6questions from Handling the big jets
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6questions from Handling the big jets
I have just bought Handling the big jets...I'm at that stage of my career now...
Read it a few times and wanted to clear up some questions I have any takers be great....
1.I have always been lead to believe that flying for endurance initially you would think highe but I was told at lower altitudes because the wings can produce lift easier..HBJ states that you go high I'm confused?
2.EAS is something I don't use in my flying but Vs EAS rises slightly as you climb for example, in what use of EAS does it become significant to talk about it in flying I use IAS or TAS but I have to say my knowledge in terms of talking about EAS in Flying is very limited ! - HELP
3.Why does a jet engine produce more thrust at higher rpm than lower RPM and a Piston doesn't say?
4.Why are those bloody performance figures for pistons only ever quoted in 65% and 75%values only? - only I have never deared to ask anyone for fear of someone saying you twit!
5.HBJ talks a lot about Vdf /Mdf I thought the fastest you could go in jet terms (remember I am still a piston pilot?!) was Vmo/Mmo so what is going on I talk Vne HBJ talks Mmo and you could goto Mdf if you want!!!!! aggghhh!!! I'm getting confused.
6.I think this is an extension of Q5 but at high Mach can you by deflecting a control surface too much put the wing through the sound barrier is this what the great D.P Davies means when he says beware mach buffet at high Mach No. etc...
7. Thankyou to those of you who may answer my thread.
Read it a few times and wanted to clear up some questions I have any takers be great....
1.I have always been lead to believe that flying for endurance initially you would think highe but I was told at lower altitudes because the wings can produce lift easier..HBJ states that you go high I'm confused?
2.EAS is something I don't use in my flying but Vs EAS rises slightly as you climb for example, in what use of EAS does it become significant to talk about it in flying I use IAS or TAS but I have to say my knowledge in terms of talking about EAS in Flying is very limited ! - HELP
3.Why does a jet engine produce more thrust at higher rpm than lower RPM and a Piston doesn't say?
4.Why are those bloody performance figures for pistons only ever quoted in 65% and 75%values only? - only I have never deared to ask anyone for fear of someone saying you twit!
5.HBJ talks a lot about Vdf /Mdf I thought the fastest you could go in jet terms (remember I am still a piston pilot?!) was Vmo/Mmo so what is going on I talk Vne HBJ talks Mmo and you could goto Mdf if you want!!!!! aggghhh!!! I'm getting confused.
6.I think this is an extension of Q5 but at high Mach can you by deflecting a control surface too much put the wing through the sound barrier is this what the great D.P Davies means when he says beware mach buffet at high Mach No. etc...
7. Thankyou to those of you who may answer my thread.
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posted by dynamite dean
1.I have always been lead to believe that flying for endurance initially you would think highe but I was told at lower altitudes because the wings can produce lift easier..HBJ states that you go high I'm confused?
1.I have always been lead to believe that flying for endurance initially you would think highe but I was told at lower altitudes because the wings can produce lift easier..HBJ states that you go high I'm confused?
Classically, jet thrust is just a function of EAS, and so is drag. So a plane which can hold 300kts EAS at 10,000ft can also cruise at 300kts EAS at 35,000ft, say. (Ignoring Mach effects for now).
But at 35,000ft your TAS will be more like 600kts, while at 10,000ft it might be only 400kts.
I'm making these numbers up, since I can't be bothered calculating the exact values.
The relationships are all you need to understand the general pointSo the aircraft at 35,000ft is actually travelling over the ground (assuming no wind) 50% faster than the one at 10,000ft. Hence higher is better.
It gets more complex when you worry about winds, and Mach limits, and time taken/wasted in climbing.
And prop engines don't come close to the classic jet thrust independent of altitude assumption, so they do tend to be better off staying lower.
2.EAS is something I don't use in my flying but Vs EAS rises slightly as you climb for example, in what use of EAS does it become significant to talk about it in flying I use IAS or TAS but I have to say my knowledge in terms of talking about EAS in Flying is very limited ! - HELP
True Air Speed (TAS) : this is your actual speed relative to the local air mass.
