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F-16GUY
12th Aug 2019, 11:35
Time and again after incidents like the S7 at Domodedovo, I wonder if during T/O liners donít have any sort of acceleration check or similar procedure, to confirm that the engines are performing as expected during the first stage of the T/O run.

In my ďoutfitĒ (I know, I know, totally different kind), we do a TOLD calculation prior every flight. This calculation will amongst other things show the expected rotation and T/O speed, T/O distance and acceleration check speed.

The acceleration check speed will normally be calculated for a distance of 1000í from brakes release. The 1000í normally equals the distance to the first distance to go marker (normally only present on military fields), but any other visible or displayable feature will work (intersection, distance from mark point generated at brakes release position). The TOLD calculation takes the following data into account: Power setting (MIL or A/B T/O), weight, temperature, pressure, wind, runway conditions (friction, slope, length).

If the expected speed is not reached (+/- 5 KTS in MIL and +/- 10 KTS in A/B) when passing the ref. point, or if rotation or T/O speed is not reached at the expected point, T/O is to be aborted (A/B, provided there is enough RWY left) or full A/B is to be selected (MIL).

This method gives an early indication if all is well, or if the pilots should increase thrust/abort. In cases like the S7, where the wrong weight is put into the system, it will be perfectly clear by the lack of proper acceleration. There most be something like this in the civil world, or?

Monarch Man
12th Aug 2019, 12:13
Itís effectively the same thing in a transport Aircraft, but itís reliant in all cases on the validity of the data input.
I use a good rule of thumb that works most times as a gross error check. Ultimately in Mil or Civ service we operate for the most part by numbers, but you canít replace experience and the seat of the pants either.
BTW, my old bang seat Jet was underpowered with both engines in reheat!

Raffles S.A.
12th Aug 2019, 12:18
The trend vector on the PFD should give a clue as well. They also didn't use full length apparently,

RetiredBA/BY
12th Aug 2019, 13:21
Itís effectively the same thing in a transport Aircraft, but itís reliant in all cases on the validity of the data input.
I use a good rule of thumb that works most times as a gross error check. Ultimately in Mil or Civ service we operate for the most part by numbers, but you canít replace experience and the seat of the pants either.
BTW, my old bang seat Jet was underpowered with both engines in reheat!
You must have been flying the Jaguar !

In the V force we did an acceleration time check to 100 knots.

Much later I suggested a system which I named TOPIS, Take Off Performance Indicating System and the concept was published in the Internationl Journal Of Air Safety many years ago. Basically the display, driven by the FMC was to drive a bug on the ASI giving the required speed. Any significant shortfall required a stop.

I thought Emirates was pursuing such a system, which modern electronics should make easy, after their take off incident at Melbourne, due I believe, to the wrong weights being entered in the FMC

Monarch Man
12th Aug 2019, 13:54
You must have been flying the Jaguar !

In the V force we did an acceleration time check to 100 knots.

Much later I suggested a system which I named TOPIS, Take Off Performance Indicating System and the concept was published in the Internationl Journal Of Air Safety many years ago. Basically the display, driven by the FMC was to drive a bug on the ASI giving the required speed. Any significant shortfall required a stop.

I thought Emirates was pursuing such a system, which modern electronics should make easy, after their take off incident at Melbourne, due I believe, to the wrong weights being entered in the FMC

The fog of time has dulled my memory but I recall we did something similar on the Jag along with a distance remaining or used check.
Far more critical however was having a place to aim the jet at should one of the afforementioned fuel to noise convertors decided to go into quiet mode.
On the B777 at EK in my time there we used a Boeing provided onboard performance tool or OPT which worked very very well, but as ever it required careful data input and cross-checking.

F-16GUY
12th Aug 2019, 14:40
Itís effectively the same thing in a transport Aircraft, but itís reliant in all cases on the validity of the data input.

Wrong data input, weight for instance, will show in a different speed when passing the ref. point. Thats way I asked, as the procedure we use will show either poor engine performance or wrong data input in time to abort the T/O. Only thing I can see is if wrong data input is used on more than one parameter (weight and temperature for instance). Then there is a chance they will cancel each other out with regard to acceleration check speed, but then the T/O speed will be off.

