V2 - reference zero = 35' or 50'
Guest
Posts: n/a
V2 - reference zero = 35' or 50'
Correct me if I'm wrong:
'reference zero' is the height at which the aircraft can maintain flight/climb out of ground effect ...
that is what the whole V2 related performance calculation is based on, not so?
why are some talking about 50 foot and others about 35 foot and should this figure not be different for all aircraft anyway?
or can it be generalised for any aircraft to be out of ground effect at 35/50 foot?
I'd really appreciate somebody enlightening me about the matter ... or, in case I'm totally on the wrong heading, request radar vectors to get back onto the right path ...
thanx in advance
cheers and beers
LF
------------------
Let there be cold beers on a hot day !!!
'reference zero' is the height at which the aircraft can maintain flight/climb out of ground effect ...
that is what the whole V2 related performance calculation is based on, not so?
why are some talking about 50 foot and others about 35 foot and should this figure not be different for all aircraft anyway?
or can it be generalised for any aircraft to be out of ground effect at 35/50 foot?
I'd really appreciate somebody enlightening me about the matter ... or, in case I'm totally on the wrong heading, request radar vectors to get back onto the right path ...
thanx in advance
cheers and beers
LF
------------------
Let there be cold beers on a hot day !!!
Guest
Posts: n/a
Shows again, there's not only ****** questions but also ****** answers ...
If you reckon I asked a stupid question, I'm happy to buy your reasoning ...
otherwise, me farts CO2 mingled with some other stuff, me thinks ... still, according to the manuals I had the pleasure to study so far, not all of the Jet-A1 farting machines seem to leave ground effect at the same height ... so don't the 100LL'ers ...
'eeeh why ??? ... anybody ???
Me don't know really what him try to tells me !?
cheers and beers
LF
------------------
Let there be cold beers on a hot day !!!
[This message has been edited by LetsFetz (edited 16 January 2001).]
If you reckon I asked a stupid question, I'm happy to buy your reasoning ...
otherwise, me farts CO2 mingled with some other stuff, me thinks ... still, according to the manuals I had the pleasure to study so far, not all of the Jet-A1 farting machines seem to leave ground effect at the same height ... so don't the 100LL'ers ...
'eeeh why ??? ... anybody ???
Me don't know really what him try to tells me !?
cheers and beers
LF
------------------
Let there be cold beers on a hot day !!!
[This message has been edited by LetsFetz (edited 16 January 2001).]
Guest
Posts: n/a
LF-
V2 is not really a "ground effect" number.
With the advent of turbojet powered aircraft, it was recognized that the certification rules had to be changed. The props had a 50' screen, turbojets have 35'. (And if "wet" today, only 15 feet.)
V2 is really a compromise, not really the best speed for getting off the runway (too fast), or for maximizing the climb gradient after liftoff (too slow).
V2 is not really a "ground effect" number.
With the advent of turbojet powered aircraft, it was recognized that the certification rules had to be changed. The props had a 50' screen, turbojets have 35'. (And if "wet" today, only 15 feet.)
V2 is really a compromise, not really the best speed for getting off the runway (too fast), or for maximizing the climb gradient after liftoff (too slow).
Guest
Posts: n/a
QUID
You are correct with regard to turbojet aircraft. However, recip powered transport aircraft rotate at V2. There is no separate Vr and certification requires them to climb at V2 to 400 feet, where METO power is selected. Ah....Douglas! The DC-6B was/is my favorite.
You are correct with regard to turbojet aircraft. However, recip powered transport aircraft rotate at V2. There is no separate Vr and certification requires them to climb at V2 to 400 feet, where METO power is selected. Ah....Douglas! The DC-6B was/is my favorite.
Guest
Posts: n/a
LetsFetz,
V2 is calculated as 1.2 x stall speed in takeoff configuration. At this speed the A/C should be able to maintain a safe rate of climb with one engine inop. This will vary with loading, temperature, pressure, etc.
35 feet is the minimum obstacle clearance required along the runway extended centerline (clearway) until reaching 1500ft.
Nothing to do with ground effect, as quid says.
I have to admit I never heard of an 50ft requirement. Is this a JAA/European reg.?
I never came across it in the FAA regs.
V2 is calculated as 1.2 x stall speed in takeoff configuration. At this speed the A/C should be able to maintain a safe rate of climb with one engine inop. This will vary with loading, temperature, pressure, etc.
35 feet is the minimum obstacle clearance required along the runway extended centerline (clearway) until reaching 1500ft.
Nothing to do with ground effect, as quid says.
I have to admit I never heard of an 50ft requirement. Is this a JAA/European reg.?
I never came across it in the FAA regs.
Guest
Posts: n/a
Thanx so far guys,
Davies in his book 'Handling the big Jets' also talks about the 15' when wet ... unfortunately without giving specific reasons in terms of aerodynamics and/or performance ...