Equivalent Air Speed (EAS) : this is the speed which would, at sea level under standard conditions, give the same kinetic pressure (1/2-rho-v-squared) as you are currently experiencing. It's useful for any time you are worrying about e.g. airframe loads, because for the same aerodynamic coefficients the forces will be the same at the same EAS. It gets significantly lower than TAS at higher altitudes.
Calibrated Air Speed (CAS) : this is the speed which an ideal airspeed indicating system would read for the current conditions. It is similar to EAS, in that it drops to lower values than TAS at altitude, for similar reasons. But there is a factor, called the "scale altitude correction" which is the difference between EAS and CAS.
Indicated Air Speed (IAS) : this is the speed actually indicated to the pilot, and differs from the theoretical CAS due to inadequacy in the design of all airspeed sensing systems. These include position errors (PEs) due to the pitot/static system not being perfect, plus computing lags and equipment errors.
The possible "fifth" airspeed I have seen used is ASI - "Air Speed Indicated". This is sometimes used when people wish to distinguish between PEs and other errors - in that case IAS is CAS plus PE effects, and ASI is IAS plus equipment errors.
When flying the only speeds that should matter to the pilot are those used in the appropriate aircraft manuals. If they are quoted in IAS (as they should be) then all the TAS/EAS/CAS stuff is irrelevant - any corrections will have been made in producing the manuals, and if you respect the IAS limits then anything which is "really" an EAS limit (loads, perhaps) will be taken care of automatically.
3.Why does a jet engine produce more thrust at higher rpm than lower RPM and a Piston doesn't say?
4.Why are those bloody performance figures for pistons only ever quoted in 65% and 75%values only? - only I have never deared to ask anyone for fear of someone saying you twit!
4.Why are those bloody performance figures for pistons only ever quoted in 65% and 75%values only? - only I have never deared to ask anyone for fear of someone saying you twit!
5.HBJ talks a lot about Vdf /Mdf I thought the fastest you could go in jet terms (remember I am still a piston pilot?!) was Vmo/Mmo so what is going on I talk Vne HBJ talks Mmo and you could goto Mdf if you want!!!!! aggghhh!!! I'm getting confused.
Vmo/Mmo: Maximum Operating speed/mach number. Not to be exceeded intentionally.
Vfc/Mfc: the maximum speed/Mach to which FAR25 handling requirements must be met
Vdf/Mdf: the maximum flight demonstrated speed/mach number
Vd/Md: maximum design speed/mach number
Vfc/Mfc lies between Vmo/Mmo and Vdf/Mdf. It's a bit complex in practice, if you think "about half way between" you'll not go far wrong.
Vdf/Mdf MUST be less than Vd/Md.
Vmo/Mmo MUST be defined such that the aircraft will never exceed Vdf/Mdf for a variety of defined "upset" cases.
The statement "you can go to Vdf" should be treated with extreme caution. You shouldn't be anywhere near that speed under any kind of normal conditions. It's a speed you go to in flight test to make sure the aircraft will not fall apart at that speed, but there may be no margin for error. If you intentionally flew there and any of several things happened (gust, control runaway, autopilot failure (if it could be engaged that is)) you could easily end up past Vdf, in a region never explored in test. That is not a good thing.
Typically I wuld expect max practical cruise speed to be a little under Mmo/Vmo. That way you won't get all kinds of warnings every time there's a slight disturbance.
6.I think this is an extension of Q5 but at high Mach can you by deflecting a control surface too much put the wing through the sound barrier is this what the great D.P Davies means when he says beware mach buffet at high Mach No. etc...
I don't think that's what he referring to though - you can get Mach-related buffeting without control deflections.
edited for style
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If I may add a comment or two to MFS's post.
(a) do run some searches in this site as these questions, quite naturally, come up time and time again as new people become interested in the site and finding out more about flying.