FlightDetent
12th Aug 2019, 18:38
IIRC the result of previous mind-melts was that due to wide spread of weights and thrust settings (down to 60% rated) a system or method that is simple enough for pilots to understand and dependably reliable is impossible to devise.

The idea is sound, just that the solution was not found.

gearlever
12th Aug 2019, 19:39
Wrong data input, weight for instance, will show in a different speed when passing the ref. point. Thats way I asked, as the procedure we use will show either poor engine performance or wrong data input in time to abort the T/O. Only thing I can see is if wrong data input is used on more than one parameter (weight and temperature for instance). Then there is a chance they will cancel each other out with regard to acceleration check speed, but then the T/O speed will be off.

Wrong flap setting?

F-16GUY
12th Aug 2019, 20:03
Wrong flap setting?

Not possible in the F-16. Flaps up/down follows the gear handle. Taking off with the gear and flaps up will probably require one more engine :-)

Doesn't liners have warning systems for wrong T/O configuration?

gearlever
12th Aug 2019, 20:16
Not possible in the F-16. Flaps up/down follows the gear handle. Taking off with the gear and flaps up will probably require one more engine :-)

Doesn't liners have warning systems for wrong T/O configuration?

Well it depends...

- no flaps, yes A&B have warnings
- too less flaps, don't know the current state of airbus T/O checklists on ECAM but I was flying more than 10 years the bus with T/O ECAM happy with ANY flap setting (1,2 and 3)....

Most of the time T/Os are performed wit F1, but if your calcs were for F2 or even F3 there was NO warning if you put the flap lever to F1 for T/O.....

Another thing is Tflex. Neither on A320 nor on A340 flex temp was asked for in a checklist on my operator. Well, on A300 it was....

F-16GUY
12th Aug 2019, 21:07
Not familiar with Tflex. Can you give me a quick introduction to this concept? Is the power setting still fixed during T/O (at a lower setting) or will it be changed somewhere along the T/O roll?

Loose rivets
13th Aug 2019, 01:49
IIRC it was in the mid 70's that I learned that my friend, Col, Carl Crane in Texas had patented a system called Tell Me. It annunciated the distance run during landing or takeoff. I immediately realised that the receiver he was hoping to market was already installed on most aircraft in the form of the Marker and that fan shaped transmissions could be installed on the edge of the runway. Remember, DME was fairly new, and distance on the runway was not generally available. There had been a few accidents in that era associated with landing well off the numbers.

Working out the acceleration was my next leap. It made sense . . . back then. I smelled an investment.

I came back to the UK with an agreement that if he had North America, I could have the rest of the world. I was already in touch with our government's 'development scheme' (or some such) who were soon in possession of my drawings. They paid half of the research and development to successful applicants.

It was only a short time later that I got a letter saying they were already funding a similar system, and mine would be in conflict. I called the folk doing the testing and a nice guy told me their hopes and woes. It seems a BAC 1-11 was charging up and down a runway with doppler looking at the runway surface. A secondary dial on the ASI showed the accelerative progress. It worked . . . sometimes, the doppler unlocking with tedious regularity. Given how quickly DME became available on the runway, I guess it's a good thing neither of us poured too much into our respective dreams.

It had been a subject close to my heart. Despite having passed my performance A in 25 minutes, I was very aware that so much of it was utter bo-lox. Quite meaningless in many situations. For me, the most memorable was Split, with my boss doing the takeoff with a well loaded BAC 1-11. I still remember the thundering of the undercarriage on the uneven concrete slabs. It was soaking up energy. The simpleton on my left said nothing, then or later, despite me having called V1 as we passed over the rocky coastline.

I put a lot of effort into getting the subject discussed and talked it over with a couple of big name electronics manufacturers. There was a lot of interest, but even more inertia. No, the pun has not gone unnoticed.

I expect my phone could do a fair job of it these days.

Check Airman
13th Aug 2019, 01:55
Well it depends...

- no flaps, yes A&B have warnings
- too less flaps, don't know the current state of airbus T/O checklists on ECAM but I was flying more than 10 years the bus with T/O ECAM happy with ANY flap setting (1,2 and 3)....

Most of the time T/Os are performed wit F1, but if your calcs were for F2 or even F3 there was NO warning if you put the flap lever to F1 for T/O.....

