Whats going on with this stab?

[Stewartb]

They'll be most effective when the stab is leading edge down (for nose-up moment), keeping the flow over the stab attached with nose-up yoke in the flare. Worst case - flow separates, stab stalls, downforce on the tail goes away, nose plunks down hard.

Nauga,
and his downforce

At second glance I believe you're correct. Those slot ducts are inverted from typical high AOA slot configuration. Good call!

 

[Pilawt]

At second glance I believe you're correct. Those slot ducts are inverted from typical high AOA slot configuration. Good call!

See Posts #5 and #31 above. A Cessna aerodynamicist and test pilot explained why the slots were put in the C-177 stabilator.

But what would he know ... ?

 

[SixPapaCharlie]

I want a thread that is just Nauga's signatures.
Those crack me up!

 

[Henning]

Try reading the material I linked so next time you use appropriate and accurate terminology to express your meaning. I still don't know whether you're talking static or dynamic stability.

Static, it can only be, I don't see how you got confused.

 

[Stewartb]

See Posts #5 and #31 above. A Cessna aerodynamicist and test pilot explained why the slots were put in the C-177 stabilator.

But what would he know ... ?

That's a fancy way of saying they needed to improve the elevator's authority in a flare. My micro VGs do the same thing to a tail that wasn't weak to start with.

We don't see many 177s in Alaska.

 

[bnt83]

See post #5 above. According to Cessna's Manager of Aerodynamics and Flight Test, they made the "decision to incorporate slots in the stabilators' leading-edges so that they could tolerate a steeper downflow of air without stalling the under-surface of the stabilator."

The stabilator control linkage was also changed with the 1969 Model 177A to adjust stick forces in the flare.

Must have been a very small adjustment because its so close between serial # 21 and # 2618 I don't notice much difference in stick forces (in pitch axis)

The biggest different I notice in controls between these two is the added drag of constant speed prop & 180 horse with engine at idle makes power off glide much shorter than the fixed pitch on the 150 horse 1968 model.

 

[bnt83]

In a passing view the only time those slots would do much would be in a slow speed/nose down control configuration. I'd think it has to do with stall recovery with an aft CG. Perhaps they needed better control response to validate a more useful aft CG limit.

The stab pivots a very long ways airplane nose up, hence the tailplane stall and added slots. The slots do jack when pitching airplane nose down.

One of my favorite moves is touch and goes and never letting the nose wheel touch. It can be done in other models but I think its easier in the 177 as long as you have a rock steady hand and very precise elevator inputs.

 

[poadeleted20]

Static, it can only be, I don't see how you got confused.

Because you said:

It gets greater diverging from center stick, normal planes try to return to neutral when you let go.

The first half of that is static stability, but the second half is dynamic. Again, please try reading some basic aerodynamics so you can discuss this in an understandable manner.

 

[Henning]

Because you said:
The first half of that is static stability, but the second half is dynamic. Again, please try reading some basic aerodynamics so you can discuss this in an understandable manner.

The stick returning to neutral when released is a function of static stability IIRC.

 

[poadeleted20]

The stick returning to neutral when released is a function of static stability IIRC.

You don't recall correctly. Please read the material. And you said the plane returns to neutral, not the stick, so please read your own posts correctly, too.

 

[kgruber]

You don't recall correctly. Please read the material. And you said the plane returns to neutral, not the stick, so please read your own posts correctly, too.

Read "Denker" is your standard response to your own confusion.

 

[Velocity173]

As Nauga and the Cessna engineer stated, it helps keep the airflow attached to the airfoil. Same principle as the leading edge slat on our helicopter. At high climb rates the increased down flow on the horizontal stab will exceed critical AOA. The added slats keep the airflow attached and allow better pitch stability in the climb.

 

[MAKG1]

Because you said:
The first half of that is static stability, but the second half is dynamic. Again, please try reading some basic aerodynamics so you can discuss this in an understandable manner.

That's funny, I understood what he was talking about right away.

What I didn't understand was your description of "negative" stability, as it sounded neutral to me. Negative stability means a change in configuration grows. I.e., stick forces push the stick away from the center. Sounds scary as hell to me. Neutral would mean no stick forces, which is what you describe far from center.

