Slip Vs. Stall Speed

[dhoward]

Does stall speed increase when an airplane is not in coordinated flight?

 

[huncowboy]

In a slip wind hits the wings at an angle, so the airfoil will not be able to produce as much lift as in normal flight, thus stall speed should be higher. But some airplanes read above or below the normal indicated airspeed depending on which side the static port is and toward which side the slip is performed (unless they have 2 static ports).

When you do a cross-controlled stall, that is exactly what you are trying to present, namely that a stall will occur earlier in an uncontrolled condition. We stalled at 65ish versus 50 the other day while performing a cross-controlled stall.

Can I have your ATP now?

 

[dhoward]

I had a CFI student ask me this question and told him the same thing that you said about the wing not producing as much lift because of less flow across the chord of the wing. Funny that no text I can find mentions this effect of a slip, it seems pretty relevant.

 

[JetBlast2000]

In a slip wind hits the wings at an angle, so the airfoil will not be able to produce as much lift as in normal flight, thus stall speed should be higher.

I’m not so sure I buy into your argument. The amount of lift garneted at any given AOA does not affect the stall speed. It only means you would need a higher AOA thereby getting you closer to your Critical AOA giving you a higher stall speed. I would say that the stall speed is higher due to the increase in drag requiring a higher AOA thus getting you closer to your Critical AOA.

JB2k

 

[ShawnC]

It's the pitot tube that is reading wrong when you are in a slip (though the static port could have an affect on it). The pitot tube was designed to read the airspeed coming straight onto it, but when you in a slip it's coming in at an angle thus not all the air is entering the tube, instead some of it's going around the tube.

 

[dhoward]

I would think that as long as the increas in drag can be overcome with power it should have no bearing on stall speed. The loss of efficiency of the wing when it loses some of its chordwise airflow because of meeting the oncoming air at an angle would cause the AOA to increase if you try to maintain altitude at a given airspeed. So the lift required to keep the airplane on altitude would require a higher AOA at any airspeed and the stall speed would be higher than normal.

 

[huncowboy]

It's the pitot tube that is reading wrong when you are in a slip (though the static port could have an affect on it). The pitot tube was designed to read the airspeed coming straight onto it, but when you in a slip it's coming in at an angle thus not all the air is entering the tube, instead some of it's going around the tube.

ShawnC,

Your airspeed indicator only measures ram pressure than?

The static tube definitely has an effect on it, it is even mentioned in some POH. The static port is also tied into the airspeed indicator, not only the pitot tube. It adjusts the airspeed reading for altitude. If you do a slip toward the side on which the static port is, it will read higher pressure thus the airspeed indicator will think the airplane is lower, and thus your airspeed will show lower as well. Opposite for the other side. In the RG we did it, we have 2 static ports, I think that averages it out but I am not sure.



Jetblast2000,

I don't see how drag is a factor any more or less than lift is. The wing stalls at a speed when the air still hits at its ideal (perpendicular for most GE wings) angle. If that angle is any different I would think that the layer separation would come earlier. Direction of relative air must have an effect.

But I am ready to learn anything if you guys can point me toward some proof.



BTW where the heck is BobbySlam or Avbug when the kids need them?

 

[ShawnC]

Huncowboy,

Next time your up pop open the alternate static source, that should not be affected by the slip, and match you IAS with GPS ground speed (not perfect but close enough), do the same with using the outside static ports. Most of the error comes from the pitot tube being cockeyed because of the slip, at least that was how I was taught in both glider and power aircraft.

Legal disclaimer: Please don't open up the alternate static source before checking with an AMT to make sure it's safe for your aircraft. (Just in case).

 

[MW44]

Do any of you guys use forward slips in airliners these days? I imagine it would be uncomfortable for the passengers and might even make the non-flying pilot a little nervous but it would probably be fun to try. I remember reading about that Canadian pilot who deadsticked the 767 from cruise altitude used a forward slip to get rid of some altitude but I don't think I've ever been on an airline flight where anybody actually used that technique in normal ops.

 

[JetBlast2000]

Huncowboy,

Hmm, my reasoning that drag plays more of a part than lift is due to Lateral and Directional Stability. The sweep back of an a/c wings will increase lift on one side due to wing meeting the air at more than 90 degrees giving us more camber per cord. The opposite would be true for the other wing. Thus the TOTAL lift generated would roughly balance out. However, this would give you and increase in parasite drag due to the additional surface area of the fuselage and horizontal stabilizer. So in order to maintain alt. you pull back on the yoke. And then, as you stated earlier its like a cross-controlled stall.

JB2k

 

[huncowboy]

Huncowboy,

Hmm, my reasoning that drag plays more of a part than lift is due to Lateral and Directional Stability. The sweep back of an a/c wings will increase lift on one side due to wing meeting the air at more than 90 degrees giving us more camber per cord. The opposite would be true for the other wing. Thus the TOTAL lift generated would roughly balance out. However, this would give you and increase in parasite drag due to the additional surface area of the fuselage and horizontal stabilizer. So in order to maintain alt. you pull back on the yoke. And then, as you stated earlier its like a cross-controlled stall.

JB2k

That seems to make sense, but for sweept back wings, and for those I would think that the wing that doe snot get the 90 relative wind would still stall. but anyways in GE airplanes, and most of us learn to fly in those, there is not much sweep back. The reason why you do a forward slip is because you lose lift, AND you increase drag. The two together will result in that desired altitude loss. IMO it is hard to argue that in a forward slip you will have to pitch up to compensate for the additional drag only. You do it for both, the lift, and the drag.

 

[Duke Elegant]

In a side-slipping manouvre , drag plays a large role. After all , the fuselage is now forced partially into wind which causes some of the downwind wing to be blanketed. Controls are all deployed with a large displacement also causing more drag. Controls are fighting one another and the airspeed indicator reads erroneously as the pilot intentionally flies in this drunken , awkward manner because he had exhausted his inventory of tools ..like flaps , appropriate power reduction and good judgement.


It is an emergency manouvre and caution should be used when employing it close to the ground.

 

[ShawnC]

It is an emergency maneuver and caution should be used when employing it close to the ground.

It's just one more tool in your bag of tricks, if you are not familiar with it, and it's ASI errors that it produces, it's worthless tool. Someone who regularly practices it, it is a very safe maneuver when it is done within limits.

It's better to have an ace up your sleeve and know how to use it, than not have one.

Particularly comes in helpful during engine off scenarios, or in tight airfield where there is simply no time to bring in the flaps (and the loss of lift that comes with flap retraction).

I think we had a rather in depth conversation or whether or not to slip a year or so ago.

 

[ShawnC]

Well back from my task from the day, I did a search here is part I of the Forward slip converstation:
http://forums.flightinfo.com/showthread.php?t=8508

And Part II:
http://forums.flightinfo.com/showthread.php?t=12400

You will see both sides of the camp duke it out.

 

[Duke Elegant]

I always taught my students how to slip. I always cautioned them on slipping when close to the ground.


I have sure seen some comical events when pilots are slipping in gusty conditions.

I find it more pleasurable to use skill and judgement in order to avoid having to fly askew.