Another thing is Tflex. Neither on A320 nor on A340 flex temp was asked for in a checklist on my operator. Well, on A300 it was....

I'm guilty of doing that. Fifi caught me. Thankfully

vilas
13th Aug 2019, 05:46
Most of the time T/Os are performed wit F1, but if your calcs were for F2 or even F3 there was NO warning if you put the flap lever to F1 for T/O.....
Another thing is Tflex. Neither on A320 nor on A340 flex temp was asked for in a checklist on my operator. Well, on A300 it was.... Airbus is generally quick to find fixes and both these issues have fixed. Flex temperature is very much a part of the C/L for a long time now. As far as config warning is concerned in conventional A320 it was just an assurance that you are not outside takeoff configuration. Now Airline can opt for new app that will compare the actual configuration with the MCDU entry. Acceleration check in my opinion is not possible when thrust settings vary by as much as 40% nor is it required when everything else is in place. Trend arrow is a good indicator it can even indicate a wind shear.

gearlever
13th Aug 2019, 10:55
Not familiar with Tflex. Can you give me a quick introduction to this concept? Is the power setting still fixed during T/O (at a lower setting) or will it be changed somewhere along the T/O roll?

Tflex or assumed temperature method is a smart and safe way to reduce engine wear, increase lifetime.
Thrust will be constant besides physical effects of increasing airspeed on jet engines.

Flex temp (https://en.wikipedia.org/wiki/Flex_temp)

FlightDetent
13th Aug 2019, 14:29
F-16 re FLEX Temp / AST method. A fictitious temperature is fed to ECU so that a reduced thurst is provided by the engine. This reduced thrust level is pre-calculated to provide exactly the amount of remaining thrust needed for the EFATO - Go case.

For the purpose of this thread, the HOW (Temp) is not relevant, also the De-Rate feature could be used (same goal). The WHAT is - for every takeoff a different thrust is used and thus a reference acceleration pattern is hard to define.

hans brinker
13th Aug 2019, 16:23
F-16 re FLEX Temp / AST method. A fictitious temperature is fed to ECU so that a reduced thurst is provided by the engine. This reduced thrust level is pre-calculated to provide exactly the amount of remaining thrust needed for the EFATO - Go case.

For the purpose of this thread, the HOW (Temp) is not relevant, also the De-Rate feature could be used (same goal). The WHAT is - for every takeoff a different thrust is used and thus a reference acceleration pattern is hard to define.

Airbus is generally quick to find fixes and both these issues have fixed. Flex temperature is very much a part of the C/L for a long time now. As far as config warning is concerned in conventional A320 it was just an assurance that you are not outside takeoff configuration. Now Airline can opt for new app that will compare the actual configuration with the MCDU entry. Acceleration check in my opinion is not possible when thrust settings vary by as much as 40% nor is it required when everything else is in place. Trend arrow is a good indicator it can even indicate a wind shear.

I am going to disagree a little with some of that. Using FLEX/De-Rate will give you closer acceleration for different weights, (for the same aircraft on the same runway length), than using TOGA for every take-off regardless of actual weight. (very light A320 with a very high FLEX temp accelerates similar to heavy A320 with low FLEX, very light A320 with TOGA accelerates much faster than very heavy A320 with TOGA.)
Not suggesting an acceleration check would work.

wiedehopf
13th Aug 2019, 17:05
With knowledge of V2 speed and runway length combined with aircraft position, you could do calculate somewhat of a minimum of acceleration required.
If that goes below the required acceleration between 40 and 80 knots (groundspeed acceleration, this system isn't for wind shear), give a warning which should produce an abort.

Even with a wrong gross weight and somewhat lower V2 input, the system would likely still catch cases where planes rotate with all engines operating scrape the approach lights.

You could get even more tricky and infer gross weight from actual acceleration given a certain EPR.
That gross weight gives you an V2 to use for the computations.

lederhosen
13th Aug 2019, 17:26
Back in my early 737 days we used assumed temperature with paper charts. Later on we got computers which then allowed us to calculate assumed plus derate and thus on short flights allowing even greater reduction in thrust. I remember the first time I used this on a 4000 meter runway being asked by the tower if we had had an engine failure, the difference was that great! The calculation was correct and the tower got used to our high idle takeoffs. Personally I was very sceptical about what the local population thought of this improvement with us thundering over their heads much lower and our noise abatement then adding thrust. An earlier chief pilot in my opinion quite rightly disagreed with this but later on we would see increased thrust at a lot of places at 1000 feet.

