AFT CG and Stall Speed... need diagram

Ian_J

Ian_J

Hubschrauber Flieger
Staff member
An aft CG lowers the stall speed and a fwd CG increases it. I was explaining this concept to a student and he was understanding that "yes, those are facts and I believe them," but really wanted to understand why. So I started sketching the side of the wing, relative wind, total aerodynamic force vector and all that, all the while happily explaining why it all works, only to be given a blank stare. So I admitted I don't know the best/ easiest way to explain it to him, and promised to revisit it next lesson.

So, anyone have a good diagram or a simple, easy to understand explanation? Many thanks!
 
ChinookDriver said:
So, anyone have a good diagram or a simple, easy to understand explanation? Many thanks!
Click to expand...


A forward CG requires more downforce on the tail. This downforce must be countered by an increased lift on the main wing, otherwise the weight plus downforce would exceed the lift. At a given airspeed, this means the AOA must be increased. So for every airspeed, the AOA is increased, meaning that an airspeed originally just above the stall would now be stalled.
 
tgrayson said:
A forward CG requires more downforce on the tail. This downforce must be countered by an increased lift on the main wing, otherwise the weight plus downforce would exceed the lift. At a given airspeed, this means the AOA must be increased. So for every airspeed, the AOA is increased, meaning that an airspeed originally just above the stall would now be stalled.
Click to expand...

That's what I told him... he's having trouble visualizing it...

Since AOA is the acute angle between the chord of an airfoil and a line representing the undisturbed relative airflow, what's changing with an aft or fwd CG? The chordline or the relative wind?
 
The down force on the tail. (set by the trim) increases the angle of attack. The increased drag induced by the increased AOA thus increases the airspeed at which a stall will occur.
 
ChinookDriver said:
Since AOA is the acute angle between the chord of an airfoil and a line representing the undisturbed relative airflow, what's changing with an aft or fwd CG? The chordline or the relative wind?
Click to expand...

When you increase the AOA, you rotate the aircraft around the lateral axis, so the chordline becomes more inclined with respect to the relative wind.

If you make no power change, then the increased induced drag will create a power deficit and the aircraft will descend, so the relative wind will change with respect to the horizon. The aircraft will again rotate around its lateral axis to maintain the same orientation of the chordline with the new relative wind as in the paragraph above. (due to longitudinal stability)
 
tgrayson said:
When you increase the AOA, you rotate the aircraft around the lateral axis, so the chordline becomes more inclined with respect to the relative wind.

If you make no power change, then the increased induced drag will create a power deficit and the aircraft will descend, so the relative wind will change with respect to the horizon. The aircraft will again rotate around its lateral axis to maintain the same orientation of the chordline with the new relative wind as in the paragraph above. (due to longitudinal stability)
Click to expand...
Got it! It just clicked. Thanks, dude.
 
desertdog71 said:
Increased drag requires increased lift. Either by increasing airspeed or AOA.
Click to expand...

I hope you will rethink that. ;)

Remember your basic diagram of the forces acting on an aircraft? Lift opposes weight, thrust opposes drag.

Increased drag requires increased thrust, not increased lift. In unaccelerated flight, lift = weight. Descents require less thrust than drag, climbs require more thrust than drag.
 
tgrayson said:
I hope you will rethink that. ;)

Remember your basic diagram of the forces acting on an aircraft? Lift opposes weight, thrust opposes drag.

Increased drag requires increased thrust, not increased lift. In unaccelerated flight, lift = weight. Descents require less thrust than drag, climbs require more thrust than drag.
Click to expand...

With forward
loading, “nose-up” trim is required in most airplanes
to maintain level cruising flight. Nose-up trim
involves setting the tail surfaces to produce a
greater down load on the aft portion of the fuselage,
which adds to the wing loading and the total lift
required from the wing if altitude is to be maintained.
This requires a higher angle of attack of
the wing, which results in more drag and, in turn,
produces a higher stalling speed.
​
​
​
​
​

 
desertdog71 said:
With forward​

loading, “nose-up” trim is required in most airplanes
to maintain level cruising flight. Nose-up trim
involves setting the tail surfaces to produce a
greater down load on the aft portion of the fuselage,
which adds to the wing loading and the total lift
required from the wing if altitude is to be maintained.
This requires a higher angle of attack of
the wing, which results in more drag and, in turn,
produces a higher stalling speed.​
Click to expand...
I think the difference here is the text you quoted says (accurately) thata higher AOA results in more drag. You had it flipped flopped above.
 
desertdog71 said:
With forward ​
​
loading, “nose-up” trim is required in most airplanes to maintain level cruising flight. Nose-up trim involves setting the tail surfaces to produce a greater down load on the aft portion of the fuselage, which adds to the wing loading and the total lift required from the wing if altitude is to be maintained. This requires a higher angle of attack of the wing, which results in more drag and, in turn, produces a higher stalling speed.​
Click to expand...

