase the angle of attack, get a bit more lift, and start climbing at a lower speed, as you say. When things stabilize (assuming no power change), your lift will become a bit less than originally, since your relative wind will be now be coming slightly down at you.
 
dmspilot said:
That's not what I disagree with. You said earlier that gravity "carries the load". The load of what? Gravity doesn't carry a load, that doesn't make sense, and gravity is not one of the four forces. In a vertical dive, the upward acting force is drag and the downward acting force is weight. Therefore, drag "carries the load" of the aircraft's weight.
Click to expand...

I agree. When going down, drag partially contributes to the vertical component of the total aerodynamic force on the wing, and is exactly balanced by total thrust (which is the sum of the engine thrust plus the gravity component pulling the plane down). So you are right in saying that drag (partially) carries the weight of the plane in a descent.

Here is a diagram of an aircraft in descent (glider or airplane at idle) that illustrates the acting forces. Notice how the lift vector is smaller than the total aerodynamic force, since drag is now helping the plane stay up:

27.jpg
 
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My very first line says "I originally learned that in a stabilized climb or descent, the load factor is 1G which for all practical intents and purposes is close enough to correct."

This thread is for those who are interested in digging a little deeper in the subject, and thanks to it I have a better understanding of the forces in flight.

Since we're talking about forces, reading the PHOK I came across this regarding thrust. "As a general rule, it is said to act parallel to the longitudinal axis. However, this is not always the case as will be explained later."

I've been trying to find the part where it explains how it's not always parallel to the longitudinal axis to no avail. All I can think of are planes with thrust vectoring or perhaps with engines mounted at a certain angle that causes thrust to not be parallel to the longitudinal axis.
 
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MrManH said:
Since we're talking about forces, reading the PHOK I came across this regarding thrust. "As a general rule, it is said to act parallel to the longitudinal axis. However, this is not always the case as will be explained later."

I've been trying to find the part where it explains how it's not always parallel to the longitudinal axis to no avail. All I can think of are planes with thrust vectoring or perhaps with engines mounted at a certain angle that causes thrust to not be parallel to the longitudinal axis.
Click to expand...

By definition, thrust is the forward pulling force along the relative wind (flight path) direction.
To produce lift, the wing chord must be aimed above the relative wind ("angle of attack").
In a given aircraft, the wing chord is typically set above the longitudinal axis
("angle of incidence"), so you'd be flying level in cruise.
So thrust in level cruise flight would be roughly along the longitudinal axis, but when your angle of attack is different from the angle of incidence (e.g. during climb or slow flight), thrust would no longer be along the longitudinal axis.
 
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MrManH said:
Since we're talking about forces, reading the PHOK I came across this regarding thrust. "As a general rule, it is said to act parallel to the longitudinal axis. However, this is not always the case as will be explained later."
Click to expand...

There are a lot of mistakes and poorly worded passages in the PHAK.
 
RotorDude said:
By definition, thrust is the forward pulling force along the relative wind (flight path) direction.
To produce lift, the wing chord must be aimed above the relative wind ("angle of attack").
In a given aircraft, the wing chord is typically set above the longitudinal axis
("angle of incidence"), so you'd be flying level in cruise.
So thrust in level cruise flight would be roughly along the longitudinal axis, but when your angle of attack is different from the angle of incidence (e.g. during climb or slow flight), thrust would no longer be along the longitudinal axis.
Click to expand...

In my mind the thrust vector would always be aligned with the engine regardless of the angle of attack. Drag, weight and lift will affect the flight path of the airplane but the engine would always pull/push the airplane in the direction the engine is pointing. I'm isolating the force vectors here.
 
MrManH said:
In my mind the thrust vector would always be aligned with the engine regardless of the angle of attack. Drag, weight and lift will affect the flight path of the airplane but the engine would always pull/push the airplane in the direction the engine is pointing. I'm isolating the force vectors here.
Click to expand...

Sure, it's a matter of semantics.
What you are referring to is "engine thrust", which is actually aligned with the engine (not necessarily the fuselage, to offset torque). So engine thrust is (roughly) aligned with the longitudinal axis, and is independent of angle of attack.
On the other hand, the term "thrust" in the fundamental aerodynamic force equation refers to forward propulsion along the flight path, or relative wind direction. This thrust does depend on angle of attack and angle of incidence, and its direction relative to the longitudinal axis will vary as I described above. And as I also noted, this thrust is there even if there is no engine (or the engine is idling), since in a descent it is supplied (partially or fully) by gravity.
 
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