Crash landings/ditchings

[wsherif1]

AirRabbit,

Your comments,

quote:
_____________________________________________
I never mentioned arresting the forward motion to zero!
_____________________________________________

"I went back and read the thread again. I apologize; your statement was “…at minimum momentum…” not zero momentum."

I really meant "reduced" momentum, my error.

"Personally, I’ve never heard of “zero weight transition."

"Zero weight transition" sounds like the point where the lift equals the force of gravity."

I think you got it! The transition occurs automatically on take-off. It occurs again on landing shortly after the flare maneuver in ground effect, out of a level flight attitude, at a tangent angle to the earth flight path.

"Let’s not go around inventing new terms and leaving everyone the daunting task of trying to figure out what the term means."

The term "zero aircraft weight transition" describes an event that occurs in aircraft operations. You were obviously able to understand its meaning. It seems self explanatory.

"...and therefore the aircraft would have "full weight" by the time it touches surface."

This statement is obviously the problem! In this technique the aircraft touches down at the instant the weight exceeds the lift, (in a level flight attitude) but the lift is still supporting the weight of the aircraft, to some extent, until the aircraft slows to taxi speed, at which time the aircraft is then at its unsupported,
actual weight.

" In order to get an “imperceptible” touchdown, this “transition” would have to occur slowly enough to be below the threshold of recognition of the occupants of the airplane. And, again, to my knowledge, humans don’t have the ability, or the tools, to control the balance of forces acting on an airplane to such a fine-tuned degree to achieve this set of circumstances, other than by pure fortune."

No inate ability or special tools required! Just hold a level flight attitude out of the flare, maintaining a tangent angle flight path, as the aircraft descends to an imperceptible touch down.

Used this technique in the CV 990 (Stiff lndg, gear) Flight attendants exclaimed they couldn't tell when the aircraft landed.

[AirRabbit]

Howdy wsherif1:

I don't really know how to say this without sounding like I'm trying to insult you -- believe me, that is not my goal.

First: I think you are reading more than one post and confusing one author with the author of another post. Or, you have not understood that when someone else is being quoted, that quote is set off from the other text between horizontal lines.

Second: I think you're using terms that apparently you have become accustomed to using, although, I certainly cannot understand where knowledgeable aviators would not question the use of these terms. As far as I know, some of the terms you are using are not recognized aeronautical terms. For example, the term "zero aircraft weight transition" is not recognized as an aviation term by anyone I know in the industry. The notion of holding an aircraft “…continually in a level flight attitude, while descending in a tangent angle, to the ground, flight path…” in addition to being difficult to understand grammatically, is beyond the understanding of anyone that I know in this business either mathematically or aerodynamically. At least one other participant in this thread has also indicated that he just does not understand what you are saying.

Third: I was not the one who said that the term “zero weight transfer” sounded like something. To me, it sounds like a term that has been "cobbled together" to sound technical, but has no basis in practicality.

Fourth: judging by your last post, I think you have probably misunderstood some of the basic concepts of aerodynamics. You indicated as untrue the idea that as the aircraft lands the earth supports the weight of the aircraft. In argument, you state your belief that lift is still “…supporting the weight of the aircraft, to some extent, until the aircraft slows to taxi speed, at which time the aircraft is then at its unsupported, actual weight.” I would respectfully suggest that you check out your theory with any good book on aerodynamics and/or physics.

Fifth: I am concerned that you really do have knowledge of aviation and you have been exercising a rather elaborate plan to inject “gobble-dy-gook” explained by additional nonsense, into this thread, and I, and others here, have swallowed the whole joke!

Sixth: I am concerned that less experienced aviators, or those who are contemplating becoming involved in aviation, who frequently visit this particular forum, may read your posts and become confused as to how an airplane actually flies. I am concerned that these folks might get frustrated to the extent that they may seek a career in another area. I would hate to have a budding aviation professional scared into being a TV weather “person” because of some irate ranting they once read on an aviation discussion forum!

[Sven Sixtoo]

Air Rabbit

On behalf of us all, thank you for your patience.

Are you a schoolteacher?

Sven

[AirRabbit]

Hey Sven:

On behalf of us all, thank you for your patience.

I’m not sure if that is an example of patience or stubbornness on my part. I really DO believe that there is a good likelihood of there being impressionable folks on this forum who might get lost in some of the George-Orwellian double-talk that surfaces once in a while when its purveyor tries to pass it along as technical lingo.

Are you a schoolteacher?

Well, actually, I used to be. I taught high school and college to eat and ab initio flight training for fun. But the last 30+ (well, almost 40, now) years, I’ve been involved in flight training and simulation as a primary interest from both the management and the worker perspective, thankfully, interspersed periodically with line operations. Admittedly, most of that has been centered around larger aircraft, and, until a couple of years ago, I enjoyed flying some of the newer, smaller stuff.

Stay inquisitive ... take everything you read on these forums with a grain of salt. Most here are genuine, intelligent, and very experienced aviators offering their views on things; or are interested, lesser experienced aviators or those who desire to become aviators – but obviously, there are some here with questionable backgrounds who still believe in George Orwell and his predictions for 1984.

[chornedsnorkack]

Fourth: judging by your last post, I think you have probably misunderstood some of the basic concepts of aerodynamics. You indicated as untrue the idea that as the aircraft lands the earth supports the weight of the aircraft. In argument, you state your belief that lift is still “…supporting the weight of the aircraft, to some extent, until the aircraft slows to taxi speed, at which time the aircraft is then at its unsupported, actual weight.” I would respectfully suggest that you check out your theory with any good book on aerodynamics and/or physics.

I do not see anything wrong with that.

Each and every object moving with respect to air at whatever nonzero speed and direction is subject to some aerodynamic forces.

Planes tend to generate upwards aerodynamic force - "lift" -when subject to airflow across the wing.

It follows that an airplane generates some lift when taxiing, or when parked at a gate and subject to wind... in short, at all times except when standing in a hangar with doors closed and ventilation shut off. (Though depending on the direction... a tailwind might generate negative lift, pressing the plane downwards).

When an aircraft lands, it starts to be in contact with and supported by the ground.

But it still moves in the same direction, and with almost the same speed as before landing. It follows that it must generate almost as much lift as before - only slightly less than before landing.

Of course, you may not want that situation for serveral reasons, including:
if your airspeed increases (a gust of headwind) or the lift coefficient increases then the lift increases and may again exceed the weight, so the aircraft becomes airborne again
if only a small part of the weight is supported by wheels, then the wheel friction cannot be big, either

surely there are more...

[wsherif1]

Crash landings/ditching.

Air Rabbit,

Your comments,

Quote:
____________________________________________________
"You indicated as untrue the idea that as the aircraft lands the earth supports the weight of the aircraft. In argument, you state your belief that lift is still “…supporting the weight of the aircraft, to some extent, until the aircraft slows to taxi speed, at which time the aircraft is then at its unsupported, actual weight.” "I would respectfully suggest that you check out your theory with any good book on aerodynamics and/or physics."
____________________________________________________

The following quote is from chornedsnorkack.
____________________________________________________
I do not see anything wrong with that.

Each and every object moving with respect to air at whatever nonzero speed and direction is subject to some aerodynamic forces.

