Quick stall speed question

[Ronbonjovi]

Prepping for checkride and going through my POH for our 1978 C152 and ran into something that is probably stupid simple, but over thinking it here.

Vso is 35 knots IAS Vs 40 knots.

Stall speed in landing config is 43 knots CAS, and standard 48 knots CAS.

I know the difference between IAS and CAS. CAS is IAS corrected for instrument and position error.

What I am confused about is when the heck does this apply to me when I am flying the airplane and using the air speed indicator as a basis for my speed? When the DPE asks me what is stall speed in clean/dirty, my answer will be referring to the green and white arc, Vs and Vso. I don't feel like I have an answer if he wants me to explain the difference between stall speeds in IAS and CAS.

 

[MauleSkinner]

It only applies if you're trying to calculate an airspeed relative to stall speed...for example, youre 20% under gross, and you want to calculate a precise 1.3 Vso for your weight. You'd make the calculations using CAS, and then correct to IAS.

I'm not sure exactly when the change was made (1978 sticks in my mind, for some reason), but airspeed markings used to be CAS, and were changed to IAS for exactly the operational reasons you mention.

 

[RyanB]

MauleSkinner is correct.

As for the checkride— If the DPE asks what the stall speeds are, just list them by what the ASI indicates as that will be the most relevant in day to day flying. There’s no need to specifically go into CAS if he or she doesn’t ask for it.

Rule of thumb for a checkride: Don’t offer more information than what the examiner asks. Answer their question and be done.

 

[luvflyin]

Prepping for checkride and going through my POH for our 1978 C152 and ran into something that is probably stupid simple, but over thinking it here.

Vso is 35 knots IAS Vs 40 knots.

Stall speed in landing config is 43 knots CAS, and standard 48 knots CAS.

I know the difference between IAS AND CAS. CAS is IAS corrected for instrument and position error.

What I am confused about is when the heck does this apply to me when I am flying the airplane and using the air speed indicator as a basis for my speed? When the DPE asks me what is stall speed in clean/dirty, my answer will be referring to the green and white arc, Vs and Vso. I don't feel like I have an answer if he wants me to explain the difference between stall speeds in IAS and CAS.

Read your post. You've already answered the question.

 

[DanWilkins]

Right before stalling, my two planes start to shake, look for rudder help and the controls become very light. I don't have a stall warning horn so I depend on the clear messages that I receive from my airplane. I rarely focus on the air speed indicator right before a stall. Watching the ball and keeping the plane coordinated is far more important than trying to keep an eye on the airspeed needle as it inches towards the "stall speed".

Knowing those critical air speed numbers is important; but, I would not count on them as gospel to warn me exactly when a stall will occur.

You might answer this way "Sir, right after the plane starts shaking around". It may sound sarcastic; however, if that is the way your plane warns you, why not heed the warning? The examiner may ask you to give actual number values. Tell him the values stated in the book; however, be sure he knows that you understand that those numbers are guidelines and the plane's messages are your best indication of stall speed.

 

[midwestpa24]

Not to mention, those numbers are at max gross. Just me in my Citabria, I am well below the book stall speed at break. My ASI is close to zero.

 

[mryan75]

Rule of thumb for a checkride: Don’t offer more information than what the examiner asks. Answer their question and be done.

THis is the best checkride advice you will ever get. If (when) he asks for stall speeds, give him the book numbers and cork it.

 

[Dan Thomas]

Not to mention, those numbers are at max gross. Just me in my Citabria, I am well below the book stall speed at break. My ASI is close to zero.

Your ASI is lying to you in that configuration. When the pitot tube isn't directly facing the relative wind, it will read lower. If you were to get it to about 50°, it would probably produce no pressure at all and beyond that it would produce suction.

Besides that, the static ports get goofy airflows at high angles of attack. If they get an increased pressure they drive the ASI down.

My old Auster (38 MPH stall at gross, 34 at typical operating weight) would have its ASI reading a bit negative at the stall break.

Designers use special pitot heads when getting stall speeds. The pitit head has a vane that pivots the tube directly into the airflow no matter what attitude the airplane is in.

Look at the CAS chart for the 172N. See that 9 kt difference at the bottom end of the speed range? Actual airspeed is 49, indicated is 40. The airplane is travelling much faster than the ASI says it is. At cruise, the speed differential disappears. It's all because of position error.

 

[midwestpa24]

Your ASI is lying to you in that configuration. When the pitot tube isn't directly facing the relative wind, it will read lower. If you were to get it to about 50°, it would probably produce no pressure at all and beyond that it would produce suction.

Fully aware of that. The split between IAS and CAS grows as you approach stall speed and below. The point being, the markings on the indicator, and the numbers in the book, are just points of reference but not absolutes. Same goes the opposite direction, stall speed goes up as G loading does. The correct answer for what speed an aircraft will stall is when the wing reaches critical angle of attack, not necessarily any given speed.

 

[Chip Sylverne]

And herein lies the value of the AOA instrument. Though airspeed is a proxy for AOA, it's the angle of attack for a given configuration that determines when the wing will stall, and that's a constant regardless of airspeed.

 

[easik]

I think you have your stall speeds correct already. Just remember they can change depending on load. I don't think your DPE will ask you about stall speeds as it relates to CAS. Personally never heard of it being asked. As long as you give a straight answer and don't dig holes for yourself you should be fine.

 

[flyingron]

Yes, understand that AIRSPEED DOES NOT DETERMINE STALL. AOA does.
Get that through your head and you will understand this part of aerodynamics more.

The so-called "stall speed" is the speed where you will exceed the critical angle trying to maintain level flight given a bunch of other conditions (weight, not otherwise accelerating (turning), even CG).

 

[skier]

L=1/2*rho*V^2*S_w*Cl_aplha*alpha

Where:
Rho=density
V=Velocity
S_w=wing area
Cl_alpha=lift cube slope ~ 2*pi
alpha=angle of attack

At stall:
L=W (unaccelerated Flight)
CL=cl_max (not quite Cl_alpha*alpha since the lift curve slope decreases near stall)

Where:
W=weight

Solving for stall speed give:
V_stall =sqrt(2*w/(rho*S_w*cl_max)
Note that Cl_max is a function of angle of attack.

The take away (for flying) is that stall speed is a function of density and aircraft weight.

For designing an airplane the takeaway is that stall speed is a function of wing area, max lift coefficient of the airfoil, and the weight of the aircraft.