Mishandling a forward slip

[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?

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.

It is loss of control: you are losing your altitude while trying to maintain altitude.

[slam525i]

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?

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.

I didn't try to abruptly haul back on the yoke and full rudder, but took a second or two to get it established as any sane person entering a slip on approach would. The point for me was to prove to myself that it's difficult to screw it up, not impossible, just like it's not impossible to roll it inverted and pull the nose into the ground on approach.

I completely agree that mushing consumes altitude, and there's precious little of it on approach, but it's not an abrupt break into a stall/spin which would take a lot more to recover from.

The point isn't that it can't be done, but that it's really quite difficult and you have to seriously work at it get screw it up that badly.

EDIT: I fully agree I was stalled, but it didn't BREAK. The story FTUs seem to give is that it's break, the nose would plunge, and horizon would start to rotate, and you'd die before you can do anything about it. Instead, it just mushes down and all you have to do is let go to immediately start flying again.

[cgzro]

. I was always taught the purpose of a forward slip is to lose altitude, therefore it defeats the purpose to have power on.

Useful also when forward visibility is limited in which case power may be on. For example you have bird guts or oil on the windscreen then a slipping approach with power improves forward vis. Also useful in planes with big engines like radials in the nose or blind tail draggers.

You can get a stall in a slip but the nice thing is the roll will be towards wings level while a skid stall takes you upside downl. This is something the unusual attitudes courses show with dramatic results.

Note that a stall/spin can be provoked much more easily with a rapid pitch change than a slow one so when un coordinated for whatever reason gentle pitch changes are wise.

[photofly]

It is loss of control: you are losing your altitude while trying to maintain altitude.

Who says I'm trying to maintain altitude? You were trying to descend, in a slip, and managing it very effectively, by the sound of it.

The "doing what I ask it to do" or "being under control" is not a relevant distinction. Suppose I ask it to descend at 3000fpm with the nose up in the air. Does it become not stalled because that's the flight path I intend?

Seriously... high descent rate, low airspeed, high angle of attack... you're stalled. I'm not saying that's necessarily a bad way to fly, or out of control, or dangerous. But the airplane doesn't care what your intentions are. It's still stalled.

[akoch]

The airspeed is stable at 60 (stall is 36), no intention to maintain altitude so the wing is not working hard (aka low angle of attack). No buffeting, uncontrolled pitch changes, stall horn is silent. I don't know... I think I just should shut up

[photofly]

Stalls don't care what speed you're going. They care only about the angle of attack.

3000 fpm descent = 30kts vertical velocity.

If your airspeed was 60kts, your flight path angle was at a 30 degrees downward angle to the horizontal. Even if the nose was level, your angle of attack would have been +30 degrees. That's about double the critical angle of attack.

no intention to maintain altitude so the wing is not working hard (aka low angle of attack)

The nice linear relationship between lift, airspeed and angle of attack breaks down in the stall. That's the whole point. So once you're stalled there's no correlation there. You get more lift at a lower angle of attack in the stall - working the wing "not very hard" can mean an extreme stall, and an extreme angle of attack.

(Complicating that picture is that at such a steep angle, a big chunk of the force holding the aircraft up is the vertical component of drag.)

[akoch]

Allright. I personally fail to see the high angle of attack, but I accept I can be stupid.

But the reason I tried it in the first place was to explore the slip characteristics, as I also had the same question - if I could stall it into spin in a slip. So the take away was that I am highly unlikely to do that as the nose up attitude with full stick aft was stunning and with full aft stick/full top rudder it still did not spin or attempt to. I would be highly unlikely to let it develop to such extent slipping in the pattern. Which I personally do a lot, it is just a fun thing to do and my plane slips well. And I know all too well that with the bottom rudder it spins all too well and for as long as you care to

[Colonel Sanders]

airspeed was 60kts

At a high AOA, one would reasonably expect
the ASI to under-read. This of course is
complicated by the slip - one wing would be
at a higher AOA than the other, and we don't
know which wing the pitot tube is on.

Anyways, ignoring the above, unless you're
pulling G, you're not stalled with 60 knots on
the ASI, if your Vs is 36 knots!

Of course, if one had AOA sensors on both
wings, you could easily see what was going on.

It is truly unfortunate, this obsession that GA
has with airspeed, instead of AOA.

The wing doesn't care how fast you are going.
The United States Navy, which does some pretty
interesting flying, doesn't care about airspeed
when you slow down for approach. They fly
their approach to a carrier landing based solely
on AOA. They couldn't care what their airspeed
is.

Meanwhile, back at the ranch, civilian aviation
concerns itself only with indicated airspeed,
and AOA is at best a curiousity.

[akoch]

Thank you for summarizing it CS.