Planes tend to generate upwards aerodynamic force - "lift" -when subject to airflow across the wing.

It follows that an airplane generates some lift when taxiing, or when parked at a gate and subject to wind... in short, at all times except when standing in a hangar with doors closed and ventilation shut off. (Though depending on the direction... a tailwind might generate negative lift, pressing the plane downwards).

When an aircraft lands, it starts to be in contact with and supported by the ground.

But it still moves in the same direction, and with almost the same speed as before landing. It follows that it must generate almost as much lift as before - only slightly less than before landing.
____________________________________________________
Your comment,

Quote:
____________________________________________________ "I would respectfully suggest that you check out your theory with any good book on aerodynamics and/or physics."
____________________________________________________

I suggest that there are some problems with your "Good book"!

[Linton Chilcott]

I heard once that the USAF test ditched a B-24 during WW2.

Apparently the crew didn't have anything to say on the imperceptibility or otherwise of the touchdown, as it went straight to the bottom, taking them all with it.

[wsherif1]

Linton Chilcott,
____________________________________________________
"I heard once that the USAF test ditched a B-24 during WW2.

Apparently the crew didn't have anything to say on the imperceptibility or otherwise of the touchdown, as it went straight to the bottom, taking them all with it."
____________________________________________________
What a tragic, avoidable loss.

Apparently the aircraft touched down at its full weight and momentum in a certain rate of descent mode.


!

[AirRabbit]

chornedsnorkack

I have thought long and hard about whether or not I should bother putting together a long, drawn-out response to you and to wsherif1. I was quite tempted to say, “OK. Fine. Believe what you will. Good Luck.” However, I began to think about the fact that I have attempted to point out some errors in this approach for a couple of weeks, and, that others may believe that by just “rolling over” without further comment, I just might be indicating that I really don’t have much of an argument against such positions. The impression I get when reading posts that decry the loss of a US Army Air Corps B-24 as “a tragic, avoidable loss” and immediately concluding that “…apparently the aircraft touched down at its full weight and momentum,” and continues with indicating the same “error” was behind the AA Little Rock and the AF Toronto accidents, is that at least some here believe (to some extent) that if only those crews had been aware of and had employed this “philosophy” we’ve been debating they would be alive to talk about it today. Would that this were true! However, I believe that such simplistic comments are not soundly based – at least not communicated effectively – and discount other relevant factors that just cannot be discounted. Judging by the number of folks who have read this particular thread, there is more than just some interest in the topic, and I don’t want the younger, or lesser-experienced participants in this forum to be beguiled into believing erroneous statements. I would hope that each such impressionable reader would NOT believe anything (necessarily) that they read here – including what I have to say (perhaps more so than others, but I guess that remains to be seen). I would rather they take up the interest and investigate the subject on their own. However, taking the time to put together a thoughtful and careful response (hopefully without insulting anyone) is not necessarily something that I can do at one sitting at the computer keyboard. I would like to provide my rebuttal, but it will take a bit of time to do so. And, unfortunately, with the complexities that would necessarily go into such a rebuttal, I may have to post it in more than one section. Stay tuned ….

(The Rabbit saunters over to the library shelf, selects 3 texts, positions himself at the keyboard, and begins to read and type….)

[wsherif1]

chornedsnorkack,

I have corrected my previous post, the two types of accidents were different in that one was a ditching and the AA and Air France were overruns.

[AirRabbit]

Hello Messrs. Chornedsnorkack and Wsherif1:

Planes tend to generate upwards aerodynamic force - "lift" -when subject to airflow across the wing. It follows that an airplane generates some lift when taxiing, or when parked at a gate and subject to wind... in short, at all times except when standing in a hangar with doors closed and ventilation shut off. (Though depending on the direction... a tailwind might generate negative lift, pressing the plane downwards).

While it is true that air moving over an airfoil will likely generate some sort of aerodynamic reaction, one must know several things (wing shape, size, camber, aspect ratio, angle of incidence, etc.) in order to know what that reaction is likely to be. I’m sure that I don’t have to provide much of a dissertation on the effect of angle of attack (AoA) and how that AoA affects the generation of lift.

I know that talking about a military bomber, the B-52, might seem out of place here, but bear with me as I do have a point. The B-52 was designed and built with a wing angle-of-incidence of 6 degrees. You probably don’t need this information, but in case there is someone here who might – the angle of incidence is the angle between the wing chord line and the longitudinal axis of the airplane. It is built into the structure and does not change. (There was considerable effort some time back to try to design an airplane that would have an adjustable angle of incidence, similar to the way the horizontal stabilizers on some aircraft are adjusted to trim elevator force – but it was abandoned as far as I know.) The 6-degree-design (and 6 degrees is a pretty “fat” amount) for the B-52 was necessary because the tandem MLG layout (two double wheels, side-by-side, under the front fuselage; and the same arrangement under the aft fuselage) did not permit the aircraft to “rotate” on takeoff. If you watch a B-52 take off, it does not “rotate.” Due to the AoA and the airflow over the wings with takeoff flaps extended, it just flies off the ground when the lift is sufficient. A lot of general aviation aircraft are constructed with the same wing angle of incidence, approximately 6 degrees. Because of that, I’m sure that if you allowed a “typical” general aviation airplane to accelerate on the ground, it would eventually reach a point where the angle of incidence (which is very close to the AoA while accelerating with all the wheels on the ground) would allow the airplane to “fly off the ground” with virtually no input from the pilot. However, transport category aircraft are not designed for that kind of operation. So lets discuss an airplane like the ones we were discussing earlier in this thread – a “generic” airplane that weights 135,000 pounds (a good, average weight for an MD-88, by the way) where all flying surfaces are sweptback, tapered, and use a “traditional” airfoil shape with wing sweepback at the quarter-chord of 25 degrees; a dihedral of 5 degrees; and a wing incidence of a little more than 3 degrees (3.2 degrees is a good, typical number), and an aspect ratio of about 10. Again, the salient point here is that the wing is mounted at 3.2 degrees of incidence with the fuselage.

If we use these values of wing geometry, assuming our generic airplane has all three gear on the ground and the longitudinal axis is parallel with the horizon, at approximately 10 knots, a nice taxi speed, there would be a “lifting force” on the airplane of 280 pounds. At 50 knots this force increases to approximately 6,800 pounds. With up to 75 knots (more than a Category I hurricane level wind) you would have a “lifting force” of a bit more than 14,000 pounds acting on the airplane. I know that 14,000 pounds sounds like a lot of weight, and, I guess it is as far as pure weight goes – but it is just about 10% of the total weight of a 135,000-pound airplane. If you want to consider that a “significant” force, be my guest. As the airplane accelerates to approximately 140 knots (just a bit above the “rotate speed” of a jet aircraft weighing approximately 135,000 pounds) the lift being generated is approximately 55,000 pounds – hardly enough to allow a 135,000 pound airplane to “fly.”

