Stalls 101

[dmccormack]

I will keep my opinions to myself - on things that are matters of opinion.

I'd be surprised if we had differences on standards though since they are spelled out pretty well. But I'm gonna start another thread on one thing that is frustrating me.

Well, that's the way it should be.

But defer to the examiner's judgment (however inane) -- as soon as you pass you won't have to deal with him ever again.

 

[nosehair]

While it may not be a good way to get a "full stall" in some types, doing stalls from constant altitude is still useful for approaching the stall very slowly so the trainee gets a feel for all the cues and sensations as the stall is approached.

This is the way I teach slow flight. Holding constant altitude, or more correctly, learning pitch control (elevator feel), yaw control (rudder feel), and roll control (aileron feel), as throttle/speed is reduced, slowly into the stall, and back out with throttle and pitch control until the student can feel it and hold it just above the stall, which is the PTS definition of slow flight.
The main purpose of PTS slow flight is the feeling of the stall.
After that, all the rest of the stalls is just learning how you want it done.

 

[Henning]

~~~~ Thanks, I thought as much but thought I'd better confirm it. And since the CP shifts aft it assures that the aircraft "breaks" nose down instead of into a deep stall/ tail stall. Aft CG is an important limit to never go beyond.

Out of curiosity, what is it you think dictates Aft CG?

 

[Jeanie]

Out of curiosity, what is it you think dictates Aft CG?

Limits as as stated in the POH w/ regard to the current W/B for that aircraft? Am I missing something, Henning?

 

[Henning]

Limits as as stated in the POH w/ regard to the current W/B for that aircraft? Am I missing something, Henning?

Operationaly you are correct, but physically, what determines that limit?

 

[Jeanie]

when you don't have enough elevator authority to affect a forward/down pitch I 'magine

something akin to running out of sufficient rudder to align w/ the runway in a crosswind...??

 

[Henning]

when you don't have enough elevator authority to affect a forward/down pitch I 'magine

something akin to running out of sufficient rudder to align w/ the runway in a crosswind...??

When would that happen? At what point in the relationship between the wing and the horizontal stab would that occur?

 

[Jeanie]

Well, let's see - power off there would be no extra wash over the wings to the horz. stab to create the normal tail down moment.... ummm,

the CP is too far forward as in the AOA is high..... ummm,

definately a back side of the power curve kinda thing....

I give up

 

[COFlyBoy]

Operationaly you are correct, but physically, what determines that limit?

Been a few years since I studied basic A/C design, but I believe most conventional aircraft will have the CG limit no further back than the center of pressure of the wing. The engineers then add some margin and move the limit forward a bit. (Not sure what the formula is here though. I'm guessing there is some rule of thumb, like the elevator should be providing a downward force of, say, 2% of the MGW)

 

[Henning]

Well, let's see - power off there would be no extra wash over the wings to the horz. stab to create the normal tail down moment.... ummm,

the CP is too far forward as in the AOA is high..... ummm,

definately a back side of the power curve kinda thing....

I give up

Does the tail always have a "down" moment"?

 

[Henning]

Been a few years since I studied basic A/C design, but I believe most conventional aircraft will have the CG limit no further back than the center of pressure of the wing. The engineers then add some margin and move the limit forward a bit. (Not sure what the formula is here though. I'm guessing there is some rule of thumb, like the elevator should be providing a downward force of, say, 2% of the MGW)

Look at this picture of a Bonanza flying and tell me if you think the CG is in front of the CP of the wing, and that pic has only one person in the plane.

 

[zaitcev]

Look at this picture of a Bonanza flying and tell me if you think the CG is in front of the CP of the wing, and that pic has only one person in the plane.

Very likely CG is in front of CP, due to the weight of the engine. It would even be so with 4 persons in the airplane. The structures are very light despite looking fat in picture. I know what you're driving at though. A statically stable airplane is not one where CG is ahed of CP, it's the one where AoA of tail is less than AoA of wing.

But the issue with of CG location regarding CP of the wing is not in front or back, but if the elevator reaches the critical AoA first, or the wing does. As CG moves aft, the elevator has to fly at greater and greater AoA, assuming the same angle of incidence for the wing (and same area for each).

A better example than Bo would be Ilyushin Il-2. Its rearward CG killed more of its pilots than Messershmitts did. Eventually the design bureau came up with swept wing panels outboard of main gear, similar in shape to those of DC-3. That moved the CP aft more than it did CG and improved the handling of the airplane dramatically.

-- Pete

 

[flyingcheesehead]

But the issue with of CG location regarding CP of the wing is not in front or back, but if the elevator reaches the critical AoA first, or the wing does. As CG moves aft, the elevator has to fly at greater and greater AoA, assuming the same angle of incidence for the wing (and same area for each).

I think you've got that backwards.

Most small airplanes, the tail provides a downward force - So, the further forward the CG is, the higher the AoA of the tail is.

 

[Henning]

I think you've got that backwards.

Most small airplanes, the tail provides a downward force - So, the further forward the CG is, the higher the AoA of the tail is.

Look at the picture of the Bonanza again and tell me which direction of lift the tail foil cambers for.

 

[Henning]

