Aug 30, 2012, 05:43 PM
Registered User
Toowoomba, QLD, AUSTRALIA
Joined Jan 2008
636 Posts
Quote:
 Originally Posted by Chophop Exactly, now as I had been arguing, the turn to downwind at or better yet, below MCA (minimum controllable airspeed) can do the same. But only if the plane's inertia is great enough to delay the resultant speed up. I say can happen but not will happen. It is a grey zone thing.
No it's not a grey zone thing. The plane isn't slingshotting around a pole fixed to the ground to perform a 180deg turn, it's pushing on the air.

So if it's windy day and I lose airspeed by turning downwind, I'd lose exactly the same airspeed if i turned exactly the same way on a calm day.

For those who still think the Earth is flat, please remember the Earth is a sphere and the air your flying in is actually spinning around this sphere at about 1000mph. The fact that the surface of the sphere is matching this speed on calm day makes no difference.
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 Aug 30, 2012, 06:13 PM You can't take the sky from me United States, IA, Hampton Joined May 2012 358 Posts You are on an airliner at 35,000' travelling across the ground at 320 kts. There is a fly travelling from the tail to the nose at 2 kts. This gives it a ground speed of 322 kts it suddenly turns 180 degrees and is now travelling at 318kts backwards. How does it not get smashed into the tail? How can it fly at all with those huge speed changes? It goes from 322kts one direction to 318kts backwards flying the other direction. Pure gibberish right? Any inertia imparted by steady wind is unchanging therefore it has no effect on the aircraft. You will have trouble when there is gusting, wind sheer, etc... but no matter what direction you are turning, to or from, you will be affected the same, or not at all, depending on the stability of the air. Not the direction of the wind.
 Aug 30, 2012, 06:21 PM You can't take the sky from me United States, IA, Hampton Joined May 2012 358 Posts This is directly from the AOPA by one of the most highly reguarded flight instructors in the industry Rod Machado. As you can see it permeates all aspects of flying. "Dear Rod, Would you do me a great favor and clear up the age-old debate about downwind turns? My opinion is that once the aircraft is airborne and moving in a steady-state air mass, provided the turn is balanced, the indicated airspeed and hence the margin above stall will not change. Granted, the aircraft will follow a different path over the ground, but when considered separately from the earth, the aircraft is still flying in a circle. In my opinion, the problem with pilots making downwind turns lies with their perception of speed in relation to the ground. The pilot might think he or she is going faster and try to compensate by reducing power and/ or increasing the angle of attack. This could cause a stall or spin. I say that once the aircraft has left the ground, it doesn't care about the ground at all, and the plane can't tell its speed in relation to the ground. I would be most grateful if you could clear this up for me. Thanks very much for your help. Travis (For those of you who are not already familiar with the downwind turn argument, it goes like this: In a steady-state wind, turning downwind [away from a headwind] can result in a loss of airspeed, possibly leading to a stall if the airplane's speed is low enough. Proponents of this argument suggest that the stronger the wind, the greater the loss of airspeed.) Greetings Travis, You're right on all counts. Excluding any effects of wind shear while climbing, descending, or turning, pilots who stall while turning downwind usually do so because of the perceptual problems you mentioned. Unfortunately, the downwind turn argument is like a vampire: It just won't die. Without going into the rather complex aerodynamics, I believe there is a simple way to put a stake through the heart of this argument once and for all. It involves only one question: If the downwind turn effect is indeed real, then why is it that no major airline simulator - not a single one to my knowledge - is programmed to represent this phenomenon? After all, if this was a genuine threat to safety and lives were supposedly at risk because of the dangers of turning downwind, then sophisticated simulators would certainly be programmed to represent this hazard. They aren't. Despite the fact that airliners turn downwind all the time, even in the jet stream, they don't fall out of the sky. If they did, an airline company would surely capitalize on the issue by adopting a name such as Bob's Discount No-Downwind-Turn Airline. The fact is that the downwind turn phenomenon occurs primarily for the reason you stated - reaction to perceived speed in relation to the ground. The downwind turn phenomenon does not exist in real life. It only exists in the heads of those who insist that the laws of physics don't apply to airplanes in flight."
 Aug 30, 2012, 08:49 PM Turn that thing around...naaaa Pleasant Valley Modelport Joined Sep 2006 9,939 Posts What would the airspeed do if you were in 10 mph headwind going 50 mph and instantly turned 180* ? Same as a head wind suddenly dropping. But most likely at or reasonably near a standard rate of turn time is allowed to compensate for those effects and keep the air speed. I always put some power into a turn. You either loose altitude or speed if not.
 Aug 30, 2012, 08:55 PM You can't take the sky from me United States, IA, Hampton Joined May 2012 358 Posts You loose speed due to the added loading and the loss of lift that is being used to turn the aircraft. The same loss is seen no matter how much or how little wind and no matter what direction.
 Aug 30, 2012, 08:58 PM You can't take the sky from me United States, IA, Hampton Joined May 2012 358 Posts As far as the instant 180 your airspeed would go from positive to negative. The wind speed would not factor into it at all. If you could do that and measure it you would find the exact same numbers with and without wind
 Aug 30, 2012, 09:31 PM Turn that thing around...naaaa Pleasant Valley Modelport Joined Sep 2006 9,939 Posts I was thinking 40 mph airspeed for a short time until the plane would equalize back to 50 mph. I can't give in because I am 95% correct in theories. No one seems to have any scientific proof in either side of it. It is usually some example or a friend said this or that, again without any scientific proof. I don't feel up to doing the math. I stay away from any terrain I can't easily climb above, add power as I go into a turn, and keep reasonable attitudes and have no problem. I only have about 550 hours solo time but I'll stick with what has worked for me. I only have a short bit of time flying around near stall turning and kicking the rudder. I can say you WILL find yourself in trouble doing stuff like that.