For this next part there are several assumptions that we’ll have to make, since this is all hypothetical. If we rotate the airplane to a pitch attitude of approximately 15 degrees nose up, the 3.2 degrees of incidence puts the wing at approximately 18.2 degrees to the horizontal. Recall the following: 1) AoA is the angle between the wing and the relative wind; 2) once out of ground effect, the airplane will be climbing at less than the airplane pitch angle; and 3) a lot of things are happening simultaneously; e.g., the continued rotation from zero to the desired angle; the continued acceleration; and, significantly, the continuing change in the lifting force generated. I’m going to avoid getting into a discussion of what happens when in ground effect and look at the airplane after it gets into “free air.” We know that the AoA is going to be less than the pitch angle. So with a pitch angle of approximately 15 degrees, the AoA will probably be something on the order of 11 to 13 degrees – say 12 degrees, maybe a skosh more. So, at the 140 knots airspeed, with this AoA, the lift generated will be approximately 146,700 pounds. This is certainly enough to support the 135,000-pound airplane in a positive rate of climb (RoC). As the airplane accelerates to a comfortable speed margin above the stall, to something like 180 knots, the lift will increase to a bit over 230,000 pounds – or a reasonable RoC for our “generic” airplane – and, as the airplane continues to accelerate to a normal climb speed, this difference continues to widen, allowing an even greater RoC.

When an aircraft lands, it starts to be in contact with and supported by the ground. But it still moves in the same direction, and with almost the same speed as before landing. It follows that it must generate almost as much lift as before - only slightly less than before landing.

OK – let’s talk about the other end of the flight – the landing. When the airplane is configured for the landing (gear and flaps down) and is approaching the runway, at the proper speed (no wind conditions would make this 130% of the stalling speed for the present configuration of the airplane – with zero thrust), it has an AoA that is defined by the chord line of the wing and the relative wind. As you have seen airplanes on final, most (but not all) have a prominent “nose up” attitude while on final approach – that is, “nose up” relative to the horizontal. A normal pitch attitude would be in the neighborhood of about 5 degrees nose up – so lets use that value for our generic airplane on final.

The normal glide slope on final approach is to track an Instrument Landing System (ILS) electronic glide slope that is typically between 2.5 and 3 degrees. Therefore, about 5 degrees nose up attitude, descending on a 2.5 degree down slope, with a 3.2 degree angle of incidence on the wing – the aircraft will have about 10.7 degrees AoA. How does this compare with the AoA on takeoff? Well, we said we’d probably have a skosh more AoA than 12 degrees during the initial portion of the takeoff and that produced about 146,700 pounds of lift – enough for a RoC. How about here? Well, here we’ll have a lot more flap extended, which, as you know, changes the shape of the wing – giving it more camber. The speed on final will be less. Why? While expressed as a percentage of the stalling speed, just like at takeoff, with more flaps the stalling speed is less and therefore the 30% addition is a smaller addition to a smaller number, yielding a lesser airspeed. We’ll still use the 135,000-pound airplane (assuming for the sake of the discussion that we don’t burn any fuel – oh, that it were true!) and final approach should be flown at something like 135 knots (no wind). Plugging all of these values in, we get lift generated of just a bit over 130,000 pounds. With a weight of 135,000 pounds, what do you suppose is happening? Yes, the airplane is descending. Hopefully, this should stay essentially undisturbed until the pilot is ready to land. Technique here is a big factor – but no matter the technique, the airplane should be landed in a level flight attitude -- level flight attitude for the configuration of the airplane and level flight attitude for the airspeed reached at the conclusion of the flare. Note, I did not say land at the airspeed reached at the conclusion of the flare; I said land in the attitude that would achieve level flight at the airspeed reached at the conclusion of the flare. Read that sentence again if you have any doubt about what I’m saying. You should also be aware of the fact that when you consider that any landing must come from at least some RoD and that when that RoD is eliminated (once on the ground you’re not going to be descending any further, we hope) the AoA necessarily changes – it becomes less. As we’ve seen here, even a very minor change in AoA can have rather dramatic change in the amount of lift being generated – less AoA; less lift.

-- Because of the length of my overall post, I'm going to have to stop here and post the remainder in one or more additional posts -- sorry if that causes any inconvenience.

AirRabbit

Hello Messrs. Chornedsnorkack and Wsherif1: (Part 2 of 3 parts)

I’m sure you’re aware of “lift induced drag;” drag that is developed as you develop lift – the more lift you develop, the more induced drag is developed as well. However, in ground effect, the drag that is “induced” is substantially less than that produced for the same lift outside of ground effect. Because of this, you don’t need the same AoA and you don’t need the same velocity to get the same lift. Therefore, if you should bring the airplane to a level attitude inside of ground effect (which you have to do to land) with either too much airspeed or too much power or both and the airplane will tend to climb or it will “float;” neither is a sign of a good approach and landing.

Again, the flare is the maneuver the pilot performs to assist in the arresting of the RoD. Ideally, the flare maneuver will put the airplane in a level flight attitude just a few inches above touching the runway. And the ideal technique will have the throttles coming back to idle, allowing the airspeed to decrease throughout the maneuver; allowing the wheels to touch within seconds of reaching level flight attitude; and having the throttles at idle at the instant the wheels touch, and this should happen at an airspeed slightly below Vref. This allows the pilot to retain complete control of the airplane while it is a flight vehicle, through its transition to a ground vehicle, and after it becomes a ground vehicle.

Oh yes, there are some (and some on this thread) who have advocated just holding the airplane in that “level flight attitude, and let the airspeed bleed off until touchdown.” Unfortunately, there are two errors that are typically made with this intention. In the first case, the pilot will forget to increase the backpressure on the controls (note – I did not say increase the pitch) as the airspeed decreases. This failure will lead to a dropping of the nose and the establishment of a RoD that will get arrested by the runway. The distance the airplane moves from when the RoD is initiated and what that RoD actually is, will determine the “ouch” level. In the second case, the pilot gets over-zealous with the backpressure on the controls and does what I cautioned against – s/he increases the pitch attitude. Again too much increase and the airplane will climb. No one would want to climb out of ground effect in this condition (low airspeed, low power setting, more flap deployment, higher pitch attitude) – so don’t do that! But those that manage to prevent a climb often continue a gradual increase in the pitch to compensate for the airspeed reduction and wind up with the nose precariously high with the airspeed precariously low and rapidly running out of airplane controllability. This is the traditional “full stall landing” that you’ve probably heard me decry for transport airplanes before – don’t do that either! You should know that it is VERY easy to strike the tail of almost any airliner before getting to the stall; and striking the tail can cause some very serious structural damage, particularly if the wheels are not on the ground and supporting at least some of the weight. If, by some chance, the pilot doesn’t make either of these errors, and s/he maintains level flight attitude with a proper increase on the backpressure to maintain that attitude, s/he will allow the airplane to float until IT gets ready to land. In my opinion, that is abdicating the responsibility of the pilot. The pilot should land the airplane, not the other way around! Also, floating down the runway is an excellent way to chew up runway that will of no use to you. The wheel brakes are the most effective stopping mechanism on the airplane, and they are useless with the wheels in the air.

