Power-On vs Power-Off Stall Speed

[Zipper730]

I'm curious about two basic things

1. Propeller-Driven Aircraft: I'm curious if there were any generalized rules of thumb for computing power-on stall speed over power-off?
2. Jet-Driven Aircraft: Since jet-aircraft don't have a slipstream, why is this figure often listed on the SAC sheets for USN aircraft that use jet-engines if they don't have blown-flaps?

 

[XBe02Drvr]

Propeller-Driven Aircraft: I'm curious if there were any generalized rules of thumb for computing power-on stall speed over power-off?

Sure, aerodynamicists could calculate projected stall speeds by all the theoretical L/D at various weights and speeds and AOAs, then test a mock-up in a wind tunnel, but you never really know til you build it and test fly it. Occasionally an aircraft performs exactly as advertised right from its first flight. Most do not. There are too many interacting variables based on extrapolated data. Errors creep in, breed, and multiply.

Since jet-aircraft don't have a slipstream, why is this figure often listed on the SAC sheets for USN aircraft that use jet-engines if they don't have blown-flaps?

Depends on your definition of slipstream. If you're going to define it as the same thing as propwash, then I suppose that's true. But the common definition of slipstream is more like relative wind, and any object in motion in the ocean of air has slipstream.
Terrestrial aviators tend to think in terms of hurling themselves at the mass of air, and focus on airspeed. Nautical aviators OTOH, tend to focus on AOA, seeing that as the ultimate defining feature of aircraft handling. Navy cockpits almost invariably feature an AOA indicator front and center in the pilot's scan, as well as an approach indexer in the line of sight through the windshield to the runway / flight deck. This is a lit indicator involving an up arrowhead and a down arrowhead, both pointing at a "donut" between them. You want to see the donut lit, signifying "on speed" (optimum AOA). If one of your arrowheads is lit, you're either at too high an AOA (slow), or too low an AOA (fast), and are in danger of becoming a statistic. Even that "teeny weeny" T34 we had in the flying club had an indexer, and we could go out and fly the ball anytime we wanted. Every runway had one.
Cheers,
Wes

 

[BiffF15]

The T-38 has (or had) the same chevron AOA indicator. Worked well, but was just part of what was focused on in the final turn. Feel, AOA, calculated speed, sight picture, spacing, winds and aircraft weight all played a part.

Great airplane, very fun to fly, extremely unforgiving in the pattern.

Cheers,
Biff

 

[Zipper730]

X XBe02Drvr ,

I'm just curious why power on/off stall speeds would vary in a jet. I don't really grasp it, unless you mean that the lack of power would cause you to hit the stall faster.

 

[fliger747]

Several reasons stall speed will vary with jet thrust... At any positive inclination of the thrust vector from horizontal, there will be a vertical component. Depending on the thrust (or thrusts) locations a pitch or yaw may be introduced (as in engine out). With underslung engines this pitch moment is quite pronounced. Reduce thrust and a nose down pitch is introduced, to counter this additional downforce on the tail will be necessary, this increases the lift required on the wing. Additionally jet thrust induces flow (just like a "jet" water or fuel pump).

 

[XBe02Drvr]

X XBe02Drvr ,

I'm just curious why power on/off stall speeds would vary in a jet. I don't really grasp it, unless you mean that the lack of power would cause you to hit the stall faster.

It's because what really matters is Angle of Attack, since that's what actually precipitates a stall. Simply put, more thrust will allow you to keep your AOA below the stalling angle down to a lower airspeed. Sink rate increases AOA if deck angle stays constant, and more thrust delays the increase in sink rate. The higher your AOA, the more angle induced error will make your ASI read low, so the perceived effect of the thrust is an even lower indicated airspeed than is actually occurring. Hope that makes sense to you. It's so much easier to explain when you can demonstrate it in flight.
Cheers,
Wes

 

[DerAdlerIstGelandet]

Ahhh, I remember my first stall flights. Not going to lie, I was expecting a lot worse than it really was. I was very nervous. After the instructor demonstrated the first one though, all apprehension was gone.

Of course my first stalls were in a very docile Cherokee...lol Very good plane to learn in.

 

[XBe02Drvr]

Of course my first stalls were in a very docile Cherokee...lol Very good plane to learn in.

Did it actually stall, or just mush rapidly earthward after stabilator authority petered out? The few Cherokees I've flown were so nose heavy (back seat placarded unoccupied for training maneuvers) you couldn't generate enough elevator authority to truly make them stall. You just got the gentle rocking chair.
Cheers,
Wes

 

[Zipper730]

It's because what really matters is Angle of Attack, since that's what actually precipitates a stall. Simply put, more thrust will allow you to keep your AOA below the stalling angle down to a lower airspeed.

By plowing into the air harder and stopping the sink angle?

For determining corner velocity, you'd compute power on stall correct?

