Aug 26, 2012, 07:38 AM Closed Account Joined Nov 2010 67 Posts It's the inertia due to mass that makes the difference. If you imagine flying into wind, and therefore at low ground speed but normal airspeed, then making a 180 turn downwind, you now have to accelerate your mass to normal airspeed plus windspeed. What normally happens is you lose airspeed until you have achieved the required mass acceleration. Same thing happens in reverse turning into wind. Your mass keeps going at normal speed but the wind now gives you a higher airspeed, so you gain lift. It's these effects that make dynamic soaring possible. Last edited by Coomkeen; Aug 26, 2012 at 07:43 AM.
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Aug 26, 2012, 08:01 AM
3D? I only got two thumbs!
United States, MA
Joined Jul 2012
3,196 Posts
Yes, there is a big difference between flying in a tail wind as you said in your first post...

Quote:
 Originally Posted by seeingeyegod The model does experience different aerodynamic effects depending on whether or not its flying in a tail wind or head wind, just like a real plane. A tail wind can both make a plane move faster relative to the ground, and reduce lift.
And getting hit with a sudden tailwind...

Quote:
 Originally Posted by seeingeyegod Exactly, the air speed is NOT the same if you get a sudden tailwind. The air flow relative to the wings can suddenly slow down, reducing lift.
Getting hit with a sudden tailwind has essentially the same effect as getting hit with a sudden calm when flying in a head wind. Or to phrase it differently, gusty wind is a problem
 Aug 26, 2012, 08:22 AM Glad I'm not a Boll Weevil Pleasant Valley Modelport Joined Sep 2006 10,008 Posts We live in an analog world, our computing is done in binary states. There are dynamic in-betweens to account for. I'll bet my life as I have in the thousands of full scale landings I have done, and was taught in flight school. Just keep the power up in a turn, especially at landing speed, 1.3X anticipated stall speed. If you build up too much speed on final, bleed it off by something in your bag of tricks. You will probably get by with not adding power in a steep enough descending turn. There is need to get to know the airplane's tendancies.
 Aug 26, 2012, 09:28 AM You can't take the sky from me United States, IA, Hampton Joined May 2012 352 Posts You need to increase power in a level steep bank to maintain speed. This is true no matter what the wind conditions. 1.3x stall speed is a general rule of thumb for a smooth day. For gusty conditions you increase speed by half of the gust speed ie: if the wind is 10 gusting to 20 you increase your speed by 5. None of this has anything to do with wind direction. If you remember back to your training when talking about wind in reference to the pattern your focus was on when to turn and how long to hold a turn to maintain your rectangular pattern. Throttle in the pattern is to be used to control descent rate and your AOA is used to control speed. If you were taught to increase power for your turns in the pattern then they were wrong. You should not be making steep turns in the pattern and shallow turns will not cause an excess descent and is part of the descent profile. Coomkeen as far as inertia when going from up wind to down wind there is no need to compensate. The thing about the inertia from the wind is it is only in reference to the ground. The easiest way to visualize this is in a crash. Inertia is basically kinetic energy. If you hit a tree flying up wind your impact will be your airspeed minus wind speed this equals ground speed. If you hit it down wind your impact will be airspeed plus wind speed or a greater impact. Now imagine you have a balloon floating along in the breeze it is moving at the same speed as the wind. If you hit that balloon from any direction the impact will be the same. What happened to that inertia? Your inertia due to wind is always moving down wind therefore there is no need to compensate. There will never be a need to catch up when turning down wind, and the plane will never balloon up when turning up wind (unless you add power or pull up too much in the turn.)
Aug 26, 2012, 10:12 AM
Registered User
United States, OR, Portland
Joined Jan 2008
2,299 Posts
Quote:
 Originally Posted by Chophop We live in an analog world, our computing is done in binary states. There are dynamic in-betweens to account for. I'll bet my life as I have in the thousands of full scale landings I have done, and was taught in flight school. Just keep the power up in a turn, especially at landing speed, 1.3X anticipated stall speed. If you build up too much speed on final, bleed it off by something in your bag of tricks. You will probably get by with not adding power in a steep enough descending turn. There is need to get to know the airplane's tendancies.
I'm trying to learn to do that in RC. I am at the point where I can usually pick my decent speed and angle pretty well but I need to learn to sideslip on the way down to burn off excess speed. I can do it in a sim all day long but in real life its scary.
Aug 26, 2012, 10:15 AM
Registered User
Joined Oct 2010
393 Posts
Quote:
 Originally Posted by sablatnic What many people think of as 'the dreaded down wind turn' is in fact a quite different phenomenon, 'ground affect'.
I didn't write ground effect, but 'ground affect', a nervousness that many people experience, when their models are close to the ground.
Me too, if I start thinking about what I am doing.
Aug 26, 2012, 11:56 AM
Tossing planes into the snow
Canada, BC, Smithers
Joined Nov 2011
3,522 Posts
Quote:
 Originally Posted by sablatnic I didn't write ground effect, but 'ground affect', a nervousness that many people experience, when their models are close to the ground. Me too, if I start thinking about what I am doing.
Sorry for the misunderstanding. I have never heard this term "ground affect" before, and assumed it was a typo.
Aug 26, 2012, 01:04 PM
Registered User
United States, OR, Portland
Joined Jan 2008
2,299 Posts
Quote:
 Originally Posted by Jovanx Sorry for the misunderstanding. I have never heard this term "ground affect" before, and assumed it was a typo.
Not sure if serious
Aug 26, 2012, 10:04 PM
Registered User
Toowoomba, QLD, AUSTRALIA
Joined Jan 2008
636 Posts
Quote:
 Originally Posted by Coomkeen It's the inertia due to mass that makes the difference. If you imagine flying into wind, and therefore at low ground speed but normal airspeed, then making a 180 turn downwind, you now have to accelerate your mass to normal airspeed plus windspeed.
People keep assuming that inertia and kinetic energy relative to the ground means something. IT DOESN'T !!!