(b) Endurance basically means looking for where we can run the aircraft with minimum, or near minimum, fuel flow (ie maximum time available for loitering flight) and it works out that pistons like low and jets like high. The specific height chosen will depend on the individual aircraft to some extent and how you go about the analysis .. what factors you include and so on. Some jets will give best endurance at a medium height (eg earlier 737s typically somewhere around FL200) .. it all comes back to doing the sums on the specific aircraft data. The generic "go high" story is only that .. generic... however, the jet does not like low ... you often find fuel flows at the holding point to be not much different to what you get at normal cruise levels and speeds.
(c) stall speed variation with altitude often confuses people. The typical lift equation we learn as pilots
lift = CL x 0.5 x rho x V-squared x S
can be reworked to something like the following to approximate the one G stall case which occurs at the maximum value of CL (CLmax) ... if this is not in your principles of flight book, then it probably is in Kermode or Davies (long time since I have read those texts) or else check out some of the websites listed in the Tech Log sticky thread.
Vs = root ((2 x W)/(rho x S x CLmax)).
This is all fine but you MUST always remember that real world processes are complex and, as engineers and scientists, we have to approximate things and make various assumptions to be able to get whatever the job is done ....
This "standard" equation that you will see so often makes the assumption that the CL curve only depends on alpha, which is reasonably accurate over a small altitude band, typically near sea level for common use. However, there are other things which come into play. Two considerations, which typically we ignore for the near SL case are Mach Number and Reynolds Number which are very important fluid flow measures. As an aircraft climbs to much higher levels these measures change with their effect on the CL curve becoming measurable and having to be considered.
In regard to stall, they reduce the CLmax value which the wing section is able to generate. If you revisit the equation above, if the value of CLmax is reduced, then the right hand side of the equation gets bigger in value and stall speed increases somewhat. In practical everyday terms it is not something to get ulcers about as the book data to which we fly the aircraft considers such matters for us.
And you need to keep in mind that the ASI is a differential pressure gauge calibrated to give an indication of speed at standard SL ... it is not a speedometer like you might have in your motorcar. Except for standard conditions at SL, the ASI tells fibs ... which is why we have all these various "airspeeds" to worry about ... they allow us to account for the various fibs which the ASI is wont to tell. Once you get used to it, it all seems to fall into place. In practical terms, we are concerned with what the gauge tells us (IAS) for operating the aircraft (for jets we need the Mach Number as well) and the TAS we calculate from that for figuring out the navigation solution. EAS is important for engineers but much less so for civil line pilots.
(d) The jet engine operates effectively only over a small RPM band due to the problems involved in getting the air to find its way through the machinery .. just consider all the little wings you have spinning around and the torture to which the airflow is subjected. The typical thrust curve shows very low thrust at low rpm, slowly increasing with increasing rpm and then rapidly increasing in the higher rpm range. These turbomachinery flow problems make turbine engine design a pretty specialised engineering discipline and the engines have all sorts of built in fixes (eg acceleration control bleed valves) to allow the engine to run at low rpm. Basically a jet engine likes to run near its optimum operating design point, which is somewhere towards the maximum rpm.
You need to consider that a jet is a thrust producing machine while the piston engine is a power producer. For the piston, the propeller takes a bunch of power and converts it rather inefficiently into thrust. The piston/propeller combination is rather complex in that the engine power is largely dictated by cylinder pressures and rpm .. increasing these increases power. However, the propeller has a big problem handling high rpm as the tips rapidly get to local sonic speeds and experience the same sorts of problems which wings do at high subsonic Mach Numbers. As a result you find things like blowers to increase pressures, geared engines to increase engine rpm while keeping the propeller rpm under control, variable pitch propellers to allow a more flexible conversion of power into thrust.
The use of percent power is just a convention .. if it bothers you, you can convert quite simply to BHP or kW according to your preferences .. makes little difference. It is also very useful to get the engine manufacturer's Operator's Manual for the particular engine, as this will give you a whole lot more information than what the airframe manufacturer chooses to tell you in the POH. Much the same sort of thing as CG being quoted in percent MAC .. useful for the aerodynamicists on the project team, but a minor nuisance pain in the neck for the line pilot.
