No bounce landing

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PilotDAR
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Re: No bounce landing

Post by PilotDAR »

Typically, elevator control against the pitch up control stop may be one indicator of a stall. This is permitted by the design requirements. It is one of several possible conditions where the pilot no longer has control to raise the nose, which should be the more broad indicator of a stalled condition. If the nose won't go up any more, or you cannot prevent it going down, the plane is stalled.

While flight testing a modified Roberston STOL Cessna 206, I demonstrated to the Transport Canada test pilot who was accompanying me that day, that the slowest speed at which I could fly varied between 42 and 37 KIAS. This was with the control wheel held fully back at forward C of G. I inquired of him as to what we would define the stall speed as being in IAS. He asked me if I could control the flight at 37 KIAS - no I could not, it bobbed there itself, I could control at 42. So 42 KIAS was the stall speed, because although it would bob to a slower speed, then speed up a little again, I could not control that motion, other than to recover the stall, and fly faster than 42 KIAS.

However, a plane which may have controls at the stop as a stall indicator in one configuration, may certainly aerodynamically stall in other configurations (C of G and flap position, in particular). There were a few planes (Ercoupe) for which elevator at the stop was always the stall indication - but then the Ercoupe had a rather narrow C of G range, so it worked. The manufacturer won't give you any more elevator control than is shown to be needed for control in all situations, but they must give you enough control, so that may be a little more than you need to stall in some configurations.
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Ki-ll
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Re: No bounce landing

Post by Ki-ll »

digits_ wrote:
Mon Dec 16, 2019 3:49 pm
Ki-ll wrote:
Mon Dec 16, 2019 2:34 pm

I’d disagree. Airplane would stall at roughly the same stick/control column position most of the time. So if you find yourself pulling hard back on the controls you should know that you are close to the stall.
Following that traing of logic, if full aft position of the controls equals "stall", why does the manufacturer allow pilots to move the controls fully aft? Why not limit the control to the "not stalling" range?
I am not sure that’s the same train of logic.
There are situations where you might need that control deflection which goes beyond the “stalling position” of the controls albeit momentary. Holding it in full aft is different than going to aft stop when you need it.
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digits_
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Re: No bounce landing

Post by digits_ »

The problem with defining a stall like that -ignoring for a moment that it is factually incorrect- is that it simplifies matters too much. Let's say you teach a student like that. He might be afraid to use full control deflections when necessary, because full aft apparently means the plane is stalling.

Teach pilots what a stall actually is. Angle of attack that exceeds the critical angle. Ok, great. Now how can you recognize that? Multiple options, stall warnings, AoA vanes if installed, you might feel buffetting, the controls will feel different, unable to hold altitude ... Cool. But you can also be stalling without any of those things. Warnings can be broken, your yoke could be almost centered, you might think you are climbing instead of falling down, etc.

Then teach all those possible options. If you dumb it down to "full aft means you are stalling", you will not be doing your students any favors.
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Ki-ll
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Re: No bounce landing

Post by Ki-ll »

That’s a fair point.
Defining a stall via stick position is not correct.
I was talking more among the lines of what pilots have readily available to them in the cockpit to determine how close they are to a stall.
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iflyforpie
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Re: No bounce landing

Post by iflyforpie »

There’s also definite cases where full aft is not stalled.

Pipers were notorious for this—especially the T tail Seminole where the stabilator is out of the prop wash. Get too slow and you’re at full deflection and can’t pull the nose up. That’s why you tend to fly spam can Pipers on instead of stalling them on like a Cessna.

The Cessna 206 was also like this if it was at the fwd limit. I’d land with full aft input all of the time and the wing was nowhere near stalling. Usually a bit of power on touch down helped it out.
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corethatthermal
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Re: No bounce landing

Post by corethatthermal »

If photofly's data is correct, the was damn well as far into the backside as he can get. No you don't need thrust to slow down more. He was descending. He would just be descending steeper, which I think is exactly what happened, as the video seemed to show.
NO ! What may be confusing you is that he was descending rather steeply. Far into the backside would involve generous amounts of power with the ability to control A/C through pitch and power. The IAS would be much lower as well ..... If a plane was going straight down and unloaded the wing, would the A/C be in a stalled condition ?
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Re: No bounce landing

Post by corethatthermal »

According to charts i have seen, It appears that the industry is denoting the beginning of a stall when the lift reaches a maximum THEN starts to decrease. That start of decrease in lift is shown as the stall.
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AuxBatOn
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Re: No bounce landing

Post by AuxBatOn »

iflyforpie wrote:
Mon Dec 16, 2019 6:52 pm
There’s also definite cases where full aft is not stalled.

