Stall speed in an established slip
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Most GA planes don't have an AOA indicator, so speed is the most indicative. That works for flying straight and level, anything else is a best guess. Most GA planes don't do aerobatics either.
So to get back on topic: in a straight and level slip with constant speed, do you think the speed at which the plane reaches critical AOA on a wing is higher, lower or the same? My guess is higher, see my reasoning above. Due to the air flow coming from a side angle, the indicated speed could be lower, higher or the same, depending on the position of the static port and pitot tube.
So to get back on topic: in a straight and level slip with constant speed, do you think the speed at which the plane reaches critical AOA on a wing is higher, lower or the same? My guess is higher, see my reasoning above. Due to the air flow coming from a side angle, the indicated speed could be lower, higher or the same, depending on the position of the static port and pitot tube.
I have seen articles claiming that in a sideslip stall speed can actually be lower than in balanced flight, because of lift produced by the fuselage. There are also articles that claim that it's higher. Conceivably it could be both, depending on the aircraft type.
Does anybody know what the actual difference is, for any reasonably common types?
Does anybody know what the actual difference is, for any reasonably common types?
When an aircraft stalls, there will be something showing on the ASI. This is the stall IAS in that aircraft, under those conditions. If the aircraft stalls under identical conditions, the stall IAS will be the same, so to suggest that there is no such thing as stall speed is unhelpful.
In a steady heading sideslip, in unaccelerated, level flight, there will be some wing blanking, some turbulence, some spanwise airflow. and some angling of the lift vector away from the vertical. For a given speed therfore, assuming for our purpose that the ASI has no position errror due to the sideslip, in order to maintain sufficient vertical lift component to support the weight, the AOA will have to be increased. This means that the aircaft will reach it's critical AOA at a higher speed.
In a sideslip, there will however, always be some element of lift generated by the fuselage, and there will be some vertical component of this lift. The question is, will this fuselage vertical lift component be sufficient to compensate for the vertical lift component lost. If it is, then there will be no increase in wing AOA to maintain the total vertical component, and the stall speed will remain the same. If it isn't, then the stall speed will be higher. If it is more than required just to compensate for the lost vertical component, then the stall speed will be lower.
In practice it will be difficult to evaluate the effect, without a special flight test pitot/static system, as the position errors in a sideslip can be huge, so I suggest that few pilots, if any, will be able to say which common types of aircraft fall into each category.
MJ
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If you slip, your wing is operating at less efficiency, so a lower coefficent of lift, so you need more speed for the same lift, or more AOA at the same speed, so my guess would be a higher stall-speed.
A better question would be what are you doing slipping with a high AOA/low-speed? It seems like a great way of getting yourself killed.
When I do a slip to lose altitude on final, and I need to lose as much as I can in as short a distance as I can, I go full flap, full rudder, and keep the speed up, since drag is speed^2. I think J. Mac McClellan (or was it Richard Collins?) wrote this in Flying Magazine a long time ago as more effective than going slow and waiting. You can go above or below best glide speed to get rid of energy faster, but going faster is safer since you're not as close to stall. Just don't get too close to the flap limits.
A better question would be what are you doing slipping with a high AOA/low-speed? It seems like a great way of getting yourself killed.
When I do a slip to lose altitude on final, and I need to lose as much as I can in as short a distance as I can, I go full flap, full rudder, and keep the speed up, since drag is speed^2. I think J. Mac McClellan (or was it Richard Collins?) wrote this in Flying Magazine a long time ago as more effective than going slow and waiting. You can go above or below best glide speed to get rid of energy faster, but going faster is safer since you're not as close to stall. Just don't get too close to the flap limits.
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Abgd,
No need to lower the nose PRIOR to uncrossing the controls It works like this.
As you enter the slip the nose will have a shallower angle. You maintain this pitch angle. The ASI will show a lower speed due to position error. As you straighten up the nose 'falls' back to the original pitch angle and the indicated speed on the ASI is restored. It never actually changed.
Slam,
Choosing a higher than normal approach speed just leaves you in ground effect with excess speed which makes the whole thing pointless. Don't see what's wrong with doing it at the proper speed.
The whole point of the exercise being to increase the drag to increase the descent angle.
No need to lower the nose PRIOR to uncrossing the controls It works like this.
As you enter the slip the nose will have a shallower angle. You maintain this pitch angle. The ASI will show a lower speed due to position error. As you straighten up the nose 'falls' back to the original pitch angle and the indicated speed on the ASI is restored. It never actually changed.
Slam,
Choosing a higher than normal approach speed just leaves you in ground effect with excess speed which makes the whole thing pointless. Don't see what's wrong with doing it at the proper speed.
The whole point of the exercise being to increase the drag to increase the descent angle.
