Mushing in a stall

[bob_albertson]

That's not unique to the onset of a stall. It always does that.

Yeah, the counterweights on most Cessna's do that.... but, we're talking about explaining something to the clueless.

Stretchiness is a nice and simple description of the feel of the controls.... twist a rubber band vs the feeling of power steering when everything is going like it should

 

[Dan Thomas]

Yeah, the counterweights on most Cessna's do that.... but, we're talking about explaining something to the clueless.

The buffet is turbulent airflow off the top of the wing striking the stabilizer. It's not the mass balance weights.

Dan

 

[Richard]

It feels like you're flying through pudding.

Oh yeah everyone gets that!

Ok, everyone thread closed. You can go back to your homes. Nick did it.

 

[gismo]

All good descriptions, but I concur with those who said "mushing" implies some residual lift -- that some part of the wing is still flying.

I believe that "mushing" means one of two things. Either there is insufficient elevator to bring the most of the wing (gradually) to it's critical angle of attack or the airplane enters a stable pitch attitude with a significant portion of the wing beyond the crititical AoA. Either way this pretty much requires that the outer portion of the wings remains at an AoA below critical (AoA of max lift) in order to provide any roll stability or control. AFaIK this behavior is typical of airplanes who's wings have two characteristics: Significant "washout" (twist of the wing along the pitch axis with reduced angle of incidence at the tips), and a rather flat curve of the lift vs AoA curve. What produces the "mushing" is the relatively small loss of lift from the stalled portion which in combination with the unstalled outer wing portion provides enough lift to prevent the AoA on the outer wing area to exceed critical.

Normally in a stall, when the inner wing exceeds critical AoA, the total loss of lift and resulting increase in vertical velocity at the wing (e.g. nose drop) shifts the AoA on the outer wing to or beyond critical. Often, once the pitch stabilizes sufficiently nose down to bring the AoA back across the peak and the lift increases again. This leads to the "falling leaf" behavior which is a continuous cycling between stalled and unstalled condition with associated pitch oscillations.

But if the elevator cannot generate enough downforce when the wing is nearly or partially stalled the condition becomes fairly stable with no substantial variations in pitch or AoA. At that point the airplane is "mushing".

 

[ebetancourt]

I believe that "mushing" means one of two things. Either there is insufficient elevator to bring the most of the wing (gradually) to it's critical angle of attack or the airplane enters a stable pitch attitude with a significant portion of the wing beyond the crititical AoA. Either way this pretty much requires that the outer portion of the wings remains at an AoA below critical (AoA of max lift) in order to provide any roll stability or control. AFaIK this behavior is typical of airplanes who's wings have two characteristics: Significant "washout" (twist of the wing along the pitch axis with reduced angle of incidence at the tips), and a rather flat curve of the lift vs AoA curve. What produces the "mushing" is the relatively small loss of lift from the stalled portion which in combination with the unstalled outer wing portion provides enough lift to prevent the AoA on the outer wing area to exceed critical.

Normally in a stall, when the inner wing exceeds critical AoA, the total loss of lift and resulting increase in vertical velocity at the wing (e.g. nose drop) shifts the AoA on the outer wing to or beyond critical. Often, once the pitch stabilizes sufficiently nose down to bring the AoA back across the peak and the lift increases again. This leads to the "falling leaf" behavior which is a continuous cycling between stalled and unstalled condition with associated pitch oscillations.

But if the elevator cannot generate enough downforce when the wing is nearly or partially stalled the condition becomes fairly stable with no substantial variations in pitch or AoA. At that point the airplane is "mushing".

Outstanding answer! Hard to believe there could be so much discussion of stalls without more discussion of AoA.

And "nose up" or "nose down" are meaningless unless one is talking about sustained flight. I can demonstrate nose straight up at virtually zero airspeed and NOT stalled or nose straight down and fully stalled. I believe the recent Air France tragedy demonstrated fully stalled "level" flight rather well, sadly.

Ernie

 

[bob_albertson]

The buffet is turbulent airflow off the top of the wing striking the stabilizer. It's not the mass balance weights.

Dan

No, we were talking about the controls returning to a position when you let go, not any shaking caused by the airflow separating and then reattaching on the elevator. The buffet, however, isn't unique to high wings as you describe it. The buffet is only made more accentuated because of that airflow from the wings striking the horizontal stabilizer and elevator. The actual cause of the buffet is the seperation and reattaching of airflow on the wings. The horizontal stabilizer increases in negative AOA as the main wings increase in their positive AOA. The loss of airflow on the horizontal stabilizer occurs on the bottom.