Now, to the issue of whether there is “an automatic zero weight transfer” when this landing occurs. I hope by now I have convinced you that the airplane always has weight – it is never weightless. It is true that if you had an infinitely long runway, and had just the right combination of approach airspeed, flare and throttle reduction technique, to arrive at a level flight attitude just a couple of inches off the runway and “held it off” until IT decided to land, as the wheels touch the runway, the weight of the airplane is assumed by the runway. No. Not all of the weight is transferred to the ground instantly. However, I surely hope you don’t think this is a several minutes long process to transition from a flying vehicle to a ground vehicle. First, the initial touchdown (even “kissing” it onto the runway surface) will generate a significant energy drain on the airplane. Also, that first “kiss” touchdown will result in a change in the AoA as there will no longer be a RoD, except for the time it takes for the airplane to travel the length of the landing gear struts. Almost any energy drain will reduce the lift being generated and a change in the AoA will have the same effect. This will erase any “kissing” tendency and put a significantly higher amount of weight on the wheels. This process, while certainly not instantaneous, is measured in portions of a second. If the nose is in what was a level flight attitude, the nose gear should not be far from the ground and as soon as it is placed on the ground (or falls to that point) the airplane will be back again in a 3-point attitude and the lift being generated will be well below what it would take to support the airplane. The numbers are similar to those we discussed for the takeoff.

However, in fact, transport category airplanes are designed with features that allow the transition from flight vehicle to ground vehicle to occur much more quickly than what I’ve just described here. Those two things are speed brakes and ground spoilers – and I’m not necessarily talking about the same surfaces used differently at different times. There are some aircraft that have speed brakes fixed to the vertical stabilizer that open equally on both sides – others have speed brakes that are extended from the lower portion of the fuselage. These serve to hold the airplane from accelerating with a larger thrust setting – so that when the speed brakes are retracted, there is less hesitancy in acceleration (like for a missed approach / go-around) or they continue to act as speed brakes when the throttles are pulled to idle at landing – and help slow the airplane on the ground. Ground spoilers are different animals. They do just what their name implies – they spoil the lift being generated over the wing. This kills most or all of the lift being generated and puts all of the weight of the airplane on the wheels where the brakes can do the most good.

The next time you’re watching airplanes land – either from a vantage point at an airport or from inside the cabin – watch carefully at the wing tips. If there is sufficient moisture content in the air, you may see contrails spinning off the wing tips. The higher the humidity the better the chance you will see them. Watch what happens to these vortices as the wheels of the airplane touchdown. They disappear. Recall that the vortices are being generated because there is lift being generated. Why do you suppose they would disappear when the wheels touch down?

But, lets go back to your statement that as an airplane lands “it still moves in the same direction, and with almost the same speed as before landing – it follows that it must generate s much lift as before, only slightly less than before landing.” Remember the description I provided of the airplane taking off; where I described a relatively high airspeed, well above the calculated “rotate” speed, the lift being generated was only a fraction of the the weight of the airplane? Well, the same thing happens just after landing. Once the AoA is decreased, the lift being generated decreases just as rapidly – and it doen’t make any difference how long after “landing” that happens.

Simply put, there is no such thing as “an automatic zero weight transfer” either at lift off or touch down. As I mentioned earlier, I’ve discussed this with several aeronautical engineers with whom I have the privilege of working every day. This term and the term “reduced momentum,” that have been tossed around as if they were meaningful terms, has generated more than a little comment from these very seasoned and very professional engineers and engineering test pilots. One comment from one of the engineers probably sums it up best: “I don’t know what this guy is talking about. I think he’s dangerous. No, I don’t think he’s dangerous; he is dangerous.”

-- Again, because of the length of my overall post, I\'ll have to stop here and post the remainder in an additional post.

AirRabbit

Hello Messrs. Chornedsnorkack and Wsherif1: (Part 3 of 3 parts)

…if your airspeed increases (a gust of headwind) or the lift coefficient increases then the lift increases and may again exceed the weight, so the aircraft becomes airborne again if only a small part of the weight is supported by wheels, then the wheel friction cannot be big, either.

From your statement, I would gather that you are of the opinion that lift is generated when air moves over a wing’s surface because of the shape of that surface – and the amount of lift is directly related to the speed of the air over that wing. There are a couple of common misconceptions about the “generation of lift,” at least one of which is probably playing into your making this statement.

The misconception I’m referring to is based on something called the principle of “equal transit times" (or ETT, for short) where side-by-side molecules of air are rumored to split at or just prior to the airfoil leading edge and meet again at the trailing edge. To provide additional information, and very probably some raised eyebrows and other reactions, let me provide a bit of information about some errors in the understanding of the generation of lift.

“Longer Path” or “Equal Transit Time” Theory:

This assumption is that airfoils are shaped with the upper surface longer than the bottom. Furthermore, because the air molecules have farther to travel over the top of the airfoil than along the bottom in order to meet up at the trailing edge, the molecules going over the top of the wing must travel faster than the molecules moving under the wing. Because the upper flow is faster, the pressure is lower. The difference in pressure across the airfoil produces the lift.

Unfortunately, this is not always correct. Think of a paper airplane. Its airfoil is a flat plate -- top and bottom exactly the same length and shape and yet they fly just fine. This part of the theory probably got started because early airfoils were curved and shaped with a longer distance along the top. Such airfoils do produce a lot of lift and flow turning, but it is the turning that\'s important, not the distance. There are modern, low-drag airfoils which produce lift on which the bottom surface is actually longer than the top. This theory also does not explain how airplanes can fly upside-down. The longer surface is then on the bottom!

If you use this theory and calculate the required lift for a typical small airplane, the distance over the top of the wing must be about 50% longer than under the bottom. You’ve seen airplane wings – does that sound like what you see? Of course not! In fact, if you look at a wing of a typical small plane, like a C-172, the distance from the leading edge to the trailing edge over the top surface of the wing is 1.5 to 2.5 percent longer than over the bottom surface. OK, it is farther, but not as far as it has to be – so now what? Well, we can increase the velocity of the air; however, we’d have to fly that C-172 at over 400 mph to generate enough lift to support its weight. You know that ain’t gonna happen. There is nothing wrong with the Bernoulli principle, or with the statement that the air goes faster over the top of the wing. But, after that, what we expect begins to be over-run with facts.

There are experiments that clearly show that airflow over the top of a curved airfoil does travel faster than the flow beneath the airfoil. But the flow is much faster than the speed that would be required to have the molecules meet up at the trailing edge. Experiments have been done where smoke is introduced into a wind tunnel periodically (i.e., introduced and stopped; re-introduced and stopped again; etc.). When this is done an observer can see that the air going over the top of the wing gets to the trailing edge considerably before the air going under the wing. Two molecules near each other at the leading edge will not end up next to each other at the trailing edge. In fact, the air over the top of the airfoil is accelerated much faster than is theorized under this assumption. Also, on close inspection you can see that the air going under the wing is actually slowed down from the "free-stream" velocity of the air.

The lift predicted by the "Equal Transit" theory is much less than the observed lift, because the velocity is too low. The actual velocity over the top of an airfoil is much faster than that predicted by the "Longer Path" theory and particles moving over the top arrive at the trailing edge before particles moving under the airfoil. The upper flow is faster and the pressure is lower. The difference in pressure across the airfoil produces the lift. Part of the theory is correct. In fact, this theory is very appealing because many parts of the theory are correct. The problem with the “Equal Transit” theory is that it attempts to explain things based on a non-physical assumption. What is that? It is that the air molecule-pairs must “re-unite” at the trailing edge of the wing.