Yes, but you went one step too far. When you said "This requires a higher angle of attack of the wing," you already identified the cause of the higher stall speed. When you said "which results in more drag and, in turn, produces a higher stalling speed," you threw in an irrelevancy.

The increase in stall speed can be demonstrated solely by the lift equation, into which drag does not enter.
 
ChinookDriver said:
I think the difference here is the text you quoted says (accurately) thata higher AOA results in more drag. You had it flipped flopped above.
Click to expand...


I will boil it down even simpler.

A given airplane will stall at the same AOA regardless of airspeed.

In a forward CG situation you have already used some of that AOA in order to maintain level flight. That being said, it will take less of an increase of AOA to induce the stall. Therefore a smaller drop in airspeed.

I know what I am trying to say, maybe just not describing it the right way. I just described it to a student pilot here and they were able to grasp the concept without any trouble.
 
desertdog71 said:
I will boil it down even simpler.

A given airplane will stall at the same AOA regardless of airspeed.

In a forward CG situation you have already used some of that AOA in order to maintain level flight. That being said, it will take less of an increase of AOA to induce the stall. Therefore a smaller drop in airspeed.

I know what I am trying to say, maybe just not describing it the right way. I just described it to a student pilot here and they were able to grasp the concept without any trouble.
Click to expand...

Nope, no worries... I got it as far as the fwd/ aft CG stall speed bit...

There was just a small bit of wording issues with the "drag requires lift" statement you made... what Tgryason was saying is that drag is not in the lift equation and that drag (induced) is a result of lift.
 
desertdog71 said:
Increased drag requires increased lift. Either by increasing airspeed or AOA.
Click to expand...

Correct or not, Desert knew what he was saying as I can completely see where this statement came from. When discussing tail down force or a heavily weighted aircraft, you will fly with a higher AoA to maintain altitude as there is a higher wing load that must be suppored which in turn can be accomplished through increasing your CL through AoA. I can see why you'd say increased AoA is needed to overcome drag. While not correct, I can see some fast thinking logic behind the comment.
 
meritflyer said:
When discussing tail down force or a heavily weighted aircraft, you will fly with a higher AoA to maintain altitude as there is a higher wing load that must be suppored which in turn can be accomplished through increasing your CL through AoA.
Click to expand...

You don't need to do anything to handle the increased load. The aircraft itself though its basic stability will always ensure the Lift = Weight. If you add weight to the airplane, it will merely restablish equilibrium at a higher airspeed.

But you will be descending, because your AOA is the same, yet your airspeed is higher with increased parasite drag.

To regain level flight, you need thrust, not lift.
 
So, you're telling me if I have an aircraft flying at max gross, at cruise altitude, all I need is thrust? A heavily weighted aircraft will be flying with a higher AoA at cruise as opposed to the same aircraft with a lighter load. Maybe we're talking the same thing here and I can see your point as to lift=weight but assuming two identical aircraft flying along, one 3800 lbs. and one 2800 lbs., the wing loading on the heavier aircraft is obviously needing to support a greater load so if both aircraft are cruising at 120 knots and 10,000', the heavier aircraft will have to fly with a greater AoA to increase our coefficient of lift as opposed to a light aircraft.

This same idea goes for Va (maneuvering speed) in the same aircraft, one heavy and one light.
 
tgrayson said:
But you will be descending, because your AOA is the same, yet your airspeed is higher with increased parasite drag.
Click to expand...

To stop such a descent, the variable that would need to be changed would be lift assuming keeping an identical AoA. This can be accomplished through increasing our coefficient of lift.
 
meritflyer said:
To stop such a descent, the variable that would need to be changed would be lift assuming keeping an identical AoA. This can be accomplished through increasing our coefficient of lift.
Click to expand...

Descents are caused by lack of thrust, not lack of lift, and therefore aren't stopped by increases in lift.

I promise, there are formulas in textbooks that show precisely the angle of descent produced by the quantity of drag and thrust available. Wait! Here's one:

Sin(descent angle) = (T-D)/Weight

Do you see lift in that equation?

When D is greater than T, the aircraft is descending. To make the aircraft level out, you need to reduce D or increase T.

Now, you can stop the descent by increasing your AOA, but that works by slowing the aircraft towards your L/D max speed. In other words, pulling back on the yoke reduces drag. In our present scenario, you would end up slower than you began. But lift would not be increased, even though your lift coefficient gets larger.
 
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