As long as the tail has reserved AOA over the wing, it does not matter which way the tail foil is working. The aircraft will gain considerably in efficiency (and speed) if the tail is providing Upward force rather than downward. Any downward force the tail produces is added to the weight that the wing is carrying where as any upward lifting force is deducted from the weight on the wing... e.g. Ramp weight 2000lbs... forward CG requiring 100 lbs of down force for the tail to balance makes for a flying weight of 2100lbs whereas with an aft CG and the tail providing upward force the same plane has an effective flying weight of 2000lbs.

The center of lift moves back as it is split between the two foils, same thing that makes a Canard equipped aircraft more efficient. You're still perfectly safe as long as the wing stalls before the tail, because the instant the wing stalls, the center of lift is immediately shifted to the tail, well behind the CG and the nose will drop. The aft limit of CG is where the wing has greater excess lift. If you have the weight back far enough, you will exceed the weight/lift load/wing loading that the tail can lift and the "stall speed" of the tail foil exceeds that of the wing. Only then in a stall will the tail drop. It's more than just the simple "weight and balance, the the CG has to be forward of the CL or the tail will fall in a stall..." that so many CFIs teach as gospel. It is true, but it is an incomplete phrase. it should be "The CG has to end up in front of CL when stall occurs".

 

[Jeanie]

Ah sooooo , danka

 

[poadeleted20]

Pitch stability. If the cg were aft of the center of lift, any pitch-up input would result in the airplane pitching up faster as it pitches, and it would pitch all the way to the stall without further input. With a pitch-up input with the cg forward of the center of lift, the airplane will find a new equilibrium at a higher pitch attitude, but the pitch rate will slow as it approaches the new equilibrium, and it will take more back-force on the yoke/stick to get it to pitch further. See http://www.av8n.com/how/htm/aoastab.html#sec-basic-stability for the details.

 

[flyingcheesehead]

Look at the picture of the Bonanza again and tell me which direction of lift the tail foil cambers for.

Like I said, "Most." I've heard the Bo is "backwards". Should help it go faster than it would otherwise, though, as the tail isn't counteracting the wing's lift.

So, in that case, a farther forward CG would make it less stable.

 

[gismo]

Like I said, "Most." I've heard the Bo is "backwards". Should help it go faster than it would otherwise, though, as the tail isn't counteracting the wing's lift.

So, in that case, a farther forward CG would make it less stable.

IIRC, only the straight tail Bonanzas generate positive lift at the tail at or near the aft CG limit. And FWIW, there are several certification requirements that may determine the published aft limit, one of the most common is stick vs g force although that can be "enhanced" by adding a bob weight that applies elevator down force. Static pitch stability (what Ron mentioned) is another requirement that can determine the aft CG limit and that can definitely be achieved with a design that allows the tail to produce upward lift in level flight.AFaIK this is because wings not only produce lift, they also generate a torque about the pitch axis and that torque can generate positive pitch stability.

 

[COFlyBoy]

AFaIK this is because wings not only produce lift, they also generate a torque about the pitch axis and that torque can generate positive pitch stability.

Yes this is how most of the tailless aircraft do it. You reflex the airfoil to provide a positive pitch moment.

 

[Dan Thomas]

Look at this picture of a Bonanza flying and tell me if you think the CG is in front of the CP of the wing, and that pic has only one person in the plane.

The ruddervators may be down a bit but so is a 172's elevator while in cruise, along with a good many other airplanes that use elevator/stab combinations. The stabilizer's incidence is angled downward by a couple of degrees with respect to the wing's chordline so the stab is always lifting downward. In cruise this downforce gets big enough that the elevator must be trimmed downward to counter it. I would not believe that the Bo's CG is aft of its CP, even if it was fully loaded, and would take some measurements on the airplane itself to find the aft CG limit on the wing and see if it's anywhere near the 40% or so chord mark.

Dan

 

[gismo]

The ruddervators may be down a bit but so is a 172's elevator while in cruise, along with a good many other airplanes that use elevator/stab combinations. The stabilizer's incidence is angled downward by a couple of degrees with respect to the wing's chordline so the stab is always lifting downward. In cruise this downforce gets big enough that the elevator must be trimmed downward to counter it. I would not believe that the Bo's CG is aft of its CP, even if it was fully loaded, and would take some measurements on the airplane itself to find the aft CG limit on the wing and see if it's anywhere near the 40% or so chord mark.

Dan

First of all, it's more convenient to talk about the CG in reference to the aerodynamic center (ac) rather than the center of pressure (CP) because the CP moves with the AoA. On the Bonanza's NACA 2300 series wing the CP is barely aft of the ac in cruise flight and moves slightly further aft as the AoA increases to the critical AoA. I do know for certain that the Bonanza's CG is aft of the ac when loaded at the aft CG limit (reference: Flying the Beech Bonanza by John Eckalbar, chapter 14). But since I don't know exactly how much further aft the CP is from the ac at the critical AoA I can't say with certainty whether or not the CG can be aft of the CP under all flight conditions within the published CG enevelope.

