IAS Question

JohnAJohnson

JohnAJohnson

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JohnAJohnson
I made a short 20 minute flight the other day, cruising at 3000' MSL heading west with 10 knots on the nose. Power was 65% (22 square) burning 8.7gph and my indicated airspeed was 125. An hour later on the way back traveling east with 10 on the tail, and using the exact same altitude and engine power settings, my indicated was 120. Weight was about the same on both legs as were OAT and pressure. I've always thought IAS was really a measure of power so I'm curious as to why the indicated airspeed was different.
 
My bet is that you didn't "get up on step" during your return flight! :goofy:


:popcorn:
 
JohnAJohnson said:
I made a short 20 minute flight the other day, cruising at 3000' MSL heading west with 10 knots on the nose. Power was 65% (22 square) burning 8.7gph and my indicated airspeed was 125. An hour later on the way back traveling east with 10 on the tail, and using the exact same altitude and engine power settings, my indicated was 120. Weight was about the same on both legs as were OAT and pressure. I've always thought IAS was really a measure of power so I'm curious as to why the indicated airspeed was different.
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Do you get compliments on the accuracy of your instruments?
 
timwinters said:
My bet is that you didn't "get up on step" during your return flight! :goofy:

Yea, but I leaned forward on both legs :)

:popcorn:
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roncachamp said:
Do you get compliments on the accuracy of your instruments?
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Not sure I follow Steven. Pressure and temperature are from AWOS, and my EDM830 gives me OAT, MAP, RPM, and FF as digital readouts and I do trust these numbers for setting up a repeatable test.
 
Yep... had a hell of a thermally day last week. Holding a level altitude resulted in wide swings of airspeed as I moved from one airmass to another.
 
flyingron said:
Yep... had a hell of a thermally day last week. Holding a level altitude resulted in wide swings of airspeed as I moved from one airmass to another.
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I ferried a draggy, slightly underpowered biplane from AZ to TN once.

I swear, over TX I'd have to alternate between full power, Vy "climbs" and then having to throttle back to stay in the green in a screaming "descent" all to just hold altitude.

It was far from a subtle effect.
 
FastEddieB said:
Additionally, air masses can both rise and subside.

That could easily account for the difference observed.
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Interesting. Both flights were smooth as glass but I can see how the faster a rising air mass, the less power needed to stay aloft. But the slower IAS was an hour later and if it was adiabatic, it seems it would have given me more speed later in the morning.
 
Although the ASI is suppose to read your speed through the moving airmass, I do remember reading somewhere (don't remember where) that if someone goes one direction with a strong enough tailwind then turns around and goes into the headwind the indicated air speeds will vary. If I recall, it has something to do with how the air acts a little different in entering the pitot tube and how the instrument is not perfect.

Don't know how true that is, and it is way too much science for my liking, but I guess it could hold true for certain plane/pitot tube setups
 
TheGolfPilot said:
Don't know how true that is, and it is way too much science for my liking, but I guess it could hold true for certain plane/pitot tube setups
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Whoever wrote it was either wrong or you are misunderstanding it. The pitot tube has no clue whether it is in a headwind or tailwind (everything's a headwind as far as it is concerned).

Changes in angle of attack will make small differences in the calibrated (or as we said in the olden days "true indicated") airspeed for a given indication. But that's not going to be a function of the winds.
 
flyingron said:
Whoever wrote it was either wrong or you are misunderstanding it. The pitot tube has no clue whether it is in a headwind or tailwind (everything's a headwind as far as it is concerned).

Changes in angle of attack will make small differences in the calibrated (or as we said in the olden days "true indicated") airspeed for a given indication. But that's not going to be a function of the winds.
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I think it was more in regards to the fact that there are possible mechanical imperfections with the pitot tube or any instrument.
 
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JohnAJohnson said:
Not sure I follow Steven. Pressure and temperature are from AWOS, and my EDM830 gives me OAT, MAP, RPM, and FF as digital readouts and I do trust these numbers for setting up a repeatable test.
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It's a running joke. Never mind.
 
All it takes is a slight change in the airplanes orientation. A little rudder, a slightly dropped wing can change how the air moves around the airplane. The pitot tube is only taking a small sample of the ram air. There WILL BE error.

So sure, maybe the instrument won't "know" that there is a headwind or tailwind or crosswind, but the pilot is making adjustments to fly the plane straight to a point. The adjustments the pilot is making to fly the plane straight will change the way air is flowing around the airplane.

Maybe the OP had a slight crosswind of sorts. If he is flying straight to a point, that will require some control inputs. Those will adjust the flow of air past the airplane and the pitot tube. So probably more pilot cause than mechanical cause, but it is plausible he had different IAS at the same altitude, going different directions.
 
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jordane93 said:
According to some people on this board you don't really need to use a checklist in a light general aviation plane.
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Not a problem, if you don't mind periodically "forgetting" something - like retracting takeoff flaps, putting down your gear or, as mentioned previously in the thread, forgetting to close one's cowl flaps.

But that's a hijack, and I apologize.
 