That was, what I was hoping to learn, the aerodynamic and/or performance related reasons for the differences in screen height ...
I know, V2 is not related to ground effect, but is the speed you (should) reach at 35/50 foot in the engine out case ...
35/50 foot however seems to be based on the consideration of the aircraft leaving ground effect ...
As far as I can remember, Davies also mentions this in his book ...
So, why the differences, which ever nature they are? ... wet=15' ... piston and/or prop (statements differ)=50' ... jets=35' ... ???
Was hoping to get some answers from the guys who develop and build the stuff we have the pleasure to fly ...
Thanx in advance for bringing it on ...
LF
------------------
Let there be cold beers on a hot day !!!
Davies in his book 'Handling the big Jets' also talks about the 15' when wet ... unfortunately without giving specific reasons in terms of aerodynamics and/or performance ...
That was, what I was hoping to learn, the aerodynamic and/or performance related reasons for the differences in screen height ...
I know, V2 is not related to ground effect, but is the speed you (should) reach at 35/50 foot in the engine out case ...
35/50 foot however seems to be based on the consideration of the aircraft leaving ground effect ...
As far as I can remember, Davies also mentions this in his book ...
So, why the differences, which ever nature they are? ... wet=15' ... piston and/or prop (statements differ)=50' ... jets=35' ... ???
Was hoping to get some answers from the guys who develop and build the stuff we have the pleasure to fly ...
Thanx in advance for bringing it on ...
LF
------------------
Let there be cold beers on a hot day !!!
Guest
Posts: n/a
I don’t know anything about 50ft, but for perf A jets the normal screen height is 35ft which is reduced to 15ft if wet or contaminated as a concession by the authorities.
The way they tie in with V2 is that the speed must be at least V2 at the 35 ft screen height, but for a 15 ft screen height, speed must consistent with achieving at least V2 at 35 ft. ie V2 does not have to be achieved at the 15 ft screen height.
S & L
The way they tie in with V2 is that the speed must be at least V2 at the 35 ft screen height, but for a 15 ft screen height, speed must consistent with achieving at least V2 at 35 ft. ie V2 does not have to be achieved at the 15 ft screen height.
S & L
Guest
Posts: n/a
411A / Quid
thanx a lot for your input ...
Davies actually also talks about CAR4b in his book ...
I'm at the moment flying similar category aircraft (RA06): screen height = 50' and V1=Vr ... in the event of an engine failure the recommended procedure is V2 to 400' - accelerate to Vyse - flaps up ...
all very well and acceptable, but still not clear to me why 15'/35'/50' ... and what about the ground effect in connection with screen height ... how is it established/certified and why does it differ from case to case?
By the way, it's correct as stated in one of the posts: if screen height reduced to 15', V2 still has to be achieved at 35' ...
but again, why the 15' foot and what is it all based on? ... what about the ground effect? ... or if not the ground effect theory, what else is it that determines screen height?
thanx
LF
------------------
Let there be cold beers on a hot day !!!
[This message has been edited by LetsFetz (edited 16 January 2001).]
thanx a lot for your input ...
Davies actually also talks about CAR4b in his book ...
I'm at the moment flying similar category aircraft (RA06): screen height = 50' and V1=Vr ... in the event of an engine failure the recommended procedure is V2 to 400' - accelerate to Vyse - flaps up ...
all very well and acceptable, but still not clear to me why 15'/35'/50' ... and what about the ground effect in connection with screen height ... how is it established/certified and why does it differ from case to case?
By the way, it's correct as stated in one of the posts: if screen height reduced to 15', V2 still has to be achieved at 35' ...
but again, why the 15' foot and what is it all based on? ... what about the ground effect? ... or if not the ground effect theory, what else is it that determines screen height?
thanx
LF
------------------
Let there be cold beers on a hot day !!!
[This message has been edited by LetsFetz (edited 16 January 2001).]
Guest
Posts: n/a
At times like this, it's often best to refer to the rulebook: -
V2, Take-off safety speed' means a referenced airspeed obtained after lift-off at which the required one-engine-inoperative climb performance can be achieved. (From JAR-1, definitions)
The rotation speed, VR, in terms of calibrated airspeed, must be selected by the applicant and must not be less than the greatest of the following:
(i) V1; or
(ii) 1.05 VMC determined under JAR 23.149 (b); or
(iii) 1.10 VS1; or
(iv) The speed that allows attaining the initial climb-out speed, V2, before reaching a height of 35 ft above the take-off surface in accordance with JAR 23.57 (c)(2).
(From JAR-23, the usual light aircraft standard)
The take-off safety speed, V2, in terms of calibrated airspeed, must be selected by the applicant so as to allow the gradient of climb required in JAR 23.67 (c)(1) and (c)(2) but must not be less than 1.10 VMC or less than 1.20 VS1.
