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cameljockey
9th Sep 2012, 04:55
Hi,

Can anyone explain to me why GD speed for a 330 on single engine is 10kts less than normal GD, given that GD is based on weight and altitude.

Would be much appreciated.

Regards

145qrh
9th Sep 2012, 06:55
I was told by an EK instructor that it was due to the fact that SE G.DOT does not assume ice accreation, but to be honest that sounds like complete rubbish. So I would assume it's because when SE you have a fair bit more drag due to a windmilling big fan and therefore best L/D will be a bit slower because of this.

SloppyJoe
9th Sep 2012, 07:45
I was under the impression that all engine green dot was artificially inflated by 10kts so as to keep speed a bit further from VLS and be a bit more speed stable.

JABBARA
9th Sep 2012, 12:43
Hi CJ

As we know green dot is the L/D max point on total drag curve.
That is the the point where the drag is minimum for a given weight. For a given weight the total drag is calculated as a sum of parasit drag (=profiile drag) and induced drag. The Parasit drag curve shape is different for dual and single engine case. In the single engine case, the parasit drag increase with increasing CAS, is more rapid and sharp. Eventually this moves total drag curve to left relative to dual engine. The result is L/Dmax point (green dot speed) corresponds to less speed than dual engine.

cameljockey
9th Sep 2012, 12:53
Jabbara,

That makes a lot of sense, having just drawn it all out.
Thank you very much.

As an aside, why doesn't Airbus include this with the A320 or A340?

CJ