The key to the OP's question is to remember that the people we are talking to have no idea about how any of this works. Therefor the answer needs to completely ignore any theory and answer ONLY the basic question. You don't want to scare anyone off from general aviation because they get the idea that this takes a Ph.D to earn a certificate. We all have been flying for long enough that we have had the opportunity to delve into information that some may or may not be interested in. With that in mind, compare how Doctor's inform family members about procedures/surgery. You rarely hear any kind of theory or medical terms, and the analogies are at the most basic level that may or may not be as accurate of an analogy as can be made. They main focus is on basic understanding, not comprehensive knowledge. Just like with the Doctors, if the person wants more information then the discussion can go there. To start with, though, we don't want to scare them with all this mumbo-jumbo using terms they have probably never heard of before

 

[bob_albertson]

That is one of the examples I used. But you know, driving in those conditions leaves little control...which immediately prompted the question, "So mushing means you are out of control?" rolls eyes..."No, I did not say that."

Try again, Bob.

That is actually a poor follow-up answer IMHO. If I was asked that same question... or if the conversation went like this, I'd answer much differently than you.... Here:

Me:The controls feel a lot like you are trying to drive on ice
Them: So mushing means you are out of control?

Me: No, just like driving on ice, you can maintain control with purposeful inputs but if you start doing the wrong things you only worsen the situation, and that is when you loose control in a car and in a plane.

Bob

 