BizJetJock
13th Aug 2019, 20:03
Falcons have had takeoff acceleration on the pfd for decades. It is part of the SOP for the PM to call it out and verify it matches the predicted within a small margin. Maybe a current Falcon pilot can tell us what is considered acceptable deviation. If you've got your sums wrong then it won't agree. It doesn't protect against incorrect flap setting though.
But the technology not only exists, it is in use every day.

FlightDetent
14th Aug 2019, 06:09
I am going to disagree a little with some of that.

If the thrust is set for a desired (2.4 OEI) climb performance, depending on the mass of the aircraft, there would be a certain Thrust/Mass ratio. Not constant but as a function of flaps and the density altitude it should be pretty coherent. Per Sir Isaac's F=-a*m, by its mathematical definition, thrust to mass ratio IS the acceleration.

So where's the problem?

Obstacles requiring a higher than 2.4 ratio? I do not think so, with two running you'd clear them anyway.

But about the gradient: it is actually achieved by Lift /Mass, contrary to the suggestion I made above. The lift does evolve from the (square of) speed, but the required speed needs only to be achieved within the constraints of TODA/ASDA, so the acceleration required varies greatly. (lederhosen's post #12).

I still think the idea is good. Understanding the reasons why all the clever and experienced minds of yesteryear had not brought forward a solution already is part of the approach on how to crack the nut.

As a recap of the previous debates, the wide range of valid accelerations is part of the problem

vilas
14th Aug 2019, 09:00
I think there is enough to monitor during take-off and no need to add anymore Ďmade-upí acceleration check. Anyway, what happens when rvr 125m?

If both pilots check the the performance data and the numbers are cross checked when inserted into whatever electronic box the aircraft has, then good to go. Gross error check is also useful too.
Agreed. This fancy vague non manufacturer recommended acceleration check will unnecessarily complicate takeoff and lead to reject. How many rejects per 10000 takeoff are caused by inadequate acceleration? If it ain't broke don't break it trying to fix it.

FlightDetent
14th Aug 2019, 09:52
S7 at Domodedovo
MK Airlines at Halifax
EK at Melbourne
QR on the West Coast
BA out of the Carribean

and many many more. It may still be a very rare occurrence given the sheer departure numbers worldwide, but the severity is absolute. In a sense we are lucky the bright people had not been asked to provide a tool as of yet, since the outcome of the above was merciful on the travelling public.

Not that sure "ain't broke" is a valid evaluation. EGPWS and PWS, as well as the much-beloved FBW with protections, were all created above the contemporary burning needs.

FullWings
14th Aug 2019, 13:14
Iíve thought for quite a while that a TOPM wouldnít be such a hard device to implement. You know where the aeroplane is, how much runway is available, what the flap setting is, aircraft mass, temperature and wind component. It could produce a caution/warning after thrust has stabilised only if there was an apparent gross error, i.e. something that was way outside normal operation. Yes, there would have to be some intelligent filtering/processing of the data but it would also function to alert that some other parameters were incorrect.

lederhosen
14th Aug 2019, 14:01
I agree with FullWings that an electronic solution is certainly possible. But given airlines unwillingness to pay for stuff that might have prevented accidents (as suggested by Boeing now including things that were options previously in the 737 Max) it would probably only work if it was mandatory. Light aircraft rules of thumb (speed at halfway point for example) are great with full thrust takeoffs and familiar airfields. On the Airbus/Boeing aircraft I flew I don't think you can eyeball it and the incidents I heard about were mainly flap setting issues e.g. people setting flaps 1 with flaps 5 speeds. With experience you should spot on medium jets if something is strange about the V speeds, but much harder to spot acceleration problems on unfamiliar runways with varying met conditions.

