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9 Answers

effect of steady state wind on airspeed

Asked by: 4438 views ,
Aerodynamics

Greetings..

I've seen the question asked several dozen different ways, so I can't believe I'm going to give it a go... but my CFI and I disagreed about this the other day in the airplane, so I wanted to give it a shot.

The other day, while in slow flight at around 3000 AGL, and after a slow 180 degree turn from a heading that was a tailwind to a heading that was now a headwind, my CFI said "See that little climb you've started?  That's because we changed directions into the wind."  -- at which point, I objected strongly, having always been taught that in a steady state wind the airplane has no idea that it's pointed into or away from the wind, and thus it has no discernible effect on airspeed.  Note that I'm not talking about gusts or shear, which certainly WOULD have a momentary effect on airspeed.  I'm also NOT talking about ground speed -- I'm talking about airspeed and the corresponding lift potential.

Given that every time this question is asked there are two answers -- "of course it doesn't affect airspeed", and "of course it affects airspeed!", with all kinds of anecdotes to go along with, I was wondering whether there was any definitive physics work on the subject.  I've poked around trying to find some, but it quickly gets intermixed with lots of opinion rather than science.. 

I feel strongly that I'm correct, that steady wind doesn't have an effect on airspeed.. beyond it making sense to me, my anecdotal reason is that if our heading with relation to the steady state wind made any difference to airspeed, we'd be mucking with the throttle all the way through S turns across a road.  However, my CFI stood his ground (though his explanation wasn't much deeper than "Well explain the climb then!") and I'd love to be able to point at some solid physics.

For reference, I'm a student pilot just about ready for checkride.

Cheers,

Joe

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9 Answers



  1. Mark Kolber on Jan 19, 2020

    “Well explain the climb then!”

    “When making a turn, we need to increase back pressure because of the change to the lift vector, I pulled back too much.”

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  2. histriosum on Jan 19, 2020

    |“When making a turn, we need to increase back pressure because of the change to the lift
    | vector, I pulled back too much.”

    Indeed, sorry, I should have included my answer to his question in the airplane.. my answer to his question about how the climb started was that I almost certainly nudged the throttle forward a touch when I started the turn, having had the airplane (Cessna 150) trimmed for hands off flight at about 55mph IAS. It would have been almost automatic for me, and I probably just forgot to un-nudge it when we rolled level more than a minute later.

    In any event, that’s how I fixed the slight climb — took a touch of throttle away.

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  3. John D Collins on Jan 19, 2020

    Certainly your instructor is not correct and probably a combination of your answer and Mark’s explains the slight climb. Although you are correct that the instructor is wrong, I would avoid a confrontation with him and get on with your check ride. Then find a new instructor who has a better understanding of the physics and mechanics of flight for your future training.

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  4. Dave M on Jan 19, 2020

    Well, I’m going to throw my 2 cents in and say that it could. However, I’ll add that at the airspeeds we’re talking about, it probably is the effect is negligible, if there at all.

    Let me explain. When flying in the flight levels, a chang of wind (such as entering/leaving the jet stream) will usually result in a change of airspeed (or mach). Now I can’t say for sure, but it seems to me that it takes time for the wind to overpower the airplane’s inertia. Kind of like throwing something that floats into a moving river… it takes a measurable amount of time for the object to match the speed of the river.

    This might be what your instructor is basing his opinion on. Again, I really don’t think that this is in play at “instructional” speeds, but here you go: my 2c!

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  5. John D Collins on Jan 20, 2020

    Dave M,

    The question is with respect to a steady wind. Your example is of a change in the wind velocity and or direction and does not apply to the question posed.

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  6. Richard Eastman on Jan 20, 2020

    John …

    I have a question for you with respect to this query …

    An airplane flying downwind will have a given angle-of-attack (AOA) on both wings due to the relative wind (which in this case, is wind from behind). As we both understand, AOA is a function of relative wind direction.

    As the airplane turns, the AOA will change on both wings as a function of bank — changing the lift vectors on both wings requiring a bit of power to maintain a steady altitude and steady heading.

    As the airplane rolls out into the wind, it will re-establish a given AOA on both wings necessary to maintain the steady level and desired airspeed. But now the wind will be coming from what is effectively a different direction than it was coming from when the airplane had a tailwind. While the AOA of the airplane must return to the same elements as were used when downwind (or for that matter, throughout the turn) … with the pitch of the airplane relative to the horizon still be the same? It seems to me that the physics of the wing moving through the same air-mass but in different directions would suggest two different (albeit minor) pitch attitudes relative to the horizon even though the airplane itself would have a constant AOA within the air-mass. So the pilot question expands to becomes how does one recognize the different pitch attitude relative to the horizon?

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  7. Russ Roslewski on Jan 20, 2020

    Richard – your whole question is based on a flawed premise.

    For an aircraft with a tailwind, the “relative wind” is not from behind. The only time a relative wind can come from behind is if the airplane is flying backwards – which usually only happens in aerobatics.

    The AOA does not change from a tailwind to a headwind because the relative wind does not change – if the airplane is flying at 100 kts (airspeed), the relative wind is 100 kts.

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  8. John D Collins on Jan 20, 2020

    Richard,

    Once the aircraft is in steady flight in a steady wind, it does not matter what direction the aircraft is heading. Assuming the power is unchanged and the aircraft enters a turn, additional lift must be generated in order to hold altitude because the lift vector must support both the weight of the aircraft plus the force due to the acceleration in the turn. The aircraft will need to increase the angle of attack to generate the addition lift to maintain altitude. The aircraft will slow due to the additional induced drag. When one rolls out, regardless of the new heading, the aircraft will accelerate to the original airspeed as altitude is maintained and the AOA is reduced back to the original steady state value. There is no additional indication on any of the flight instruments regardless of the direction at roll in or roll out. During the turn the airspeed will be lower and the AOA will be higher, but the indications on the flight instruments will give no indication of whether one is flying into a head wind or tail wind. Roll out will always require a pitch reduction to hold altitude and there will always be an acceleration back to the original speed. If wind direction made a difference, then the amount of acceleration would vary based on the wind, but it does not. The basic lift equation is defined as the coefficient of lift C times 1/2(p)(V**2)(S), where p is the actual air density, S is the wing area, and V is the airspeed. Nowhere in the lift equation is a term for ground speed, so a constant wind that is not accelerating has no effect on the amount of lift generated.

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  9. Mark Kolber on Jan 20, 2020

    Some myths simply live in longer than others. The “downwind turn” myth will continue to be with us, so long as we continue only to see activities with reference to the ground,

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