“Skipping Stone” Theory:

This theory is based on the idea that lift is generated from the reaction of air molecules striking the bottom of the airfoil as it moves through the air. Because this is similar to the way in which a flat rock thrown at a shallow angle skips across a body of water, it is called the "Skipping Stone" theory of lift. This theory assumes that lift is produced by the interaction of the lower surface of the airfoil and the air; i.e., air molecules striking the bottom of the airfoil.

Unfortunately, this theory neglects the action / reaction of molecules striking the upper surface of the airfoil. I hope that everyone recognizes that there is no “vacuum” on top of the airfoil. Molecules are still in constant, random motion on the upper surface (as well as the lower surface), and these molecules strike the surface and impart momentum to the airfoil on the top as well as on the lower surface. With this theory, the upper airfoil surface isn’t considered at all. Unfortunately, this presents a really gaping problem when we recognize that many airliners use “plates” on the upper surface, between the leading and trailing edges; they’re called spoilers. They are used to change the lift of the wing to maneuver the aircraft by disrupting the flow over the upper surface. If the lift were being generated by the impinging air on the lower airfoil surface, how does the spoiler panels change any lift?

However, this theory is not totally inaccurate. In certain flight regimes, where the velocity of the aircraft is very high and the air density is very low, few molecules can strike the upper airfoil surface and the Newtonian theory provides very accurate predictions. These are the conditions encountered by the Space Shuttle during the early phases of its re-entry. But, here, we’re talking over 250,000 feet and speeds at and above 10,000 mph. For most normal flight conditions, like those on an airliner, this theory does not give the right answer.

“Venturi” Theory:

Not entirely different from the ETT theory, this one is based on the idea that the airfoil upper surface is shaped to act as a Venturi nozzle to accelerate the airflow. But an airfoil is not a Venturi nozzle. The first problem is that there is no “phantom surface” to produce the other half of the nozzle.

When air flows over the top surface of an airfoil, the velocity gradually decreases as you move away from the airfoil, eventually approaching the free stream velocity. However, this does not happen in a Venturi nozzle. The velocity of air flowing along the centerline of a Venturi nozzle (as far away from the edge wall of the Venturi as you can get) is typically higher than the velocity along the wall of the nozzle.

Then, there is that stubborn flat plat acting as an airfoil. The leading edge of a flat plate presents no constriction to the flow so there is no “nozzle” formed. This theory deals with only the pressure and velocity along the upper surface of the airfoil. It neglects the shape of the lower surface completely.

If this theory were correct, you could have any shape you want for the lower surface of the airfoil, and the lift would be the same. Obviously, this is not true. In fact, the “skipping stone” theory proposed that only the lower surface produces lift.

Here is a quote from Tom Benson from the Glenn Research Center, NASA, that may provide you with some comfort or it may cause you some additional anxiety about “lift.”

“It\'s easy to debunk the very obvious incorrect theories, but the real explanation of lift is quite complex. The lift on the body is simple...it\'s the re-action of the solid body to the turning of a moving fluid. The "turning" implies an acceleration (change in vector velocity) of the fluid so a force must be applied to the fluid (Newton\'s first law). In response, a force is applied to the body by the fluid (Newton\'s third law) and that force is resolved into a force in the direction of the initial fluid motion (drag) and a force perpendicular to the fluid motion (lift).

“The cause for the flow turning is the simultaneous conservation of mass, momentum (both linear and angular), and energy by the fluid. And it\'s confusing for a fluid because the mass can move and redistribute itself (unlike a solid), but can only do so in ways that conserve momentum (mass times velocity) and energy (mass times velocity squared). Velocity and momentum are vector quantities, so there are actually three spatial momentums that must be conserved; and they (and the mass) are all interdependent. A change in velocity in one direction can cause a change in velocity in a perpendicular direction in a fluid, which doesn\'t occur in solid mechanics.

“So exactly describing how the flow turns is a complex problem; too complex for most people to visualize. So we make up simplified "models". And when we simplify, we leave something out. So the model is flawed. Most of the arguments about lift generation come down to people finding the flaws in the various models, and so the arguments are usually very legitimate.

“The key to understanding creation of lift is that it is a mechanical force. To be a mechanical force, there must be interaction and contact of a solid body (airplane or wing) with a fluid (air). \'Contact\' is the keyword because that is were the air molecules crash [collide] into the wing/airplane, transferring their momentum to the surface. Similarly, the effects of lift are also present; like the pressure variation around the object, velocity variation around the object, downwash, and shed vorticity.”

AirRabbit

[chornedsnorkack]

Pitch on ground

So. When an airplane is stationary on ground and at a zero airspeed, the pitch of the airplane is fully determined by its landing gear - it is supported at three or more points, some ahead and some behind its centre of gravity. There are no aerodynamic forces in the play.

When an airplane is moving on the ground on takeoff or landing, and supported in 3 points, the pitch is still given by the landing gear. Therefore the angle of attack is determined by the angle of incidence, and is normally much smaller than the stalling AOA. So, the airspeed is the main variable determining the lift of the main wing. However, there are also the movable control surfaces, like flaps, spoilers and others, so the lift depends on their position. Except that if the airspeed is zero or negligible, all aerodynamic forces are small, wherever the controls may be, and therefore the aircraft must stay in the attitude given by the 3 points of support.

Now, near the end of the takeoff roll, the aircraft still has low AoA, the lift is appreciable but much smaller than the weight. Then the aircraft "rotates". What it means is that the nose of aircraft is lifted off the ground while the rear remains supported by the ground for a while; the increased pitch angle and therefore the angle of attack causes a large increase of lift, until the lift exceeds weight and the rear of the aircraft also becomes airborne.

Right?

Again, when an aircraft lands, as you describe the normal landing, so first the rear wheels touch ground lightly - but then the aircraft nose is brought down as fast as feasible, decreasing AOA, flaps are withdrawn, spoilers are extended - all of which rapidly decrease lift until much of the weight is supported by the runway, and then the brakes on the landing gear are put to the best use.

So, I understand that if the landing gear is functional, on a good runway surface, rapidly getting rid of lift and applying the brakes is the best way to get rid of kinetic energy.

Now, what are the alternatives?

If a plane delays touchdown and uses the high induced drag at high angles of attack to slow down before touching down, then as you mentioned, there are problems. One is that planes often do not have landing gear good for landing near the stall AOA - the tail behind the rear landing gear may hit the runway first. Another is that the braking forces in air, even near stall, seem to be smaller than on a good runway with good brakes, so delaying touchdown means spending more runway.

A plane in takeoff at large speeds can use control aerodynamic forces to lift the nose up. It follows that an airplane on landing, with rear wheels on the ground, might also try and use the aerodynamic forces to keep the nose up. What would the outcme be? The nose must eventually fall down before the plane stops, since all aerodynamic forces decrease. Before then, while a plane succeeded in keeping the nose up, the lift would limit the effectiveness of brakes.

Now, as stated, brakes are the most effective way of braking a plane as long as they are functional on a good runway surface.

Depending on the runway surface, this may be less important - e. g. a small plane with low landing speed and distance landing on a runway long enough for liners.

And then there are the crash landings and ditchings. By definition not on normal runways, brakes cannot be used anyway, fuselage damage occurs anyway.

