Strange headwind phenomenon
Strange headwind phenomenon
Another naive question!
Flying circuits with 11 - 15kn of wind reasonably aligned with the runway (runway 07, wind around 050) I found that I had to significantly increase the power to achieve the desired airspeed on finals. I would normally expect to trickle along at 70kn IAS with around 1,700rpm as I descend on finals, this time I was using > 2,000rpm. It felt reasonable at the time as I was battling a headwind, but in retrospect I find it hard to understand. Of course I expect the groundspeed to fall in proportion to the headwind, but why is the IAS apparently declining for the same power setting? I can't work this out. I guess my sink rate has to also reduce as my groundspeed falls, because I am flying slower with respect to the glide-slope. So I guess I am maintaining more power to descend more slowly? Can that explain the whole of what seemed to be a big effect? Or was power required to counter increased drag from the modest amount of crabbing I must have been doing?
(It was great fun coping with the occasional blasts and swirls, and the headwind made for some sweet, short landings)
Flying circuits with 11 - 15kn of wind reasonably aligned with the runway (runway 07, wind around 050) I found that I had to significantly increase the power to achieve the desired airspeed on finals. I would normally expect to trickle along at 70kn IAS with around 1,700rpm as I descend on finals, this time I was using > 2,000rpm. It felt reasonable at the time as I was battling a headwind, but in retrospect I find it hard to understand. Of course I expect the groundspeed to fall in proportion to the headwind, but why is the IAS apparently declining for the same power setting? I can't work this out. I guess my sink rate has to also reduce as my groundspeed falls, because I am flying slower with respect to the glide-slope. So I guess I am maintaining more power to descend more slowly? Can that explain the whole of what seemed to be a big effect? Or was power required to counter increased drag from the modest amount of crabbing I must have been doing?
(It was great fun coping with the occasional blasts and swirls, and the headwind made for some sweet, short landings)
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You are right in the sense that the aerodynamics don't change. A certain RPM gives you a certain IAS, independent of the wind. However, you were doing a ground-referenced maneuver (descending along a fixed path to the threshold) and because of the headwind, your air-referenced path was more shallow than usual. And to maintain that shallower glide path you need more power.
Think of it like this. In still air, your glide path, both ground-referenced and through the air, is maybe 5 degrees (I'm assuming a VFR circuit here, and not an IFR approach). Once you get a headwind, your ground-referenced glide path is still 5 degrees (the picture outside is the same), but your glide path through the moving air is now maybe 3 degrees. The stronger the headwind, the shallower your glide path through the air. And that accounts for the difference in power setting.
In addition to this, crabbing also has a modest effect, like you suspect. Because of the crab you fly a longer distance through the air, while covering the same distance over the ground. This leads through a shallower glide through the air, and thus a higher power setting.
As far as flying technique is concerned, in a strong headwind I tend to keep up the speed on final, otherwise you'll never get there. I reduce speed to Vref much later than I would normally do, and apply full flaps a lot later than I would normally do. Because of the headwind, it turns out you've got plenty time for that anyway.
Think of it like this. In still air, your glide path, both ground-referenced and through the air, is maybe 5 degrees (I'm assuming a VFR circuit here, and not an IFR approach). Once you get a headwind, your ground-referenced glide path is still 5 degrees (the picture outside is the same), but your glide path through the moving air is now maybe 3 degrees. The stronger the headwind, the shallower your glide path through the air. And that accounts for the difference in power setting.
In addition to this, crabbing also has a modest effect, like you suspect. Because of the crab you fly a longer distance through the air, while covering the same distance over the ground. This leads through a shallower glide through the air, and thus a higher power setting.
As far as flying technique is concerned, in a strong headwind I tend to keep up the speed on final, otherwise you'll never get there. I reduce speed to Vref much later than I would normally do, and apply full flaps a lot later than I would normally do. Because of the headwind, it turns out you've got plenty time for that anyway.
Thanks, that makes sense. Interesting that I was not aware of flying a descent that was shallower through the air, but as you say, I must have been! I guess that's a good sign! I must have been automatically using the runway appearance as a guide.
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Much easier to answer it in these simple terms in my view:
Power controls rate of descent. Pitch attitude controls speed. So you've set the pitch attitude for 70kts. Your normal power setting of 1700rpm gives you a rate of descent that'll take you to the runway in zero or light wind. But in a strong headwind, your ground speed is reduced, so 1700 rpm will result in you reaching the ground short of the runway. You can see that by looking forwards and seeing the runway numbers going up the windscreen. So you need to increase the power.
NS
Power controls rate of descent. Pitch attitude controls speed. So you've set the pitch attitude for 70kts. Your normal power setting of 1700rpm gives you a rate of descent that'll take you to the runway in zero or light wind. But in a strong headwind, your ground speed is reduced, so 1700 rpm will result in you reaching the ground short of the runway. You can see that by looking forwards and seeing the runway numbers going up the windscreen. So you need to increase the power.
NS
