Submitted by maddogdriver on January 10, 2012
I just want to throw this topic out into the ether for discussion, but mainly for clarification.There are some who remember the old discussion about the turn to downwind and the loss of airspeed that resulted from the increasing tailwind component. This has thuroughly been discussed and debunked. However, there are still some who think that the aircraft feels an acceleration as the groundspeed increases during the turn to down wind.Example: Assume you are flying in a 50kt direct headwind at 5,000ft with a 100kt TAS. You currently have a 50kt ground speed- we can all agree on that. Now assume you make a 180 degree turn to downwind. Your new ground speed is now 150kts (100kt TAS + 50kt wind.) The aircraft accelerated 100kts over the ground, yet the TAS remained the same throughout the turn. There are some that think that that 100kt acceleration can be felt by the pilot and the aircraft physically experiences this acceleration. Obviously we are talking apples and oranges here, as the frames of reference are mutually exclusive. Fying in IMC with no reference to the ground, doing a 360 degree turn, you would not feel yourself going faster as you turned down wind and slower as you turned upwind (we are not talking about turns around a point, in which this would absolutely be the case)The only acceleration felt in the turn to down wind (or upwind) is the acceleration inherent in the turn itself, as acceleration is defined as a change in direction and/or speed. However, the forces felt are the same in a steady wind and a no wind (shearing wind is a different subject.)Does anybody disagree with was written here?
Comments (13)
Post a comment
All Comments
January 27, 2012
Aerodynamically the airplane sees no difference, there we agree. In terms of mass "feeling" inertia though - I would think possible if the turn rate was rapid enough (as in a canyon turn?) in a strong enough wind. We do not feel the absolute speed in a closed cockpit, only a relatively rapid net change -acceleration or deceleration- on a given axis. On crosswind & base we might not be paying a huge amount of attention to how much lateral movement relative to the ground is happening in this scenario (unless we are crabbing significantly) so that should tend to make the transition more gradual and less noticeable. But have you ever tried walking on those long people conveyor belts at some airport passenger terminals? The moving belt is somewhat analogous to the wind. Your "airspeed" (your walking speed) remains the same if you keep walking (unlike most of the lazy masses who just stand), but your ground speed very suddenly changes and you certainly feel the acceleration & deceleration as you step onto & off the belt, don't you? Imagine the belt was 50 feet wide and you made a wide gradual u-turn. The acceleration wouldn't be quite so noticeable. You'd just be feeling foolish for walking and not going anywhere! The suddenness makes a big difference in our perception. And this question is all about perception- the "can we feel". We rely on our inner ear & kinesthetic feedback to sense these kind of changes- changes in our absolute velocity. Meaning sudden changes in altitude or ground speed (pick your axis). A boat in a current is very similar. I've certainly noticed the "push" the current can suddenly provide.
Inertial navigation counts on sensors being able to detect tiny changes in inertia- change of absolute speed in each axis. If they can sense small changes in those airplanes so equipped, shouldn't we be able to feel big changes? Of course this particular scenario is not one I would often encounter, or at least I didn't when I was current.
February 08, 2012
Disagree------
C G of the earth should be the reference point. Level flight is circular motion because of the shape of the earth. Thrust and drag determine the airspeed.
When turning downwind the drag tries to decrease--but thrust will accelerate the aircraft
relative to the Great Circle around the earth if the time span is long enough. If the time span is to short---there will be a loss of airspeed.
In a rate one turn (3 degrees / sec) it takes one minute to turn 180 degrees. In the example above the aircraft would accelerate 100 mph in one minute relative to earth. If the ball in the turn and bank is centered--the g force is going to be on the seat of your pants. You are already pulling about 1.15 G's ---my old body would not be able pick up the small difference in G force.
In comment 2 (aerodynamically the airplane sees no difference). IMHO the aircraft experiences changes in the drag which causes acceleration if turning downwind and negative
acceleration in an upwind turn relative to the earth.
February 17, 2012
The above responses are incorrect. No matter how sensitive the instrument or the person, NO acceleration will be experienced in a steady wind. No wind or a steady 100kts, the aircraft, pilot or accelerometer will not detect any acclelration. There just physically is not any to be detected. End of story. This may sound counter-intuitive, but it is a physics fact.
February 24, 2012
I stand by my points. Physics? Absolutely, but only if you get your points of reference right! Acceleration can mean an increase of airspeed, but it more correctly should be understood as a change in inertia. This can be a positive or negative change, and can be in any direction, not just forward. An aircraft can accelerate ground speed while decreasing airspeed or vice versa in certain circumstances. Or accelerate/decelerate up/down, or laterally (usually in a turn). What the airplane "feels" (indicated airspeed) and what WE feel are two different things sometimes. This question has everything to do with point of reference- and there are two: Airspeed is relative to air - whether the air itself is moving or still. Seat of the pants perceptions generally derived from inertia.