(Also from JAR-23, 23.67 is very complicated but depending on conditions the minimum climb gradient is generally between 1 and 2%)
And if you thought that JAR-23 was complicated, try the following from JAR-25, the airliner standard...
) V2MIN, in terms of calibrated airspeed, may not be less than--
(1) 1.13 VSR for--
(i) Two-engined and three-engined turbo-propeller powered aeroplanes; and
(ii) Turbojet powered aeroplanes without provisions for obtaining a significant reduction in the one-engine-inoperative power-on stall speed;]
[(2) 1.08 VSR for--
(i) Turbo-propeller powered aeroplanes with more than three engines; and
(ii) Turbojet powered aeroplanes with provisions for obtaining a significant reduction in the one-engine-inoperative power-on stall speed: and
(3) 1.10 times VMC established under JAR 25.149.]
(c) V2 in terms of calibrated airspeed, must be selected by the applicant to provide at least the gradient of climb required by JAR 25.121(b) but may not be less than--
[(1) V2MIN;
(2) VR plus the speed increment attained (in accordance with JAR 25.111(c)(2)) before reaching a height of 35 ft above the take-off surface; and
(3) A speed that provides the manoeuvring capability specified in JAR 15.143(g).]
Commenting upon what other people have said before. V2 is the speed that should be established by a given height, which varies according to conditions (although 35ft seems normal). This is a separate (but inevitably related) issue to the take-off distance, which is the distance to reach the screen height, which is (in my experience) usually 15m (or 50ft) using the recommended take-off speeds and methods.
Changes in screen height are unlikely to be due to ground effect (which is generally considered to become negligible at about 2 wingspans), and would probably be associated with a matched increasingly stringent set of requirements for airport approval.
G
[This message has been edited by Genghis the Engineer (edited 16 January 2001).]
V2, Take-off safety speed' means a referenced airspeed obtained after lift-off at which the required one-engine-inoperative climb performance can be achieved. (From JAR-1, definitions)
The rotation speed, VR, in terms of calibrated airspeed, must be selected by the applicant and must not be less than the greatest of the following:
(i) V1; or
(ii) 1.05 VMC determined under JAR 23.149 (b); or
(iii) 1.10 VS1; or
(iv) The speed that allows attaining the initial climb-out speed, V2, before reaching a height of 35 ft above the take-off surface in accordance with JAR 23.57 (c)(2).
(From JAR-23, the usual light aircraft standard)
The take-off safety speed, V2, in terms of calibrated airspeed, must be selected by the applicant so as to allow the gradient of climb required in JAR 23.67 (c)(1) and (c)(2) but must not be less than 1.10 VMC or less than 1.20 VS1.
(Also from JAR-23, 23.67 is very complicated but depending on conditions the minimum climb gradient is generally between 1 and 2%)
And if you thought that JAR-23 was complicated, try the following from JAR-25, the airliner standard...
) V2MIN, in terms of calibrated airspeed, may not be less than--
(1) 1.13 VSR for--
(i) Two-engined and three-engined turbo-propeller powered aeroplanes; and
(ii) Turbojet powered aeroplanes without provisions for obtaining a significant reduction in the one-engine-inoperative power-on stall speed;]
[(2) 1.08 VSR for--
(i) Turbo-propeller powered aeroplanes with more than three engines; and
(ii) Turbojet powered aeroplanes with provisions for obtaining a significant reduction in the one-engine-inoperative power-on stall speed: and
(3) 1.10 times VMC established under JAR 25.149.]
(c) V2 in terms of calibrated airspeed, must be selected by the applicant to provide at least the gradient of climb required by JAR 25.121(b) but may not be less than--
[(1) V2MIN;
(2) VR plus the speed increment attained (in accordance with JAR 25.111(c)(2)) before reaching a height of 35 ft above the take-off surface; and
(3) A speed that provides the manoeuvring capability specified in JAR 15.143(g).]
Commenting upon what other people have said before. V2 is the speed that should be established by a given height, which varies according to conditions (although 35ft seems normal). This is a separate (but inevitably related) issue to the take-off distance, which is the distance to reach the screen height, which is (in my experience) usually 15m (or 50ft) using the recommended take-off speeds and methods.
Changes in screen height are unlikely to be due to ground effect (which is generally considered to become negligible at about 2 wingspans), and would probably be associated with a matched increasingly stringent set of requirements for airport approval.
G
[This message has been edited by Genghis the Engineer (edited 16 January 2001).]
Guest
Posts: n/a
LetsFetz,
Going out on a limb here, but I think the 'screen height' figure is just an arbitrary number to provide wingtip clearance. Engine-out procedures may require a slight bank into the 'good' engine side, and the 35' is the nominal height required.