Vessbot
14th Aug 2019, 14:28
But about the gradient: it is actually achieved by Lift /Mass, contrary to the suggestion I made above.
Climb gradient is a function of excess thrust, nothing to do with lift. (Some are surprised to learn that the steeper you climb, the less lift is actually required or produced by the wing.)

hans brinker
14th Aug 2019, 16:16
If the thrust is set for a desired (2.4 OEI) climb performance, depending on the mass of the aircraft, there would be a certain Thrust/Mass ratio. Not constant but as a function of flaps and the density altitude it should be pretty coherent. Per Sir Isaac's F=-a*m, by its mathematical definition, thrust to mass ratio IS the acceleration.

So where's the problem?

Obstacles requiring a higher than 2.4 ratio? I do not think so, with two running you'd clear them anyway.

But about the gradient: it is actually achieved by Lift /Mass, contrary to the suggestion I made above. The lift does evolve from the (square of) speed, but the required speed needs only to be achieved within the constraints of TODA/ASDA, so the acceleration required varies greatly. (lederhosen's post #12).

I still think the idea is good. Understanding the reasons why all the clever and experienced minds of yesteryear had not brought forward a solution already is part of the approach on how to crack the nut.

As a recap of the previous debates, the wide range of valid accelerations is part of the problem

I am nowhere near as knowledgeable as you on this subject, the only thing I tried to point out is that using FLEX instead of TOGA for will probably result in acceleration numbers that have a lower spread instead of a higher spread.

FlightDetent
14th Aug 2019, 16:17
@vessbot
Perhaps I ventured too far. Hope the other message remains standing: for a given V2, there would be different acceleration schedules depending on runway length. Longer tarmac, less thrust, valid slower acceleration, satisfactory V2+10 at screen height by the end of CWY anyway.

Would you check your statement applies well in the second segment case, for climb angle of 1.4 deg with flaps out and perhaps 11 deg pitch?
The AFM data confirms that for the same thrust the climb gradient is better if IAS is increased, the so-called "improved climb V2 schedule". Without thinking too much about it, I was building on the classical vector cross with L in the upward sense opposing the W, and L = c(l) * area * 1/2 * density * v^2. It has got to work somehow, nay?

OTOH that thing I uttered above about obstacles is rubbish. Higher required gradient, more thrust needed and usually lower speeds. Which underlines your point.

Where have all my marbles are gone? :\ (M. Dittrich tune)

(...) I tried to point out is that using FLEX instead of TOGA for will probably result in acceleration numbers that have a lower spread instead of a higher spread. That, for sure, is a physical necessity. I did not put the effort in to read your message properly before posting, apologies.

F-16GUY
14th Aug 2019, 17:12
gearlever and FlightDetent, thanks for the explanation. So basically flex is another word for reduced power T/O.

Disregarding climb performance requirements or wrong configuration situations, as the power setting is fixed (but reduced), it should be fairly simple to calculate an acceleration check speed after a certain distance, if the weight, configuration and atmospheric conditions are known. I understand the fact that if the numbers put into the calculation are wrong, the acceleration check speed would be wrong to, but the entire point of this check was to confirm that the aircraft is performing as expected. So putting a lower than actual weight into the equation, or having an engine problem, will reveal itself to the pilots at an early stage, by the fact that the aircraft is not meeting the expected performance.

I don't buy the the comment by small cog and vilas, that the T/O phase is too "high workload" for a two man crew, that there won't be time to perform a quick "how gets it" with the acceleration check. Especially since a reduced power T/O gives the crew much more time between brakes release and T/O. If the vis is pure, surly the distance used can be derived from the systems (INU or GPS).

I acknowledge the fact that liner pilots take off from many different airfields, under many different conditions with varying loads and varying power settings , but I think that simplicity is the key for an acceleration check.

One more question: When punching all the numbers into the FMS, to get the flex power setting, doesn't the system then also display the required distance from brakes release to rotation or T/O, so that the crew can do a sanity check whether the runway a head will suffice?

F-16GUY
14th Aug 2019, 17:17
Climb gradient is a function of excess thrust, nothing to do with lift. (Some are surprised to learn that the steeper you climb, the less lift is actually required or produced by the wing.)

Hence the saying, with enough thrust lift is irrelevant...
https://upload.wikimedia.org/wikipedia/commons/thumb/c/ce/F16_vertical_climb.png/610px-F16_vertical_climb.png

hans brinker
14th Aug 2019, 17:26
That, for sure, is a physical necessity. I did not put the effort in to read your message properly before posting, apologies.
No problem, and thanks for all your explanations for all of us.