Pipers were notorious for this—especially the T tail Seminole where the stabilator is out of the prop wash. Get too slow and you’re at full deflection and can’t pull the nose up. That’s why you tend to fly spam can Pipers on instead of stalling them on like a Cessna.

The Cessna 206 was also like this if it was at the fwd limit. I’d land with full aft input all of the time and the wing was nowhere near stalling. Usually a bit of power on touch down helped it out.
It is ONE way to define stalls. Every aircraft will have different behaviour. Full aft stick doesn’t apply to all aircraft. Your AFM should tell you what the stall characteristics of your aircraft are.
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digits_
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Re: No bounce landing

Post by digits_ »

AuxBatOn wrote:
Mon Dec 16, 2019 7:16 pm


It is ONE way to define stalls.
No it isn't. The definition of a stall is independent of aircarft type. At best it is a possible symptom of a stall for some aircraft.
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Re: No bounce landing

Post by corethatthermal »

It is ONE way to define stalls
Well, I define a stall as a small place to house a horse or a cow or a goat !!

Looking for clues in a control column or an AOA or even a stall transmitter is like testing the water to determine how your daughter got pregnant!

My Definition : When the wing airflow starts to detach and the consequential reduction in lift starts, you have entered into the beginning of the stall region!
Stall : stop or cause to stop making progress.
A stall is a condition in aerodynamics and aviation such that if the angle of attack increases beyond a certain point then lift begins to decrease. The angle at which this occurs is called the critical angle of attack.
Technically, a wing is stalling when the reduction in lift from maximum ( in that particular case ) is as little as 1 billionth of 1% or less! remember the encylopedia definition above !
Stall : stop or cause to stop making progress.
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digits_
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Re: No bounce landing

Post by digits_ »

corethatthermal wrote:
Mon Dec 16, 2019 7:02 pm
If photofly's data is correct, the was damn well as far into the backside as he can get. No you don't need thrust to slow down more. He was descending. He would just be descending steeper, which I think is exactly what happened, as the video seemed to show.
NO ! What may be confusing you is that he was descending rather steeply. Far into the backside would involve generous amounts of power with the ability to control A/C through pitch and power. The IAS would be much lower as well ..... If a plane was going straight down and unloaded the wing, would the A/C be in a stalled condition ?
Oh I'm not confused at all. You only need power if you want to fly level on the backside of the power curve. If you are ok with losing altitude, you can go almost as slow without as you can with power. You are right you can go a little bit slower if you add full power, basically the difference between the power on and power off stall speed of the airplane. The power pulls a bit more, but mainly increases the airflow over the wing and control surfaces, thus making control inputs more efficient.

But all that doesn't change that the plane in the movie -assuming photofly's data is correct- was almost stalling and well on the backside of the power curve, even if the engines might have been shut off.

Image
On the above image, you can go below 60 kts with the power off. You can also go above 60 kts with the power off. Just depends on what you do with your altitude.


corethatthermal wrote:
Mon Dec 16, 2019 7:57 pm

My Definition : When the wing airflow starts to detach and the consequential reduction in lift starts, you have entered into the beginning of the stall region!
The 2 mentioned events (airflow starts to detach) and (reduction in lift) happen at different times. The airflow start to detach pretty quickly, on a typical aerofoil around 10 degrees. The actuall stall happens around 14 degree AoA. Only the second part in your definition matters.
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Re: No bounce landing

Post by corethatthermal »

If you have driven a car in snow or ice and have turned the steering wheel and the car failed to turn you have demonstrated how a wing stalls. You “stalled” the tire. You can do this on dry pavement which is much faster, more dramatic…and expensive!

I hope this helps the original poster.

For more detail…with relatively simple physics.

Wings generate a lift vector (a force in a direction) by flying at an “angle of attack” (AOA). That is the angle between where the wing is pointed and where it is actually going. Up to the point of stall, more AOA equals more lift (vector). When maximum AOA is exceeded lift decreases. That is a stalled condition.

With tires AOA is called Slip Angle. If you graph wing AOA vs. lift and tire Slip Angle vs. cornering force, the curves are remarkably similar. Passenger car tires act similarly to general aviation wings, and race car tires act similarly to fighter plane wings. Mother Nature is one wise lady.
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Re: No bounce landing

Post by corethatthermal »

When a wing "stalls" the A/C doesn't fall out of the sky as non-pilots assume. It simply starts to produce less lift for the configuration present! The twin in in our subject, would have reached its critical ( maximum ) AOA IF even 1 millionth of 1 % pitch up produces less lift. The accident A/C with further reflection on the definition of "stall" was likely within the first 10% of the stall "envelope" ( not yet a "deep stall" ) but I could be wrong ! IF the gear was up and the props feathered, perhaps there would be still a very small margin before the critical AOA !
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Re: No bounce landing

Post by corethatthermal »

