Steep Spiral Descent

[Rockymountain]

As mentioned above, a steep spiral is NOT about getting down faster. It is a maneuver that tests a lot of things in a pilot about maintaining precise control of the aircraft. Just a couple of comments. If you want to get down fast, you don't want flaps, because in most aircraft flaps will decrease your descent speed (Vfe much less than Vne). AND flaps decrease the strength and load handling ability of the wing, and you will see most POH's will have lower g-limits for any flap configuration. So you just might break something while you are concentrating on your emergency. The way to get down is the fastest of Vne smooth, Vno if in other then smooth air, or Vturbnulence in turbulent air, at a bank angle, usually around 45 degrees. IMHO.

If you have drag, spoilers, retract gear or speed brakes, may be helpful POH willing to throw them out.

 

[Cap'n Jack]

When you bank an airplane you are no longer at 1G due to centrifugal force and you have a corresponding increase in stall speed. The fact you are descending does not change the forces. This is why we can descend so rapidly in the emergency descent.
http://media.aero.und.edu/interactive-trainers/forces-in-a-turn/

Play with this and you will see why your CFIs emergency descent is dangerous.

From the Private pilot ACS of emergency descents. Use bank angle between 30° and 45° to maintain positive load factors during the descent.

Yes, you are correct, and I edited my post to reflect the correct information. Once one is in a steady-state turn, centripetal force stabilizes based on the bank angle.

Your post caused me to think about it more carefully.

The post below did not cause me to think about it more carefully and is merely noise.

Incorrect on all counts.

 

[ArnoldPalmer]

CFIs rationale - if you have an emergency, chances are you will have to land. No point doing spiral turns and not get a landing in. Usually the maneuver is started no flap, then pick landing runway, add flaps and call it good.

Not sure what the DPE wants to see. CFI has had 2 other applicants for commercial in the recent past.

 

[MauleSkinner]

CFIs rationale - if you have an emergency, chances are you will have to land. No point doing spiral turns and not get a landing in. Usually the maneuver is started no flap, then pick landing runway, add flaps and call it good.

Not sure what the DPE wants to see. CFI has had 2 other applicants for commercial in the recent past.

Most likely the DPE wants to see what’s in the ACS and the POH.

 

[Matthew Rogers]

The 172 stall speed @ 60 degrees bank 40 degree flap gross weight and forward CG is ~58 KIAS and you are close to the ground. With 30 degrees flaps its ~59 KIAS. With a little wind shear you are there.
.

That stall speed is for a level turn and does not apply in a dive, I think.

Stalling while in a dive is much different that stalling in climb or level flight. While stall speed in the turn is increased if you keep holding back pressure, any release of that pressure and you are no longer in a G loaded flight condition. Just because the wings are at 60 degrees does not mean that you are always pulling 2 Gs. If the wings are not loaded at 2g, then the stall speed is not increased in the same way.

But please correct me on this one. And I know both sides will claim I am wrong and others correct.

 

[Clip4]

That stall speed is for a level turn and does not apply in a dive, I think.

Stalling while in a dive is much different that stalling in climb or level flight. While stall speed in the turn is increased if you keep holding back pressure, any release of that pressure and you are no longer in a G loaded flight condition. Just because the wings are at 60 degrees does not mean that you are always pulling 2 Gs. If the wings are not loaded at 2g, then the stall speed is not increased in the same way.

But please correct me on this one. And I know both sides will claim I am wrong and others correct.

If you refer to performance in Section 5 of the 172 POH and find the stall speed table, all of the stall speeds are for power off. How does one not descend in a 60 degree power off turn?

You might also notice the Vs1 and Vso on your airspeed indicator are the same values as 0 degree bank with no flaps and full flaps in the stall table. As you bank, the stall speed is accelerated (higher).

 

[FastEddieB]

I always thought of the steep spiral as a coordination exercise, not simulating anything you’d do in real life.

A while back I was practicing Commercial maneuvers in my Sky Arrow and stuck a GoPro on the bill of my baseball cap. If anyone’s interested the steep spiral is at around 4:20 in the below video.

