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Question

I have thought myself into a conundrum:

A hypothetical (DHC-8-106) airplane has the following limitations at MGTOW:

V_A 164 KIAS
V_FE 148 KIAS
V_S 80 KIAS (flaps 15°/bank 0°)
Load factor limit of 2 G with flaps extended

If the airplane is banked at 60° in level flight, it takes 2 G to make a level turn, which is the load limit of the plane with flaps extended. A 60°/2 G turn increases stall speed about 1.41 times, so the new stall speed is about 112.8 KIAS. Let's say I'm flying this turn at 148 KIAS, the max speed for the flap setting and well above the stall speed.

If I pull the yoke back a bit more (let's say a lot more), can I over-G the airplane? I am well above stall speed, so it would seem to me that I could increase ? quite a bit and therefore increase G quite a bit, which implies to me that I could overstress the airplane 16 KIAS slower than V_A. Am I missing something? I know that an airplane stalls when it exceeds critical ?, and I thought V_A is meant to be the "airplane stalls before it breaks" speed. I am just stuck. Thanks!

Answer 1

Don't be stuck, look at the flight envelope diagram in your POH

Remember, we are talking about acceleration here (G's).

Increasing Angle of Attack will accelerate the plane in a new direction.

Look at the Lift equation:

Lift = Density × Area × Coefficient of Lift × V$^2$

It's the rate of acceleration in the new direction which determines G's. What happens below Va is one never reaches the G limit before the AoA limit (stall) because the plane isn't going fast enough to generate sufficient additional Lift by changing AoA.

Above Va, additional Lift will "tear the wings off" before stall AoA is reached because Lift is determined by AoA and V$^2$.

However, flap configuration may affect Va, because adding flaps will increase the coefficient of lift for a given AoA.

So, the answer is YES you can, because G limit is lower with flaps extended, and coefficient of lift is higher.

Answer 2

Not sure if I misunderstood, but you seem to be describing a spiral dive! In your steep turn, by pulling back you decreased the turn radius of the aircraft, which increased the centrifugal force on the aircraft - supposing you kept the speed constant!

The formula for centrifugal acceleration is w= V^2 / R

Where w= centrifugal accerlation in m/s^2 V= tangential speed in m/s R= turn radius in m

Centrifugal force is mass times the acceleration w.

At some point, the centrifugal force will exceed the lift force provided by the wings and the aircraft will lose Altitude!

You could try keeping 148 KIAS and overstress the aircraft with the flaps down, but it's probably unlikely because you'll run out of something making the overstress condition possible before you get there: power to maintain speed, speed will drop, bank angle will change, lift, Altitude, etc...

Ps. It doesn't "take 2G to make a level turn", instead, the aircraft will experience a load equivalent to 2Gs because of the turn speed and radius! It is a real force that the wings must overcome.