Factors that affect the change in stall speed.

[romaria]

Dear all member!

Weight greatly affects stall speed. And I can follow the maths to prove that it does. I understand that the increase in weight requires an increase in lift to counter. I understand that to increase the lift we have the option to increase v or CL (AoA).

But why does the stall speed increase? Reverse logic suggests we will hit the critical AoA at a faster speed since I know the stall speed increases but my question is why?

What am I missing? Every explanation on the web just explains that we need to increase lift because of the additional weight and therefore stall speed increases.

I'm missing the jump between the two. Anyone got a nice simple answer?

I don't know what you guys think.
Thanks for the suggestions.

[KayPam]

Just use the math.
You stall always at the same aoa, so at the same CL.
Since there is a direct correlation between weight and speed²*Cl in level flight, then if weight increases, Cl at stall is the same, then v must increase, so the stall speed increases.

[Duchess_Driver]

Put some numbers in... it becomes easier - as does prefixing the equation with W=L=0.5rhoV2ClS.

you can simplify if a bit more by assuming that 0.5, Rho and S are not going to change and therefore can be ignored. So

W=L=V2Cl

If Cl can’t increase - as it’s already at Max before the stall then as W increases to maintain steady state, unaccelerated straight and level flight L must increase and the only way that we can do that is by speeding up - otherwise W is bigger than L and we descend

[ATPMBA]

I believe stall speed increases by the square root of the load factor.
Ex. 50 kts stall speed, when the airplane is under a 2G load, square route of 2 is 1.41.
So 50 kts X 1.41 = 70.5 kts.

[markkal]

When maneuvering apparent weight increases due to the effects of gravity, you need to use the elevator to increase AoA to maintain level flight otherwise you descend.
But you can only increase AoA up to a point until you reach max Alpha which is a fixed value around 18 deg. before it stalls

Say you are in a 60 deg turn. in an aircraft with a stall speed of 60kt

At 60 deg angle of bank, provided you exert enough back pressure on elevator to maintain altitude -otherwise due to increased apparent weight you descend- then you weigh 2G's ( 1/Cos 60 deg) and you feel that by the seat of your pants.

As noted on previous posts your accelerated stall speed increases by the square root of your 1 G stall speed you will then stall at ( Square root of 2 is 1,41 times your stall speed 1g= 60x 1,41) = 85 Kts

Since you will probably not be flying a jet, not having excess power even firewalling the throttle on a 60 deg constant turn due to drag of increased AoA and the resultant of gravity and centrifugal accelerations, as your weight increases two fold, your speed in the turn while you pull to maintain altitude decreases.

That is why when we first learn steep turns we all have a tendency to descend, we need to PULL.

So say we start the turn at 100 kt, in the turn to maintain altitude we need to pull on elevator, but we can only pull up to a point before stalling (say max 18 deg).

To avoid loosing altitude at 60 deg of bank we increase pressure (up elevator) to maintain altitude and there is where our weight increses (2 g's).

Unless we start our 60 deg turn at a higher speed or we have excess power to prevent speed from decaying at a certain point we will reach those 85 kts and we will stall. So its easy to understand that at twice your weight you will stall with at a speed 40% higher, it has to do with the lasw of physics.

This is also valid in a dive; The fundamental issue here is that we have to dissociate AoA ( Angle between wing cord line and where the nose is posinting) with attitude ( Angle where the nose is pointing and horizon).

Operating in a two dimensional environment, in straight level flight, we maintain 1 G, and or AoA and Attitude are basically the same, this changes in a 3 dimensional environment and under load.

Our attitude varies with respects to te horizon, say you make a loop BUT our AoA with its fixed excursion ( say between 1,5 and 18 deg's) and it stays fixed during this loop.

If one is able to "visualise" this difference, easy to "feel" if you do a loop you will experience to have nose pointed up 50 degrees and not stall or 50 degrees pointing down and stall.

Stall speed is meaningless unless it is associated with weight and AoA. And excursion of AoA with respects to stall is a constant, while Attitude can vary between 0 to 90 degrees in our typical 2 dimensional environment and urange between 0 and 360 degrees in a 3 dimensional environment (Aerobatics)