Second, the notion that the CG must be forward of the ac to achieve positive longitudinal stability only applies to a single lifting surface. The addition of a tail can produce pitch stability when the CG is aft of the ac. Perhaps another way of looking at this is to consider the tail as a means of shifting the CP aft of the wing's CP and as long as the aircraft CG is ahead of the combined CP (wing + tail), positive longitudinal stability can be achieved.

 

[Dan Thomas]

The CP will move forward a bit as AoA increases. If the CG was at or behind it, the flare would become problematic, requiring forward push on the elevator as the nose came up. Stories abound of guys who have paid no attention to loading and have had to try to get the thing back on the ground while holding the nose down, and needing a bunch of power to do it.

On the other hand, increasing cruise would shove the nose down as speed increased if the tail had any lifting action. The V35 I flew demonstrated none of this.

YMMV. Maybe I'm just mistaken, but somewhere I once read that lifting tails had been outlawed for normal/utility category airplanes many years ago. Some modern airliners are close to neutral while in cruise, but that's about it. They have computers to keep their pilots out of trouble with stability issues; part 23 airplanes don't.

Dan

 

[gismo]

The CP will move forward a bit as AoA increases. If the CG was at or behind it, the flare would become problematic, requiring forward push on the elevator as the nose came up. Stories abound of guys who have paid no attention to loading and have had to try to get the thing back on the ground while holding the nose down, and needing a bunch of power to do it.

On the other hand, increasing cruise would shove the nose down as speed increased if the tail had any lifting action. The V35 I flew demonstrated none of this.

YMMV. Maybe I'm just mistaken, but somewhere I once read that lifting tails had been outlawed for normal/utility category airplanes many years ago. Some modern airliners are close to neutral while in cruise, but that's about it. They have computers to keep their pilots out of trouble with stability issues; part 23 airplanes don't.

Dan

Again, I believe the CG will be ahead of the CP for the entire airplane but not necessarily the CP of the wing.

 

[skidoo]

Again, I believe the CG will be ahead of the CP for the entire airplane but not necessarily the CP of the wing.

If that was true, would not the entire airplane continuously rotate?

 

[gismo]

If that was true, would not the entire airplane continuously rotate?

I was a bit simplistic. The aircraft CP has to be behind the CG with the elevator in a nose down position. The point I was trying to make is that static stability can be achieved when the CG isn't ahead of the wing's CP by virtue of the tail's contribution.

 

[Capt. Geoffrey Thorpe]

If that was true, would not the entire airplane continuously rotate?

No.

Think of an arrow. The center of gravity is in the front half due to the heavy pont. The center of pressure is well aft. It is this difference between the Cp and the Cg that keep it straight.

Same thing for an airplane.

If the centerline of the airplane deviates from the apparent wind, the fact that the Cp is behind the Cg will cause the airplane to rotate and "line up" with the apparent wind. This is where static stability comes from.

 

[poadeleted20]

If that was true, would not the entire airplane continuously rotate?

Well, if you sum all of the moments of the aero forces in the Z-axis (including the h-tail force), they will balance right at the cg whenever pitch is constant. However, as a matter of convenience, for such discussions, we generally talk about the "center of lift" as being at the center of wing lift, with the tail force being considered a third, balancing force against the weight at the cg, with the center of wing lift being the fulcrum. Here's how that plays out...

As long as tail downforce moment (downforce times the arm of the tail from the center of wing lift) equals the weight moment (weight times the distance from cg to center of wing lift), pitch is constant. If you increase tail downforce (say, by pulling back on the stick/yoke, which increases the curvature of the tail, thus increasing its CL, and therefore downforce), the nose begins to pitch up because the tail downforce moment is now greater than the weight moment. However, with the increase in pitch comes an increase in AoA, and thus an increase in wing lift. If we were now to view the system as rotating around the cg, we'd see an increasing upforce at the center of wing lift, which at some point will create an increased wing lift moment that balances out the increased tail downforce moment, and the pitch will stabilize at that new equilibrium.

 

[skidoo]

Well, if you sum all of the moments of the aero forces in the Z-axis (including the h-tail force), they will balance right at the cg whenever pitch is constant. ...snip...

And that sums up my point. If they are not all balanced at the same point, then I think there would be rotation. This was in response to the comment that stated

"Again, I believe the CG will be ahead of the CP for the entire airplane"

Correct me if wrong, but I interpreted the "CP for the entire airplane" as being the sum of all the lifting moments.

 

[poadeleted20]

And that sums up my point. If they are not all balanced at the same point, then I think there would be rotation. This was in response to the comment that stated

"Again, I believe the CG will be ahead of the CP for the entire airplane"

Correct me if wrong, but I interpreted the "CP for the entire airplane" as being the sum of all the lifting moments.

In the context of the statement you quoted, you are correct -- unless the plane is actively changing pitch attitude, the forces are all in balance, and the cg must be in the same place as the sum of all the lifting forces (positive lift from the wing and negative lift from the tail). However, in these discussions, the term "center of lift" usually refers to the center of wing lift, with the tail downforce being considered a separate balancing force. That's why we always talk about the cg remaining ahead of the "center of lift" in order to maintain positive static stability.