If a plane is certainly going to have tailstrike, because e. g. landing on a runway, there is no rear landing gear (it is stuck and cannot be extended), is it better to land as in the normal procedure, at a relatively high speed and low AoA, or rather slow down and have the tail strike, and belly strike after that, at a minimum speed and maximally nose-up attitude? Same question if the plane is landing on a surface off-runway, or into water.

[barit1]

Really interesting subject - and some more examples are in order.

The B-52 was quoted as an airplane with a high angle of incidence, evidenced by its landing attitude. The BAC111 is another - it seems to almost touch down nosewheel first (a phenomenon known as "wheelbarrowing" - in which more than a few light aircraft have been damaged by the directional instability of nosewheel-only contact...)

At the other extreme - the Lockheed Constellation landed in a very nose-high attitude - come to think of it, so does the TriStar.

And I recall watching delta-wing aircraft (F-102, F-106, B-58) land nose-high then hold the nose up for a surprisingly long time, for aerodynamic braking. I guess this illustrates the chornedsnorkack
remark "But it still moves in the same direction, and with almost the same speed as before landing. It follows that it must generate almost as much lift as before - only slightly less than before landing." But it happens INTENTIONALLY because the pilot held the nose up. If the pilot dumped the nose, the lift would go to near zero very quickly.

The "aero braking" routine is discouraged by the airplane builders, because while it is kinder on the brakes, too many planes have overrun under this practice.

[AirRabbit]

Chornedsnorkack wrote:

So. When an airplane is stationary on ground and at a zero airspeed, the pitch of the airplane is fully determined by its landing gear - it is supported at three or more points, some ahead and some behind its centre of gravity. There are no aerodynamic forces in the play.

Yes. In fact, with some airplanes, the “3-point” attitude has the airplane slightly nose down.

Chornedsnorkack wrote:

When an airplane is moving on the ground on takeoff or landing, and supported in 3 points, the pitch is still given by the landing gear. Therefore the angle of attack is determined by the angle of incidence, and is normally much smaller than the stalling AOA. So, the airspeed is the main variable determining the lift of the main wing. However, there are also the movable control surfaces, like flaps, spoilers and others, so the lift depends on their position. Except that if the airspeed is zero or negligible, all aerodynamic forces are small, wherever the controls may be, and therefore the aircraft must stay in the attitude given by the 3 points of support.

Well, almost. The AoA is angle between the chord line of the airfoil and the relative wind. IF the chord line is parallel with the horizontal axis of the airplane and the airplane is horizontal when the nose gear is on the ground, THEN, yes, as the airplane begins its acceleration toward takeoff the angle between the relative wind (horizontal) and the chord line of the wing (at the angle of incidence) would be the AoA. The stalling AoA is on the other end of the pitch position. It occurs when the pitch is too much, the airflow separates, and the wing “stalls” (fails to produce lift).

Chornedsnorkack wrote:

Now, near the end of the takeoff roll, the aircraft still has low AoA, the lift is appreciable but much smaller than the weight. Then the aircraft "rotates". What it means is that the nose of aircraft is lifted off the ground while the rear remains supported by the ground for a while; the increased pitch angle and therefore the angle of attack causes a large increase of lift, until the lift exceeds weight and the rear of the aircraft also becomes airborne. Right?

Yes. Typically, as the airplane continues to accelerate and begins rotating, the lift will increase as a function of both increasing airspeed and increasing AoA.

Chornedsnorkack wrote:

Again, when an aircraft lands, as you describe the normal landing, so first the rear wheels touch ground lightly - but then the aircraft nose is brought down as fast as feasible, decreasing AOA, flaps are withdrawn, spoilers are extended - all of which rapidly decrease lift until much of the weight is supported by the runway, and then the brakes on the landing gear are put to the best use.

Yes, however, when the airplane touches down, the RoD is changed (goes to zero) immediately, so the AoA immediately changes as well. In addition, with the main wheels on the ground there is an immediate increase in drag, slowing the airplane. Of course, if autobrakes are selected or if the pilot “gets on the brakes” immediately, that increase in drag goes up rather substantially. Both of these actions reduce the effective lift. On some airplanes, in some conditions, flaps are raised to reduce lift and put more weight on the wheels. However, on transport category airplanes, while still contributing to the lift, the last couple of flap settings result primarily in more drag. That is why when the pilot selects the last increment of flaps during an approach to landing you will normally hear the engines spool up (to overcome the additional drag). Therefore, in the cases of transport category airplanes, retracting the flaps is not necessary. You would be decreasing the lift – but you would also be decreasing the drag even more – until the flaps were into and beyond the typical takeoff range. Of course, if the spoilers extend, the amount of lift is dramatically reduced, to almost zero (I say “almost” because the spoiler panels do not cover the entire length of the wing) and the drag from the landing gear (and brakes) is increased.

Chornedsnorkack wrote:

So, I understand that if the landing gear is functional, on a good runway surface, rapidly getting rid of lift and applying the brakes is the best way to get rid of kinetic energy.

Yes.

Chornedsnorkack wrote:

Now, what are the alternatives?
If a plane delays touchdown and uses the high induced drag at high angles of attack to slow down before touching down, then as you mentioned, there are problems. One is that planes often do not have landing gear good for landing near the stall AOA - the tail behind the rear landing gear may hit the runway first. Another is that the braking forces in air, even near stall, seem to be smaller than on a good runway with good brakes, so delaying touchdown means spending more runway.

Well, almost, again. I was not necessarily describing the “quality” of the landing gear. What I was saying was that if the airplane was stalled while still some distance from the ground and the airplane were to “fall” that distance, some structural damage is likely. This would probably affect the landing gear system, but it is likely to affect other components of the airframe as well. And yes, if the approach is too steep and the pilot attempts to arrest that rate of descent with a very rapid increase in pitch attitude (usually well above the “level flight attitude” that I’m always harping on) there is a greater chance that a tail strike will occur. Tail strikes can be very serious, can cause lots of structural damage, and are required to be reported so that an inspection may be made. My comment about the wheel brakes being useless while airborne was strictly “tongue-in-cheek.”

Chornedsnorkack wrote:

A plane in takeoff at large speeds can use control aerodynamic forces to lift the nose up. It follows that an airplane on landing, with rear wheels on the ground, might also try and use the aerodynamic forces to keep the nose up. What would the outcme be? The nose must eventually fall down before the plane stops, since all aerodynamic forces decrease. Before then, while a plane succeeded in keeping the nose up, the lift would limit the effectiveness of brakes.

There are some circumstances where using “aerodynamic braking” is an appropriate thing to do. As barit1 has said, military aircraft are often seen performing this maneuver. However, it is usually because fighter aircraft have smaller wheels and tires and therefore do not have much brake surface. Using aero-braking helps reduce wear and tear on these smaller wheel/brake assemblies. This procedure is not warranted for transport category airplanes.

Your statement/assumption that “...while a plane succeeded in keeping the nose up, the lift would limit the effectiveness of the brakes...” is not completely correct. Any limiting of the effectiveness of the brakes would be due to whatever minimum lift is still being generated by the wings. The only way the nose may be held up is with sufficient airflow over the deflected elevator, pushing/pulling the tail down. The weight of the front portion of the airplane is being held “up” (less whatever weight any minimum lift still existing at the wings would offset) by the “down” force being generated by the elevator. You have to remember that the position around which this rotation occurs is the landing gear (fulcrum and lever – physics).