 

[Matthew Rogers]

You can still either pull back or push forward regardless of power application. It is when you are in the pattern at low power and pulling back on the yoke absentmindedly at a high bank that you stall and enter a spin. If you are descending, you are not loading the wings in the same way and at low risk of stall.

 

[dmspilot]

You can still either pull back or push forward regardless of power application. It is when you are in the pattern at low power and pulling back on the yoke absentmindedly at a high bank that you stall and enter a spin. If you are descending, you are not loading the wings in the same way and at low risk of stall.

What is the g-load during a 3000fpm constant rate descent straight ahead?

 

[Matthew Rogers]

1g, right? But that does not take into account the loads in a turn. However, my point is that a descending turn has different loads than a level turn as the necessary component of lift is lower. Lower lift means less back pressure, which means lower G loading, which mean lower AOA, which should mean lower stall speed.

Obviously, free falling at terminal velocity would be 0g - but wait, would that be at 1g as well? It is 0g as soon as you exit the jump plane, and then you would have gravity pulling down, but drag pushing upwards so it would change as your air speed and drag changes. When drag equals gravity, you are back at 1g.

 

[dtuuri]

1g, right? But that does not take into account the loads in a turn. However, my point is that a descending turn has different loads than a level turn as the necessary component of lift is lower. Lower lift means less back pressure, which means lower G loading, which mean lower AOA, which should mean lower stall speed.

Obviously, free falling at terminal velocity would be 0g - but wait, would that be at 1g as well? It is 0g as soon as you exit the jump plane, and then you would have gravity pulling down, but drag pushing upwards so it would change as your air speed and drag changes. When drag equals gravity, you are back at 1g.

I think there at the end, because you're getting some good tutelage here, you might be catching on. The g load is due to back pressure, as is the turn rate, it isn't dependent upon climb or descent. In fact, in a climb, there's less lift required from the wing because the engine provides a vertical component of lift. Vice versa, the required lift for a descent is more than in straight and level, so the stalling speed will be higher not lower than in a climb. Dumping the stick also dumps your turn, which won't do you any good while trying to keep a constant radius around a reference point.

 

[Clip4]

You can still either pull back or push forward regardless of power application. It is when you are in the pattern at low power and pulling back on the yoke absentmindedly at a high bank that you stall and enter a spin. If you are descending, you are not loading the wings in the same way and at low risk of stall.

When in a wings level descent, you are at 1g. I will agree the pilot abusing the elevator is the cause of stalls in the pattern, but a spin requires the plane to be in an uncoordinated condition along with the wing exceeding critical AOA. In a descending turn, the stall speed increases due to increased G loading.

 

[tspear]

I believe there is a fair amount of over thinking going on here.
The angle of attack which causes a stall does not materially change if you are in a bank, or pulling up to arrest a descent or start a climb.
Because historically, AOA has not been available pilots used IAS as a proxy to determine the effective AOA in straight and level conditions. To account for non-straight and level conditions, we have learned such rules increasing stall speed by a factor of two for a 60% bank. However, this is based on the premise that you are attempting to maintain the same vertical lift component.

In a descending turn (spiral) the vertical lift component is decreased. The amount it is decreased, is beyond my math/knowledge kills. However, this lowers the stall speed, so unless you have an AOA, you are guessing how much it decreases the stall speed. Which is a poor proxy anyways for AOA.

Tim

 

[dtuuri]

In a descending turn (spiral) the vertical lift component is decreased. The amount it is decreased, is beyond my math/knowledge kills. However, this lowers the stall speed...

If it's a uniform descent the four forces are in equilibrium, the same as in level flight or a climb. The difference is the engine, if there is one. It helps "lift" when aimed upward, lessening the angle of attack needed to support the weight. A power-off descent has more angle of attack than a climb and is nearer the stall, all other factors like bank angle and IAS being equal, so the stall speed is increased not decreased in a descent.