I can quite happily pedal around in circles riding a bicycle on the deck of an aircraft carrier doing 30 knots (with a 30kn tailwind) with my mate flying a plane over my head, while a man on land watches me go past yelling at me to pedal harder as I turn back towards the bow because to him I need accelerate to a higher speed, which I'm sure from his statonary position is what it looks like I need to do, but we all know is completely wrong.

My tyres interact with the deck of the ship to turn in constant speed circles, while the plane interacts with molecules in the body of air to fly constant airspeed circles while the man on land thinks we're both speeding up and slowing down....
 Aug 26, 2012, 10:18 PM You can't take the sky from me United States, IA, Hampton Joined May 2012 352 Posts The whole inertia thing is rampant in full scale pilots as well.
 Aug 26, 2012, 10:19 PM Registered User Toowoomba, QLD, AUSTRALIA Joined Jan 2008 636 Posts Actually for those that still try to keep bringing up this extra momentum or kinetic energy rubbish ask yourself why a glider doing a low, high ground-speed, downwind pass over the strip can't pull up any higher than a glider performing a the same pass into the wind at the same airspeed (but lower ground speed)???? In both cases kinetic energy will be converted to gravitational potential energy, but the final GPE in both cases will be the same, even though the glider doing the downind pass has a much higher ground speed....
Aug 26, 2012, 10:44 PM
Tossing planes into the snow
Canada, BC, Smithers
Joined Nov 2011
3,522 Posts
Here is a short video of slope soaring that demonstrates some of the principles discussed here. I am no expert, as this was only my second time slope soaring, but you can see that it is possible to go from zero ground speed and then turn downwind without stalling the plane. The downwind turns are in the second half of the video.

 Radian Pro slope soaring at ski hill (4 min 18 sec)
 Aug 26, 2012, 11:38 PM Glad I'm not a Boll Weevil Pleasant Valley Modelport Joined Sep 2006 10,008 Posts What's the stall speed of that glider, 3-5 mph ? You couldn't see it in that case. Simple way out, just carry more power in a level or climbing turn. I thermal at near zero ground speed, turn around and get in a stall. I just kick some power up for the turn then pull it back off. This not with a glider but a 2 3/4 lb 14oz/sq in plane in heavy thermals. Editidied the wing loading, it's 14.5oz/sq ft not 10, the 10 was a guess from memory. Last edited by Chophop; Aug 26, 2012 at 11:47 PM.
 Aug 27, 2012, 08:29 AM You can't take the sky from me United States, IA, Hampton Joined May 2012 352 Posts Chop I have on countless occasions flown a C-150 with the stall horn chirping on the edge of stall flying backwards, and was still able to turn 180 degrees without adding power or losing altitude and never once stalled. How do you explain that?
 Aug 27, 2012, 09:50 AM Glad I'm not a Boll Weevil Pleasant Valley Modelport Joined Sep 2006 10,008 Posts The conditions for a stall were probably not met. If the inertia of the airplane is enough to prevent accelerating back up to flying airspeed and over the time needed to develop a stall, it will stall. It happens to mine now and then but not every time. I went to flight school and was taught the procedure. I am not going to deviate no matter what anyone other than a group certified flight instructors say. What's the big effort to add a bit of power when slow and turning downwind ? It's a part of flying to me, keeping track of the situation. And that little plastic guy is a space case, I wouldn't trust him.

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