[Jaybird180]

~~~~~~~~ and since you were in a Citabria did you apply full rudder just at the stall and get to spin too ?

Of courseJeanie. Left and right spins. About the day after, I started to feel spin withdrawal.

 

[bob_albertson]

Outstanding answer! Hard to believe there could be so much discussion of stalls without more discussion of AoA.

And "nose up" or "nose down" are meaningless unless one is talking about sustained flight. I can demonstrate nose straight up at virtually zero airspeed and NOT stalled or nose straight down and fully stalled. I believe the recent Air France tragedy demonstrated fully stalled "level" flight rather well, sadly.

Ernie

We are talking to a passenger who is neither a student nor a person who has been around aviation before, right? Then throwing in aviation jargon would be like the teacher speaking only in French on the first day of taking French in school.

Just like a lot of the passenger's on our flights that have questions, if I started telling them a detailed explanation of why we have 14 vortex generators on the underside of the right horizontal stabilizer and none on the left horizontal stabilizer then I'm pretty sure I'd lose them at the same point that I said, horizontal stabilizer.

What is the medulla oblongata --- Said the Patient to the Doctor

VS.

You have a brain tumor in a location that is inoperable said the Dr to the Patient.

 

[Everskyward]

How about an educated engineer type who has no first hand experience with anything aviation? No familiarity to aviation terms, or principals other than the understanding of physics as required by his non-aviation career.

What is wrong with the word "mushing"? Is that not an engineering term?

I would just say that since the airflow over the wings is slower the control deflections need to be greater.

 

[Dan Thomas]

The horizontal stabilizer increases in negative AOA as the main wings increase in their positive AOA. The loss of airflow on the horizontal stabilizer occurs on the bottom.

If the elevator/stab suffer airflow separation, that stalls them and the airplane goes forward over onto its back. It's a really nasty scenario. As the tail comes up its AoA increases and deepens the stall on it.

The only airplane I can recall having any such issue was the early Cardinal, where the stabilator suffered some stall issues in the landing flare and would drop the nosewheel hard enough to break stuff. Cessna slotted the stabilator to stop that.

Dan

 

[bob_albertson]

If the elevator/stab suffer airflow separation, that stalls them and the airplane goes forward over onto its back. It's a really nasty scenario. As the tail comes up its AoA increases and deepens the stall on it.

The only airplane I can recall having any such issue was the early Cardinal, where the stabilator suffered some stall issues in the landing flare and would drop the nosewheel hard enough to break stuff. Cessna slotted the stabilator to stop that.

Dan

No, that is incorrect.... Might I suggest you read AC 91-74A and the description of a tailplane stall caused by icing. It won't make it onto it's back, and didn't in any of the tests NASA conducted with full and deep stalls of the tail.

You have the AOA backwards, also. As the tail comes up the NEGATIVE AOA increases. To recover from a tailplane stall you MUST pull BACK on the yoke. Think of it this way.... the horizontal stabilizer is an upside down wing. Everything is opposite of what you would expect from a wing. As you pull back on the yoke the AOA increases since the high pressure area is on the bottom of the horizontal stabilizer. Therefor, moving the elevator up towards the sky increases the AOA by increasing the chord line. The plane will not flip over on it's back if the tail stalls.... It will, however, nose dive towards the ground... see the AC I cited

This is also covered in the Instrument Flying Handbook in Chapter 2 as I seem to recall.

Also, all Cessna's have the elevator Flutter problem. It's one reason why flaps 40 was STC'ed inop on many of the early models and not available in the later models. That flutter was what we refer to as buffering on the main wings. The Wings would blanket out the airflow over the horizontal stabilizer and cause the airflow to become separated with the greater wing downwash from higher AOA and the associated lift being produced.

 

[dmccormack]

Also, all Cessna's have the elevator Flutter problem. It's one reason why flaps 40 was STC'ed inop on many of the early models and not available in the later models. That flutter was what we refer to as buffering on the main wings. The Wings would blanket out the airflow over the horizontal stabilizer and cause the airflow to become separated with the greater wing downwash from higher AOA and the associated lift being produced.

Is there a Cessna which has had the 40 degree flap setting rendered inoperative by an STC??

I think you're confusing "flutter" with tail surface "buffeting."

Flutter is a function of speed can cause rapid destruction of all or part of a control surface.

A related FAA AC is found here

In some conditions (slipping with 40 degrees of flaps) a Cessna will exhibit minor buffeting and slight pitch oscillations.

 

[gismo]

We are talking to a passenger who is neither a student nor a person who has been around aviation before, right? Then throwing in aviation jargon would be like the teacher speaking only in French on the first day of taking French in school.

Just like a lot of the passenger's on our flights that have questions, if I started telling them a detailed explanation of why we have 14 vortex generators on the underside of the right horizontal stabilizer and none on the left horizontal stabilizer then I'm pretty sure I'd lose them at the same point that I said, horizontal stabilizer.

Sorry, I was addressing the PoA discussion of what's involved in "mushing", not suggesting how to explain mushing, stalling, or slow flight to a non-pilot. This was in the spirit of my belief that one should understand an issue fairly well before trying to explain it in any terms. Otherwise you might as well just make stuff up.

 

[ebetancourt]

We are talking to a passenger who is neither a student nor a person who has been around aviation before, right? Then throwing in aviation jargon would be like the teacher speaking only in French on the first day of taking French in school.
SNIP

If an educated person, e.g. an engineer, asks me about a stall, I wouldn't know how to respond without discussing lift and AoA. At their basic level, they are actually both simple to explain. I consider AoA a key concept and not aviation jargon, but obviously that is my opinion and everyone is entitled to their own.

I will admit I consider Stick and Rudder to be the key text for learning to fly well.

Ernie

Edit: And for me,"mushing" is hard to explain without discussing the concept of a stall.

 

[Jaybird180]

No, that is incorrect.... Might I suggest you read AC 91-74A and the description of a tailplane stall caused by icing. It won't make it onto it's back, and didn't in any of the tests NASA conducted with full and deep stalls of the tail.

You have the AOA backwards, also. As the tail comes up the NEGATIVE AOA increases. To recover from a tailplane stall you MUST pull BACK on the yoke. Think of it this way.... the horizontal stabilizer is an upside down wing. Everything is opposite of what you would expect from a wing. As you pull back on the yoke the AOA increases since the high pressure area is on the bottom of the horizontal stabilizer. Therefor, moving the elevator up towards the sky increases the AOA by increasing the chord line. The plane will not flip over on it's back if the tail stalls.... It will, however, nose dive towards the ground... see the AC I cited

