Mishandling a forward slip

[High Flyin]

I've recently started my first instructor job, and I have to say, I'm shocked at how poor the knowledge is with students surrounding the forward slip.

I was always taught the purpose of a forward slip is to lose altitude, therefore it defeats the purpose to have power on. So the procedure I learned was power to idle, trim to maintain an airspeed, enter the slip and hold the nose in a constant attitude to maintain that airspeed.

Anyway, based off what I've seen thus far, my question is, can you mishandle a forward slip so poorly that it results in a stall/spin situation?

[photofly]

So the procedure I learned was power to idle, trim to maintain an airspeed, enter the slip and hold the nose in a constant attitude to maintain that airspeed.

The extra drag means you'll want to lower the nose to maintain airspeed.
But note, depending on the location and quantity of your static ports, the ASI will read too low in a slip (instructive to think about why).

Yes you can stall, but since you're using it to descend faster, it's not a huge issue. Lower the nose to recover.
Spins from a slip: From a forward slip it would be very difficult, as long as you're smooth with the rudder on the entry and exit - sudden yaw can make a wing drop.

Over to the experts.

[Colonel Sanders]

can you mishandle a forward slip so poorly that it results in a stall/spin situation?

Never seen (or even heard of) one in the last 40 years.

Slips are good. Skids are bad.

My only advice is to get the nose down in the slip.

[akoch]

My only advice is to get the nose down in the slip.

But if you lower it (even a little bit) the airplane accelerates very quickly. In mine to maintain the constant speed I need to keep the nose failrly high, much higher than a normal approach. I do like to put the nose lower for short moment when recovering from a forward slip, to compensate for the increased angle of attack on recovery.

So not sure how you do it, unless there is a trick.... or the airplane itself has considerable drag so it won't matter.

[trampbike]

But if you lower it (even a little bit) the airplane accelerates very quickly. In mine to maintain the constant speed I need to keep the nose failrly high, much higher than a normal approach.

I don't get this. Said like that it sounds as though your aircraft has less drag during a slip than during coordinated flight.

[Rookie50]

But if you lower it (even a little bit) the airplane accelerates very quickly. In mine to maintain the constant speed I need to keep the nose failrly high, much higher than a normal approach.

I don't get this. Said like that it sounds as though your aircraft has less drag during a slip than during coordinated flight.

Wow. So then I should just use hold those inputs all the time and my asi will increase. Amazing

[Colonel Sanders]

Don't joke about this!! People will post mathematical
proofs, and invent a perpetual motion machine, and that
will be it for Alberta. No more oil patch. Are you really
ready to take responsibility for that?

[PilotDAR]

Though a forward slip has the control inputs which tend toward a spin entry, in is uncommon for a forward slip to be flown at speeds so slow as to promote a stall. It could be done, but it kinda defeats the reason for being in the forward slip to begin with!

If you are slipping to loose altitude, about the last thing you're going to do is to reduce speed in an attempt to reduce the sink rate! You'll recover the slip first! Now, if you add a turn into that forward slip, so it's not forward anymore, things can change dramatically, and your chances of a spin entry go way up. It's not that you cannot do it, but much more skill and caution are needed....

[trampbike]

Too much nonsense in that last post. A slip is quite anti-spin. A skid in quite different though.

[trampbike]

But if you lower it (even a little bit) the airplane accelerates very quickly. In mine to maintain the constant speed I need to keep the nose failrly high, much higher than a normal approach.

I don't get this. Said like that it sounds as though your aircraft has less drag during a slip than during coordinated flight.

Wow. So then I should just use hold those inputs all the time and my asi will increase. Amazing

Yep, the engine really is just there in case you don't like to fly uncoordinated.

[Shiny Side Up]

can you mishandle a forward slip so poorly that it results in a stall/spin situation?

Never seen (or even heard of) one in the last 40 years.

Slips are good. Skids are bad.

My only advice is to get the nose down in the slip.

The usual problem is people don't know what a slip is and skid the airplane instead. Technically the airplane is probably not slipping when someone gets into trouble with a stall/spin, but the pilot probably thinks he is. I suspect many pilots have gotten themselves into this bit of trouble, soiled themselves, and then erroneusly fear slips from that point on, when they really should be fearing skids. I also suspect that there are enough of these people that have gotten into the flight training establishment in this country (and to be fair, other countries as well, there's a lot of American slipophobes that I've encountered) to form a strong movement against slip training and are contributing to the overall pilot population lacking the ability to do it.

[Beefitarian]

To slip the mighty 172, I tilt the wings left and step on the right rudder pedal to keep it from drifting or turning left, that basically points the left wing a bit toward where I'm trying to get the plane to go. As was mentioned, the power is off because I was trying to decend, most likely to land. In the slip I can basically still control the drift left or right and I'm trying to lose height without diving.

