scsmith
Well Known Member
Hi All, I'm fairly new to the forum, building an RV-8QB, working on canopy skirt and FWF. I'm also a high-time sailplane pilot.
But more to the point, I am a Research Engineer at NASA with a PhD in applied aerodynamics.
I frequently see examples of minor to major misunderstandings of aerodynamics, and often a post will assert an argument based on "almost correct" understanding that leads to the wrong conclusion.
There is a current thread on the safety forum on turning back to the airport after engine failure, which is a really excellent thread by the way, with valid arguments on both sides. But some of the arguments are based on misunderstandings. So I thought I would really stick my neck out and try to clarify some points:
To keep the posts short, I will do this in three parts.
Part 1 On turning down wind:
There are three elements to this. The third element is the really important one:
a) In a UNIFORM wind, the airplane can not tell. As one poster said, its a reference frame issue. The airplane can not tell that the ground is drifting underneath it.
b) Near the ground, the pilot CAN sense the drift, and it often causes pilots to mis-coordinate the turn. When turning from downwind to upwind, there is a tendency to use too much rudder. This is no doubt a contributer to stall-spin accidents from turning near the ground. It takes a strong effort to resist the temptation.
c) In DESCENDING turns, the airplane is strongly affected by the wind, because it is not UNIFORM. There is a strong wind GRADIENT - as you get closer to the ground, the wind speed drops. The result is that descending through the wind gradient (whether turning or not) robs the airplane of energy - it has to accelerate to offset the loss in wind speed in order to maintain airspeed. The effect is most pronounced for airplanes that are heavy, and very noticeable in descending turns. One poster is a crop duster pilot who pointed this out. A pilot is likely to sense the higher sink rate and pull back on the stick. This also contributes to stall-spin accidents.
The take-away here is that the age-old debate about down-wind turns has valid observations on both sides that are not conflicting. Near the ground, making descending turns in strong wind is dangerous because of the pilot mis-queing and because energy is bled faster than normal. You must fly coordinated, and you must maintain airspeed.
Part 2 will be on the effect of descending turn on expected g-load and stall speed.
But more to the point, I am a Research Engineer at NASA with a PhD in applied aerodynamics.
I frequently see examples of minor to major misunderstandings of aerodynamics, and often a post will assert an argument based on "almost correct" understanding that leads to the wrong conclusion.
There is a current thread on the safety forum on turning back to the airport after engine failure, which is a really excellent thread by the way, with valid arguments on both sides. But some of the arguments are based on misunderstandings. So I thought I would really stick my neck out and try to clarify some points:
To keep the posts short, I will do this in three parts.
Part 1 On turning down wind:
There are three elements to this. The third element is the really important one:
a) In a UNIFORM wind, the airplane can not tell. As one poster said, its a reference frame issue. The airplane can not tell that the ground is drifting underneath it.
b) Near the ground, the pilot CAN sense the drift, and it often causes pilots to mis-coordinate the turn. When turning from downwind to upwind, there is a tendency to use too much rudder. This is no doubt a contributer to stall-spin accidents from turning near the ground. It takes a strong effort to resist the temptation.
c) In DESCENDING turns, the airplane is strongly affected by the wind, because it is not UNIFORM. There is a strong wind GRADIENT - as you get closer to the ground, the wind speed drops. The result is that descending through the wind gradient (whether turning or not) robs the airplane of energy - it has to accelerate to offset the loss in wind speed in order to maintain airspeed. The effect is most pronounced for airplanes that are heavy, and very noticeable in descending turns. One poster is a crop duster pilot who pointed this out. A pilot is likely to sense the higher sink rate and pull back on the stick. This also contributes to stall-spin accidents.
The take-away here is that the age-old debate about down-wind turns has valid observations on both sides that are not conflicting. Near the ground, making descending turns in strong wind is dangerous because of the pilot mis-queing and because energy is bled faster than normal. You must fly coordinated, and you must maintain airspeed.
Part 2 will be on the effect of descending turn on expected g-load and stall speed.