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Shaggy sheep. Gibbering wreck indeed. Well thanks for that, excuse me for my existence. You are coming across as one of the head up his ass instructors who never listen to any body.As for stall speed/angle of attack. I do know the difference but you obviously can't grasp that. So get on with it. Good night.
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Choosing a higher than normal approach speed just leaves you in ground effect with excess speed which makes the whole thing pointless. Don't see what's wrong with doing it at the proper speed.
You straighten out on short final, slow down, and land normally at normal speeds. The whole point is to have the right energy as you come over the fence in a normal landing config. Isn't that obvious? One would have to be a complete, 100%, undiluted moron to think anyone advocates actually touching down while still in a full slip (other than a forward slip for cross-winds) or while carrying excess speed.(We've gone from a nuanced discussion about the effects of a slip to stall-speed into stating the bloody obvious apparently.)
By the way, I'd much rather float a little carrying 5-10 knots extra over the numbers on a long runway than miss the fecking runway completely. If you think coming out of a slip a few knots fast "makes the whole thing pointless" compared with not slipping... I don't even know what to say to that.
I think I'm done with this discussion. Too many people stating the bloody blatantly obvious. (Not you, Shaggy, or the OP.)
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pitot errors prevent any correct indication of air speed
Probably one need not worry too much about the pitot. However, the static PEC can be expected to be shot to pieces unless it was addressed in the FT program for some strange reason.
Probably one need not worry too much about the pitot. However, the static PEC can be expected to be shot to pieces unless it was addressed in the FT program for some strange reason.
Thread Starter
An example of an article that claims that stall speed is reduced in a slip.
Aerodynamics of a Final Approach Slip Explained | Aviation International News
Incidentally, for a thought experiment imagine a cylindrical fuselage and a normal wing, rotated from normal flight to 45 degrees to the direction of the airflow.
The angle of attack of the wing will be reduced, but the angle of attack of the fuselage will stay the same - zero (assuming initial airflow was axial).
Now increase the angle of attack of the wings until it is the same as the original. Obviously there will be some blanking by the fuselage, and the wingspan will be effectively reduced (and chord increased). However, the angle of attack of the fuselage will now no longer be zero.
Ergo we may expect the amount of lift caused by the fuselage to increase from zero when the angle of attack was zero, to something. Assuming the amount of lift created by the wing remains constant, and given that the amount of lift contributed by the fuselage has risen from nothing to something, we can now reduce the angle of attack of the wing by a small amount until the total amount of lift (fuselage plus wings) is equal to the weight of the aircraft - i.e. the same as in the initial unslipped condition.
And putting some numbers into it, let's take a more reasonable slip of 30 degrees (still probably rather a lot).
The height of the leading edge above the trailing edge will be identical for an aircraft with a horizontal wing.
The chord will be 1.15 x greater.
Now, if your initial angle of attack was 10 degrees just before the stall, it will now be about 1.2 degrees less, so you will perhaps have to increase the angle of attack by another 1.2 degrees to compensate. You will have to increase the angle of attack of the fuselage by about the same amount to compensate for lost lift, and you will generate a little lift by doing this.
Looking at a top view of a Cessna 172 the fuselage seems to have about half the surface area of its wings, so assuming that it is equally efficaceous in producing lift per unit area (which it probably won't be - and I don't mean efficient because I'm expecting lots of drag) you might expect that it would now be producing about 6% extra lift relative to the unslipped condition.
In practice, I'm betting that the wings are likely to produce at least 6% less lift in a slip due to shadowing, turbulence caused by the fuselage so on reflection it seems unlikely to me that stall speed is lower in a slip.
Aerodynamics of a Final Approach Slip Explained | Aviation International News
Incidentally, for a thought experiment imagine a cylindrical fuselage and a normal wing, rotated from normal flight to 45 degrees to the direction of the airflow.
The angle of attack of the wing will be reduced, but the angle of attack of the fuselage will stay the same - zero (assuming initial airflow was axial).
Now increase the angle of attack of the wings until it is the same as the original. Obviously there will be some blanking by the fuselage, and the wingspan will be effectively reduced (and chord increased). However, the angle of attack of the fuselage will now no longer be zero.
Ergo we may expect the amount of lift caused by the fuselage to increase from zero when the angle of attack was zero, to something. Assuming the amount of lift created by the wing remains constant, and given that the amount of lift contributed by the fuselage has risen from nothing to something, we can now reduce the angle of attack of the wing by a small amount until the total amount of lift (fuselage plus wings) is equal to the weight of the aircraft - i.e. the same as in the initial unslipped condition.
And putting some numbers into it, let's take a more reasonable slip of 30 degrees (still probably rather a lot).
The height of the leading edge above the trailing edge will be identical for an aircraft with a horizontal wing.
The chord will be 1.15 x greater.
