AFM Stall Speed vs. AoB Graph

[ROB-x38]

As the AoB increases the value of the stall speed read off the graph differs more and more with the calculated stall speed value based on the load factor.

My question is.......Why?

Is this simply due to pressure / position / instrument errors associated with the ASI? Would I be correct in saying that if the ouput of the graph was KCAS the values should agree exactly with the formula?

Cheers

[john_tullamarine]

.. some sample data might help you get a response ...

[ROB-x38]

Okay John i've got the camera out... :}

Graph (http://www.imagestation.com/mypictures/inbox/view.html?id=4165425950&url=http://www.imagestation.com/picture/sraid121/p92fbc90c3b89606f1c1482a3096cd090/f8475b1e.jpg&caption=stall%20speed%20vs%20aob)

Take a low AoB -

eg: AoB = 20 degrees, flaps up, MTOW, stall speed = 59 KIAS which is equal to 57 * square root [1 / cos20 ]

Take a higher AoB -

eg: AoB = 60 degrees, flaps up, MTOW, stall speed = 87 KIAS which is not equal to 57 * square root [2]

[john_tullamarine]

... tad rugged for old eyes ... could I be a pain and ask for you to email a copy to me, please, so that I can blow it up a bit ...

[Mad (Flt) Scientist]

Is it possible that the aircraft has an underlying stall speed variation with Mach number. Either through SPS functionality with Mach number or direct aerodynamic effect of Mach.

Then it's not just a case of applying the square root of load factor - you actually may have a (slightly) lower stall AoA at higher Mach, and hence a slightly different stall speed on which you have to base the *sqrt(nz) part of the equation.

There may be little or no effect at lower bank angles because the Mach is sufficiently low that there is no variation, but higher bank angles (and hence speeds) do introduce a Mach effect.

edit: on inspection, the flap 40 data are closer to agreeing with the classical relationship (to the accuracy one can read the graph) certainly to 50 degrees of bank at least; the two lines can be seen to be different in shape, too

[john_tullamarine]

.. also, not having a copy of the relevant POH, could you post a copy of the PECs as that may be relevant to the discussion - not much point doing the sums on IAS.

[ROB-x38]

G'day John

Not sure how to attach files using the pprune email. Here's a copy of the PEC chart (http://www.imagestation.com/mypictures/inbox/view.html?id=4164673374&url=http%3A%2F%2Fwww.imagestation.com%2Fpicture%2Fsraid122%2 Fp316092ef7d7a8912b603bdd8781bc0ae%2Ff83bdf5e.jpg&caption=pecs). If you click "original image" the bigger shot should show up.

Mad Scientist, given that 140 KIAS at sea level is only a shade over M0.21 would mach no. effects be significant? I don't know anything about the variation of stall speed with mach no. - could you expand a bit?

Cheers :ok:

Rob.

[Mad (Flt) Scientist]

Although the aircraft Mach number is relatively low, that doesn't necessarily mean that the Mach number in the accelerated flow near the leading edge is equally low; you can start getting Mach effects on stall aerodynamics at surprisingly low speeds.

On "our" products - which admittedly are swept wing, which the subject at hand isn't - we see an effect on stall angles and CLmax starting at M0.20 or earlier. It's not a minor effect, either - the stall angle can reduce by as much as 25% by Mach 0.30-0.35.

It all rather depends on your stall mechanism; if you're tripping the flow artificially you'd expect that to conceal any (or at least some) Mach effect. If you've got a completely clean natural stall then you will likley see something.