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Old Apr 27, 2005, 03:32 PM
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Clear Lake City Metroport, Texas, United States
Joined Aug 2004
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Downwind Turn

I keep reading posts with the term "Downwind Turn" and "it's having a struggle when flying into the wind".

Let's get one thing straight. There is NO SUCH THING as a "downwind turn" or a "dowwind turn stall". This is basic physics 101.

Let the arguments begin....
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Old Apr 27, 2005, 03:41 PM
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davidfee's Avatar
San Diego, CA, USA
Joined Mar 2001
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All we can do is hope... but, unfortunately, "conventional wisdom" is more prevalent in this world than an education in basic physics. And yes, it's amazing that people who have been flying models for years will still insist that the conventional wisdom is correct.

Oh well, let them crash their models and blame it on "interference," "that darned carbon fiber stuff," "flux discontinuities" and, of course, the dreaded "downwind turn."

-David
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Old Apr 27, 2005, 07:28 PM
RIP Ric
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Marietta, GA
Joined Jun 1999
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or "tip stalls"..
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Old Apr 27, 2005, 07:43 PM
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Blacksburg, VA 24060 USA
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There are no such things as "tip stalls?"

Jim R
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Old Apr 27, 2005, 08:54 PM
RIP Ric
Andy W's Avatar
Marietta, GA
Joined Jun 1999
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Find me an aviation reference that describes them and I'll amend my postion.
..a
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Old Apr 27, 2005, 10:38 PM
Deletedfor proving Nauga wrong
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Quote:
Originally Posted by Andy W
or "tip stalls"..
"Tip Stalls" are a proveable thing... it is when the wingtip stalls before the center of the wing or (more common) one wing stalls before the other. It CAN and does happen. If it didn't happen, and wasn't predictable, you wouldn't be able to do a snap-roll using just rudder and elevator. (and they've been doing that for more than 60 years on purpose... longer by accident.)

The entry to a spin involves having one wing stall before the other.. (and a snap roll is sort of a spin... both are stall based maneuvers causing fast rotation of the aircraft.)

*******

The "downwind turn" problems are an effect of perspective. If you maintain a constant pabnk angle and rate of turn.. then the airplane doesn't care about the wind... but the "center" of the turn will drift downwind. By trying to maintain the turn in a constant radius about a fixed point on the ground, you have to vary the bank angle and turn rate. When you increase the bank and pull to increase rate of turn going from downwind to upwind, you increase the G load on the aircraft and add drag. This can be asking for a stall if you were already flying slow.

********

Full scale pilot's handbooks (Stick and Rudder... and the AOPA's "The Proficient Pilot") discuss the phenomenon we CALL "Tip-Stall" and the effects of wind relatied to making a turn about a FIXED point on the ground.

We might be using different words... but the physics of them are the same for the models and the airplanes people ride in.

*********

Maintain thy AIRSPEED (not ground speed..) lest the ground rise up and smite thee.

You are standing on the GROUND and observing GROUND speed. And that often has very little to do with the model's airspeed.
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Old Apr 28, 2005, 07:57 AM
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http://www.wfu.edu/albatross/atwork/dynamic_soaring.htm
For days to weeks the albatross don't stall with repeated, again and again, downwind turn!!!
DS R/C flyers do the same:
http://members.tripod.com/douglasturner/id27.htm

With stalling margin of airspeed (not ground speed) there in not a downwind turn problem!!!

When gusty wind, increase stalling margin of airspeed.

A bad flyer confuses ground speed with airspeed!!!
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Last edited by Ollie; Apr 28, 2005 at 08:27 AM.
Old Apr 28, 2005, 08:47 AM
All under control, Grommit!
leccyflyer's Avatar
United Kingdom, Aberdeen
Joined Sep 2000
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Quote:
Originally Posted by Andy W
Find me an aviation reference that describes them and I'll amend my postion.
..a
Will this do a. ?

http://aerodyn.org/Wings/larw.html
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Old Apr 28, 2005, 10:21 AM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
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The really is a downwind turn problem

Downwind Turn, Weather Vaning, Ballooning etc.