Vessbot
14th Aug 2019, 18:03
@vessbot
Perhaps I ventured too far. Hope the other message remains standing: for a given V2, there would be different acceleration schedules depending on runway length. Longer tarmac, less thrust, valid slower acceleration, satisfactory V2+10 at screen height by the end of CWY anyway.


Agreed. And to clarify, I'm not really addressing the original discussion about acceleration checks down the runway. I took a tangent just from your statement about what physically determines climb angle.

Would you check your statement applies well in the second segment case, for climb angle of 1.4 deg with flaps out and perhaps 11 deg pitch?
The AFM data confirms that for the same thrust the climb gradient is better if IAS is increased, the so-called "improved climb V2 schedule". Without thinking too much about it, I was building on the classical vector cross with L in the upward sense opposing the W, and L = c(l) * area * 1/2 * density * v^2. It has got to work somehow, nay?

My statement applies in any steady state condition, which this example of climbing at V2+10, is.(assuming the pitch is such that airspeed stays steady).

Looking at the vector diagram, it's easy but naive to look at L going up, and figure that what goes up on the page must make the airplane go up, hence L obviously figures into climb angle... common sense, no? ;)

But you gotta remember that in steady state conditions (even in a steep climb) all forces sum to zero, so L cannot be more than W. If it is, then the flight path will be curving up (which is true, temporarily, as the climb is established from an initially level path). And since in a climb W is not opposing L, (it's angled back, pointing straight at the ground) then equilibrium L is equal to W times the cosine of the climb angle... in other words, slightly less than W. This is negligible in bugsmasher and airliner climb angles, but significant as you get more extreme. As in F-16GUY's picture above, where the cosine of the climb angle is zero, and so is L!

https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/455x237/imagecjq_42c6000fe2a6aa077f376f686a87e27d0d7e451d.jpg


However what is significant for all airplanes, is that now there's a component of weight pointing aft along with drag, so thrust is not just pulling against the drag, but rather drag + the sine of the climb angle. Hopefully this makes it a little more intuitively easy to grasp why climb angle is only determined by excess thrust (i.e., thrust minus drag) and weight.

​​​​​​​Hence the saying, with enough thrust lift is irrelevant...
https://upload.wikimedia.org/wikipedia/commons/thumb/c/ce/F16_vertical_climb.png/610px-F16_vertical_climb.png

Yes from a "fighter jets are cool" perspective, but if we're being serious than what I wrote above is as true for the J-3 Cub as for the F-16.

FlightDetent
14th Aug 2019, 18:19
One more question: When punching all the numbers into the FMS, to get the flex power setting, doesn't the system then also display the required distance from brakes release to rotation or T/O, so that the crew can do a sanity check whether the runway a head will suffice? The calculation of the reduced thrust level to be used is done by the Flt Ops Performance Engineering department. They will use the manufacturer's certified data taken from the Aeroplane Flight Manual and run them against a set of unique parameters of the day. These would include typically, for any single takeoff
- runway physical characteristic and state: dry / wet
- obstacles under the pre-determined OEI lateral path
- wind, temperature and QFE
- TOW, flap setting and even CG
- perhaps some airframe deviations that may have a degrading effect.
- bleed and anti-ice configuration of the engine.

Somehow this is delivered to pilots, these days mostly run directly on the flightdeck by them. Results are the V-speeds (1, R, 2) and a temperature.

Speeds are bugged, and the temperature fed (via the FMS keypad) to the engine control units.

That resulting temperature is an assumed OAT, under which the take-off would be limiting.

To answer what was asked: Some of the modern software does have an option to display the margins of TODA / ASDA / etc. which would be there if it really was as hot outside as the resulting temp. Normally though, a sanity check is not done because the calc is run with a given runway as one of the inputs.

FlightDetent
14th Aug 2019, 19:31
Vessbot Not in perfect agreement, mostly with the interpretation how that applies in our case but things may have gotten lost in translation.

Along these lines: if that aeroplane from the graphics had a better wing (longer L vector) it would climb faster - I really wish not to be wrong about that.

