Question re “inclinometer” in old-style Turn-and-Slip Indicator

[eetrojan]

Hey all,

I'm confused about this. The excerpt below is from pages 5-20 to 5-21 of the FAA’s Instrument Flying Handbook. I am confused by the highlighted assertion that “[t]he inclinometer … does not indicate slip.”

Is this true? Or, is this whole sentence a confusing mashup of needle operation and ball operation?


I though the inclinometer was just a fancy way of referring to the little ball in the curved tube and, since it’s often called a “slip/skid” indicator, it’s all about indicating slip (and skid).

Am I missing something?

Is there any difference between the inclinometer (ball and tube) in an older Turn-and-Slip Indicator versus the the inclinometer (ball and tube) in a newer Turn Coordinator? I thought they were the same, regardless of the top part of the two instruments.

 

[dtuuri]

Right, it indicates slip, but it doesn't mean it's really in a slip. (I know.) Think of a multiengine airplane with one out--to get zero sideslip the plane needs to bank up to 5° which would put the ball off to the side. Or, you can think of a climb in a single engine airplane with "P" factor and off-setting right rudder working to mainain a heading--the ball will be centered with zero bank, but a yaw string will show a sideslip.

dtuuri

 

[FastEddieB]

Confusing.

Slips and skids can be defined in terms of "the relationship between the angle of bank and the rate of yaw"*.

Hence, the instrument does show slips and skids.


*I have normally heard the two variables as angle of bank and rate of turn - not rate of yaw. Not sure if that's a distinction without a difference.

 

[MauleSkinner]

Confusing.

Slips and skids can be defined in terms of "the relationship between the angle of bank and the rate of yaw"*.

Hence, the instrument does show slips and skids.


*I have normally heard the two variables as angle of bank and rate of turn - not rate of yaw. Not sure if that's a distinction without a difference.

just not excessively accurately, as dturri pointed out.

This instrument is normally over-simplified to the point of negative training, IMO.

In addition to the multi-engine application, centering the ball in a single-engine airplane high-power, high-aoa situation also results in a slip or skid (depending on how you define them) due to P-factor.

As far as the "turn" vs. "yaw" terminology, yes, in this case probably a distinction without a difference, but yaw change is what the needle actually senses. If you could do a true knife-edge turn, the needle would stay in the center.

 

[FastEddieB]

Good discussion.

In addition to the multi-engine application, centering the ball in a single-engine airplane high-power, high-aoa situation also results in a slip or skid (depending on how you define them) due to P-factor.

I was under the impression that one held right rudder in a climb to keep from slipping, and that can be verified with the ball in the center.

Is that not right?

Anyway, my working definitions:

Skid: too little bank for the rate of turn, or too much rate of turn for a given bank.

Slip: too much bank for the rate of turn, or too little rate of turn for a given bank.

 

[MauleSkinner]

Good discussion.



I was under the impression that one held right rudder in a climb to keep from slipping, and that can be verified with the ball in the center.

Is that not right?

No, it's not. if the airplane is wings level flying steadily Through the air in a direction other than its heading, the ball will still be centered.

 

[dtuuri]

Good discussion.



I was under the impression that one held right rudder in a climb to keep from slipping, and that can be verified with the ball in the center.

Is that not right?

Anyway, my working definitions:

Skid: too little bank for the rate of turn, or too much rate of turn for a given bank.

Slip: too much bank for the rate of turn, or too little rate of turn for a given bank.

Your definitions are spot on, but if you pull an airplane forward by one blade of the prop on some glare ice (carefully!) you will see what we mean. Since the pulling force is asymmetrical, the plane won't go straight forward (the ball will be centered) it'll start to turn. If you then have a friend push the tail so as to stay pointed in the original direction it'll slide obliquely (ball still centered), as a slip would be through the air.

dtuuri

 

[FastEddieB]

The most powerful singles I've flown have been Cessna 210's, Cherokee 6's, some cropdusters and my Cirrus.