That was my whole point, pretty much. In a slip, I'm "pulling" 1G at best realistically less than 1G allowing the aircraft to sink as it likes. The ASI is known to have minimal error in a slip (let's say it was slip to the left, the pitot/static is an integrated sensor on the left wing). I know it is accurate as when I roll back to straight the ASI does not change appreciably. I don't know if I can install the AOA sensors easily, that' would be nice to have.

[photofly]

It's neither a sin or a crime to not see something. I don't know if this helps with the angle of attack thing:

steep_descent.png (12.21 KiB) Viewed 1522 times

I don't believe you can get into a spin from a slip. I think you could, just about, if you mishandle the rudder in a slow slip, because of too much yaw. I think you'd have to work very hard at it though, and you could argue that once you start yawing, you're alternately slipping and skidding, and it's the skidding that will get you into trouble.

This of course is
complicated by the slip - one wing would be
at a higher AOA than the other,...

Only if you have dihedral. Otherwise, no, unless you're in turning flight, they're at exactly the same AoA.

you're not stalled with 60 knots on
the ASI, if your Vs is 36 knots!

Sorry boss, that's not right.

You're basing that on the fact that normally, at 60kts you're "obviously" able to develop enough lift to hold the aircraft up at an AoA less than the stalling AoA. The argument fails in a deep stall though. If the angle of attack is high enough you can fly (sink!) at any speed, and still not have enough lift to hold up the aircraft. The deficit in vertical force is made up by the vertical component of the drag.

[Colonel Sanders]

The argument fails in a deep stall though

I see your point, though I am having trouble
believing that he has a powerful enough elevator
to actually do that. I've never flown an aircraft
with an elevator that wickedly effective, with no
slipstream passing over it.

Was your stall warning light/horn on? That should
tell us - ignoring various asymmetries - which side
of Clmax he was on, with 60 on the ASI.

[photofly]

The argument fails in a deep stall though

I see your point, though I am having trouble
believing that he has a powerful enough elevator
to actually do that. I've never flown an aircraft
with an elevator that powerful.

Was your stall warning light/horn on? That should
tell us - ignoring various asymmetries - which side
of Clmax he was on.

He says the stall warning wasn't on.

He also says that he was flying at an indicated 60 kts, and descending at 2-3000 fpm. Even allowing for an artificially low ASI that's still a descent angle well above the critical angle of attack. To not be stalled, the nose would have had to have been well below the horizon.

I deduce that if the stall horn wasn't sounding, then the stall horn isn't reliable in this scenario.

I think you overestimate the power of elevator needed: A deep stall doesn't need a lot of nose up, just a low forward speed and steep descent rate. The falling leaf is exactly this scenario.

I've never flown an aircraft
with an elevator that wickedly effective, with no
slipstream passing over it.

There was still 60kts+ airspeed over the elevator; that's quite a lot of airflow.

[photofly]

Complicating factor: in a serious sideslip, the flow isn't spanwise. So you effectively have two swept wings (one forward, one backward) with increased chord and reduced span. It's doing my head in to work out what that does to the stall characteristics.

[trampbike]

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.

In a straight constant rate of descent or climb, lift=weight... exactly like when you are straight and level.
If lift did not equal weight, then you would either be accelerating downward or upward.

[photofly]

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.

In a straight constant rate of descent or climb, lift=weight... exactly like when you are straight and level.
If lift did not equal weight, then you would either be accelerating downward or upward.

The point you're making is a good one; unless there's acceleration then the forces are all in balance.

Do remember though that lift is perpendicular to the relative wind, so lift=weight really only works when the flight path is horizontal, or sufficiently close to it.

This guy is descending at 30 degrees to the horizontal. In which case the weight of the aircraft is supported by 86% of the lift and 50% of the drag.

[trampbike]

Do remember though that lift is perpendicular to the relative wind, so lift=weight really only works when the flight path is horizontal, or sufficiently close to it.

This guy is descending at 30 degrees to the horizontal. In which case the weight of the aircraft is supported by 86% of the lift and 50% of the drag.

I do remember this, but I did not think it'd be pertinent at this point of the discussion.
We could even play semantic and say that drag forces pointing upward are part of the total lift couldn't we?

The very basic point I think akoch needs to get from my previous intervention is effectively this :

unless there's acceleration then the forces are all in balance.

[photofly]

We could even play semantic and say that drag forces pointing upward are part of the total lift couldn't we?

Only if we wanted to be wrong. It's non-trivial to point out that being semantically correct is not the same as being trivial. It's also both semantically correct and non-trivial to point out that lift acts at right angles to drag, so drag forces aren't in any way ever part of the lift, be it total, partial or otherwise-qualified.

If we wanted to be correct, we could say that the vertical component of the drag force supports part of the weight of the aircraft.

As for pertinence: it's extremely pertinent in a discussion in which "how hard" the wing is working to generate lift is a relevant theme, to point out that the lift generated by the wing is in fact not equal to the whole weight of the aircraft. Even though all forces are in balance and the net acceleration is zero.