(e) The problem with ill-considered aileron deflection at high speed is that the airflow may accelerate sufficiently to put the local wing section into a shock separation problem area. This is rather different to the more commonly discussed mach tuck problem. I suspect that what you read was more in the nature of a generic discussion point rather than specific guidance.
(a) do run some searches in this site as these questions, quite naturally, come up time and time again as new people become interested in the site and finding out more about flying.
(b) Endurance basically means looking for where we can run the aircraft with minimum, or near minimum, fuel flow (ie maximum time available for loitering flight) and it works out that pistons like low and jets like high. The specific height chosen will depend on the individual aircraft to some extent and how you go about the analysis .. what factors you include and so on. Some jets will give best endurance at a medium height (eg earlier 737s typically somewhere around FL200) .. it all comes back to doing the sums on the specific aircraft data. The generic "go high" story is only that .. generic... however, the jet does not like low ... you often find fuel flows at the holding point to be not much different to what you get at normal cruise levels and speeds.
(c) stall speed variation with altitude often confuses people. The typical lift equation we learn as pilots
lift = CL x 0.5 x rho x V-squared x S
can be reworked to something like the following to approximate the one G stall case which occurs at the maximum value of CL (CLmax) ... if this is not in your principles of flight book, then it probably is in Kermode or Davies (long time since I have read those texts) or else check out some of the websites listed in the Tech Log sticky thread.
Vs = root ((2 x W)/(rho x S x CLmax)).
This is all fine but you MUST always remember that real world processes are complex and, as engineers and scientists, we have to approximate things and make various assumptions to be able to get whatever the job is done ....
This "standard" equation that you will see so often makes the assumption that the CL curve only depends on alpha, which is reasonably accurate over a small altitude band, typically near sea level for common use. However, there are other things which come into play. Two considerations, which typically we ignore for the near SL case are Mach Number and Reynolds Number which are very important fluid flow measures. As an aircraft climbs to much higher levels these measures change with their effect on the CL curve becoming measurable and having to be considered.
In regard to stall, they reduce the CLmax value which the wing section is able to generate. If you revisit the equation above, if the value of CLmax is reduced, then the right hand side of the equation gets bigger in value and stall speed increases somewhat. In practical everyday terms it is not something to get ulcers about as the book data to which we fly the aircraft considers such matters for us.
And you need to keep in mind that the ASI is a differential pressure gauge calibrated to give an indication of speed at standard SL ... it is not a speedometer like you might have in your motorcar. Except for standard conditions at SL, the ASI tells fibs ... which is why we have all these various "airspeeds" to worry about ... they allow us to account for the various fibs which the ASI is wont to tell. Once you get used to it, it all seems to fall into place. In practical terms, we are concerned with what the gauge tells us (IAS) for operating the aircraft (for jets we need the Mach Number as well) and the TAS we calculate from that for figuring out the navigation solution. EAS is important for engineers but much less so for civil line pilots.
(d) The jet engine operates effectively only over a small RPM band due to the problems involved in getting the air to find its way through the machinery .. just consider all the little wings you have spinning around and the torture to which the airflow is subjected. The typical thrust curve shows very low thrust at low rpm, slowly increasing with increasing rpm and then rapidly increasing in the higher rpm range. These turbomachinery flow problems make turbine engine design a pretty specialised engineering discipline and the engines have all sorts of built in fixes (eg acceleration control bleed valves) to allow the engine to run at low rpm. Basically a jet engine likes to run near its optimum operating design point, which is somewhere towards the maximum rpm.
You need to consider that a jet is a thrust producing machine while the piston engine is a power producer. For the piston, the propeller takes a bunch of power and converts it rather inefficiently into thrust. The piston/propeller combination is rather complex in that the engine power is largely dictated by cylinder pressures and rpm .. increasing these increases power. However, the propeller has a big problem handling high rpm as the tips rapidly get to local sonic speeds and experience the same sorts of problems which wings do at high subsonic Mach Numbers. As a result you find things like blowers to increase pressures, geared engines to increase engine rpm while keeping the propeller rpm under control, variable pitch propellers to allow a more flexible conversion of power into thrust.