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TheGolfPilot said:
So sure, maybe the instrument won't "know" that there is a headwind or tailwind or crosswind, but the pilot is making adjustments to fly the plane straight to a point. The adjustments the pilot is making to fly the plane straight will change the way air is flowing around the airplane.

Maybe the OP had a slight crosswind of sorts. If he is flying straight to a point, that will require some control inputs. Those will adjust the flow of air past the airplane and the pitot tube. So probably more pilot cause than mechanical cause, but it is plausible he had different IAS at the same altitude, going different directions.
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Have your read my linked post?

You do either have a fundamental misunderstanding, or are not expressing yourself well.

The air hitting the pitot tube will be identical with a 50 kt headwind, 50 kt tailwind or 50 kt crosswind, as long as the plane is coordinated. No control inputs are needed once a crosswind correction angle is established - your post implies that you think there is - "If he is flying straight to a point, that will require some control inputs" which is not correct.
 
FastEddieB said:
Not a problem, if you don't mind periodically "forgetting" something - like retracting takeoff flaps, putting down your gear or, as mentioned perviously in the thread, forgetting to close one's cowl flaps.

But that's a hijack, and I apologize.
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Yes. Back on track!
 
This is a Cardinal?

Leaving the cowl flaps open will be more than 5 knots. 5 knots works for a 182 at 23/2400/12.5.

You could be flying in a slight slip, or the air could be descending. It only takes a little, and in stable conditions, it will be smooth and difficult to feel.

Something else to check is that the Cardinal fuel "flow" gauge is actually a fuel pressure gauge. Any chance you left the fuel pump on, leading to a mixture different from what you thought?
 
FastEddieB said:
Have your read my linked post?

You do either have a fundamental misunderstanding, or are not expressing yourself well.

The air hitting the pitot tube will be identical with a 50 kt headwind, 50 kt tailwind or 50 kt crosswind, as long as the plane is coordinated. No control inputs are needed once a crosswind correction angle is established - your post implies that you think there is - "If he is flying straight to a point, that will require some control inputs" which is not correct.
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Yes, YOU ARE RIGHT. In a perfect environment the wind will not make and difference. The TAS and IAS will not change from a change of wind if all other variables do not change.

My point is that although the TAS may remain constant, the IAS might not, or vice versa. This inconsistency can be caused by hundreds of thousands of different variables because our real world is infinitely complex. To say that an instrument can not vary 5%, because of a very basic physics principle they read in a book, is just plain ignorant.
 
TheGolfPilot said:
My point is that although the TAS may remain constant, the IAS might not, or vice versa. This inconsistency can be caused by hundreds of thousands of different variables because our real world is infinitely complex. To say that an instrument can not vary 5%, because of a very basic physics principle they read in a book, is just plain ignorant.
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Yes, but the presence of a head or tail wind is not one of them.

We've certainly enumerated tons of them:

Density Altitude (temperature, barometric pressure, humidity), power setting changes, mixture changes, not flying coordinated, not closing cowl flaps, failure to fully retract the flaps, weight changes, thermals, etc...
 
JohnAJohnson said:
Interesting. Both flights were smooth as glass but I can see how the faster a rising air mass, the less power needed to stay aloft. But the slower IAS was an hour later and if it was adiabatic, it seems it would have given me more speed later in the morning.
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Rising/descending air isn't necessarily thermally-related. Bob Buck talks about this in Weather Flying. Low pressure is a rising airmass...high pressure is a descending airmass. If the pressure was lower on the outbound trip, the airmass would either be rising faster or descending slower...either way, a higher airspeed would be the result, even in smooth air.
 
Cowl flaps can sap a few knots, yes. But on long cross-country flights I regularly notice my IAS vary by +/- 5 knots, sometimes more. You'd theoretically expect IAS to remain about the same given the same altitude, OAT, config and power settings but it doesn't. It varies even on non-thermally winter days - and on those days it's a pretty gradual change most times. I have just chalked it up to differences in parcels of air, especially relative humidity.

Other times I have noticed an area of disturbed air characterized by some degree of chop, then an IAS and perhaps a correction angle change, then a GS change, then smooth for a while, then repeat. It's areas of differing winds, with chop at the boundaries.
 
MAKG1 said:
5 knots works for a 182 at 23/2400/12.5.
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I've lost that 5 knots for ten minutes with an unused departure checklist sitting under my right thumb on the laminated card and a CFII on board watching.

You know what?

There's just no damned way to be casual about reaching down ten minutes into the flight to close the cowl flaps on a 182.

They're just too far away. LOL
 
denverpilot said:
I've lost that 5 knots for ten minutes with an unused departure checklist sitting under my right thumb on the laminated card and a CFII on board watching.

You know what?

There's just no damned way to be casual about reaching down ten minutes into the flight to close the cowl flaps on a 182.

They're just too far away. LOL
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Well, there's a reason I know the numbers.....

I think just about everyone has done that.
 
MAKG1 said:
Well, there's a reason I know the numbers.....

I think just about everyone has done that.
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LOL. Yup.

No wait, we're all perfect pilots here! Don't let anyone know our fowl flap secrets!

Damn I wish I could just pull the chute like 6PC! ;-)
 
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