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Can any pilots on here give me a simplified explanation of Q corner? I was talking to a couple pilots last night that were trying to explain it, but they were getting very technical. The conversation was brought up when someone started talking about "doctor killers"(no offense to any doctor pilots ). They were referring to high performance pistons that can fly at high altitudes, and how a lot of these GA pilots don't have the experience or understanding to fly in that environment.
My understanding was that this phenomenon really only affected high performance jets flying at very high altitudes. Is this correct? The U2 comes to mind. I know it has to do with air density, but I'm not clear on the IAS, TAS, and Mach part and how it affects weight.
...as your altitude increases, your minimum allowable speed increases and your maximum allowable speed decreases. Where these two speeds join is the Coffin Corner.
Minimum speed is where the airflow separates from the top of the wing (Stall Speed). Maximum speed is where the airflow on the top of the wing exceeds the speed of sound causing nose-down pitching moment(Mach Tuck). Only affects jet aircraft or very fast propeller airplanes.
You don't really need to know about IAS, TAS & Mach to understand it. Mach is just a percentage of speed of sound. True airspeed is Indicated airspeed corrected for air density.
I'm not sure why the piston pilots were talking about Qc. The critical mach (Mcrit) for a common general aviation NACA airfoil is around 0.7 M with a straight, unswept wing. At 20,000 feet that's around 320 knots indicated airspeed, well above the stall speed of any aircraft in question.
Mcrit is based on level, undisturbed, 1-G flight, and is a function of the wing's lifting force; raising the required lift (weight) makes the Mcrit goes down. Stall speed goes up with weight as well. So the worst condition for a U-2 pilot would be early in the mission (heavier with fuel) with a low Mcrit AND a higher stall speed. So much so that the two approach each other. A touch of turbulence or a banked, level turn may effectively add to the weight of the aircraft (reality, it forces the wings to work harder) and the resultant higher lifting force may reduce Mcrit to the stall speed (which also rises as a function of load factor, or G), resulting in an accelerated stall.
There's a pretty good diagram of the Qc for the U-2. I didn't know their stall speed (indicated) was so slow, but watching them land at Patrick AFB in the 80s, they WERE pretty slow.
"...as your altitude increases, your minimum allowable speed increases and your maximum allowable speed decreases. Where these two speeds join is the Coffin Corner."
Thank you. I know it's more involved, but that is a good simplified explanation.
"[quote=SluggoF16;38803130]I'm not sure why the piston pilots were talking about Qc. The critical mach (Mcrit) for a common general aviation NACA airfoil is around 0.7 M with a straight, unswept wing. At 20,000 feet that's around 320 knots indicated airspeed, well above the stall speed of any aircraft in question."
They were airline pilots. The conversation turned to high performance GA aircraft that have a higher ceiling than other GA aircraft. A Mooney Acclaim is a good example. I don't think many pilots actually fly these types of aircraft at that altitude though.
"There's a pretty good diagram of the Qc for the U-2. I didn't know their stall speed (indicated) was so slow, but watching them land at Patrick AFB in the 80s, they WERE pretty slow."
I've seen this diagram for the U2. I wasn't sure if it was legit because those speeds seemed really low.
It's interesting to see the corner tighten on the speed tape as you climb. You'll notice the barber pole (Mmo) slowly creep down, while the low speed awareness tape creeps up. Climb high enough and there isn't much space between the two. In addition, the pli, pitch limit indicator, will pop into view on the pitch ladder as you get into the corner. Say you are cruising at a very high altitude for your weight. The pli will be there, but as you burn fuel and get lighter, the pli will rise higher on the pitch ladder, eventually disappearing; an indication you can likely climb higher for your new lighter weight. On a long flight where range is an issue for the fuel onboard, we'll fly as high as we can for our current weight to save fuel, which can be tucked up near the corner. This is only feasible in smooth air. Encountering turbulence in the corner has the potential to turn bad. The airspeed fluctuations can easily go from stall to overspeed and back without allowing you room to compensate with thrust adjustments. So if we encounter turbulence in such a scenario, the only option is to descend to increase our buffet margins. If turbulence is expected at that altitude, flight near coffin corner isn't an option.
I'm taking a guess here... if you have to nose down to recover from a stall. Nose down makes you go faster. You are already going close to never exceed speed. So it'd be pretty easy to go faster than you should in attempt to recovering from a stall at high altitude. is that close?
I'm taking a guess here... if you have to nose down to recover from a stall. Nose down makes you go faster. You are already going close to never exceed speed. So it'd be pretty easy to go faster than you should in attempt to recovering from a stall at high altitude. is that close?
Very close. One other thing to keep in mind, Mcrit is also a function of coefficient of lift (or CL). Higher CL, for most lower-speed (subsonic) airfoils, results in lower Mcrit. So a slight unloading of the aircraft (a reduction in angle of attack and a corresponding reduction in CL), which is what one does in an aerodynamic stall recovery, has the added benefit of temporarily increasing Mcrit. However, the improvement in performance is only transient. Eventually the aircraft will probably accelerate to Mcrit, the pilot will arrest the descent with an increase in angle of attack/CL, and the whole scenario starts over, maybe even faster than before, as stated in the previous post. The solution would be to gradually level off with a slight power reduction or drag increase, with a lot of finesse.
It's interesting to see the corner tighten on the speed tape as you climb. You'll notice the barber pole (Mmo) slowly creep down, while the low speed awareness tape creeps up. Climb high enough and there isn't much space between the two. In addition, the pli, pitch limit indicator, will pop into view on the pitch ladder as you get into the corner. Say you are cruising at a very high altitude for your weight. The pli will be there, but as you burn fuel and get lighter, the pli will rise higher on the pitch ladder, eventually disappearing; an indication you can likely climb higher for your new lighter weight. On a long flight where range is an issue for the fuel onboard, we'll fly as high as we can for our current weight to save fuel, which can be tucked up near the corner. This is only feasible in smooth air. Encountering turbulence in the corner has the potential to turn bad. The airspeed fluctuations can easily go from stall to overspeed and back without allowing you room to compensate with thrust adjustments. So if we encounter turbulence in such a scenario, the only option is to descend to increase our buffet margins. If turbulence is expected at that altitude, flight near coffin corner isn't an option.
What aircraft are you describing here? Apparently an airliner of some sort?
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). They were referring to high performance pistons that can fly at high altitudes, and how a lot of these GA pilots don't have the experience or understanding to fly in that environment. 