The use of percent power is just a convention .. if it bothers you, you can convert quite simply to BHP or kW according to your preferences .. makes little difference. It is also very useful to get the engine manufacturer's Operator's Manual for the particular engine, as this will give you a whole lot more information than what the airframe manufacturer chooses to tell you in the POH. Much the same sort of thing as CG being quoted in percent MAC .. useful for the aerodynamicists on the project team, but a minor nuisance pain in the neck for the line pilot.
(e) The problem with ill-considered aileron deflection at high speed is that the airflow may accelerate sufficiently to put the local wing section into a shock separation problem area. This is rather different to the more commonly discussed mach tuck problem. I suspect that what you read was more in the nature of a generic discussion point rather than specific guidance.
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Small piston-powered aircraft generally use 65% or 75% only for convenience.
However, large radial piston engines used on airliners of the past generally cruised at much lower power settings. For example, the Pratt&Whitney R2800CB16 engine fitted to the DC-6 had a takeoff power rating (wet) of 2400 BHP, but was operated in cruise at 1100 BHP, approximately 45% of rated power.
The reason was maxium engine life.
In addition, piston engine airliners were ALWAYS cruised at a constant power setting, unlike a jet which is cruised at a constant speed, ATC permitting of course.
However, large radial piston engines used on airliners of the past generally cruised at much lower power settings. For example, the Pratt&Whitney R2800CB16 engine fitted to the DC-6 had a takeoff power rating (wet) of 2400 BHP, but was operated in cruise at 1100 BHP, approximately 45% of rated power.
The reason was maxium engine life.
In addition, piston engine airliners were ALWAYS cruised at a constant power setting, unlike a jet which is cruised at a constant speed, ATC permitting of course.
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A piston engine produces more power as engine speed increases. It is constrained a bit by volumetric efficency factors - at high revs air flow inertia reduces BMEP, but the main problem is the capability of the engine to cope with physical loads - there are a lot of reciprocating and unbalanced forces, unlike a jet.
Dynamite Dean: Vfd/Mfd etc?? Do people in your country need to understand this to be a test pilot, or a regular line pilot? Was the author trying to over-impress his readers with such terms and does the Commercial or ATP over there require such knowledge? If for line flying, what kind of goobers (to be polite) are in charge of the licensing programs? Whether before or during 17 years on 100 to 194-seat fanjets (whether as Captain or FO), we have never been expected to tackle or even read up on such trivia. Do many countries out there want to produce pilots or flight test engineers?
If your book says to climb out at about 290/310 knots until .72/.74 Mach, how many types of airspeed/mach indicators do those planes have? Our book also says to 'pad' our approach speed by a maximum of 5 knots in gusty conditions, but tonight landing north with a wind of 060/20 knots in fairly heavy rain, we used an extra ten-fifteen knots or so (tower reported a gain/loss of ten), as most pilots would (and on a very long runway with a 'light' (82,000 #) airplane-so much for book theory. I've never heard our widebody pilots use those other terms either.
Note-I mean no disrespect by questioning other country's unusual methods, however there is plenty of valuable info to study and review each year without trying to memorize such deep theory (for example, we no longer need to be concerned about 4 vs 20 Joule ignition from AC bus or DC Transfer Bus, as in past years) which would require more time than most people have available, even without young kids around. Worrying about too many hidden details might distract one from remembering to read a checklist, look up limitations/procedures in the MEL book, or wonder whether we should accept the plane with an inop APU.
If your book says to climb out at about 290/310 knots until .72/.74 Mach, how many types of airspeed/mach indicators do those planes have? Our book also says to 'pad' our approach speed by a maximum of 5 knots in gusty conditions, but tonight landing north with a wind of 060/20 knots in fairly heavy rain, we used an extra ten-fifteen knots or so (tower reported a gain/loss of ten), as most pilots would (and on a very long runway with a 'light' (82,000 #) airplane-so much for book theory. I've never heard our widebody pilots use those other terms either.