I do not mean to “talk down” to anyone, but in case it is necessary ... place a coffee cup on the end of a 12-inch rule. Place your left index finger on a table top under the rule at the 6-inch position. Press down on the opposite end of the rule from the coffee cup with your right index finger. Bring the coffee cup to a height that will place the rule in a horizontal position. (yeah, I know, it’ll probably fall off, but use your imagination) What is holding the rule (and the coffee cup at one end) off the table surface? Is it your right index finger pushing down? No. Is it the coffee cup being pulled down by gravity? No. It is the fulcrum, your left index finger, doing the holding, bearing the weight. Now, substitute the coffee cup with the front of the airplane fuselage. Substitute your left index finger with the MLG. Finally, substitute your right index finger with the aerodynamic force pushing/pulling the tail down. The weight on the wheels (the fulcrum) will be the weight of the airplane (again, less whatever minimum lift may still exist at the wings).

If the fulcrum (the MLG) is exactly at the mid-point (between the weight of the airplane in front of the MLG and the weight of the airplane behind the MLG), the airplane would be “balanced” on the MLG. The MLG would be supporting the entire weight of the airplane (again, except for whatever amount may be offset by the lift still being generated at the wings – and as I pointed out in my earlier post, this is probably not very much and it will be getting less and less). However, the MLG is normally mounted aft of the center of mass on a “tri-cycle gear” airplane. In order to raise the nose a force has to be generated at the opposite end of the lever; at the tail. The farther aft of the center of mass the MLG is placed, the greater the down force that will have to be generated on the tail to raise the nose. The actual amount of force that must be generated is directly proportional to the amount of weight in front of the fulcrum. If the MLG were placed immediately in front of the tail surfaces, the force that would have to be generated to lift all of the mass of the airplane in front of the MLG, which would approach the entire weight of the airplane, would approach the entire weight of the airplane in an equal manner. The force (weight) felt at the fulcrum would be almost double the weight of the airplane. Recall that the fulcrum is the MLG – so the “weight” being supported by the wheels (actually the runway is doing the supporting) would be at least equal to the weight of the airplane – and it may be more depending how far aft the MLG are located. Again, less any amount of residual lift being generated at the wings.

As we have discussed, once on the ground the RoD is less (zero) so the AoA is less. The speed will be lessening as friction is increased, slowing the airplane. Both combine to reduce the lift being generated. If we have spoiler panels extended, all, or certainly most, of the lift being generated by the wings will be “spoiled.” If we do not have spoiler panels extended, the airplane will continue to slow, and it will slow more quickly if either wheel brakes or reverse thrust is used. If the pilot holds the nose off the runway, the entire weight will be transferred to the runway more slowly than if the spoilers were used, but the difference is not very long – small, single digit seconds; one, two, maybe three – not more. The landings made without spoilers and that also manage to slow that transition of weight to 3 seconds are very few and very far between.

(End part 1 of 2)

[AirRabbit]

(part 2 of 2)

If you watched any of the footage of the recent JetBlue A320 landing at LAX with the nose gear problem, you saw perhaps the best example of touching down an airplane as “softly” as possible, transitioning the weight to the runway over as long a period as possible, and keeping the nose gear off the runway as long as possible. They landed on a runway that is just a bit over 12,000 feet long. The captain elected not to deploy the spoilers (apparently didn’t want to take the chance of “driving” the nose gear into the runway) and did not use reverse thrust. I don’t know about the use of wheel brakes. If you noticed, the airplane came to a stop just about 1,000 feet from the departure end of the runway. Just about 11,000 of the 12,000 feet of the runway were used to get the airplane landed and stopped. I’ll let someone else figure out the coefficient of friction (from the nose gear tires, followed quickly by the nose gear wheels, and ultimately by grinding down to the nose gear strut) generated to help stop the airplane (just kidding).

Chornedsnorkack wrote:

Now, as stated, brakes are the most effective way of braking a plane as long as they are functional on a good runway surface. Depending on the runway surface, this may be less important - e. g. a small plane with low landing speed and distance landing on a runway long enough for liners.

Well, most of the time anyway. Most of the time airport builders have this incessant quality of always wanting the runways to have a beginning and an end – and they want both inside the boundaries of the airport. Of course, the Navy has found another solution. They string these very strong cables across the approach end of a runway, the ends of which are usually connected to very high resistance interia reels. The aircraft lands, catches the wire, and is “trapped” by that wire. All of the kinetic energy is disapated through these reels and the airplane is stopped in a VERY SHORT distance. (some have to glue their eyeballs back in place)

Chornedsnorkack wrote:

And then there are the crash landings and ditchings. By definition not on normal runways, brakes cannot be used anyway, fuselage damage occurs anyway. If a plane is certainly going to have tailstrike, because e. g. landing on a runway, there is no rear landing gear (it is stuck and cannot be extended), is it better to land as in the normal procedure, at a relatively high speed and low AoA, or rather slow down and have the tail strike, and belly strike after that, at a minimum speed and maximally nose-up attitude? Same question if the plane is landing on a surface off-runway, or into water.

Well, crashes occur wherever they occur; and a lot of them occur on airport runways. Mostly because the only time you are intentionally flying close to the ground is just after takeoff or before landing. In fact, I believe most pilots would prefer to “crash land” on a very long runway where s/he knows there is CRF services available (Crash/Fire/Rescue). Take the JetBlue flight as an example again. The Captain would have preferred to land at Long Beach when he was told that the warning lights were only an “indicator problem.” But when they confirmed the angle of the nose wheel, he elected to land on the longest runway at LAX. Why didn’t they go over to Edwards AFB and land on the dry lakebed? Well, they were told that the problem was an indicator problem and that the landing would be uneventful. So, they flew around to burn off fuel – for something like 3 hours. When they found out that it wasn’t just an indicator problem, they may not have had enough fuel to get anyplace other than LAX. I don’t know what the Captain was considering, but it seems to me that he made the right decision.

Any time you have to make an emergency “crash landing” on something other than water – it is always preferable to extend as many of the landing gear as you are able, as it will provide something else to help absorb the energy. This is true if landing at LAX on a 12,000 foot runway, or a deserted field in Pennsylvania. Unless you are cart-wheeling (like United 232 at Sioux City) any pilot is probably going to try to use whatever braking s/he may have available. However, you do not want to land on water with any landing gear extended. The gear winds up acting like that Navy airplane with the tail hook, except that the Navy arrested landings are done on solid surfaces (runways or aircraft carriers). The gear contact with the water imparts a rotational moment around the gear, rotationally accelerating the nose down, into the water, with a lot of additional energy. Water landings are always made with the gear retracted when given the option.

If you want to consider landing on a runway when the MLG is stuck in the UP position ... OK. You would probably do just exactly what the JetBlue captain did. Either dump or burn fuel down to a lesser amount. How much? Usually, whatever you think is best – and you probably would want to consider what anyone else might have to say on the subject. Remember, you’re going to be “scraping” along the runway and the wing (with fuel tank inside) is likely to get scraped as well. The approach and landing would be made normally – with the flaps set to the appropriate landing flap setting. Some airplanes have a flap/gear warning horn that won’t allow you to set landing flaps unless the gear is down and locked. In such a case you would have to pull the circuit breaker for the horn or just ignore it. The flare would be conducted normally. The airplane should be flown on-speed and be “landed” (although on its belly) from a level flight attitude, just like you would do it if the wheels were down. If you were able to lower the nose gear –and some pilots would not care to do that – but if you did, when you touch down on the belly (approximately where the lower portion of the fuselage angles up toward the tail) you may touch down the nose gear at the same time – or very shortly afterward – and that’s not all bad either.

