jungle

Got it, if I am flying in an aircraft with defective software and or an inoperative sensor(s) and decide to fly near stall speed at 40,000 feet this info will come in handy.

To the OP in this thread, the answer is still no change with altitude for all practical purposes, as far as light GA aircraft go.

Having flown a variety of swept wing military aircraft into and out of stalls at altitude many times, I will tell you from a practical standpoint that I never looked at the airspeed but flew the entry and exit based on buffet and performance. Ignorance on my part was bliss in this case.

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joepilot

One thing that I notice missing from this discussion is the effect of c.g. on stall speed. For any given flight conditions, stall speed will always be higher for a forward cg than for an aft cg.

Joe

shdw

Maybe I missed this but scanning through it seems like this has been made incredibly complicated when it is fairly easy.

lift = 1/2 (p v2 A Cl) where p = air density. As air density goes down for the given altitude total lift possible will go down. CL is limited by AOA. Lift has to remain equal to weight so put the aircraft at say 50 knots (assume it is at stall) at critical AOA with lift and weight both equaling 3000 pounds.

Now take this aircraft and increase its altitude which lowers its total lift, lets say 2800 just to give it a number. Now we are already at critical AOA, the area of the wing can't change, and this leaves only speed left. We must increase our speed in this situation to regain 3000 pounds of lift to equal weight.


I have never read this airflow separation from thinner air before till it was posted here. This obviously doesn't mean it's not true, but I would love to know more if someone can break it down barney style for me. Thanks.

joepilot

Shdw,

The question is about indicated airspeed. The formula you are quoting uses true airspeed.

Joe

shdw

Joe,

I am talking to those that have been talking about CAS/TAS/EAS and going into airflow separation differences, pressure changes, and decreased resistance in the boundary area merely to describe a simple stall speed increase with altitude (actual speed). If I am not mistaken the question was answered on the first reply and since the topic has veered off.

KIAS doesn't change period, the designers figured that would make sense. Landing at an airport at 10,000 feet would still be approach at 60 in 172, the actual aircrafts speed is however faster. Who cares whether you call that actual speed CAS/EAS/TAS? Sure EAS is the most realistic or "accurate" aircraft speed. In actuality that argument was merely a search for the most accurate aircraft speed and a distraction from the point, actual speed goes up with altitude and indicated does not. This applies across the board for all, with few exceptions as always, other V-speeds.

For us piston guys, EAS is worthless and accounts for at most a knot difference.

~Brian

Edit: PS that whole bit wasn't to you Joe, only the explaining that I was replying to other posts not the original topic. After that is just my opinion on the original topic.

ryan1234

Indicated stall speed does change because CLmax decreases (i.e. critical AoA) with an increase in altitude, thus a higher stall "speed".

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shdw

Will redo this later I need to do some research on CLmax. You are wrong about indicated airspeed though, the formula is regarding actual not indicated airspeed. That is why the pitot tube is hooked to the static system.

ryan1234

sorry we're boring you

I'm not sure what formula you're talking about. Some engineering textbooks and other aerodynamic books state a little about fluid dynamics (this is after all what we're talking about, when we talk about a "stall" and "lift"). A stall is when a fluid (in this case air and assuming a prior force 'lift' was created) in the boundary layer does not have enough energy to overcome the adverse pressure gradient (or reverse flow). Fluid forces depend on these variables:

(a) mass density
(b) a characteristic area
(c) velocity squared
(d) Reynolds Number (Re) (viscosity)
(e) Mach No (M) (compressibility)

When engineers are windtunnel testing airfoils, they pay special attention to Reynolds numbers because when they vary, they will give different values in CLmax and thus critical AoA. Mach number is not as important to GA aircraft (pertaining to stalls). Reynolds is generally for lowspeed, Mach is for higher.

Reynolds is basically the ratio of inertia to viscosity, especially seen in the boundary layer. Basically it describes part of how much energy is in the boundary layer (and x-amount of energy is needed to overcome the adverse pressure gradient).

Reynolds has a 2D and 3D effect on a stall:
The 2D effect is that a larger Re value means a greater CLmax value, and a higher AoA where it occurs - because it changes the shape of boundary layer (less or more molecules 'stick' to wing).
The 3D effect is not really relevant to what we're talking about and pertains more towards high-aspect ratio design (depends on planform).

Absolute viscosity is a function of temperature and independent (practically speaking) of pressure.



Indicated airspeed measures stagnation pressure (dynamic / static pressure). Pressure can be constant, but if CLmax changes, the required pressure changes as well resulting in a different indicated stall speed.

To break it down simple:

Lower Reynolds = Lower CLmax value = Lower critical Angle of Attack = Higher required pressure = higher indicated stall speed

Most of you may think this is all just stupid - making things too technical blah blah blah

...But stuff like this explains how vortex generators and some other high-lift devices work. As VGs provide turbulent (high energy) flow to the boundary layer delaying seperation, etc.

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shdw

The formula for lift I used in my original post, read that post through and you will see what I mean I think.

Here so others can see the formula:

Re = (qVL/μ)= VL/v = QL/vA

where:
V is the mean fluid velocity (SI units: m/s)
L is a length of the object that the flow is going through or around (m)
μ is the dynamic viscosity of the fluid (Pa·s or N·s/m²)
ν is the kinematic viscosity (ν = μ / ρ) (m²/s)
is the density of the fluid (kg/m³)
Q is the volumetric flow rate (m³/s)
A is the pipe cross-sectional area (m²)

Now I haven't done the calculations for this, but this is what I meant by going over the top. The change in density is only a small amount of what the Reynolds number is made up of and the Reynolds number is only a small a portion of Clmax. That being said the change of a knot or two indicated that would result from such a change as say 0 to 10,000 feet IMO is not worth the time for pilots and will only confuse the crap out of most of them. The intricacies of fluid dynamics is above and beyond what one needs to consider in when discussing the change of indicated airspeed with altitude IMO.

Edit: Here to see the varying changes in air density and how little their values are, http://en.wikipedia.org/wiki/Air_density. The difference from -25 to +25 (25,000 feet of altitude change at standard lapse rate) is 0.239 difference in density. Compare that to velocity being a part of the formula and in m/s 50 knots would be 84 m/s, the wing of a typical aircraft that would stall around 50 knots would be around 10 meters in length, and the viscosity of air at 0.00001827 Pa·s. Hopefully these numbers make it pretty evident that the change in air density has a very little impact on Clmax.

PS If this boring, technical, stuff didn't exist and wasn't discussed we would still be pushing around carts with square tires.

shdw

Just thought some of you might find this interesting and it demonstrates how speed in the Reynolds formula above has a great effect on separation.

http://www.nar-associates.com/techni...ide_screen.pdf

These flight tests in an arrow with and without vortex generators:

With VGs: Vs = 53 and Vso = 50
Without VGs: Vs = 60 and Vso = 53

Even with only a slightly faster speed, only about 10 knots, the difference with and without was 7 where as the slower speed was only 3.

Great article though, good explanation of how VGs work and fairly non technical.