The 2 mentioned events (airflow starts to detach) and (reduction in lift) happen at different times. The airflow start to detach pretty quickly, on a typical aerofoil around 10 degrees. The actuall stall happens around 14 degree AoA. Only the second part in your definition matters.
I stand corrected !
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Re: No bounce landing

Post by corethatthermal »

basically the difference between the power on and power off stall speed of the airplane. The power pulls a bit more, but mainly increases the airflow over the wing and control surfaces, thus making control inputs more efficient.
Correct! The aircraft basically becomes a blown wing ( where airflow is generated by the props and conforms OR re-attaches to the wing profile AND of course the vertical thrust component is a strong part of overall lift. Ironically, when looking at a Russian fighter demo or putting your hand out of a car window, Form drag also becomes a valued component in the drag scenario to further slow down the plane as thrust increases!
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photofly
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Re: No bounce landing

Post by photofly »

This may help.
STALL.png
STALL.png (187.66 KiB) Viewed 281 times
I think the next graphic in the series discusses the effect of flaps on which direction the aircraft can fly without being "stalled", relative to the longitudinal axis.
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corethatthermal
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Re: No bounce landing

Post by corethatthermal »

If wing airflow starts detaching at 10 degrees and Critical AOA is at 15 degrees, then the wing is LOSING lift and at the same time gaining more lift between 10 and 15 degrees ! Hence the lift centre of pressure moving forward. I would speculate that the fwd 1/5 of the wing is increasing in lift as the rest of the wing is decreasing / stalling therefore the net lift is a loss in the front envelope of a stall Theoretically, with enough thrust, an aircraft could stay stationary in the sky ! With high wing wash-out and high thrust, the critical AOA is delayed due to the propwash being the direction of airflow and not the A/C path AND the outer wing being at a low AOA relative to an unblown inboard section. With this in mind, the critical AOA is not reached until perhaps in the 20+ degree AOA, calculated without propwash flow ! For these and other reasons, the approach speed can be reduced SIGNIFICANTLY when using the backside approach technique !
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photofly
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Re: No bounce landing

Post by photofly »

I found part 2:
stall_flaps.png
stall_flaps.png (279.69 KiB) Viewed 261 times
part 3 is about why the critical angle of attack changes with power.
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trey kule
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Re: No bounce landing

Post by trey kule »

I believe the plane in the video is a Seneca. Regardless though, does its wing have a single airfoil, or any twists ,or any other aerodynamic properties that would affect the wing stalling. On many aircraft the wings will partially stall as the AOA is different for different parts of the wing.
The result is a pilot can get well onto the backside of the power curve on a descent but not have enough elevator authority to pitch up in the flare.
I am a bit surprised,that in the discussion of stalls no one has mentioned the movement of the C of P. Ultimately, that will cause the wing to pitch down from the relative air flow.
Prior to the C of P movement what you have is the drag /lift ratio changing on the backside of the curve.

An interesting discussion. I think AuxBat had it pretty well nailed in general.

My thoughts on these type os accidents is that students spend to much time flogging around with the stall warning horn bleeting, rather than learning how to recover before it gets to that stage. And most of the school lift/drag or power/as charts are all in level flight.
The accidents more often occur during climb out or on the approach where the pilot does not recognize the state of flight they are in.
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trey kule
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Re: No bounce landing

Post by trey kule »

I believe the plane in the video is a Seneca. Regardless though, does its wing have a single airfoil, or any twists ,or any other aerodynamic properties that would affect the wing stalling. On many aircraft the wings will partially stall as the AOA is different for different parts of the wing.
The result is a pilot can get well onto the backside of the power curve on a descent but not have enough elevator authority to pitch up in the flare.
I am a bit surprised,that in the discussion of stalls no one has mentioned the movement of the C of P. Ultimately, that will cause the wing to pitch down from the relative air flow.
Prior to the rapid C of P movement what you have is the drag /lift ratio changing on the backside of the curve and a relative slow forward creep on the C of P, providing changes are not to rapid.

An interesting discussion. I think AuxBat had it pretty well nailed in general.

My thoughts on these type of accidents is that students spend to much time flogging around with the stall warning horn bleeting, rather than learning how to recognize and recover before it gets to that stage. And most of the school lift/drag or power/as charts are all in level flight.
The accidents more often occur during climb out or on the approach where the pilot does not recognize the state of flight they are in until it is to late to recover
When that obstacle is approaching on a climb out, with full power, it takes a real applied understanding of the lift/drag not to intuitively raise the nose...or on an approach that is a little short, to once again, not just raise the nose.
In the olddays when we were not so book smart we were told that on an approach, pitch for speed, power for altitude. Of course we now know there is more to it, but that did work pretty well.
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Re: No bounce landing

Post by PilotDAR »

Thanks for all of he stall discussion colleagues, It's a good refresher for me as I have two days of stall testing starting today. It's going to be fine, but compliance must be demonstrated for the modification.