This is also covered in the Instrument Flying Handbook in Chapter 2 as I seem to recall.

Also, all Cessna's have the elevator Flutter problem. It's one reason why flaps 40 was STC'ed inop on many of the early models and not available in the later models. That flutter was what we refer to as buffering on the main wings. The Wings would blanket out the airflow over the horizontal stabilizer and cause the airflow to become separated with the greater wing downwash from higher AOA and the associated lift being produced.

Careful when you say that. The C172P I've flown is not placarded against slips with full flaps. It also doesn't have 40deg. For this reason, I can slip the heck out of it on final without any elevator flutter.

 

[eMKay]

Is there a Cessna which has had the 40 degree flap setting rendered inoperative by an STC??

I flew one, the 172N the flap control has 40 degrees on it, but when you select it, nothing happens, it's been modified to only go 30 degrees. The M model I normally fly which is one year older can go to 40 degrees, used it today as a matter of fact to review slow flight, stalls, and short field landings.

Since it's fresh in my mind I would describe "mushiness" as lighter control feel with larger inputs required.

 

[Dan Thomas]

The Cessna's that had the "No Slips With Flaps Applied" placards had a habit of dropping the nose if the slip was aggressive enough. Flap downwash in a 172 hits the stab and raises the nose, and slipping can remove the stab from the downwash and let the nose drop suddenly. It has nothing at all to do with flutter. There are no STCs or ADs limiting flap travels on any 172 model. I should know: I maintain several 172s in a flight school. And used to instruct in them.

As far as tailplane stall goes, it WILL drop the nose and as the tail rises its AoA increases because the relative wind comes at it from a higher angle.

The Type Certificate Data Sheets for every aircraft model list the control surface travels, and those specs are really tight for good reason. For instance, the 172M's elevator travel is up 28° + 1° -0° and down 23°, +1° -0°. Too little travel leads to inadequate control authority. Too much can stall the control surface and cause control reversal. Ice on the stab disrupts the airflow, critical especially over the lower surface, and downforce is lost. If it stalls, most downforce is lost and therefore the nose will drop.

Dan

 

[Dan Thomas]

I flew one, the 172N the flap control has 40 degrees on it, but when you select it, nothing happens, it's been modified to only go 30 degrees. The M model I normally fly which is one year older can go to 40 degrees, used it today as a matter of fact to review slow flight, stalls, and short field landings.

Since it's fresh in my mind I would describe "mushiness" as lighter control feel with larger inputs required.

For the 172N, the landplane version has 40 degrees of flap. The seaplane has 30 degrees. That one you flew might have been a seaplane model when it left the factory. Look and see if the front float attach fittings are sticking out of the lower cowling at the firewall. It would also have the diagonal cross brace tubes extending from the top of the instrument panel up to the wing roots.

Dan

 

[eMKay]

For the 172N, the landplane version has 40 degrees of flap. The seaplane has 30 degrees. That one you flew might have been a seaplane model when it left the factory. Look and see if the front float attach fittings are sticking out of the lower cowling at the firewall. It would also have the diagonal cross brace tubes extending from the top of the instrument panel up to the wing roots.

Dan

I didn't look at the gear, but it certainly didn't have any extra tubes sticking out of the instrument panel.

 

[Dan Thomas]

I didn't look at the gear, but it certainly didn't have any extra tubes sticking out of the instrument panel.

Then someone had moved the down limit switch on the flap actuator in the right wing to stop the flaps at 30°.

Dan

 

[eMKay]

Then someone had moved the down limit switch on the flap actuator in the right wing to stop the flaps at 30°.

Dan

Yes, as told to by Cessna.

 

[Dan Thomas]

Yes, as told to by Cessna.

Now, why would Cessna do that? I can find no AD and have never seen an SB on that, and the Type Certificate Data sheets still lists 40° flaps. The TCDS would have been amended to 30 if there'd been a legally-mandated change.

Dan

 

[eMKay]

Dunno, maybe you should contact them since it's your job to work on them. I'm just going by what I saw and was told by the CFI that uses that plane for training every day. I was told if you slip it with 40 degrees of flaps the horizontal stabilizer loses effectiveness and not to do it in 79H

 

[dmccormack]

I flew one, the 172N the flap control has 40 degrees on it, but when you select it, nothing happens, it's been modified to only go 30 degrees. The M model I normally fly which is one year older can go to 40 degrees, used it today as a matter of fact to review slow flight, stalls, and short field landings.

Since it's fresh in my mind I would describe "mushiness" as lighter control feel with larger inputs required.

I have about 50 hours in an N model -- 40 degrees of fun.

Yours was broke.

 

[dmccormack]

Dunno, maybe you should contact them since it's your job to work on them. I'm just going by what I saw and was told by the CFI that uses that plane for training every day. I was told if you slip it with 40 degrees of flaps the horizontal stabilizer loses effectiveness and not to do it in 79H

He's wrong. Surprise.

 

[PilotAlan]

Now, why would Cessna do that? I can find no AD and have never seen an SB on that, and the Type Certificate Data sheets still lists 40° flaps. The TCDS would have been amended to 30 if there'd been a legally-mandated change.

There is a gross weight increase STC that is mostly paperwork, just with a 30* flap limit. It has to do with go-around performance at higher weights and with 40* flaps.

 

[Clark1961]

Dunno, maybe you should contact them since it's your job to work on them. I'm just going by what I saw and was told by the CFI that uses that plane for training every day. I was told if you slip it with 40 degrees of flaps the horizontal stabilizer loses effectiveness and not to do it in 79H

Read the POH and find out what it says. This is an OWT. Note that "not recommended" does not equal "prohibited" in any way shape or form.

 

[azure]

Read the POH and find out what it says. This is an OWT. Note that "not recommended" does not equal "prohibited" in any way shape or form.

That's my understanding too. Anyway the placard only said "Avoid slips with flaps extended" or something like that.

Just before my PPL checkride I made sure to experience the "buffet" for myself in case it happened in front of the DPE (since slips to a landing are (were?) part of the PTS). I went up to 6000 feet, put in 40 of flaps and tried to make it happen. It took about 10 tries with different degrees of nose-down attitude, was very easy to get out of when it finally happened, and then I couldn't make it happen again.

So maybe not quite an OWT, but not really dangerous either unless you panic and stop flying the plane.

 

[Dan Thomas]

There is a gross weight increase STC that is mostly paperwork, just with a 30* flap limit. It has to do with go-around performance at higher weights and with 40* flaps.

There it is, then.

Dan