To make it climb steeper as it is, tilting the vectors, can only be achieved by adding more thrust to keep the equilibrium. No dispute there, the fact that showing throttles forward is needed to go up is not yet lost on this magenta child. There's hope yet :ok:

Similar to the extreme example of viper's temporary rocket dynamics, but on the other end: I think both of the above points need to be accepted to call our understanding complete - even before looking at a drawing where T = 1/4 L and the gamma angle is 1.4 deg.

Vessbot
14th Aug 2019, 22:18
For a certain weight, climb angle is proportional to excess thrust, which means thrust minus drag. Increase thrust, or decrease drag, and excess thrust is increased.


Along these lines: if that aeroplane from the graphics had a better wing (longer L vector) it would climb faster - I really wish not to be wrong about that.

No matter the wing, L would have to exactly equal W cos gamma, no more no less. Any longer, and we would no longer be in equilibrium. (We would accelerate upward, yes, temporarily - a transient - but a better wing is not necessary for that: a longer L is already available, so long as we're not stalling, by merely pulling the yoke back. If we were already at Vx, then drag would increase, excess thrust would decrease, and we would either be forced to pitch down to capture the new slower speed and shallower climb angle, or naively hold the nose up and decay the speed until we stall). However, a better wing (i.e., better L/D ratio) would yield a steeper climb due its lower drag.

To make it climb steeper as it is, tilting the vectors, can only be achieved by adding more thrust to keep the equilibrium. No dispute there, the fact that showing throttles forward is needed to go up is not yet lost on this magenta child. There's hope yet :ok:

Not sure what condition you're fixing by the phrase "as it is," but to climb steeper (aka increase excess thrust), in addition to increaseing thrust, we can also decrease drag. Say we're starting from a fast equilibrium condition, way on the front side of the drag curve: climbing at a shallow angle (or flying level, or descending... it doesn't matter) at 300 knots. We pitch up a little bit, and slow from 300 to 250. Two things happen:

A) The transient - short term - L is greater than W cos gamma: We pitch up, accelerate upward, and slow down
B) The new equilibrium - long term - L is equal to W cos gamma: At the new slower speed, thrust is the same but drag is less, therefore excess thrust and climb angle are greater.

Both things have to happen, but it's waay too easy to only think about A. It seems deceptively obvious, as a part of the "vehicle goes where the nose is pointed" intuitive mentality that is so hard to break people out of, and it may seem like a waste of neurons to consider it past "duh, if you pull up you'll climb steeper;" but it's the wrong explanation (and a deadly one; aviation history is red with the blood of pilots who intuitively pulled up sans thrust, and their passengers). A is really only the mechanism by which B is established, and B is the real reason we're climbing steeper now.

HarryMann
14th Aug 2019, 23:55
Wrong flap setting?
Can't see that affecting acceleration which basically is being used to check thrust and weight match up.

FlightDetent
15th Aug 2019, 05:35
Vessbot I am sure we're on the same page, perhaps viewing from slightly different angles, like two people sharing a read would.

That graphic is maybe the first figure in all of the Principles of Flight books out there, and cannot be wrong. You will find a thread where I argued against several more experienced people that it is with thrust how I control sink rate and chase the G/S on approach, even on a swept-wing jet.

However, it describes a static state with constant speed and AoA (which is why the latter is missing in the depiction). An attempt to use it to explain what happens in different places of the envelope as you change from one to another would not work. There are full volumes of aerodynamic theory beyond this drawing for this very reason, the drag polar being just the first next one.

E.g. the graphics cannot explain the difference between Vx and Vy, and we both know how vitally important those are especially if someone finds himself well slow of both.

I had a bit of resilience accepting "Climb gradient is a function of excess thrust, nothing to do with lift." Took it the wrong way, not realising against the missing "For a constant speed and stable AoA,,.. " was there, just silent. Culpa maxima.

The climb gradient after takeoff has massive amounts to do with how much lift you have to begin with, and that depends on the speed (squared) at the beginning of the second segment. Sure, drag does too.

During the take-off performance analysis we can select the V2 speed higher above the low limit, closer up towards best L/D speed, which then yields a steeper climb with the same amount of thrust applied, over-ruling the drag increase.