(Of course, I could be wrong...!)
Going out on a limb here, but I think the 'screen height' figure is just an arbitrary number to provide wingtip clearance. Engine-out procedures may require a slight bank into the 'good' engine side, and the 35' is the nominal height required.
(Of course, I could be wrong...!)
Guest
Posts: n/a
Screen height may be reduced to 15' when wet. This takes into account the reduction of V1 on wet runways, with an engine failure at V1.
The decrease in acceleration with an engine inop at an earlier speed accounts for the decrease in screen ht.
If a turn is required after T/O with an engine inop, a 50' screen ht is required.
The decrease in acceleration with an engine inop at an earlier speed accounts for the decrease in screen ht.
If a turn is required after T/O with an engine inop, a 50' screen ht is required.
Guest
Posts: n/a
Not sure about 35'-50'. Does not matter (actually I have no idea) Ground effect is a function of wingspan. V2 will give you the best ANGLE of climb at set conditions with a engine failure. Hope this helps
j
------------------
Here's to cheating, stealing, fighting, and drinking.
If you cheat, may you cheat death.
If you steal, may you steal a woman's heart.
If you fight, may you fight for a brother.
And if you drink, may you drink with me.
j
------------------
Here's to cheating, stealing, fighting, and drinking.
If you cheat, may you cheat death.
If you steal, may you steal a woman's heart.
If you fight, may you fight for a brother.
And if you drink, may you drink with me.
Guest
Posts: n/a
Concur with quid, V2 is a compromise value.
Screen height is all about obstacle clearance and nothing to do with ground effect. The rules on airport licensing are tied into screen height - they require a certain clearway, that co-ordinates with the take-off requirements (plus obviously a fairly hefty safety factor).
When I did my aero degree, we did a lot of work on take-off performance, but I can't say I recall ground effect ever being taken into account at-all. In my personal experience, ground effect is very much a function of crosswind, surface conditions, etc. (this is personal, I've never read this in a textbook) and it's not safe to take-it into account.
G
Screen height is all about obstacle clearance and nothing to do with ground effect. The rules on airport licensing are tied into screen height - they require a certain clearway, that co-ordinates with the take-off requirements (plus obviously a fairly hefty safety factor).
When I did my aero degree, we did a lot of work on take-off performance, but I can't say I recall ground effect ever being taken into account at-all. In my personal experience, ground effect is very much a function of crosswind, surface conditions, etc. (this is personal, I've never read this in a textbook) and it's not safe to take-it into account.
G
Guest
Posts: n/a
As has been posted, V2 is below Vmd on (big Boeings at least). That's why there is such a thing as "improved climb" or "overspeed" to enable you to use surplus field length to go faster and improve your 2nd segment climb and hence lift more weight out of the field.
As I understand it, screen heights are just a number agreed upon by ICAO to ensure that you are actually in the air at the end of the certified distances, a fiddle factor if you like. I don't think there's any magic in the 35ft, 50ft or 15ft.
As I understand it, screen heights are just a number agreed upon by ICAO to ensure that you are actually in the air at the end of the certified distances, a fiddle factor if you like. I don't think there's any magic in the 35ft, 50ft or 15ft.
Guest
Posts: n/a
As was mentioned earlier, V2 is a mathematical function of Vstall and VMCA. It is derived in oredr to give a minimum safe climb speed for the second sector climb with one engine inoperative. That climb may be performed with up to 5 degrees aob into the live engines, as allowed for in the derivation of VMCA.
The difference between the 35ft and 50ft screen heights is historical, and largely irrelevant. Basically, aircraft certificated by the CAA/FAA before a certain date (sometime in the early '60s, I think) were required to achieve a 50ft screen height by the end of the TORA. After that date, and no doubt because of manufacturer pressure, the screen requirement for large transport aircraft (jet or prop) was reduced to 35ft. The 15ft screen in the wet is a pragmatic reduction to allow for the possibility of an earlier 'go' decision (at wet V1), and the consequent extended period of reduced acceleration to VR due to both power loss and water drag.
Ground effect is no factor in any of this, as it relies on demonstrated performance, factored for safety.
The difference between the 35ft and 50ft screen heights is historical, and largely irrelevant. Basically, aircraft certificated by the CAA/FAA before a certain date (sometime in the early '60s, I think) were required to achieve a 50ft screen height by the end of the TORA. After that date, and no doubt because of manufacturer pressure, the screen requirement for large transport aircraft (jet or prop) was reduced to 35ft. The 15ft screen in the wet is a pragmatic reduction to allow for the possibility of an earlier 'go' decision (at wet V1), and the consequent extended period of reduced acceleration to VR due to both power loss and water drag.
Ground effect is no factor in any of this, as it relies on demonstrated performance, factored for safety.