Drag curve and Vref.
 

Do modern commercial transport aircraft operate on the back side of the drag curve during the approach or are their typical approach speeds, say 1.23x VS1G/ 1.3x VS0 + wind corrections, found at the front or bottom of the curve, rather than the back?

How does flap affect the curve itself? I know I should remember this from my ATPL studies but I don't. :rolleyes:

If your flying on the back side of the drag curve then pitch and speed are backwards. What I mean is that speed is controlled by pitch and altitude via power/thrust. On jets Thrust controls speed and pitch controls altitude which means it would be on the positive side of the curve.

That was my understanding, but Im welcome for some one to correct me if I'm wrong.

On second thoughts, I withdraw.

25.173 and 25.175, the regulations on static longitudinal stability, repeatedly mention the need to demonstrate a stable relationship of the controls, and impose limits on the free return speed as well. 25.173(d) appears to give a mild alleviation, as it talks of not returning to trim being acceptable provided the achievement of the desired trim speed doesn't require "exceptional attention" from the pilot. But that's about all the alleviation there is.

Operating well on the backside of the power curve would appear to make meeting the requirements a challenge - the 173(d) alleviation probably allows operation at the bottom of the curve but wouldn't help a lot once well on the back side of the curve.

So, barring either a specific alleviation for given type, or a fairly sophisticated control scheme, the regs seem to written to force operation no further back than the bottom of the curve.

When landing Config 3 in an A320 series aircraft, the VAPP is always lower than F speed. I always wondered if that meant the aircraft was flying behind the drag curve.

Many thanks for the replies.

I found the following Airbus document. Page 5 seems to show it quite clearly.

http://www.airbus.com/fileadmin/medi...APPR-SEQ03.pdf

It states that the minimum thrust speed usually equals about 1.35 to 1.4 V stall in landing configuration.

It also states that the minimum final approach speed is slightly on the back side of the power curve.

At low levels, Embraer's E-jets have 'Green Dots' at minimum drag speed. On approach, Vapp is typically very similar to Green Dot and Vref is normally five knots or so less. Also, should you ever find yourself in a position where you just have to land NOW, then treat Green Dot as Vref and fly it in.

PM

In the back side of the curve, basic longitudinal stability is still the same.

It is speed stability that becomes less and less stable and then in the back of the curve becomes slightly unstable, and then more and more as you get into it.

To my knowledge, most jet airliners have VREFs in the backside of the curve, but only slightly. That is why sometimes, with manual thrust, if you see speed decaying and take too long to fix it, speed decays more, and then you add some more thrust, but if you are too slow it is not enough and in this way you can end up with maximum thrust and still some knots below VREF. It is because of that speed instability. The way to fix that is to promptly add enough thrust to reverse the trend. THen the instability will play for you.

Interesting. They provide a graph of "thrust required" and then refer to it as the 'power curve'. Seriously - what am I missing here?

Possibly just a language error?

I suppose 'thrust required' makes sense in a jet or?