[Colonel Sanders]

if the stall horn wasn't sounding, then the stall horn isn't reliable

I'm having trouble believing that. I think the much
more likely explanation is that the wing had an AOA
substantially less than Clmax, which is confirmed by
the 60 knots on the ASI with +1G.

He was merely descending at 60 knots with some
drag, caused by the sideslip.

I descend all the time at 3,000 fpm from downwind
in a continuous U base-to-final with 120 mph indicated,
and I'm not in a deep stall. Sometimes on final I will
use a sideslip of 90 degrees, and again the wings aren't
stalled - I just have some drag on.

In the co-ordinated case it's caused by the 3-blade
constant speed prop at full fine pitch. With the 90
degree banked sideslip I additionally have the drag
of the side of the fuselage and canopy. But no deep
stall - just a descent rate approaching 10,000 fpm,
caused by the drag.

http://en.wikipedia.org/wiki/Occam's_razor

[akoch]

Ah, could it be this? In that forward slip my left wing was yawed forward, to an optical effect of flying completely sideways (45-50 degree yaw to the right?). Also, the plane is banked to 35-45 degrees into the slip. So this might be responsible for the lower angle of relative airflow? The stall horn certainly did not activate, an upon recovery the speed was the same 60 knot as indicated in the slip.

[photofly]

OK, I now think you're absolutely right. You're not stalled at all, and the AoA isn't 40 degrees. It's a geometry issue. Let me think of a diagram that illustrates it. Thank heavens for model airplanes.

[Colonel Sanders]

not sure what you mean by 90 degrees of sideslip

Simple. In a continuous descending U-turn
from downwind to final, as you start to line
up with the runway on short final, increase
the bank to 90 degrees. Top rudder as
required. Marvellous drag produced by the
lower side of the fuselage and canopy, which
you can instantly eliminate by rolling wings
level just before you pass over the runway
threshold for touchdown.

Stall warning horn never comes on. Low AOA.

There are all sorts of fun things you can do, if
you are not afraid of a little bank angle. One of
my favorite ways to join the circuit is to fly overhead
the runway, then as you roll onto downwind
keep banking until you are inverted straight and
level, and fly the downwind inverted.

there are two different AoA's that produce the same lift - one in the stalled regime, and one not

Of course - the Cl curve tells you that. But I
suspect it's a little harder than you think, in
most aircraft, to keep it in the deep stall. You
run out of elevator effectiveness.

This is easy to demonstrate in the falling leaf.

Put some flaps on to get the nose down, and
leave some power on (say 1250 rpm) and if
you gradually pull the control column all the
way back, you can stall it, and try to walk to
down with the rudders. But what often happens
is that you get an oscillation in pitch going,
and the wings will unstall, which is annoying -
even with full aft control column!

I agree with your theory, but in practice, I really
doubt you would have the elevator effectiveness
to keep it deeply stalled - at least, not without
a very unusual aircraft.

Remember, chalk and stick.

[photofly]

Chalk and stick agree. You're not stalled. Akoch wasn't stalled. I was holding the wrong piece of chalk.

Aircraft descends towards camera in a straight descent, with longitudinal axis horizontal: note high angle of attack:

photo copy.JPG (50.1 KiB) Viewed 1722 times

Aircraft rotated in yaw (to the pilot's right) and bank (to the pilot's left). It still has its longitudinal axis horizontal. Now while descending towards the camera the wings are edge-on and it has zero angle of attack.

photo copy 2.JPG (48.51 KiB) Viewed 1722 times

[akoch]

What is chalk and stick?

[Colonel Sanders]

Chalk and stick is a lot less annoying than
Occam's Razor, which you shouldn't use to
shave your legs, because they itch something
terrible when it grows back in.

See Reddit:

http://www.reddit.com/r/flying/comments ... s_texting/

[tkdowell]

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.

Hey Akoch,

I am curious as to what you use to determine the aircraft's acceleration.

I don't know you at all, but please understand that many pilots (incorrectly) use the airspeed indicator for reference to that aircraft's speed. It does work for trend - after you are fully established in the forward slip. For example, if you start at 70 KIAS before the slip and enter a right wing low forward slip, you'll airspeed will increase to...say...80 KIAS. This is assuming the static port is on the left side of the fuselage. Of course this isn't the case on all aircraft. As you probably already know, this "apparent acceleration" is due to ram effect (affect) or lack thereoff on the static port.

In this case, many pilots pull the nose up to "slow" back down to 70 KIAS.

Some people will say that this is "proper" because a forward slip "is most effective when performed at a slower speed", as it gives your closure rate to the field is slower and thus you have more time to lose altitude. Others will say there is a disproportionate increase in sink rate with airspeed (i.e. the increased airspeed, and greater closure rate, is overcome by an even greater sink rate).

I have to be careful, because I'm not the best at explaining things and I don't want Colonel Sanders to make fun of my theoretical post

Food for thought anyways.

TD