But the question at hand- seat of the pants perceptions- are pegged onto significant changes in inertia, not necessarily the airspeed indicator. At least apart from wind noise. This feeling of acceleration from turning into or away from the wind (not the G force acceleration because of the turn, OK?) will SELDOM ever be noticed. But could it potentially in more extreme situations? YES! The reality will be the reality regardless of what any of us think. This is a bit difficult to empirically test unless we have strong winds aloft but mild enough winds to safely get airborne. But it is easy to test on a river with a decent current with a small boat. Or even an airport "moving sidewalk" conveyor as I mentioned before. And RCH, drag has everything to do with airspeed & nothing to do with upwind/downwind, right?
February 28, 2012
to maddogdriver
From-----http://physics.info/acceleration/
Acceleration occurs anytime an object’s speed increases, decreases, or changes direction.
If an airplane is flying straight and level in a no wind situation –it is experiencing acceleration due to change in direction, caused by the curvature of the earth.
If the airplane is flying level and turning in a no wind situation---it is experiencing a second acceleration due to change in direction horizontally.
If the airplane is flying level and there is a wind blowing horizontally---the airplane is experiencing three accelerations—two directional and one increase in speed if turning downwind or a decrease in speed if turning upwind relative to the earth. All these accelerations can happen at a constant airspeed if the time span is long enough and the airplane flown correctly.
These accelerations and some other ones are used in an Inertial Navigation System.
February 28, 2012
To AF6IT
Hope this will help answer your question------
An airplane is cruising at 120 knots (2 degrees of the Great Circle per hour), flying straight and level (circular motion). It is equipped with standard flight instruments and an Inertial Navigation System (accelerometers are the sensors for this unit). There is no wind. The airspeed and the INS indicated speed in the gravitational field of the earth are the same. Some time later we notice during a thirty second time period the indicated INS speed has increased to almost 150 knots (2.5 degrees/hr). The airspeed has remained constant. The airplane has flown into a 30 knot tailwind. The airplane has accelerated, proven by the accelerometers in the INS. Thrust and drag determine the airspeed. Thrust and drag act as a governor –seeking equilibrium –if the drag tries to decrease, thrust will accelerate the airplane relative to the Great Circle so the airspeed remains almost constant.
March 01, 2012
AF6IT, RCH-
You both still fail to understand the frame of reference relative to this particular question of TURNING IN CONSTANT WIND. Yes, there is acceleration while following the curve of the earth, although this is not relavant to my topic, nor is the acceleration induced by the turn itself.
Let me put it this way, assume a 2G level turn in zero wind. Now introduce the same level 2G turn with a steady 100kt wind. Is the turn still only 2G's? (remember, we are not talking about turns about a point- assume this is all done IMC)
Answer is yes, the aircraft- and you the pilot- don't know or care what the wind is doing when banking for 2G's. The bank will ALWAYS be the same, the IAS will ALWAYS be the same. The only thing that changes with a wind is the ground track and ground speed. (you would actually track a curly-Q over the ground)
So, when turning downwind, say at a 60 degree bank (2G's) the only thing that changes is the ground speed, not the G's and therefore no acceleration RELATIVE to the air mass. You, nor an INS- NO MATTER HOW SENSITIVE would feel any acceleration.
Same is true for driving a boat on a moving river. Take away the shore and you would have NO indication that the water was moving, and no feeling of acceleration when going up/down stream.
But where did the increase in inertia RELATIVE to the EARTH come from? you might ask. Answer is the initial speed of the river/air mass. Remember, this was already moving at a constant velocity when you put yourself inside its medium. If you were to jump in a moving river, you would feel an acceleration from zero to the speed of the river, but swim around in the river, and there is no more acceleration. To get out, you must decelerate RELATIVE to the earth. Same holds true for a plane; once inside the airmass, there is no more acceleration felt turning downwind than there is turning upwind.
March 02, 2012
AF6IT, I am very curious to hear your explination for using the moving walkway to augment your point. Are you suggesting that you experience more acceleration depending on which way you are walking on a moving walkway?? (Stepping on and off, and wind resistance not included)
April 04, 2012
It is a bit head-ache inducing but the truth is that in the case of manoeuvring within a uniform parcel of air, irrespective of whether it is moving relative to the ground or not, your turn does not involve a longitudinal acceleration. The proof is in the IAS though - if anyone is doubting it, check this next chance you get. Turn into and away from the wind and watch the ASI.
I think the confusion here (and I have had it too) is probably more likely to hit people with sailing experience. In that case you are operating at the interface of the water and wind but your vessel is moving through the water. Turning into or away from the the wind has an immediate and noticeable effect.
May 07, 2012
If you turn low enough, in something with some wingspan, you discover that the wind does matter to the aircraft. That's because, if you're low and have span, in a turn into wind the lower wingtip is down in the wind shear and has a lower airspeed than the fuselage does, while the higher wingtip is higher up and has a greater airspeed. The combination results in the aircraft trying to roll into the turn. Turning downwind you get the opposite effect: the aircraft wants to roll out of the turn.
Top Rated
Your Comment
All submitted comments are subject to the license terms set forth in our Privacy Policy and Terms of Use
