Stall speed in an established slip
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Miserlou:
I said:
Which means I'm slipping and increasing the airspeed, not just diving, which would do nothing except make the runway go by faster.
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 said:
I go full flap, full rudder, and keep the speed up, since drag is speed^2
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I'm sure there's less lift in a slip, because it's going down like a lift.
I wonder whether this could be a real reason to expect stalling speed to increase when converting from a slip to level flight - your rate of descent is liable to decrease in level flight, so whilst transitioning you will require more lift and therefore a greater angle of attack than when in an established glide. You're effectively pulling out of a dive.
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It actually needs a slight amount of up-elevator to stop it speeding up in the slip.
As regards the lack of AoA indicators in aeroplanes, it's always struck me as a major omission. Fast jets, Concorde, and gliders (the ubiquitous 'wool tuft') have them. Not many other aeroplanes I'm aware of do.
There are several instruments in the aerobatic aeroplanes I've flown I'd gladly trade for one.
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Less lift in a slip?
Ok, I'll bite on this one. Yes, there is.
As the effective lift acts perpendicular to the leading edge (check your swept wing aerodynamics) then there will be slightly less lift.
However, I believe it's the significant increase in drag which is the major factor.
Abgd,
As one is releasing the slightly increased back pressure on the controls, the risk of stalling is not an issue when re-establishing straight flight.
Ok, I'll bite on this one. Yes, there is.
As the effective lift acts perpendicular to the leading edge (check your swept wing aerodynamics) then there will be slightly less lift.
However, I believe it's the significant increase in drag which is the major factor.
Abgd,
As one is releasing the slightly increased back pressure on the controls, the risk of stalling is not an issue when re-establishing straight flight.
Thread Starter
I'm not quite sure what you're getting at regarding lift perpendicular to the leading edge?
If an aircraft is descending at a steady rate, ascending at a steady rate or flat on the tarmac, the force counteracting gravity must be the same - whether it's due to the wings, terra firma, or helium bladders in the fuselage. For our purposes it's likely to come from airflow over the aircraft, so the question isn't how much lift there should be - we know the answer to that - but how it's produced.
Perhaps we're talking about slightly different things - lift defined as force perpendicular to the airflow will change slightly because the angle of descent changes in a slip. In fact, it must increase as we need the vertical components to remain equal.
If an aircraft is descending at a steady rate, ascending at a steady rate or flat on the tarmac, the force counteracting gravity must be the same - whether it's due to the wings, terra firma, or helium bladders in the fuselage. For our purposes it's likely to come from airflow over the aircraft, so the question isn't how much lift there should be - we know the answer to that - but how it's produced.
Perhaps we're talking about slightly different things - lift defined as force perpendicular to the airflow will change slightly because the angle of descent changes in a slip. In fact, it must increase as we need the vertical components to remain equal.
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Sorry Glum, but if your supposition was true then logically, knife-edge flight by the aerobatic chaps wouldn't be possible, and it clearly is.
In a side slip, this 'lift' simply becomes a drag component to the flight direction
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Fuselage lift is so inefficient you need a lot of pull from the donkey to stay in knife edge flight for any appreciable amount of time. Couldn't tell you what the stalling AoA is on the fuselage but it does plummet the same when you stall on it!
For anyone that thinks you can't fly on the fuselage have a read up on slow rolls.
For anyone that thinks you can't fly on the fuselage have a read up on slow rolls.
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I wonder how you would actually deduct the airspeed when you are in a slip. It's hard enough when flying clean with all the temp/pressure errors this thing has.
When in a slip the airflow is at an angle on the pitot tube yielding errors. And depending on the design of the static port(s), airflow could be forced in here giving errors as well. The C172 is notorious for this as there is only one static port on the left side.
In total the ASI reading is very unpredictable when slipping. Without a correct reading ASI it can be a bit tricky to determine if you are actually gaining or loosing lift, as airflow speed is part of the equation...
When in a slip the airflow is at an angle on the pitot tube yielding errors. And depending on the design of the static port(s), airflow could be forced in here giving errors as well. The C172 is notorious for this as there is only one static port on the left side.
In total the ASI reading is very unpredictable when slipping. Without a correct reading ASI it can be a bit tricky to determine if you are actually gaining or loosing lift, as airflow speed is part of the equation...
Bearing in mind all the above caveats & that no two a/c are exactly the same, one does in my sort of light a/c rely on the ASI with usually 1.3 X stall for a normal approach.
That's quite a big fudge factor to help deal with all sorts of errors & local wind effects in the airs over the hedge.
Consequently when folk start discussing the finer points of changed direction of airflow - sort of angled across the pitot - one assumes they mean it will read lower than the actual forward speed of the 'plane. They also should simultaneously note that the wing generally sees the same angled airflow.
In both cases the pitot and wing experience much the same net airflow at approx. right angles to the leading edge.
In view of the relative crudity of the pressure sensing system used as a speed measuring device I am of the opinion that when usefully side slipping, if one only keeps to the usual margin over the IAS it a) won't be closer to the stall than normal
and b) one's actual forwards speed is higher, thus requiring a certain amount of care to reduce momentum, restore a normal attitude & a slow enough speed shortly before touch down.
mike hallam.
That's quite a big fudge factor to help deal with all sorts of errors & local wind effects in the airs over the hedge.
Consequently when folk start discussing the finer points of changed direction of airflow - sort of angled across the pitot - one assumes they mean it will read lower than the actual forward speed of the 'plane. They also should simultaneously note that the wing generally sees the same angled airflow.
In both cases the pitot and wing experience much the same net airflow at approx. right angles to the leading edge.
In view of the relative crudity of the pressure sensing system used as a speed measuring device I am of the opinion that when usefully side slipping, if one only keeps to the usual margin over the IAS it a) won't be closer to the stall than normal
and b) one's actual forwards speed is higher, thus requiring a certain amount of care to reduce momentum, restore a normal attitude & a slow enough speed shortly before touch down.
mike hallam.
Re AoA indicators, the wool tuft on a glider is there to indicate yaw. It won't give you any other useful indication, but is more sensitive than a slip ball. (and cheaper)
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Cover your ASI and practice without it every now and then!
Does nobody fly by attitude and feel? On finals I very rarely look at the ASI unless in a new type I'm unfamiliar with. I say again that a slip is very intuitive and the aeroplane will tell you if it's happy or not. The buffeting it creates gives a good indication of speed and though it's a different picture attitude is still relevant. What do you need other than your sight line to landing point to tell if it's having the desired effect on descent rate? In most of the types I fly you do need less nose down attitude in the slip to maintain the same speed and the fact that you require a bit more back pressure on the stick would indicate less lift is being generated by the wings maybe? As you straighten up the back pressure on the stick needs to be relaxed or if you are slipping at a minimum speed to get into a short strip for instance ... you will be close to the stall for sure.
SS
Does nobody fly by attitude and feel? On finals I very rarely look at the ASI unless in a new type I'm unfamiliar with. I say again that a slip is very intuitive and the aeroplane will tell you if it's happy or not. The buffeting it creates gives a good indication of speed and though it's a different picture attitude is still relevant. What do you need other than your sight line to landing point to tell if it's having the desired effect on descent rate? In most of the types I fly you do need less nose down attitude in the slip to maintain the same speed and the fact that you require a bit more back pressure on the stick would indicate less lift is being generated by the wings maybe? As you straighten up the back pressure on the stick needs to be relaxed or if you are slipping at a minimum speed to get into a short strip for instance ... you will be close to the stall for sure.
SS