Now, if your initial angle of attack was 10 degrees just before the stall, it will now be about 1.2 degrees less, so you will perhaps have to increase the angle of attack by another 1.2 degrees to compensate. You will have to increase the angle of attack of the fuselage by about the same amount to compensate for lost lift, and you will generate a little lift by doing this.
Looking at a top view of a Cessna 172 the fuselage seems to have about half the surface area of its wings, so assuming that it is equally efficaceous in producing lift per unit area (which it probably won't be - and I don't mean efficient because I'm expecting lots of drag) you might expect that it would now be producing about 6% extra lift relative to the unslipped condition.
In practice, I'm betting that the wings are likely to produce at least 6% less lift in a slip due to shadowing, turbulence caused by the fuselage so on reflection it seems unlikely to me that stall speed is lower in a slip.
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I was under the impression that angle of incidence was the angle of the wing chord line to the fuselage mean line, and would be difficult to alter without major surgery.
Thread Starter
OK - made 2 small edits for clarification.
But obviously the relative angles of attack of the fuselage and wings can change, whilst the angle of incidence remains the same.
But obviously the relative angles of attack of the fuselage and wings can change, whilst the angle of incidence remains the same.
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Slam525i
You've taken Miserlou's perfectly reasonable reply and jumped on him. I don't for a minute think he meant you were landing in such a configuration. I think he meant that if flown the way you describe, you straighten out just before land then have that extra speed to lose in ground effect and float down the runway. That's fine on a long runway, but if like often the case with me, I need to slip coming into a very short strip over woods to lose that last bit of height. Any float would make the exercise pointless!
You'll probably think I'm having a dig too, but it's really just constructive criticism .... Which is the whole point of these threads surely?
SS
You've taken Miserlou's perfectly reasonable reply and jumped on him. I don't for a minute think he meant you were landing in such a configuration. I think he meant that if flown the way you describe, you straighten out just before land then have that extra speed to lose in ground effect and float down the runway. That's fine on a long runway, but if like often the case with me, I need to slip coming into a very short strip over woods to lose that last bit of height. Any float would make the exercise pointless!
You'll probably think I'm having a dig too, but it's really just constructive criticism .... Which is the whole point of these threads surely?
SS
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You're right, shortstripper. I was already worked up over the huge fuss everyone is making of the AOA/stall-speed thing. (It's such a simple concept.)
My apologies, Miserlou.
I will say though, that I don't see anything wrong with pushing the airplane down with a full slip at higher than normal approach speeds. It bleeds off more energy in a shorter distance, and will give you the same landing roll if you're doing it right (not carrying extra speed as you get to the runway). In fact, you're less likely to end up carrying extra speed because you'll have bled off that energy earlier in the approach.
The alternative of doing it at normal approach speeds (basically best-glide speed), will give you a shallower angle of descent. At slower than normal approach speed, you'll again get better angle of descent, but you're flirting with a stall.
My apologies, Miserlou.
I will say though, that I don't see anything wrong with pushing the airplane down with a full slip at higher than normal approach speeds. It bleeds off more energy in a shorter distance, and will give you the same landing roll if you're doing it right (not carrying extra speed as you get to the runway). In fact, you're less likely to end up carrying extra speed because you'll have bled off that energy earlier in the approach.
The alternative of doing it at normal approach speeds (basically best-glide speed), will give you a shallower angle of descent. At slower than normal approach speed, you'll again get better angle of descent, but you're flirting with a stall.
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Hi Slam,
Yes I see your point as a pure height losing exercise and without thinking about it too hard I think you're right and reasonable on that score. I suppose the only concern might be a little extra strain on the rudder post, but that is only if taking it to the extreme and I know that's not what you meant.
SS
Yes I see your point as a pure height losing exercise and without thinking about it too hard I think you're right and reasonable on that score. I suppose the only concern might be a little extra strain on the rudder post, but that is only if taking it to the extreme and I know that's not what you meant.
SS
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Let's look at it differently. If you're flying straight and level at critical AOA, what would the effect of entering a side slip be? To stall or not to stall?
Another one. Let's say you're just passed the critical AOA in a straight and level flight and are in a stall. Will entering a side slip recover you?
In both cases I wouldn't bet on the side slip being a help. Maybe that's my inexperience as a fresh PPL holder, but I find it hard to believe that in the same circumstances a side slip will reduce the AOA. If only for the roll input required to compensate for the yaw induced roll. Roll input at critical AOA will make the wing with down aeleron stall. That's why you only use rudder during recovery. People die from mistakes such as this, especially on final.
Another one. Let's say you're just passed the critical AOA in a straight and level flight and are in a stall. Will entering a side slip recover you?