As I read the preceding, I can easily find agreement with most everything presented, because there are many truths throughout. A major truth is that various planes will exhibit differences in their stall, and even condition changes in the plane will affect the characteristics. It's kind of pointless to try to assign one characteristic to define a stall for every type, it's just not that way, It's not even that way for variations of condition of the same type!

But, if I had to pick one condition to be my indicator of being in the regime of the stall, it would be: Do I have adequate control of the plane in pitch?" If I can't adequately control pitch, then I need to be thinking about preventing/recovering a stall to enable resumption of control. Stall recovery techniques are well understood, (and in my opinion, adding power should not be one of them, that's done after you fly the plane).

Center of pressure change - yes, though I think it was less a factor in the plane in the video, but yes, it should be considered and appreciated. A glaring example I have experienced several times while flying stall testing has been with wing modified Cessnas. Think about this: The Center of Pressure and wing pitching moment are a product of the airfoil as factored by the wing area. With the exception of the 206, Cessna has designed its singles to have very good pitch control at slow speeds. The elevator and horizontal stabilizer are right (the 206, not so much). So we take a wing to tail well balanced plane, and change the wing by adding a STOL cuff, and wing extensions, both of which change the wing area, and airfoil - wing works better at slow speed - that's why we spent the money! Did we also change the tail to balance the effect? Not commonly.

So to TK's and Pie's comment, yes, you can run out of up elevator, and I've had a few post mod Cessnas do that at forward C of G landings. I've included vortex generator installations in this case, and they have mad the difference in control effectiveness.

This is a thread drift from the accident video, but a good discussion anyway

I brought a bag of VG's with me this week, just in case.... I'm off to go flying....
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Re: No bounce landing

Post by photofly »

trey kule wrote:
Tue Dec 17, 2019 12:30 am
The result is a pilot can get well onto the backside of the power curve on a descent but not have enough elevator authority to pitch up in the flare.
If you were well into the backside of the power curve, and DID have enough elevator authority to flare, you still may not be able to arrest your descent. Raising the nose At that point will only spare the nosewheel the first impact, but not spare the airframe from damage.

To reduce the rate of descent you need to impart a quantity of upward momentum to the aircraft. Momentum is force, times time. Let’s examine each of these, as you raise the nose.

Force increases (lift), mostly in a vertical direction) when you raise the nose because of an increased angle of attack, but the increase is transient because the aircraft is slowing down, which reduces the lift.

The upward impulse that you can generate - the momentum change in the vertical direction that you need to round out to level flight is limited by two factors.

The closer you are to peak lift (stalling AoA), the less margin you have to generate more lift by raising the nose. Obviously if you are at peak lift AoA already, neither raising NOR lowering the nose will generate any upward impulse. Raising or lowering the nose from peak AoA will reduce the lift, not increase it.

Secondly element: TIME. The rate at which the airplane slows down limits the upward momentum you can generate. If you could raise the angle of attack to peak lift (stalling AoA) and maintain your airspeed indefinitely you could generate as much upward momentum as you like. Alas, the aircraft is slowing down, and the lift, even at peak lift AoA, will consequently decrease. The faster the airspeed bleeds off, the less time any extra lift you can generate has to add upward momentum to your flight path and the less of change in your rate of descent you can get.

The airplane will decelerate fastest when it has high drag (due to say full flaps, and or windmilling props). If you fly in at nearly peak lift AoA (on the back of the power curve, close to Vs0 rather than 1.3 x Vs0) but have enough power to mitigate drag for a long enough time and so decelerate very slowly, then you can achieve a nice landing.

If you have no power (make a glide descent) and fly a high drag aircraft (like one with floats or or extra weird survey antennas) then you will need more than the traditional 1.3Vs0 airspeed before you begin a roundout to be able to generate enough upward momentum to transition to level flight above the runway.

If you have very low drag airplane (slippery airframe, flaps up), then less than the traditional 1.3Vs0 might work out ok for you.

Either way, as up TK says, if you can’t raise the nose because your elevator isn’t designed to have enough authority at low airspeed then you can’t generate any upward momentum at all.
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corethatthermal
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Re: No bounce landing

Post by corethatthermal »

2 A/C in my life which had not enough elevator/stabilator authority are the Zenith CH 200-250-300 series and the present one a cessna 175 with a 180 HP conversion,,,,appears the modifiers forgot about ballast when that heavy CS prop went on! Both aircraft types have the wing nicely flying when the stick goes all the way back with no pitch up moment at reduced/idle power
A long time ago, I used to do up to about 70% power approaches and then power up to near full power for the flare. Do not try this at home !
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