 

[Clip4]

I believe there is a fair amount of over thinking going on here.
The angle of attack which causes a stall does not materially change if you are in a bank, or pulling up to arrest a descent or start a climb.
Because historically, AOA has not been available pilots used IAS as a proxy to determine the effective AOA in straight and level conditions. To account for non-straight and level conditions, we have learned such rules increasing stall speed by a factor of two for a 60% bank. However, this is based on the premise that you are attempting to maintain the same vertical lift component.

In a descending turn (spiral) the vertical lift component is decreased. The amount it is decreased, is beyond my math/knowledge kills. However, this lowers the stall speed, so unless you have an AOA, you are guessing how much it decreases the stall speed. Which is a poor proxy anyways for AOA.

Tim

If you can hold altitude to demonstrate a 60 degree bank power off stall, god bless you.

 

[FastEddieB]

If you can hold altitude to demonstrate a 60 degree bank power off stall, god bless you.

I don't think its that hard - slow to maneuvering speed, roll briskly into a 60º bank and quickly pull back hard on the yoke, maybe with some aileron to prevent overbanking. Recover at the buffet/stall. I don't think you'd lose any altitude if done rapidly enough.

 

[MauleSkinner]

If you can hold altitude to demonstrate a 60 degree bank power off stall, god bless you.

If you can’t, I’d suggest practice.

 

[dtuuri]

If you can hold altitude to demonstrate a 60 degree bank power off stall, god bless you.

Isn't that the truth! I'm sure I'm not the only CFI who was embarrassed when attempting to show the increase in stalling speed due to a steep bank. The plane shaking, the stick against the stop and ... nothing else, just that. You can't get enough elevator authority to stall the wing in a tight radius turn in most light airplanes.

 

[Clip4]

If you can’t, I’d suggest practice.

I'll just schedule you for a demo with you and watch the altimeter between entry an buffet.

 

[Clip4]

I don't think its that hard - slow to maneuvering speed, roll briskly into a 60º bank and quickly pull back hard on the yoke, maybe with some aileron to prevent overbanking. Recover at the buffet/stall. I don't think you'd lose any altitude if done rapidly enough.

Can you do that while holding a sick sack and giving a good explanation while performing said maneuver? Better have the pitch attitude correct or you will stall before 60 degrees.

 

[MauleSkinner]

I'll just schedule you for a demo with you and watch the altimeter between entry an buffet.

Anywhere in the Wichita area is fine. Your treat.

 

[Clip4]

Anywhere in the Wichita area is fine. Your treat.

Anywhere in the Wichita area is fine. Your treat.

on reevaluation, 60 bank and then power of slowly might not be that bad.

 

[tspear]

If it's a uniform descent the four forces are in equilibrium, the same as in level flight or a climb. The difference is the engine, if there is one. It helps "lift" when aimed upward, lessening the angle of attack needed to support the weight. A power-off descent has more angle of attack than a climb and is nearer the stall, all other factors like bank angle and IAS being equal, so the stall speed is increased not decreased in a descent.

Ah, but in a spiral descent forces are not in equilibrium. The turn is caused by unequal forces.
In a straight decent you may lose the lift provided by the engine; but you now have drag vector behind the plane slowing the descent and providing a "component" of lift. Albeit, not enough to "lift" the plane".
Otherwise, we need an aeronautical engineer (assuming you are not one, I do not know) to run calculations.

But my overall point still applies. IAS sucks as a proxy...

Tim

 

[dtuuri]

Ah, but in a spiral descent forces are not in equilibrium. The turn is caused by unequal forces.
In a straight decent you may lose the lift provided by the engine; but you now have drag vector behind the plane slowing the descent and providing a "component" of lift. Albeit, not enough to "lift" the plane".
Otherwise, we need an aeronautical engineer (assuming you are not one, I do not know) to run calculations.

But my overall point still applies. IAS sucks as a proxy...

Tim

It's better we argue it out between ourselves, neither one of us would understand an engineer. According to Newton, if opposing forces aren't balanced an acceleration occurs until they are. So, what forces in a turn are unbalanced and at what point do they again balance?