After reading one of these 'downwind turn' discussions I went to our field and watched the wind sock for a while. It was the usual New Mexico spring breeze. I would have described it as constant SW at 5-10 mph. However, after watching the wind sock for a few minutes I noticed that air was moving across the field in packets or bubbles. The wind was always slowly but constantly accelerating (signed velocity).

So, a model is not simply in a moving mass of air. It is constantly adjusting to changes in air velocity. Due to inertia, there is a lag in the model's response. This inertial lag results in a net change in the flow of air over the model's surfaces. A model with dihedral that turns right angle to an accelerating air mass will experience a roll force. A model running down wind in an accelerating air mass will experience a loss of lift, and control. A model that turns up wind to an accelerating air mass will balloon and even stall.

However, if the air mass were laminar and moving at a constant rate the model would not respond to it. Such air movement is rare.
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Old Apr 28, 2005, 10:28 AM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
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Ollie,

This mode of flight demonstrates that there is a downwind turn 'problem'. The Albatross is constantly changing it's rigging in order to gain energy by diving in and out of the boundary layer. If the Albatross were simply static in moving air mass it's flight would end after a few seconds.
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Old Apr 28, 2005, 11:50 AM
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When sloping we see gust effects on the planes all the time.. particularly when going across the slope. One wing can totally stall, and the plane roll 90 degrees instantly.
The air does come in packets... watching the grass down the slope move as Tom mentions with the wind sock shows the effect of the packets relative to the larger moving mass.
But the proverbial "downwind turn" is purely a perception problem due to the moving plane and the fixed pilot.
The albatross uses the essentially fixed sea wave for its reference point to get the dynamic soaring effect it thrives on.
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Old Apr 28, 2005, 11:50 AM
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The good flying albatross has a small head. The small head can't stuff the "downwind turn problem" in the small head. A good flyer allows the stalling margin airspeed above the stalling speed.

In a RC flyer in a floating a hot air balloon. When wind is high, even with the balloon drifts along. The RC flier flies perfect circles at constant speed and constant angle of bank. The same RC flyer on the ground and at the same wind has a problem in his head because of the reference to the ground speed.

The ground speed is referenced to the maneuver for power training. Sailplane flier is trained to the air speed and other gusts and air changes.

"TK was dragged out to a power field on a Sunday morning to see an old
friends immaculate 1/3 scale Piper Cub that he had just finished over the
winter fly for the first time. Well after they got the motor and
everything straightened out on the ground, he asks TK to take the first
flight, "just in case". He wasn't expecting to do the test flying as it
was a big power club and there were lots of "experts" in attendance, but he
didn't want to see his friend break his model either so he agreed to take
the first flight. So here is a senior guy, a brand new 1/3 scale Piper
Cub, a glider guy to do the test flying, and the club "experts" have now
broken away from running gas thru their engines on the ground to checking
this out like a bunch of vultures.

TK taxis the Cub out, takes off, trims it out, starts backing off the
throttle a bit and the engine sputters and quits. Next thing he knows guys
are screaming up and down the flight line, "DEAD STICK - - DEAD
STICK" He's like thinking to himself, no big deal, got flying speed, got
altitude and over the field, why are these guys screaming... so he goes to
set up an approach and the wing gives a bobble. He sets the Cub in a
thermal turn and spends the next 10 minutes specking it out. Brings it
back down, sets up a nice approach on their paved strip, and rolls it out
to his feet... and yells DEAD STICK."

Thoughts in some heads and not in others.
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Last edited by Ollie; Apr 28, 2005 at 02:05 PM.
Old Apr 28, 2005, 11:53 AM
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Palmdale, CA
Joined Oct 2000
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Ollie....
All power fliers need glider experience.. "Dead stick" from takeoff to landing..
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Old Apr 28, 2005, 12:07 PM
All under control, Grommit!
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United Kingdom, Aberdeen
Joined Sep 2000
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Great story Ollie!
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Old Apr 28, 2005, 02:38 PM
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Punta Gorda, FL
Joined Apr 2002
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It is a matter of attitude. Some use the air wind gust energy flying with finesse. Others can carry energy in the planes to stomp the air into submission.

The small downwind turn problem is part of a flier's attitude.
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