In all cases, it sure seemed like if I was in a full power climb with the right amount of right rudder, my wings were level and the path of the plane through the air was the same as where the nose was pointed, as long as the ball was centered.

Wasn't it?

 

[MauleSkinner]

The most powerful singles I've flown have been Cessna 210's, Cherokee 6's, some cropdusters and my Cirrus.

In all cases, it sure seemed like if I was in a full power climb with the right amount of right rudder, my wings were level and the path of the plane through the air was the same as where the nose was pointed, as long as the ball was centered.

Wasn't it?

.

 

[dtuuri]

The most powerful singles I've flown have been Cessna 210's, Cherokee 6's, some cropdusters and my Cirrus.

In all cases, it sure seemed like if I was in a full power climb with the right amount of right rudder, my wings were level and the path of the plane through the air was the same as where the nose was pointed, as long as the ball was centered.

Wasn't it?

Nope. Have you stil got that yaw string?

dtuuri

 

[FastEddieB]

Nope. Have you stil got that yaw string?

dtuuri

I do, but my Sky Arrow needs virtually no rudder in a climb. In theory, though, with the pusher configuration I would need left rudder in a climb.

Let me see if any of my videos catch that.

Edited to add: seems pretty much straight back once established in the climb on the first takeoff here:

 

[MauleSkinner]

Look at figure 12-19 on page 12-23, along with the appropriate discussion, of the Airplane Flying Handbook. Note that the offset thrust line of a multi-engine airplane with one engine inop is just an exaggerated version of the offset thrust line on a single due to P-factor.

Note also the statement

In fact, there is no instrument at all that will directly tell the pilot the flight conditions for zero sideslip.

...a yaw string, apparently, not being an "instrument".

 

[eetrojan]

Thanks all. I'm still wondering about this particular question...

Is there any difference between the inclinometer (ball and tube) in an older Turn-and-Slip Indicator versus the the inclinometer (ball and tube) in a newer Turn Coordinator?

 

[MauleSkinner]

Thanks all. I'm still wondering about this particular question...

Is there any difference between the inclinometer (ball and tube) in an older Turn-and-Slip Indicator versus the the inclinometer (ball and tube) in a newer Turn Coordinator?

Nope...the ball is the same.

 

[Lindberg]

Your definitions are spot on, but if you pull an airplane forward by one blade of the prop on some glare ice (carefully!) you will see what we mean. Since the pulling force is asymmetrical, the plane won't go straight forward (the ball will be centered) it'll start to turn. If you then have a friend push the tail so as to stay pointed in the original direction it'll slide obliquely (ball still centered), as a slip would be through the air.

dtuuri

But that's not what happens when you correct for the various left-turning tendencies. In that case, you have a turning force applied in one direction by the prop, and an opposite force applied by the rudder at the other end. As a result, you don't turn and you fly straight. That's different than a slip, where the opposing forces are on different axes. Of course you can apply the correction in a manner that results in a slip.

 

[MauleSkinner]

But that's not what happens when you correct for the various left-turning tendencies. In that case, you have a turning force applied in one direction by the prop, and an opposite force applied by the rudder at the other end. As a result, you don't turn and you fly straight. That's different than a slip, where the opposing forces are on different axes. Of course you can apply the correction in a manner that results in a slip.

They're not on the same axis...the prop force is centered so where on the downward-moving blade. Hence the NEED for rudder input.

 

[dtuuri]

But that's not what happens when you correct for the various left-turning tendencies. In that case, you have a turning force applied in one direction by the prop, and an opposite force applied by the rudder at the other end. As a result, you don't turn and you fly straight. That's different than a slip, where the opposing forces are on different axes. Of course you can apply the correction in a manner that results in a slip.

Take a plastic model plane sitting on it's landing gear and pull it along a slick countertop from the right side of the prop with a string while pushing the tail to keep it straight. You will experience an epiphany.

dtuuri