 

[DerAdlerIstGelandet]

Did it actually stall, or just mush rapidly earthward after stabilator authority petered out? The few Cherokees I've flown were so nose heavy (back seat placarded unoccupied for training maneuvers) you couldn't generate enough elevator authority to truly make them stall. You just got the gentle rocking chair.
Cheers,
Wes

Yeah once she she stalled, the nose would lower, and that was it. Not like in other aircraft. Very docile.

You literally can take your hands off the yoke, and the back pressure relief caused by that will take you out of the stall.

 

[special ed]

My first stalls were in an Aeronca Tri Traveler. Since the instructor was a slim man and I weighed 140 pounds, it wouldn't stall so he just shoved the stick forward. Later stalls in the 150 were real.

 

[FLYBOYJ]

Stall stories -

When first learning to fly I had issues with stalls, was always worried about spinning. Long story short, I later discovered the 152 I was learning in survived a mid-air and when it was put back together the wing asymmetry was off causing the right wing to really drop. My final primary instructor figured this out and after doing some high AoA stalls and spins, never had an issue. Used the same plane for my check ride and passed with "flying colors."

I've flown a T-41B with a fixed pitch prop, lightly loaded, you could almost make the aircraft stand on it's nose when doing power on stalls.

180 HP Super Cub - Power on stalls could be problematic as you really had to honk the nose up high but risked the aircraft from flipping over. Was told by a few cub drivers who had a lot more time than me just avoid doing high AoA stalls in a Super Cub.

The C 172s I fly these days are hard to get a clean break if they are lightly loaded, they tend to "mush" and "falling leaf" if you try to stall with the nose mildly pitched up.

The 65 C 150 I owned could stall with just trim input and even with power on broke pretty straight with no control input. I spent weeks rigging this plane and got pretty lucky.

 

[fliger747]

Part of the certification of aircraft is defining the stall at forward and aft CG values. A civil trainer aircraft is generally expected to have gentle stall manners and if not spin resistant, to at least enable an easy recovery using normal technique. Transport aircraft variously have stick shakers and stick pushers to avoid a full stall, some fly by wire systems have AOA and G limits built in.

In my earliest days of flying in a Cessna 150 I would go out to practice stalls and spins solo. At that CG loading a stall could only be made to break with some acceleration by snapping the yoke aft the last little bit. Heavier or more aft CG it would easily break into a classic stall and nose drop. Swept wing jets often have rather bad stall manners so we would only practice recovery from a heavy buffet. A full stall could take many thousands of feet, if available.

 

[DerAdlerIstGelandet]

Stall stories -

When first learning to fly I had issues with stalls, was always worried about spinning. Long story short, I later discovered the 152 I was learning in survived a mid-air and when it was put back together the wing asymmetry was off causing the right wing to really drop. My final primary instructor figured this out and after doing some high AoA stalls and spins, never had an issue. Used the same plane for my check ride and passed with "flying colors."

I've flown a T-41B with a fixed pitch prop, lightly loaded, you could almost make the aircraft stand on it's nose when doing power on stalls.

180 HP Super Cub - Power on stalls could be problematic as you really had to honk the nose up high but risked the aircraft from flipping over. Was told by a few cub drivers who had a lot more time than me just avoid doing high AoA stalls in a Super Cub.

The C 172s I fly these days are hard to get a clean break if they are lightly loaded, they tend to "mush" and "falling leaf" if you try to stall with the nose mildly pitched up.

The 65 C 150 I owned could stall with just trim input and even with power on broke pretty straight with no control input. I spent weeks rigging this plane and got pretty lucky.

Yeah I was nervouse and scared at first as well about getting into a spin (I now think spin training should be required). The Cherokee really just sort of gets mushy on the controls and then buffets (porpous) a but. Then the nose breaks down and drops, and the aircraft recovers. Very, very docile.

 

[Zipper730]

Yeah I was nervouse and scared at first as well about getting into a spin (I now think spin training should be required).

I thought spin recovery was part of pilot training? Especially with propellers and torque & p-factor.

 

[FLYBOYJ]

I thought spin recovery was part of pilot training? Especially with propellers and torque & p-factor.

Not any more for a PPL - it was many years ago. When I was instructing primary students I did do spins regardless.

 

[Zipper730]

Not any more for a PPL - it was many years ago.

When was it taken out of the curriculum?

 

[FLYBOYJ]

When was it taken out of the curriculum?

Curriculum = Practical Test Standards is the proper term but now known as "Airman Certification Standards." Removed sometime in the 1970s IIRC

 

[DerAdlerIstGelandet]

I thought spin recovery was part of pilot training? Especially with propellers and torque & p-factor.

Not for the PPL. They teach you how to not get into a spin. And then go over the theory of spin recovery.

 

[Zipper730]

Recovery from a spin is power off, opposite rudder, nose down right?