If you were “landing” on a vacant field (like the state of Kansas or something) you would do exactly the same thing, except you wouldn’t necessarily be worried about running out of Kansas. However, the fact is, you wouldn’t know what you would be running into either. Here, the mechanics would be essentially the same as landing on a runway. My counsel would be to go for the airport runway. There are a lot more folks there to help than in the middle of Kansas (please note, I’m not picking on Kansas – its a very nice state! Its open fields are not as populated as most major city airports.)

However, some on this thread apparently would advocate keeping the airplane in the air until it slowed down quite a bit – in any of the above situations. Unfortunately, that process chews up runway (or Kansas) much more quickly than you might imagine (remember the runway or Kansas behind you does you no good). Touching down and sliding off the end of the departure end of the runway into whatever lay beyond the airport boundary is not necessarily a good thing – and usually is a bad thing. There is another problem with trying to slow down while still airborne. Depending on the pitch attitude of the airplane at initial touchdown of the fuselage, you may impart a rotational moment centered at the touchdown point (just like I described for the water landing, although not quite as substantial), and initiate a similar rotational acceleration of the nose toward the ground and impact with higher energy.

barit1 wrote:

Really interesting subject - and some more examples are in order.
The B-52 was quoted as an airplane with a high angle of incidence, evidenced by its landing attitude. The BAC111 is another - it seems to almost touch down nosewheel first (a phenomenon known as "wheelbarrowing" - in which more than a few light aircraft have been damaged by the directional instability of nosewheel-only contact...)

I don’t know what the angle of incidence is on the BAC111, and not having flown the BAC, I certainly won’t speak from experience; but if memory serves, the BAC didn’t have leading edge flaps/slats. If correct, this probably accounts for the more “nose down” attitude on approach. The airplane should still have been landed from a level flight attitude and at an airspeed slightly below Vref with the throttles in idle (unless there is some peculiarity with that airplane about which I am not aware).

barit1 wrote:

At the other extreme - the Lockheed Constellation landed in a very nose-high attitude - come to think of it, so does the TriStar.

Yes. And the L-1011 had DLC, direct lift control, and was a nicely flying machine. I’m sorry to see as many of them parked in the desert as there are.

barit1 wrote:

And I recall watching delta-wing aircraft (F-102, F-106, B-58) land nose-high then hold the nose up for a surprisingly long time, for aerodynamic braking. I guess this illustrates the chornedsnorkack remark "But it still moves in the same direction, and with almost the same speed as before landing. It follows that it must generate almost as much lift as before - only slightly less than before landing." But it happens INTENTIONALLY because the pilot held the nose up. If the pilot dumped the nose, the lift would go to near zero very quickly.

Actually, the point I was making is that if the airplane were traveling sufficiently fast and the pitch attitude during the “aero-braking” maneuver was not in excess of the stalling AoA, the airplane would become airborne again. Not a good idea. Aero-braking on delta-winged aircraft is probably more conservative (i.e., not increasing the pitch attitude after touchdown – merely holding the nose-high landing attitude longer) than on a fighter with a separate tail surface. But even here, a fighter pilot does not smoothly increase the pitch as a continuing movement from the round-out or flare up to the aero-braking attitude. There is a pause and the airplane is pitched up carefully. And, as I’ve said before, this is done in limited conditions – for example, I don’t think you’re going to see a lot of aero-braking on a day with a good crosswind.

barit1 wrote:

The "aero braking" routine is discouraged by the airplane builders, because while it is kinder on the brakes, too many planes have overrun under this practice.

Yes. There is no advantage to it. As you point out, some have run off the departure end of the runway. And, you would be putting yourself, your airplane, and your passengers in a state in which you are deliberately handing control of the airplane off to Mother Nature. As you loose directional control with the rudder and your nose wheel is still in the air, its kind of hard to steer.

___________
AirRabbit

[wsherif1]

Crash landings and Ditchings

AirRabbit

Your comment,
____________________________________________________ "The notion of holding an aircraft “…continually in a level flight attitude, while descending in a tangent angle, to the ground, flight path…” in addition to being difficult to understand grammatically, is beyond the understanding of anyone that I know in this business either mathematically or aerodynamically."
____________________________________________________To maintain a level flight attitude, while descending in a flight path tangent to the ground, hold forward pitch control pressure out of the flare maneuver, to counter the increasing lift as the aircraft descends in ground effect.

This technique is based on the pilot actually flying the aircraft down the approach path, as opposed to mechcanically controlling a rate of sink. ("Constant velocity, e.g. a constant rate of sink, has little ability to stimulate"!, A.J.Hudspeth, PhD, MD, of the Rockefeller Institute, an Audiologist).

Having expended the excess kinetic energy in the flare maneuvrer, the aircraft contacts the surface in a level flight attitude at the moment the aircraft's weight exceeds the available lift.

[AirRabbit]

Howdy again, Mr. Wsherif1:

Well, I think I’ve said all that I can say to you about this “zero aircraft weight transition” thing you keep talking about. Perhaps you are correct. Perhaps one day the whole world will change the way they describe ending a flight. They might no longer call it “landing;” they’ll call it “sherifing,” or something like that. Perhaps not.

Your description of “rolling the wheels onto the surface” is something I try to do each time out. I would love the touchdown to be “imperceptible,” but I’m not going to sacrifice controllability for “imperceptibility,” particularly in an airplane.

And as for A.J. Hudspeth, PhD, MD, Rockefeller Institute, an inner ear scientist … Ask him to climb up to the top floor of the Rockefeller Institute, walk to edge and step off into space. He will undoubtedly reach a “constant velocity, e.g., a constant rate of sink.” Ask him then if such parameters fail to stimulate!

Happy landings … or … whatever it is you do.
_______
AirRabbit

[chornedsnorkack]

Controllability

Is it then the case that a plane that adopts a high but less than stalling AoA so that the rudder gets in the disturbed airflow loses all yaw control, whether near runway or in free air?

After touching down with both main landing gear, but while keeping nose up, an aircraft might not have steering by nose wheel. But once the main landing gear has touched down on both sides, the roll degree of freedom is gone. So, cannot the yaw be controlled by things like braking on one side or changing the flap settings on one side (the flaps are in strong airflow, so they have a great effect on drag!)?

[Capt Pit Bull]

To maintain a level flight attitude, while descending in a flight path tangent to the ground, hold forward pitch control pressure out of the flare maneuver, to counter the increasing lift as the aircraft descends in ground effect.

PLEASE SOMEBODY MAKE THE SUFFERING STOP.

cpb

[Rhodie]

AirRabbit - I only fly little aircraft, with and without engines and with one, two or three wheels, centre. nose and tail mounted as applicable - but I have followed this with interest..!

I say in all honesty and sincerity that I have learnt a great deal from your facinating discourse and thank you for that.

Cheers

R