Once airborne in the equilibrium state, all that ammo is gone and the basic principles take over - it is the thrust (excess or lack of) which defines the changes of the climb rate if we were keeping the airspeed steady. No disputing there.

I'll wait for your closing public remarks and then we really should continue elsewhere to avoid floggin'.

HarryMann
15th Aug 2019, 10:39
Crews started using adhoc quick-check acceleration times.with a stopwatch back in 70s at least AFAIK.

Surely the point is that with 3 crew cockpit, it wasn't a big issue.
And a rough check is all that's required... are we nominally accelerating as expected?
or not ?

if not quick check around...
Not exactly same as a V1 stop/go decision

gearlever
15th Aug 2019, 11:45
Can't see that affecting acceleration which basically is being used to check thrust and weight match up.

Indeed, brain fart I would say:\

Vessbot
16th Aug 2019, 05:20
Vessbot I am sure we're on the same page, perhaps viewing from slightly different angles, like two people sharing a read would.

That graphic is maybe the first figure in all of the Principles of Flight books out there, and cannot be wrong. You will find a thread where I argued against several more experienced people that it is with thrust how I control sink rate and chase the G/S on approach, even on a swept-wing jet.

However, it describes a static state with constant speed and AoA (which is why the latter is missing in the depiction). An attempt to use it to explain what happens in different places of the envelope as you change from one to another would not work. There are full volumes of aerodynamic theory beyond this drawing for this very reason, the drag polar being just the first next one.

E.g. the graphics cannot explain the difference between Vx and Vy, and we both know how vitally important those are especially if someone finds himself well slow of both.

I had a bit of resilience accepting "Climb gradient is a function of excess thrust, nothing to do with lift." Took it the wrong way, not realising against the missing "For a constant speed and stable AoA,,.. " was there, just silent. Culpa maxima.

The climb gradient after takeoff has massive amounts to do with how much lift you have to begin with, and that depends on the speed (squared) at the beginning of the second segment. Sure, drag does too.

During the take-off performance analysis we can select the V2 speed higher above the low limit, closer up towards best L/D speed, which then yields a steeper climb with the same amount of thrust applied, over-ruling the drag increase.

Once airborne in the equilibrium state, all that ammo is gone and the basic principles take over - it is the thrust (excess or lack of) which defines the changes of the climb rate if we were keeping the airspeed steady. No disputing there.

I'll wait for your closing public remarks and then we really should continue elsewhere to avoid floggin'.

Sounds like we're in agreement... or at least if there's disagreement somewhere, I can't find it anymore.

If I'm reading you right, you agree that for steady states it's excess thrust that determines climb angle, and lift has no effect. And it's steady states that I was talking about, as I clarified in my last post. Also I was addressing your specific comment where you laid out a situation with a specific climb angle, pitch angle, and speed (V2) which is a steady state.

Actually, I think I found the remaining disagreement halfway through your post. At the risk of seeming flogging.... I hope you reconsider and come back, as why are we talking about flying on the internet to begin with? :)

Anyway, you seem to be treating the second segment climb, or flying at V2, as a transient condition and therefore not subject to the excess thrust rule. Now I'm treading into ground where I'm less sure than my previous posting in this thread, but I disagree with that too. It may not feel "steady state" as, depending on our SOP's we may only be holding that speed for a few seconds, some or all of which may be taken up by overshooting, corrections, etc. However, any of that is merely slop in execution, and real-life imperfections away from the platonic perfectly executed maneuver where we rotate exactly into the speed, freeze the pitch, and the speed then doesn't waver. If the thrust and speed are steady, then it's a steady state condition, and lift is irrelevant. It was relevant a few seconds ago in the pitch up (and vertical acceleration) off the runway and into the V2 attitude, but not once V2 is established.

Additionally, you bring up changes to V2, where it's selectable form a range. Then, it's the consideration of different steady state speed targets. You raise an example where we select a higher V2, closer to best L/D, that gives a steeper climb "overruling the drag increase," but actually the drag decreases, not increases in that case so there's nothing to overrule. Remember that the lowest drag is at best L/D, so by flying closer to it, we're flying at less drag, and thus in exact accordance with excess thrust yielding the higher climb angle.

Again, I hope you do come back, I'm thoroughly enjoying this discussion.