Note-I mean no disrespect by questioning other country's unusual methods, however there is plenty of valuable info to study and review each year without trying to memorize such deep theory (for example, we no longer need to be concerned about 4 vs 20 Joule ignition from AC bus or DC Transfer Bus, as in past years) which would require more time than most people have available, even without young kids around. Worrying about too many hidden details might distract one from remembering to read a checklist, look up limitations/procedures in the MEL book, or wonder whether we should accept the plane with an inop APU.
Last edited by Ignition Override; 25th Dec 2002 at 04:28.
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You gotta remember Ignition Override, that when you ask a Brit pilot what time it is, generally you are told how a clock works.
Now having said all this...and having read Davies book in 1968 (when I started in big jets), he was one smart fellow, thats for sure.
Have always wondered if the young guys reading it today really have any idea of the handling qualities of the earlier jet transport aircraft, early 707's especially.
Suspect that they would be in for a big surprise if they had one strapped to their backside.
And...NO FMS either. Shock, horror.
Now having said all this...and having read Davies book in 1968 (when I started in big jets), he was one smart fellow, thats for sure.
Have always wondered if the young guys reading it today really have any idea of the handling qualities of the earlier jet transport aircraft, early 707's especially.
Suspect that they would be in for a big surprise if they had one strapped to their backside.
And...NO FMS either. Shock, horror.
Simple speeds explained (without clockworks!)
Indicated airspeed (IAS) = what's on the dial.
Rectified Airspeed (RAS) = IAS corrected for instrument and pressure error.
Equivalent Airspeed (EAS) = RAS corrected for compressability error (an ideal reference for expalining what happens to an aircraft flying faster than Mach 0.4 hence DP Davies' use of this reference)
True Airspeed (TAS) = EAS corrected for temperature - the true speed of the aircraft.
and also;
Calibrated Airspeed (CAS) = IAS corrected for pressure and instrument system errors by an Air Data computer and what you see on the dial or screen on an aircraft with an ADC.
Hope this is simple enough for you Americans!
And 411A - I take it you mean early American jet aircraft. If the readers were lucky to have flown a well designed early jet transport such as the VC10 (the nicest big aircraft I've ever flown) they wouldn't have such a suprise!
Merry Christmas.
Indicated airspeed (IAS) = what's on the dial.
Rectified Airspeed (RAS) = IAS corrected for instrument and pressure error.
Equivalent Airspeed (EAS) = RAS corrected for compressability error (an ideal reference for expalining what happens to an aircraft flying faster than Mach 0.4 hence DP Davies' use of this reference)
True Airspeed (TAS) = EAS corrected for temperature - the true speed of the aircraft.
and also;
Calibrated Airspeed (CAS) = IAS corrected for pressure and instrument system errors by an Air Data computer and what you see on the dial or screen on an aircraft with an ADC.
Hope this is simple enough for you Americans!
And 411A - I take it you mean early American jet aircraft. If the readers were lucky to have flown a well designed early jet transport such as the VC10 (the nicest big aircraft I've ever flown) they wouldn't have such a suprise!
Merry Christmas.
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Dean,
When considering Davies' comments concerning Vdf and Mdf you need to remember that he was talking about (and flying in) the days before flight simulators. In order to be prepared to deal with overspeeds caused by upsets, pilots had to practice flying at those speeds. They (presumably) did this with no passengers on board! Once qualified current-day line pilots are unlikely to get to do any non-revenue practice flying, let alone be permitted to practice flying above Vmo/Mmo.
When considering Davies' comments concerning Vdf and Mdf you need to remember that he was talking about (and flying in) the days before flight simulators. In order to be prepared to deal with overspeeds caused by upsets, pilots had to practice flying at those speeds. They (presumably) did this with no passengers on board! Once qualified current-day line pilots are unlikely to get to do any non-revenue practice flying, let alone be permitted to practice flying above Vmo/Mmo.