There are many kinds of stability, and Denker's isn't the only one. The terminology in the field is confused, but no one would describe a control whose feedback forces vanished at large deviation as unstable. Stability is a function of the direction of dynamical forces, much more than their magnitude.

Now, let's get back to 177 stabilators, rather than arguing about who's wrong. I know it's the Internet, but it's also a nice plane.

 

[PPC1052]

Am I the only one who does not like the way the 177 flies?

It feels like a 152 (fixed gear, fixed prop C177). I'd like to think the C177 CS, RG is much better.

Not a complaint I've had in a 177RG. It's been a long time since I've been in a 177B, but I don't recall it being all that weak, even taking off from a 6000 foot field (KCOS). 177B is also CS.

The original 177 with the 150 hp engine and no CS prop was definitely thought by most to be underpowered. One thing that didn't help is that when people flew it the same way as they did a 172, it really didn't perform well. The V speeds for take off were different, and if you flew them by the book speeds in the POH, they did better than if you flew them by the 172 speeds.

The 180 hp versions were pretty nice. My 177B (180hp w/ CS prop) climbs really well.

 

[PPC1052]

Must have been a very small adjustment because its so close between serial # 21 and # 2618 I don't notice much difference in stick forces (in pitch axis)

The biggest different I notice in controls between these two is the added drag of constant speed prop & 180 horse with engine at idle makes power off glide much shorter than the fixed pitch on the 150 horse 1968 model.

a 177A doesn't have a CS prop, unless it was modified. The CS prop was added for the 177B version.

 

[poadeleted20]

What I didn't understand was your description of "negative" stability, as it sounded neutral to me. Negative stability means a change in configuration grows. I.e., stick forces push the stick away from the center. Sounds scary as hell to me. Neutral would mean no stick forces, which is what you describe far from center.

Then you've been misinformed. Please read some basic aero texts such as Denker's on-line version regarding longitudinal stability.

 

[MAKG1]

Then you've been misinformed. Please read some basic aero texts such as Denker's on-line version regarding longitudinal stability.

I have read it. In detail, and several times. It's a good book, but I'm asking what YOU mean, not him.

He says longitudinal stability is positive, neutral, and negative, depending on what your bank angle is. What does that have to do with control stability?

He also defines "negative stability" (frankly, I prefer to call it INstability, as it's related to positive feedbacks...) as growing deviations, directly. Not what you said.

Denker's book is not a control system book, though it touches on several related topics. Instability is a feedback AWAY from equilibrium. It is not a lack of feedback.

 

[bnt83]

a 177A doesn't have a CS prop, unless it was modified. The CS prop was added for the 177B version.

I understand that. Pilawt was saying stab control was modified 177A and later.

The forces between the 177B and 177 seems pretty much the same to me.

I flew a B about 5 hours over the Thanks Giving holiday including about a dozen landings. The family owns a 1968 177 so I've got about 200 hours in it.

I probably have 15 hours in the B, or more.

So far I'm working towards FG Cardinal guru. Maybe someday I'll add the RGs to my A&P and pilot logbook.

 

[poadeleted20]

I have read it. In detail, and several times. It's a good book, but I'm asking what YOU mean, not him.

I'm still trying to figure out what Henning means, so you'd better ask him.

He says longitudinal stability is positive, neutral, and negative, depending on what your bank angle is. What does that have to do with control stability?

I think I'll need you to quote the specific section which states that longitudinal stability can change based solely on bank angle.

He also defines "negative stability" (frankly, I prefer to call it INstability, as it's related to positive feedbacks...)

You can call it that if you like, but instability is not the same as negative static or dynamic stability in this context.

That said, in a nutshell, positive static longitudinal stability means that when you apply a force to the stick in the longitudinal axis, the aircraft will pitch only so far, and the pitch rate will decrease as displacement increases until a new equilibrium is reached, and more stick force is required to obtain a greater displacement in pitch than the new equilibrium. Neutral static stability is when the pitch displacement will occur at a rate proportional to the stick force applied and continue at the same rate until the force is removed. Negative static stability is when the rate of pitch change with a fixed force applied increases as displacement increases and there is no limit to displacement -- it will continue until control is lost. See this accident report for a case where that actually occurred.

http://www.airdisaster.com/reports/ntsb/AAR78-01.pdf

Dynamic longitudinal stability refers to the way the aircraft reacts when a pitch force is briefly applied and then removed. Positive dynamic stability means the aircraft will return to its original state via a series of damped oscillations, i.e., each oscillations will have smaller amplitude than the one before it. Neutral dynamic stability means the aircraft will continue to oscillate in pitch with constant amplitude without stopping in the original state. Negative dynamic stability means the oscillations will increase in amplitude until control is eventually lost.