In both cases I wouldn't bet on the side slip being a help. Maybe that's my inexperience as a fresh PPL holder, but I find it hard to believe that in the same circumstances a side slip will reduce the AOA. If only for the roll input required to compensate for the yaw induced roll. Roll input at critical AOA will make the wing with down aeleron stall. That's why you only use rudder during recovery. People die from mistakes such as this, especially on final.
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Another one. Let's say you're just passed the critical AOA in a straight and level flight and are in a stall. Will entering a side slip recover you?
But I am a 130-hour PPL myself so let's wait and see what the big boys have to say.
/h88
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Hi Pirke,
It's actually not easy to spin from a slip if that's what you were implying?
The upper wing will generally stall before the lower but will induce corrective yaw by doing so and pull you out of the slip and full stall unless you are really ham fisted. I'm rubbish at explaining this but try it sometime. A skid is a very different beast and will spin you at the drop of a hat!
Once learned and ingrained, slipping becomes instinctive and very intuitive as you get to just know by feel how much back pressure is required and the natural buffeting created by the airflow disruption gives you a good idea of speed / AoA
SS
Edited to add
The first bit of what you write is interesting. When on the point of stalling, almost any control imput could tip you over the edge and possibly into a spin and so you are correct. However, when already established in a slip it changes the dynamic completely and with a few caveats is a very safe manoeuvre.
It's actually not easy to spin from a slip if that's what you were implying?
The upper wing will generally stall before the lower but will induce corrective yaw by doing so and pull you out of the slip and full stall unless you are really ham fisted. I'm rubbish at explaining this but try it sometime. A skid is a very different beast and will spin you at the drop of a hat!
Once learned and ingrained, slipping becomes instinctive and very intuitive as you get to just know by feel how much back pressure is required and the natural buffeting created by the airflow disruption gives you a good idea of speed / AoA
SS
Edited to add
The first bit of what you write is interesting. When on the point of stalling, almost any control imput could tip you over the edge and possibly into a spin and so you are correct. However, when already established in a slip it changes the dynamic completely and with a few caveats is a very safe manoeuvre.
Last edited by shortstripper; 17th Apr 2014 at 07:45. Reason: to add a bit
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I'm really intrigued by the second part...is this a "trick" answer...IE it's not stalled atzero airspeed, simply because there's no airflow to start any lift?
If there's no load on the wing it won't stall as the stalling angle of attack will not be reached. Conversely, pulling out of the 4th quarter of a loop the speed may be very high but pulling too hard can cause the critical angle to be exceeded (usually on one wing because the ball may not be exactly in the middle), and the result is that wing stalls and the aeroplane flicks, quite violently.
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Slam,
Apology accepted but……
What you say about flying higher speed, although theoretically correct, requires a greater horizontal distance where you are already 'too close' ('too high' equals 'too close') and has a much less significant effect than slipping.
I still don't see what's wrong with doing the whole process at the correct speed.
Happiness is……
ending the final turn in a side slip and straightening up in the flare to a 3 pointer!
Apology accepted but……
What you say about flying higher speed, although theoretically correct, requires a greater horizontal distance where you are already 'too close' ('too high' equals 'too close') and has a much less significant effect than slipping.
I still don't see what's wrong with doing the whole process at the correct speed.
Happiness is……
ending the final turn in a side slip and straightening up in the flare to a 3 pointer!
Last edited by Miserlou; 17th Apr 2014 at 16:07.
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An interesting thread!
The little Jodel I fly these days doesn't have an AOA indicator - indeed, it has very few instruments. So I use the ASI to tell me how things are going. It also has no flaps, and glides quite well for an old lady. It will stall, but complains through the controls first.
The standard method of losing height quickly is to sideslip it. It behaves well doing that, and fortunately (or not, depending on your point of view) the ASI indications when slipping are useable. I find that keeping the same IAS while slipping works reasonably well. Roll out of the slip, and the IAS remains unchanged. It actually needs a slight amount of up-elevator to stop it speeding up in the slip.
I'm sure there's less lift in a slip, because it's going down like a lift. As to what the "stalling speed" is, I have no idea. 80kph indicated seems to work well.
The little Jodel I fly these days doesn't have an AOA indicator - indeed, it has very few instruments. So I use the ASI to tell me how things are going. It also has no flaps, and glides quite well for an old lady. It will stall, but complains through the controls first.
The standard method of losing height quickly is to sideslip it. It behaves well doing that, and fortunately (or not, depending on your point of view) the ASI indications when slipping are useable. I find that keeping the same IAS while slipping works reasonably well. Roll out of the slip, and the IAS remains unchanged. It actually needs a slight amount of up-elevator to stop it speeding up in the slip.
I'm sure there's less lift in a slip, because it's going down like a lift. As to what the "stalling speed" is, I have no idea. 80kph indicated seems to work well.