Similar questions have been asked before, but in life-and-death questions like this, I want to be certain. This question is about airplane behavior close to the ground in normal and/or near slow flight.

I know that based on the angle of attack, a stall can occur at any airspeed. Alas, what should/does the IAS measure? Presumably, it is from a pitot tube extending from the wing, straight into the relevant wind that is to fly over the wing, so if the angle of attack becomes very high, presumably the IAS dives, too.

For a small 2-seater or 4-seater with a modern wing (like a Diamond or Cirrus or Flight Design), is an indicated airspeed of 10 knots above stall speed a sufficient guarantor for not stalling?

For example, in a slip near the ground, if my configured stall speed is 60, and I fly it so that my IAS steadily drops down to 70 in my uncoordinated slip (at which point I relax back pressure to keep IAS at 70), am I then assured that the airplane cannot enter a stall?

  • $\begingroup$ You should read How it flies?. It provides insight about flight dynamics and speak about IAS. $\endgroup$ – Manu H Aug 25 '19 at 17:56
  • $\begingroup$ I changed the title to be more specific and to be a question. Fell free to re edit it if it does not fit with what you meant (in which case you may consider editing your question to be more clear) $\endgroup$ – Manu H Aug 25 '19 at 17:59
  • $\begingroup$ it's a good title that describes what I ultimately want to know. $\endgroup$ – ivo Welch Aug 26 '19 at 18:30

When I side slip my glider "correctly" the airspeed indicator drops to zero or at least shows large fluctuations between 30 to 90% of the actual airspeed. So the airspeed indicator essentially becomes worthless during a large side slip and can't give any estimate whether the aircraft is approaching a stall or not. And I wouldn't rely on it to judge if the aircraft is approaching a stall. I can somewhat judge the speed from the sound of the wind and also how much rudder input creates what kind of side slip (slow usually creates more slip angle for full rudder deflection) and also the back pressure needed but the best indicator for the approaching stall is still the elevator buffeting or an angle of attack indicator or angle of attack warning. If you're planning on doing short field landings with low approach speeds then an angle of attack indicator is probably a good investment.

From a physical standpoint the indicated airspeed in a side slip condition should always be less than or equal to the airspeed that you're actually flying. So if you see an airspeed indication in a side slip configuration you're going to have at least that figure as actual airspeed. This is because the airspeed indicator just shows the difference between atmospheric pressure and total pressure (sum of dynamic and atmospheric pressure). And if you rotate your pitot tube from 0 degrees to 90 degrees off the direction of incoming air it will show less and less dynamic pressure and work more like the static port, thus the readout drops. But it can never show more than what the dynamic pressure actually is (disregarding small vibrations due to unstable inflow). So if you look at the airspeed indicator and judge by the speed it reads you should be on the safe side.

Whether or not the 10kt increment is enough depends on how steep your side slip is and what you are flying. Ground effect could also influence how much angle of attack is left before the stall. Only a numerical simulation would give you an exact number here.

But you can always experiment high up in the air, do a side slip down to what feels as low as you would want to go for a safe landing and then pull up even more and see how far you can go without it stalling or see how it departs (relax controls immediately to avoid any nasty spins) if it actually does anything. Maybe it will show some significant buffeting before it actually stalls (great indicator!), maybe it will just lower the nose (not ideal close to the ground but still safe) or maybe it drops a wing (not so good, add more safety margin).

  • $\begingroup$ I think the short answer in here is that IAS is not a good indicator of AoA. (I read up...it requires a second pitot hole to get an AoA.) Thus, in the absence of an AoA in the airplane, the only warning may well be the stall buffeting for a well-designed aircraft, and near-nada for a more dangerous one. true? if so, please add for marking this question as answered. $\endgroup$ – ivo Welch Aug 26 '19 at 5:27

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