A321 green dot speed (again)

[Lotetu]

Good morning,

on A321 the green dot speed should be increased by 1 Kt/1000' above FL200. My observation is that the actual rate of the change is higher, about 1.3 Kt/1000'

On A320 I never saw such.

Does anyone have information about this?

To minimize the effect of the fuel change I prefer to watch it during descent instead of climb.

Another question about: if the clean wings are the same on 320 and 321 than why they have different green dot speed for the same weight?
They match at 60 tons, but the rate of change by the weight is different.
4 Kt/2tons on 320 and 3Kt/2tons on 321.

Thank you for the replies

[Cough]

321 clean wing isn't the same as the 320 wing. If my memory serves me right, there is another 4 square metres of wing area due to reprofiling of the inboard wing trailing edge. As for the GD speed change, I've never been worried enough to watch it that closely...

[Owain Glyndwr]

I'm afraid that isn't true - apart from minor local strengthening to cater for the extra MTOW the A320 and A321 wings are identical.
The green dot speed is the speed at which one gets best lift/drag ratio (clean aircraft).
Best L/D is at a speed where zero lift (profile) drag equals drag due to lift.
The A321 fuselage is 7m longer than the A320, leading to a higher value of profile drag and therefore a higher lift dependent drag for best L/D. (Actual value of the best L/D will of course be lower on the A321). This in turn means that green dot conditions are found at a higher lift coefficient on the A321 than the A320 so that with the same wing and at the same weight the green dot speeds will differ.

[Cough]

Owain, There is a reference to the above on the Wiki entry for the A321...

[Owain Glyndwr]

Cough, you are correct, but that same Wiki entry gives a table (which I used as reference) quoting both aircraft with the same wing area of 122.4 m^2.
Other references I have checked also give the A321 wing area as 122.4 m^2. Maybe it is a case of keeping the reference wing area the same even though there have been minor changes.
Whatever, I still believe that the basic reason for differences between A320 and A321 green dot speeds is an increase in zero lift drag, to which the increased wing area skin friction would contribute

[Lotetu]

Thanks Owain,

sounds good. Would be nice to see how it is on 319 and 318, but I'm not flying them.

About the 1 kt/1000' (or more) I think that it is the reason that the 321 looks heavier at high altitudes than it is.

On the 320 if the weight entry is correct you can fly 2000' above the FMGC calculated optimum FL, on the 321 it is maximum 500'.
On 321 the max recommended FL is unreachable.

[vilas]

Also A320 GD speed with IAE differs from CFM by 5kts.

[Lotetu]

Thanks Vilas, I heard it but I never flew with CFM. All I wrote was about IAE.

[IcePack]

Trouble is green dot on the A321 is often at the top of VLS. So think the idea of best lift drag went out the window on that type.

[Sidestick_n_Rudder]

Trouble is green dot on the A321 is often at the top of VLS. So think the idea of best lift drag went out the window on that type.

Well, it still is best lift/drag AVAILABLE and consistent with appropriate safety margins...

[vilas]

Sidestick_n_Rudder
What you say is only half true because at altitude although VLS remains same Valphaprot is g dependant and can overtake VLs triggering protection.