And the FAA will not certify a Part 23 aircraft which does not have positive longitudinal stability, both statically and dynamically. See 23.175 and 23.181.

 

[bnt83]

Full nose down pitch input


Full nose up pitch input



Trim was set at full nose up at all times

 

[nauga]

And the FAA will not certify a Part 23 aircraft which does not have positive longitudinal stability, both statically and dynamically. See 23.175 and 23.181.

It is possible to meet the requirements of 23.175 and 23.181 with a statically unstable airplane and a decent flight control system that makes it feel and appear stable to a pilot. That's why places like the test pilot school across the bay from you teach the characteristics you describe (albeit in a convoluted way) as "APPARENT" static longitudinal stability, and true longitudinal static stability as the variation in pitching moment with AOA lift coefficient.

Nauga,
and his pole placement.

 

[SixPapaCharlie]

Full nose down pitch input

That's gonna whistle I think.

 

[Checkout_my_Six]

That's gonna whistle I think.

dixie?

 

[Pilawt]

I understand that. Pilawt was saying stab control was modified 177A and later.

The forces between the 177B and 177 seems pretty much the same to me.

Yes, they do now, because the "Cardinal Rule" service letter SE68-14, which by now has presumably been accomplished fleetwide, retrofitted those modifications in addition to the slots. It increased the counterbalance from 7 to 11 pounds, and changed the gearing ratio on the anti-servo tab.

All these changes were incorporated on new airplanes at the factory beginning with the C-177A (s/n 17701165).

 

[Pilawt]

Full nose up pitch input

That photo makes it easy to understand how a C-177 stabilator would be susceptible to stall with full flap. The stabilator is lower than the flaps, right in the area of heavy downwash from the flaps. That creates a substantial angle of attack at the leading edge of the stabilator, greater than it would be if it were a fixed stab with hinged elevator.

That's why when Cessna issued the "Cardinal Rule" service letter, it required that flap extension be limited to 15º until the stabilator slots and other mods were completed.

With a low-wing airplane such as a PA-28, the stabilator is above the flaps, where downwash is not as great.

 

[bnt83]

Yes, they do now, because the "Cardinal Rule" service letter SE68-14, which by now has presumably been accomplished fleetwide, retrofitted those modifications in addition to the slots. It increased the counterbalance from 7 to 11 pounds, and changed the gearing ratio on the anti-servo tab.

All these changes were incorporated on new airplanes at the factory beginning with the C-177A (s/n 17701165).

It looks like the last few 1968s came with slots from the factory since the AD on the flaps only applies to 177-00001 through 177-01152.

TCDS serial # eligibility:
177 - 661, 17700001 and 1770003 through 17701164
177A - 17701165 through 17701370


Its interesting that 17700002 is a 1971 B model per the TCDS.

 

[Pilawt]

Its interesting that 17700002 is a 1971 B model per the TCDS.

17700002 was probably a "mule" that the company kept around for R&D. Wings of Cessna by Edward Phillips says it was the prototype 177B, N1902F (below), the one used for 1970 ad photos.

Thompson says the first flight of the prototype 177B was on 12/4/1968.

Meanwhile, FAA Registration says 17700002, a 177B, was registered since 7/31/1970 as N1910F, to the University of Kansas, in the "Experimental - Research & Development" category. It shows a manufacture date of 1968, and that the engine was a Continental (!) "Model 17025", whatever that is.

 

[poadeleted20]

It is possible to meet the requirements of 23.175 and 23.181 with a statically unstable airplane and a decent flight control system that makes it feel and appear stable to a pilot. That's why places like the test pilot school across the bay from you teach the characteristics you describe (albeit in a convoluted way) as "APPARENT" static longitudinal stability, and true longitudinal static stability as the variation in pitching moment with AOA lift coefficient.

While that is true (and I've flown such aircraft including the F-111 and F-16), I think such a system would cost more than the light plane it controls would be worth. In any event, I think the classical description of pitch stability terms is appropriate for the aircraft under discussion here.