The speed doesn't seem too difficult to control, I want to be going about the same forward speed as I would any other time I'm approaching to land. I probably tend to cause the plane to go very slightly faster because I'm trying to decend and might push the yoke slightly, I usually have the plane mostly trimmed for approach before I start to slip.

Once I get the plane near the spot I want to be on approach and I want to be done slipping, I just reduce the pressures on the controls and let the plane straighten out, as it gets near to straight, I start to add what ever control imputs I need to make it do the next thing I want it to. If I was paying enough attention to everything the plane is decending toward the threshold in a line straight down the center line of the runway at my desired approach speed and the numbers are staying in the same spot on the windscreen and getting larger.

I might be doing it all wrong. Slips seem kind of easy and very natural.

[Rookie50]

It's an interesting subject.

Goes back to the FTU taught fear of a steep turn in the circuit, which in a base to final turn where the wind is behind you, unconsciously leads to the temptation to apply too much inside rudder at a low airspeed, to avoid overshooting the centerline.

Dangerous, much more than a coordinated steep turn IMO. I think this is a weakness of FTU's, to properly teach coordinated turns and use them in the circuit, certainly to 30 degrees anyway. If the nose is relaxed a little, allowed to drop, steeper turns are very safe as well.

Some of the training I received by some folks working with confined and terrain airstrips, all turns are 45's.

I am not sure if this is completely accurate, but I would think a descending turn at low power, at least in any cessna, should need almost no inside rudder, and should be safer this way. Correct?

[akoch]

But if you lower it (even a little bit) the airplane accelerates very quickly. In mine to maintain the constant speed I need to keep the nose failrly high, much higher than a normal approach.

I don't get this. Said like that it sounds as though your aircraft has less drag during a slip than during coordinated flight.

OK, then what is the reason I need to keep the nose up to prevent the airspeed from increasing in the slip? This is the fact of life: with the nose in the same attitude it accelerates, with the nose lower than the stabilized approach (as CS suggested) it accelerates quickly. Keeping the nose up it slips happily and effectively.

To prevent point out in the wrong direction I'll say that the static port is integrated with the pitot port, and has minimal ASI error in the slip.

[Colonel Sanders]

what is the reason I need to keep the nose up to prevent the airspeed from increasing in the slip?

My first guess would be limited rudder authority.

Second guess is the usual combination of plate techtonics and coriolis force.

[akoch]

Limited rudder authority - this is always possible. Though it is rigged to the spec. Full rudder slip typically results in about 45-degree nose deflection from the forward trajectory.

Also, to the point of the original question - I intentionally tried to stall in a forward slip (at 4000feet), and pretty much failed. The nose up attitude was ridiculous (something like well over 45 degrees nose up), and it would simply not stall... I guesss in a way it did, the VSI would read 2-3,000 descent rate but the airplane would continue to "fly" like this until the pro-slip or full stick back input is removed.

[photofly]

Limited rudder authority - this is always possible. Though it is rigged to the spec. Full rudder slip typically results in about 45-degree nose deflection from the forward trajectory.

Also, to the point of the original question - I intentionally tried to stall in a forward slip (at 4000feet), and pretty much failed. The nose up attitude was ridiculous (something like well over 45 degrees nose up), and it would simply not stall... I guesss in a way it did, the VSI would read 2-3,000 descent rate

What is the definition of a stall? What was your angle of attack? And how much more stalled you would like it to be!? And what other indications of a stall would you expect?

[akoch]

To me stall is a loss of control (besides the purely aerodynamic abrupt loss of lift effect): both directional, vertical direction, or airspeed or vertical speed.

In that case the airplane continued to be fully controllable in every axis, including the vertical speed that would be linearly modulated by the aft stick pressure. I was not looking for an airplane to maintain the altitude, it was descending and flying as far as I could tell.

[photofly]

To me stall is a loss of control (besides the purely aerodynamic abrupt loss of lift effect)

I hate to break it to you my friend, but a stall doesn't involve either a loss of control or an abrupt loss of lift.

[akoch]

How's that? If the aircraft doing exactly what I'm asking it to do - there is no loss of control, aistream separation over wings or control surfaces, how's it is in a stall? Remember, we are not asking the wings to provide much lift in a slip, hence the angle of attack is shallow, despite the optics.

Example - when you attempt to maintain your altitude, and stall the wing(s) - it looses lift and altitude. You lost control of it minimally in vertical axis until you get it back flying again. That's a stall.

[trampbike]

Also, to the point of the original question - I intentionally tried to stall in a forward slip (at 4000feet), and pretty much failed. The nose up attitude was ridiculous (something like well over 45 degrees nose up), and it would simply not stall... I guesss in a way it did, the VSI would read 2-3,000 descent rate but the airplane would continue to "fly" like this until the pro-slip or full stick back input is removed.

Yep, you were stalled...
I highy doubt the "well above 45 degrees nose up" figure. Would love to see a video of this.