Aug 30, 2012, 09:40 PM
3D? I only got two thumbs!
United States, MA
Joined Jul 2012
3,513 Posts
Quote:
 Originally Posted by Chophop What would the airspeed do if you were in 10 mph headwind going 50 mph and instantly turned 180* ? Same as a head wind suddenly dropping. But most likely at or reasonably near a standard rate of turn time is allowed to compensate for those effects and keep the air speed
if you're in a 10mph headwind going 50mph, the first thing we need to clarify is if that 50mph is the airspeed or the ground speed, because they are not the same. Let's say it's the air speed. That means your ground speed is 40. You instantly turn 180 like you bounced off a wall. The key point though is the wall you bounced off of is in the air, moving right along with that 10 mph breeze, it's not a wall attached to the ground.. Your air speed is now 50mph in the opposite direction. Your ground speed is now 60. The ground speed is irrelevant as far as keeping the plane in the air - its only airspeed that counts.
Aug 30, 2012, 09:46 PM
3D? I only got two thumbs!
United States, MA
Joined Jul 2012
3,513 Posts
Quote:
 Originally Posted by Chophop I was thinking 40 mph airspeed for a short time until the plane would equalize back to 50 mph. I can't give in because I am 95% correct in theories. No one seems to have any scientific proof in either side of it. It is usually some example or a friend said this or that, again without any scientific proof.
See if you can find the myth busters episode that covered the myth that a plane can't take off from a conveyor belt if the conveyor belt is traveling in the opposite direction. It's a very similar situation which similarly seems to confuse people. The myth busters dealt with it in classic myth buster style - real airplane, real pilot, both sitting on a huge tarp being towed by a truck in the opposite direction. It's fun to watch no matter what you think, and you might find it enlightening.
 Aug 30, 2012, 09:52 PM Turn that thing around...naaaa Pleasant Valley Modelport Joined Sep 2006 9,939 Posts Why on takeoff for no aparent reason you can sudenly drop a few feet ? Probably because some of your headwind dropped and you lost some lift for a short time. Maybe it was a thermal, or God wanted to scare you and pushed it down. It does that. So waht happens on a downwind takeoff , you are 5 mph IAS above stall, the tailwind kicks up suddenly 10 more mph ? You all say that plane has enough section for the air to instantly accelerate the plane by 10 mph groundspeed and without fluctuation maintain that 5 mph ias above stall speed ?
 Aug 30, 2012, 10:01 PM Turn that thing around...naaaa Pleasant Valley Modelport Joined Sep 2006 9,939 Posts I'm insulted that anyone thought they had a chance of making me fall for that conveyor belt routine. It's like rolling on stacked wheels, the plane will move across it anyway, but a wheel driven vehicle could hold still while matching the belt's linear motion with the wheel's circular motion.
 Aug 30, 2012, 10:04 PM You can't take the sky from me United States, IA, Hampton Joined May 2012 358 Posts Nobody ever said that. You are confusing gusting (non steady state winds) with the down wind turn problem. Any time the wind velocity or direction changes and you are near stall you can enter a stall due to that change. The point is that that will happen at any point of flight and the down wind turn is not more or less susceptable. It can happen in level flight, up wind turns, down wind turns and cross wind turns. The turn is not the problem nor is the steady wind, only gusting, or more often the pilots confusion between ground speed and air speed.
 Aug 30, 2012, 10:12 PM Turn that thing around...naaaa Pleasant Valley Modelport Joined Sep 2006 9,939 Posts Well then the question comes from fear of turning downwind during normal flying and then entering a stall. From what I was taught, when using standard turns, no. But as they demonstrated, you go goofing around near stall and pushing it, you will get in a bad situation. Do what they say, don't push the plane to the edge and you'll be ok. If you want to go flying, do what the flight instructor says. It comes time to worry about those grey zone things when you go for aerobatics instruction. It's been a wonderful debate and I loved every word of it. chuckle
Aug 31, 2012, 07:30 AM
3D? I only got two thumbs!
United States, MA
Joined Jul 2012
3,513 Posts
Quote:
 Originally Posted by Chophop I'm insulted that anyone thought they had a chance of making me fall for that conveyor belt routine. It's like rolling on stacked wheels, the plane will move across it anyway, but a wheel driven vehicle could hold still while matching the belt's linear motion with the wheel's circular motion.
Yup. It's also exactly the same situation as the downwind turn - only difference is the ground is moving and the air is still. In both cases the plane only cares about motion relative to the air, not the ground.
Aug 31, 2012, 07:38 AM
An itch?. Scratch build.
South Wales U.K.
Joined Mar 2003
16,897 Posts
Quote:
 Originally Posted by cfircav8r You are on an airliner at 35,000' travelling across the ground at 320 kts. There is a fly travelling from the tail to the nose at 2 kts. This gives it a ground speed of 322 kts it suddenly turns 180 degrees and is now travelling at 318kts backwards. How does it not get smashed into the tail? How can it fly at all with those huge speed changes? It goes from 322kts one direction to 318kts backwards flying the other direction. Pure gibberish right? Any inertia imparted by steady wind is unchanging therefore it has no effect on the aircraft. You will have trouble when there is gusting, wind sheer, etc... but no matter what direction you are turning, to or from, you will be affected the same, or not at all, depending on the stability of the air. Not the direction of the wind.
Quite right. There are just too many 'what if...' being quoted here to try and justify some answers.
How the wind can suddenly go from 30mph to zero, just ...well there is no answer I can think of. What if gravity suddenly doubles....., what if a brick wall suddenly appeared in front to the plane.... could it brake in time.
This is getting just like all the plane on a conveyor threads, ...what if........

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