Will it stall if I simply bring back throttle without holding the control yoke?
Thread Starter
Join Date: Apr 2013
Location: HK
Posts: 11
Likes: 0
Received 0 Likes
on
0 Posts
Will it stall if I simply bring back throttle without holding the control yoke?
I'm studying ground school but have almost no experience in flying.
I have some questions about STALL.
Let's say I'm flying a piper warrior straight and level, what happen if I bring back throttle without holding the yoke?
I assume it will start to decelerate and descend due to insufficient lift, and then it will just keep diving and accelerate again, and it won't stall, is it correct?
What I'm being told is plane will only stall when it exceeds critical AoA, is there any possible scenario that the plane stalls without exceeding the critical AoA?
thank you very much
I have some questions about STALL.
Let's say I'm flying a piper warrior straight and level, what happen if I bring back throttle without holding the yoke?
I assume it will start to decelerate and descend due to insufficient lift, and then it will just keep diving and accelerate again, and it won't stall, is it correct?
What I'm being told is plane will only stall when it exceeds critical AoA, is there any possible scenario that the plane stalls without exceeding the critical AoA?
thank you very much
Join Date: Dec 2015
Location: France
Posts: 507
Likes: 0
Received 0 Likes
on
0 Posts
It will actually just descend.
If you pull back the throttle, you will lose a bit of speed, then this bit of speed lost will induce a slight imbalance in the longitudinal axis (which is the one going through the wings) and the nose will slightly drop, and the aircraft will establish itself in a descent at the same speed as your previous cruise.
Because you're basically trimming for a speed (or rather, an AoA, which is almost equivalent on the short term)
Of course if you transition sharply from 100% power cruise (that's forbidden but imagine) to feathering propeller, you'll see more speed lost, but the idea is the same, you'll still transition to a descent. And you won't stall.
Let's keep it that simple for now.
Stalling is by definition the moment when the air is no longer able to stick to the airfoil's shape. It will occur only at one AoA in normal conditions of flying for you piper warrior.
However, if you change the Reynolds number or Mach number your critical AoA could change significantly
For instance if you fly an Airbus at high mach numbers, your critical aoa will drastically reduce.
If you tried to build an airplane for ants, you would have a very different Reynolds number and this would lead to a different critical AoA even while keeping the same airplane and airfoil shape.
To sum it up simply --> your piper warrior always stalls at the same AoA, given constant flaps configuration
If you pull back the throttle, you will lose a bit of speed, then this bit of speed lost will induce a slight imbalance in the longitudinal axis (which is the one going through the wings) and the nose will slightly drop, and the aircraft will establish itself in a descent at the same speed as your previous cruise.
Because you're basically trimming for a speed (or rather, an AoA, which is almost equivalent on the short term)
Of course if you transition sharply from 100% power cruise (that's forbidden but imagine) to feathering propeller, you'll see more speed lost, but the idea is the same, you'll still transition to a descent. And you won't stall.
Let's keep it that simple for now.
Stalling is by definition the moment when the air is no longer able to stick to the airfoil's shape. It will occur only at one AoA in normal conditions of flying for you piper warrior.
However, if you change the Reynolds number or Mach number your critical AoA could change significantly
For instance if you fly an Airbus at high mach numbers, your critical aoa will drastically reduce.
If you tried to build an airplane for ants, you would have a very different Reynolds number and this would lead to a different critical AoA even while keeping the same airplane and airfoil shape.
To sum it up simply --> your piper warrior always stalls at the same AoA, given constant flaps configuration
Join Date: Sep 2016
Location: USA
Posts: 803
Likes: 0
Received 0 Likes
on
0 Posts
I'll start with the simple version of what will happen (with a theoretically "perfect" airplane), and then add some real-life complications.
The simple version: the elevator is trimmed to fly the wing at a particular AOA, and as long as you don't touch the elevator, you will stay at that AOA, and therefore the same speed. You may be climbing or descending, depending on the power, but always at a fixed AOA and airspeed. So if you reduce the power, the nose will lower, the flight path angle will lower (by the same amount) and therefore AOA and speed will be maintained.
A more detailed view of all the events that will happen: (we're still in the simple scenario)
0. All the forces on the airplane are in equilibrium, and you're in straight and level flight.
1. Thrust is reduced
2. Drag is now greater than thrust, therefore airspeed decreases slightly.
3. Due to the lower airspeed, lift decreases slightly.
4. Weight is now greater than lift, therefore the flight path curves downward.
5. Due to the downward inclined flight path, a component of weight now aligns with thrust. Therefore speed increases back to its original value. (Thrust and drag are in equilibrium again)
6. Due to the greater speed, lift is increased to the original value and fully supports the weight of the airplane. (Lift and weight are in equilibrium again)
So it's kind of a long domino chain of steps, each one setting off the next. Since the later steps change airspeed and lift in the opposite direction of the initial steps, it's a self-regulating cycle. But how does the speed "know" where to settle down in step 5? It does, because the speed changes the lift and it stops at the value that makes the lift balance the weight. If it reached any other value, the lift-weight difference would shallow or steepen the descent until the opposite thing happens with the speed. So it always self-corrects around the speed. (In other words, it's a negative feedback cycle and works exactly like a closed-loop controller, in engineering terms.)
First complication: The flight path won't cleanly change into the new equilibrium decent. It will initially overshot into a steeper one than that, then overshoot again up into a shallower one, again and again, each time the overshoots getting smaller until it settles into the final stabilized descent. This is called a phugoid oscillation. It happens because each step along the chain is not instantaneous but rather takes some time, and all of the subsequent steps have to "catch up." This can be controlled to some extent.