To me stall is a loss of control (besides the purely aerodynamic abrupt loss of lift effect): both directional, vertical direction, or airspeed or vertical speed.

That is not what a stall is. A stall is simply the separation of the airflow from the upper lifting surface when the critical angle of attack is exceeded.

In that case the airplane continued to be fully controllable in every axis, including the vertical speed that would be linearly modulated by the aft stick pressure. I was not looking for an airplane to maintain the altitude, it was descending and flying as far as I could tell.

Then you were flying an airplane that has very gentle stall characteristics.

Remember, we are not asking the wings to provide much lift in a slip, hence the angle of attack is shallow, despite the optics.

What do you mean by that? If you are not vertically accelerating (either downward or upward) and are not turning, then the wings provide as much lift during a slip (let's ignore the small vertical drag component of the fuselage) as they do during coordinated unnaccelerated flight (straight and level or constant descent or climb)...

Example - when you attempt to maintain your altitude, and stall the wing(s) - it looses lift and altitude. You lost control of it minimally in vertical axis until you get it back flying again. That's a stall.

Wrong axis there...

[akoch]

What do you mean by that? If you are not vertically accelerating (either downward or upward) and are not turning, then the wings provide as much lift during a slip (let's ignore the small vertical drag component of the fuselage) as they do during coordinated unnaccelerated flight (straight and level or constant descent or climb)...

I admit it is difficult to me to defend the point. We seem to be talking about different things, or I just don't understand it.

I thought we were discussing a forward slip. The maneuver where loosing altitude in a controllable manner was the target. Hence it is a very different manoeuvre aerodynamically than a straight and level flight, where you're asking the wing to provide lift equal to the weight of the airplane. So my point is directly contradicting your point above in the quote, and to be honest with you I think mine is the correct one. The angle of attack in a slip is low, and the aircraft was not in the stall when I tried it.

Example, perhaps more extreme but hopefully illustrative case: a wingover. You're at 90 degree bank angle, at the top of the turn. Speed is 15-25 knots, optically things look horrible (90 degree bank, sense of free-fall in the stomach). Yet you could not be further from the stall, and the aircraft is fully controllable at all axis, and there is no flow separation anywhere. In fact all is gentle and good with the world. Don't know if CS would agree with me here.

[photofly]

How's that? If the aircraft doing exactly what I'm asking it to do - there is no loss of control, aistream separation over wings or control surfaces, how's it is in a stall?

Airstream separation doesn't imply loss of control.

Do the falling leaf exercise. Full control of attitude, in a full stall. All the way down, just like AF447.

[slam525i]

Disclaimer: I'm a weekend warrior, PPL, blah blah blah.

This is one of those things that had always bugged me. You read all about stall-spin accidents on approach and you figure "If someone with much more experience than I can screw it up, I could screw it up that way too!" That plus I've had (terrible) FTU instructors go into a panic when I slip, talking about spinning it in. Last time I had my 6-month rental review, I tried it with a (good) instructor on board.

The setup: from 4000 feet; full flaps; 2 on board with 1/2 tanks; power-off. (The idea being that I'm approaching too high and hot, and need to lose energy as quickly as possible.)
The result: You can't lose enough speed in "normal" approach angles to stall it. To try to get it to stall, I pulled the nose higher and higher until we're clearly nose up (10-15 degrees), but still no "break". It just mushes on down, nose-up, even with full rudder and full aft-elevator. The attitude is definitely not something even a student pilot would miss.
The conclusion: To stall-spin on approach in a 172, you really have to be screwing the pooch. To "save" it, all you have to do it let-go, and it goes back to the trimmed approach speed, straight and level. I even tried to hoof it on a steep, nose-up, low speed, "base-to-final turn", and it just drops the nose gently with no spin.

Of course, this is aircraft specific, and a 172 is pretty much spin resistant normally, but it seems like the whole stall-spin on approach line from FTUs is hysteria. (Of course, one could conjecture that they know it's just hysteria, but prepares students for less benign aircraft.)

Side note: stall-spin on approach may be hysteria, but I don't think stall-spin on departure is.

[photofly]

but it seems like the whole stall-spin on approach line from FTUs is hysteria

Did you try a slow descending turn, then haul back on the yoke and add full rudder? Or were you doing it without the turn?

The result: You can't lose enough speed in "normal" approach angles to stall it. To try to get it to stall, I pulled the nose higher and higher until we're clearly nose up (10-15 degrees), but still no "break".

You were stalled. You just didn't get a break to announce it.

Second thing about stall on approach: at 4000', the recovery from a stall doesn't appear to consume any visible altitude. Whereas at 400', that gentle and benign "mushing" makes the ground come up towards you really really fast. And getting the nose down to regain some speed is alarmingly difficult.

That plus I've had (terrible) FTU instructors go into a panic when I slip

Slips are good, skids are bad etc. etc.