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

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 !!!

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.

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

standing by ...

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

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.

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.

... thanx quid

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

cheer and beers
LF

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.

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

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.

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

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)

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