Let's look at the beginning. When you reduce the thrust, if you suddenly jerk the throttle to idle, you'll instantly create a big drag-thrust difference in step 2. That will more quickly effect your speed, which will more quickly reduce your lift in step 3. That will have a strong downward effect on your flight path. It will "whip" down. It will of course be self-corrected in the later steps, but the strong phugoid has been set up, which will take more cycles to settle down.
Let's say, instead, that you pull back the throttle very gradually. The initial drag-thrust difference will be smaller, so the airplane will slow down more gradually. By the time the throttle is at idle, the later steps have already had time to change their values and partially complete. They therefore have smaller changes to make. There is less "whipping" action at all the stages, the phugoid starts smaller, and damps out in fewer cycles.
Second complication: There are things, besides the elevator, that impart a pitching moment to the airplane. Imagine a jet with engines under the wings. When they increase power, it will cause the nose to pitch up, in addition to whatever you're commanding with the elevator. Conversely, when it powers back, it wants to pitch down. So, if power is changed, it's as if the trim setting is changed. I like to call this "virtual trim." (For a plane with high mounted engines like the DC-9, the same thing happens, but the "virtual trim" is the opposite. Increased power will tend to pitch down.)
Well, a typical prop trainer has the same effect as the low-engine jet, but for a different reason: Airflow washes down over the wing, and his the elevator slightly from above, thus pushing the tail down and the nose up. Increased power increases the prop blast, which energises this air, which pitches the nose up.
Bottom line: when you reduce power in your scenario, it adds a little bit of downward "virtual trim," which lowers the AOA and increases the airspeed. So the final stabilized descent will be at a higher airspeed than when you start.
This also means that when you increase power, like in a go-around, the new trim condition will be at a higher AOA and lower airspeed than when you started! That requires you to counteract that with some forward yoke pressure initially, and retrimming when you can get to it. It gets students in trouble in light trainers, and it gets airline pilots in trouble in 737s.
The simple version: the elevator is trimmed to fly the wing at a particular AOA, and as long as you don't touch the elevator, you will stay at that AOA, and therefore the same speed. You may be climbing or descending, depending on the power, but always at a fixed AOA and airspeed. So if you reduce the power, the nose will lower, the flight path angle will lower (by the same amount) and therefore AOA and speed will be maintained.
A more detailed view of all the events that will happen: (we're still in the simple scenario)
0. All the forces on the airplane are in equilibrium, and you're in straight and level flight.
1. Thrust is reduced
2. Drag is now greater than thrust, therefore airspeed decreases slightly.
3. Due to the lower airspeed, lift decreases slightly.
4. Weight is now greater than lift, therefore the flight path curves downward.
5. Due to the downward inclined flight path, a component of weight now aligns with thrust. Therefore speed increases back to its original value. (Thrust and drag are in equilibrium again)
6. Due to the greater speed, lift is increased to the original value and fully supports the weight of the airplane. (Lift and weight are in equilibrium again)
So it's kind of a long domino chain of steps, each one setting off the next. Since the later steps change airspeed and lift in the opposite direction of the initial steps, it's a self-regulating cycle. But how does the speed "know" where to settle down in step 5? It does, because the speed changes the lift and it stops at the value that makes the lift balance the weight. If it reached any other value, the lift-weight difference would shallow or steepen the descent until the opposite thing happens with the speed. So it always self-corrects around the speed. (In other words, it's a negative feedback cycle and works exactly like a closed-loop controller, in engineering terms.)
First complication: The flight path won't cleanly change into the new equilibrium decent. It will initially overshot into a steeper one than that, then overshoot again up into a shallower one, again and again, each time the overshoots getting smaller until it settles into the final stabilized descent. This is called a phugoid oscillation. It happens because each step along the chain is not instantaneous but rather takes some time, and all of the subsequent steps have to "catch up." This can be controlled to some extent.
Let's look at the beginning. When you reduce the thrust, if you suddenly jerk the throttle to idle, you'll instantly create a big drag-thrust difference in step 2. That will more quickly effect your speed, which will more quickly reduce your lift in step 3. That will have a strong downward effect on your flight path. It will "whip" down. It will of course be self-corrected in the later steps, but the strong phugoid has been set up, which will take more cycles to settle down.
Let's say, instead, that you pull back the throttle very gradually. The initial drag-thrust difference will be smaller, so the airplane will slow down more gradually. By the time the throttle is at idle, the later steps have already had time to change their values and partially complete. They therefore have smaller changes to make. There is less "whipping" action at all the stages, the phugoid starts smaller, and damps out in fewer cycles.
Second complication: There are things, besides the elevator, that impart a pitching moment to the airplane. Imagine a jet with engines under the wings. When they increase power, it will cause the nose to pitch up, in addition to whatever you're commanding with the elevator. Conversely, when it powers back, it wants to pitch down. So, if power is changed, it's as if the trim setting is changed. I like to call this "virtual trim." (For a plane with high mounted engines like the DC-9, the same thing happens, but the "virtual trim" is the opposite. Increased power will tend to pitch down.)
Well, a typical prop trainer has the same effect as the low-engine jet, but for a different reason: Airflow washes down over the wing, and his the elevator slightly from above, thus pushing the tail down and the nose up. Increased power increases the prop blast, which energises this air, which pitches the nose up.
Bottom line: when you reduce power in your scenario, it adds a little bit of downward "virtual trim," which lowers the AOA and increases the airspeed. So the final stabilized descent will be at a higher airspeed than when you start.
This also means that when you increase power, like in a go-around, the new trim condition will be at a higher AOA and lower airspeed than when you started! That requires you to counteract that with some forward yoke pressure initially, and retrimming when you can get to it. It gets students in trouble in light trainers, and it gets airline pilots in trouble in 737s.
