V2 - reference zero = 35' or 50'

[LetsFetz]

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

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Let there be cold beers on a hot day !!!

[askop]

Correct me if I`m wrong, but:

Depends on what you fart,

35ft for JET-A1
50ft for 100LL

[LetsFetz]

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 !? http://www.pprune.org/ubb/NonCGI/confused.gif

cheers and beers
LF

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Let there be cold beers on a hot day !!!

[This message has been edited by LetsFetz (edited 16 January 2001).]

[quid]

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).

[411A]

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.

[NIMBUS]

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.

[LetsFetz]

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

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Let there be cold beers on a hot day !!!

[CaptainSandL]

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

[411A]

The 50 foot screen height was a requirement for recip transport catagory aircraft certificated under CAR4b. QUID is quite correct. Notice that many on this forum are junior guys. This is good as they will (hopefully) learn from the seniors.

[LetsFetz]

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

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Let there be cold beers on a hot day !!!

[This message has been edited by LetsFetz (edited 16 January 2001).]

[Genghis the Engineer]

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).]

[LetsFetz]

Thanx Genghis

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Let there be cold beers on a hot day !!!

[mutt]

LetsFetz

I dont know of any connection between the V2 screen heights and ground effect.

Ground effect is usually 1.5 times your wingspan, therefore for most jet aircraft it will be a lot greater than 50 feet.

Mutt

[NIMBUS]

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...!)

[olivasnooze]

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.

[before landing check list]

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

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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.

[quid]

BLCL-

Nope. V2 is a compromise between getting off the runway and missing the obstacles (if any). Best angle of climb is much faster.

[Genghis the Engineer]

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

[mustafagander]

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.

[scroggs]

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.

[before landing check list]

Isn't missing the obstacles (assuming straight out departure) a function of angle of climb? I know that best angle per se is much faster. But INITIALLY with a engine failure V2 will give you in initial best angle till you get things sorted out after the engine problem.
j

[LetsFetz]

Interesting ... isn't it?!

Seems to be one of those why is it that green = go and red = stop ...

about 20 answers and almost 20 different ones ... well, I am glad I could raise some thoughts at least ...

I've thought so much about it meanwhile, that I'm now totally confused ... but I tend to believe in the "no significance" theory ...

looks like it's really all about clearing obstacles only and establishing TORA when testing and factoring with a nominated screen height to make sure the machine is actually airborne and clear of a certain reference height ...

thanx everybody ... I'd be still glad if you bring it on ...

cheers and beers
LF

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Let there be cold beers on a hot day !!!

[mountain man]

I thought it was 35' in straight flight and 50' in a turning flight path.
My Bible on this subject arrives tomorrow morning with the rest of my personal effects...I'll get back to ya!!

MM.

[mustafagander]

Mountainman,
That's how it is in Oz. CAO 20.7.1b which we all know and love.

[LetsFetz]

standing by ...

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Let there be cold beers on a hot day !!!

[Roman]

So is the term "screen height" improperly used when referring to achieving V2, ie should it only be used in relation to Take Off Distance?
Fascinating discussion, guys - my ignorance of Perf never fails to surprise me.

[quid]

LF-

20 different answers? Heck, the way I read them, we pretty much all agree:

1. Screen heights have nothing to do with "ground effect".

2. Screen heights are arbitrary values set forth for certification. FAR 121.177 (a),(2) spells out the 50' requirement for recips. For turbojet a/c certified after August 26, 1957 (SR422), FAR 121.189 (d)(1), requires only 35 feet.

3. For any given weight, a lesser flap setting or a faster speed will yield a steeper climb gradient.

4. V2 is NEVER the best angle of climb speed. A lower V2 will get you off the runway in less distance, and you might need that to miss a "close-in" obstacle, but you'll sacrifice gradient capability to do so. (BLCL, are you listening? <grin> ).

5. FAR 121.189 (f) assumes the turbojet a/c will not be banked below 50' for purposes of this discussion.

Roman- The takeoff run is not complete until the turbojet has reached 35'. At that height it is assumed the a/c has accelerated to V2. They are tied together.

[LetsFetz]

... thanx quid

still don't agree, that nearly everybody agrees ...

cheer and beers
LF

[carpe.d.m]

KISS

35 foot screen height for climbing straight out i.e. nil turning flight path.

50 foot screen height for a turning flight path.This to allow 15 feet for the downward wing resulting effectively in 35 foot clearance.

[KIFIS]

The practice I used for many years when a balanced field length was involved on a dry runway was to extract the V1 from the performance card and then expect to be 35’ over the end of the runway if an engine failed at V1. If the runway was wet and the same balanced field length was involved ( making stopping more critical ) then I reduced the V1 by a maximum of 10 kts. This now means that if an engine failed at the reduced V1 the aeroplane is required to accelerate on the ground on three engines to VR. The result is not 35’ at the end but 15’. The company I worked for fully approved this procedure and considered it the Captains’ prerogative. I've often wondered if this procedure would work for the " Big Twins "?

KIFIS

[mountain man]

LetzFetz,
Yup ANO 20.7.1b is the one, 50' in turning flight when more than 15 degrees of heading change.
V2 has nothing whatsoever to do with ground effect.
V2 must not be less than 1.2 Vs and 1.1 Vmca.
For field length considerations V2 will often be chosen as low as possible with the aforementioned limits in mind but not always as it's the climb speed for the first and second segments with one(or more) engines inoperative.For minimising field length requirements it can be slower than best climb gradient speed.
V1 and Vr also have an impact on V2 as V1 is effected by weight, runway length and runway condition ie wet/dry and Vr cannot be less than V1 nor greater than V2.Vr is also effected by VMU and sometimes tyre limit speeds. Vr is a speed which allows attainment of V2 at 35' on one engine or slightly exceed V2 on two engines if rotation is carried out at the proper rate.

MM.

[mutt]

KIFIS,

The answer is NO, your method wont work for the big twins. They are generally limited by the One Engine Accelerate Go, whereas your 4 engine aircraft is limited by the 1.15% all engine distance.
You are therefore able to apply a general Vspeed decrement, they are not.

Mutt

[Engineer]

Believe 50' height was applied to wartime aircraft When performance was calculated only on a balance field length in the post war era

In the early fifties 35'height introduced when performance calculations took into consideration unbalancd field length (i.e clearways and overrun)

[LetsFetz]

You got it MountainMan

20,7,1B is the one ... that's where I picked up the bug with the ground effect, too

" ... the a/c has to be able to climb at V2 and out of ground effect"

that was I think the wording there ... might have mixed it up a bit ...

definitely an interesting and educational CAO, that 20.7.1b

cheers all
LF